Graphene2014 Conference Book

Graphene2014 May 06-09, 2014 Toulouse (France) 3

OREWORD

On behalf of the Organising, Scientific and Local Committees we take great pleasure in welcoming you to Toulouse for the fourth edition of the Graphene International Conference & Exhibition.

A plenary session with internationally renowned speakers, extensive thematic workshops in parallel, one-to-one meetings (Brokerage Event) and a significant industrial exhibition featuring current and future Graphene developments will be highlighted at the event.

Graphene 2014 is now an established event, attracting global participants intent on sharing, exchanging and exploring new avenues of graphene-related scientific and commercial developments. The event is raising great interest and is now considered as the Graphene meeting point in 2014.

We truly hope that Graphene 2014 serves as an international platform for communication between science and business.

We are also indebted to the following Scientific Institutions, Companies and Government Agencies for their help and/or financial support: Phantoms Foundation, Université Catholique de Louvain, ICN2 (ICN-CSIC), Centre National de la Recherche Scientifique, CEMES, CIRIMAT, Université de Montpellier 2, LCC Ensiacet, Université de Bordeaux, Grafoid Inc., Aixtron, Thermo Scientific, Fondation AIRBUS Group, AIRBUS, HORIBA Scientific, The Nano, EXtreme measurements & Theory (NEXT) project, ONERA, Donostia International Physics Center (DIPC) & Materials Physics Center (CFM), SO Toulouse, Galeries Lafayette, Mairie de Toulouse, EuroPhysics Letters (epl), INSA Toulouse, Solvay, Center for Nanostructured Graphene, GDRI: Graphene-Nanotubes, American Elements, PRACE, Université Toulouse III, Paul Sabatier, Groupe Français d’Etude des Carbones (GFEC), Région Midi-Pyrénées, European Physical Society (EPS), Cambridge University Press and Air France / KLM.

We also would like to thank all the exhibitors and participants that join us this year.

One thing we have for granted: very few industries, one way or another, will escape from the influence of Graphene and the impact on businesses is here to stay.

Hope to see you again in the next edition of Graphene 2015 to be held during ImagineNano event (www.imaginenano.com) in Spain.

Graphene 2014 Organising Committee

F

4 May 06-09, 2014 Toulouse (France) Graphene2014

NDEX

PAGE

5 COMMITTEES

7 SPONSORS

13 EXHIBITORS

15 SPEAKERS

23 ABSTRACTS

I


OMMITTEES

Organising Committee

Phantoms Foundation (Spain) Antonio Correia

Universite Catholique de Louvain (Belgium) Jean-Christophe Charlier

ICN2 (Spain) Stephan Roche

Local Organising Committee

CEMES CNRS (France) Erik Dujardin

LAAS/CNRS (France) George Deligeorgis

CIRIMAT/CNRS (France) Emmanuel Flahaut

Université de Montpellier 2 (France) Jean-Louis Sauvajol

LCC Ensiacet (France) Philippe Serp

Université de Bordeaux (France) Alain Penicaud

International Scientific Committee

National University of Singapore (Singapore) Antonio Castro Neto

IIT, Graphene Labs (Italy) Francesco Bonaccorso

Institute of Metal Research (China) Hui-Ming Cheng

Texas Instruments (USA) Luigi Colombo

TITECH (Japan) Toshiaki Enoki

Cambridge University (UK) Andrea Ferrari

FORTH/ ICE-HT (Greece) Costas Galiotis

ICMM-CSIC (Spain) Paco Guinea

University of Chalmers (Sweden) Jari Kinaret

SungKyunKwan University (Korea) Young-Hee Lee

Academia Sinica (Taiwan) Lain-Jong Li

ONERA (France) Annick Loiseau

MPI for Polymer Research (Germany) Klaus Mullen

CNR-ISOF (Italy) Vincenzo Palermo

Airbus (Spain) Jose Sanchez-Gomez

C


PONSORS

Platinum Sponsor

www.grafoid.com

Gold Sponsors

www.aixtron.com

www.fondation.airbus-group.com

www.thermoscientific.com/carbon

Silver Sponsors

www.airbus.com

www.horiba.com/scientific

www.next-toulouse.fr

Bronze Sponsor

www.onera.fr

http://dipc.ehu.es

http://cfm.ehu.es/

S

2014

SEPT 1-3NINGBO, CHINA

China Innovation Alliance of the

Graphene Industry (CGIA) is the first

national alliance ever in China,

comprising dozens of top research

institutions and a many inbound and

outbound enterprises. CGIA is

designed to provide opportunity to

knowledge sharing and networking

for those engaged in research and

development of Graphene based

industries with the investors across

the globe.

WWW.CONFWWW.CONFWWW.CONFWWW.CONF----ANN.COMANN.COMANN.COMANN.COM

TOPICSSymposia A: Symposia A: Symposia A: Symposia A: Fundamental of Graphene, other

2D materials and Related Devices

SymposiaSymposiaSymposiaSymposia B: B: B: B: Production of Graphene Materials

SymposiaSymposiaSymposiaSymposia C: C: C: C: Applications Exploration of

Graphene-based Materials

Symposia Symposia Symposia Symposia D: D: D: D: Applications and Commercialization

of Graphene-based Materials

Symposia E: Symposia E: Symposia E: Symposia E: Characterization and

Standardization of Graphene

Symposia Symposia Symposia Symposia F: F: F: F: Global Initiatives and Cooperation

of Graphene Industrialization

Symposia G: Symposia G: Symposia G: Symposia G: Graphene Resource Cooperation

and Utilization

Symposia Symposia Symposia Symposia H: H: H: H: Graphene Investment

Hosted byHosted byHosted byHosted by: CONTACT US:

Tel: +86-10-62773655

[email protected] Innovation Alliance of the Graphene Industry

Phantoms Foundation

Ningbo City Government


Lanyards Sponsor

http://grafoid.com/

Other Sponsors

www.so-toulouse.com

www.galerieslafayette.com

www.epljournal.org

www.insa-toulouse.fr

www.solvay.com

www.cng.dtu.dk

www.graphene-nanotubes.org

www.cnrs.fr

www.americanelements.com

www.prace-ri.eu

www.univ-tlse3.fr

www.gfec.net

www.midipyrenees.fr


Awards Sponsors

www.eps.org

www.epljournal.org

www.cambridge.org

www.next-toulouse.fr

Official carriers

www.airfrance.com

Advertisement


Platinum Sponsor

Grafoid Inc. is a privately held Canadian graphene development company with a global presence. As a developer and producer of high-performing few layer graphene, we resolve issues, adapt and engineer our trademarked MesoGraf™ graphene to both industrial-scale material and product applications. As a leader in graphene’s market development we are on the cusp of graphene’s commercialization. Our MesoGraf™ production is economically scalable, environmentally sustainable, reproducible on a mass scale and safe. We believe MesoGraf™ sets the global standard for graphene; our science is proven and we are currently in production in Canada, in Singapore and plan production in the United States in late 2014. Grafoid is a complete solutions graphene company.

Gold Sponsors

AIXTRON is a leading provider of deposition equipment to R&D and the semiconductor industry. The Company's technology solutions are used by a diverse range of customers worldwide to build advanced components for electronic and opto-electronic applications based on compound, silicon, or organic semiconductor materials, as well as carbon nanotubes (CNT), graphene and other nanomaterials. Our equipments are used today to manufacture high performance thin films for fiber optic communication systems, wireless and mobile telephony applications, optical and electronic storage devices, computing, signalling and lighting, as well as a range of other leading-edge technologies.

Under the Thermo Scientific brand of Thermo Fisher Scientific, Inc., we help scientists meet the challenges they face in the lab or in the field every day. Our high-end analytical instruments, laboratory equipment, software, services, consumables and reagents help our customers solve complex analytical challenges, improve patient diagnostics and increase laboratory productivity. Thermo Scientific solutions provide the most comprehensive suite of techniques, including imaging, spectroscopy, material characterization, and data management. In particular, Raman spectroscopy can be used to gain insight into graphene-based materials provides insight into material properties such as structural uniformity, film quality, degree of functionalization, and electrical properties.


Silver Sponsors

The Labex NEXT (Nano, EXtreme Measurements & Theory) gathers 400 physicists and chemists, with complementary theoretical and experimental skills from six regional laboratories of CNRS, INSA Toulouse and Paul Sabatier University: www.next-toulouse.eu/. The NEXT community is dedicated to frontier science in nano-physics and nano-chemistry, condensed and “soft” matter physics, optics and atomic/cluster physics. The trademark of NEXT is the combination of materials design and production and the study of matter in extreme conditions (very high magnetic field, very low temperature, ultra-high vacuum, atomic-scale and ultra-fast resolution...). To this end, NEXT strongly invests brings a financial support to ambitious trans-disciplinary scientific projects, promotes the attractiveness of local masters and doctoral schools and contributes to significant innovation actions through start-up companies.

Advertisement


XHIBITORS

E

Advertisement


Graphene Research Centers Pavilion

France Pavilion

China Pavilion


peakers

alphabetical order PAGE Mie Andersen (iNANO, Aarhus University, Denmark) "Intercalating graphene on Ir(111)"

Oral Parallel 23

Charalampos Androulidakis (ICEHT-FORTH, Greece) "Mechanical behaviour of embedded monolayer graphene sheets under axial loading"

Oral Parallel Students

25

Adrian Bachtold (ICFO, Spain) “Coupling graphene mechanical resonators to superconducting microwave cavities”

Oral Plenary 27

Adrian Balan (University of Pennsylvania, USA) "Hybrid graphene nanoribbon-nanopore devices for biolomolecule detection and DNA sequencing"

Oral Plenary 28

Bruno Beccard (Thermo Fisher Scientific, France) Abstract not available

Thermo Scientific Workshop

- Giuseppe Valerio Bianco (CNR-IMIP, Italy) "Functionalization of Graphene by Plasma Treatments for Tailoring Electrical Properties"

Oral Parallel 31

Christophe Bichara (CNRS and AMU, France) "Growing perfect monolayer of graphene from nickel surfaces"

Oral Parallel 33

Volker Blum (Duke University, USA) "Surface Thermodynamic Equilibrium Conditions and the Growth of Monolayer Graphene Films on SiC"

Invited Parallel 35

Francesco Bonaccorso (IIT, Graphene Labs@IIT, Italy) “Graphene for Energy Storage”

Invited Plenary 36

Vincent Bouchiat (Neel Institute, CNRS, France) "Collapse of superconductivity in a hybrid tin-graphene Josephson junction array"

Oral Plenary 37

Louis Bouet (LPCNO INSA-CNRS-UPS, France) "Carrier and polarization dynamics in monolayer MoS2 studied by time resolved photoluminescence"

Invited Parallel 39

Romain Bourrellier (Université Paris Sud 11, France) "Nanometric resolved cathodoluminescence on few layers h-BN flakes"


42 Matteo Bruna (Cambridge Graphene Centre, UK) "Doping dependence of the Raman signatures of defects in graphene"

Oral Parallel 44

Ivan Buckley (Nat. Graphene Institute at the Univ. of Manchester, UK) "The National Graphene Institute at the University of Manchester"

Oral Parallel 46

Damien Cabosart (Université Catholique de Louvain (UCL), Belgium) "Imaging coherent transport in a graphene quantum ring"

Oral Parallel Students 47

Seymur Cahangirov (Universidad del País Vasco, Spain) "Model of √3x√3 phases of silicene and its multilayers"

Oral Parallel 49

Eduardo Carrasco (Ecole Polytechnique Fédérale de Lausanne, Switzerland) "Graphene-Based Plasmonic Arrays for Dynamic Light Bending"

Oral Parallel 50

S


PAGE Antonio Castro Neto (National University of Singapore, Singapore) “Two-dimensional crystals: the next steps ahead”

Plenary Lecture 52

Iñigo Charola (Graphenea, Spain) Abstract not available " Graphenea: Crossing the Chasm in a High-Tech Market”

Invest in Graphene -

Chun-Wei Chen (National Taiwan University, Taiwan) "Tunable graphene based optics, electronics and photonics"

Invited Plenary 53

Kuei-Hsien Chen (Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan) "Graphene Oxide and its Hybrids as Photocatalysts for Solar Fuels"

Taiwan Landscape Workshop

54

Manish Chhowalla (Rutgers University, USA) "Phase Engineering in 2D Transition Metal Dichalcogenides"

Invited Plenary 55

Seungmin Cho (Samsung Techwin Co., Korea) “Improving the quality of a graphene film by process innovation”

Invited Plenary 56

Min Sup Choi (SKKU Institute of Nano-Technology (SAINT), Korea) "Chemical p- and n-doping for MoS2 transistor and its application"


57 Sung-Yool Choi (KAIST, Korea) “Dry-transfer process and interface engineering for high performance graphene transistor”

Invited Plenary 59

Sung-Yool Choi (KAIST Graphene Research Center, Korea) “Convergence Research at KAIST Graphene Research Center”

Oral Parallel 60

Mei-Yin Chou (IAMS, Academia Sinica, Taiwan) "Computational Studies of Two-Dimensional Materials: From Graphene to Few-Layer Graphene and Beyond"


61

Jonathan N. Coleman (Trinity College Dublin, Ireland) "Liquid exfoliation of 2D materials: From scaleup to applications"

Invited Plenary 63

Luigi Colombo (Texas Instruments, USA) "Graphene and Other 2D Materials: Challenges and Opportunities"

Invited Parallel 64

Dominique Coquillat (CNRS-Université Montpellier 2, France) "Broadband terahertz imaging with sensitive graphene field-effect-transistors"

Invited Parallel 66

Alessandro Cresti (IMEP-LAHC (UMR 5130), Grenoble-INP, Minatec, France) "Topological insulator graphene by heavy atom adsorption: Impact of segregation"

Oral Parallel 68

Aron Cummings (Institut Català de Nanociència i Nanotecnologia, Spain) "Grain Boundary Resistivity in Polycrystalline Graphene"

Oral Plenary 70

Lun Dai (School of Physics, Peking University, China) "Novel Pattern Graphene Fabrication Methods and Their Application in Graphene Based Nano-Optoelectronic Devices"

Oral Parallel 72

Yohan Dall'Agnese (Université Paul Sabatier, France) "Two-Dimensional Early Transition Metal Carbides as Electrode Materials for Energy Storage"


74

G.P. Das (Indian Association for the Cultivation of Science, India) “Epitaxial Silicene on Semiconductor Substrates: a Density Functional Study”

Oral Parallel 76

Georg Duesberg (Trinity College Dublin , Ireland) "Processing 2D Materials: From synthesis to devices"

Oral Plenary 77

Dinh-Loc Duong (Sungkyunkwan University, Korea) "Graphene/piezoelectric hybrid for Coulomb drag of graphene bilayer system"

Oral Plenary 78

Siegfried Eigler (Friedrich-Alexander Universität Erlangen-Nürnberg, Germany) "Synthesis of graphene oxide with an almost intact carbon framework - a precursor for graphene and an entry to functionalized graphene derivatives"

Oral Plenary 79


PAGE Toshiaki Enoki (Tokyo Institute of Technology, Japan) "How does the magnetic edge-state vary in the fusion of nanographene sheets?"

Oral Plenary 82

Jonathan Eroms (University of Regensburg, Germany) "Towards superlattices: Lateral bipolar multibarriers in graphene"

Oral Plenary 84

Andrea C. Ferrari (University of Cambridge, UK) “Raman Spectroscopy in Graphene and Layered Materials”

Keynote Plenary 86

Silvia Ferrari (Invest in Toulouse, France) Abstract not available "Toulouse and the graphene: connections, actors and business opportunities"


Albert Fert (Université Paris Sud & CNRS/Thales, France) "Spin transport and spintronics with graphene"

Plenary Lecture 87

Marco Fiocco (Politecnico di Milano - LNESS, Italy) "Integrated graphene high-gain voltage amplifiers and ring oscillators"

Oral Parallel 89

Rahul Fotedar (Graphene Batteries AS, Norway) "Graphene-Silicon Composites for Li-ion Battery Anodes"

Oral Parallel 92

Wangyang Fu (Basel University, Switzerland) "High-frequency measurement of graphene transistor for biosensing"

Oral Parallel 94

Philippe Gaillard (University of Namur, Belgium) "Numerical simulations of the growth of graphene on Cu (111)"


96 Costas Galiotis (FORTH/ ICE-HT, Greece) "FORTH Graphene Centre: Research Activities and Perspectives"

Oral Parallel 97

José Hugo Garcia Aguilar (Universidade Federal do Rio de Janeiro, Brazil) "Heavy adatoms and Anderson localization in graphene"

Oral Parallel 98

Aran Garcia Lekue (DIPC, Spain) “Graphene nanoislands on Ni(111): structural and scattering properties”

Invited Parallel 100

Fernando Gargiulo (EPFL, Switzerland) "Electronic transport in graphene with aggregated hydrogen adatoms"


102 David Goldhaber-Gordon (Stanford University, USA) “Novel Phenomena Driven by Interactions and Symmetry-breaking in Graphene”

Keynote Plenary 104

Stijn Goossens (ICFO, Spain) "Prototyping CMOS-compatible ultrasensitive photo detectors for visible and infrared light"

Oral Parallel 105

Gong Gu (University of Tennessee, USA) "Grow Up and Grow Out: van der Waals Epitaxy and In-Plane Epitaxy of Two-Dimensional Materials"

Oral Parallel 107

Sophie Gueron (Université Paris-Sud, France) “Unipolar supercurrent through graphene grafted with Pt-Porphyrins: Signature of gate voltage dependent magnetism”

Oral Plenary 109

Bharati Gupta (Queensland University of Technology, Australia) "An UHV study of epitaxial growth of graphene on SiC/Si substrates by Si sublimation"


110

Ling Hao (National Physical Laboratory, UK) "Development of Near-field Microwave Methods for Graphene NEMS Resonators"

Oral Plenary 112

Masataka Hasegawa (AIST, Japan) “Graphene Synthesis by Plasma Technique for Transparent Conductive Film Applications”


Ana Heras (ICEX España Exportación e Inversiones-Invest in Spain, Spain) “Why Spain for innovative industries”

Invest in Graphene 116


PAGE Po-Hsun Ho (National Taiwan University, Taiwan) "Wavelength-Selective n- and p-typed carrier transport of a graphene transistor by organic/inorganic Hybrid Doping Platform"


117

James Hone (Columbia University, USA) "Fabrication, Properties, and Applications of Ultrahigh Performance Graphene"

Invited Plenary 118

Suklyun Hong (Sejong University, Korea) "Graphene Research Institute at Sejong University"

Oral Parallel 120

David Horsell (University of Exeter, United Kingdom) "Electron cooling mechanisms in graphene"

Oral Plenary 121

Andrew Houghton (European Commission, Belgium) "Graphene in the European Union Future and Emerging Technologies Flagships Programme: Status and Future Plans"


Jian Huang (University of Oxford, UK) "Role of disorder in temperature-dependent magneto-transport of epitaxial graphene near the Dirac point"


123

Giuseppe Iannaccone (University of Pisa, Italy) "Performance assessment of graphene-based lateral and vertical heterostructure FETs"

Oral Parallel 125

Susanne Irmer (University of Regensburg, Germany) "Spin-orbit coupling in dilute fluorinated graphene"


128 Antti-Pekka Jauho (DTU Nanotech, Denmark) "CNG – Center for Nanostructured Graphene"

Oral Parallel 130

Antti-Pekka Jauho (DTU Nanotech, Denmark) "Transport phenomena in nanostructured graphene"


Debdeep Jena (University of Notre Dame, USA) “Electron Transport in Graphene based 2D Crystals for Novel Electronic Devices”


Christian Joachim (CEMES/CNRS, France) “Nanographene: wires, analogue adder and molecule logic gates”

Invited Plenary 135

Benoit Jouault (CNRS, France) "Transport properties and quantum Hall effect of graphene films grown by CVD on SiC(0001) with in-situ hydrogenation"

Oral Plenary 136

Frantisek Karlicky (Palacky University, Czech Republic) "On the Electronic and Optical Properties of Fluorographene, Chlorographene, and Graphane"

Oral Parallel 138

Anupama Kaul (National Science Foundation, USA) "Prospects of Beyond-Graphene Materials and Devices"


Morris (Ming-Dou) Ker (National Chiao Tung University, Taiwan) "Introduction of National Program on Nano Technology (NPNT)"

Taiwan Landscape Workshop 141

Jari Kinaret (Chalmers University of Technology, Sweden) “Graphene Flagship"


Frank Koppens (ICFO, Spain) "Graphene@ICFO"

Oral Parallel 144

Arkady Krasheninnikov (Aalto University, Finland) "Defects in two-dimensional transition-metal dichalcogenides and silica bilayers"

Oral Plenary 145

Norio Kumada (NTT Corporation, Japan) "Edge Magnetoplasmon in Graphene Investigated by Frequency and Time Domain Measurements"

Oral Plenary 147


PAGE Fabien Lafont (LNE, France) "Unusual backscattering between quantum Hall edge states in CVD graphene"


149

Uzi Landman (Georgia Tech, USA) Abstract not available

Invited Parallel -

Vincent Larat (HORIBA Scientific, France) "Graphene and Raman spectroscopy: new instrumental developments"

Oral Parallel 152

Bo W. Laursen (University of Copenhagen , Denmark) "Application of Graphene Materials as Electrodes for Molecular Electronic Devices"

Oral Parallel 153

James Lawlor (Trinity College Dublin, Ireland) "Sublattice asymmetry of substitutionally doped impurities in graphene"


Guy Le Lay (Aix-Marseille University, France) "After silicene, epitaxial germanene: a newborn in the graphene family"

Oral Plenary 156

Silvia Lazcano (Airbus, Spain) "Graphene for Advanced Aircraft Airframe"


Nicolas Leconte (ICN2, Spain) “PRACE: how an european research infrastructure supports the graphene Community”

Oral Parallel 158

Gun-Do Lee (Seoul National University, Korea) "Stability and Dynamics of Defects in Graphene: Combinatorial Study of HR-TEM and Simulation methods"

Oral Plenary 159

Seung Hwan Lee ((SAINT) Sungkyunkwan University, Korea) "Enhancement of rectifying behavior for hetero-structured graphene tunneling diodes by chemical doping"


Lain-Jong Li (Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan) "Production and Applications of High Quality Graphene Flakes and 2D Monolayers"


163

Wu Li (Scientific Computing & Modelling NV, The Netherlands) "Thermal conductivity and Phonon Linewidths of Monolayer MoS2 from First Principles"

Oral Parallel 165

Tom Lindfors (Åbo Akademi University, Finland) "Composites materials of graphene derivatives and electrically conducting polymers and their application in solid-state ion-selective electrodes"

Oral Parallel 166

Zhongfan Liu (Peking University, China) "CVD Growth of Graphene and Its 2D Hybrids: Attraction, Reality and Future"

Invited Plenary 168

Annick Loiseau (LEM, ONERA - CNRS, France) "Spectroscopic properties of BN layers"

Oral Plenary 169

Maria Losurdo (IMIP-CNR, Italy) "The Copper for CVD Graphene: its cleaning and oxidation in relation to graphene quality"

Oral Parallel 171

Tony Low (IBM, Yale & Columbia Universities, USA) "Light-Matter Interactions in 2D Materials"


Grzegorz Lupina (IHP, Germany) "Challenges in integrating graphene into CMOS technology platform"

Oral Plenary 174


PAGE Teng Ma (Shenyang National Laboratory for Materials Science, China) "Edge-Controlled Growth and Kinetics of Single-Crystal Graphene Domains by Chemical Vapor Deposition"


176

Sami Makharza (IFW Dresden, Germany) "Graphene Oxide Mediated Carboplatin Delivery and its Anticancer Activity"


178

Teresa Martínez González (Ministerio de Hacienda y AAPP, Spain) "Financing Through European Territorial Cooperation Funds"


Rafael Martinez Gordillo (ICN2, Spain) "Transport Fingerprints at Graphene Superlattice Dirac Points Induced by Boron-Nitride Substrate"


180

Laëtitia Marty (Institut Néel, France) "Strain superlattices and suspended graphene at the macroscale"

Oral Plenary 181

Riccardo Mazzarello (RWTH Aachen, Germany) "Electronic and magnetic properties of graphene nanoribbons deposited on metallic substrates"

Oral Plenary 183

Pere Miro (Jacobs University Bremen, Germany) "Two-Dimensional Materials Beyond MoS2"

Oral Parallel 184

Martin Mittendorff (Helmholtz-Zentrum Dresden-Rossendorf, Germany) "Ultrafast detection from 0.6 THz to 33 THz employing graphene flakes"

Oral Parallel 185

Niclas Mueller (Freie Universitaet Berlin, Germany) "Growing graphene on polycrystalline copper foils by ultra-high vacuum chemical vapor deposition"


187

Sananda Nag (European University of Brittany, France) "Application of Reduced Graphene Oxide Based Hybrid Functional Nanomaterials in Vapour Sensors for Human Health Monitoring"


189

Zhenhua Ni (Southeast University, China) "Modulating the properties of MoS2 by plasma thinning and defect engineering"

Oral Plenary 191

Nikan Noorbehesht (University of Sydney, Australia) "Superb Electrocatalytic Activity for the Oxygen Reduction Reaction at N-doped CNT-Graphene Composite Electrodes"


193

Marc Nuñez (CEMES - CNRS, France) "Towards integrated atomically smoothed graphene nanoribbons devices"


Il-Kwon Oh (KAIST, Republic of Korea) "Graphene Hybrid Materials for Energy Storage and Actuator Devices"


Hanako Okuno (CEA-Grenoble, France) "A new route towards low temperature production of continuous graphene film"

Oral Parallel 200

Pablo Ordejon (ICN2, Spain) "Graphene research at ICN2"

Oral Parallel 202

Luca Ortolani (CNR-IMM Bologna, Italy) "Interferometric TEM characterization of graphene materials"


Juan José Palacios (Universidad Autónoma de Madrid, Spain) "Intrinsic ferromagnetism induced by hydrogen adsorption on graphite surfaces"

Oral Parallel 205

Seongjun Park (Samsung Advanced Inst. of Tech. (SAIT), Korea) Abstract not available

Invited Parallel -

Keith Paton (Thomas Swan & Co. Ltd., UK) "A New Industrially Relevant Solvent Exfoliation Route to Graphene"

Oral Parallel 206


PAGE Vittorio Pellegrini (IIT & CNR-NANO Pisa, Italy) “Graphene and 2d crystals research @ Istituto Italiano di Tecnologia, Graphene Labs”

Oral Parallel 208

Vitor M. Pereira (National Univ. of Singapore, Singapore) "Designing electronic properties of two-dimensional crystals through optimization of deformations"


Sergio Pezzini (Universitá Degli Studi di Pavia, Italy) "High-mobility h-BN/graphene/h-BN devices: zero-field electrical transport and quantum Hall criticality"


211

Marcos Pimenta (UFMG, Brazil) "Resonance Raman studies of twisted bilayer graphene and 2D transition metal dichalcogenides"

Invited Plenary 214

Andreas Pospischil (Vienna University of Technology, Austria) "Photovoltaic Energy Conversion and Electrically Driven Light Emission in a WSe2 monolayer"

Oral Plenary 215

Renaud Puybaret (Georgia Institute of Technology, USA) "Scalable control of graphene growth on 4H-SiC C-face using decomposing silicon nitride masks"

Oral Parallel 218

Tatiana Rappoport (Universidade Federal do Rio de Janeiro, Brazil) "Spin Hall Effect Induced by Resonant Skew Scattering in Graphene"

Oral Plenary 221

Bertrand Raquet (LNCMI- Toulouse, France) "Quantum magnetotransport phenomena on ultra-narrow graphene nano-ribbons"

Invited Plenary 222

Wencai Ren (IMR Chinese Academy of Sciences, China “Synthesis and applications of graphene materials”


Wencai Ren (IMR Chinese Academy of Sciences, China) "Graphene Research Activities at the Institute of Metal Research, Chinese Academy of Sciences"

Oral Parallel 225

Stéphane Requena (GENCI/PRACE, France) "PRACE: how an european research infrastructure supports the graphene community"

Oral Parallel 227

Massimiliano Rocchia (Thermo Fisher Scientific, Italy) Abstract not available

Thermo Scientific Workshop

- Rod Ruoff (CMCM, IBS (UNIST), Republic of Korea) "Future Carbon Materials"

Plenary Lecture 228

Tapani Ryhänen (Nokia Research Center, Finland) Abstract not available

Invited Plenary -

Toby Sainsbury (National Physical Laboratory, UK) "Carbene Functionalization of Exfoliated Graphene: Towards Scalable Dispersion and Integration of Chemically Modified Graphene"

Oral Parallel 229

Mikkel Settnes (Technical University of Denmark, Denmark) "Conductance mapping of graphene using dual-probe STM"


231 Young-Woo Son (Korea Inst. for Advanced Study, Korea) "Interactions inside interlayer spaces of layered materials"


Daniele Stradi (Technical University of Denmark, Denmark) "Magnetic functionalities in epitaxial graphene structures by means of molecular deposition"

Oral Plenary 234

Emmanuel Stratakis (IESL-FORTH , Greece) "Laser photochemical synthesis of novel graphene oxide derivatives for organic electronics"

Oral Parallel 236


PAGE Mauricio Terrones (The Pennsylvania State University, USA) "Going Beyond Graphene: Doped Graphene, Chalcogenide Monolayers and van der Waals Solids"

Invited Plenary 238

Antoine Tiberj (Université Montpellier 2, France) "Reversible optical doping of graphene"


Klaas-Jan Tielrooij (ICFO, Spain) "Electrically controllable strong light-matter interactions with graphene"

Oral Plenary 241

Valentina Tozzini (CNR, Italy) "Multi-Scale Simulations of Graphene for Energy Applications"

Oral Parallel 242

Guy Trambly de Laissardière (LPTM, Université de Cergy-Pontoise, France) "Conductivity of graphene and rotated graphene bilayers with point defects"

Oral Parallel 244

Emanuel Tutuc (The Univ. of Texas at Austin, USA) " Electron Interaction and Tunneling in Graphene-Based Heterostructures"

Invited Plenary 247

Yonhua Tzeng (National Cheng Kung University, Taiwan) "CVD and Applications of Standing, Dendritic and Continuous Graphene and Their Hybrids"


249

Dinh Van Tuan (Institut Catala de Nanociencia i Nanotecnologia, Spain) "Spin-Pseudospin Entanglement and Spin Relaxation in Graphene"


251 Lin Wang (University of Science and Technology of China, China) "Electron spin relaxation in bilayer graphene and monolayer MoS2"


253 Shengnan Wang (NTT Basic Research Laboratories, Japan) "Chemical Identification of Topological Defects in Graphene by Carbon Isotope labeling"

Oral Parallel 255

Heiko Weber (University of Erlangen, Germany) "Magnetoresistance of large-area epitaxial graphene: interactions and dislocations"

Oral Plenary 256

Kung-Hwa Wei (National Chiao Tung University, Taiwan) "Plasma-assisted Electrochemical Exfoliation of Graphite for Rapid Production of Graphene Sheets"

Taiwan Landscape Workshop 257

Wei Wei (IEMN, CNRS, France) "Back-gated Microwave Field-Effect Transistors Based on Transferred CVD-Grown Graphene"


Yasodinee Wimalasiri (University of South Australia, Australia) "Layered assembly of hierarchical graphene/Ni-Al hydroxide composites for supercapacitors"


261

Xiaoyue Xiao (China Innovation Alliance of the Graphene Industry, China) "Graphene in China"

Oral Parallel 263

Heejun Yang (Sungkyunkwan University, Korea) "Integrated Graphene Researches at Sungkyunkwan University"

Oral Parallel 265

Ting Yu (Nanyang Technological Univ., Singapore) "Light-matter interaction in 2D materials: from graphene to TMDs"


Yanfeng Zhang (Peking University, China) "Plasmonic Graphene-Antenna Photodetector and Transistor"

Oral Plenary 267

Paul Zomer (University of Groningen, The Netherlands) "Spin transport in high mobility graphene devices"

Oral Plenary 268


ntercalating graphene on Ir(111)

MIE ANDERSEN Interdisciplinary Nanoscience Center (iNANO) Denmark

Epitaxial graphene can be grown routinely and with high crystalline quality on transition metal surfaces such as Ir(111) [1]. However, in order to access the extraordinary electronic properties of the graphene itself, it is necessary to either transfer the graphene to an insulating substrate or to decouple it from the metal surface after growth [2-3]. Intercalation of atomic or molecular species has shown promising re-sults towards both strategies, as the decoupling from the metal surface in some cases is large enough to enable peeling of graphene flakes from its substrate [4]. Furthermore, intercalation is a flexible way of controlling the doping level in the graphene. Using a combination of angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM) and density functional theory (DFT) we investigate graphene on Ir(111) intercalated by oxygen, which is electronically similar to p-doped freestanding graphene. From a second intercalation step of rubidium atoms we counter-dope the graphene with electrons, see Figure 1. This approach leads to coexisting domains of p- and n-doped graphene on the surface. The resulting intercalation structures, see Figure 2, are characterized by DFT calculations, obtaining doping levels in good agreement with the experiments. Theoretically, we further extend our studies to a range of other intercalation structures including CO, H, alkali and halogen atoms.

References [1] Alpha T N’Diaye, Johann Coraux, Tim

N. Plasa, Carsten Busse and Thomas Michely, New Journal of Physics, 10 (2008) 043033.

[2] Sukang Bae, Hyeongkeun Kim, Youngbin Lee, Xiangfan Xu, Jae-Sung Park, Yi Zheng, Jayakumar Balakrishnan, Tian Lei, Hye Ri Kim, Young Il Song, Young-Jin Kim, Kwang S. Kim, Barbaros Özyilmaz, Jong-Hyun Ahn, Byung Hee Hong and Sumio Iijima. Nature Nanotechnolgy, 5, (2010), 574.

[3] Silvano Lizzit, Rosanna Larciprete, Paolo Lacovig, Matteo Dalmiglio, Fabrizio Orlando, Alessandro Baraldi, Lauge Gammelgaard, Lucas Barreto, Marco Bianchi, Edward Perkins and Philip Hofmann, Nano Letters, 12, (2012) 4503.

[4] Charlotte Herbig, Markus Kaiser, Nedjma Bendiab, Stefan Schumacher, Daniel F. Förster, Johann Coraux, Klaus Meerholz, Thomas Michely and Carsten Busse, Journal of Physics: Condensed Matter, 24, (2012) 314208.

Mie Andersen, Søren Ulstrup, Lucas Barreto, Marco Bianchi, Philip Hofmann, Liv Hornekær and Bjørk Hammer Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark [email protected]

I


Figures

Figure 1: a) High resolution ARPES data of graphene on Ir(111) (GR/Ir), the O-intercalated structure (GR/O/Ir) and the mixed O/Rb-intercalated structure (GR/Rb/O/Ir). b) Doping chart of the transition from p-doped GR/O/Ir to n-doped GR/Rb/O/Ir via two separate doping phases.

Figure 2: Structures (top view and side view) of a) graphene on Ir(111), b) the O-intercalated structure and c) the mixed O/Rb-intercalated structure.


echanical behaviour of embedded monolayer graphene sheets under axial loading

CHARALAMPOS ANDROULIDAKIS Foundation for Research and Technology – Hellas (FORTH) Greece

Monolayer graphene sheets were prepared by mechanical exfoliation with the scotch tape method and transferred on polymeric substrate (PMMA bar with a layer of SU8 photoresist coated on the top) [1,2,3]. For efficient stress transfer the graphene sheets were covered on the top with an additional layer of PMMA. Using the cantilever beam and four point bending configurations the samples were subjected to axial loading both in tension and compression and Raman measurements were taken in situ at every load step. By monitoring the shift of the 2D Raman band with the applied strain the mechanical behaviour of the graphene sheets can be addressed for strain level of ~ 1.5%. When an embedded monolayer graphene is subjected to compressive loading, the behaviour is non linear as can be seen from the plotted curve of the position of the 2D band vs strain (Figure 1), which can be captured with a second order polynomial fit. The plateau of this curve corresponds to the strain that the graphene sheet fails to buckling. Graphene sheets with various dimension’s ratio were examined experimentally and was observed that critical strain to buckling is independent on the flake’s size. All the samples failed at a average critical strain of εcr~-0.6% (Figure 2). The problem was also treated analytically by modeling the interaction of the polymer and graphene with linear elastic springs (Winkler type). Based on the experimental results the Winkler’s foundation modulus was estimated and the form of failure of the buckled was examined. The analytical approach predicts that an embedded graphene buckles with a buckling

wavelength of ~1-2 nm, in contrast to a free graphene, which Euler theory predicts wavelength of three orders of magnitude larger. Based on the slope of ω2D/ε we are able to identify the efficiency of the stress transfer. Furthermore it was found that for efficient stress transfer a minimum length of about ~4 µm is required. Also DFT analysis was performed in order to gain insight in the interaction between graphene and polymer matrix. Under tension no failure was observed until ~1.5 % of strain level (Figure 3). Ch. Androulidakis1,3, C. Galiotis1, E. Koukaras1, Otakar Frank4, G. Tsoukleri1,2, D. Sfyris1, J. Parthenios1,2 and K. Papagelis1,3 1Institute of Chemical Engineering and High Temperature Chemical Processes, Foundation for Research and Technology – Hellas (FORTH), P.O. BOX 1414, Patras 265 04, Greece 2Interdeparmental Programme in Polymer Science & Technology, University of Patras, Patras 26504, Greece 3Department of Materials Science, University of Patras, Patras 26504, Greece 4J. Heyrovsky Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, 182 23 Prague 8, Czech Republic [email protected]

M


References [1] Tsoukleri G., Parthenios J., Papagelis

K., Jalil R., Ferrari A. C., Geim A. K., Novoselov K. S., Galiotis C., Small 5(21) (2009), 2397–2402

[2] Frank O., Tsoukleri G., Parthenios J., Papagelis K., Riaz I., Jalil R., Novoselov K. S., Galiotis C., Acs Nano 4 (6) (2010), 3131-3138.

[3] Charalampos Androulidakis, Emmanuel N. Koukaras, Otakar Frank, Georgia Tsoukleri, Dimitris Sfyris, John Parthenios, Nicola Pugno, Konstantinos Papagelis, Kostya S. Novoselov, Costas Galiotis, (2014), arXiv.org > cond-mat > arXiv:1401.0280

Figures

Figure 1: The position of the 2D band vs applied strain. All the samples buckle at -0.6%.

Corrected (graphene) strain

Experimental data

Experimental Average Mean

0 1 2 3 4 5 60.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

-0.9

-1.0

-1.1

-1.2ε c

r(%)

Dimension Ratio (l/w)

Figure 2: The critical strain is plotted vs the dimension’s ratio.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

2580

2600

2620

2640

2660

2680

Pos 2D, cm-1

Strain, %

Figure 3: Pos 2D vs strain for strain level ~1.5% under tension. No failure is observed.


oupling graphene mechanical resonators to superconducting microwave cavities

ADRIAN BACHTOLD ICFO - Institut de Ciencies Fotoniques Spain

Graphene is an attractive material for nanomechanical devices because it allows for exceptional properties, such as high frequencies and quality factors, and low mass. An outstanding challenge, however, has been to obtain large coupling between the motion and external systems for efficient readout and manipulation. Here, we report on a novel approach, in which we capacitively couple a high-Q graphene mechanical resonator (Q ~ 105) to a superconducting microwave cavity [1]. The initial devices exhibit a large single-photon coupling of ~ 10 Hz. Remarkably, the large capacitive coupling also enables strong tuning of the graphene equilibrium position, renormalization of the single-photon coupling, and softening of the mechanical resonance frequency. We also observe a Duffing nonlinearity whose sign and magnitude can be controlled via a constant voltage applied to the graphene. With realistic improvements, it should be possible to enter the regime of quantum optomechanics. Adrian Bachtold ICFO - Institut de Ciencies Fotoniques, 08860 Castelldefels (Barcelona), Spain [email protected]

References [1] P. Weber, J. Güttinger, I. Tsioutsios, D.E.

Chang, A. Bachtold, arXiv:1403.4792

C


ybrid graphene nanoribbon-nanopore devices for biolomolecule detection and DNA sequencing

ADRIAN BALAN University of Pennsylvania USA

We present a study of hybrid graphene nanoribbons-nanopore devices[1] for biomolecule detection and ultimately DNA sequencing. When a graphene nanoribbon is constricted to nm sizes, the variation of the potential created by the different bases of the DNA strand passing through the adjacent nanopore will create a variation of the ribbon conductivity, enabling an electrical discrimination between DNA bases. We realized devices (Figure 1) comprised of nanopores with diameters in the range of 2−10 nm at the edge or in the center of graphene nanoribbons (GNRs), with widths between 5nm and 200 nm, on 40 nm thick silicon nitride (SiNx) membranes. We discuss the challenges encountered in the manufacturing of these nanoconstrictions (by lithograph or by electron beam sculpting) and the irradiation effects of the electron beam during the nanopore formation. GNR conductance is monitored in situ during electron irradiation-induced nanopore formation inside a transmission electron microscope (TEM) operating at 200 kV. We identify and study a linear and a supralinear regime for the increase of GNR resistance with the electron dose(Figure 2a), and correlate with the decrease by a factor of ten or more in mobility(Figure 3b) when GNRs are imaged at relatively high magnification with a broad beam prior to making a nanopore. Based on our findings we devise a scanning TEM procedure in which the position of the converged electron beam can be controlled with high spatial precision via

automated feedback and we are able to quantify GNR electron induced damage (Figure 2b). This method minimizes the exposure of the GNRs to the beam before and during nanopore formation. A statistic of the resistances of the GNR-NP devices obtained by the TEM and STEM method(Figure 3a) shows that the TEM method severely damage the ribbons (increase in resistance on average 15 times), while the resistance of the STEM drilled ribbons remains virtually unchanged during the process. The resulting STEM GNRs can sustain microampere currents at low voltages (∼50 mV) in buffered electrolyte solution and exhibit high sensitivity and mobility, similar to pristine GNRs without nanopores(figure 3b). We finally present the operation of this sensor for biomolecule detection and DNA sequencing, correlating the electric signal measured in the GNR to the ionic current measured through the nanopore. The higher current(~uA) which can be driven through a GNR compared to the ionic current(~nA)[2] enable us to obtain a hundredfold increase in the measuring speed, making possible DNA sequencing without slowing the molecules, for a projected 10 minutes gull genome sequencing. Adrian Balan1, Matthew Puster1, 2, Julio Alejandro Rodriguez-Manzo1 and Marija Drndic1 1 Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA, United States 2 Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, United States [email protected]

H


References [1] Towards sensitive graphene

nanoribbon-nanopore devices by preventing electron beam induced damage. M. Puster*, J. A. Rodríguez- Manzo*, A. Balan*, M. Drndić. ACS Nano, 7(12) 2013 pp 11283-11289 , *equal contribution

[2] Differentiation of short, single-stranded DNA homopolymers in solid-state nanopores. K. Venta, G.Shemer, M. Puster, J. A. Rodríguez- Manzo, A. Balan, J. K. Rosenstein, K. Shepard, M. Drndić. ACS Nano, 7(5) 2013 pp 4629-4636.

Figures

Figure 1: a) TEM images of GNR devices. The dark gray areas are graphene covered with a 15 nm thick layer of hydrogen silsesquioxane (HSQ). Light gray areas are the bare 40 nm thick supporting silicon nitride (SiNx) membrane. Inset: Nanopore formed in the center of the GNR b) Schematic showing the GNR-NP device and the circuit diagram used for electrolytic gating in KCl solution.

Figure 2: a) In situ TEM electrical measurement of GNR resistance vs. time for broad beam TEM imaging showing a linear increase. Top-left inset: the rate of change of resistance increases with current density (j1, j2, and j3 are 3, 9, and 23×104 A m-2, respectively). Bottom-right inset: illustration of a GNR exposed to a broad beam (red circle) in TEM imaging mode. b) In situ STEM electrical measurement of GNR resistance vs. time for converged beam STEM imaging. GNR resistance increases in a step-like fashion after each 330 ms scan in between the four steps, indicated by arrows. Top-left inset: average increase of resistance (∆R) per STEM scan exposure as a function of average dose (Davg). Bottom-right inset: illustration of the STEM scan over a GNR.


Figure 3: Comparison of GNR electrical properties after TEM and STEM nanopore formation methods. (a) Relative increase in resistance before (Ri) and after (Rf) nanopore formation for 28 GNR-NP devices made with a TEM method (17, blue squares) and STEM method (11, red circles), as a function of initial resistance Ri. (b) GNR conductance vs. gate voltage (Vg) measured in 1M KCl solution for representative devices before (black curves) and after nanopore formation with TEM (blue) and STEM (red) methods. For clarity, these curves were shifted so that the charge neutrality point is at Vg = 0 V.


unctionalization of Graphene by Plasma Treatments for Tailoring Electrical Properties

GIUSEPPE VALERIO BIANCO CNR-IMIP Italy

Graphene holds several peculiar properties such as high carrier mobility, optical transparency, flexibility and high chemical resistance which have stimulated a vast amount of research in several technological fields. However, the diffusion of graphene technologies is still limited by the difficulties (i) in opening a gap in the graphene band-structure, (ii) in modulating its work function, and (iii) in the processing of graphene itself being a relatively inert material. Chemical functionalization has been reported to be effective in addressing these issues. Literature on graphene presents a variety of experimental work exploring its decoration by several functional groups ranging from simple hydrogen, for tailoring graphene structural and electrical properties [1], to complex organic molecules, for increasing the graphene reactivity toward specific chemical species [2]. However, the control of the graphene functionalization processes as well as the reduction of induced defects and related structural damage are still challenging. In this contribution, we present chemical and plasma-chemical routes for the tailoring of electrical properties in large area chemical vapor deposition (CVD) graphene [3] by functionalization with several chemical groups including oxygen, nitrogen [4] and sulfur groups as well as hydrogen and fluorine atoms. Our functionalization processes have been developed and optimized with the twofold aim: the fine tuning of graphene electrical properties and the strong minimization of induced structural damage. To this purpose, we perform “mild modulated plasma processes” for the covalent binding

of functional groups without introducing structural defects related to ion radiative damaging. We also exploit intrinsic defects in CVD graphene which acting as preferential reactive sites allows the reduction of the operating temperature. This, in combination with the real time monitoring of graphene optical properties by spectroscopic ellipsometry, allows for an unprecedented control over the degree of functionalization. Acknowledgement: The authors acknowledge funding from the European Community's 7th Framework Programme under grant agreement no. 314578 MEM4WIN (www.mem4win.org). Giuseppe Valerio Bianco, Maria Losurdo, Maria M. Giangregorio, Alberto Sacchetti, Pio Capezzuto and Giovanni Bruno Institute of Inorganic Methodologies and of Plasmas, CNR-IMIP, Chemistry Department of University of Bari “Aldo Moro”, Via Orabona 4, 70126 Bari, Italy [email protected]

F


References [1] D. C. Elias, R. R. Nair, T. M. G.

Mohiuddin, S. V. Morozov, P. Blake, M. P. Halsall, A. C. Ferrari, D. W. Boukhvalov, M. I. Katsnelson, A. K. Geim, K. S. Novoselov, Science, 323 (2009) 610.

[2] J. M. Englert, C. Dotzer, G. Yang, M. Schmid, C. Papp, J. M. Gottfried, H. P. Steinruck, E. Spiecker, F. Hauke, A. Hirsch, Nature Chemistry, 3 (2011) 279.

[3] M. Losurdo, M. M. Giangregorio, P, Capezzuto, G. Bruno, Phys. Chem. Chem. Phys., 12 (2011) 20836.

[4] G. V. Bianco, M. Losurdo, M. M. Giangregorio, P. Capezzuto, G. Bruno, Phys. Chem. Chem. Phys., 16 (2014), 3632.

Figures

Figure 1: Time evolution of (a) the Raman spectra and (b) ellipsometric spectra of the pseudoextinction coefficient of CVD graphene on glass substrate during hydrogen plasma treatment. (c) Temperature dependence of the sheet resistance for pristine and hydrogenated graphene.

1200 1600 2000 2400 2800

Raman shift (cm-1)

(a)

2 3 4 50.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

H p

lasm

a

Energy (eV)

(b)

3 4 5 6 7 86.5

7.0

18.5

19.0

19.5

20.0

20.5

21.0

21.5

103/T (K-1)

Rsh

(10

3O

hm

/)

graphene

Hydrogenated

graphene

(c)


rowing perfect monolayer of graphene from nickel surfaces

CHRISTOPHE BICHARA CINaM - CNRS France

Growing graphene on a metal surface is one possible way to obtain a high quality graphene, with a controllable number of layers. The synthesis usually relies on a chemical vapor deposition of a carbon bearing gas on the surface of a metal such as Ir, Cu, or Ni [1]. In the present work, we investigate the latter case of graphene on Ni that is of particular interest because the role of carbon solubility in subsurface layers is both difficult to investigate experimentally and important to understand for the production of high quality graphene [2]. To understand the interaction of carbon with nickel at the atomic level, we have developed a tight binding model [3] implemented in a Grand Canonical Monte Carlo (GCMC) code. It has been used to investigate the nucleation and growth of carbon nanotubes in CVD processes [4, 5]. In this latter case, we study the chemical and physical states of the metal catalyst as a function of size, temperature and carbon chemical potential conditions corresponding to growth of SWNTs. We also study the interfacial properties of the NPs with respect to sp2 carbon walls, show that they strongly depend on the amount of carbon dissolved, and emphasize their role in the growth of tubes [6]. With the same approach, we investigate the CVD synthesis of graphene on Ni (111). We identify thermodynamic conditions (temperature and carbon chemical potential) to obtain a graphene monolayer. Owing to the significant technical improvements of our grand canonical Monte Carlo code (CPU time speedup by two or three orders of magnitude, as explained in [7]), we can extend the range

of our previous calculations [8] to present carbon adsorption isotherms to slab of nickel containing 1000 atoms, and temperatures ranging from 800 to 1400 K. A possible scenario is the following : during the course of the GCMC simulations, the adsorption of C atoms begins at the surface while some atoms are incorporated in the interstitial sites between the Ni layers. When the solubility limits is reached, a surface C structure begins to develop in the form of chains creeping on the surface and eventually crossing each other. At their intersections, threefold coordinated C atoms act as nucleation centers for C sp2 structures that develop on the surface. These sp2 C atoms interact weakly with the underlying Ni atoms and can detach from the surface to form graphene (see Figure 1). Moreover, depending on the growth conditions, we show that variable amounts of carbon atoms can be found in the subsurface layers, while the first subsurface layer shows a tendency for carbon depletion when graphene covers the Ni surface. This result is surprizing since the subsurface interstitial sites have been identified as most favorable for carbon incorporation on Ni(111) surfaces. To confirm this effect, we have performed static TB and DFT calculations to predict the stability of C atom located at different depths from the Ni slab surface covered or not with a graphene overlayer. To conclude, such depletion is probably at the root of interesting catalytic property of Ni. Indeed, this lower stability of carbon close to the surface can be used to control the number of layers formed. This lack of C atoms in subsurface sites can play the role of a barrier limiting the presence of C atoms on the surface and suggesting that the control of one layer is possible [9].

G


C. Bichara1 H. Amara2 A. Zappelli1 and F. Ducastelle2 1 Centre Interdisciplinaire de Nanoscience de Marseille, CNRS, 13288 Marseille, France 2 Laboratoire d’Etude des Microstructures, ONERA-CNRS, BP 72, 92322 Châtillon Cedex, France [email protected]

References [1] M. Batzill, Surface Science Report 67,

83 (2012). [2] R. S. Weatherup, B. Dlubak and S.

Hofmann, ACS Nano 6, 9996 (2012) [3] H. Amara, J.-M. Roussel, C. Bichara, J.-

P. Gaspard and F. Ducastelle Phys. Rev. B 79, 014109 (2009)

[4] H. Amara, C. Bichara and F. Ducastelle, Phys. Rev. Lett. 100, 056105 (2008)

[5] M.-F. C. Fiawoo, A.-M. Bonnot, H. Amara, C. Bichara, J. Thibault-Pénisson and A. Loiseau, Phys. Rev. Lett. 108, 195503 (2012)

[6] M. Diarra, A. Zappelli, H. Amara, F. Ducastelle and C. Bichara Phys. Rev. Lett. 109, 185501 (2012)

[7] J. H. Los, C. Bichara and R. Pellenq, Phys. Rev. B 84, 085455 (2011).

[8] H. Amara, C. Bichara and F. Ducastelle, Phys. Rev. B 73, 113404 (2006).

[9] M. Bahri, M. Diarra, H. Amara, C. Bichara and F. Ducastelle (in preparation)

Figures

Figure 1: Adsorption isotherms of Carbon atoms on a Ni(111) slab at different temperatures


urface Thermodynamic Equilibrium Conditions and the Growth of Monolayer Graphene Films on SiC

VOLKER BLUM Duke University USA

The Si side of SiC substrates is known for the epitaxial growth of large-scale, near-perfect monolayer graphene films by Si sublimation. Curiously, the same is not true for the C side of the substrate, where rotated few-layer graphene films are observed instead. In either case, straightforward theoretical simulations from first principles are made difficult by the lattice mismatch between graphene and the SiC substrate, leading to rather large, experimentally observed (6√3×6√3)-R30° superstructures on the Si side of SiC. We here present accurate, all-electron simulations of graphene and competing phases on SiC that explain the observed high-quality monolayer graphene growth and its precursors on the Si side by the existence of a narrow range of (near) thermal equilibrium conditions [1], equivalent to specific settings of growth temperature and Si background pressure in experiment. These calculations are based on a van der Waals corrected generalized-gradient approximation to density functional theory, encompassing up to ~2,700 atoms in a slab model of graphene on SiC. In contrast, smaller model unit cell sizes in the surface energy calculations would introduce significant strain, enough to erroneously stabilize artificial defects. On the C side, we do not find a thermodynamic equilibrium regime for monolayer graphene. Instead, two non-graphene surface phases ((3×3) and (2×2)C) are found to coexist with a monolayer graphene covered surface right at the stability limit of bulk graphite, explaining the experimentally observed features. Overall, our results indicate that thermodynamic equilibrium conditions appear to govern the local atomic

arrangements observed in the growth of graphene on SiC. Accurate, large-scale low-strain first-principles models of 2D materials on an intended substrate are thus indeed a desirable and viable way forward to distinguish which potential growth paths to near-perfect layers are promising and which are not. Work carried out with Lydia Nemec, Patrick Rinke, and Matthias Scheffler at Fritz Haber Institute, Berlin. Volker Blum

Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, U.S.A. [email protected] References [1] L. Nemec, V. Blum, P. Rinke, M.

Scheffler, Thermodynamic Equilibrium Conditions of Graphene Films on SiC. Phys. Rev. Lett. 111, 065502 (2013).

S


raphene for Energy Storage

FRANCESCO BONACCORSO Istituto Italiano di Tecnologia Italy

Energy conversion and storage are two of the grand challenges that our society is facing. New materials and processes can improve the performance of existing devices or enable new ones that are also environmentally benign. In this talk I will start by reviewing recent progress on the application of graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage [1,2]. The versatility of graphene and related materials can lead to new power management solutions for portable and flexible devices, as well as integration in living environments. I will then focus on our recent development of a new class of lithium-ion batteries based on a graphene ink anode and a lithium iron phosphate cathode that displays an estimated energy density of about 200 Whkg-1 and a stable operation for over 80 charge-discharge cycles [3]. I will argue that these unique properties are linked to the graphene nanoflake anode displaying crystalline order and high uptake of lithium at the edges, as well as to its structural and morphological optimization in relation to the overall battery composition. Our approach, compatible with any printing technologies, is cheap and scalable and opens up new opportunities for the development of high-capacity Li-ion batteries. Work done in collaboration with Jusef Hassoun, Maria Grazia Betti, Roberto Cingolani, Mauro Gemmi, Carlo Mariani, Bruno Scrosati, Valentina Tozzini and Vittorio Pellegrini

Francesco Bonaccorso and Vittorio Pellegrini Istituto Italiano di Tecnologia, Graphene Labs Via Morego 30, 16163 Genova, Italy

[email protected] References [1] F. Bonaccorso, L. Colombo, G. Yu, M.

Stoller, V. Tozzini, A.C. Ferrari, R.S. Ruoff, V. Pellegrini, submitted.

[2] V. Tozzini, V. Pellegrini Phys. Chem. Chem. Phys. 15, 80-89 (2013)

[3] J. Hassoun, F. Bonaccorso et al. arXiv:1403.2161

G


ollapse of superconductivity in a hybrid tin-graphene Josephson junction array

VINCENT BOUCHIAT Univ. Grenoble Alpes, Institut Néel, CNRS France

Graphene has a great potential for implementation of tunable and high temperature 2D superconductor. The accessible and surface exposed 2D electron gas offered by graphene provides indeed an ideal platform on which to tune, via application of an electrostatic gate, the coupling between adsorbates deposited on its surface. This situation is particularly interesting when the network of adsorbates can induce some electronic order within the underlying graphene substrate, such as magnetic or superconducting correlations [1]. We have experimentally studied the case of macroscopic graphene decorated with an array of superconducting tin clusters [2], which induce via percolation of proximity effect a global but tunable 2D superconducting state which critical temperature Tc can be tuned by gate voltage. In these systems the transition towards a truly zero-resistance state exhibiting a well developed supercurrent, is strongly gate-tunable and is quantitatively described by Berezinskii-Kosterlitz-Thouless vortex unbinding, typical of a 2D superconductor [1]. Depending on the graphene disorder and charge carrier density on one side , density and order of the superconducting islands on the other side, many parameter controlling the transition can be independently adjusted allow to test different regimes. When the Graphene show strong disorder, it is possible to tune via the applied gate voltage the system towards an insulating state, demonstrating the possibility to trigger a superconducting to insulator transition [2], which features ressembles

those found in granular superconductors. The extension is for diluted arrays in which superconducting dots covers less than 20% of the graphene surface. Interpretation of this metallic state in terms of quantum fluctuations is proposed. We will show recent experimental results involving a set of dot deposited according to triangular arrays sparsely distributed on graphene, in which superconductivity is suddenly destroyed for a critical gate value caused by quantum fluctuations of the phase giving rise to an intermediate metallic state [4]. Zheng Han1,2, Adrien Allain1,2, Hadi Arjmandi-Tash1,2, Konstantin Tikhonov3,4, Mikhail Feigel’man3,5, Benjamin Sacépé1,2 and Vincent Bouchiat1,2 1Univ. Grenoble Alpes, Institut Néel, F-38042 Grenoble, France 2CNRS, Institut Néel, F-38042 Grenoble, France 3L. D. Landau Institute for Theoretical Physics, Kosygin street 2, Moscow 119334, Russia 4Dept. of Condensed Matter Physics, The Weizmann Institute of Science, 76100 Rehovot, Israel 5Moscow Institute of Physics and Technology, Moscow 141700, Russia

[email protected]

C


References [1] M. Feigel’man et al. JETP Lett. , 88,

747, (2008). [2] B.M.Kessler, et al., Phys. Rev. Lett 104,

047001 (2010). [3] Adrien Allain, et al. Nature Materials,

11, 590–594, (2012). [4] Zheng Han, et al., Nature Physics, in

press, 2014).

Figures

Figure 1: Gate tuned superconductor to insulator transition in Tin decorated graphene. Resistance Vs backgate voltage of a macroscopic graphene sheet decorated with array of tin nanoparticles. A global superconducting state is formed by percolation of proximity effect. Note that the auantum phase transition has a temperature independent critical point at the quantum of resistance. Adapted from ref3.


arrier and polarization dynamics in monolayer MoS2 studied by time resolved photoluminescence

LOUIS BOUET Toulouse University France

Transition metal dichalcogenides such as MoS2 emerge as an exciting class of atomically flat, two-dimensional materials for electronics, optics and optoelectronics [1]. In contrast to graphene, monolayer (ML) MoS2 has a direct bandgap in the visible region of the optical spectrum. Inversion symmetry breaking (usually absent in graphene) together with the spin-orbit interaction leads to a unique coupling of carrier spin and k-space valley physics. The circular polarization (σ+ or σ-) of the absorbed or emitted light can be directly associated with selective carrier excitation in one of the two non-equivalent K valleys (K+ or K-, respectively) in momentum space [2]. The chiral optical selection rules open up very exciting possibilities of manipulating carriers in valleys with contrasting Berry phase curvatures, aiming for experimental manifestations of the predicted valley Hall effect [4]. Also stable spin states have been predicted for valence and conduction states for this material.

Up to now optical valley initialization in ML MoS2 is based on the analysis of the large circular polarization degree Pc of the emitted light from the direct bandgap observed in time-integrated measurements following circularly polarized laser excitation [2,5]. An important drawback seemed to be the drastic decrease of Pc as the temperature is raised to 300K [6]. In a simple approach, the time integrated polarisation is determined by the initially created polarization P0, the lifetime of the electron-hole pair τ and the polarization decay time τs through Pc=P0/(1+τ/τs). We emphasize that the polarization decay time does not correspond directly to the carrier spin flip time as in most semiconductors like GaAs, but it includes the scattering time between the two non-equivalent K valleys (K+ or K-) [4].

Here we present time resolved polarization measurements in MoS2 monolayers, providing vital information on the valley dynamics from 4K to room temperature [7]. We determine the key parameters that govern the stationary polarization degree Pc: Using quasi-resonant excitation of the A-exciton transitions, we can infer from Figure 1a that the photoluminescence (PL) decays within τ ≈ 4.5ps. For pulsed laser excitation, we observe a decrease of Pc with increasing laser power. We show that the PL polarisation remains nearly constant within our time resolution for experiments from 4K up to 300K (see Figure 1b and 1c), a necessary condition for the success of future Valley Hall effect experiments based on optically initialized K-valley polarization [4]. In addition, τ does not vary significantly over this temperature range [7]. These results are surprising when considering the decrease of Pc in time-integrated experiments when going from 4K to 300K reported in the literature [5,6]. By tuning the laser following the shift of the A-exciton resonance with temperature we are able to recover at room temperature 80% of the polarization observed at 4K in our sample, see Figures 2a and 2b. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization P0, which dominates the steady state PL polarization, is responsible for this observation.

The role of strong excitonic effects (electron-hole pairs bound by Coulomb Interaction) merits further investigation in this context [3], since the broad monolayer PL emission originates from charged and/or neutral exciton recombination. Neutral excitons from the K- and K+ valleys can couple via the Coulomb exchange interaction [8]. The role of this coupling as a mechanism for valley depolarization is discussed.

C


L. Bouet1, D. Lagarde1, X. Marie1, G. Wang1, C.R. Zhu2, B.L. Liu2, T. Amand1, P.H. Tan3 and B. Urbaszek1 1Toulouse University, INSA-CNRS-UPS, LPCNO, Toulouse, France 2Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, China 3State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China [email protected]

References [1] Qing Hua Wang et al, Nat.

Nanotechnol, 7 (2013) 699 and references therein

[2] T. Cao et al , Nat. Commun. 3 (2012) 887.

[3] T. Cheiwchanchamnangij et al , PRB 85 (2012) 205302; Diana Y. Qiu et al, PRL 111 (2013) 216805

[4] D. Xiao et al, Phys. Rev. Lett. 108 (2012) 196802

[5] for example K. F. Mak, K. He, J. Shan, and T. F. Heinz, Nat. Nanotechnol. 7 (2012) 494.

[6] for example G. Sallen, L. Bouet, X. Marie et al, Phys. Rev. B. 86 (2013) 081301(R)

[7] D. Lagarde, L. Bouet et al Phys. Rev. Lett. 112, (2014) 047401.

[8] T. Yu et al, arxiv 1401.0047 and H. Yu et al arxiv 1401.0667


Figures

Figure 1: Time resolved photoluminescence of A-exciton in monolayer MoS2. (a) Laser pulse (blue line) and PL emission (black line) intensity at T = 4K detected at maximum of A-exciton PL EDet=1.867eV as a function of time. Inset: Chiral optical selection rules in 1ML MoS2 (b) T=4K, ELaser=1.965eV, EDet=1.867eV. Laser polarization σ+. Left axis: σ+ (σ-) polarized PL emission intensity presented in black (red) as a function of time. Right axis: Circular polarisation degree during exciton emission. blue hollow squares: excitation power PLaser=550µW/µm2, green full squares:0.01 PLaser, errors bars take into account uncertainty in time origin ∆t ∼ 0.7ps. (c) same as (b), but for T=300K, ELaser=1.937eV, EDet=1.828eV.

Figure 2: PL polarization as a function of laser excitation energy in monolayer MoS2. (a) T = 4K, PLaser=550µW/µm2

black squares: PL circular polarization degree detected on the A-exciton PL maximum EDet=1.867 eV. Blue spectrum: The A-exciton emission is shown. (b) same as (a) but for T = 300 K, PLaser=550µW/µm2, EDet=1.828 eV


anometric resolved cathodoluminescence on few layers h-BN flakes

ROMAIN BOURRELLIER Univ. Paris-Sud, CNRS France

Within the latest years number of layered materials at reduced dimensions have demonstrated remarkable optical properties. However most studies focused on perfect system and the role of defects as optical active centers remain still largely unexplored. Hexagonal boron nitride (h-BN) is one of the most promising candidates for light emitting devices in the far UV region, presenting a single strong excitonic emission at 5.8 eV. However, a single line appears only in extremely pure monocrystals that can hardly be obtained only though complex synthesis processes. Common h-BN samples present more complex emission spectra that have been generally attributed to the presence of structural defects. Despite a large number of experimental studies up to now it was not possible to attribute specific emission features to well identify defective structures.

Here we address this fundamental questions by adopting a theoretical and experimental approach combining few nanometer resolved cathodoluminescence techniques with high resolution transmission electron microscopy images and state of the art quantum mechanical simulations.

Very recently, the Orsay team has developed a cathodoluminescence detection system integrated within a scanning transmission electron microscope. This unique experimental set up is now able to provide full emission spectra with a resolution as low as few tens of meV associated with an electron probe size of one nanometer. A cathodoluminescence spectrum-image can thus be recorded in parallel with an HAADF image.

Nanometric resolved cathodoluminescence on few-layer chemically exfoliated h-BN

crystals have shown that emission spectra are strongly inhomogeneous within individual flakes. Emission peaks close to the free exciton appear in extended regions. Complementary investigations through high resolution transmission electron microscopy allow to associate these emission lines with extended crystal deformation such as stacking faults and folds of the planes.

By means of ab-initio calculations in the framework of Many Body Perturbation Theory (GW approximation and Bethe-Salpeter equation) we provide an in-depth description of the electronic structure and spectroscopic response of bulk hexagonal boron nitride in the presence of extended morphological modifications. In particular we show that, in a good agreement with the experimental results, additional excitons are associated to local symmetry changes occurring at crystal stacking faults.

Additional features appearing within the band gap present a high spatial localization, typically less than 100 nm, and thus they can be related to individual point defects. When addressed individually through a highly focused electron probe they might have a single photon emitter quantum character. This hypothesis has been recently confirmed by experiments combining our cathodoluminescence system with an Hanbury Brown and Twiss (HBT) interferometer [4].

N


Romain Bourrellier1 Michele Amato1, 2 Sophie Meuret1 Luiz Henrique Galvão Tizei1 Christine Giorgetti2 Alexandre Gloter1 Malcolm I. Heggie3 Katia March1 Odile Stephan1 Lucia Reining2 Mathieu Kociak1 and Alberto Zobelli1 1Laboratoire de Physique des Solides, Univ. Paris-Sud, CNRS UMR 8502, F-91405, Orsay, France 2Laboratoire des Solides Irradies, Ecole Polytechnique, Route de Saclay, F-91128 Palaiseau and European Theoretical Spectroscopy Facility (ETSF), France 3Department of Chemistry, University of Surrey, Guildford GU2 7XH, United Kingdom

[email protected]

References [1] K. Watanabe et al, Nat. Mater. 3

(2004) p. 404. [2] L. Zagonel et al Nano Lett. 11 (2011) p.

56. [3] R Bourrellier et al, arXiv:cond-

mat/1401.1948 (2014) [4] R Bourrellier et al. in preparation.

Figures

Figure 1: a Bright field and b dark field images of an individual BN flake. c Overall emission spectrum of the flake and individual spectra taken at specific probe positions indicated in panel b. d-h Emission maps for individual emission peak. Intensity is normalized independently within each individual map.


oping dependence of the Raman signatures of defects in graphene

MATTEO BRUNA Cambridge Graphene Centre UK

Raman spectroscopy is a fast, non-destructive way to probe the structural and electronic properties of graphene [1,2]. Doping and structural defects can strongly affect the properties of graphene, therefore the ability to characterize them efficiently is of crucial importance in this phase of the development of graphene-based technology. While a significant effort was devoted to understand the effect of defects in samples with negligible doping[3], and the effect of doping in samples with negligible defects [4,5], the combined effect of doping and defects on the Raman spectrum of graphene has received little attention to date. But, most samples produced by either micromechanical exfoliation, or chemical vapour deposition or liquid phase exfoliation or carbon segregation from SiC or metal substrates are doped, and many of them have also defects, and defects may appear during processing for device integration. It is thus critical to understand if and how the defects can be detected and quantified by Raman spectroscopy in doped samples. Here, we report the dependence of the defect-related Raman peaks in graphene on the position of the Fermi level, by combining polymer electrolyte gating [4] with in situ Hall-effect measurements and Raman spectroscopy (see fig.1 a)) at different excitation wavelengths. We find that the intensity of the D and D’ peaks varies strongly with the doping level, as shown in fig. 1 b). This highlights the importance of taking into account the doping level when determining the amount and the type of defects in graphene from the intensity of the D-peak. We interpret the doping-induced intensity variation as an arising

from the increased broadening of the electronic states due to electron-electron interaction. We present a formula for the determination of the defect concentration for samples with non-negligible doping. M. Bruna, A. K. Ott , M. K. Ijäs, D. Yoon , U. Sassi, and A. C. Ferrari Cambridge Graphene Centre, 9 JJ Thomsonon Avenue, Cambridge CB3 0FA, UK

[email protected]

D


References [1] A. C. Ferrari, J. C. Meyer, V. Scardaci,

C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, Phys. Rev. Lett. 97 (2006), 187401

[2] A. C. Ferrari and D. M. Basko. Nature Nanotechnology, 8 (2013), 235

[3] L. G. Cancado, A. Jorio, E. H. Martins Ferreira, F. Stavale, C. A. Achete, R. B. Capaz, M. V. O. Moutinho, A. Lombardo, T. S. Kulmala, and A. C. Ferrari, Nano Letters,11 (2011) 3190–3196

[4] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. K. Saha, U. V. Waghmare, K. S. Novoselov, H. R. Krishnamurthy, A. K. Geim, A. C. Ferrari, and A. K. Sood, Nature Nanotechnology, 3 (2008) 210–215

[5] A. Das, B. Chakraborty, S. Piscanec, S. Pisana, A. K. Sood and A. C. Ferrari Phys. Rev. B 79 (2009) 155417

Figures

Figure 1: a) Scheme of polymer electrolyte-gated graphene transistor. B) doping dependence of the intensity ratio between D and G peak, I(D)/I(G)


he National Graphene Institute at the Univerity of Manchester

IVAN BUCKLEY National Graphene Institute at University of Manchester UK

The National Graphene Institute (NGI) at The University of Manchester will be the world’s leading centre of graphene research and commercialisation. The NGI will uniquely offer a collaborative environment where industry and science can work side by side on developing new and exciting graphene applications. Set to open in early 2015, the £61m NGI is funded by the Engineering and Physical Sciences Research Council (EPSRC) and the European Regional Development Fund (ERDF).

• £61m expenditure o Building and Infrastructure - £50m o Equipment - £11m

• 7,825m2 world class facility over 5 floors

• 2 x Class 100/1000 Clean rooms (c1,500m2)

• Labs – c1,500m2 • Partner Space - c900m2 (Clean

Room and Labs) • Employing ~125 people (110

research) • Building to be completed and

occupied - 1st Qtr 2015

Ivan Buckley

National Graphene Institute at University of Manchester; Oxford Road, Manchester, M13 9PL, UK Tel: +44 161 275 2441

[email protected] www.graphene.manchester.ac.uk Figures

T


maging coherent transport in a graphene quantum ring

DAMIEN CABOSART Universite Catholique de Louvain Belgium

The understanding and the control of electronic transport inside mesoscopic devices is an active investigation field, which benefited recently from advances in scanning probe imaging techniques. One of them is Scanning Gate Microscopy (SGM), a powerful technique allowing direct imaging of electronic transport at a local (nm) scale. The principle is to introduce inside a mesoscopic structure a local electrostatic perturbation induced by an electrically-biased Atomic Force Microscopy (AFM) tip, positioned few nanometers away from the device. Hence, by sweeping the tip over the structure and recording the induced change of the sample electrical conductance (G) as a function of the tip position, one obtains a SGM map.

To date, SGM investigations of graphene have already highlighted the influence of defects on the electronic transport inside a graphene sheet, which generates localized states identifiable by Coulomb blockade [1,2]. Moreover, SGM mapping has also been employed to image in real space two coherent effects: the universal conductance fluctuations and the weak localization [3]. Nevertheless, so far, no experimental SGM study has been performed on the Aharonov-Bohm (AB) effect inside a graphene quantum ring (QR).

Here, we report on SGM measurements performed on a mesoscopic graphene QR. The graphene monolayer has been extracted from natural graphite by mechanical exfoliation on a 90nm-thick silicon dioxide thermally grown on the top of a highly doped silicon substrate (SiO2/Si(n++)). The number of layers has been confirmed by Raman spectroscopy. The graphene sheet has been electrically contacted by Electron

Beam Lithography (EBL) patterning followed by metallization (Ti(5nm)/Au(30nm)) and lift-off. To define the geometry of the QR we used a new functionalization technique aiming to convert graphene regions into electrically-insulating zones: the fluorination [4-6]. The latter step is realized using the combination of EBL with a CF4 plasma process [7]. The final device is presented on the AFM picture in Figure 1a.

First, in magnetotransport measurements on the QR at very low temperature (down to 30 mK) and in the coherent regime of transport, we find clear AB conductance oscillations confirming that the device operates properly [8-13]. Secondly, thanks to SGM, we can pinpoint the position of defectinduced quantum dots inside the device [1,2]. Thirdly, spatial signatures of the AB effect have been found, which surprisingly differs from previous results in semiconductors QRs. Finally, radial fringes located inside the QR in SGM maps (Figure 1b) are reminiscent of previous SGM observations in semiconductor QRs [14,15] and comparable to SGM mapping simulations on mesoscopic QRs with charged defects [15,16]. Those fringes are ascribed to the presence of resonant structures scarring the local density of states in the QR. Damien Cabosart1, Sebastien Faniel1, Frederico Martins1, Alexandre Felten2, Vincent Bayot1,3 and Benoit Hackens1

1 Nanoscopic Physics (NAPS), Institute of Condensed Matter and Nanosciences (IMCN), Universite Catholique de Louvain (UCL), Louvain-la-Neuve, Belgium 2 Research Center in Physics of Matter and Radiation (PMR), Universite de Namur (UNamur), Namur, Belgium 3 Institut Neel CNRS, Universite Joseph Fourrier, Grenoble, France

[email protected]

I


References [1] N. Pascher et al., Appl. Phys. Lett., 101

(2012) 063101. [2] M. R. Connolly et al., Phys. Rev. B, 83

(2011) 115441. [3] J. Berezovsky et al., Nanotechnology,

21 (2010) 274013. [4] R. R. Nair et al., Small, 6 (2010) 2877. [5] J. T. Robinson et al., Nano. Lett., 10

(2010) 3001. [6] F. Withers et al., Phys. Rev. B, 82 (2010)

073403. [7] A. Felten et al., Nanotechnology, 24

(2013) 355705. [8] S. Russo et al., Phys. Rev. B, 77 (2008)

085413. [9] J. S. Yoo, Appl. Phys. Lett., 96 (2010)

143112. [10] D. Smirnov, Appl. Phys. Lett., 100 (2012)

203114. [11] M. Huefner, New J. Phys., 12 (2010)

043054. [12] Y. Nam et al., Carbon, 15 (2012) 5562. [13] A. Rahman et al., Phys. Rev. B, 87

(2013) 081401. [14] B. Hackens et al., Nat. Phys, 2 (2006)

626. [15] F. Martins et al., Phys. Rev. Lett., 99

(2007) 136807. [16] M. Pala et al., Nanotechnology, 20

(2009) 264021.

Figures

Figure 1: (a) AFM image of the graphene QR. White dashed lines indicate the fluorinated graphene areas (b) SGM map in the vicinity of the QR for Vg = -5 V and a tip-sample distance of 80 nm. The arrows highlight the position of radial fringes inside the QR. White scale bar represents in both cases 400 nm.


odel of √3x√3 phases of silicene and its multilayers

SEYMUR CAHANGIROV Unidad de Materiales Centro Mixto CSIC-UPV/EHU Spain

Silicene, a monolayer of silicon atoms arranged in a honeycomb structure, received an enormous interest for being a candidate two-dimensional material that could bring the exotic electronic structure of graphene to the well-developed silicon-based technology [1-2]. Experiments have shown that silicene synthesized on Ag substrates can acquire various reconstructions. In particular, structures having √3x√3 reconstruction have been frequently observed but yet poorly understood [3,5]. Here we provide a compelling high-resolution angle resolved photoemission (ARPES) study together with first-principles calculations and scanning tunneling microscopy (STM), which unambiguously prove the existence of a particular two-dimensional arrangement of silicon atoms that, gives rise to two different phases with √3x√3 periodicity. We propose a new mechanism for explaining the spontaneous and consequential formation of both phases. We show that unlike others the √3x√3 reconstruction is intrinsic and is not dictated by the interaction with the Ag substrate [3]. The proposed mechanism opens the path to the understanding of multilayer silicon [3,5]. Seymur Cahangirov, V. Ongun, Özçelik, Salim Ciraci, María C. Asensio and Angel Rubio Nano-Bio Spectroscopy, Departamento de Fisica de Materiales, Unidad de Materiales Centro Mixto CSIC-UPV/EHU, Universidad del Pais Vasco, Avd. Tolosa 72, E-20018 Donostia, Spain

[email protected]

References [1] S. Cahangirov et al., Phys. Rev. Lett. 102

(2009) 236804 [2] P. Vogt et al., Phys. Rev. Lett. 108 (2012)

155501 [3] B. Feng et al., Nano letters 12 (2012)

3507-3511 [4] L. Chen et al., Phys. Rev. Lett. 109 (2012)

056804 [5] P. De Padova et al., Appl. Phys. Lett.

102 (2013) 163106

M


raphene-Based Plasmonic Arrays for Dynamic Light Bending

EDUARDO CARRASCO Ecole Polytechnique Fédérale de Lausanne France

Plasmonic arrays based on patterned graphene allow long-lived plasmonic resonances, strong light-matter interaction and dynamic tunability. These features pave the way to unprecedented applications in the spectral regions from low-terahertz to mid-infrared. In this contribution, the possibility of utilizing reflective-type graphene arrays for dynamic light bending is discussed. The basic working principle relies on manipulating the spatial variations of the phase-shifts of the reflected beam, achieved through optimizing the size of the array elements and gate-tuning of the plasmonic resonance. Fig. 1(a) shows the conceptual principle. Based on experimentally feasible designs, accurate electromagnetic simulations show that it is possible to bend the impinging beam over a wide range of angular directions. It is well known that the complex conductivity of graphene can be modeled through the Kubo formalism [1] which depends, among other parameters, on the chemical potential (µc) and the relaxation time (τ). The value of µc can be controlled by chemically doping graphene or by applying an external electrostatic field. While in the former case the value of µc is fixed, in the latter case the conductivity of graphene can be dynamically controlled allowing, for instance, adjusting the phase of the reflection coefficient along the surface of the array. On the other hand, τ is somehow a measure of the graphene quality dependent on the fabrication process. Two kinds of topologies for the elementary cell forming the reflective-type array are used: square patches [2] and ribbons [3]. In the first case a periodic array formed by equally-sized patches is optimized and the dynamic phase-shift in reflection is produced

by varying the chemical potential in graphene. In the second case, an aperiodic array with optimized widths is designed and at the same time the chemical potential (µc) is varied for producing the desired phase profile. The obtained results are very promising, demonstrating the possibility of efficiently bending the beam towards the desired spatial direction, when the graphene is properly patterned and a simple biasing is implemented. An example of the resulting bent beam, when a laser-beam illuminates the plasmonic structure is shown in Fig. 1(b). Acknowledgements: This work was supported by the Hasler Foundation (Project 11149), the Swiss National Science Foundation (Project 133583), and the European Union (Marie Curie IEF 300934 RASTREO). Eduardo Carrasco1, Tony Low2 and Julien Perruisseau-Carrier1 1Adaptive MicroNano Wave Systems, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland 2IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA

[email protected]

G


References [1] V. P. Gusynin, S. G. Sharapov, and J. B.

Carbotte, New J. Phys. 11, (2009), 095013.

[2] E. Carrasco, M. Tamagnone and J. Perruisseau-Carrier, Applied Physics Letters, 102, (2013), 104103.

[3] H. Yan, T. Low, W. Zhu, Y. Wu, M. Freitag, X. Li, F. Guinea, P. Avouris, F. Xia, Nature Photonics, 7, (2013), 394–399.

Figures (a)

(b)

Figure 1: Proposed reflective-type array based on graphene for ligth bending. (a) Architecture of the whole structure. (b) Some examples of the spatial ligth bending.


wo-dimensional crystals: the next steps ahead

ANTONIO H. CASTRO NETO National University of Singapore Singapore

With the advent of graphene the new science and technology of two-dimensional crystals was created. The physical properties of these new materials are very different from its three dimensional counterparts for several different important reasons including their soft nature due to extreme thickness, their sensitivity to environmental conditions since they are pure surfaces, and the strong role played by electron-electron interactions due to lack of screening. I will discuss some of the progress in this exciting area of research and the challenges that remain to be conquered before one reaches the holy grail of using these materials in three dimensional heterostructures.

Antonio H. Castro Neto

Graphene Research Centre Physics Department Department of Electrical and Computer Engineering National University of Singapore Block S14, Level 6 6 Science Drive 2, Singapore 117546

[email protected]

T


unable graphene based optics, electronics and photonics

CHUN-WEI CHEN National Taiwan University Taiwan

Graphene, which consists of a single atom-thick plane of carbon atoms arranged in a honeycomb lattice, exhibits unique both “bulk” and “surface” properties of materials due to its tunable electronic structure. In this talk, I would like to present the tunable platform of graphene-based materials including graphene and graphene oxide in optical, electronic and photonic applications by manipulating their corresponding atomic and interfacial structures. By controlling the sp2/sp3 ratio of graphene oxide (GO), interesting PL emission of GO and r-GO can be tuned from blue to red color with a wide spectrum [1,2]. Through tuning the atomic structures, several interesting applications of GO in optoelectronic or photovoltaic devices will be also presented as a result of tunable electrical conductivity from a thin tunneling layer of GO to a transparent electrode of graphene[3,4]. The tunable workfunction of graphene makes it an ideal candidate as an “active” electrode. In addition, we would like to demonstrate an interesting tunable doping mechanism in graphene using so-called self-encapsulated doping or organic/inorganic hybrid doping platform,[5,6] which allows us to fabricate air-stable n- and p-type graphene based transistors with excellent tunability by using chemical or optical ways. I would like to demonstrate the wavelength-selective p- and n-typed carrier transport behaviors of a graphene transistor based on the organic/inorganic hybrid doping platform, which enables us to control the dual carrier-typed transport behaviors of a graphene transistor by wavelength-selective illumination. Finally, we would like to present a unique graphene transfer technique called “clean-lifting transfer (CLT)“, which was just

recently developed in our lab.[7], which enables the fabrication of clean and residue-free graphene films with excellent scalability. Chun-Wei Chen

Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan

[email protected] References [1] Advanced Materials, Vol.22, 505, (2010) [2] Angew. Chem. Int. Ed., Vol. 51, 6662,

(2012) [3] ACS Nano, Vol. 4 , 3169, (2010) [4] ACS Nano, Vol. 8,6564, (2011) [5] Nano Letters, Vol. 12, 964,(2012) [6] ACS Nano, 6, 6215, (2012) [7] Advanced Materials, Vol.25, 4521,

(2013)

T


raphene oxide and its hybrids as photocatalysts for solar fuels

KUEI-HSIEN CHEN Academia Sinica and National Taiwan University Taiwan

Photocatalytic conversion of carbon dioxide (CO2) to hydrocarbons such as methanol and ethanol makes possible simultaneous solar energy harvesting and CO2 reduction, two birds with one stone for the energy and environmental issues. This work describes a high photocatalytic conversion of CO2 to methanol using graphene oxides (GOs) as a promising photocatalyst. Modified Hummer’s method has been applied to synthesize the GO based photocatalyst for the enhanced catalytic activity. The photocatalytic CO2 to methanol conversion rate on modified graphene oxide (GO-3) is 0.172 mole g-cat-1 h-1 under visible light, which is six-fold higher than the pure TiO2 (P-25). Further, Cu and MoS2 nanoparticles were deposited on GO as co-catalysts to enhanced the photocatalysis reaction. Not only methanol, but also acetaldehyde were detected. Total solar to fuel yield of 6.8 mole g-cat-1 h-1 have been achieved, which is 240 times enhancement relative to the commercial P-25 photocatalyst. Detailed study one the mechanism and selectivity of the products will be addressed in this paper. Kuei-Hsien Chen

Institute of Atomic and Molecular Sciences, Academia Sinica; and Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan

[email protected]

References [1] H.C. Hsu et al., Nanoscale 5, 262-268

(2013).

G


hase Engineering in 2D Transition Metal Dichalcogenides

MANISH CHHOWALLA Rutgers University USA

Two-dimensional transition metal dichalcogenides (2D TMDs) — whose generalized formula is MX2, where M is a transition metal of groups 4–7 and X is a chalcogen — exhibit versatile chemistry and consist of a family of over 40 compounds that range from complex metals to semiconductors to insulator. Complex metal TMDs assume the 1T phase where the transition metal atom coordination is octahedral. The 2H phase is stable in semiconducting TMDs where the coordination of metal atoms is trigonal prismatic. Unlike mechanical exfoliation and chemical vapor deposition, chemical exfoliation of semiconducting layered TMDs yields monolayered nanosheets with heterogeneous atomic structure consisting of metallic (1T) and semiconducting (2H) phases (Figure 1). Metal (1T phase) to semiconductor (2H phase) transition can be achieved via mild annealing of exfoliated materials. Semiconductor to metal transitions can be achieved via chemistry. The 1T phase in semiconducting TMDs has scarcely been studied but it deserves urgent attention as it exhibits promise as a hydrogen evolution catalyst and as contact electrode in electronic devices. We will describe these phase transitions in semiconducting TMDs and provide examples of how we have learned to exploit them for enhanced catalytic and electronic performance.

Manish Chhowalla

Rutgers - The State University of New Jersey Department of Materials Science and Engineering 607 Taylor Road, Piscataway, NJ 08854,USA

[email protected] Figures

Figure 1: : STEM image of single layer MoS2 showing the richness of phases in chemically exfoliated nanosheets. 2H (red) and 1T phases (yellow) along with the strained 1T phase (blue) are shown. The corresponding atomic resolution image and diffraction patterns for 2H (top right) and 1T (middle right) phases are shown. The bottom right image also shows strain map of the distorted 1T phase.

P


mproving the quality of a graphene film by process innovation

SEUNGMIN CHO Samsung Techwin Korea

Graphene is a mono-atomic layer of sp2 carbons and its extraordinary properties are attracting interests from industries of display, semiconductor and composites. The typical manufacturing process for large area graphene film include synthesis of graphene on a copper foil, lamination of graphene to carrier film, removal of copper foil, and dry transfer process. In this talk, recent advances on graphene synthesis, lamination/transfer and removal of copper foil are discussed. A new equipment to reduce growth time for high-throughput and to achieve size, uniformity needed for industrial applications is designed and fabricated. Special attention is given to minimize thermal variations to provide uniform synthesis conditions. Higher ramp rate resulted in favorable effect on the morphology and microstructures of copper foil. As synthesized graphene gets larger, lamination and transferring process need to be customized. Cu etching is one of the key processes to produce large-area graphene through chemical vapor deposition. The Cu etchant generally includes a strong oxidizing agent that converts metallic Cu to Cu2+ in a short period of time, which deteriorates graphene quality if not suppressed properly. The addition of metal-chelating agents such as benzimidazole (BI) to etching solution reduces the reactivity of Cu-etching solution by forming a coordination compound between BI and Cu2+. The BI is well known as a heterocyclic molecular with strong electron affinity and excellent chemical and thermal stability.

The resulting graphene film exhibits a sheet resistance as lows as ~200 Ohm/sq. The improved electrical conductivity of graphene remained stable for more than 4 months at ambient conditions. Seungmin Cho

Samsung Techwin 8, Seongju-dong, Seongsan-gu Changwon, Gyeongnam 641-716, South Korea

I


hemical p- and n-doping for MoS2 transistor and its application

MIN SUP CHOI Sungkyunkwan University Korea

The discovery of isolation of the monolayer graphene using mechanically exfoliation has triggered the intense studies on the other two-dimensional (2D) materials, e.g., semiconducting molybdenum disulphide (MoS2) and insulating hexagonal boron nitride (h-BN) [1]-[3]. MoS2, one of the transition metal dichalcogenides (TMDCs), has a great potential for replacing the Si-based electronics due to its comparable band gap as large as 1.8 eV for monolayer and 1.2 eV for multilayers. For more versatile transistor applications, however, the doping process is inevitably necessary since various practical devices such as tunneling, transistors, logic circuits, and memory devices are composed of p-n junctions. Although there have been some reports about surface charge transfer doping effects of MoS2 using potassium [4], MoO3 [5], and molecules [6] recently, it is still quite ambiguous to apply for practical junction devices since they just demonstrated slight changes of electrical properties or improvement of contact resistance. In this study, we demonstrate the chemical p- and n-doping for MoS2 using AuCl3 and benzyl viologen (BV), respectively. Figure 1 shows the electrical properties (ID-VG) of MoS2 transistors after (a) BV and (b) AuCl3 doping. Before these doping, the MoS2 transistor shows n-type semiconducting property as previously reported (see Figure 1(a)) [2]. However, it becomes to degenerate n-type and p-type semiconductor after BV and AuCl3 chemical doping, respectively. Using potassium, which is a strong electron donor, there has been similar behavior compared with BV doping [4]. While the clear p-type semiconducting property using AuCl3 has not been reported from our literature survey

so far. Furthermore, it seems feasible to apply for the p-n diodes toward various optoelectronics. For achieving this, we stacked h-BN on a partial area of MoS2 and dope the other area using AuCl3 as shown in Figure 2(a). When we measured the photo-response of this prototype diode, it showed the unique properties of the conventional diode such as open circuit voltage and short circuit current as shown in Figure 2(b). Consequently, we demonstrated the proper chemical doping method for MoS2 toward various optoelectronics. Min Sup Choi, Deshun Qu, Jia Lee, Xiaochi Liu, Hua-Min Li, Chang Ho Ra, Seung Hwan Lee, and Won Jong Yoo SKKU Advanced Institute of Nano-Technology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do, 440-746, Korea [email protected]

C


References [1] Novoselov, K.S., et al. Science, 306,

(2004) 666-669. [2] Radisavljevic, B., et al. Nat. Nanotech.,

6, (2011) 147-150. [3] Dean, C.R., et al. Nat. Nanotech., 5,

(2010) 722-726. [4] Fang, H., et al., Nano Lett., 13, (2013)

1991-1995. [5] Lin, J., et al. Appl. Phys. Lett., 103, (2013)

063109. [6] Du, Y., et al. IEEE Elec. Dev. Lett., 34,

(2013) 1328-1330. Figures

Figure 1: The electrical properties (ID-VG) of MoS2 transistors (a) before and after BV doping, and (b) after AuCl3 doping.

Figure 2: (a) Cross-sectional diagram of MoS2 p-n diode. (b) The photo-response of a fabricated diode.


ry-transfer process and interface engineering for high performance graphene transistor

SUNG-YOOL CHOI Korea Advanced Institute of Science and Technology South Korea

Graphene has attracted explosive attentions due to its unique and outstanding electrical, optical, and mechanical properties. Chemical vapor deposition (CVD) has been most widely utilized among various production methods of high quality graphene, because it enables low-cost growth of large area, high quality graphene with good electrical properties. In this talk, I will discuss recent advances in the dry-transfer process for CVD-grown graphene and the interface engineering between graphene channel and dielectrics for graphene transistors with high electrical performance. Applying CVD-grown graphene channels to electronic devices requires a transfer process to dielectric substrates. Although PMMA-assisted wet transfer of graphene has been used, ionic impurities from etchant and by-product of metal etching trapped at the interface between graphene and target substrate result in the degradation of electrical performance and the reliability of the fabricated devices [1]. For realizing high-performance graphene transistor, the integration of high quality gate dielectrics on graphene is also a key technical challenge because the quality of interface between the dielectrics and graphene channel affects the electrical characteristics of graphene devices, such as operating voltage, scaling capability, and device reliability [2]. To address these issues, we propose a novel method for graphene transfer on various substrates with direct delamination of graphene and a new approach for integration of high-k dielectrics on graphene using a functionalized graphene monolayer as an

ultrathin seed layer on top of the graphene channel [3]. Graphene transistors with top gate structure fabricated on SiO2 using these methods show narrower distribution of Dirac voltages and enhanced device performances. In addition, the related issues for transfer and device fabrication on flexible substrate will be discussed. Sung-Yool Choi

Dept. of Electrical Engineering and Graphene Research Center, KAIST 373-1 Guseong-dong, Yuseong-gu, Daejeon, South Korea [email protected] References [1] W.C. Shin et al., Doping suppression

and mobility enhancement of graphene transistors fabricated using an adhesion promoting dry transfer process, Applied Physics Letters, 103, 243504 (2013)

[2] W.C. Shin et al., Functionalized Graphene as an Ultrathin Seed Layer for the Atomic Layer Deposition of Conformal High‑k Dielectrics on Graphene, ACS Appl. Mater. Interfaces, 5, 11515 (2013)

[3] S. Y. Yang et al., Metal etching-free, direct delamination and transfer method of single layer graphene, (in review)

D


onvergence Research at KAIST Graphene Research Center

SUNG-YOOL CHOI Korea Advanced Institute of Science and Technology South Korea

In 2012, KAIST Graphene Research Center was established with its founding goal, to be a key research center of excellence as well as to be the global research hub in graphene research. To realize this goal, 15 professors and more than 100 students from 7 departments are joining in this center. Among them, the key members, 6 professors, gathered in the KAIST Institute (KI) Building to promote the convergence research on graphene materials and devices. Our main research have been focused on the novel synthesis methods for high-quality graphene and the device applications of graphene for future information and energy technology. Now we are actively participating to a variety of national or industrial research projects on the science and technology of graphene and related materials. By pursuing the world-class excellence through the cooperative research activities between outstanding research groups from different disciplines inside or outside of KAIST, the Graphene Research Center will be a national and global research hub in graphene science and technology. Sung-Yool Choi

Dept. of Electrical Engineering and Graphene Research Center, KAIST 373-1 Guseong-dong, Yuseong-gu, Daejeon, South Korea [email protected]

Figures

Figure 1: Members of KAIST Graphene Research Center at the KI Building Lobby (2012.9.10).

C


omputational Studies of Two-Dimensional Materials: From Graphene to Few-Layer Graphene and Beyond

MEI-YIN CHOU Academia Sinica Taiwan

It has become possible in recent years to fabricate and manipulate two-dimensional (2D) nanomaterials in the laboratory that are as thin as one to few atomic layers. A well-known example is graphene, where the Dirac-Weyl Hamiltonian for massless fermions describes the low-energy quasiparticles. Intriguing physics has been found in these few-layer systems, and phenomena originally associated with particle physics can now be observed in condensed matter systems. In this talk, I will focus on our recent theoretical and computational studies of a few representative systems. These two-dimensional atomic layer systems provide a unique platform to probe the rich physics involving multiple interacting massless fermions. It has been found that graphene layers grown epitaxially on SiC or by the chemical vapor deposition method on metal substrates display a stacking pattern with adjacent layers rotated by an angle with respect to each other. Our calculation shows that anisotropic transport properties manifest in a specific energy window, which is accessible experimentally in twisted bilayer graphene. The quasiparticle states in two distinct graphene layers act as neutrinos with two flavors, and the interlayer interaction between them induces an appreciable coupling between these two “flavors” of massless fermions, leading to neutrino-like oscillations [1]. The energy spectrum under an external magnetic field in twisted bilayer graphene also exhibits intriguing properties. The Hofstadter butterfly spectrum for Landau levels in a two-dimensional periodic lattice is a rare example exhibiting fractal properties in a truly quantum system. However, the

observation of this physical phenomenon in a conventional material will require magnetic field strengths several orders of magnitude larger than what can be produced in a modern laboratory. It turns out that for a specific range of rotational angles twisted bilayer graphene serves as a special system with a fractal energy spectrum under laboratory accessible magnetic field strengths. This unique feature arises from an intriguing electronic structure induced by the interlayer coupling. Using a recursive tightbinding method we systematically map out the spectra of these Landau levels as a function of the rotational angle [2]. Our results give a complete description of LLs in twisted bilayer graphene for both commensurate and incommensurate rotational angles and provide quantitative predictions of magnetic field strengths for observing the fractal spectra in these graphene systems. In addition, it has been shown recently that silicene, a 2D graphene-like form of silicon, may be synthesized epitaxially on the surface of a silver substrate [3] or on diboride thin films grown on silicon wafers [4]. This suggests new perspectives for the applications of massless fermions in materials that are compatible with Si-based electronics. Silicene has a buckled honeycomb arrangement of Si atoms with an electronic dispersion resembling that of graphene. It is expected that many of the unique electronic properties of graphene can also be realized in this new 2D system. Most likely the samples will be synthesized on some forms of substrates, and the possibilities are abundant. We have performed first-principles calculations of silicene on graphene in order to understand the effect of substrate interaction on the physical properties of these systems [5]. The phonon properties and electron-phonon interaction of freestanding silicene [6] will also be discussed.

C


Mei-Yin Chou1,2,3, Lede Xian3, Zhengfei Wang3, Chih-Piao Chuu1, Yongmao Cai1, Ching-Ming Wei1 1Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan 2Department of Physics, National Taiwan University, Taipei 106, Taiwan 3School of Physics, Georgia Tech, Atlanta, 30332, USA [email protected]

References [1] L. Xian, Z. F. Wang, and M. Y. Chou,

Nano Lett. 13 (2013) 5159. [2] [2] Z. F. Wang, F. Liu, and M. Y. Chou,

Nano Lett. 12 (2012) 3833. [3] For example, Vogt, P.; De Padova P.;

Quaresima, C.; Avila, J.; Frantzeskakis, E.; Asensio, M.C.; Resta, A.; Ealet, B.; Le Lay, G. Phys. Rev. Lett. 108 (2012) 155501.

[4] Fleurence, A.; Friedlein, R.; Ozaki, T.; Kawai, H.; Wang, Y.; Yamada-Takamura, Y. Phys. Rev. Lett. 108 (2012) 245501.

[5] Y. Cai, C.-P. Chuu, C. M. Wei, and M. Y. Chou, Phys. Rev. B 88 (2013) 245408.

[6] J.-A. Yan, R. Stein, D. M. Schaefer, X.-Q. Wang, and M. Y. Chou, Phys. Rev. B (Rapid Communications) 88 (2013) 121403.

Figures

Figure 1: Schematic illustration of interlayer interaction in twisted bilayer graphene. (a) A plane cutting through the Dirac points of the two Dirac cones associated with the two twisted layers. The energy bands on the cross section shown in (a) are drawn in (b) and (c) for cases without and with interlayer interaction, respectively. (d) A plane cutting through the two Dirac cones without including the two Dirac points. The energy bands on the cross section shown in (d) are drawn in (e) and (f) for cases without and with interlayer interaction, respectively.

Figure 2: Band structure of Si(√3)/G(√7): (a) the projected states on Si are highlighted; (b) the projected states on C are highlighted; and (c) the projected bands in (a) and (b) are combined. The substrateinduced gap is about 26 meV for Si (Γ) and 2 meV for graphene (K), respectively.


iquid exfoliation of 2D materials: From scaleup to applications

JONATHAN N. COLEMAN Trinity College Dublin Ireland

In this talk I will describe liquid exfoliation of 2D materials. The simplest way to do this is to sonicate graphite in certain, stabilising solvents. When the solvent surface energy matches that of graphene the energetic cost of exfoliation is minimised and some of the graphite is converted to graphene. Graphite can also be exfoliated to give graphene by sonication in surfactants or polymer solutions. The resulting graphene is free of oxides and basal-plane defects and consists of nanosheets with lateral size of 200-2000 nm and thickness from 1-10 layers. This material can be used in arrange of applications, such as reinforced composites or strain sensors. This process can be extended to a host of other layered crystals including BN, MoS2, MoO3 and GaS. The exfoliation of MoS2 will be used as an example. In liquid exfoliation, it is important to know both the lateral size and thickness of the material being exfoliated. In the case of MoS2, we will show that this information is contained within the optical absorption spectrum. I will describe simple metrics which can be extracted from the MoS2 absorption spectrum and used to give the flake length and thickness directly. Access to such metrics allows the exfoliation process to be optimised to maximise monolayer content. As a result we can obtain monolayer-rich MoS2 dispersions which are photoluminescent. These methods can be applied to a range of 2D materials. To illustrate this, I will describe the production of MoTe2 dispersions which are monolayer-rich and photoluminescent. The availability of dispersions of a range of 2D materials allows the demonstration of a

range of applications. We will demonstrate that liquid exfoliated nanosheets can be used to reinforce composites, replace Pt as the counter-electrode in dye-sensitised solar cells, act as catalysts to generate hydrogen, produce high capacitance supercaps and develop solution processed photodetectors. For any of the applications described above to be successful, a method of producing very large quantities of exfoliated nanosheets will be required. We have developed such a method in collaboration with Thomas Swan ltd. This method is based on high shear mixing. We have demonstrated the scalability of this method and envisaged the exfoliation mechanism. I will describe the production of batches of liquid exfoliated graphene with volumes of up to (but not limited to) 300 L. Jonathan N. Coleman

School of Physics & CRANN, Trinity College Dublin, Ireland [email protected]

L


raphene and Other 2D Materials: Challenges and Opportunities

LUIGI COLOMBO Texas Instruments Incorporated USA

Over the past decade many applications using graphene and more recently other rediscovered two-dimensional materials (2DM) have been extensively studied. The numerous potential graphene applications dictate a variety of materials preparation processes, from monolayer films to high surface area graphene. Nano-electronics will need the highest quality materials and there are still many challenges in growing flat graphene single crystal films on dielectric surfaces. Other applications will need graphene of varying quality nevertheless equally demanding. The level of research effort as demonstrated by the number of publications as a function of year, Fig. 1, shows that there is an increasing number of papers published across the world with Europe and the US having a higher percentage in the initial years and the Asian countries having a higher percentage in the later years. What is perhaps more important from an implementation and product development perspective is the level of investments in basic sciences versus technology. Fig. 2 shows that science and technology papers are increasing as are materials science and chemistry, both equally critical in making graphene technology a reality. While physics papers are not increasing as much the absolute number is still higher than other disciplines. The emerging new 2DM, in comparison to graphene while providing us with many new opportunities due to their layered nature, the existence of a bandgap and many more, are technologically very challenging and thus there are many opportunities is growing these films to meet the applications requirements. The principal challenges in transition metal dichalcogenides (TMDs)

are in the area of materials growth, point defect control, carrier type control, contacts and potential reactivity with other materials used in the process of device integration and controlled growth of multilayers. We are now at stage in the development cycle of graphene where we are realizing that it will take longer for graphene to make into products and even longer for TMDs given the greater difficulty in preparing them. In this presentation I will present the state of the art of graphene and TMD materials and highlight the challenges that need to overcome and dedicate funding in order for these materials to become technologically relevant. Luigi Colombo

Texas Instruments Incorporated, Dallas, TX 75243, USA [email protected]

G


Figures

Figure 1: Percentage of total papers for the various territories plotted as a function of year. The data presented in this graph was extracted from Thomson Reuters’ Web of Science.

Figure 2: Percentage of total papers published as a function of year for different disciplines. The data presented in this graph was extracted from Thomson Reuters’ Web of Science.

0

5

10

15

20

25

30

35

40

45

2004 2006 2008 2010 2012 2014

Pe

rce

nta

ge

of

tota

l p

ap

ers

Year

World

China

EU

USA

S Korea

Japan

Singapore

0

5

10

15

20

25

30

35

40

2004 2009 2014

Pe

rce

nta

ge

of

tota

l p

ub

lish

ed

pa

pe

rs

Year

Physics

Chemistry

Materials Science

Science and technology


roadband terahertz imaging with sensitive graphene field-effect-transistors

DOMINIQUE COQUILLAT CNRS-Université Montpellier 2 France

Interest in terahertz (THz) systems and technology has grown significantly over the past 10 years for their potential in non-invasive imaging, sensing and high-data-rate wireless communication. Waves at THz frequencies present an alternative to x-rays for imaging through paper, cloth, wood, concrete, plastic and many other materials. In contrast to x-rays they are non-ionizing and therefore inherently safe. Applications of THz radiations range from nondestructive testing to medical imaging, security screening of objects and persons [1]. Several groups have also considered using THz waves to transmit data in wireless communications. Wireless THz communications for which THz waves are the free-space carrier of data are recognized as the promising breakthrough solution to achieve data-rates up to 100 Gbps [2]. THz imaging and wireless communication applications suffer, however, from the lack of fast and low-cost detectors operating at room temperature and in this work we show that graphene based plasma nanotransistors can be a good alternative. Nanotransistors offer great prospect for the development of innovative THz detectors. The interest in using field-effect transistors for THz applications was initiated by the theoretical work of Dyakonov and Shur, who predicted that the nonlinear properties of the 2D plasma in the transistor channel can be used for detection of THz waves at frequencies significantly higher than the transistor cut-off frequency [3,4].

Graphene field-effect nanotransistors were recently demonstrated showing maturity of graphene microelectronics. In this paper, we present extensive studies on first THz detectors based on monolayer and bilayer graphene field effect transistors. The specific detection sign reversal related to the graphene Dirac point change of electron to hole conductivity is clearly demonstrated. We show that the detectors consisting of a gated 2D massless fermion gas as rectifying element and an integrated coupling antenna achieve a responsivity above 1.2 V/W (1.3 mA/W) in photovoltage and photocurrent mode respectively, and a noise equivalent power below 2 10-9 W/Hz0.5 We show also that these detectors can operating as sensitive room-temperature broadband THz detectors in THz imaging systems [5,6]. Feasibility of THz food industry quality control (Fig.1a-c) and agriculture watering control (Fig.1d) imagers using graphene nanotransitor sensors/detectors is demonstrated. D. Coquillat1, L. Vicarelli2, D. Spirito2, S. L. De Bonis2, A. Lombardo3, M. Bruna3, A. C. Ferrari3, M. Polini3, V. Pellegrini2, A. Tredicucci2, M. S. Vitiello2 and W. Knap1 1Laboratoire Charles Coulomb UMR 5221 CNRS-Université Montpellier 2, F-34095 Montpellier, France 2NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, I-56127 Pisa, Italy 3Department of Engineering, Cambridge University, Cambridge CB3 0FA, UK [email protected]

B


References [1] D. Saeedkia (Editor),

Handbook of terahertz technology for imaging, sensing and communications, Woodhead Publishing Series in Electronic and Optical Materials, 34 (2013).

[2] S. Blin, L. Tohme, D. Coquillat, S. Horiguchi, Y. Minamikata, S. Hisatake, P. Nouvel, T. Cohen, A. Penarier, F. Cano, L. Varani, W. Knap, T. Nagatsuma, Journal of Communications and Networks, 15 (2013) 559.

[3] M. Dyakonov and M. Shur, IEEE Transactions on Electron Devices, 43 (1996) 380.

[4] W. Knap, S. Rumyantsev, M. S. Vitiello, D. Coquillat, S. Blin, N. Dyakonova, M. Shur, F. Teppe, A. Tredicucci, and T. Nagatsuma, Nanotechnology, 24 (2013) 214002.

[5] L. Vicarelli, M. S. Vitiello, D. Coquillat, A. Lombardo, A. C. Ferrari, W. Knap, M. Polini, V. Pellegrini, A. Tredicucci, nature materials, 11 (2012) 865.

[6] D. Spirito, D. Coquillat, S. L. De Bonis, A. Lombardo, M. Bruna, A. C. Ferrari, V. Pellegrini, A. Tredicucci, W. Knap, M.S. Vitiello, Appl. Phys. Lett., 104, (2014) 061111.

Figures

Figure 1: Fast, large-area, THz imaging. a), Photograph and 0.3 THz transmission mode image of a closed coffee-capsule box with a metallic knife blade inside. b) For visible light illumination the contents cannot be seen, either by naked eye or by the CCD camera used to take the picture. b) Same box as in a), but with one side removed. This allows the inside of the box to be seen, but requires the destruction of the packaging. c) 0.3 THz transmission image of the sealed, intact box mounted on a XY stage, with spatial resolution 0:5 µm. Our graphene-based terahertz detector allows one to monitor the contents of the closed package. d) 0.3 THz image of a leaf revealing the veins.


opological insulator graphene by heavy atom adsorption: Impact of segregation

ALESSANDRO CRESTI Grenoble INP, Minatec France

Since the prediction of the existence of topological insulators in 2005 [1,2], graphene has been considered a promising platform for their observation. However, due to the extremely weak intrinsic spin-orbit coupling, realizing a topological phase in pristine graphene is experimentally unattainable. There are several recent proposals to artificially increase the spin-orbit coupling by chemical modifications of graphene or the realization of hybrid structures. In particular, the adsorption of heavy adatoms has been predicted to introduce locally strong spin-orbit coupling [3]. This would allow the engineering the topological phase and the observation of the quantum spin Hall regime in graphene. However, all experimental attempts to confirm such a new possibility have remained unsuccessful to date. Here, we illustrate how this failure might be determined by adatom segregation, which is experimentally unavoidable. Moreover, we show that the adsorbate clustering can induce bulk extended states of original nature. In the present contribution, we consider thallium adatoms randomly distributed (non-segregated or clustered) over graphene, see fig.1. Thallium adatoms place above the center of carbon hexagons and induce an effective spin-orbit coupling between the carbon atoms of the involved plaquettes (R). The system is described by a standard effective tight-binding model [1-3]

where γ =2.7 eV is the nearest-neighbor coupling, λ=54 meV the spin-orbit coupling, µ=270 meV the local energy shift and we considered the possible presence of a superimposed potential V. We investigate transport in both a two-terminal configuration

(by using the Green’s function formalism) and a 2D configuration (by using the Kubo-Greenwood approach). Our results for non-segregated adatoms confirm the onset of a topological phase with the observation of a quantum spin-Hall regime, where the two-terminal conductance is quantized to 2e2/h in the region of the topological gap and spin-polarized currents flow at the ribbon edges, see figs.2(a,b). Upon segregation of adatoms into islands with radius between 2 and 3 nm, the conductance is no more quantized in the region of the topological gap, where it becomes higher and fluctuating, see fig.2a. This breakdown of the spin-Hall phase can be understood by looking at the local density-of-states of the injected electrons reported in fig.2(c). Contrary to the case of scattered adatoms, electrons flow through the bulk. The explanation of such a behavior is that the induced spin-orbit coupling vanishes in the region between the islands, where electrons can flow as in a normal conductor. The distance between the islands, though short, does not allow the proximity effect observed for non-segregated atoms to take place. We can conclude that segregation has a detrimental effect on the quantum spin Hall phase and leads to the failure of its observation [4]. When considering the conductivity of a two-dimensional system, we observe the transition from topological insulator (for non-segregated adatoms) to a metal (for clustered adatoms) with a minimum conductivity at the charge neutrality point of about 4e2/h, see fig.3(a). The value of the minimum conductivity is stable in the presence of long-range disorder of different strengths, and, more importantly, the quantum diffusion coefficient does not undergo localization, see fig.3(b), thus indicating the extended nature of the states.

T


Alessandro Cresti1, Dinh Van Tuan2, David Soriano2 and Stephan Roche2 1IMEP-LAHC (UMR 5130), Grenoble INP, Minatec, 3 Parvis Louis Néel, F-38016 Grenoble, France 2CIN2 (ICN-CSIC) and Universitat Autonoma de Barcelona, E-08193 Bellaterra, Spain [email protected]

References [1] C.L. Kane and E.J. Mele, Phys. Rev.

Lett., 95 (2005) 226801. [2] C.L. Kane and E.J. Mele, Phys. Rev.

Lett., 95 (2005) 146802. [3] C. Weeks, J. Hu, J. Alicea, M. Franz,

and R. Wu, Phys. Rev. X, 1 (2011) 021001.

[4] A. Cresti, D. Van Tuan, D. Soriano, and S. Roche, submitted to Phys. Rev. Lett.

Figures

(a)

(b)

Figure 1: (a) Non-segregated thallium adatoms adsorbed over graphene. (b) Thallium adatoms clustered into islands with a radius of 2 nm. For given adatoms density, the distance between the islands increases as their radius increases.

a)

b)

c)

Figure 2: (a) Zero-temperature differential conductance of a graphene ribbon of width 50 nm with a section of length 50 nm functionalized with a density n=15% of scattered or clustered (with island radius between 2 and 3 nm) thallium adatoms. (b) Local density-of-states for spin down electrons injected from the right contact at energy -100 meV. Spin-down electrons flow along the top edge. Spin-up electrons, not shown here, flow along the bottom edge. Such a spatial chirality gives rise to a quantum spin-Hall phase. (c) Same as (b) for clustered adatoms.

a)

b)

Figure 3: (a) Kubo-Greenwood conductivity for 2D graphene with thallium adatoms segregated into islands with radius up to 3 nm. Different concentrations of long-range disorder (with strength ∆=2.7 eV) have been considered. Note the minimum conductivity of about 4e2/h. (b) Time-dependent diffusion coefficient when spin-orbit coupling is active or inactive in the system. Extended states are observed in the presence of spin-orbit coupling.


rain Boundary Resistivity in Polycrystalline Graphene

ARON W. CUMMINGS Institut Català de Nanociència i Nanotecnologia Spain

In recent years, graphene has emerged as a favorable material for a wide range of applications [1]. While single-crystal graphene would be ideal, the most promising approach for the mass production of graphene is chemical vapor deposition (CVD), which results in a material that is polycrystalline [2,3]. This polycrystallinity arises due to the nucleation of growth sites at random positions and orientations during the CVD process. In order to accommodate the lattice mismatch between misoriented grains, the grain boundaries in polycrystalline graphene consist of a variety of non-hexagonal carbon rings, which can serve as a source of carrier scattering [4,5]. Indeed, several experimental works have demonstrated that grain boundaries add an extra resistance compared to single-grain samples [6-9]. In addition, a variety of studies have demonstrated the high chemical reactivity of the grain boundaries, compared to pristine graphene [6,10]. This has led to the possibility of using polycrystalline graphene as efficient chemical sensors [11]. Thus, in order to understand the large-scale transport properties of polycrystalline graphene, it is important to understand charge transport through the grain boundaries, and how this is altered by chemical functionalization. In this work, we use numerical simulations to examine the role that grain boundaries play in charge transport through polycrystalline graphene. We find that grain boundaries increase the sheet resistance of graphene samples, and with a simple scaling law we extract the intrinsic grain boundary resistivity. The calculated grain boundary resistivity is 1-2 orders of magnitude smaller than what is

obtained from most measurements, suggesting that scattering due to the non-hexagonal structure of the grain boundaries is relatively small, and that another mechanism must be responsible for most of the experimentally-measured grain boundary resistivity. We examine this by progressively adding chemical adsorbates to the grain boundaries, as depicted in Fig. 1. We find that functionalization can tune the grain boundary resistivity by more than one order of magnitude, as shown in Fig. 2, bringing the simulations within the range of experimental measurements [6-9,12,13]. These results have strong implications for CVD-grown graphene, as they indicate that chemical functionalization can play a strong role in the electrical properties of this material. We also present recent experimental measurements that demonstrate the impact of chemical functionalization on an individual grain boundary. Aron W. Cummings1, Dinh Loc Duong2, Eduardo Barrios1, Dinh Van Tuan1, Jani Kotakoski3,4, Young Hee Lee2 and Stephan Roche1,5 1 ICN2 - Institut Català de Nanociència i Nanotecnologia, Campus UAB, 08193 Bellaterra (Barcelona), Spain 2 IBS Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science, Department of Energy Science, Department of Physics, Sungkyunkwan University, Suwon 440-746, South Korea 3 Department of Physics, University of Vienna, Boltzmanngasse 5, 1090 Wien, Austria 4 Department of Physics, University of Helsinki, P.O. Box 43, 00014 University of Helsinki, Finland 5 ICREA - Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain [email protected]

G


References [1] K.S. Novoselov et al., Nature, 490

(2012), 192-200. [2] P. Y. Huang et al., Nature 469 (2011),

389-392. [3] K. Kim et al., ACS Nano 5 (2011), 2142-

2146. [4] O.V. Yazyev and S.G. Louie, Nat.

Mater. 9 (2010), 806-809. [5] D. van Tuan et al., Nano Lett. 13

(2013), 1730-1735. [6] D.L. Duong et al., Nature 11 (2012),

235-240. [7] Q. Yu et al., Nat. Mater. 10 (2011), 443-

449.

[8] Jauregi et al., Solid State Commun. 151 (2011), 1100-1104.

[9] A.W. Tsen et al., Science 336 (2012), 1143-1146.

[10] P. Nemes-Incze et al., Appl. Phys. Lett. 99 (2011), 023104.

[11] A. Salehi-Khojin et al., Adv. Mater. 24 (2012), 53-57.

[12] Clark et al., ACS Nano 7 (2013), 7956-7966.

[13] Vlassiouk et al., Nanotechnology 22 (2011), 275716.

Figures

Figure 1: Schematic representation of H adsorbates on a graphene grain boundary. Gray atoms belong to the graphene grains, blue atoms belong to the grain boundaries, and white atoms are the H adsorbates. Adsorbate concentrations are (a) 1%, (b) 10%, (c) 50%, and (d) 100%.

Figure 2: Effect of chemical functionalization on grain boundary resistivity. Panel (a) shows the grain boundary resistivity as a function of adsorbate concentration. Red squares are for epoxide, green circles are for hydrogen, and blue triangles are for hydroxyl adsorbates. Panel (b) shows the grain boundary resistivity from a variety of sources. The first set of data (red open circles, far left) gives our numerical simulations, and the rest are the experimental measurements of other groups.


ovel Pattern Graphene Fabrication Methods And Their Application in Graphene Based Nano-Optoelectronic Devices

LUN DAI Peking University China

A promising site-controllable patterned graphene transfer method, which can economize graphene material and requires no additional etching process, was developed.

A simple and scalable graphene patterning method was also invented, which employed electron-beam or ultraviolet lithography followed by a lift-off process. This method, with the merits of: high pattern resolution and high alignment accuracy, without additional harsh process, universal to arbitrary substrates, compatible to Si microelectronic technology, can be easily applied to array-based device applications.

Based on the above approaches, various graphene / semiconductor nanowire hybrid optoelectronic devices were fabricated, including high-performance graphene / CdS semiconductor nanowire (SNW) Schottky junction solar cells and novel graphene nanoribbon (GNR)/ SNW heterojunction light-emitting diodes (LEDs). In former work, Au (5 nm)/graphene composite electrode was used as the Schottky contact electrode to the NW. Typical as-fabricated solar cells showed excellent photovoltaic behavior with an energy conversion efficiency up to ~1.65%. In latter work, ZnO, CdS, and CdSe NWs were employed as representatives. At forward biases, the GNR/SNW heterjunction LEDs could emit light with wavelengths varying from ultraviolet (380 nm) to green (513 nm) to red (705 nm), which were determined by the band-gaps of the involved SNWs. The mechanism of light emitting for the GNR/SNW heterojunction LEDs was discussed. Our work pioneers new routes to developing diverse graphene-based nano-optoelectronic devices, which are basic components in integrated optoelectronic system.

Lun Dai, Yu Ye, and Guogang Qin State Key Lab for Mesoscopic Physics and School of Physics, Peking University, Beijing, 100871, China; Collaborative Innovation Center of Quantum Matter, Beijing 100871, China [email protected] References [1] Yu Ye, Yu Dai, Lun Dai*, Zujin Shi, Nan

Liu, Fei Wang, Lei Fu, Ruomin Peng, Xiaonan Wen, Zhijian Chen, Zhongfan Liu, and Guogang Qin, “High-performance Single CdS nanowire (nanobelt) Schottky junction solar cells with Au/graphene Schottky electrodes”, ACS applied materials & interfaces 2, (2010) 3406.

[2] Yu Ye, Lin Gan, Lun Dai*, Yu Dai, Xuefeng Guo, Hu Meng, Bin Yu, Zujin Shi, and Guogang Qin, “A Simple and Scalable Graphene Patterning Method and Its Application in CdSe Nanobelt/Graphene Schottky Junction Solar Cells”, Nanoscale 3 (2011) 1477.

[3] Yu Ye, Lin Gan, Lun Dai*, Hu Meng, Feng Wei, Yu Dai, Zujin Shi, Bin Yu, Xuefeng Guo, and Guogang Qin, “Multicolor Graphene Nanoribbon/Semiconductor Nanowire Heterojunction Light-Emitting Diodes” , J. Mater. Chem. 21 (2011) 11760.

N


Figures

Figure 1: The method to transfer a patterned graphene to desired position on a device substrate.

Figure 2: A simple and scalable graphene patterning method is invented (Patent No. 201010215355. 4).

Figure 3: a)-(c)The optical images of the GNR/SNW (ZnO, CdS, CdSe, respectively) heterojunction LEDs at a forward bias of 5 V. (d)-(f). Room-temperature EL spectra for GNR/SNW (ZnO, CdS, CdSe, respectively) heterojunction LEDs at various forward biases.


wo-Dimensional Early Transition Metal Carbides as Electrode Materials for Energy Storage

YOHAN DALL’AGNESE Université Paul Sabatier France

Recently, a new family of two-dimensional materials called MXene was discovered. MXenes are derived from the MAX phases, which are a class of conductive, layered ternary carbides and/or nitrides composed of an early transition metal M, an A-group element and carbon and/or nitrogen noted X, with the general formula Mn+1AXn. MXenes are synthetized by selectively etching out the A layers from the MAX phases. To date, about 10 MXenes - that include, among others, Ti3C2, V2C, Ti2C Ta4C3, Nb2C, TiNbC and Ti3CN,- have been successfully synthetized [1]. MXenes have quickly attracted the attention as promising candidates for energy storage applications because they have good conductivities, hydrophilic surfaces, a variety of surface chemistries and can host a variety of cations. Preliminary results on their use as electrodes materials for lithium-ion batteries and electrochemical capacitors are quite promising [2-4].

Here, we report on MXene’s performances as electrode for electrochemical capacitors. Various electrochemical techniques were used to characterize functionalized, f-Ti3C2. Using cyclic voltammetry, we demonstrate high cyclability rates and obtain reversible volumetric capacitances of the order of 350/cm3 by galvanostatic charge-discharge at 1 A.g-1. Intercalation between the f-Ti3C2 layers of a large variety of cations such as Li+, Na+, Mg2+ was confirmed by in situ Xray diffraction [2]. We also report on applications of MXenes as anode materials for Li -ion batteries. In

the case of delaminated f-Ti3C2, we measured a capacity of 410 mAh.g-1 at 1C cycling rate and more than 100 mAh.g-1 at 36 C (2 min charge) cycling rate for 700 cycles [3]. Yohan Dall’Agnese1,2,3,4, Michael Naguib3,4, Maria R. Lukastkaya3,4, Olha Mashtalir3,4, Chang E. Ren3,4, Patrick Rozier1, Michel W. Barsoum3,4, Pierre Louis Taberna1, Yury Gogotsi3,4 and Patrice Simon1,2 1Université Paul Sabatier, CIRIMAT UMR CNRS 5085, 118 route de Narbonne, 31062 Toulouse, France 2Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, France. 3Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA 4A. J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, PA 19104, USA [email protected]

T


References [1] Michael Naguib, Vadym N. Mochalin,

Michel W. Barsoum, and Yury Gogotsi, '25th Anniversary Article: Mxenes: A New Family of Two-Dimensional Materials', Advanced Materials (2013)

[2] M. R. Lukatskaya, O. Mashtalir, C. E. Ren, Y. Dall'Agnese, P. Rozier, P. L. Taberna, M. Naguib, P. Simon, M. W. Barsoum, and Y. Gogotsi, 'Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide', Science, 341 (2013), 1502-05.

[3] O. Mashtalir, M. Naguib, V. N. Mochalin, Y. Dall'Agnese, M. Heon, M. W. Barsoum, and Y. Gogotsi, 'Intercalation and Delamination of Layered Carbides and Carbonitrides', Nature Communications, 4 (2013).

[4] M. Naguib, J. Come, B. Dyatkin, V. Presser, P. L. Taberna, P. Simon, M. W. Barsoum, and Y. Gogotsi, 'Mxene: A Promising Transition Metal Carbide Anode for Lithium-Ion Batteries', Electrochemistry Communications, 16 (2012), 61-64.

Figures

Figure 1: (Left) Schematic for the exfoliation process of MAX phases and formation of MXenes. (Right). Secondary electron SEM micrographs for (A) Ti3AlC2 particle before treatment, which is typical of unreacted MAX phases, (B) Ti3AlC2 after HF treatment, (C) Ti2AlC after HF treatment, (D) Ta4AlC3 after HF treatment, (E) TiNbAlC after HF treatment, and (F) Ti3AlCN after HF treatment. In (B-F), the exfoliation is obvious [1].

Figure 2: (A) Comparison of the performance of exfoliated and delaminated Ti3C2 as anode material in Li-ion batteries. Inset shows SEM image of an additive-free film of delaminated f-Ti 3C 2 filtered through the membrane. (B) Capacitance retention test of delaminated Ti3C2 paper in KOH. Inset: Galvanostatic cycling data collected at 1 A/g [2,3].


pitaxial Silicene on Semiconductor Substrates: a Density Functional Study

G.P. DAS Indian Association for the Cultivation of Science India

In spite of the uniqueness of carbon to form pristine fullerene, nanotube and graphene, there have been attempts to replicate these nanostructures with silicon. The latest in this game is the quasi-2D silicene whose free-standing honeycomb form has been predicted to be stable with linear band dispersion and Dirac cone feature similar to graphene. Epitaxial silicene on Ag(110) and on ZrB2(0001) substrates have been reported recently [1,2]. We have carried out first principles density functional investigation of the structural and electronic properties of silicene monolayer on various wurzite structured III-V and II-VI semiconducting substrates, with metal terminated (MT) as well as non-metal terminated (NMT) top surface [3]. The binding energies of silicene on MT semiconductors are in the range ~0.56±0.12 eV/atom and their behavior can be metallic, semi-metallic or even magnetic, depending on the choice of substrates. The silicene overlayer undergoes n-type (p-type) doping on MT (NMT) semiconductor surface, depending upon the direction of the charge transfer. G.P. Das, A. Bhattacharya*, S. Bhattacharya* Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India * Currently at Fritz-Haber-Institute, Berlin, Germany [email protected]

References [1] P. Vogt, P. D. Padova, C. Quaresima, J.

Avila, E. Frantzeskakis, M. C. Asensio, A. Resta, B. Ealet, and G. L. Lay, Phys. Rev. Lett. 108, (2012) 155501.

[2] A. Fleurence R. Friedlein, T. Ozaki, H. Kawai, Y. Wang, and Y. Yamada-Takamura,, Phys. Rev. Lett. 108, (2012) 245501.

[3] A. Bhattacharya, S. Bhattacharya and G.P. Das, Appl. Phys. Lett. 103, (2013) 123113.

E


rocessing 2D Materials: From synthesis to devices

GEORG DUESBERG Trinity College Dublin Ireland

The reduction of dimensionality has revealed exciting electronic behavior of 2D materials such as graphene and transition metal dichalcogenides. In order to successfully exploit those techniques synthesis on an industrial scale and integration into functional devices must be developed. In this talk examples for the fabrication of hybrid electric devices with films produced by scalable CVD synthesis are presented. Namely, diodes were fabricated by transferring large area functional 2D layers onto pre-patterned silicon substrates [1, 2]. Monolayer graphene diodes, in particular, serve as a platform for a new type of chemical sensor [1, 4]. Exposure to a range chemicals alternated the conductivity and ideality factor of these devices by doping the graphene layer reversibly. Furthermore, p-n heterojunction diodes between MoS2 and p-Si where fabricated [3, 4]. These devices exhibited impressive photoconductivity displaying a broad spectral response with an extended range in the visible region. Our approach of integrating novel 2D materials with traditional silicon technology represents a significant step towards scalable fabrication of devices and opens up a wide range of novel functionalities of the achieved heterostacks.

Georg Duesberg, Nina Berner, Riley Gatensby, Toby Hallam, Hye-Young Kim, Kangho Lee, Niall McEvoy, Hugo Nolan, Maria O’Brien, Ehsan Rezvani, Sinéad Winters, Christian Wirtz, and Chanyoung Yim School of Chemistry Centre for Adaptive Nanostructures and Nanodevices (CRANN) Advanced Materials BioEngineering Research Centre (AMBER) College Green Trinity College Dublin Dublin 2, Ireland Duesberg@ tcd.ie References [1] Yim, C.; McEvoy, N.; Kim, H. Y.; Rezvani, E.;

Duesberg, G. S. Acs Applied Materials & Interfaces 2013, 5, (15), 6951-6958.

[2] Yim, C.; McEvoy, N.; Rezvani, E.; Kumar, S.; Duesberg, G. S. Small 2012, 8, (9), 1360-1364.

[3] Lee, K.; Gatensby, R.; McEvoy, N.; Hallam, T.; Duesberg, G. S. Advanced Materials %@ 1521-4095 2013.

[4] Kim, H. Y.; Lee, K.; McEvoy, N.; Yim, C.; Duesberg, G. S. Nano Letters 2013, 13, (5), 2182-2188.

Figures

Figure 1: Schematic Silicon-Graphene Diode for Chemical Sensing.

P


raphene/piezoelectric hybrid for Coulomb drag of graphene bilayer system

DINH-LOC DUONG Sungkyunkwan University Korea

Excitonic superfuid and Bose-Einstein condensation are expected to accur at higher temperature compared to atomic system due to their low effective mass [1,2,3]. However, excitonic system is unequilibrium and has a short lifetime. The indirect excitonic system which consists of 2 layers insulated by an ultrathin insulator between them to prevent the recombination of electron-hole pair is, therefore, proposed to overcome this issue. The challenges of this approach is how to fabricate the device with an ultrathin insulator thin enough to maintain the interaction between two layers but thick enough to avoid tunnelling phenomena.

The explosion of 2D materials opens new opportunities for making devices with well-controlled thickness. Although room temperature excitonic superfluid of graphene bilayer system is still debated from theoretical prediction [4,5], a system of combination between graphene/BN/graphene is ideal structure for indirect excitonic study. However, no evidence of experiments are observed [6,7]. Using both top and botton gate reduces interaction of carrier between two layers. Therefore, new device designs are required

In this presentation, we proposed a combination between Graphene/Piezo-electric-layer/Graphene for excitonc study. Hole and electron on two graphene layers are generated by the strong dipole of piezoelectric layer was simulated by density functional theoretical calculation [8,9]. This hybrid structured was also realized by experiments with PVDF thin layer made by Langmuir-Blodgett method. The simulation by is consistent with experimental results from Raman spectroscopy with doping concentration up to 1013 cm-2. Our study

opens a new design for studuing bylayer system not only for graphene but also for another two dimensional materials. Dinh-Loc Duong, Young Hee Lee Centre for Integrated Nanostructure Physics (CINAP). Institute for Basic Science (IBS), Sungkunkwan -University, Suwon 446-746, Korea [email protected] References [1] Eisenstien, J. P. & MacDonals, A. H.

Bose-Eistein condensation of excitons in bilayer electron systems. Nature 432, (2004), 691-694.

[2] Lecture from Prof. Leonid Butov. [3] David W. Snoke. Dipole excitons in

coupled quantum wells: toward anequilibrium exciton condensate. asXiv: 1208.1213v1.

[4] Min, H., Bistritzer, R., Su, J. J. & MacDonald, A. H. Room-temperature superfuidity in fraphene bilayers. Phys. Rev. B 78 (2008) 121401.

[5] Karitonov, M. Y. & Efetov, K. B. Excitonic condensation in a double-layer graphene system. Semicond. Sci. Technol. 25, (2010)034004.

[6] R. V. Gorbachex et. al. Strong Coulomb drag and broken symmetry in double-layer graphene. Nature Physics 8 (2012) 896-901.

[7] Kim, S. et al. Coulomb drag of massless fermions in graphene. Phys. Rev. B 83, (2011) 161401.

[8] Dinh Loc Duong et. al. Band-gap engineering in chemically conjugated bilayer graphene: Ab initio calculations. Phys. Rev. B 85, (2012) 205413.

[9] Dinh Loc Duong et. al. Graphen/piezoelectric hybrid. In preparation.

G


ynthesis of graphene oxide with an almost intact carbon framework - a precursor for graphene and an entry to functionalized graphene derivatives

SIEGFRIED EIGLER Friedrich-Alexander-Universität Erlangen-Nürnberg Germany

Graphene, a single layer of graphite, has created a completely new interdisciplinary research field.[1-3] Chemists of all disciplines, physicists and materials scientists constantly recognize new fascinating properties of graphene and its derivatives.

Nevertheless, graphene must become available on large-scale for realizing applications.[2] Graphene oxide (GO) has a long history and can be produced in large quantities and provides access to individual layers of carbon.[4] However, the honeycomb lattice generally degrades under the harsh conditions applied for synthesis.[5, 6] However, several methods are described in the literature, none reaching the quality of graphene prepared by alternative routes like chemical vapor deposition.[7] Our work intends to increase the understanding of the chemical reactivity of functionalized graphene derivatives. A better understanding is crucial to benefit of the full potential of graphene.

Here, we present a wet-chemical synthetic method that allows for the synthesis of a type of GO whose carbon framework remains almost intact (ai-GO) after oxidation of graphite.[8] This means that the honeycomb lattice is not damaged severely by the oxidative functionalization procedure. After chemical reduction by hydriodic acid, the graphene is of high quality, which means that the residual defect density is as low as 0.01%.[9] Statistical Raman microscopy was used to systematically characterize the quality of samples. Furthermore, we prepared transport devices of ai-GO. Therefore, we applied the Langmuir-Blodgett technique to deposit ai-GO flakes onto SiO2/Si

substrates followed by chemical reduction. We identified flakes of high quality by Raman spectroscopy (ID/IG ratio < 1) and used electron-beam lithography to pattern Hall bars of those flakes. The Hall mobility measurements we conducted in magnetic fields up to 14 T and temperatures down to 0.3 K.[8] For the best quality of flakes we identified samples of graphene with a mobility of charge carriers up to 2000 cm2/Vs. This is the highest mobility ever measured using any GO as precursor for the preparation of graphene. Furthermore, we conducted high resolution transmission electron microscopy on ai-GO and can visualize the almost intact carbon framework for the first time. The results confirm that GO with an almost intact carbon framework can be successfully prepared by preventing CO2 formation during synthesis. Hence, our ai-GO reflects a type of chemically functionalized graphene, since there is a low amount of σ-defects.

With this material in hand we evaluated the stability of the carbon framework by statistical Raman microscopy for the very first time. We investigated the thermal stability of the carbon framework of ai-GO and found that it remains stable up to 100 °C in contrast to conventionally prepared GO.[10] Functional groups are thermally less stable than the carbon framework. Consequently, one has to discriminate between the stability of functional groups and the stability of the carbon framework. Conclusions about the integrity of the carbon framework in graphene oxide are possible after reduction using scanning Raman spectroscopy. In addition the visualization of the heterogeneity of samples is possible by combining scanning Raman and atomic force microscopy that we additionally correlated with atomic force microscopy.

S


Using this method we were able to compare and determine the efficiency of different reducing agents for ai-GO.[9] Furthermore, we evaluated the chemical structure of GO that was prepared in sulfuric acid using an oxidant. Our investigations reveal that organosulfate groups are chemically bound and their amount approaches one organosulfate group on about 30 carbon atoms[11]. These organosulfate groups can explain the acidity of graphene oxide that bears an almost intact carbon framework.

Moreover, we could demonstrate that the carbon framework is stable enough to enable chemical reactions, e. g., with sodium hydroxide without degradation of the carbon framework.[12] This is possible only under certain reaction conditions, while others result in degradation. These findings are a prerequisite to make controlled chemistry with ai-GO. Thus, we used organosulfate groups as a leaving group to introduce anionic nucleophiles, like azide groups above and below the basal plane for the first time.[13] Establishing this type of reaction represents the ability to functionalize ai-GO with more complex organic structures to fine tune the properties of functionalized graphene. Siegfried Eigler1, Stefan Grimm1, Ferdinand Hof1, Christoph Dotzer1, Michael Enzelberger-Heim2, Philipp Hofmann2, Benjamin Butz3, Christian Dolle3, Erdmann Spiecker3, Yichen Hu4, Yoshitaka Ishii4,and Andreas Hirsch1 1Department of Chemistry and Pharmacy and Institute of Advanced Materials and Processes (ZMP) Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Fürth, Germany 2Department of Physics and Interdisciplinary, Center for Molecular Materials, University Erlangen-Nürnberg, Erlangen, Germany 3Center for Nanoanalysis and Electron Microscopy, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany 4Department of Chemistry, University of Illinois at Chicago, Chicago, USA and Center for Structural Biology, University of Illinois at Chicago, Chicago, USA [email protected]

References [1] K. S. Novoselov, V. I. Fal'ko, L. Colombo, P. R.

Gellert, M. G. Schwab, K. Kim, Nature, 7419 (2012), 192.

[2] M. J. Allen, V. C. Tung, R. B. Kaner, Chem. Rev., 1 (2010), 132.

[3] V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P. Hobza, R. Zboril, K. S. Kim, Chem. Rev., 11 (2012), 6156.

[4] D. Chen, H. Feng, J. Li, Chem. Rev., 11 (2012), 6027.

[5] S. Eigler, C. Dotzer, A. Hirsch, M. Enzelberger, P. Müller, Chem. Mater., 7 (2012), 1276.

[6] S. Eigler, C. Dotzer, A. Hirsch, Carbon, 10 (2012), 3666.

[7] C. K. Chua, M. Pumera, Chem. Soc. Rev., 1 (2014), 291.

[8] S. Eigler, M. Enzelberger-Heim, S. Grimm, P. Hofmann, W. Kroener, A. Geworski, C. Dotzer, M. Rockert, J. Xiao, C. Papp, O. Lytken, H. P. Steinruck, P. Muller, A. Hirsch, Adv. Mater., 26 (2013), 3583.

[9] S. Eigler, S. Grimm, M. Enzelberger-Heim, P. Muller, A. Hirsch, Chem. Commun., 67 (2013), 7391.

[10] S. Eigler, S. Grimm, A. Hirsch, Chem. Eur. J., 4 (2014), 984.

[11] S. Eigler, C. Dotzer, F. Hof, W. Bauer, A. Hirsch, Chem. Eur. J., 29 (2013), 9490.

[12] S. Eigler, S. Grimm, F. Hof, A. Hirsch, J. Mater. Chem. A, 38 (2013), 11559.

[13] S. Eigler, Y. Hu, Y. Ishii, A. Hirsch, Nanoscale, 24 (2013), 12136.


Figures

Figure 1: A) Illustration of graphene with defects and without defects; B) Raman spectra of defective graphene and graphene of high quality; C) derivatives of ai-GO with different chemical structures.


ow does the magnetic edge-state vary in the fusion of nanogrphene sheets?

TOSHIAKI ENOKI Tokyo Institute of Technology Japan

When a graphene sheet is cut into nanofragments (nanographene), the created edges in nanographene work as a boundary condition to modify the electronic structure depending on the edge geometry [1,2]. In the armchair shaped edges, the electron wave interference takes place, resulting in the aromatic stability of the electronic structures. In contrast, localized nonbonding edge state of π-electron origin is created in the zigzag edges, in which the large spatial electron population at the edges with strong spin polarization causes electronic, magnetic and chemical activities. In the meantime, heat-treatment annealing at high temperatures makes assembled nanographene sheets fused at the expense of edges, ensuing the growth of nanographene sizes. Here the stability and activities of nanographene, for which the edge-geometry dependence in the electronic structure is responsible, are expected to govern the fusion process. In addition, the oxygen-containing functional groups bonded to the edge carbon atoms in the nanographene samples handled in the ambient atmosphere are concerned in the process. We investigated the fusion process of assembled nanographene sheets with a focus on the variation of the electronic and magnetic structure, in which the magnetic edge state plays an important role, using in-situ measurements of X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), magnetic susceptibility, electrical conductance, together with temperature-programmed desorption (TPD) measurements. The sample is activated carbon fibers (ACFs), whose structure

consists of disordered network of nanographene sheets [3,4]. The TPD results indicate that the edges of nanographene sheets are terminated with oxygen-containing functional groups and hydrogen atoms in the pristine sample. Oxygen-containing functional groups such as –COOH, >C=O, -OH are almost completely decomposed under heat treatment up to 1300−1500 K, resulting in the formation of edges primarily terminated by hydrogen. The removal of the oxygen-containing groups enhances the conductance owing to the decrease in the energy barriers in the electron transport between nanographene sheets, which is governed by the Coulomb-gap type variable range hopping process. Heat treatment above 1500 K removes also the hydrogen atoms from the edges, promoting the successive fusion of nanographene sheets at the expense of edges. The decrease in the π* peak width in NEXAFS and the increase in the orbital susceptibility indicate the progress of the fusion reaction, that is, the extension of the π-conjugation. The fusion leads to the random formation of local π/sp2 bridges between nanographene sheets in the percolative manner and brings about an insulator-to-metal transition at 1500−1600 K, at which the bridge network becomes infinite. The onset of the insulator-to-metal transition is accompanied by the change in the magnetism from the localized spin magnetism to the Pauli paramagnetism, for the former and the latter of which the edge-state spins forming a superparamagnetic structure and π-conduction carriers in the individual nanographene sheet are responsible, respectively. Interestingly spin glass state appears in the vicinity of the insulator-to-metal transition. The intensity of the edge state peak in NEXAFS, which corresponds to

H


the number of the spin-polarized edge states, decreases above 1500 K, demonstrating the successive disappearance of the edges, though the thermal average of the edge-state spin <S> estimated from the magnetic susceptibility starts decreasing about 200 K lower than the temperature of the edge state peak change. This disagreement indicates the development of antiferromagnetic short range ordering as a precursor of a spin glass state near the insulator-metal transition, at which the random network of inter-nanographene-sheet antiferromagnetic exchange interactions strengthened with the formation of the π/sp2 bridges becomes infinite. This is evidenced by the molecular field treatment that gives the estimate of the inter-nanographene-sheet exchange interaction to be -1600 K in the vicinity of the insulator-to-metal transition, which is comparable to the strengths of intra-nanographene sheet exchange interactions. Above 1500-1600 K, the edge state peak in NEXAFS tends to disappear, demonstrating that the sizes of nanographene sheets grow swiftly at the expense of edges. Toshiaki Enoki1, Jun-ichi Takashiro1, Yasuhiko Kudo1,aSatoshi Kaneko1, Kazuyuki Takai1, Takafumi Ishii2, Takashi Kyotani2, Manabu Kiguchi1 1Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan 2Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Aoba-ku, Sendai, Miyagi 980-8577, Japan [email protected]

References [1] T. Enoki, Physica Scripta T 146 (2012)

014008, Proc. Nobel Symp. on Graphene and Quantum Matter.

[2] S. Fujii and T. Enoki, Acc. Chem. Res. 46 (2013) 2202.

[3] Y. Shibayama, H. Sato, T. Enoki, M. Endo, Phys. Rev. Lett. 84 (2000) 1744.

[4] V. L. Joseph Joly, K. Takahara, K. Takai, K. Sugihara, M. Koshino, H. Tanaka, T. Enoki, Phys. Rev. B.81 (2010) 115408.


owards superlattices: Lateral bipolar multibarriers in graphene

JONATHAN EROMS University of Regensburg Germany

Superlattice effects in graphene are currently under active investigation. Unlike moiré superlattices, where a modulation of fixed strength and fixed lattice period is achieved by the interplay of graphene and a thin layer of hexagonal boron nitride, a tunable modulation of arbitrary period is highly desired. Here we report on a detailed experimental and theoretical study [1] of graphene samples with a global back gate and patterned top gate. The latter consists of a periodic stripe array with a lattice period between 100 nm and 200 nm. Tuning back and top gates independently, we can achieve both a unipolar transport regime, where carriers below and in between top gate stripes are of the same polarity, and a bipolar regime, with carriers of opposite types. In the latter, pronounced single and multibarrier Fabry-Pérot (FP) resonances occur. Our data on different devices with different numbers of top gate stripes and lattice spacings are compared to a detailed transport simulation based on a tightbinding model. The gate coupling of the modulated top gate is modeled accurately by taking into account both the geometric and the quantum capacitance. We can explain the resistance oscillations observed in the experiment by individual FP cavities stringed together, without invoking a superlattice effect. This is in contrast to a recent publication [2] where similar data on similar devices was ascribed to an artificial band structure, even though the mean free path did not exceed the superlattice period.

Jonathan Eroms1, Martin Drienovsky1, Franz-Xaver Schrettenbrunner1, Andreas Sandner1, Dieter Weiss1, Ming-Hao Liu( ) 2, Fedor Tkatschenko2, Klaus Richter2 1Institute for Experimental and Applied Physics, University of Regensburg, Regensburg, Germany 2Institute for Theoretical Physics, University of Regensburg, Regensburg, Germany [email protected]

T


References [1] M. Drienovsky, et al., arXiv:1401.1955 [2] S. Dubey, et al., Nano Lett. 13, (2013)

3990.

Figures

Figure 1: (a): Four-point resistance of a graphene sample with 3 top gate stripes, with 100 nm lattice period. In the bipolar regime (higher resistance) FP oscillations with 2 periods (dashed lines) are clearly visible. (b): Inverse transmission from the corresponding transport simulation. The calculation closely matches the experimental data.


aman Spectroscopy in Graphene and Layered Material

ANDREA C. FERRARI Cambridge Graphene Centre UK

Raman spectroscopy is an integral part of graphene research[1-3]. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp2-bonded carbon allotropes, because graphene is their fundamental building block. I will review the state of the art, future directions and open questions in Raman spectroscopy of graphene. The essential physical processes will be described, in particular those only recently been recognized, such as the various types of resonance at play, and the role of quantum interference[3-6]. I will update all basic concepts and notations, and propose a terminology that is able to describe any result in literature[3]. Few layer graphene (FLG) with less than 10 layers do each show a distinctive band structure. There is thus an increasing interest in the physics and applications of FLG. I will discuss the interlayer shear mode of FLG, and show that the corresponding Raman peak, named C, measures the interlayer coupling[7]. A variety of layered materials can also be exfoliated to produce a whole range of two dimensional crystals [8,9]. Similar shear and layer breathing modes are present in all these materials, and their detection provides a direct probe of interlayer interactions. A simple chain model can be used to explain the results, with general applicability to any layered material [10].

Andrea C. Ferrari Cambridge Graphene Centre, Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 OFA, UK [email protected] References [1] A. C. Ferrari et al. Phys. Rev. Lett 97,

187401 (2006). [2] A. C. Ferrari et al. Solid State Comm.

143, 47 (2007). [3] A. C. Ferrari, D. M. Basko Nature

Nanotech. 8, 235 (2013). [4] D. M. Basko, New J. Phys. 11, 095011

(2009). [5] M. Kalbac et al. ACS Nano 10, 6055

(2010). [6] C. F. Chen et al. Nature 471, 617

(2011). [7] P. H. Tan et al. Nature Materials 11, 294

(2012). [8] J. N. Coleman, et al. Science 331, 568

(2011). [9] F. Bonaccorso et al. Materials Today

15, 564 (2012) [10] X. Zhang et al. Phys. Rev. B 87, 115413

(2013).

R


pin transport and spintronics with graphene

A. FERT Université Paris Sud & CNRS/Thales France

Spintronics is a paradigm focusing on spin as the information vector in fast and ultra-low-power non volatile devices such as the new STT-MRAM. In particular spintronics is expected to provide beyond CMOS solutions for the realization of spin logic circuits in which logic gates acting on the spin process an information coded and propagated by spin currents. The recent discovery of graphene has opened novel exciting opportunities for such devices as long spin diffusion lengths can be expected from the small spin-orbit coupling of carbon and its large Fermi velocities. Multiple possibilities of spin manipulation inside graphene by proximity effects with other materials can also be anticipated. We will successively discuss these two issues, propagation of spin currents to long distance (spin relaxation time) and spin manipulation. The published experimental spin relaxation times, in most cases derived from the analysis of Hanle effects in lateral spin valves, are scattered in a relatively broad range and generally smaller than was intitially expected for graphene. We will show that some uniformisation of the data can be obtained by taking into account the spin absorption by the contact between graphene and magnetic electrodes in the interpretation of Hanle experiments. In transport measurments on lateral spin valves the spin diffusion length can be derived from the dependence of the MR signal on the contact resistances. We will show that highly efficient spin transport can occur in epitaxial graphene grown on the C-face of SiC leading to large spin signals and macroscopic spin diffusion lengths (~100 microns), a key enabler for the advent of envisioned

beyond-CMOS spin-based logic architectures [1]. On the technical side of the preparation of lateral spin valves, we will also show that a thin graphene passivation layer can prevent the oxidation of a ferromagnet, enabling its use in novel humide/ambient low-cost processes for spintronics devices, while keeping its highly surface sensitive spin current polarizer/analyzer behavior and adding new enhanced spin filtering property [2]. The experimental studies on mechanisms of spin manipulation in graphene circuits are still in their initial stage. However some very interesting results have already been obtained, as, for example, those of Balakrishnan et al who have demonstrated the introduction of Spin-Orbit Coupling (SOC) in hydrogenated graphene and the resulting generation of spin current by SHE [3]. Local introduction of SOC could also be used manipulate the polarization of spin currents. The road is open to the exploration of various similar effects induced by impurities, defects, edges, adatoms, absorbed molecules or interfaces with other materials, and we will discuss the resulting perspective for technology. We will be able to conclude that many different experiments begin to unveil promising uses of graphene for spintronic devices. A. Fert1, B. Dlubak1,3, M.-B. Martin1, H. Yang1, R.

Weatherup3, M. Sprinkle2, B. Servet1, S. Xavier1,

C. Berger2, W. de Heer2, S. Hoffman3, J. Robertson3, C. Deranlot1, R. Mattana1, H. Jaffres, A. Anane1, F. Petroff1, P. Seneor1 1Unité Mixte de Physique CNRS/Thales,

Palaiseau, France, & Université Paris-Sud, Orsay, France 2GeorgiaTech, Atlanta/Institut Néel and 3University of Cambridge [email protected]

S


References [1] B. Dlubak et al., Nature Physics 8, 557

(2012); P. Seneor, et al., MRS Bulletin 37, 1245 (2012).

[2] B. Dlubak et al., ACS Nano 6, 10930 (2012); R. Weatherup, et al., ACS Nano 6, 9996 (2012).

[3] Balakrishnan et al, Nature Physics 9, 284 (2013).


ntegrated graphene high-gain voltage amplifiers and ring oscillators

MARCO FIOCCO Politecnico di Milano Italy

Graphene has been investigated as a possible contender in high-frequency electronics due to its high charge carrier mobility, which is larger than that of conventional high-mobility semiconductors, such as InP or SiGe. However, the applications of graphene in commercial electronics are virtually non-existent, as only laboratory prototypes have been demonstrated so far. This apparent discrepancy occurs because of a very low voltage gain Av exhibited by typical graphene electronic devices and by large device to device variations. Without |Av| > 1 and reproducible device characteristics it is not possible to realize more complex (i.e., realistic) electronic circuits relying on the amplification of signals and on cascading of different stages. As a consequence, early graphene electronics was entirely based on proof-of-principle operation of single-transistor and single-stage circuits. Such circuits are unusable in real applications in which a large number of transistors and stages are required to obtain the required functionality. The main reason for a low voltage gain in graphene electronic devices is the absence of a bandgap in graphene. This prevents the depletion of charge carriers in graphene field effect transistors (GFETs) and therefore increases the output conductance gd due to the absence of drain current saturation. In order to increase the intrinsic gain A = gm/gd of GFETs and consequently the voltage gain Av in graphene circuits, it is necessary to maximize the transconductance gm of GFETs. To this end very thin high-k gate oxides, high-quality graphene materials, and metal contacts exhibiting a low

contact resistance have been implemented in GFETs. In this way, |Av| in the range of 1.7 to 7 has been demonstrated both in exfoliated [1-3] and large-scale graphene samples [4-8]. However, there have been no previous demonstrations of |Av| > 10 (i.e., > 20 dB) in monolayer graphene electronic circuits, which would represent an important milestone of achieving a signal amplification of an order of magnitude. Here we demonstrate the first voltage amplifiers integrated on a large-scale monolayer graphene exhibiting |Av| > 10. Graphene ring oscillators (ROs), circuits often used as a test-bench for new technologies, have been realized using such inverters with submicron gate lengths, proving the reproducibility of device characteristics. The oscillation frequency was 1.8 GHz, which is the highest oscillation frequency reported to date in an oscillator made of any low-dimensional material. Graphene amplifiers were realized in a complementary configuration (Fig. 1). Equivalent oxide thickness (EOT) of only ~ 2 nm was obtained by direct evaporation of Al on graphene, which upon exposure to air naturally forms a very thin AlOx insulator at the interface with graphene. High mobility of graphene samples grown by chemical vapor deposition was preserved by careful transfer from Cu foils to float-zone Si substrates (resistivity 5 kΩcm) with 1 µm thick SiO2 on top. Low contact resistance was obtained by contacting the graphene with purely Au contacts, without the use of any adhesion layers. Low EOT and contact resistance together with the high mobility of the graphene resulted in high voltage gain of the fabricated inverters obtained already at low voltage supplies (VDD < 2 V), as evidenced by their DC characteristics (Fig. 2). High voltage gain of Av ~ -11 was directly measured in

I


the AC regime (Fig. 3), demonstrating highly stable device characteristics of the fabricated devices. Graphene ROs were realized by cascading three graphene inverters in a loop, with the fourth inverter decoupling the RO from the measurement equipment connected to the output (Fig. 1). High voltage gain and very small Dirac voltages in the fabricated inverters allowed in/out signal matching and induced oscillations at high frequencies (Fig. 4). The highest oscillation frequency was 1.8 GHz and it was obtained in ROs with the gate length of 800 nm, demonstrating intrinsic voltage gain A > 1 in submicron GFETs. Demonstrated graphene electronic circuits are an important step toward the application of graphene in electronics. This research was supported by Fondazione Cariplo (grant no. 2011-0373), PRIN project GRAF, and Graphene Flagship (grant no. 604391). Marco Fiocco,1 Erica Guerriero,1 Abhay A.

Sagade, 2 Daniel Neumaier,2 Roman Sordan1 1L-NESS, Department of Physics, Politecnico di Milano, Via Anzani 42, 22100 Como, Italy 2AMO GmbH, Otto-Blumenthal-Strasse 25, 52074 Aachen, Germany [email protected]

References [1] A. Sagar et al, Appl. Phys. Lett., 99

(2011) 043307. [2] E. Guerriero et al., Small, 8 (2012) 357. [3] B. N. Szafranek et al., Nano Lett., 12

(2012) 1324. [4] S.-J. Han et al., Nano Lett., 11 (2011),

3690. [5] L. G. Rizzi et al., Nano Lett., 12 (2012)

3948. [6] Y. Wu et al., Nano Lett., 12 (2012) 3062. [7] E. Guerriero et al., ACS Nano, 7 (2013)

5588. [8] D. Schall et al., Sci. Rep., 3 (2013) 2592. Figures

Figure 1: Integrated monolayer graphene voltage amplifiers and ROs. a) Circuit diagram of a three-stage RO with a fourth stage acting as a buffer. Single inverting stages were separately investigated as voltage amplifiers. b) An optical image of a graphene RO with the gate length of 800 nm.


Figure 2: DC transfer curves and voltage gain Av of one of the fabricated graphene inverters at different supply voltages VDD under ambient conditions.

Figure 3: AC components of the input and output voltage signals measured in a graphene inverter in ambient air at a frequency f = 1 kHz for VDD = 3 V. The voltage gain is Av = −11.3. .

Figure 4: Output signal of one of the fabricated graphene ROs at VDD = 1.9 V. a) Waveform. b) Power spectrum.


raphene-Silicon Composites for Li-ion Battery Anodes

RAHUL FOTEDAR Graphene Batteries AS Norway

Silicon has remarkable properties to replace the presently used graphite as an anode in Li-ion batteries. There is abundance of Si in nature, it can potentially be produced at a low cost, it is environmentally benign, and has a high theoretical capacity which is an order of magnitude higher than that of the conventionally used graphite [1]. However, in practice, silicon suffers from massive cracking and degradation when it is being employed as an anode. The rapid fade is also accelerated by the continuous generation of SEI (Solid electrolyte interface) layer on the silicon surface. Carbon coating or scaffolding of silicon particles has proved to be the most reliable way to avoid the SEI formation. The formation of such an effective carbon scaffold for silicon can be an extremely challenging yet the most important step towards the utilisation of silicon. Amongst all existing forms of carbon, graphene has the most exceptional set of properties. It possesses almost metallic conductivity [2], very high flexibility [3] and a large specific surface area [4]. These properties can perfectly alleviate the inherent problems of low conductivity and cracking associated with the cycling of silicon. Graphene will additionally benefit the rate capability of Si based anode by increasing the transport of ions and electrons within the electrode microstructure. Last but not the least the exceptionally high thermal conductivity of graphene [5] will reduce the risk of any thermal run away in a cell and significantly improve the safety of high energy Li-ion cells. We have synthesised composite Si-graphene anodes using nano-silicon and

reduced graphene oxide. The electrodes exhibit reduced internal resistance during cycling compared to electrodes without graphene which in turn might lead to improved overall performance. Rahul Fotedar a, Jan Petter Maehlen b, Hanne F.

Andersen b, Preben J.S. Vie b, Sameer Fotedar a,

Rune Wendelbo a a Graphene Batteries AS, Oslo, Norway; b Institute for Energy Technology, Kjeller, Norway [email protected]

G


References [1] Dominique Larcher, Shane Beattie,

Mathieu Morcrette, Kristina Edström, Jean-Claude Jumas and Jean-Marie Tarascon, J. Mater. Chem., 17, 2007, 3759 – 3772.

[2] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science , 306, 2004 ,666.

[3] C. Lee, X. D. Wei, J. W. Kysar, J. Hone, Science, 321, 2008, 321, 385.

[4] M. D. Stoller, S. Park, Y. Zhu, J. An, R. S. Ruoff, Nano Lett., 8, 2008, 3498.

[5] A. A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C. N. Lau, Nano Lett., 8, 2008, 902.

Figures

Figure 1: Internal resistance versus cycle number measured for selected Si-C based materials with electrode composition in wt.% as indicated in the figure legend (Silicon/Carbon-black/Binder (CMC)/reduced graphene oxide). The electrode containing just small amounts of reduced graphene oxide shows significant less internal resistance after cycling. Typical loading of the active material was 0.2-0.5 mg/cm2.


igh-frequency measurement of graphene transistor for biosensing

WANGYANG FU Basel University Switzerland

Owing to its high carrier mobility, large surface-to-volume ratio, and chemical stability, graphene have drawn considerable attention as the building blocks for next generation label-free electrical biochemical sensors. Previously, we revealed that ideal defect-free graphene is inert to electrolyte composition changes in solution and non-covalent functionalization of graphene with chemically active groups has to be introduced for practical sensing applications [1, 2]. However, under physiological conditions, graphene sensors typically can not detect a biological stimuli occurring at a distance larger than a nanometer from its surface due to the present of movable ions (the so called “Debye screening length”). Consequently, graphene is therefore generally disregarded as a potent biological sensor. The present work [3] serves to shed light on this distance-dependent sensing paradigm. We performed radiofrequency (RF) measurements at 2-4 GHz on electrolyte-gated graphene field-effect transistors (GFETs). At these high frequencies the ions in the electrolyte start to lag behind the alternating current (AC) electric field due to the viscosity of the solution. As a result, the Debye screening effect is canceled and the electrolyte behaves as a pure dielectric at RF/microwave frequencies. Consequently, the biomolecules could be probed beyond the Debye screening length. In this work, we started with chemical vapor deposition (CVD) graphene [4] grown on 25 um thick copper foils. The CVD graphene allows for the transfer of high-quality graphene with lateral scale of many centimeters on

arbitrary substrates. Further characterizations indicate that we achieved predominant uniform, monolayer graphene with high mobility ~3000cm2/Vs. As shown in Fig. 1a and b, liquid-gated GFETs with reliable performance are developed. Then we performed RF measurements on the electrolyte-gated GFETs. In Fig. 1c, we demonstrated that the gate voltage dependent RF resistivity of graphene can be deduced (which is found to be consistent with its direct current (DC) counterpart in the full gate voltage range) even in the presence of the electrolyte which is in direct contact with the graphene layer. A time-dependent gating in solution with nanosecond time resolution, has also been demonstrate to access the potential of high-frequency sensing for real-time applications. We believe this work sets the ground for a novel RF approach to deeply probe biological pathways beyond the Debye screening length and has the potential to lead to groundbreaking changes in the field of biosensing.. Wangyang Fu, Maria El Abbassi, Cornelia Nef,

Michel Calame, and Christian Schönenberger Department of Physics, Basel University, 4054-

Basel, Switzerland [email protected], [email protected]

H


References [1] W. Fu, et al, “Graphene Transistors Are

Insensitive to pH Changes in Solution”, Nano Lett., 11 (2011) 3597.

[2] W. Fu, et al, “High Mobility Graphene Ion-Sensitive Field-Effect Transistors by Noncovalent Functionalization”, Nanoscale, 5 (2013) 12104.

[3] W. Fu, et al, “Electrolyte Gate Dependent High-Frequency Measurement of Graphene FETs for Sensing Applications”, Appl. Phys. Lett., accepted (2014).

[4] X. Li, et al, “Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils”, Science, 324 (2009) 1312.

Figures

(a)

Epoxy

Photoresist

Electrolyte

solution Pt

SiO2/Si

Vsd

Au/Ti

Graphene

Vref

Isd

VPt

0.2 0.4 0.6

250

300

350

Vdirac

pH 7

pH 7B

fitting

Gsd (µS)

Vref (V)

(b)

(c)

(a)

Epoxy

Photoresist

Electrolyte

solution Pt

SiO2/Si

Vsd

Au/Ti

Graphene

Vref

Isd

VPt

0.2 0.4 0.6

250

300

350

Vdirac

pH 7

pH 7B

fitting

Gsd (µS)

Vref (V)

(b)

(c)

Figure 1: (a) Schematics of the experimental setup and the electrical circuitry of the liquid-gated GFET. The gate voltage VPt was applied to the solution via a Pt wire. The electrostatic potential in solution Vref was monitored by the reference calomel electrode. (b) Two data sets obtained for pH = 7 are shown to illustrate the excellent degree of reproducibility. Here a bipolar transfer curve is observed corresponding to different type of charge carriers that can continuously be tuned from holes (left) to electrons (right) with the charge-neutral point VCNP at minimum Gsd. (c) The measured DC resistivity ρDC (black squares) and the extracted small-signal RF resistance ρRF (red circles) as a function of the applied liquid gate voltage Vref . Inset: the relative deviation between ρDC and ρRF as a function of the applied liquid gate voltage Vref.


umerical simulations of the growth of graphene on Cu (111)

PHILIPPE GAILLARD University of Namur Belgium

Graphene is a material that has attracted a great deal of attention recently, through its exceptional electric, optical and mechanical properties. Foreseen applications such as transparent conducting layers or sensors depend on growing large areas of crystalline graphene with controlled quality. Chemical vapour deposition is the most promising method to reach this goal, but improving film quality requires a better understanding of film growth. This work presents the modelling of the growth of a graphene film on Cu (111) through the use of kinetic Monte Carlo (KMC) simulations, through a program that is described in [1]. These simulations require the knowledge of activation energies for the more stable adsorption sites and of the diffusion barriers between those sites. Based on detailed parameters obtained from ab-initio simulations, larger scale KMC simulations were performed as a function of the carbon partial pressure and the temperature. We studied the grain size and shape of graphene as a function of the carbon partial pressure and of temperature. In particular, we focussed on the conditions of the formation of grain boundaries when two graphene crystallites merge. The figure shows an example of a simulation of graphene growth on copper by CVD. The grey dots are carbon atoms, and the orange dots represent the hexagonal graphene lattice with additional “bridge” sites included.

P. Gaillard, T. Chanier, P. Moskovkin, L. Henrard,

S. Lucas University of Namur, Research center for the Physics of Matter and Radiation (PMR), 61 Rue de Bruxelles, B-5000, Namur, Belgium

[email protected] References [1] S. Lucas, P.Moskovkin, Thin Solids Films,

Volume 518, Issue 18 (2010), Pages 5355-5361.

Figures

N


ORTH Graphene Centre: Research Activities and Perspectives

COSTAS GALIOTIS FORTH/ICE-HT Greece

In Greece three Institutes of Foundation for Research and Technology (http:// www.forth.gr, FORTH / ICE-HT, IESL and ICAM), have joint forces and established in 2011 the FORTH Graphene Centre which is the main pillar for graphene research in Greece and actively participates in the Graphene FLAGSHIP. The main areas of research activities and expertise are nicely spread between institutes. In particular ICE-HT is involved in CVD production, physico-mechanical characterization and modelling, IESL in nano and opto-electronic applications and IACM in modelling. The main aim of the Center is to consolidate research in graphene in Greece in a cohesive manner. More details can be found in http://graphene.forth.gr. C. Galiotis Foundation for Research & Technology Hellas, Institute of Chemical Engineering Science, Greece [email protected]

References [1] "From Graphene to Carbon Fibres:

Mechanical and development of a universal stress sensor", Otakar F., Tsoukleri G., Riaz I., Papagelis K., Parthenios J., Ferrari C., Geim A., Novoselov K., Galiotis C. Nature Comms DOI: 10.1038/ncomms1247 (2011)

[2] “Phonon and Structural Changes in Deformed Bernal Stacked Bilayer Graphene”, O. Frank, M. Bouša, I. Riaz, R. Jalil, K. S. Novoselov, G. Tsoukleri, J. Parthenios, L. Kavan, K. Papagelis, and C. Galiotis Nano Lett. 12, 687–693 (2011).

[3] "Raman 2D-Band Splitting in Graphene: Theory and Experiment", O. Frank, M. Mohr, J. Maultzsch, C. Thomsen, I. Riaz, R. Jalil, K.S. Novoselov, G. Tsoukleri, J. Parthenios, K. Papagelis, L. Kavan, C. Galiotis:. ACS Nano 5 (3), pp.2231-2239 (2011)

[4] "Compression Behaviour of Single-layer Graphenes" by O. Frank, G. Tsoukleri, J. Parthenios, K. Papagelis, K. S. Novoselov , and Costas Galiotis, ACS Nano, 4(6), pp.3131-3138(2010)

[5] "Subjecting a Graphene Monolayer to Tension and Compression" by Georgia Tsoukleri, John Parthenios, Konstantinos Papagelis, Rashid Jalil, Andrea C. Ferrari, Andre K. Geim, Kostya S. Novoselov, and Costas Galiotis, Small, 5/21 (2009), 2397-2402

F


eavy adatoms and Anderson localization in graphene

JOSE H. GARCIA A. Universidade Federal do Rio de Janeiro Brazil

We analyze electronic localization in a graphene layer doped with adatoms sitting in the center of the honeycomb hexagon, as happens with the heaviest adatoms. In this configuration, the hybridization between the adatom orbitals and its neighboring carbon atoms mediate hopping processes that connect all six vertices of the honeycomb hexagon around the impurity(see Illustration 1.a). The amplitudes of the hopping depend on the symmetry of the orbital that hybridizes with graphene, leading to an orbital-dependent “plaquete disorder”. To capture the physics of localization, we propose an effective graphene-only Hamiltonian that preserves the associated orbital symmetries and conduct a scaling analysis of the local density of states(LDOS) for large system sizes. We show that some adatoms will form a zero-energy resonant state(See illustration 1.b) in the plaquette of the impurity that leads to Anderson Localization in the vicinity of the Dirac point and to a metal-insulator transition at a well defined energy. We show that when the discrete rotational symmetry of graphene is broken, charge puddles with non characteristic sizes appears at energies near the Dirac point, which also happens for a strong scalar disorder(On the plaquete) (See Illustration 2.a and 2.b) in agreement with the Anderson Localization scenario. But when the symmetry is preserved, destructive interference between the different hybridization paths in the plaquette of the adatom suppresses Anderson Localization and leads to a non-homogeneous metallic state with large charge puddles that localizes only at the Dirac point (See Illustration 2.c and 2.d). Therefore, the quest for finding Anderson Localization in graphene hence relies on

empirically identifying species of adatoms that sit at the center of the honeycomb plaquettes and do have a resonance at the Dirac point. Jose H. Garcia A., Bruno Uchoa, Lucian Covaci,

Tatiana G. Rappoport Universidade Federal do Rio de Janeiro. Av. Athos da Silveira Ramos, 149

Centro de Tecnologia - bloco A - Cidade Universitária, Rio de Janeiro, Brazil j [email protected]

H


Figures

Figure 1: a) Impurity plaquette for an adatom(center) stting at an H-site, with six carbon atoms: white circle(sublattice A); black circles(sublaticce B). Hopping processes mediated by the adatom:Solid lines(Nearest Neighbors), dashed (Next Nearest Neighbors), dot-dashed(Next Next Nearest Neighbors). b) DOS at the Dirac point vs effective hopping parameter for different orbital symmetries for a concentrarion of 0.5% adatoms per carbon and 10^7 atoms.

Figure 2Normalized LDOS at the Dirac point for a) Scalar plaquete disorder and b) asymmetric plaquette disorder for d orbitals. c) Symmetric plaquette disorder(s-wave orbital), at the Dirac point and d) Away from it (E=0.1t).


raphene nanoislands on Ni(111): structural and scattering properties

ARAN GARCIA-LEKUE DIPC & IKERBASQUE Spain

The scattering of electrons at the interface between graphene and metal contacts determines the charge and spin injection efficiency into graphene and, consequently, it is a fundamental issue for the performance of graphene-based devices. Weakly interacting metal contacts simply dope the Dirac bands. The interface with more reactive metals, however, is usually characterized by significant electronic reconstruction, which defines a complex scenario for scattering. The graphene-Ni interface represents an interesting case where the interaction with the ferromagnetic substrate opens hybridization gaps and induces magnetic moments.[1,2] Consequently, graphene is predicted to behave as a perfect spin filter in contact with a magnetic Ni electrode. In this talk, I will report on the results of a recent combined theoretical and experimental study of graphene nanoislands on Ni(111). Using pin-polarized density functional (DFT) calculations combined with scanning tunneling microscopy (STM) and spectroscopy (STS) measurements, we have studied the structural and electronic properties of this system. We show that the substrate induces the stabilization and reconstruction of the zigzag edges, being the edge structure strongly dependent on the registry of the edge C atoms with the Ni atoms underneath. These results allow us to understand the experimental observation of nanoislands with either triangular or hexagonal shape.[3] Moreover, we find that the electron scattering at the graphene edges depends on the electron spin, on the atomic structure of the edges, and on the orbital character and energy of the surface states.[4] This behavior is attributed to the

strong distortion of the electronic structure at the interface, which opens a gap and spin-polarizes the Dirac bands of graphene. This suggests a lateral 2D spin filtering for graphene layers, similar to that occurring across the interface. Aran Garcia-Lekue

Donostia International Physics Center (DIPC), San Sebastian, Spain IKERBASQUE, Basque Foundation for Science, Bilbao, Spain [email protected] References [1] V.M. Karpan et al., Phys. Rev. Lett. 99,

176602 (2007) [2] M. Weser et al., Appl. Phys. Lett. 96,

012504 (2010) [3] M. Olle et al., (submitted) [4] A. Garcia-Lekue et al., Phys. Rev. Lett.

112, 066802 (2014)

G


Figures

Figure 1: (a) Topographic (Vb = 0.1V) and constant current dI/dV map showing the interference pattern of the S1 surface state scattered from graphene islands. Setpoint current: I = 0:3 nA. Image size: 30 x 37 nm2. (b) Top: calculated energy of the majority interface states (IFS) as a function of the distance between the graphene layer and the Ni surface. The energy of the S1 surface state for pristine Ni(111) is represented by the red line. Bottom: electron density associated with the majority IFS averaged over a plane at the equilibrium distance (2.1Å).


lectronic transport in graphene with aggregated hydrogen adatoms

FERNANDO GARGIULO Ecole Polytechnique Fédérale de Lausanne (EPFL) Switzerland

Scattering by resonant impurities, such as covalently bonded hydrogen adatoms, is an important mechanism governing charge conductance in realistic graphene [1]. This type of disorder in 2D systems is known to trigger a non-perturbative Anderson transition for the whole electronic spectrum with an emergence of strongly energy-dependent localization length [2,3]. So far, the effect of resonant scatterers at finite concentrations has been treated as a mere sum of scattering contributions due to individual impurities. However, even the species covalently bonded to graphene are able to diffuse at room temperature, leading to their aggregation due to attractive interactions. We investigate the realistic spatial distribution of hydrogen adatoms on graphene by performing Monte-Carlo simulation ns based on interaction potentials between adatoms parameterized with the help of first-principles calculations. Hydrogen adatoms show a strong tendency to form small clusters thus ruling out almost entirely the occurrence of isolated adatoms (Fig.1a). The scattering behavior of adatom clusters is very different from single atoms, especially for even-membered clusters that do not show zero-energy resonances [4]. The spectral and transport properties of the constructed models of disordered graphene have been assessed by combining the Landauer-Büttiker approach [5] with the linear response Kernel Polynomial Method (KPM) [6]. A comparison with the case of randomly distributed (non-interacting) adatoms shows that the formation of clusters increases conductance and reduces the extent of the wave-function localization (Fig. 1b-d). Based on the calculated cluster size distributions we introduce a notion of effective concentration

of resonant impurities, which provides accurate description of both random and correlated cases [7]. Fernando Gargiulo1, Stefan Barthel2, Tim O.

Wehling2, Oleg V. Yazyev1 1 Institute of Theoretical Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland 2 Institut für Theoretische Physik, Universität

Bremen, D-28359 Bremen, Germany [email protected]

E


References [1] T. O. Wehling, S. Yuan, A. I.

Lichtenstein, A. K. Geim, M. I. Katsnelson, Phys. Rev. Lett., 105 (2010) 056802.

[2] A. Bostwick et al., Phys. Rev. Lett., 103 (2009) 056404.

[3] F. Evers, A. D. Mirlin, Rev. Mod. Phys., 80 (2008) 1355.

[4] G. Trambly de Laissardière, D. Mayou, Phys. Rev. Lett., 111 (2013) 146601.

[5] S. Datta, Electronic transport in mesoscopic systems (Cambridge University Press, 1997).

[6] A. Weisse, G. Wellein, A. Alvermann, H. Fehske, Rev. Mod. Phys., 78 (2006) 275.

[7] F. Gargiulo et al., in preparation.

Figures

Figure 1: (a) Cluster size distribution for random and correlated hydrogen adatoms at 5% concentration. (b,c) Average conductivity as a function of sample length L calculated for random and aggregated adatoms, respectively, at 5% concentration. (d) Localization length as a function of charge-carrier energy for the two cases.


ovel Phenomena Driven by Interactions and Symmetry-breaking in Graphene

DAVID GOLDHABER-GORDON Stanford University USA

Recent developments have led to monolayer graphene with extremely high mobility, corresponding to mean free paths exceeding 10 microns [1,2]. Interactions and substrate-induced symmetry breaking lead to novel phenomena in these ultraclean samples. I will speak about a few examples of such phenomena, including: Insulating behavior at the charge neutrality point at zero magnetic field [3]. Spin-selective equilibration of quantum Hall edge states [4]. A large set of fractional quantum Hall states, at denominators up to 9, with unexpected patterns of stability and spin polarization [5]. In addition to the physics, I will discuss some special fabrication approaches, and future electronic devices they may enable.. David Goldhaber-Gordon

Geballe Laboratory for Advanced Materials McCullough Building Rm. 346 476 Lomita Mall Stanford, California 94305-4045, USA

References [1] C. R. Dean, A. F. Young, I.Meric, C. Lee,

L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard and J. Hone, "Boron nitride substrates for high-quality graphene electronics," Nature Nanotechnology 5 , 722 - 726 (2010).

[2] Lei Wang, Inanc Meric, Pinshane Y. Huang, Qun Gao, Yuanda Gao, Helen Tran, Taniguchi Takashi, Watanabe Kenji, Luis M. Campos, David A. Muller, Jing Guo, Philip Kim, James Hone, Ken L. Shepard, Cory R. Dean

[3] F. Amet, J. R. Williams, K. Watanabe, T. Taniguchi, D. Goldhaber-Gordon, "Insulating Behavior at the Neutrality Point in Single-Layer Graphene" Physical Review Letters 110, 216601 (2013)

[4] F. Amet, J. R. Williams, K. Watanabe, T. Taniguchi, D. Goldhaber-Gordon, "Gate control of spin and valley polarized quantum Hall edge states in graphene," arXiv:1307.4408. To be published in PRL.

[5] F. Amet, A. J. Bestwick, J. R. Williams, K. Watanabe, T. Taniguchi, D. Goldhaber-Gordon Composite Fermions and Broken Symmetries in Graphene. Under review

N


rototyping CMOS-compatible ultrasensitive photo detectors for visible and infrared light

A.M. GOOSSENS ICFO – The Institute of Photonic Sciences Spain

Human beings are virtually blind during the night. Surveillance, military and car driver safety benefit enormously from the ability to observe in the dark. Currently there are different technologies available that enable people to see in the night. The most standard technology uses image intensifier tubes that amplify the remaining visible light. Image intensifier tubes are only sensitive to visible light and near infrared. With a camera that can detect short wave infrared radiation (SWIR), it is possible to benefit from illumination by night glow (also called air glow) [1]. Moreover fog is transparent for these wavelengths. A camera technology that combines responsivity in visible and SWIR spectral ranges would be enormously attractive. Current technologies relying on InGaAs cameras are prohibitively expensive for consumer electronics markets. Thus a need for low cost SWIR-Vis image sensors is immense. Previously we demonstrated a hybrid graphene quantum dot photo detector that is sensitive to both visible light and short wave infrared radiation [2]. The broad spectral range combined with its extremely low noise-equivalent power smaller than a fW make it a promising sensing technology for future night vision devices. The high dynamic range enables imaging under day and night conditions. Recent developments make it possible to operate the detector at video frame rate. The detectors are fabricated by depositing PbS colloidal quantum dots on top of the graphene that induce a photogating effect when exposed to radiation. By tuning the size of the quantum dots, the band gap can be tuned and hence the absorption

range. We have demonstrated absorption from the visible up to 1.6 µm (see Fig. 1b). As a proof of concept for the robustness and facile integrability of the photo detection platform we have developed a demonstrator. We designed a custom read-out board with integrated amplification, video frame rate imaging capabilities. With this custom board we will perform a live demonstration of a single pixel ultrasensitive hybrid graphene quantum dot photo detector during the talk (see Fig. 1 for the setup). We will demonstrate for the first time a prototype photo detector device enabled by graphene’s unique properties. The hybrid graphene quantum dot photo detector is based on large area chemical vapour deposition (CVD) - graphene. Moreover the colloidal quantum dots are compatible with large volume synthesis wet-chemistry methodologies and can be deposited atop substrates using standard solution-processed large area deposition techniques. These factors make it possible to integrate the photo detectors at the end of the line of a CMOS fabrication process. This paves the way for volume production and commercialization A.M. Goossens, I. Nikitskiy, J. C. Cifuentes

González , G. Konstantatos, F. Koppens ICFO – The Institute of Photonic Sciences, Av. Carl Friedrich Gauss, 3, 08860 Castelldefels (Barcelona), Spain [email protected]

P


References [1] M.P. Hansen and D.S. Malchow, Proc.

SPIE, 6939 (March 2008). Overview of SWIR detectors, cameras, and applications.

[2] G. Konstantatos, M. Badioli, L. Gaudreau, J. Osmond, M. Bernechea, F. P. Garcia de Arquer, F. Gatti and F. H. L. Koppens, Nature Nanotechnol., 7 (June 2012). Hybrid graphene-quantum dot detectors with ultrahigh gain.

Figures

Figure 1: Demonstration setup with custom read-out board with integrated amplifier and signal visualized on an iPad. The chip with the detector is mounted in the chip carrier on read-out board.


row Up and Grow Out: van der Waals Epitaxy and In-Plane Epitaxy of Two-Dimensional Materials

GONG GU University of Tennessee USA

Both van der Waals (vdW) solids [1] consisting of two-dimensional (2D) sheets and in-plane heterostructures [2,3] of 2D materials promise new physics and novel applications. The synthesis and characterization of these novel structures and, more importantly, the physical insight into their growth are prerequisites to the experimental exploration of the new physics. Here, we report a combined experimental-theoretical study that has gained insight into the determination of orientational order in vdW epitaxy, as well as a demonstration of in-plane heterostructures of 2D materials by adapting common wisdom accrued in the art of conventional epitaxy. We experimentally show that hexagonal boron nitride (h-BN) grown on a (100)-oriented Cu foil surface strictly aligns to the underlying Cu lattice (Fig. 1). This behavior is in sharp contrast to the orientational disorder of the graphene/Cu(100) system observed in previous work, [4] despite the close crystallographic similarity between h-BN and graphene. Our first-principles calculations reveal the origin of this curious contrast between the two crystallographically similar 2D materials. We show that strong C-Cu interactions (relative to B- and N-Cu interactions) actually lead to misalignment, a conclusion that runs counter to the conventional wisdom that stronger epilayer-substrate interactions enhance orientational order. The choice of the h-BN/Cu(100) system as the platform for this case study of vdW epitaxy provides the following advantages: (1) The epilayer and substrate are of different symmetries, therefore the physical picture we get from this study is more general than vdW epitaxies of epilayer-substrate pairs of the same symmetry already documented in the

literature. (2) Comparison between h-BN and crystallographically similar graphene leads to physical insight: In vdW epitaxy, interactions between the cluster edges and the substrate at the early nucleation stage steer the cluster’s orientation, which later determines the orientation of the crystallite that the cluster grows into. In this picture, stronger edgesubstrate interactions may reduce orientational order. Using the same archetypical 2D materials – graphene and h-BN, we also demonstrate a single-atomic layer, in-plane heterostructure, by projecting the concept of heteroepitaxy to 2D space. [5] Monolayer h- BN grows from fresh edges of monolayer graphene with lattice coherence, forming an abrupt boundary, or 1D interface (Fig. 2). More importantly, the crystallography of the h-BN is solely determined by that of the graphene seed; in this 2D heterostructure, the h-BN forgoes the aforementioned orientations it would assume if grown independently on the supporting Cu substrate. This demonstration is of fundamental significance. Due to the inherent 3D nature of the interactions between the 2D crystals and their surroundings, it is not clear whether a 2D crystal can grow from the edge of a 2D seed crystal epitaxially, i.e., adapting to the crystallography of the seed, while overcoming energetic factors in its 3D environment. Our work unambiguously answers this fundamental question. On the technical side, our method (Fig. 2a) was inspired by the common practice in conventional epitaxy in 3D. We view the graphene seed crystal as the “substrate” in 2D space, while the Cu supporting substrate merely confines the growth process to its surface – the 2D space where the epitaxy happens. This new vantage point inspires us to adapt the good practice commonly used in conventional epitaxy simply by reduced-dimension analogy.

G


Moreover, by adapting homoepitaxy to 2D space, we also demonstrate multiple millimeter-sized graphene single crystals. Gong Gu1, Lei Liu,1 D. A. Siegel,2 Jewook Park,3

Wei Chen,1,4 A. Mohsin,1 Peizhi Liu,1 I. N. Ivanov,3 K. F. McCarty,2 An-Ping Li,3 Zhenyu Zhang4

1 University of Tennessee, 1 Circle Park, Knoxville, USA; 2 Sandia National Laboratories, Livermore, USA; 3 Oak Ridge National Laboratory, Oak Ridge, USA; 4 University of Science and Technology, Hefei, China

[email protected] References [1] A. K. Geim and I. V. Grigorieva, Nature,

499 (2013) 419. [2] Y. Gao, Y. Zhang, P. Chen, Y. Li, M. Liu, T.

Gao, D. Ma, Y. Chen, Z. Cheng, X. Qiu, W. Duan, and Z. Liu, Nano Lett., 13 (2013) 3439.

[3] P. Sutter, R. Cortes, J. Lahiri, E. Sutter, Nano Lett., 12 (2012) 4869.

[4] J. M. Wofford, S. Nie, K. F. McCarty, N. C. Bartelt, O. D. Dubon, Nano Lett., 10 (2010) 4890.

[5] L. Liu et al, Science, 343 (2014) 163.

Figures

Figure 1: (a) Low-energy electron microscopy (LEEM) image (25 µm diameter field of view) of h-BN crystallites on (100) surface of Cu foil. (c-e) Selected-area LEED patterns obtained in circled areas labeled c-e in (a), respectively. (b) Real space model of h-BN crystals on Cu(100). Four and only four equivalent orientations are observed.

Figure 2: Cartoon illustration of in-plane h- BN/graphene heterostructure growth. Hydrogen etch results in fresh edges of graphene, analogous to fresh surfaces in conventional epitaxy. (b,c) Atomic resolution STM images of two h-BN/graphene boudaries. In (b), although the overall boundary orientation is armchair, each segment is zigzag. In (c), a zigzag boundary is shown.


nipolar supercurrent through graphene grafted with Pt-Porphyrins: Signature of gate voltage dependent magnetism

SOPHIE GUÉRON Univ. Paris-sud France

The superconducting proximity effect is a sensitive probe of mesoscopic systems. Here we show how it can detect a magnetic order induced in graphene. We have grafted graphene with Pt-porphyrin molecules which interact with graphene’s delocalized electrons. Neutral Pt-porphyrins are non-magnetic, but the ionized form carriers a magnetic moment of roughly one Bohr magneton. At room temperature we find that the molecules electron-dope the graphene and there is a hysteresis in gate voltage, demonstrating that electron transfer occurs. More surprisingly, the grafted graphene’s mobility increases. At low temperature, we show how superconducting contact electrodes can uniquely reveal the magnetic order induced in a mesoscopic, one micron-long graphene sheet. The unipolar nature of the induced supercurrent, which is enhanced at negative gate voltage but suppressed at positive gate voltage, may be the evidence for the Fermi-level controlled exchange interaction between localized spins and graphene. We have also found signatures of magnetic moments in graphene grafted by porphyrin using non superconducting contacts, most notably in the asymmetric magnetoresistance in parallel field. Sophie Guéron1, C. Li1, K.Komatsu1, G.Clavé2, S.

Campidelli2, A. Filoramo2, S. Guéron1 and H. Bouchiat1 1LPS, Univ. Paris-sud, CNRS, UMR 8502, F-91405 Orsay Cedex, France, 2 LEM CEA 91191 Saclay France [email protected]

References [1] Chuan Li, Katsuyoshi Komatsu, G.

Clave, S. Campidelli, A. Filoramo, S. Gueron, H. Bouchiat, arXiv:1304.7089.

Figures

Figure 1: Comparison between the proximity effect in graphene connected to Pd/Nb electrodes before (upper) and after (lower) deposition of porphyrin molecules.

U


n UHV study of epitaxial growth of graphene on SiC/Si substrates by Si sublimation

BHARATI GUPTA Queensland University of Technology Australia

Graphene extraordinary physical and electrical properties, like the high mobility and near ballistic transport at room temperature [1] attracted brilliant researchers to explore its potential application towards sensors, optoelectronics, and nanoelectronic devices. The successful development of graphene based nanoelectronics and sensors depends upon the availability of high quality and large area graphene layers directly grown on a wafer. Epitaxial graphene can be obtained from thermal decomposition of SiC, but the use of SiC wafers is very expensive. Growth of graphene on 3C SiC/Si is more affordable, and at the same time the most efficient route towards the integration of this 2D material in the microelectronic industry based on Si. The thermal decomposition of SiC in ultra-high vacuum (UHV) gives origin to a contaminant free surface. High temperature annealing of SiC produces the diffusion and sublimation of Si atoms leading to different surface reconstructions depending on the temperature. The challenge in UHV annealing of SiC/Si is to control the Si diffusion. The rate of Si out-diffusionis higher compared to furnace annealing under Ar at atmospheric pressure. 250 nm of 3C SiC was grown on p-doped Si (111) and Si(100) substrate by an alternating supply epitaxy (ASE) process. This process was undertaken in a hot-wall low pressure CVD reactor using the vapour precursors silane (SiH4) and acetylene (C2H2) [2]. Epitaxial Graphene was grown by annealing 3C SiC/Si (111) and 3C SiC (100) substrates in UHV for about 10 mins [3]. We investigated the diffusion of Si in SiC/Si during UHV annealing at temperatures ranging from 1125°C to 1375°C in order to produce epitaxial graphene. We have also studied the effect

of the exposure to Si flux at 900°C for various times before the final annealing stage. Chemical composition and surface morphology characterizations were investigated in-situ by using XPS and STM. Ex-situ analysis of the graphene layers was performed by Raman spectroscopy. Single layer graphene regions have been visualised by Scanning Tunneling Microscopy (STM) in samples annealed at 1250 °C. The graphene layers appears to be continuous on the substrate, although wrinkled because of the steps and defects in the underlying SiC/Si(111). STM images of graphene obtained after annealing at 1300 °C are shown in Fig. 1. Figure 1a is a 20x20 nm STM image showing a step where a Moiré pattern is visible like a shadow around the center of the image, while in two areas (top and bottom part of the image) it is even possible to observe the honeycomb graphene structure. This difference is due to the presence of multiple/single graphene layers in different areas of the sample. Fig 1b shows the C-rich (6√3×6√3) R30° reconstruction caused by the 30°rotation of the graphene overlayer with respect to the unreconstructed 3C SiC (111) surface. In our case the typical structure due to the Bernal stacking is visible, confirming the presence of more than one graphene layer (Fig 1d). The number of graphene layers was calculated by using the intensity ratio of XPS graphitic and carbidic peak of SiC. The results show an exponential increase in the number of graphene layers with the increase of temperature, which was fitted to an Arrhenius law (Fig 2). This behaviour is discussed in terms of the Si atom diffusion mechanism through the atomic lattice. The diffusion rate of Si atoms was moderately minimized by depositing Si at 900°C before the final annealing. Furthermore, STM analysis reveals different types of surface reconstructions during graphene formation. Raman 2D band intensity behaviour

A


confirmed the increase of the graphitic layers leading to a similar Arrhenius law [4]. Further studies are in progress. B. Gupta1, M. Notarianni1, W. Macaskill1, N.

Mishra2, M. Shafiei1, F. Iacopi2, N. Motta1 1 Queensland University of Technology, 2

George Street, Brisbane 4001, QLD, Australia 2 Queensland Micro and Nano-electronic Centre, Griffith University, Nathan Campus 4111, QLD Australia [email protected] References [1] A K.Geim, Science, 324 (2009) 1530-

1534. [2] L.Wang et al, Thin Solid films, 519 (2011),

6443-6446 [3] A.Ouerghi, et al. Applied Physics Letters,

96 (2010) 191910-1 – 191910-3 [4] B Gupta et al. Carbon, 68 (2014) 563-

572.

Figures

Figure 1: STM images of graphene obtained by annealing SiC/Si(111) at 1300 °C. (a) 20x20 nm2 area with a step showing a shadow of Moire pattern (V=70 mV, I=0.3 nA), (b) high resolution Moirè pattern with hexagonal symmetry (V=50 mV, I=0.2 nA). A (6 √3×6 √3) R 30° unit cell (blue insert) is also shown, (c) FFT of image (b) showing 27° rotation of graphene layer with respect to the buffer layer and (d) high resolution STM image of bi/few layer graphene (V= 50 mV, I= 0.2 nA) with graphene unit cell (red insert) [4].

Figure 2: Number of graphene layers developed in 10’ versus annealing temperature, as obtained from XPS analysis (red circles). The values are fitted to an Arrhenius function (solid line). Blue squares: area of 2D Raman peak versus the annealing temperature.


evelopment of Near-field Microwave Methods for Graphene NEMS Resonators

LING HAO National Physical Laboratory UK

As electromechanical devices become smaller, approaching the nanoscale, the oscillation displacement amplitude scales down in proportion to size. Thus new ultra-sensitive transducer techniques and low dissipation excitation schemes are needed to operate NEMS sensors. Microwave measurement using high Q resonators becomes attractive due to the high sensitivity of frequency measurement and the very low phase noise from synthesized microwave sources, in contrast with optical systems. We have developed a novel near-field microwave probe system which is able to simultaneously excite and readout the oscillation of a range of mechanical resonators, from hundreds of microns to sub-micron size. By using a quarter wave microwave coaxial resonator with the open end connected to a sharp tip we can produce a very localized intense microwave field in a very limited region close to the tip. The spatial range of this high field region is on the order of the radius of curvature of the tip, which can be, comparable to the smallest mechanical resonator dimensions. In this paper we discuss fabrication and measurement of a number of different graphene mechanical resonators based on transferred material onto different lithographically patterned substrates. CVD grown graphene films are of increasingly good quality and, following growth on a metal thin film catalyst, can then be transferred to any arbitrary supporting substrate. The transfer process is critical. The quality and performance of the graphene mechanical resonators depend on how fully the polymer support layer has been removed from the graphene, following

transfer. AFM scanning has been used to investigate the purity of the graphene and the strain in the transferred films and to test the breaking stress of the material. The main result of this paper is to present experimental data on graphene mechanical resonators using a variety of microwave excitation and readout methods, including a novel self-sustaining microwave interrogated oscillator with excellent frequency stability. An important issue is that there is strong coupling between graphene and microwave fields. This relates to the relatively close matching of the impedance of free space, or a confined geometry like a microwave resonator, and the sheet resistance of high quality graphene [1]. This fact makes the microwave method particularly suitable for application to graphene NEMS resonators. We have modelled the performance of single graphene layer drumhead membrane mechanical resonators and compared these results with those of experiments as a further means of determining the strain present in the NEMS systems. An SEM image of one of our graphene NEMS resonators is shown in Fig. 1a. The microwave method seems to be particularly suitable for implementation in an array of sensors. As a start towards development of this technique we show, in Fig. 1b, an array of various sized circular graphene drum resonators on a Si3N4 membrane. We present designs for a scanned array system which allows a single microwave source to interrogate a range of membrane resonators. This has potential for biosensor and other future applications. Further details will be provided at the conference. In our latest results, in order to try to increase the mechanical Q factor of the graphene NEMS resonators we have begun to move away from the use of

D


drilled Si3N4 membranes as support, towards more rigid SiO2 or Si support structures. Ling Hao, Stefan Goniszewski, Trupti Patel & John

Gallop National Physical Laboratory, Hampton Rd.,

Teddington TW11 0LW, United Kingdom [email protected]

References [1] L. Hao, J. Gallop, S. Goniszewski, O.

Shaforost, N. Klein and R. Yakimova, ‘ Non-contact method for measurement of the microwave conductivity of graphene’, Appl. Phys. Lett. Vol. 103, 123103 (2013).

Figures

Figure 1 a: A SEM image of CVD graphene covering one hole in the Si3N4 membrane.

Figure 1 b: SEM image of Si3N4 membrane drilled with array of holes with diameter from 1 µm to 3µm.


raphene Synthesis by Plasma Technique for Transparent Conductive Film Applications

MASATAKA HASEGAWA National Institute of Advance Industrial Science and Technology (AIST) Japan

We developed the large-area microwave plasma chemical vapor deposition (CVD) of graphene for transparent electrode applications [1]. This technique has been successfully combined with the roll-to-roll process to synthesize graphene on Cu substrates [2]. High growth rate and nucleation density of plasma CVD suppresses two-dimensional growth of graphene and leads to graphene flakes of several nanometers stacking in multiple layers. This causes low electrical conductivity of the synthesized graphene. The low concentration of carbon source is effective to reduce the growth rate and the nucleation density which is expected to enlarge the crystal size. In this study, we utilize small amount of carbon delivered from Cu foil and/or the ambient in the reaction chamber as extremely low-concentration of carbon source and perform the synthesis of graphene of higher crystalline quality. The carbon atoms precipitate on the Cu surface by the heat treatment of the Cu foil for 15 minutes at about 800°C. Then the copper foil was exposed to the hydrogen plasma for about 30 seconds under 5Pa to synthesize graphene on the foil. We did not use any carbon-contained gas such as CH4. Fig.1 (a) shows the Raman spectrum of synthesized graphene film. On the other hand, fig.1 (b) indicates Raman spectrum of graphene film synthesized by the plasma CVD using CH4 as the carbon source as a reference. In fig.1 (a) the D band is much smaller and the 2D band is sharper and stronger than in fig.1 (b). These results indicate that the crystalline quality of graphene was successfully improved by

using extremely low-concentration of carbon source. The synthesized graphene on Cu foil was transferred to the quartz or silicon substrate. After that, the van der Pauw devices for Hall Effect measurements were fabricated using conventional photolithography, metal deposition and lift-off processes. The mobility was estimated to be more than 1000 cm2/Vs, which was dramatically improved from the previous plasma CVD using CH4 as source gas. This work was partially supported by the Basic research and development of high quality graphene funded by NEDO. Masataka Hasegawa1,2, Ryuichi Kato2, Yuki

Okigawa1,2, Masatou Isihara1,2, Takatoshi Yamada1,2

1Nanotube Research Center, National Institute of Advance Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. 2Technology Research Association for Single

Wall Carbon Nanotubes (TASC), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan [email protected]

G


References [1] J. Kim, M. Ishihara, Y. Koga, K. Tsugawa,

M. Hasegawa, S. Iijima: Appl. Phys. Lett. 98 (2011) 091502.

[2] T. Yamada, M. Ishihara, J. Kim, M. Hasegawa, S. Iijima: Carbon 50 (2012) 2615.

Figures

Figure 1: Raman spectra of graphene synthesized by plasma treatment of copper foil by using (a) extremely low concentration of carbon source, and (b) using CH4 gas.


hy spain for innovative industries

ANA ELENA HERAS AZNAR ICEX España Exportación e Inversiones-Invest in Spain Spain

Spain is the 5th largest industrial economy in the EU with privileged market access to Europe, Latin-America, North Africa and Mediterranean countries. Modern Industrial sectors, qualified and competitive human resources, excellent scientific base and technological infrastructures and a favorable business climate, are key competitive factors driving foreign investment in innovative technologies and products into Spain. Spanish Industrial Innovation System is prepared to be at the forefront of new Nanotechnologies and Industrial production of Nano-materials (in particular Graphene) and Nano-products in Europe. Ana Elena Heras Aznar ICEX España Exportación e Inversiones-Invest in Spain Calle Orense, 58 28020 Madrid (Spain) [email protected]

W


avelength-Selective n- and p-typed carrier transport of a graphene transistor by organic/inorganic Hybrid Doping Platform

PO-HSUN HO National Taiwan University Taiwan

Recently, one promising approach to modulate the electrical properties of a graphene transistor using optical excitation has been demonstrated, resulting in controllable p- or n-doping processes by light illumination. However, most of these optically induced graphene-based electronic devices reported in literatures typically show a single-typed carrier transport under light illumination (either n-type or p-type). In this work, we propose a novel device structure based on organic/inorganic hybrid doping (OIHD) platform with a graphene transistor sandwiched with two photoactive layers with complementary absorption spectra, where one layer is only sensitized under visible light excitation and the other layer is only sensitized by UV illumination. When this graphene device is under illumination with selective wavelengths, controllable n-type or p-type doping of graphene with two opposite carriers can be achieved. The concept of this device structure based on OIHD platform enables us to control the dual carrier-typed transport of a graphene transistor simply by wavelength-selective illumination. Based on this novel device platform, we are further able to demonstrate the graphene p-n junction transistor controlled by wavelength-selective patterned illumination. This building block thus provides a great potential in the future development of large-area optically controlled graphene-based integrated circuits.

o-Hsun Ho, Yi-Ting Liou, Shao-Sian Li, Yun-Chieh

Yeh, Chun-Wei Chen Department of Materials Science and

Engineering, National Taiwan University, Taipei, Taiwan [email protected]

W


abrication, Properties, and Applications of Ultrahigh Performance Graphene

JAMES HONE Columbia University USA

Two-dimensional materials such as graphene can achieve spectacular performance but are highly sensitive to disorder from the environment. We have developed techniques to controllably ‘stack’ graphene on insulating hexagonal boron nitride, which dramatically reduces disorder and increases electronic mobility, [1] which in turn leads to superior performance in graphene FETs and emergence of new low-T physics. [2]. In addition, these heterostructures can display novel behavior due to the presence of ‘superlattice’ potentials arising from the graphene-BN stacking. [3] In recent work, we have extended these techniques to create fully encapsulated devices whose performance approaches the ideal behavior of graphene. [4] At room temperature, the electrical transport behavior is near the limit set by acoustic phonon scattering: mean free path is near one µm, correspoinding to mobility of 30,000-100,000 cm2/Vs, across a wide range in carrier density. At low temperature, fully ballistic transport is seen in devices as large as 15 µm in size, and phenomena such as magnetic focusing can be observed. BN-encapsulated bilayer graphene shows a well-developed fractional quantum Hall spectrum that can be tuned by an applied displacement field. These devices show high performance in a range of practical applications such as photonic devices and sensors. These techniques can be used to create heterostructures of other 2D materials such as MoS2 and WSe2, which also show improved performance.

J. Hone, L. Wang, Y. Gao, P. Maher, C.R. Dean,

P. Kim, K.L. Shepard Columbia University, New York 10027 USA

[email protected] References [1] Dean, C. R., Young, A. F., Meric, I., Lee,

C., Wang, L., Sorgenfrei, S., Watanabe, K., Taniguchi, T., Kim, P., Shepard, K. L. & Hone, J. Boron nitride substrates for high-quality graphene electronics. Nature Nanotechnology 5, 722-726, (2010).

[2] Dean, C., Young, A. F., Wang, L., Meric, I., Lee, G. H., Watanabe, K., Taniguchi, T., Shepard, K., Kim, P. & Hone, J. Graphene based heterostructures. Solid State Communications 152, 1275-1282, (2012).

[3] Dean, C. R., Wang, L., Maher, P., Forsythe, C., Ghahari, F., Gao, Y., Katoch, J., Ishigami, M., Moon, P., Koshino, M., Taniguchi, T., Watanabe, K., Shepard, K. L., Hone, J. & Kim, P. Hofstadter's butterfly and the fractal quantum Hall effect in moire superlattices. Nature 497, 598-602, (2013).

[4] Wang, L., Meric, I., Huang, P. Y., Gao, Q., Gao, Y., Tran, H., Taniguchi, T., Watanabe, K., Campos, L. M., Muller, D. A., Guo, J., Kim, P., Hone, J., Shepard, K. L. & Dean, C. R. One-Dimensional Electrical Contact to a Two-Dimensional Material. Science 342, 614-617, (2013).

F


Figures

Figure 1: BN-encapsulated graphene. Top, fabrication process flow. Bottom left, optical micrograph of large device. Bottom center, TEM cross-section showing edge contact. Bottom right, HRTEM cross-section showing clean interface between atomic planes of BN and graphene.

Figure 2: Transport properties of BN-encapsulated graphene. (a) Room-T resistivity and conductivity of 15-µm device. (b) Low-T conductivity vs. carrier density for 15 µm and 2 µm encapsulated devices, and device made by PMMA transfer.1 (c) Measured low-T mean free path as a function of device size.


raphene Research Institute at Sejong University

SUKLYUN HONG Graphene Research Institute Korea

Graphene Research Institute (GRI) at Sejong University is the first government-funded institute on graphene research in Korea. GRI was established in 2010 to take a lead in the research on the growth of high quality graphene and the development of novel graphene devices by promoting collaborative research among the experts in related fields. GRI has two main projects. One project is supported by Priority Research Centers Program of the National Research Foundation (NRF) of Korea government. The project title is “Graphene Nanostructures and Electronic Devices”. Eleven professors at Sejong participate in the project. The research fund is approximately a half million dollars per year from the government during 2010-2019, whereas matching fund from Sejong University is about one million dollars. The other project is supported by Nano•Material Technology Development Program funded by the Korean Ministry of Science, ICT and Future Planning, of which title is “Growth and defect control in graphene and development of innovative graphene-based electronic devices”. The research fund is about 1.5 million per year during 2012-2017. Fifteen professors from 9 institutes including Harvard University, Cornell University and Seoul National University are participating in the project. Graphene research activities at GRI include (i) graphene growth and synthesis using molecular beam epitaxy (MBE) and chemical vapor deposition (CVD), (ii) evaluation and process development for band gap engineering, transport property, and nanostructured lithography, and (iii) device fabrication and other applications for graphene-based electronic devices, high-frequency devices, biosensors, and

gas sensors. These research activities have been published in many high-profile professional journals. GRI hopes that it becomes a worldwide research center for PhDs and graduate students doing graphene research, provides skilled human resources to graphene industrial community, and hosts graphene-related meetings and conferences. In this presentation, we will mainly introduce graphene research activities at GRI and its accomplishments until now, as well as research projects operated by GRI. Suklyun Hong

Graphene Research Institute and Department of Physics, Sejong University, Seoul 143-747, Korea

[email protected] Figures

Opening ceremony of GRI after being selected as one of Priority Research Centers in 2010.

G


lectron cooling mechanisms in graphene

DAVID W. HORSELL University of Exeter UK

An important consideration in graphene-based devices is thermal cooling of charge carriers, which can be overheated by the current applied to operate them. This heat must be dissipated to avoid thermal breakdown [1]. The main cooling mechanisms are: direct transfer of heat to the metallic contacts forming the source and drain of the device via diffusion of electrons [2]; transfer of heat to the graphene lattice via scattering of electrons by acoustic phonons [3]; and transfer of heat directly to the underlying substrate via scattering of electrons by surface mode phonons of the substrate [4]. We probe the heat dissipation mechanisms in monolayer graphene devices supported on silicon dioxide [5] through measurements of the (differential) resistance, R, as a function of DC source-drain bias, V, and temperature, T. R(V) is shown to become increasingly non-linear as the temperature of the surrounding system is decreased from 300 K down to 4 K. By combining numerical modelling with measurements of R(T), we demonstrate that this non-linearity is caused by significant overheating of the electron gas above the lattice temperature. The form of the bias dependence is shown to contain information about the influence of the different dissipation mechanisms on electron cooling. D. W. Horsell, S. M. Hornett, A. S Price, A. V.

Shytov and E. Hendry School of Physics, University of Exeter, Exeter, EX4 4QL, UK

[email protected]

References [1] E. Pop, Nano. Res. 3 (2010) 147. [2] V. I. Kozub and A. M. Rudin, Phys. Rev. B

52 (1995) 7853. [3] A. A. Balandin et al. Nano Lett. 8 (2008)

902. [4] M. Freitag et al. Nano Lett. 9 (2009)

1883. [5] A. S. Price et al. Phys. Rev. B 85 (2012)

161411(R).

E


raphene in the European Union Future and Emerging Technologies Flagships Programme: Status and Future Plans

ANDREW HOUGHTON European Commission Belgium

The presentation reviews the current status of the EU Future and Emerging Technologies Flagships programme and outlines the model for the implementation of the Flagships in the H2020 programme, using Framework Partnership Agreements (FPAs). FET Flagships are large-scale long-term research initiatives which address challenges of major scientific, economic and societal importance. Their concept has been developed over the past four years within the EU's Future and Emerging Technologies (FET) research programme, which is part of the ICT Technologies programme. The FET Flagships reflect the fact that digital technologies are now embedded in all other areas of science. Two FET Flagships, "Graphene and Related 2D Materials" and the "Human Brain Project" were launched in October 2013, following a Call for Proposals within the FP7 research programme, and will follow a long-term (10-year) scientific road-map. In contrast to other large-scale EU research initiatives such as public-private partnerships (PPPs), the Flagships are science-driven from the outset and will build on novel scientific ideas and breakthroughs. Their continuation is implemented within the H2020 research programme, which started in January 2014 and will run until 2020. In the H2020 programme there is a stronger emphasis on innovation and on the economic and societal exploitation of research results than in previous research programmes,. This emphasis on innovation will apply equally to the Flagships, which will take novel ideas and scientific discoveries from proof-of-concept through to maturity and industrial exploitation. The implementation of the Flagships in H2020 will make use of a type of contract called a Framework Partnership Agreement (FPA). The FPA provides long-term continuity for the

duration of H2020 and provides a framework in which the each Flagship consortium agrees to follow a strategic research agenda. In each Flagship the research work of this core consortium will be funded in a sequence of projects, covering the seven years of H2020. The contracts of this sequence of core projects are in the form of Specific Grant Agreements (SGAs), for which the project consortium is the same as in the FPA. Andrew Houghton European Commission, Brussels

[email protected]

G


ole of disorder in temperature-dependent magneto-transport of epitaxial graphene near the Dirac point

JIAN HUANG University of Oxford UK

Many of the exceptional electronic properties of graphene arise from its linear dispersion relation. However, when the Fermi energy approaches the Dirac point, its properties can be dominated by the effects of disorder. To date, there are very few studies of disorder in epitaxial graphene grown on SiC due to the high level of doping from the substrate. Using extremely low carrier density (~ 1010 cm-2) polymer gated epitaxial graphene we describe the role of disorder in governing the temperature dependent magneto-transport. At such low carrier densities we observe a ν = 2 quantum Hall state beginning at just 0.6 T, with a maximum critical current at 1.2 T. Due to magnetic field dependent charge transfer from the SiC substrate [1,2], the ν = 2 quantum Hall state extends to B > 15 T (Fig. 1a), suggesting a substantial increase in carrier density. We have studied the effects of disorder in both the low and high field regimes at both low and high temperatures. The carrier density derived from the low-field Hall coefficients for a two-carrier system shows a quadratic increase as a function of temperature (Fig. 2a), which can be well modelled by intrinsic excitation combined with disorder-induced electron-hole puddles [3]. The characteristic strength of the potential variation is found to be about 12 meV. In the quantum Hall regime we fit the temperature dependence of the longitudinal conductivity with an activated transport model at temperatures which are above the variable-range hopping regime (> 20 K). We assume the extended states make up a constant fraction of each Landau level, and are evenly distributed over a width of 2Eµ, giving a thermally activated conductivity

∝ ∙ e| |

∙ (e

− e

), (1) where En and EF are the nth Landau level and the Fermi level respectively. Eµ is found to be 12.3 meV, in good agreement with the standard deviation of the potential fluctuations discussed above, suggesting that the majority of states in the zeroth Landau level are extended. The energy difference between the Fermi level and the mobility edge (Fig 1b) tends towards zero at high magnetic fields, consistent with the increasing Rxx and deviation of the Hall resistance from the quantized value as shown in the inset of Fig. 1a. These results are important for determining the properties of epitaxial graphene at the low carrier densities required in quantum electrical resistance standards operating at low magnetic fields [1]. J. Huang1, J. A. Alexander-Webber1, T. J. B. M.

Janssen2, A. Tzalenchuk2,3, V. Antonov3, T. Yager4, S. Lara-Avila4, S. Kubatkin4, R. Yakimova5, and R. J. Nicholas1,* 1Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1

3PU, United Kingdom 2National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom 3Department of Physics, Royal Holloway, University of London, Egham TW20 0EX, United Kingdom 4Department of Microtechnology and Nanoscience, Chalmers University of Technology, S-412 96 Göteborg, Sweden 5Department of Physics, Chemistry and Biology (IFM), Linköping University, S-581 83 Linköping, Sweden

*[email protected]

R


References [1] J. A. Alexander-Webber, et al., Phys.

Rev. Lett. 111, 096601 (2013). [2] T. J. B. M. Janssen, et al., Phys. Rev. B 83,

233402 (2011). [3] Q. Li, E. H. Hwang, and S. Das Sarma,

Phys. Rev. B 84, 115442 (2011).

Figures

Figure 1: (a) Longitudinal resistance Rxx (blue) and Hall resistance Rxy (red) as a function of magnetic field up to 19 T at 1.4 K showing the v = 2 quantum Hall state. The inset shows the increasing Rxx and deviation from the quantization ∆Rxy = Rxy - h/2e2 for B > 15 T. (b) Energy difference between the Fermi level EF and the mobility edge Eµ = 12.3 meV.

Figure 2: (a) Temperature-dependent low-field effective carrier density. The solid line is a fit to the model from Ref. 3, yielding potential fluctuation strength of about 12 meV. (b) Temperature dependence of the longitudinal resistance at high magnetic fields. Also shown are the fits using Eqn. (1).


erformance assessment of graphene-based lateral and vertical heterostructure FETs

GIUSEPPE IANNACCONE Universita’ di Pisa Italy

Native graphene has a zero energy gap and it is therefore not suitable as a transistor channel material for digital electronics [1]. However, recent advances based on materials engineering have demonstrated graphene-based “materials on demand”, with tailored properties [2,3]. Vertical graphene heterostructures have been proven to be suitable for FETs [4,5] and hot-electron transistors [6] exhibiting large current modulation [7]. Inspired by recent progress in the growth of seamless lateral graphene heterostructures [8-10], graphene-based lateral heterostructure (LH)-FETs have been proposed [11-13], exhibiting extremely promising switching behavior in terms of leakage current, propagation delay, and power-delay product. Despite the interest and the huge expectations of the research community shown towards these new devices, a comparative analysis of the performance against ITRS requirements for next-generation devices has never been done so far. In this work, we explore the performance potential of state-of-the-art graphene based devices, which have already been demonstrated to provide large Ion/Ioff ratios. In particular, we will focus on the lateral heterostructure FET (LH-FET), and two FETs based on vertical graphene-based heterostructures: one proposed by Britnell et al. [4], that we call VH-FET, and the “barristor” proposed in [14]. The three device structures are shown in Figs. 1a-c. We do not consider transistors dedicated to analog RF applications [6]. Vertical Heterostructure FET Performance - We consider the VH-FET shown in Fig. 1b, experimentally demonstrated in [4-5] and also analyzed in [7]. The barrier consists of three atomic layers of boron-carbon-nitride, AB-

stacked on graphene. Fig. 2a shows the pFET transfer characteristics for different valence band edge barriers BV. Performance figures shown are poor: the Ion/Ioff ratio is always smaller than 20 and the delay time is four orders of magnitude larger than that expected from Internation Technology Roadmap for Semiconductors (ITRS) [15]. Barristor Performance - The device structure is shown in Fig. 1c. Silicon has a donor doping ND and the gate dielectric has effective oxide thickness EOT. The gate voltage modulates the Schottky barrier between graphene and silicon exploiting the partial graphene screening properties. Fig. 3 shows that performance is again much worse as compared to ITRS requirements CMOS transistors, i.e. poor Ion/Ioff, larger Power Delay Product (PDP) and intrinsic delay time (). LH-FET Optimization and performance – Finally, we consider the double-gate p-channel LH-FET illustrated in Fig. 1a, BC2N is lattice-matched to graphene and has a bandgap of 1.6 eV, offering a barrier to holes from graphene of 0.64 eV. As can be seen in Fig. 4a, the transfer characteristics are almost independent of tB, and all performance parameters are optimized when tB = L (Figs. 4b). As can be seen, in this case, Ion/Ioff is very large (104) and complying with ITRS requirements, outperforming VH-FET and the barristor device. Conclusion - Finally, we compare in Fig. 5 the requirements of ITRS 2012 for high performance logic CMOS [12] with graphene-based heterostructure FETs. As can bee seen, LH-FETs exhibit lowe intrinsic delay time and lower PDP than CMOS for the same gate length (considering 10 nm for 2020, and 7 nm for 2024). VH-FET and barristor cannot be included in the comparison since they exhibit larger delay times by at least four orders of magnitude. We are aware that our simulations do not consider the impact of dissipation in the channel, that typically reduces Ion by a factor of two. Even taking

P


this aspect into account, graphene-based lateral heterostructure FETs stand out as the most promising graphene-based transistors for digital electronics. G. Fiori, D. Logoteta, G. Iannaccone Dipartimento Ingegneria dell’Informazione,

Universita’ di Pisa, Via Caruso 16, 56126 Pisa, Italy

[email protected] References [1] G. Iannaccone et al., IEDM Tech. Dig.

2009 [2] F. Bonaccorso et al, Materials Today Vol. 15, pp. 564, 2012.

[2] F. Bonaccorso et al, Materials Today Vol. 15, pp. 564, 2012

[3] K.S. Novoselov, A. H. Castro Neto, Phys. Scr. T146, 014006, 2012.L. Br

[4] itnell et al., Science, Vol. 335 (2012), 947. [5] T. Georgiou, et al, Nature Nanotech.

Vol. 8, 100, 2013. [6] S. Vaziri et al. Nano Lett. Vol. 13, pp.

1435-1439, 2012. [7] G. Fiori et al. , IEEE-TED Vol. 60, n. 1, pp.

268-2703, 2013. [8] L. Ci, et al, Nat. Materials, 9 (2010), 430. [9] M. P. Levendorf et al. Nature 488, 627

(2012). [10] Z. Liu, et al. Nature Nanotechnology, 8,

119 (2013). [11] G. Iannaccone, G. Fiori, Patent WO

2013080237. [12] G. Fiori et al., IEDM Tech. Dig. 2011. [13] G. Fiori et al. ACS Nano 6, 2642-2648

(2012). [14] H. Yang, Science Vol. 336, pp. 1140-

1143 (2012). [15] ITRS, available at http://itrs.net.

Figures

Figure 1: Device structure of a) LH-FET; b) VH-FET; c) Barristor.

Figure 2: transfer characteristics of the VH-FET of Fig. 1b, for different values of the barrier BV at the valence band edge. The device has EOT = 0.62 nm (4nm HfO2), barrier thickness of three atomic layers.


Figure 3: a) [b)] Ion/Ioff, c) [d)] τ, and e) [f)] PDP of the barristor as a function of ND for EOT = 0.61 nm [EOT for ND = 1022 m-3].

Figure 4: a) Transfer characteristics of the LH-FET for different values of tB. b) Ion/Ioff ratio as a function of tb. The device has the structure showninFig.1a, withL=10nm,tox =1nm,Vdd =0.6V,f=0.01.

Figure 5: a) Intrinsic delay time τ and b) PDP as a function of year of shipment according to the ITRS 2012. On the same plot: comparison with simulation results for the LH-FETs with metal gate length of 10 nm (year 2020) and 7 nm (year 2024).


pin-orbit coupling in dilute fluorinated graphene

SUSANNE IRMER University of Regensburg Germany

Graphene and its various functionalized derivatives combine remarkable electronic and mechanical properties that make them feasible for novel electronic applications. From the point of view of spintronics it is crucial to understand spin-dependent phenomena that can be tuned and harvested from the newly engineered graphene-based materials. One of such materials is fluorinated graphene. In the talk we present detailed ab-initio and tight-binding model studies focusing on spin-orbit coupling (SOC) effects in the fluorinated graphene. We discuss a new SOC mechanism which appears due to structural sp3 distortion caused by chemisorption of fluorine and provide realistic values of induced SOC strengths. The underlying tight-binding model is derived from symmetry considerations and its parameters are fixed by fitting to the ab-initio computed electronic band structure. The first-principles calculations were performed by means of full potential density functional theory (DFT) code [1]. We tested various supercell configurations, going from 5 x 5 supercell (fluorine-carbon ratio of 1 : 50) to 10 x 10 (fluorine-carbon ratio of 1 : 200). Our orbital and spin-orbital tight-binding models seem to be very robust. Their comparison with the first-principles results for the 10 x 10 supercell are shown in Fig. 1. Figure 1(a) displays the orbital band structure and Fig. 1(b) the corresponding SOC splittings of valence, impurity and conduction band with respect to the Fermi level. These splittings are of the order of 100 _eV. Based on the symmetry arguments we identify the main mechanism capable to explain SOC in the vicinity of fluorine. Moreover, testing our assumptions, we have turned off the SOC interaction on the fluorine atom in the DFT-calculation reaching typical graphene values,

(order of 10 µeV, see [2]), for more details consult Fig. 1(b). This reflects the fingerprint the SOC splitting of fluorine's atomic p orbitals leave in the SOC mechanisms and justifies our concentration on the impurity region. The dominant contribution to SOC comes from the Bychkov-Rashba interaction and the new contribution which we called PIA SOC, see [3]. A similar Hamiltonian in the hydrogenated case was recently used in [3] to describe dilute hydrogenated graphene. We stress that the introduced SOC parameters are of the order of 10 meV and are thus remarkably larger than pristine graphene's intrinsic spin-orbit coupling of about 10-2 meV [2]. We conclude that the fluorine adatoms in small concentration induce a large SOC in graphene being about 1000 times larger than the intrinsic contribution of pristine graphene. This offers an inspiring initial point for further studies of SOC in dilute fluorinated graphene. This work has been supported by the DFG SFB 689, GRK 1570, SPP1285, and the European Union Seventh Framework Programme under grant agreement 604391 Graphene Flagship. Susanne Irmer, Tobias Frank, Denis Kochan,

Martin Gmitra, Jaroslav Fabian

Institute for Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany [email protected]

S


References [1] FLEUR. http://www.flapw.de/. [2] S. Konschuh, M. Gmitra and J. Fabian,

Phys. Rev. B, 82 (2010) 245412. [3] M. Gmitra, D. Kochan and J. Fabian,

Phys. Rev. Lett., 110 (2013) 246602.

Figures

Figure 1: Band structure and spin-orbit coupling splitting of a 10 x10 supercell calculation of fluorinated graphene. (a) Band structure around the Fermi level: The ab-initio data (black dotted lines) is fitted well by our tight-binding model (solid blue lines) around the Fermi level. (b) Band-splittings of corresponding valence, impurity and conduction band (bottom to top): Our tight-binding model (solid blue line) concentrating on the SOC in the vicinity of fluorine recovers the first-principles result (black dotted). Turning off the SOC on the fluorine adatom in the DFT calculation significantly reduces the splitting from about 100 µeV (dotted) to 10 µeV (dashed), which is of the order of the SOC induced splitting in pristine graphene.


NG – Center for Nanostructured Graphene

ANTTI-PEKKA JAUHO DTU Nanotech Denmark

CNG – Center for Nanostructured Graphene – is funded by the Danish National Research Foundation with a 54 MDKr (7.2 M€) grant, starting in February 2012, and running initially for six years with a possibility for a four year extension, pending a successful evaluation. The main stake-holder in CNG is DTU Nanotech, and additional partners include two other departments from DTU campus, DTU Fotonik and DTU Fysik, the center for electron nanoscopy DTU Cen, and the Physics Department from Aalborg University. DTU Danchip, the state-of-the-art clean room facility on DTU Campus is an important component in CNG’s experimental research. In addition, many other researchers on the DTU campus are independently financed stake-holders in CNG’s research program, so that all in all more than sixty persons contribute towards CNG’s goals. The graphene research program at DTU Nanotech also receives important support from various EU instruments, including the Graphene Flagship. CNG focuses on basic research, but all its research projects have long-time perspectives with the aim of applications. Its research profile has a broad range: it involves polymer chemists, nanofabrication specialists, experimental physicists, and condensed matter theorists using a wide palette of analytical and numerical tools, including large scale simulations of nanodevices, ab initio electronic structure calculations, and theory of quantum transport. The key word in CNG’s research program is “control”: we want to achieve an increased control of the electrical, thermal, and optical properties of graphene by using additional, carefully designed nanoscale features to the pristine material.

Antti-Pekka Jauho

CNG, DTU Nanotech, Technical University of Denmark, Building 345 East, Oersteds Plads, Kongens Lyngby, DK 2800, Denmark

[email protected]

C


ransport phenomena in nanostructured graphene

ANTTI-PEKKA JAUHO DTU Nanotech Denmark

Graphene samples in the lab are not ideal: disorder, defects, or deliberate nanostructuring is always present. Here we describe three recent developments in modeling of such systems. STM is a widely used tool to analyze mesoscopic samples: it yields direct information about the density of states. We have recently analyzed a dual-probe STM setup [1]; here current is injected from one probe, and collected by another, adjacent probe. Present technology allows probe separations below one hundred nanometers – well below dephasing lengths at low temperatures – and thus the conductance between such probes displays quantum interference phenomena. The signal, being a transport quantity, contains more information than the conventional STM measurement which reflects local properties. For example, probes separated along armchair or zigzag directions on pristine graphene yield very different results because of the underlying anisotropy of the graphene lattice. Calculations have been performed for a large number of nanostructured graphene samples: single or several defects, adsorbents, or vacancies as well as samples with edges. We envisage experiments where one probe is kept fixed while the other scans along the sample surface, and examples of such conductance maps as well as their Fourier transforms (which yield information about intra- and intervalley scattering) will be analyzed during the talk. We have also considered [2] a setup where the separation of the two probes is kept fixed but the voltage is varied; this spectroscopic mode is also shown to be very versatile.

A paradigm for creating a gap in graphene is to fabricate periodic perforations of the graphene sheet – graphene antidot lattices (GAL) [3]. The band gaps can be used as potential barriers so as to guide charge carriers [4], much in analog with photonic crystals. Here, we report on simulations of charge transport for two coupled GAL waveguides in Coulomb drag geometry [5]. The lateral geometry of our suggested device offers many technological advantages as compared to conventional stacking (where the two graphene layers are separated by a thin isolating material). We make several predictions for the temperature dependence of the measured drag signal: this displays a complex behavior due to an interplay between the available phase-space for scattering, and the screened Coulomb interaction. Finally, we assess the role of disorder in GALs as a limiting factor for carrier confinement. Our previous work used a Kubo-formula approach to consider large samples [6], here we investigate mesoscopic devices with a Green’s function formalism: transistors and GAL waveguides [7]. We have performed extensive numerical work and show that the barriers predicted for perfect GALS may in fact become leaky for disordered GALs, depending of the character of the disorder. Also, we show that the edge character of the antidots (armchair, zigzag, disordered) plays an important role in the transmission properties of GAL wave guides. We believe that our numerical work will give important guidelines for the optimization of devices based on GAL waveguides. Acknowledgement: CNG is supported by the Danish National Research Foundation, Project No DNRF58.

T


A. P. Jauho, M. Settnes, S. R Power, A. A. Shylau,

and D. H. Petersen

Center for Nanostructured Graphene CNG, Department for Micro and Nanotechnology, DTU Nanotech, Technical University of Denmark, Ørsteds Plads, Bldg 345E, 2800 Kongens Lyngby, Denmark

[email protected] References [1] M. Settnes, S. R. Power, D. H. Petersen,

and A. P. Jauho, Phys. Rev. Lett. (in press); arXiv:1401.8156.

[2] M. Settnes, S. R. Power, D. H. Petersen, and A. P. Jauho, in preparation.

[3] T. G. Pedersen, C. Flindt, J. Pedersen, N. A. Mortensen, A. P. Jauho, and K Pedersen, Phys. Rev. Lett. 100, 136804 (2008).

[4] J. G. Pedersen, T. Gunst, T. Markussen, and T. G. Pedersen, Phys. Rev. B 86, 245410 (2012).

[5] A. A. Shylau and A. P. Jauho, submitted to Phys. Rev. Lett. (February 2014).

[6] S. Yuan, R. Roldan, A. P. Jauho, and M. I. Katsnelson, Phys. Rev. B 87, 085430 (2013).

[7] S. R. Power and A. P. Jauho, in preparation.

Figures

Figure 1: Dual probe STM setup on pristine graphene. The conductance is constant along armchair probe separations, and oscillatory along zigzag separations.

Figure 2: Temperature dependence of drag resistance. Shown are results for unscreened case (“bare”), and RPA screened Coulomb interaction. Inset shows a drag enhancement which can occur at low temperatures for certain parameters.

Figure 3: An example of a ”leaky” GAL barrier (current path shown as a color map). Disorder (varying antidot size) opens pinholes through a GAL structure which would be impenetrable for antidots of equal size.


lectron Transport in Graphene based 2D Crystals for Novel Electronic Devices

DEBDEEP JENA University of Notre Dame USA

Reproducing electronic device functionalities that have been successfully realized, and gainfully employed for many decades in traditional semiconductor devices made of Silicon, or III-V heterostructures is a difficult long-term survival strategy for 2D crystal material electronic device applications. They sure are the most sensitive tests of the physics of transport, electrostatic control, and electronic properties of the materials. However, for realistic long-term applications, 2D crystals must transcend the traditional device paradigm and exploit what is truly unique in them. Novel features of the electron transport properties and electrostatic control will be the driver for realistic electronic device applications of graphene and related 2D crystal semiconductors. An immediate, and potential opening’ for realistic application for logic devices is in ultra low-power tunneling transistors [1]. For tunneling transistors, Graphene and the transition metal dichalcogenides (TMDs) such as MoS2, WS2, etc due to their atomically thin nature have the potential to achieve what is considered very difficult with traditional 3D semiconductors [2]. The physics, and material challenges of 2D crystals for in-plane tunneling transistors [3], and out-of-plane interlayer tunneling transistors such as the proposed SymFET [4] and the THIN-TFET [5] will be discussed. Recent experimental progress in the realization of the proposed devices, such as the SymFET [6], and graphene nanoribbon TFETs [7] will be presented, and their performance compared to the predicted and desired characteristics. There is danger in clubbing too much under the umbrella of already understood’, or ‘simple’ physics. Case in point: the low-field

transport properties of TMD semiconductors are poorly understood. Because of the potential for low dangling bonds, they are considered attractive for scaled traditional transistors as well [8], as has been experimentally demonstrated [9]. However, carrier mobilities in these crystals suffer from very strong coupling to surroundings, which simultaneously are a challenge, and an opportunity for boosting the performance [10]. Mobilities far exceeding current numbers are predicted in the near future, based on the analysis [11]. Finally, the contribution of d-orbitals to the electronic band-edge states are predicted to be key to exploiting correlated effects in these semiconductors, seeding ideas for devices that transcend the current state of the ar. Debdeep Jena University of Notre Dame, Notre Dame, IN, USA [email protected]

E


References [1] A. Seabaugh, Q. Zhang, “Low-voltage

tunnel transistors for beyond CMOS logic”, Proc. of the IEEE, v.98, p.2095, 2010.

[2] D. Jena, “Tunneling Transistors Based on Graphene and 2D Crystals”, Proc. of the IEEE, v.101, p.1585, 2013.

[3] N. Ma, D. Jena, “Interband tunneling in 2D crystal semiconductors,” Appl. Phys. Lett., v.102, p.132102, 2013.

[4] P. Zhao at al. “SymFET: A proposed symmetric graphene tunneling field-effect transistor”, IEEE. Trans. Electron. Dev. v.60, p.951, 2013.

[5] M. Li et al., “Single particle transport in 2D heterojunction interlayer TFET (THIN-TFET)”, J. Appl. Phys. v.115, p.074508, 2014.

[6] L. Britnell at al. “Resonant tunneling and negative differential conductance in graphene transistors”, Nature Comm. v.4, p.1794, 2013. .

[7] W. S. Hwang et al., in preparation. [8] D. Jimenez, “Drift-diffusion model for

single layer TMD FETs”, Appl. Phys. Lett, v.101, p.243501, 2012.

[9] S, Kim et al., “High-mobility, low-power thin-film transistors based on multilayer MoS2 crystals,” Nature Communications, v.3, p.1011, 2012.

[10] D. Jena and A. Konar, “”Enhancement of carrier mobility in semiconductor nanostructures by dielectric engineering”, Phys. Rev. Lett., v.98, p.136805, 2007.

[11] N. Ma, D. Jena, “Charge Scattering and Mobility in Atomically Thin Semiconductors”, Phys. Rev. X, v.4, p.011043, 2014.


anographene: wires, analogue adder and molecule logic gates

CHRISTIAN JOACHIM CEMES CNRS France

LT-UHV-STM single atom manipulations, sub-monolayer UHV molecule sublimation and on-surface synthesis [1] are now used to explore the down limits of electronic devices at the atomic scale. The on-surface synthesis of perfect 1 nm in width graphene nanorubans had permitted to explore very long range and low voltage tunnel transport through a single molecule [2]. STM single molecule dI/dV images and manipulations of coronene, hexabenzocoronene and larger nanographene like molecules have been used to evaluate how many classical inputs can be converted in quantum information by the electronic quantum system of a single molecule [3, 4]. Starphene like molecules were shown to perform a NOR Boolean logic function using single metal atom logical inputs [5]. This open the way to design and construct atom by atom more complex Boolean logic function in large nanographene like molecules and more generally at the surface of 2D like MoS2 [6] or pseudo 2D materials like Si(100)H [7]. C. Joachim GNS & MANA Satellite CEMES CNRS Toulouse

(France) A*STAR VIP Atom Tech (Singapore) [email protected]

References [1] L. Grill et al., Nature Nano., 2, 687 (2007) [2] M. Koch, F. Ample, C. Joachim, L. Grill,

Nature Nano., 7, 713 (2012). [3] C.Manzano et al., Chem. Phys. Lett, 587,

35 (2013). [4] W. H. Soe et al., ACS Nano, 6, 3230 (2012). [5] W.H. Soe et al., ACS Nano,5 ,1436

(2011). [6] N. Kodama et al., J. J. Appl. Phys., 49,

08LB01 (2010). [7] M. Kolmer et al., Appl. Surf. Sci., 288,

83 (2014).

N


ransport properties and quantum Hall effect of graphene films grown by CVD on SiC(0001) with in-situ hydrogenation

BENOIT JOUAULT CNRS France

One of the most promising techniques to produce graphene at the industrial level is silicon sublimation of a silicon carbide (SiC) wafer. In this way, homogeneous epitaxial graphene layers have been obtained on the silicon face of SiC substrates, but the graphene resides on top of the so-called buffer layer or zero layer graphene (ZLG). The ZLG is a reconstructed graphene-like honeycomb lattice with covalent bonds between some of its carbon atoms and the underlying silicon ones. The presence of the ZLG leads to low mobility and high electron concentration in the upper monolayer graphene (MLG). To overcome this problem, various approaches have been developed to dissociate the graphene film from the substrate. The most successful one is post-growth hydrogen intercalation, which drastically decreases the carrier concentration and increases the mobility. Because of the reduced interaction with the substrate, MLG on H-passivated SiC surface has been called "quasi-free standing monolayer graphene" (QFMLG).

As an alternative to SiC sublimation, it has also been shown that graphene can be grown on SiC from an external carbon source. Demonstration of the direct growth of graphene on (0001)SiC has been done by chemical vapor deposition (CVD) with argon as carrier gas (Ar-CVD). In spite of the different carbon supply mechanisms, graphene films obtained by Ar-CVD appear similar to those obtained by the sublimation method. Particularly, both processes lead to the formation of a ZLG on the Si-face and to a strong doping. However, graphene can also be grown by CVD on SiC using hydrogen/argon mixtures as carrier gas. In this case, the presence of

hydrogen during the growth has a strong influence on the carbon supply mechanism and the graphene properties. We will present a detailed study of graphene films grown by these CVD methods. The results of angle resolved photoemission spectroscopy (ARPES) measurements, before and after UHV annealing, validate clearly the in-situ hydrogenation of the graphene/SiC interface during the growth. We show that H/Ar-CVD allows to grow either standard epitaxial graphene or quasi-free standing graphene, depending closely on the growth conditions. The key parameters are the growth temperature and, of course, the admixture of argon and hydrogen. Remarkably, a small elevation of the growth temperature allows changing the nature of the graphene film from QFMLG to MLG on a 6R3 interface.

A large part of this presentation will be focused on transport measurements. In our samples, the carrier concentration can be, to some extent, modulated. MLG on ZLG are n-doped, while QFMLG are p-doped. QFMLG have a higher mobility, and the temperature dependence of their electrical resistance is dominated by the graphene acoustical phonons (Fig. 1c). Normally, substrate phonons perturb the graphene magnetoresistance (MR). This is not the case here for p-doped samples, and at moderate magnetic fields, MR reveals a rich physics, including weak-localization (Fig. 1c,1d) and electron-electron interactions. At high magnetic fields and low temperatures, the QHE is observed, both for MLG on ZLG and QFMLG samples. In some case, the QHE is observed with sample sizes of the order of the centimeter. This is a first indication of the good homogeneity of the graphene, which is also confirmed by Raman spectroscopy. Finally, the temperature dependence of the quantum Hall plateaus,

T


as well as the transitions between the plateaus, will be presented and studied. The influence of the disorder and more particularly the influence of the SiC steps (Fig. 1a) on the transport properties will be discussed. B. Jouault1 and A. Michon2 1 Laboratoire Charles Coulomb, CNRS-Université de Montpellier II, Pl. Eugène

Bataillon, 34095 Montpellier Cedex, France 2 CNRS-CRHEA, rue Bernard Grégory, 06560 Valbonne, France [email protected]

References [1] B. Jabakhanji, A. Michon, C. Consejo, W.

Desrat, M. Portail, A. Tiberj, M. Paillet, A. Zahab, F. Cheynis, F. Lafont, F. Schopfer, W. Poirier, F. Bertran, P. Le Fevre, A. Taleb-Ibrahimi, D. Kazazis, B. C. Camargo, Y. Kopelevich, J. Camassel and B. Jouault, accepted in Phys. Rev. B, 2014.

Figures

Figure 1: (a) AFM image of the hydrogenated sample surface; (b) The measurements are performed with these Hall bars of width 100 µm and length 500 µm. (c) Temperature dependence of the resistivity, from T=1.7 K up to 290 K. The insets show the MR induced by weak localization at T=1.7 K, 40 K and 250 K. (b) Coherence length as a function of temperature, extracted from fits of the weak localization peak. (c) MR at low temperature (T= 2.3 K) up to B= 58 T, revealing the half-integer quantum Hall effect.


n the Electronic and Optical Properties of Fluorographene, Chlorographene, and Graphane

FRANTISEK KARLICKY Palacky University Czech Republic

New two-dimensional materials derived from graphene by attachment of hydrogen and halogens have attracted considerable interest over the past few years because of their potential applications (e.g., in electronic devices). [1] Here, we consider presence of point defects and the effect of electron-electron and electron-hole correlation on the electronic/optical properties of materials under study. Especially, large difference between the experimental optical gap and the electronic band gap from many-body GW theory for fluorographene [2,3] was explained by unusual large binding energies of excitons, whereas point defects lowered band gaps and absorption energies only slightly. [4] Similar effects are predicted for chlorographene, which stability is, however, still questionable. [5]. Frantisek Karlicky and Michal Otyepka

Regional Centre of Advanced Technologies and Materials, Department of Physical

Chemistry, Faculty of Science, Palacky University, 17. listopadu 12, 771 46 Olomouc, Czech Republic [email protected]

References [1] Karlicky F., Datta KKR., Otyepka M., Zboril

R., ACS Nano 7 (2013) 6434. [2] Nair RR., Ren WC., Jalil R., Riaz I., Kravets

VG., Britnell L., Blake P., Schedin F., Mayorov AS.,Yuan SJ., Katsnelson MI., Cheng HM., Strupinski W., Bulusheva LG., Okotrub AV., Grigorieva IV.,Grigorenko AN., Novoselov KS., Geim AK., Small 6 (2010) 2877.

[3] Zboril R., Karlicky F., Bourlinos AB., Steriotis TA., Stubos AK., Georgakilas V., Safarova K., Jancik D., Trapalis C., Otyepka M., Small 6 (2010) 2885.

[4] Karlicky F., Otyepka M., J. Chem. Theory Comput. 9 (2013) 4155.

[5] Karlicky F., Zboril R., Otyepka M., J. Chem. Phys. 137 (2012) 034709.

Figures

O


rospects of Beyond-Graphene Materials and Devices

ANUPAMA B. KAUL National Science Foundation USA

Carbon is truly a remarkable material, for not only sustaining life on earth, but for the promising materials properties it encompasses that emerge from its diverse and rich physical structures. Carbon based nanomaterials, such as graphene and carbon nanotubes, have been proposed for a wide variety of applications including high-frequency transistors, [1,2] interconnects,[3] stretchable electronics,[4] photo-voltaics,[5,6] and plasmonics,[7]. In particular, although graphene has been shown to exhibit remarkable electronic, thermal, mechanical and optical properties, the absence of a band-gap poses concerns for its attractiveness in some applications, particularly in digital electronics where high ON/OFF ratios are desired. While a band-gap is induced in graphene through quantum confinement by creating graphene nanoribbons, the band gaps nonetheless are small (few hundred meV) and it is challenging to maintain pristine edge chirality due to defects that are induced during nanofabrication of the ribbons. Recently, layered 2D crystals of other materials similar to graphene have been realized which include insulating hexagonal-BN (band gap ~5.5 eV) and transition metal di-chalcogenides which display properties ranging from superconducting, semiconducting, metallic to insulating. The device applications of such systems show promising characteristics where MoS2 transistors have been formed on flexible and transparent substrates, and transistors derived from 2D monolayers of MoS2 show ON/OFF ratios many orders of magnitude larger than the best graphene transistors. In this talk, I will provide an overview of the Electronics, Photonics and Magnetic

Devices (EPMD) program in the ECCS division where graphene, as well as other layered 2D nanomaterials, are playing an important role for enabling innovative device applications in electronics, photonics and sensing. I will also provide an overview of the NSF 2-DARE initiative under the Emerging Frontiers in Research and Innovation (EFRI) program. Anupama B. Kaul ECCS Division, Engineering Directorate, National Science Foundation, Arlington, VA 22203 [email protected]

P


References [1] Y. M. Lin, C. Dimitrakopoulos, K. A. Jenkins,

D. B. Farmer, H. Y. Chiu, A. Grill, P. Avouris: 100 GHz transistors from wafer-scale epitaxial graphene. Science 327(5966), 662 (2010).

[2] Y. Q. Wu, K. A. Jenkins, A Valdes-Garcia, D. B. Farmer, Y. Zhu, A. A. Bol, C. Dimitrakopoulos, W. J. Zhu, F. N. Xia, P. Avouris, and Y. M. Lin: State-of-the-art graphene high-frequency electronics. Nano Lett. 12(6), 3062 (2012).

[3] Y. Khatami, H. Li, C. Xu, K. Banerjee: Metal-to-multilayer-graphene contact – Part 1: Contact resistance modeling. IEEE Transactions on Electron Devices 59(9), 2444 (2012).

[4] K. S. Kim, Y. Zhao, H. Jang. S. Y. Lee, J. M. Kim, K. S. Kim, J. H. Ahn, P. Kim, J. Y. Choi, and B. H. Hong: Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 457(7230), 706 (2009).

[5] X. Miao, S. Tongay, M. K. Petterson, K. Berke, A. G. Rinzler, B. R. Appleton, and A. F. Hebard: High efficiency graphene solar cells by chemical doping. Nano Lett. 12(6), 2745 (2012).

[6] X. Dang, H. Yi, M. Ham, J. Qi, D. Yun, R. Ladewski, M. S. Strano, P. T. Hammond, and A. M. Belcher: Virus-templated self-assembled single-walled carbon nanotubes for highly efficient electron collection in photovoltaic devices. Nature Nano. 6(6), 377 (2011)

[7] A. Vakil and N. Engheta: Transformation optics using graphene. Science 332(6035), 1291 (2011).


ntroduction of National Program on Nano Technology (NPNT)

MORRIS (MING-DOU) KER National Program on Nano Technology Taiwan

National Program on Nano Technology (NPNT) launched in 2003 is a landmark project in Taiwan to promote the scientific and technological development of the country’s academic and industrial sectors. Through the effort made by the NPNT, the promotion of industrial investments and economic competitiveness will be fostered by a continuous industry revolution in terms of resource development and realization of academic research results in practical products. The ultimate goal of the commercialization of nanotechnology can be accomplished in Taiwan. To enhance the application of nanotechnology in Taiwan’s industries and increase investment and output values, NPNT phase ii (2009–2014) is devoted to promoting commercialization in seven main areas: (1) advance studies on nano science & nano technology, (2) biomedical & agricultural applications, (3) nanoelectronics and optoelectronics techniques, (4) energy and environmental applications, (5) nanomaterials and traditional industry, (6) instrument and equipment development, and (7) government policy and education projects. Moreover, significant projects of government agencies and departments are continuously promoted, including projects on environment, health, and safety (EHS) crisis control and projects on nano education and training, nano scales and standardization, nanomark, academia–industry, and international collaborations. To accomplish the program’s mission, the NPNT interfaces and coordinates efforts from various government agencies, including the National Science Council, the ministry of economic affairs, the ministry of

education, the atomic energy Council, the environmental Protection administration, the Department of health, and the Council of labor affairs, to enhance interdisciplinary research and technology development by the execution of the funded projects. The total budget in phase ii (2009–2014) is US$595.6 million, allocated for the industrialization of nanotechnology (~68%), the advancement of fundamental research (~22%), and the execution of strategic projects (~10%), such as EHS and nanomark promotions. In addition, nanomark, established by the Nanotechnology industrialization Promotion Project steered by the industrial Development Bureau, has brought up the development of nano-related industries to create a quality image of nano products. Hence, nanomark can increase the competitive strength in both domestic and international markets. Morris (Ming-Dou) Ker National Program on Nano Technology, Taiwan National Chiao Tung University Room701 CPT Building, No.1001 University Rd. Hsinchu 30010, Taiwan, R.O.C.

[email protected]

I


Figures

Figure 1: The logo of NPNT.

Figure 2: The operation structure for the NPNT network.

Figure 3: The pie chart for the budget allocation in NPNT Phase II (2009 ~ 2014).


raphene Flagship

JARI KINARET Chalmers University of Technology Sweden

The Graphene Flagship is a ten year research program funded by the European Commission, EU member states and project participants. It originates from the science of graphene and related layered materials and targets a disruptive technology shift, bringing these material from academic laboratories to society as new products, employment opportunities and economic growth. Realizing this ambition is only possible by integrating the entire value chain from basic research to applied and industrial research, which requires a large consortium with corresponding resources – the total project cost is about one billion euros. In this presentation I will cover the origins, motivations and plans for the flagship. I will describe the research program and plans to develop the consortium as well as the evolution of the flagship concept. The flagship covers a wide range of topics ranging from fundamental research and spintronics to flexible electronics and nanocomposites, and I will not be able to go into the technical details of any of the 16 work packages. Jari Kinaret Department of Applied Physics

Chalmers University of Technology SE-41296 Gothenburg Sweden Tel. +46-31-772 3668 FAX +46-31-772 3202

G


raphene@ICFO

FRANK KOPPENS ICFO Spain

ICFO, The Institute of Photonic Sciences, is a world-leading research center in Photonics hosting more than 250 researchers organized in 23 research groups, working in 60 state-of-the-art research laboratories. Ten research groups are involved in graphene related activities: Opto-electronics Photo-detection and phototransistors Transparant conductors, Coatings, Extraordinary graphene-substrate interactions, Organic LEDs Fundamentals Attosecond dynamics in Graphene Ultra-fast carrier dynamics, Non-linear optics Nano-photonics Plasmonics, metamaterials Novel 2d materials Heterostructures of graphene and 2d materials Quantum optics Single-photon non-linear optics, Coupling single emitters to nano-photonic systems Light harvesting Novel power generation concepts Nano-mechanics Graphene quantum nanomechanics Force and mass sensing Graphene quantum opto-mechanics Quantum simulations Artificial graphene, Ultra-fast phenomena Artificial graphene Ultra-cold atom gases Sensing Force and mass sensing, Bio-sensing Key facts • Mix of fundamental and applied research

• Numerous Industrial collaborations, patents and prototype-development projects • Combination of expertize in opto-electronics, nanotechnology, solid-state physics, quantum optics, and nano-photonics. • 14 European Research Council (ERC) projects • Leading worldwide research institute in “Mapping Scientific Excellence” ranking and Nature Publishing Index. • Graphene flagship: WP optoelectronics (deputy leader) and WP sensors Access to resources • Cleanroom with state-of-the-art fabrication tools for nano-electronic and nano-photonic devices (e.g. 2 EBL systems, Laser writer, 3 Evaporation systems, ALD, RIE, etc.) • Fabrication facilities for graphene and 2d materials: Aixtron CVD machine, deterministic transfer setups, solution deposition, liquid phase and hot press transfer equipment. • Advanced imaging characterization tools: near-field plasmon imaging, low-temperature and high-resolution opto-electronic characterization for visible and infrared light • Characterization of optical materials and components for fundamental and industrial research projects. Frank Kkoppens

ICFO, Spain Graphene.icfo.eu; www.icfo.eu [email protected]

G


efects in two-dimensional transition-metal dichalcogenides and silica bilayers

ARKADY V. KRASHENINNIKOV Aalto University Finland

Isolation of a single sheet of graphene indicated that strictly two-dimensional (2D) materials can exist at finite temperatures. Indeed, inorganic 2D systems such as hexagonal BN sheets, transition metal dichalcogenides (TMD) with a common structural formula MeX2, where Me stands for transition metals (Mo, W, Ti, etc.), X for chalcogens (S, Se, Te), and SiO2 layers were later manufactured by various methods. All these materials have defects, which naturally affect their properties. Moreover, defects can deliberately be introduced by ion and electron irradiation to tailor the properties of the system [1]. In my talk, I will present the results of our first-principles theoretical studies of defects in inorganic 2D systems -- TMDs and silica obtained in collaboration with several experimental groups [2-4]. I will also touch upon defect production in 2D systems under impacts of energetic electrons [2,3]. I will further discuss defect and impurity-mediated engineering of the electronic structure of 2D materials such as TMDs [3-4] and BN [5,6]. Besides, I will discuss mixed TMDs, such as MoS2x Se2(1−x), which can be referred to as 2D random alloys [7]. Our simulations indicate that 2D mixed ternary random alloy MoS2/MoSe2/MoTe2 compounds are thermodynamically stable at room temperature, so that such materials can be manufactured by CVD or exfoliation techniques. Moreover, our simulations predicted that the direct gap in these materials can continuously be tuned depending on relative component concentration, as confirmed later on by several experimental groups.

I will finally touch upon defects in bilayer 2D silica [8] and show that defects are strikingly similar to those in graphene [9] with their morphology governed by the hexagonal symmetry of the lattice [10,11]. Arkady V. Krasheninnikov

Department of Applied Physics, Aalto University, Finland [email protected]

D


References [1] A.V. Krasheninnikov and F. Banhart,

Nature Materials, 6 (2007) 723. [2] H.-P. Komsa, J. Kotakoski, S. Kurasch, O.

Lehtinen, U. Kaiser, and A. V. Krasheninnikov, Phys. Rev. Lett. 109 (2012) 035503.

[3] H.-P. Komsa, S. Kurasch, O. Lehtinen, U. Kaiser, and A. V. Krasheninnikov, Phys. Rev. B 88 (2013) 035301..

[4] Y.-C. Lin, D.O. Dumcenco, H.-P. Komsa, Y. Niimi, A.V. Krasheninnikov, Y.-S. Huang, and K. Suenaga, Advanced Materials (2014) in press.

[5] N. Berseneva, A. V. Krasheninnikov, and R.M. Nieminen, Phys. Rev. Lett. 107 (2011) 035501.4.

[6] H.-P. Komsa, N. Berseneva, A. V. Krasheninnikov, and R.M. Nieminen, submitted.

[7] H.-P. Komsa and A. V. Krasheninnikov, J. Phys. Chem. Lett. 3 (2012) 3652.

[8] P. Y. Huang, S. Kurasch, A. Srivastava, V. Skakalova, J. Kotakoski, A. V. Krasheninnikov, R. Hovden, Q. Mao, J. C. Meyer, J. H. Smet, D.A. Muller, and U. Kaiser, Nano Letters 12 (2012) 1081.

[9] F. Banhart, J. Kotakoski and A. V. Krasheninnikov, ACS Nano, 5 (2011) 26.

[10] T. Björkman, S. Kurasch, O. Lehtinen, J. Kotakoski, O.Yazyev, A. Srivastava, V. Skakalova, J. Smet, U. Kaiser, and A.V. Krasheninnikov, Scientific Reports 3 (2013) 3482.

[11] F. Ben Romdhane, T. Bjorkman, J.A. Rodrıguez-Manzo, O. Cretu, A.V. Krasheninnikov, and F. Banhart, ACS Nano 7 (2013) 5175.


dge Magnetoplasmon in Graphene Investigated by Frequency and Time Domain Measurements

NORIO KUMADA NTT Basic Research Laboratories Japan

Chiral edge channels in a quantum Hall effect regime can be regarded as ideal one-dimensional channels, in which charge carriers are immune to backscattering. Edge magnetoplasmons (EMPs), which are collective charge excitations in the QH edge channels, can travel more than a few millimeters coherently and have a potential for the applications to plasmonic devices. One expects that graphene has advantages over conventional semiconductor two-dimensional electron systems by the lower transport loss and narrower transverse width of EMPs. However, even fundamental aspects of EMPs in graphene such as dispersion and decay are yet to be measured. In this work, we show the velocity [1,2], the dispersion relation, and the decay time of EMPs in graphene measured by high-frequency electronic techniques with the frequency range up to 50 GHz. We fabricated a resonator of EMPs in graphene and measured the orbital motion of EMPs in either frequency or time domain. Frequency domain measurement shows resonant frequencies of the fundamental and harmonic modes of EMPs. Dispersion relation derived from the transmission spectra agrees with theory for a steep edge potential, where EMPs are confined in a narrow strip with a width less than 10 nm. The time domain measurement directly shows the time evolution of the EMP pulse, from which the EMP decay time is obtained. At low temperature (T < 10 K), the decay time is much larger than the period of the EMP modes. The EMP decay time increases with decreasing frequency, indicating that the dissipation is caused through the capacitive coupling between EMPs and the localized states induced by

potential fluctuations. At higher temperature (T > 10 K), on the other hand, the decay time is no longer frequency dependent. This can be explained by finite longitudinal conductance in the interior of graphene. These pieces of information are essential for plasmonic applications and understanding high-frequency charge dynamics in graphene. N. Kumada1,2, B. Roche1, M. Hashisaka3, H.

Hibino1, I. Petkovic2, P. Roulleau2, and D. C. Glattli2 1NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi,

Japan 2Nanoelectronics group, SPEC, CEA Saclay, F-91191 Gif-Sur-Yvette, France 3Department of Physics, Tokyo Institute of Technology, Ookayama, Meguro, Tokyo, Japan

[email protected]

E


References [1] I. Petkovic et al. Phys. Rev. Lett. 110 (2013)

016801. [2] N. Kumada et al. Nature Commun. 4

(2013) 1363.

Figures

Figure 1: (a) Device structure. Graphene is circular shaped with the perimeter of 200 or 1000 µm. Two high-frequency lines to inject and detect EMPs are capacitively coupled to graphene. (b) Dispersion relation of EMPs determined by frequency domain measurement. The magnetic field is set to form the ν = 2 quantum Hall state. (c) Example of time domain measurement. Orbital motion with decay of EMPs is recorded as a function of time.


nusual backscattering between quantum Hall edge states in CVD graphene

FABIEN LAFONT Laboratoire national de métrologie et d’essais France

The quantum Hall effect (QHE) observed in graphene is promising for an application to resistance metrology thanks to the energy spacing ∆E(B) = 36 √[] meV between the first two Landau levels (Lls)much larger than in GaAs (1.7B[T]meV). This is an advantage to develop a resistance standard surpassing the GaAs-based ones by operating at B < 4 T and T > 4 K. This would ease the dissemination of the quantum resistance standard. Achieving this goal requires the fabrication of large graphene monolayer (a few 10000 µm2) with an homogeneous low carrier density ( < 2 × 1011cm− 2) and a carrier mobility higher than 5000 cm2V−1s−1 [1]. The quantization of the Hall resistance was checked with an uncertainty of 9 parts in 1011 in a large monolayer sample (160 × 35 µm2) made by sublimation of SiC[2]. Growth based on chemical vapor deposition (CVD) appears as another promising route to produce large graphene monolayers required for the development of a quantum resistance standard. Carrier mobilities up to 25000 cm2V−1s−1 and 60000 cm2V−1s−1 have been obtained in CVD grapheme transferred on SiO2 and Boron nitride substrates respectively[3]. We studied the transport properties of Hall bars of 200 µm × 400 µm size made of polycrystalline graphene grown by CVD on copper and then transferred on SiO2/Si substrate. The Hall resistance reported as a function of gate voltage Vg at T=0.3 K and B=19 T features well developed Hall resistance plateaus at values h ⁄ νe2 for integer Ll filling factors ν = − 10, ±6, ±2 (see fig. 1b). They coincide with minima of the longitudinal resistance. One can also observe specific behaviors of Rxx and RH at

the charge neutrality point (resistance peaks higher than h ⁄ e2) and at Vg ~ 8 V (resistance peaks of lower amplitude). They corresponds to transverse conductivity plateaus σxy = 0 and σxy = e2 ⁄ h respectively (as seen more clearly in inset of fig. 2). Such conductivity plateaus, usually observed in higher mobility graphene, are explained by the degeneracy lifting of the n=0 Landau level [4]. Although nice plateaus are observed, it turns out that the Hall resistance even on the ν = ±2 plateau deviates from RK ⁄ 2 theoretically equal to h/2e2 by more than 1 percent at a current of 1 µA. Rxx, that measures the dissipation of the system, is high even at low currents values resulting from backscattering of carriers between counter-propagating edge states (see inset of fig. 1 ). To characterize this lack of quantization, σxx, was measured versus the filling factor ν, between T=0.3 K and T=40 K (see fig. 2) and B=5 T and B=19 T. It appears that backscattering σxx(T) does not follow an activated behavior (nor a variable range hopping behavior) except at the minimal value of σxx, and the highest magnetic field (19 T) (see fig. 3a). Even at this particular point, the Arhenius-law behavior is characterized by a very low activation temperature of about 2.4 K which is much smaller than the expected characteristic temperature ∆E ⁄ kB ~ 1834 K where ∆E ~ 158 meV is the energy gap at B=19 T. For all other filling factors and/or magnetic fields the temperature dependence is smoother. More precisely, σxx(T) and (σxx(B)) follow power-laws dependences versus T (versus B). More importantly, it turns out that the temperature dependence is similar at ν values corresponding to minima and maxima (ν = − 4 in fig. 3) of σxx. Since conductivity peaks are due to transport through extended states existing close to Lls

U


energies, this observation suggests that the strong backscattering observed at ν values around ν = ±6, ±2 is caused by the existence of delocalized states at energies in between Lls. Structural characterizations (Raman spectroscopy, optical and atomic force microscopy) were performed. They confirm a high density of line defects (GB, wrinkles) that form continuous networks connecting Hall bar edges. It is mandatory for carriers moving from source to drain to cross some of these line defects. Besides the impact on the Hall quantization of a grain boundary crossing a Hall bar was evaluated by performing numerical simulations. It is shown that non-chiral edge channels along the defect at Fermi energy value in between Lls short-circuit the counter-propagating edge states of the conductor. This supports that linear defects (grain boundaries and probably also pleats) are responsible for the strong backscattering observed in polycrystalline CVD graphene. References [1] F. Schopfer and W. Poirier, Graphene-

based quantum Hall effect metrology,

MRS bulletin, vol. 37,no. 12, pp. 1255 11264, November 2012.

[2] T. J. B. M. Janssen et al, Graphene, universality of the quantum Hall effect and redefinition of the SI system, New. J. Phys., vol. 13, pp. 093026,September 2011.

[3] N. Petrone et al, Chemical Vapor Deposition-Derived Graphene with Electrical Performance of Exfoliated Graphene, Nano lett., vol. 12, pp.2751-2756, May 2012.

[4] M. Kharitonov, Phase diagram for the ν = 0 quantum Hall state in monolayer graphene, Phys. Rev. B, vol. 85, pp. 155439, April 2012.

F. Lafont1, R. Ribeiro-Palau1, Z. Han2, A. Cresti3, A. W. Cummings4, S. Roche5, V. Bouchiat2, S. Ducourtieux1, F. Schopfer1, and W. Poirier1 1 Laboratoire national de métrologie et d’essais, 29, av Roger Hennequin, 78197 Trappes, France 2 CNRS, Institut Néel, Grenoble, France 3 IMEP-LAHC, INP Minatec, Grenoble, France 4 ICN2, Barcelona, Spain 5 ICREA, Barcelona, Spain [email protected]

Figures

Figure 1: Hall and longitudinal resistance versus gate voltage. Inset: Rxx vs I.

Figure 2: σxx (σxy in inset) versus the filling factor υ between 0.3 K and 40 K


Figure 3: σxx versus 1/T (versus T in log-log scale in inset).


raphene and Raman spectroscopy: new instrumental developments

VINCENT LARAT HORIBA Scientific France

In this contribution, we would like to present two latest instrumental developments particularly well adapted to the analysis of graphene and other carbon materials. The first presented option will be the low-frequency filters which allow analysis of Raman spectra down to < 10cm-1. Such filters allow analysis of shear and compressive modes of graphene on a single stage instrument with all the advantages of such a system in comparison with triple systems: easy of use and high throughput, making it accessible for less advanced spectroscopists. In the second part, we will present the latest developments in terms of Tip Enhanced Raman spectroscopy (TERS) that make possible nanoscale imaging of chemical

and physical properties of graphene and other carbon species: innovative integration of technologies brings high-throughput optics and high-resolution scanning for high-speed imaging without interferences between the techniques. The two presented options, the low-frequency filters and latest developments in near-field optical probes can be combined and provide reliable solutions for academic and industrial researchers alike to easily get started with spectroscopic analysis of carbon materials. R. Lewandowska, O. Lancry, E. Leroy, E. Froigneux, J. Schreiber, A Krayev, S Saunin, V. Larat HORIBA Scientific, 16 rue du Canal, 91160 Longjumeau, France AIST-NT Inc., Novato, CA 94949, US

G


pplication of Graphene Materials as Electrodes for Molecular Electronic Devices

BO W. LAURSEN University of Copenhagen Denmark

Vertical devices based on molecular monolayers deposited as SAMs or Langmuir Blodgett (LB) films between top and bottom electrodes are very attractive architectures for molecular electronics. However, this device layout in general suffers from ill-defined junctions when vapor deposited metal top-contacts are applied.[1] A promising solution to this top-contact problem is the application of graphene materials as the actual contacts or as protective interlayers between molecules and metals. Deposition of a dense LB film of single layer graphene oxide flakes is shown by x-ray reflectometry to provide an efficient protection of fragile molecular LB films from vapor deposited titanium-gold top electrodes.[2] Large are thin films (≈5 nm thickness) of reduced graphene oxide (rGO) can be deposited as top contacts to molecular SAM’s grown in lithography defined micro wells on an array of bottom electrodes (Figure 1). The conductive rGO film complete a devices with two molecular junctions in series (junction I), or after vapor deposition of a metal cross wire a well-defined permanent monolayer device (junction II).[3] In this way rGO is demonstrated to function as both a protective layer and as soft electrodes for molecular devices. The transparency of the rGO film is further exploited to fabricate a light switchable electronic device (junction I) from a SAM of photo chemical active molecules.[4] References [1] Christian R. Hansen, Thomas J.

Sørensen, Magni Glyvradal, Jacob

Larsen, Sara H. Eisenhardt, Thomas Bjørnholm, Martin M. Nielsen, Robert Feidenhans’l, and Bo W. Laursen, “Structure of the Buried Metal-Molecule Interface in Organic Thin Film Devices”, Nano Letters, 2009, 9, 1052-1057.

[2] Søren Petersen, Magni Glyvradal, Peter Bøggild, Wenping Hu, Robert Feidenhans´l and Bo W. Laursen, ACS Nano, 2012, 6, 8022–8029.

[3] Tao Li, Jonas Rahlf Hauptmann, Zhongming Wei, Søren Petersen, Nicolas Bovet, Tom Vosch, Jesper Nygård, Wenping Hu, Yunqi Liu, Thomas Bjørnholm, Kasper Nørgaard, and Bo W. Laursen, Advanced Materials, 2012, 24, 1333–1339.

[4] Tao Li, Martyn Jevric, Jonas R. Hauptmann, Rune Hviid, Zhongming Wei, Rui Wang, Nini E. A. Reeler, Erling Thyrhaug, Søren Petersen, Jakob A. S. Meyer, Nicolas Bovet, Tom Vosch, Jesper Nygård, Xiaohui Qiu, Wenping Hu,Yunqi Liu, Gemma C. Solomon, Henrik G. Kjaergaard, Thomas Bjørnholm, Mogens Brøndsted Nielsen, Bo W. Laursen, Kasper Nørgaard, “Ultrathin Reduced Graphene Oxide Films as Transparent Top-Contacts for Light Switchable Solid-State Molecular Junctions”, Advanced Materials, 2013, 25, 4164–4170.

Bo W. Laursen, Tao Li, Søren Petersen, Jonas Rahlf Hauptmann, Magni Glyvradal, Martyn Jevric, Jesper Nygård, Mogens Brøndsted, Peter Bøggild, Kasper Nørgaard Nano-Science Center & Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark [email protected]

A


Figures

Figure 3: Ultra-thin films of reduced graphene oxide may serve as transparent top electrodes and connector for a device with two molecular junctions in series (junction I) or as a protective inter layer between molecules and vapor deposited top contacts (junction II).


ublattice asymmetry of substitutionally doped impurities in graphene

JAMES A. LAWLOR Trinity College Dublin Ireland

Motivated by the recently oberved asymmetry in the spatial distribution of substitutional nitrogen impurities in graphene[1,2,3,4], we explain why substitutional impurities may favour one of the sublattices of graphene even though both are absolutely equivalent. We show that oscillations in the local density of states that arise as a result of the presence of substitutional impurities [5] are responsible for breaking the sublattice symmetry displayed by pristine graphene. While these oscillations are normally averaged out in the case of randomly dispersed impurities, in graphene they have either the same periodicity of the lattice or are very close to it. As a result, the total interaction energy of randomly distributed impurities embedded in a conduction-electron-filled medium does not vanish and is lowered when their spatial distribution is sublattice-asymmetric. Furthermore, we argue that this effect should be more ubiquitous and seen with a plethora of other impurities if suitable substrates are identified.

References [1] Zhao et al., Science 999 (2011) 333. [2] Lv et al., Scientific Reports 2 (2012)

586. [3] Lherbier et al., Nanoletters 13 (2013)

1446. [4] Zhao et al., Nanoletters 13 (2013) 4659. [5] Lawlor et al., Phys. Rev. B 88 (2013)

205416. J. A. Lawlor, M. S. Ferreira, S. R. Power and C. G. Bezerra

Trinity College Dublin, Ireland

S


fter silicene, epitaxial germanene: a newborn in the graphene family

GUY LE LAY Aix-Marseille University France

Silicene has been called graphene’s cousin [1]. Germanene, its germanium counterpart, is predicted to have extremely high mobilities of its charge carriers, to behave as a two-dimensional topological insulator, nearly up to room temperature, and to be possibly a high temperature superconductor. After summarizing our realization of single layer [2] and multilayer [3,4,5,6] epitaxial silicene on silver (111) substrates we will present hints of the synthesis of epitaxial single layer germanene, obtained through a synergetic combination of STM imaging and synchrotron radiation photoelectron spectroscopy measurements. Clearly, these novel synthetic two-dimensional materials, which do not exist in nature, might open the way to practical applications because of their direct compatibility with the current nano/micro electronic technologies. References [1] G. Brumfiel, Nature, 495, (2013) 153;

Nature 485, (2012) 9. [2] P. Vogt et al., Phys. Rev. Lett.,108,

(2012) 155501. [3] A. Resta et al., Scientific Reports,3,

(2013) 2399. [4] P. Vogt et al., Appl. Phys. Lett.,104,

(2014) 021602. [5] E. Salomon et al., submitted. [6] P. De Padova et al., Appl. Phys.

Lett.,102 (2013) 163106.

Guy Le Lay1, María Eugenia Dávila2 1Aix-Marseille University, PIIM-CNRS,13397 Marseille Cedex, France 2Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco 28049 Madrid, Spain [email protected] Figures

Figure 1: STM topograph of the germanene monolayer -1.12 eV; 1.58 nA.

A


raphene for Advanced Aircraft Airframe

SILVIA LAZCANO UREÑA Airbus Operations Spain

Future aircraft airframe faces important challenges in terms of eco-efficiency, performance, cost reduction, maintenance and industrialization. Airbus has taken up these challenges and started to consider a large variety of technologies and approaches to develop the next gen-eration smart airframe concept. Particular challenges emerge from the accelerating shift in materials usage that started some 40 years ago. With aircraft composite structure becoming more and more dominant, the set of requirements to be fulfilled is ever increasing. While nanotechnologies, including graphene, could not yet compete in all challenging areas of future airframe, several key applications exist, where improvements by nano-reinforcement could not only be shown but were also deemed to offer an economic feasibility. Among them, three key composite airframe application fields have been prioritized:

• Barrier Properties: As an example, to limit water absorption of composite materials will allow designing lighter structures

• Functional Properties: Inclusion of the electrical functionality within the aircraft composite structure is of particular interest, as low values turn out to be one of the main disadvantages of the composite use in airframe, which is especially important when it comes to last generation CFRP structure.

Other opportunities include electrical curing of epoxy systems, anti / de-icing, self-sensing, etc.

• Mechanical Behaviour: For resin systems used in CFRP structure, it is of particular concern the fracture toughness, as it directly influences the material behaviour against impact threat. An improvement in matrix fracture toughness by graphene application is immediately translatable into the weight saving of airframe areas subject to increased impact threats. Additionally, other foreseen composite mechanical improvements by nanomaterial addition also show interest from the structural point of view: enhanced strength, modulus, hardness. Silvia Lazcano Ureña1 & Tamara Blanco Varela2 1 R&T Program, Airbus Operations, S.L., Spain 2 R&T Materials & Processes, Airbus Operations, S.L., Spain

Figures

Charge dissipa tionvia wingtips

Nose sect ion

Nacelles ,Pylon

Tailplane

Lightning exit preferably at wingtips

G


RACE: how an european research infrastructure supports the graphene community

NICOLAS LECONTE ICN2 Spain

PRACE (Partnership for Advanced Computing in Europe) is a pan European research infrastructure spanning over 25 countries which aims to offers to European scientists access to world-class resources and services in HPC (High Performance Computing) and advanced numerical simulation. Established since April 2010 with a seat in Brussels (Belgium), PRACE is providing in 2014 a unique computing capacity of more than 15 Petaflops across 6 complementary supercomputers based in France, Germany, Italy and Spain. By offering this unique aggregated computing power and services upon a single peer-review based on scientific excellence, PRACE is allowing its scientific and industrial users to have access to similar capacities and services like their competitors in USA, China, Japan or Russia. Since this level of resources and diversity of HPC architectures was clearly unreachable for any single European country, THE rationale of PRACE was to unite efforts from European countries in order to sustain scientific and industrial competitiveness of Europe. Since mid 2010 PRACE has been able to allocate close to 7 billion cpu core hours on 259 research projects, allowing major breakthroughs in climate modelling,

astrophysics, chemistry, materials, biology and medicine or combustion to name a few. The study of graphene-based materials and other families of two-dimensional materials crucially demands for advanced simulation techniques to explore realistic models of materials of technology and industrial relevance. To that end, beyond the development of suitable numerical approaches for studying large-scale models, the access, implementation and use of high-performance computing has become a strategic value for Europe. In this talk, the European High Performance Computing infrastructure (PRACE) will be briefly presented, and illustrated with an undergoing scientific project focused on simulation of Hall Kubo conductivity in graphene-based materials. The scalability and performance of supercomputers will be shown on a concrete study of quantum Hall effect in structurally and chemically disordered graphene, with new physics revealed thanks to such computing resources, and out of reach otherwise. Nicolas Leconte1 Stéphane Requena2 1 ICN2, Spain 2 Member of the PRACE Board of Directors

http://www.prace-ri.eu/

P


tability and Dynamics of Defects in Graphene: Combinatorial Study of HR-TEM and Simulation methods

GUN-DO LEE Seoul National University Korea

Defects in graphene have become a subject of intensive investigation because those affect the mechanical and electronic properties of graphene. In order to observe and control the defects in graphene, many state-of-the-art techniques such as high resolution transmission electron microscopy (HR-TEM) have been devoted to the study of the structure and formation process. However, it is very difficult to observe the detail of the formation process even within the state-of-the-art microscopy methods because the dynamics of defective structures such as vacancy, adatom, and edge atoms is completed in very short time. Various simulation methods have been employed to elucidate the hidden process of defect formation and dynamics [1]. In the study of defect formation and dynamics in graphene, we performed the cooperative research of HR-TEM and simulation methods. In the simulation methods, we used the tight-binding molecular dynamics simulation and density functional theory (DFT) calculation. In this study, it is found the hydrogen-free graphene edges and our TBMD simulation results are in excellent agreement with images from HR-TEM. These results are expected to make an effective way toward the functionalized graphene [2]. We also found from tight-binding calculation and HR-TEM study that the dislocation core with pentagon-heptagon pair originates ripples which are an out of plane distortion that help stabilize suspended monolayer graphene [3]. We also show that the introduction of atomic vacancies in graphene disrupts the uniformity of C-C bond lengths immediately

surrounding linear arm-chair defects in graphene. The measured changes in C-C bond lengths are related to DFT calculations of charge density variation and corresponding DFT calculated structural models [4]. If time allows, we will introduce briefly our recent results on metal dopants in graphene which show interesting results on magnetic property. Gun-Do Lee1, Euijoon Yoon1, Alex Robertson2, Kuang He2, Zhengyu He2, Angus Kirkland2, Jamie Warner2 1Department of Materials Science and Engineering, Seoul National University, 151-742, Korea 2Department of Materials, University of Oxford, Parks Rd, Oxford, OX1 3PH, United Kingdom [email protected] References [1] G.-D. Lee, C. Z. Wang, E. Yoon, N.-M.

Hwang, D.-Y. Kim, and K. M. Ho, Phys Rev Lett 95 (2005) 205501.

[2] K. He, G.-D. Lee, A. W. Robertson, E. Yoon, and J. H. Warner, Nature Communications 5:3040 (2014).

[3] J. H. Warner, Y. Fan, A. W. Robertson, K. He, E. Yoon, and G.-D. Lee, Nano Lett 13 (2013) 4937.

[4] J. H. Warner, G.-D. Lee, K. He, A. W. Robertson, E. Yoon, and A. I. Kirkland, ACS Nano 7 (2013) 9860.

S


Figures

Figure 1: Simulation of ripples in graphene from dislocation addition and their analysis. (a)-(f) Atomic models obtained from tight-binding calculations of graphene (top, side and 3D views) for (a)-(c) two dislocations and (d)-(f) four dislocations.

Figure 2: Correlation of charge density and bond length in linear arm-chair defect. Total charge density and bond length determined using DFT.

Figure 3: Tight-binding Molecular Dynamics (TBMD) simulation and time-series of TEM images showing the triangle structure formation in edge of graphene.


nhancement of rectifying behavior for hetero-structured graphene tunneling diodes by chemical doping

SEUNG HWAN LEE SKKU Advanced Institute of Nano-Technology (SAINT) Korea

In this study, a tunneling rectifier prepared from vertically stacked two-dimensional (2D) materials composed of chemically doped graphene and h-BN is demonstrated. [1] Figure 1(a) shows the fabricated p-doped top graphene (p-GrT)/ h-BN/ n-doped bottom graphene (n-GrB) tunneling diode (TD). [2] The changes in the properties of the CVD graphene before and after doping were characterized by fabricating a back-gate graphene FET (GFET) on a 90 nm SiO2 wafer. As shown in Figure 1(b), the applied drain voltage (VD) was 20 mV and the Dirac point of the n-doped graphene, obtained using BV, was shifted by about 80 V toward more negative VG values, enhancing the flow of electrons. In contrast, the Dirac point of p-doped graphene, obtained using AuCl3, was shifted by more than 100 V toward positive VG values, suppressing the flow of electrons whereas enhancing the flow of holes. [3], [4] The tunneling current induced by the change in the Fermi level due to the chemical doping was estimated by simulations and was found to agree well with the experimental results. Figure 2(b) shows the calculated current as a function of bias voltage. The tunneling probability was calculated using the equation

−−= ∫

d

dzzVEm

ET0 2

)(2

exp)(h

where d is the width of the potential barrier, m is the effective mass of tunneling electrons (0.5 m0, where m0 is the free electron mass), h is Planck’s constant, E is the energy of the electrons, and

[ ] dzeVzdzVeh

/)()()( −Φ+−Φ=

where e is the electron charge, V is the applied voltage, and Φh and Φe are the

work-functions of the p- and n-doped graphene, respectively. The calculated current was normalized by J (7V) as the current at V = – 7V. In this calculation, the thickness of the potential barrier (h-BN) was 6 nm and the barrier height was 3.5 eV. [5] By applying high bias, the asymmetric current can increase relative to the change in the interfacial potential barrier depending on the direction of bias. Under a forward bias, the n-Gr is at a lower potential, and the interfacial insulating barrier width becomes narrower than that obtained in the unbiased case. This effect gives rise to a higher current because the electrons tunnel through lower barrier or across shorter tunneling distance, obeying the F-N tunneling mechanism. In contrast, under a reverse bias, the current will be much smaller because the width of the interfacial potential barrier does not change. This current is described by the direct tunneling mechanism. The asymmetric 2D tunneling diode is expected to be a key building block for the preparation of future flexible and transparent electronic devices, as it enables electronic devices to operate through a functioning p-n junction. Seung Hwan Lee, Min Sup Choi, Jia Lee, Xiaochi Liu, Chang Ho Ra and Won Jong Yoo

SKKU Advanced Institute of Nano-Technology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Korea [email protected] References [1] C. R. Dean, et al. Nat. Nanotechnol.

2010, 5, 722-726. [2] L. Britnell, et al. 12, (2012), 1707-1710.

E


[3] Y. -J. Yu, et al. Nano Lett. 9, (2009), 3430-3434.

[4] H. -J. Shin, et al. Am. Chem. Soc. 132, (2010), 15603-15609.

[5] G. -H. Lee, et al. J. Appl. Phys. Lett. 99, (2011), 243114.

Figures

Figure 1: (a) A scanning electron microscopic picture of the fabricated p-type doped top graphene (p-GrT)/ h-BN/ n-type doped bottom graphene (n-GrB) hetero-structure. (b) Transfer curves (ID-VG) of a graphene field effect transistor (GFET) before and after doping

Figure 2: (a) Energy band diagrams under zero, reverse, and forward bias conditions. (b) Simulation results of the GBG-TDs before and after p-doping of the top graphene layer.


roduction and Applications of High Quality Graphene Flakes and 2D Monolayers

LAIN-JONG LI Institute of Atomic and Molecular Sciences, Academia Sinica Taiwan

Single- and few-layer graphene films are promising for immediate applications such as heat dissipation for electronic devices. One of the most urgent issues is to develop a scalable method for producing high quality graphene flakes. A highly efficient method to produce high quality graphene flakes based on electrochemical method has been developed.[1] This technology produces a new class of graphene with very active electrochemical performance, which is very different from those obtained by other methods such as mechanical or chemical exfoliation.[2] This type of graphene is perfect as a conducting support for catalytic reaction as well as the heat conduction components. Although graphene films may be are promising for post-silicon electronics due to their high carrier mobility and excellent stability, the lack of energy band gap issue still hinders their applications in electronics. Hence, other 2D materials such as MoS2 and WSe2 with a direct-gap are attractive for optoelectronics, energy harvesting and even for transistors.[3] Here I would like to discuss the synthetic approach to obtain MoS2 (WSe2, MoSe2 and WS2) monolayers directly on arbitrary insulating substrates using vapor phase reaction between metal oxides and S or Se powders.[4,5] The bandgap tunable monolayer alloy such as MoSxSey or WSxSey can be successfully obtained by the replacement reaction between Se and S. These layer materials can be transferred to desired substrates, making them suitable building blocks for constructing multilayer stacking structures, which never exist in nature and may exhibit unique and unexplored physical properties.

The 2D emiconducting material molybdenum disulfide (MoS2) is also known as light- sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a chemical vapor deposition method and graphene is transferable onto MoS2. The significance of charge movement in the emerging field of 2D heterostructures, and the charge distribution strongly affects the properties of the 2D heterostructures. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 108 Lain-Jong Li (Lance Li) Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan [email protected] References [1] C. Y. Su, A. Y. Lu, Y. Xu, F-R. Chen, A.

Khlobystov, and L.-J. Li, ACS Nano 5 (2011) 2332.

[2] See http://www.nitronix.com for large scale graphene production.

[3] M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh, and H. Zhang, Nature Chem. 5 (2013) 263. (2013).

[4] Y.-H. Lee, X.Q. Zhang, W. Zhang, M.-T. Chang, C.-T. Lin, K.-D. Chang, Y.-C. Yu, J. T.-W. Wang, C.-S. Chang, L.-J. Li and T.-W. Lin, Adv. Mater. 24 (2012) 2320.

[5] J.-K. Huang, J. Pu, C.-L. Hsu, M.-H. Chiu, Z.-Y. Juang, Y.-H. Chang, W.-H. Chang, Y. Iwasa, T. Takenobu and L.-J. Li, ACS Nano (2014) DOI: 10.1021/nn405719x.

[6] W. Zhang, C.-P. Chuu, J.-K. Huang, C.‐H. Chen, M.-L. Tsai, Y.-H. Chang, C.-T. Liang, Y.-Z. Chen, Y.-L. Chueh, J.-H. He, M.-Y. Chou and L.-J. Li, Scientific Reports (2014) SREP-13-05483A.

P


Figures

Figure 1: Schematic illustration of graphene flake and ink production. The electrically and thermally conducting films can be made by graphene inks.

Figure 2: Schematic illustration of the growth of WSe2 single crystalline flakes using chemical vapor deposition methods.


hermal conductivity and Phonon Linewidths of Monolayer MoS2 from First Principles

WU LI Scientific Computing & Modelling NV The Netherlands

MoS2 is regarded as a promising material for next-generation electronic applications. Heat generated in electronic devices must be dissipated in order to maintain the devices’ reliability. Thermal conductivity is a crucial property determining the ability of heat dissipation. Despite the great importance, the thermal conductivity of monolayer MoS2 is not yet clear. In this work, we have investigated the thermal conductivity and phonon linewidths of monolayer MoS2 by using first principle calculations. We find that the thermal conductivity for a typical sample size of 1 micrometer is 83 W/m K at room temperature, suggesting the thermal conductivity is not a limiting factor for the potential electronic application of monolayer MoS2. The thermal conductivity can be further increased by 30% in 10 micrometer sized samples. Due to strong

anharmonicity, isotope enhancement of room temperature thermal conductivity is only 10% for 1 micrometer sized samples. However, linewidths can be significantly reduced, for instance, for Raman active modes A_1g and E^1_2g, in isotopically pure samples Wu Li1, Jesus Carrete2, and Natalio Mingo2

1Scientific Computing & Modelling NV, De Boelelaan 1083, Amsterdam, The Netherlands 2CEA-Grenoble, 17 Rue des Martyrs, Grenoble, France

[email protected] References [1] W. Li, J. Carrete, and N. Mingo, Appl.

Phys. Lett. 103(2013) 253103.

Figures

T


omposites materials of graphene derivatives and electrically conducting polymers and their application in solid-state ion-selective electrodes

TOM LINDFORS Åbo Akademi University Finland

We report the electrochemical and chemical synthesis of different types of composite materials consisting of either graphene oxide (GO), reduced GO or exfoliated graphite and conducting polymers, such as poly(3,4-ethylenedioxythiophene) [1,2], polypyrrole [1], poly(N-methylaniline) [3] and polyaniline [4,5]. The presentation focuses on their synthesis, electrochemical reduction of graphene oxide in the electrically conducting polymer matrix, the improved electron transfer and capacitive properties, and long-term potential cycling stability. It is shown that the electrochemical reduction improved the electron transfer in the composite materials [4,5]. Especially for composites with polyaniline, the electroreduction improved the charging/discharging properties of the composite film with 30% and its redox capacitance (pseudocapacitance) with 15% [5]. Moreover, some of the composites were applied as ion-to-electron transducers in potentiometric solid-state ion-selective electrodes (ISEs). It has been shown that a low water uptake of the membrane materials (high hydrophobicity) is a crucial factor for obtaining solid-state ISEs with stable response characteristics, reproducible standard potential and good long-term stability [5]. We have studied with potentiometry and FTIR spectroscopy how the hydrophobicity of the transducer layer influences the potential stability of the solid-state ISEs during the initial contacting of the electrodes with aqueous electrolyte solutions for 24 h. The final goal of our studies is the fabrication of printable capacitors and

conditioning- and calibration-free solid-state ISEs. Tom Lindfors, Anna Österholm1,§, Zhanna Boeva1,¥, Jussi Kauppila2, Patrycja Bober1,*, Konstantin Milakin1,¥, Róbert E. Gyurcsányi3 1 Åbo Akademi University, Process Chemistry Centre, Dept. of Chemical Engineering, Laboratory of Analytical Chemistry, Turku, Finland 2 University of Turku, Turku University Centre for Materials and Surfaces (MATSURF), Laboratory of Materials Chemistry and Chemical Analysis, Turku, Finland 3 Budapest University of Technology and Economics, Department of Inorganic and Analytical Chemistry and Research Group of Technical Analytical Chemistry of the Hungarian Academy of Sciences, Budapest, Hungary Permanent addresses: § Georgia Institute of Technology, School of Chemistry and Biochemistry, Atlanta, GA, USA ¥ M.V. Lomonosov Moscow State University, Chemistry Department, Polymer Division, Moscow, Russia * Academy of Sciences of the Czech Republic, Institute of Macromolecular Chemistry, Prague, Czech Republic

[email protected] References [1] A. Österholm, T. Lindfors, J. Kauppila,

P. Damlin, C. Kvarnström, Electrochim. Acta, 83 (2012) 463.

[2] T. Lindfors, A. Österholm, J. Kauppila, M. Pesonen, Electrochim. Acta, 110 (2013) 428.

[3] T. Lindfors, A.Österholm, J. Kauppila, R.E. Gyurcsányi, Carbon, 63 (2013) 588.

C


[4] Z.A. Boeva, K. Milakin, M. Pesonen, V.G. Sergeyev, T. Lindfors, Manuscript in preparation.

[5] T. Lindfors, Rose-Marie Latonen, 69 (2014) 122.

[6] T. Lindfors, L. Höfler, G. Jágerszki, R.E. Gyurcsányi, Anal. Chem., 83 (2011) 4902.

Figures

Figure 1:. PEDOT-rGO (1)



Figure 4:. PANI-Cl-GO


VD Growth of Graphene and Its 2D Hybrids: Attraction, Reality and Future

ZHONGFAN LIU Peking University China

Graphene, the atomic thin carbon film with honeycomb lattice, holds great promise in a wide range of applications, which is however determined by the development of scalable preparation technique. Among various emerging techniques, chemical vapor deposition (CVD) has received the fastest advances in the last few years. For the CVD growth of graphene, the ultimate goal is to achieve the highest quality in the largest scale and lowest cost with a precise control of layer thickness, stacking order and crystallinity. This talk focuses on our recent progresses towards the controlled surface catalytic growth of graphene and its two-dimensional (2D) hybrids via CVD process engineering. Our general strategy involves the rational design of growth catalysts as well as the control of the elementary steps of CVD process for achieving a precise control of layer thickness, stacking order, domain size, doping and energy band structure. For instance, with a designed binary alloy, such as Ni/Mo, Co/Mo, or Fe/Mo, we effectively suppressed the carbon precipitation step and achieved perfect single layer graphene with 100% surface coverage. We also discovered the groups IVB-VIB early transition metal catalysts, which work well for high-quality graphene growth via carbide formation. The stacking structures of bilayer graphene were successfully modulated by using van de Waals epitaxy and process control. Very recently, we further succeeded in growing the two dimensional hybrid materials of graphene with h-BN and/or doped graphene. These graphene hybrids have demonstrated unique energy conversion properties with high efficiency.

Zhongfan Liu Center for Nanochemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

[email protected] References [1] K Yan, L Fu, HL Peng, ZF Liu, Designed

CVD Growth of Graphene via Process Engineering, Acc. Chem. Res., 10(2013) 2263-2274.

[2] K Yan, D Wu, HL Peng, L Jin, Q Fu, XH Bao, ZF Liu, Modulation-doped growth of mosaic graphene with single-crystalline p-n junctions for efficient photocurrent generation, Nature Comm., 3(2012) 1280-1286.

[3] BY Dai, L Fu, ZY Zou, M Wang, HT Xu, S Wang, ZF Liu,Rational design of a binary metal alloy for chemical vapour deposition growth of uniform single-layer graphene, Nature Comm., 2(2011) 522-527.

[4] LM Zhang, JW Yu, MM Yang, Q Xie, HL Peng, ZF Liu, Janus graphene from asymmetric two-dimensional chemistry, Nature Comm., 4(2013) 1443-1449.

[5] W Yan, WY He, ZD Chu, MX Liu, L Meng, RF Dou, YF Zhang, ZF Liu, JC Nie, L He, Strain and curvature induced evolution of electronic band structures in twisted graphene bilayer, Nature Comm., 4(2013) 2159-2165.

C


pectroscopic properties of BN layers

ANNICK LOISEAU LEM, ONERA-CNRS France

Hexagonal boron nitride (h-BN) is a wide band gap semiconductor (~ 6.5 eV), which can be synthesized, as graphite, its carbon analog, as bulk crystallites, nanotubes and layers. These structures meet a growing interest for deep UV LED and graphene engineering [1]. For instance electron mobility of graphene has been shown to be preserved when graphene is supported by a h-BN film. Until recently, properties of h-BN materials were poorly known due to both the scarcity of crystals and suitable investigation tools. This situation has changed thanks, first, to the development of dedicated photoluminescence (PL) and cathodoluminescence (CL) experiments running at 4K and adapted to the detection in the far UV range [2, 3, 4], and second to the avaibility of high quality single crystals [5]. Thanks to these tools, h-BN has been shown to display original optical properties, governed, in the energy range 5.5 – 6 eV, by strong excitonic effects [2, 3, 6], in agreement with the most reliable theoretical calculations [7, 8]. Furthermore, experimental investigations combining cathodoluminescence measurements and transmission electron microscopy (TEM) observations have revealed that excitonic luminescence is highly sensitive to their environment and are easily perturbed by structural defects such as dislocations [4]. In this talk, we will examine the interplay between structure, defects and spectroscopic properties of BN layers and how these properties can be further exploited for the characterization of these nanostructures. We carry out optical and structural characterizations of this material

by combining PL, CL measurements at 4K in the UV range (up to 7eV), HRTEM observations and Electron Energy Loss Spectroscopy (EELS) in TEM and STEM modes. Thin layers have been obtained by mechanically exfoliating small crystallites or synthesized by CVD techniques. Exfoliated flakes were reported first on SiO2 substrates for AFM thickness measurements, as described in [9] and second on TEM grids. CVD made layers were transferred on gold TEM grids. We will show first, that, whatever the structure, bulk or nanoscale, excitonic luminescence consists of two series of lines called S and D. S excitons are found to be self-trapped, due to a Jahn-Teller effect [3]. Thanks to the imaging capability of the CL, emission, related to D lines, is found to be localized on defects, identified by TEM as grain boundaries (Figure 1). In defect free areas of thin layers, D lines completely vanish and S lines only are observed. D/S ratio can therefore be used as a qualification parameter of the defect densities present in the layers [10]. Second, we will show that EELS provides an alternative approach to the nature of electronic excitations by inspecting the low losses in the 0 – 20 eV range. Pioneering work performed on BN SWNT has shown the potentialities of this approach [11]. One can indeed access under controlled illumination conditions, to the onset of optical transitions. Progress made recently in TEM instrumentation makes now possible the investigation of these transitions at a nm scale and with an energy resolution below 100 meV. We will discuss how these possibilities can be exploited for comparing structural properties of different BN layers.

S


L.Schué1, A. Pierret1,3, J. Barjon2, F. Fossard1, F. Ducastelle1 and A. Loiseau1 1 LEM, ONERA-CNRS, Châtillon, France 2 Group ‘Nanophysique et Semi-conducteurs’, CEA/INAC –CNRS-UJF, Grenoble, France 3 GEMAC, Université Versailles St Quentin – CNRS, Versailles, France

[email protected]

References [1] C.R. Dean et al. Nature

Nanotechnology 5 (2010) 722. [2] P. Jaffrennou el al., Phys. Rev. B 77

(2008) 235422. [3] K. Watanabe et al., Phys. Rev. B 79

(2009) 193104. [4] P. Jaffrennou el al., J. Appl. Phys. 102

(2007) 116102. [5] Y. Kubota et al.. Science 317, (2007) 932 [6] L. Museur et al., Phys. Stat. Sol. rrl, 5

(2011) 414. [7] B. Arnaud, et al. Phys. Rev. Lett. 96

(2006) 026402. [8] C.-H. Park et al., Phys. Rev Lett. 96

126105 (2006). [9] G. F. Schneider, et al, Nano. Lett. 10

(2010) 1912. [10] A. Pierret et al, Phys. Rev. B, 89 (2014)

035414. [11] R. Arenal et al, Phys. Rev. Lett. 95 (2005)

127601.

Figures

Figure 1: Left: a) MEB image of BN crystallite, b) and c) CL images recorded at the energy of the D4 and S3-S4 lines respectively, d) D/S ratio mapping. Right: CL spectra recorded in the areas labeled #1 (top) and #2 of the crystallite (bottom). Area #1 is a grain boundary.


he Copper for CVD Graphene: its cleaning and oxidation in relation to graphene quality

MARIA LOSURDO IMIP-CNR Italy

The efforts toward mass production of large area graphene have stressed even more the need to control and improve the quality and reproducibility of the CVD growth and transfer of graphene [1]. Copper foil is the most common substrate and a variety of work has already allowed to progress in increasing grain size and controlling the nucleation site. Nevertheless, there are still unsolved issues and challenges that make CVD graphene growth and quality irreproducible dependent on copper foil supplier and batch. Furthermore, recently some contradictory results are out about the graphene as a barrier to oxidation of metals underneath. This latter aspect is important in relation also to the adhesion of graphene on copper and consequently to the quality of its transferring. It is clear that a correlation exists between the copper substrate, the growth, the graphene quality and goodness of transferring [2]. This contribution aims at elucidating and controlling this correlation. We show significant improvement in reproducibility and control of CVD graphene growth and quality by a H2 plasma processing of copper. Implementation of commercial CVD reactors with a simple H2 remote plasma configuration will be shown, which is also applicable to other 2D materials. Various copper foils and wafers from different suppliers and with different morphology are tested and discussed. The significant advantage over the common H2-Ar annealing will be presented, which leads to consistent growth conditions and high-quality continuous graphene.

This step will also be related to properties of graphene as oxidation resistance layer [3], discussing the role of the copper oxides on the adhesion of graphene to the Cu foil and consequences for the quality of the transferring. Therefore, we also correlate results on copper/graphene oxidation to the copper processing and graphene quality. Acknowledgements We acknowledge support from the European Commission FP7 Project MEM4WIN under grant agreement GA314578.

Maria Losurdo, M.M. Giangregorio, G.V. Bianco, P. Capezzuto, G. Bruno

Institute of Inorganic Methodologies and of Plasmas, IMIP-CNR, via Orabona 4, 70126 Bari, Italy [email protected] References [1] M. Losurdo, et al. Phys. Chem. Chem.

Phys. 13 (2011) 20836. [2] M. Losurdo, J. Phys. Chem. C 115 (2011)

21804. [3] M. Losurdo et al. Adv. Funct. Mater.

(2013) DOI: 10.1002/adfm.201303135.

T


Figures

Figure 1: (a) Raman spectra of various Cu foils after common annealing and after developed H2 plasma treatment (blue line); (b) Raman spectra of the graphene-on-Cu foils (same as in (a)); (c) corresponding optical quality as evaluated by spectroscopic ellipsometry spectra of the imaginary part of the dielectric function, which is sensitive to both graphene quality and Cu oxidation underneath.


ight-Matter Interactions in 2D Materials

TONY LOW IBM, Yale University & Columbia University USA

Recent discovery of a new class of two-dimensional (2D) crystals, with widely diverse and unique electrical, mechanical and optical properties, carries great potentials for major advancements in nano-electronics and nano-photonics. In this talk, I will describe our recent work on the understanding and engineering of light-matter interactions in graphene, drawing upon both theoretical and experimental studies [1-8]. I will discuss how strong coupling of electromagnetic waves with an electric dipole carrying excitation, such as plasmon, phonons or excitons, can lead to various types of surface polaritons in 2D materials. In particular, I will describe how plasmons in graphene disperses and damps, and how the coupling with remote phonons can strongly modify the plasmonic character. Furthermore, these plasmon- and phonon-polaritons resides within the technologically important terahertz to mid-infrared spectrum, allowing for interesting active devices such as light benders, photodetectors, and sensing of molecular layers. Lastly, I will discuss light-matter interaction in new 2D materials beyond graphene. I will illustrate how going from monolayer to bilayer graphene can lead to interesting plasmonic effects such as phonon-induced transparency and an optical-like plasmonic mode. Black phosphorus, a layered material like graphene, was also re-discovered recently. This material exhibits optical properties that vary sensitively with thickness, doping, and light polarization across the mid- to near-infrared spectrum. In addition, it exhibits a highly anisotropic plasmonic mode not seen in any other plasmonic materials.

Tony Low

IBM Thomas J. Watson Research Center, USA Electrical and Computer Engineering, Yale University, USA Electrical and Computer Engineering, Columbia University, USA References [1] M. Freitag, T. Low, W. Zhu, H. Yan, F. Xia

and P. Avouris, "Photocurrent in graphene harnessed by tunable intrinsic plasmons," Nature Communications 4:1951 (2013).

[2] M. Freitag, T. Low, F. Xia and P. Avouris, "Photoconductivity of biased graphene," Nature Photonics 7, 53 (2013).

[3] T. Low, A. S. Rodin, A. Carvalho, Y. Jiang, H. Wang, F. Xia and A. H. Castro Neto, "Tunable optical properties of multilayers black phosphorus," Unpublished (2014).

[4] T. Low and P. Avouris, "Graphene plasmonics for terahertz to mid-infrared applications," ACS Nano 8, 1086 (2014).

[5] H. Yan, T. Low, W. Zhu, Y. Wu et al, "Damping pathways of mid-infrared plasmons in graphene nanostructures," Nature Photonics 7, 394 (2013).

[6] T. Low, F. Guinea, H. Yan, F. Xia and P. Avouris, "Novel mid-infrared plasmonic properties of bilayer graphene," Physical Review Letters 112, 116801 (2014).

[7] H. Yan, T. Low, F. Guinea, F. Xia and P. Avouris, "Tunable phonon-induced transparency in bilayer graphene nanoribbons," arXiv:1310.4394v1 (2013).

[8] T. Low et al, "Screening and plasmons in black phosphorus," Unpublished (2014).

L


hallenges in integrating graphene into CMOS technology platform

GRZEGORZ LUPINA IHP Germany

Graphene and graphene-based devices have a great potential to considerably extend the use and functionality of Si CMOS technology [1,2]. Graphene-enhanced modules (e.g. RF communication, optoelectronic, sensing) with superior performance improving the interaction with the user and the outside world can find application in many branches such as consumer electronics, automotive electronics, medical applications etc. Combination of the graphene-enabled non-digital functionalities with the digital CMOS world on one chip will require integration of this new material into the existing Si platform [2]. Here, we describe the results of first graphene integration trials into a professional Si wafer fabrication line located in a class-1 cleanroom and point out to challenges which should be addressed in the future. We have developed a graphene device fabrication scheme which can be used to manufacture a wide range of graphene devices such as transistors, sensors, or optoelectronic components. To demonstrate feasibility of the process in our initial experiments we focus on the realization of graphene base transistors [3,4]. We use commercially available CVD graphene which is transferred from Cu onto target 8-inch wafers. Fabrication begins with the preparation of patterned substrates (Fig. 1a) equipped with doped Si emitter areas (E) and metal contacts (M). Directly before graphene deposition, the substrate is etched to remove surface oxides and enable formation of graphene-Si Schottky emitter junction and ohmic metal-graphene contacts. Chip-size graphene layers (~20 mm2) are placed on the prepared wafers

(Fig. 1b) using wet transfer techniques. This process is scalable to larger graphene layer sizes. Transfer of graphene is followed by the deposition and patterning (photolithography, dry-etching, Fig. 1c) of the collector stack comprised of a high-k insulator (e.g. HfO2) [5] or a semiconductor layer (e.g. Si) [6] and a metal contact layer C (Fig. 1d). In this way the structure of the vertical graphene base transistor is realized [3,7,8]. The terminals of the transistor are connected to Al signal-ground-signal measurement pads (Fig. 1e,f) using tungsten vias (Fig. 1g). Electrical characterization of the fabricated devices indicates that monolayer graphene preserves its characteristics even after many steps of harsh technological processing showing sheet resistance of about 2kOhm/sq. Well performing graphene-Si Schottky diodes and graphene/high-k/metal capacitor stacks are demonstrated. Among the challenges ahead is the reduction of graphene-metal contact resistance as well as finding solution to the problems associated with graphene transfer. The latter include well known inevitable cracks, folds and polymer residuals but also a considerable amount of Cu contamination which may be dangerous in CMOS fabrication lines. This underlines the need for a CMOS-compatible method enabling direct graphene growth on arbitrary dielectric and semiconducting substrates. Our attempts to achieve this challenging goal will be also presented here [9,10]. Grzegorz Lupina, Andre Wolff, Mindaugas Lukosius, Julia Kitzmann, Gunther Lippert, David Kaiser, Jarek Dabrowski, Chafik Meliani, Christian Wenger, Ioan Costina, Markus Andreas Schubert, Wolfgang Mehr IHP, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany

[email protected]

C


References [1] K. Novoselov, V. Falko, L. Colombo, P.

Gellert, M. Schwab, and K. Kim, Nature 490 (2012) 192.

[2] K. Kim, J.-Y. Choi, T. Kim, S.-H. Cho, and H.-J. Chung, Nature 479 (2011) 338.

[3] W. Mehr, J. Dabrowski, C. Scheytt, G. Lippert, Y. Xie, M. Lemme, M. Ostling, G. Lupina, Electron Dev. Lett., 33, 691 (2012).

[4] S. Vaziri, G.Lupina, Ch.Henkel, A.Smith, G.Lippert, W.Mehr, M. Lemme, Nano Lett., 13, 1435 (2013).

[5] G. Lupina, M. Lukosius, J. Kitzmann, A. Wolff, W. Mehr, Appl. Phys. Lett., 103 (2013) 183116.

[6] G. Lupina, J. Kitzmann, M. Lukosius, J. Dabrowski, W. Mehr, Appl. Phys. Lett., 103 (2013) 263101.

[7] V. D. Lecce, R. Grassi, A. Gnudi, E. Gnani, S. Reggiani, and G. Baccarani, TED 60 (2013) 4263.

[8] S. Vaziri, G. Lupina, A. Paussa, A.D. Smith, C. Henkel, G. Lippert, J. Dabrowski, W. Mehr, M. Östling, M.C. Lemme, Solid State Electronics, 84 (2013) 185.

[9] G. Lippert, J. Dabrowski, T. Schroeder, Y. Yamamoto, F. Herziger, J. Maultzsch, J. Baringhaus, Ch. Tegenkamp, M. C. Asensio, J. Avila, G. Lupina, arXiv:1312.5425, (2013).

[10] G. Lippert, J. Dabrowski, Y. Yamamoto, F. Herziger, J. Maultzsch, M. Lemme, W. Mehr, G. Lupina, Carbon 52 (2013) 40.

Figures

Figure 1: Fabrication of graphene devices in a 8-inch Si wafer pilot line. (a) Preparation of patterned substrates. (b) Graphene transfer. (c) Processing in the pilot line. (d) graphene base transistor with collector high-k/metal stack deposited on graphene and patterned. (e-f) fabricated transistors and test structures. (g) STEM-EDX investigation of a graphene device fabricated using only Si technology compatible materials and processes.


dge-Controlled Growth and Kinetics of Single-Crystal Graphene Domains by Chemical Vapor Deposition

TENG MA Shenyang National Laboratory for Materials Science China

The controlled growth of large-area, high-quality, single-crystal graphene is highly desired for applications in electronics and optoelectronics; however, the production of this material remains challenging because the atomistic mechanism that governs graphene growth is not well understood. The edges of graphene, which are the sites at which carbon accumulates in the sp2 lattice, influence many properties, including the electronic properties and chemical reactivity of graphene [1-5], and they are expected to significantly influence its growth. However, this influence of edge structure has not been verified experimentally due to the difficulty of growing graphene with controlled edges. For instance, only zigzag or randomly oriented edges have been fabricated by chemical vapor deposition (CVD). Here, we demonstrate the growth of single-crystal graphene domains with controlled edges that range from zigzag to armchair orientations via growth–etching–regrowth in a CVD process (Figure 1) [6]. We have observed that both the growth and etching rates of a single-crystal graphene domain increase linearly with the slanted angle of its edges from 0 to ~19° and that the rates for an armchair edge are faster than those for a zigzag edge (Figure 2). Such edge-structure-dependent growth/etching kinetics of graphene can be well explained at the atomic level based on the concentrations of the kinks on various edges. In addition, we found that the graphene edges and morphology are not determined by the nucleation but are kinetically controlled, following the classical kinetic Wulff construction theory, during the CVD process, and that defects in the graphene can be healed through an

etching–regrowth process (Figure 3). Using these findings, we propose several strategies for the fabrication of wafer-sized, high-quality, single-crystal graphene. Teng Ma1, Wencai Ren1*, Xiuyun Zhang2, Zhibo

Liu1, Yang Gao1, Lichang Yin1, Xiuliang Ma1, Feng Ding2,3, Hui-Ming Cheng1 1Shenyang National Laboratory for Materials

Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P. R. China 2Institute of Textiles and Clothing, Hong Kong Polytechnic University, Kowloon, Hong Kong, P. R. China 3Beijing Computational Science Research Center, No. 3 He-Qing Road, Hai-Dian District, Beijing 100084, P. R. China *[email protected] References [1] Son, Y. W., Cohen, M. L. & Louie, S. G.,

Nature, 444 (2006) 347-349. [2] Nakada, K., Fujita, M., Dresselhaus, G.

& Dresselhaus, M. S., Phys. Rev. B, 54 (1996) 17954-17961.

[3] Radovic, L. R. & Bockrath, B., J. Am. Chem. Soc., 127 (2005) 5917-5927.

[4] Girit, C. O. et al., Science, 323 (2009)1705-1708.

[5] Jia, X. et al., Science, 323 (2009) 1701-1705.

[6] Ma, T. et al., Proc Natl Acad Sci USA, 110 (2013) 20386-20391.

E


Figures

Figure 1: Morphology and edge evolution of single-crystal graphene domains grown on a Pt surface during the growth-etching-regrowth process.

Figure 3: Regrowth of defect-free single-crystal graphene domains.

Figure 2: Edge-structure-dependent growth/etching of single-crystal graphene domains.


raphene Oxide Mediated Carboplatin Delivery and its Anticancer Activity

SAMI A MAKHARZA IFW Dresden Germany

Graphene and its derivative, graphene oxide (GO), have been considered as promising materials for drug loading and delivery. Nanoscaled materials, such as liposomes, microspheres, polymeric shells, and nanoparticles, are mostly used as drug loading agents acting through different mechanisms (e.g. embedding, surface absorption, hydrogen bonding, van der Waals interactions, etc.). However these materials have a low loading capacity for drug molecules [1-3]. Therefore, for efficient drug activity, drug loading efficiency needs to be improved in drug carrier research. For the first time, nanographene oxide (NGO) used as nanocarrier for drug loading and delivery [4,5], the loading capacity increased dramatically and graphene did not provoke cytotoxicity in human fibroblast cells. Prior drug loading, graphene has to be functionalized via covalent or noncovalent interaction with other organic materials, the functionalization provides high dispersability and biocompatibility in physiological media. Several studies emphasized that numerous factors such as chemical composition, size, shape, contaminants, concentration, and cell types will influence the cellular uptake and the cytotoxicity of graphene. A variety of characterization techniques have been used to utilize the structure and properties of GO. These techniques are classified into spectroscopic and microscopic approaches. The spectroscopic approaches are used to identify the chemical structure of GO, and include Raman, FTIR, and XPS. Microscopic tools are used to map out the structure of GO at various heights and lateral dimensions, for instance, AFM, SEM, TEM, and STM. For graphene 2014 conference in Toulouse, the subject talk will be concentrated on

NGO functionalized Polyamido amide (PAMAM) for carboplatin (CP) delivery (figure 1). The results revealed that NGO-PAMAM as a platform was found to be able to deliver carboplatin to the cancer cells, by enhancing the drug anticancer efficiency. Moreover, the carboplatin loaded NGO carrier shows no significant effect on the viability of mesenchymal stem cells (hMSCs) even at high concentration (100 mg ml-1) Figure 2. Sami A Makharza, Giuseppe Cirillo, Orazio

Vittorio, Steffen Oswald, Silke Hampel Leibniz Institute for Solid State and Materials

Research Dresden PF 27011601171 Dresden Germany [email protected] References [1] C. Soldano, A. Mahmood and E.

Dujardin, Carbon, 48, (2010) 2127–2150.

[2] F. Wang, Y. Zhang, C. Tian, C. Girit, A. Zettl, M. Crommie and Y. R. Shen, Science, 320 (2008) 206–209.

[3] D. Garcia-Sanchez, M. van der Zande, S. Paulo, B. Lassagne, P. L. McEuen and A. Bachtold, Nano Lett., 8 (2008) 1399–1403.

[4] X. Sun, Z. Liu, K. Welsher, J. T. Robinson, A. Goodwin, S. Zaric and H. Dai, Nano Res., 1 (2008) 203–212.

[5] Z. Liu, J. T. Robinson, X. Sun and H. Dai, J. Am. Chem. Soc., 130 (2008) 10876–10877.

G


Figures

Figure 1: Schematic illustration of NGO–PAMAM/CP: (1) chemical oxidation exfoliation reaction, (2) size reduction and functionalization by PAMAM, and (3) carboplatin loading.

Figure 2: Cell viability (WST-8) of HeLa and hMSCs after 24 h of incubation: (a) pristine NGO, CP–NGO, and CP/PAMAM–NGO. The concentrations tested were: CP (10 mgml_1), PAMAM (20 mgml_1), and NGO (100 mgml_1); (b) PAMAM, NGO and PAMAM–NGO.


ransport Fingerprints at Graphene Superlattice Dirac Points Induced by Boron-Nitride Substrate

RAFAEL MARTINEZ-GORDILLO

ICN2 Spain

We report peculiar transport fingerprints at the secondary Dirac points created by the interaction between graphene and boron-nitride layers. By performing ab-initio calculations, the electronic characteristics of the Moiré patterns produced by the interaction between layers are first shown to be in good agreement with experimental data, and further used to calibrate the tight-binding model implemented for the transport study. By means of a real-space order N quantum transport (Kubo) methodology, low-energy (Dirac point) transport properties are contrasted with those of high-energy (secondary) Dirac points, including both Anderson disorder and Gaussian impurities to respectively mimic short range and long range scattering potentials. Mean free paths at the secondary Dirac points are found to range from 10 nm to few hundreds of nm depending on the static disorder, while the observation of satellite resistivity peaks depends on the strength of quantum interferences and localization effects.

Rafael Martinez-Gordillo1,2, Stephan Roche1,3,

Frank Ortmann1,4, Miguel Pruneda1,2 1ICN2 - Institut Catala de Nanociencia Nanotecnologia, Campus UAB, 08193 Bellaterra (Barcelona), Spain 2CSIC - Consejo Superior de Investigaciones Cientificas, ICN2 Building , Campus UAB ,08193 Bellaterra (Barcelona), Spain

3ICREA - Institucio Catalana de Recerca i Estudis Avancats, 08010 Barcelona, Spain

4 Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, 01062 Dresden, Germany

[email protected]

T


train superlattices and suspended graphene at the macroscale

LAËTITIA MARTY Institut Néel France

The development of graphene growth over macroscopic areas and the improvement of transfer techniques increase the need to control the shape and geometry of graphene once deposited onto the destination substrate. After being transferred on flat surface or suspended [1], graphene membranes always display unwanted ripples that limit electrical, thermal and mechanical properties [2]. Indeed, ripples in graphene-based transistor can alter their electrical conductivity [2]. Nevertheless, it offers interesting ways to locally tune strain in graphene in order to influences its electronic, magnetic and vibrational properties [3,4]. Local bending of graphene is a mean to induce an electrical gap or create high pseudo-magnetic fields [3], so that a controlled strain can allow designing "stresstronic" devices. When fully suspended, graphene membranes generally exhibit high electrical mobility and can be used as nano-electromechanical systems [5] or optomechanical devices [6]. Before reaching such control, it appears necessary to understand the formation of graphene ripples during transfer. For this purpose, we investigate here the formation process of strain and ripples in CVD graphene layers by spatially resolved Raman spectroscopy. To do so, we transferred graphene onto a corrugated substrate formed by an array of SiO2 nano-pillars with varying spacing and apex radius. This ordered corrugated substrate defines strain domains of parallel ripples. By varying the pitch of the array and sharpness of the pillars, different regimes

can be found. We explore both limits of low-density arrays where graphene exhibits ripples domains, and of very dense arrays for which no ripples are formed, and so graphene stays fully suspended. Spatially resolved Raman spectroscopy reveals uniaxial strain domains in the transferred graphene, which are induced and controlled by the array. For the tightest arrays, this technique demonstrates a method to obtain macroscopically suspended graphene membranes with minimal interaction with the supporting substrate. It also offers a platform to tailor stress in graphene layers and offer perspectives for electron transport and nano mechanical applications. This work was supported by the Agence Nationale de la Recherche (ANR projects: Supergraph, Allucinan and Trico), European Research Council (ERC advanced grant no. 226558), the Nanosciences Foundation of Grenoble and Region Rhône-Alpes. Laëtitia Marty1, Antoine Reserbat-Plantey1,

Dipankar Kalita1, Laurence Ferlazzo3, Katsuyoshi Komatsu2, Chuan Li2, Raphaël Weil2, Zheng Han1, Arnaud Ralko1, Sophie Guéron2, Nedjma Bendiab1, Hélène Bouchiat2, Vincent Bouchiat1 1Institut Néel, Grenoble University-CNRS, 25 rue des Martyrs, Grenoble, France 2LPS, Université Paris-Sud-CNRS, Orsay, France 3LPN, CNRS, Marcoussis, France [email protected]

S


References [1] Meyer, J. C.; Geim, A. K.; Katsnelson,

M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. Nature, 446 (2007) 60.

[2] Ni, G.-X.; Zheng, Y.; Bae, S.; Kim, H. R.; Pachoud, A.; Kim, Y. S.; Tan, C.-L.; Im, D.;Ahn, J.-H.; Hong, B. H.; Ozyilmaz, B. ACS nano, 6 (2012) 1158 ; Chen, C.-C.; Bao, W.; Theiss, J.; Dames, C.; Lau, C. N.; Cronin, S. B. Nano Letters, 9 (2009) 4172 ; Bao, W.; Myhro, K.; Zhao, Z.; Chen, Z.; Jang, W.; Jing, L.; Miao, F.; Zhang, H.; Dames, C.; Lau, C. N. Nano Letters, 12 (2012) 5470.

[3] Levy, N.; Burke, S. A.; Meaker, K. L.; Panlasigui, M.; Zettl, A.; Guinea, F.; Castro Neto, a. H.;Crommie, M. F. Science, 329 (2010) 544.

[4] Frank, O.; Tsoukleri, G.; Riaz, I.; Papagelis, K.; Parthenios, J.; Ferrari, A. C.; Geim, A. K.;Novoselov, K. S.; Galiotis, C. Nature Communications, 2 (2011) 255.

[5] Bunch, J. S.; van Der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science, 315 (2007) 490.

[6] Eichler, A.; Moser, J.; Chaste, J.; Zdrojek, M.;Wilson-Rae, I.; Bachtold, A. Nature Nanotechnology, 6 (2011) 339; A. Reserbat-Plantey L. Marty, O. Arcizet, N. Bendiab and V. Bouchiat, Nature Nanotechnology 7 (2012) 151.

Figures

Figure 1: top: principle of the graphene deposition on silicon oxide pillars: depending on thepillars network, the graphene will either collapse or get suspended macroscopically Bottom: SEM image of a fully suspended graphene monolayer over silicon oxide pillars.


lectronic and magnetic properties of graphene nanoribbons deposited on metallic substrates

RICCARDO MAZZARELLO RWTH Aachen University Germany

Graphene is a fascinating two-dimensional system with unique electronic and transport properties. Nevertheless, the absence of an energy gap in its band structure limits its applicability in semiconductor technology. A route to induce the required band gap is nanostructuring. Recent efforts have focused on one-dimensional graphene nanoribbons and zero-dimensional graphene quantum dots.

According to mean-field calculations, zigzag-terminated nanostructures possess magnetic electronic states localized at the edge. This property has been investigated intensively recently, due to the potential applications in the field of spintronics. However, the robustness of this phenomenon is hotly debated, particularly in the case of supported nanostructures.

In this work, we have carried out a density functional theory study of the electronic and magnetic properties of graphene nanoribbons on the (111) surface of several metallic substrates, namely Ir [1], Au, Ag and Cu [2]. Ir and Cu substrates are routinely used for the growth of graphene flakes by chemical vapor deposition. Ag and Au substrates have been successfully employed to fabricate graphene nanoribbons by thermally-induced polymerization of suitable precursor molecules.

We have considered both H-free and H-passivated nanostructures. In the case of the Ir(111) surface, we do not find states localized at the nanoribbon edges. We explain this surprising result by the interplay between a strong and intricate hybridization of the graphene d orbitals with Ir d orbitals and a lattice-mismatch driven geometrical relaxation at the edges.

These findings are in agreement with STM experiments performed on graphene islands on Ir(111) [1,3].

In the case of Au, Ag and Au substrates, the nanoribbons possess edge states. In spite of this, they do not exhibit a significant magnetization at the edge, with the exception of H-terminated nanoribbons on Au(111), whose zero-temperature magnetic properties are comparable to those of free-standing nanoribbons. These results are explained by the different hybridization between the graphene π states and those of the substrates and, for some models, also by the charge transfer between the surface and the nanoribbon [2]. Riccardo Mazzarello RWTH Aachen, Otto-Blumenthal-Strasse, D-52056 Aachen, Germany

[email protected] References [1] Y. Li, D. Subramaniam, N. Atodiresei, P.

Lazić, V. Caciuc, C. Pauly, A. Georgi, C. Busse, M. Liebmann, S. Blügel, M. Pratzer, M. Morgenstern, and R. Mazzarello, Adv. Mater. 25 (2013) 1967.

[2] Y. Li, W. Zhang, M. Morgenstern, and R. Mazzarello, Phys. Rev. Lett. 110 (2013) 216804.

[3] D. Subramaniam, F. Libisch, Y. Li, C. Pauly, V. Geringer, R. Reiter, T. Mashoff, M. Liebmann, J. Burgdoerfer, C. Busse, T. Michely, R. Mazzarello, M. Pratzer, and M. Morgenstern, Phys. Rev. Lett. 108 (2012) 046801.

E


wo-Dimensional Materials Beyond MoS2

PERE MIRÓ Jacobs University Bremen Germany

The development of small electronic components is fundamental in our highly technology dependent society. Currently, the electronic industry is rapidly approaching the limit of silicon-based complementary metal-oxide-semiconductor (CMOS) technology. In consequence, the development of new technologies to replace silicon has rapidly become a hot topic not only in the academic community but also in industry. This new Holy Grail of electronic materials has to perform better than silicon at smaller scales (>10 nm) and if possible add new functionalities for electronic devices such as flexible electronics. In this direction, single layer transition metal chalcogenides (TMCs) have recently emerged in nanoelectronics, although the study of layered TMCs beyond MoX2/WX2 is still incipient.

The electronic structure of nickel, palladium and platinum TMDs with sulfur, selenium and tellurium as the chalcogenide were investigated via periodic density functional theory calculations. All disulfide monolayers are indirect band gap semiconductors with band gaps of 0.5, 1.1 and 1.7eV for NiS2 PdS2 and PtS2, respectively. The MSe2 and MTe2 analogues present significantly smaller band gaps and can even become semimetallic or metallic materials. Under mechanical strain these MX2 materials become quasi-direct band-gap semiconductors. The mechanical-deformation and electron-transport properties of these materials indicate their potential application in flexible nanoelectronics [1].

In the second part, we will present the electronic properties of the most interesting 2D materials beyond graphene, in particular of all transition metal chalcogenides and

halides. We concentrate on the dependency of the electronic band gap on the number of layers (magnitude and character), and the effective hole and electron masses [2]. Pere Miró, Thomas Heine and Mahdi Ghorbani-Asl

Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany

[email protected] References [1] P. Miró, M. Ghorbani-Asl, T. Heine,

Angewandte Chemie Intl. Ed. (2014) in press.

[2] P. Miró, M. Audiffred, T. Heine, Chem. Soc. Rev. (2014) submitted.

Figures

Figure 1: Left. Structure of TX2, T=Ni, Pd, Pt, X=S, Se, Te. Right: band structure of these materials a monolayer (top) and bilayer (bottom). Note that PdS2 is semiconducting as monolayer, but metallic as bilayer.

T


ltrafast detection from 0.6 THz to 33 THz employing graphene flakes

MARTIN MITTENDORFF Helmholtz-Zentrum Dresden-Rossendorf Germany

Graphene can serve as an excellent active material for the development of ultrafast electro-optic devices. With the vanishing bandgap, photons can be absorbed via interband processes over an extremely wide spectral range (from ultraviolet to far-infrared). However, in the regime of non-zero Fermi energy and very low photon energies, interband absorption can be prohibited. In this case intraband absorption is an efficient process. T. Müller et al. [1] presented an ultrafast detector for the near-infrared range. Their device was operated at room temperature and reached frequencies of up to 16 GHz. Field-effect transistors made of graphene flakes have been employed for the detection of THz radiation. Vicarelli et al. [2] developed a very sensitive detector for cw radiation at room temperature, while Yan et al. [3] presented an ultrafast bolometer which was cooled to 4 K.

We present a detector based on a graphene flake for a very broad spectral range from 0.6 THz to 33 THz, corresponding to wavelengths of 500 µm to 9 µm, respectively. To couple the far-infrared radiation efficiently to the flake, which is orders of magnitude smaller than the largest wavelengths, a logarithmic periodic antenna [5] is patterned on top of the substrate. The antenna is connected to the graphene flake by an interdigitated structure (see fig. 1). A coaxial cable, bonded to the outer part of the two antenna arms, serves as signal line. The signal is amplified by a high-frequency amplifier and recorded with a fast sampling oscilloscope with a bandwidth of 30 GHz.

The free-electron laser (FEL) FELBE at the Dresden lab served as radiation source for the characterizing the detectors at

wavelengths of up to 220 µm. Additional data were obtained using a THz gas laser at the University of Regensburg providing radiation pulses with wavelengths of up to 500 µm. The response time of the devices is about 50 ps, which highlights the potential of this detector for timing measurements of intense THz pulses. The signal of two FEL pulses with a temporal delay of 500 ps is shown in fig. 2. The pulse energy of each of the pulses was about 40 nJ, which lead to a signal amplitude of 30 mV. Despite a low responsivity of about 5 nA/W, pulses with energies down to 1 nJ can be resolved. For high pulse energies, the signal amplitude saturates strongly. While this saturation limits the dynamic range for linear detection, it can be exploited in autocorrelation measurements [6]. In this regime the response time is not limited by the RC time constant but by the intrinsic response time of the graphene flake (< 10 ps).

Furthermore we demonstrate the important role of the substrate for these devices. Our first devices were produced on SiO2 on Si [7]. When a low-resistivity substrate is used, the high-speed performance of the device is strongly deteriorated. The antenna forms a capacitor with the conductive substrate material and therefore increases the RC time constant of the detector. Devices on high-resistive Si could resolve fast signals only for wavelengths above 20 µm. This can be attributed to phonon-related absorption in the Si substrate resulting in higher substrate conductivity due to thermally activated carriers. To overcome this restriction, a new set of detectors has been fabricated on semi-insulating SiC. As graphene is nearly invisible on top of SiC, graphene grown by chemical vapor deposition on copper was transferred to the new substrate and located by Raman mapping. With these new devices FEL pulses can be measured down to a wavelength of 9 µm.

U


Martin Mittendorff1,2, Stephan Winnerl1, Josef

Kamann3, Jonathan Eroms3, Dieter Weiss3, Christoph Drexler3, Sergey Ganichev3, Harald Schneider1 and Manfred Helm1,2

1Helmholtz-Zentrum Dresden-Rossendorf, P.O. Box 510119, 01314 Dresden, Germany 2Technische Universität Dresden, 01062 Dresden, Germany 3Universität Regensburg, 93040 Regensburg,

Germany

[email protected]

References [1] T. Müller et al., Nature Photon. 4, 297

(2010). [2] L. Vicarelli et al., Nature Mat. 11, 865

(2012). [3] J. Yan et al., Nature Nanotechnol. 7,

472 (2012). [4] R. Mendis et al., IEEE Antennas and

Propag. Lett. 4, 85 (2005). [5] S. Winnerl et al., Appl. Phys. Lett. 73,

2983 (1998). [6] M. Mittendorff et al. Appl. Phys. Lett.

103, 021113 (2013).

Figures

Figure 1: Design of the detector: The graphene flake is in the blue box in the center, the antenna is connected to the flake by an interdigitated structure.

Figure 2: Signal of two FEL pulses with a temporal delay of 500 ps at a wavelength of 42 µm.


rowing graphene on polycrystalline copper foils by ultra-high vacuum chemical vapor deposition

NICLAS S. MUELLER Freie Universitaet Berlin Germany

The growth of graphene by chemical vapour deposition (CVD) on polycrystalline copper foils has become very popular owing to its scalability, high yield, inexpensiveness and suitability for industrial implementation [1, 2]. A detailed understanding of underlying growth mechanisms and the influence of growth parameters is crucial to obtain a quality of CVD graphene comparable to that of exfoliated graphene. The CVD growth process is usually performed at atmospheric pressure (APCVD), or at low pressures down to 10-3 mbar (LPCVD). Reports of grapheme growth on copper in the high- and ultrahigh vacuum regime (UHV-CVD) are rare [3-7] and often utilize difficult growth processes, such as thermal cycling [3, 7] or ion irradiation [4] to obtain sufficient grapheme coverage (though these are currently limited to nanometer-scale domains). However the growth of graphene at UHV pressures benefits from low contamination before and during the growth process. It is therefore of high interest for studying fundamental growth mechanisms. Furthermore it enables the investigation with in-situ techniques, such as low energy electron microscopy and diffraction, which have been successfully used for studying graphene growth on ruthenium [8] and platinum [9]. We show that monolayer graphene, with a low structural defect density, can be grown on polycrystalline copper foils with ultra-high vacuum chemical vapor deposition using acetylene as a carbon precursor. Our experimental results demonstrate that acetylene, compared to methane, is more suitable as a carbon precursor in this pressure regime, which we attribute to a

higher adsorption energy on a copper surface. A simple one step growth process is demonstrated, in which the copper foil is exposed to acetylene isothermally at 900°C. Partial pressures are monitored with in-situ quadrupole mass spectroscopy, while chamber pressures during growth are maintained below 10-6 mbar with controlled partial pressures of acetylene. The controlled growth of graphene domains and continuous graphene film formation is demonstrated with representative data from Raman imaging, scanning electron microscopy and electron backscattering diffraction. A systematic study of the influence of growth parameters on the grapheme growth, such as growth time, growth pressure, effects of additional hydrogen, is presented. Investigating the time dependence of the graphene area coverage at fixed temperature and pressure enables us to propose a kinetic growth model, which covers all stages of growth (Figure 1a-c). A dependency of the growth kinetics on the surface orientation of the copper grains is observed and attributed to the surface orientation dependent adsorption energy of acetylene on copper. Confocal Raman spectroscopy and mapping is used to characterize the properties of the as-grown graphene after transfer to an adequate substrate and proves that the graphene domains are monolayer with low structural defect density (Figure 1d-f). Furthermore we show that the graphene domains grow into a continuous film by increasing the partial pressure of acetylene during growth. A manuscript is currently submitted for publication.

G


Niclas S. Mueller1, Anthony J. Morfa1, Daniel

Abou-Ras2, Michael Giersig1 1 Freie Universitaet Berlin, Department of Physics, Arnimallee 14, D-14195 Berlin, Germany 2 Solar Energy Research, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin,

Germany [email protected]

References [1] X. S. Li, et al., Science, 5932 (2009)

1312-1314.Phys. Rev. Lett. 111, 065502 (2013).

[2] T. Kobayashi, et al., Applied Physics Letters, 2 (2013).

[3] L. Gao, et al., Nano Letters, 9 (2010) 3512-3516.

[4] A. J. Martinez-Galera, et al., Nano Letters, 9 (2011) 3576-3580.

[5] Z. R. Robinson, et al., Physical Review B, 23 (2012).

[6] L. Zhao, et al., Solid State Communications, 7 (2011) 509-513.

[7] T. Niu, et al., Journal of the American Chemical Society, 22 (2013) 8409-8414.

[8] P. W. Sutter, et al., Nat Mater, 5 (2008) 406-411.

[9] P. Sutter, et al., Physical Review B, 24 (2009) 245411.

Figures

Figure 1: Characterization of UHV-CVD grown graphene by scanning electron microscopy (SEM) and Raman imaging. (a-c) SEM images of graphene domains of (a) round and (b) rectangular shapes for different growth durations and (c) evaluation of the time dependence of the area coverage (scale bars: 2 µm). (d) Raman map of the 2D/G peak height ratio and (e) D/G height ratio of a transferred, continuous graphene film (excitation with 532 nm). (f) Typical Raman spectrum of a transferred graphene domain (inset shows 2D peak fit with a single- Lorentzian curve).


pplication of Reduced Graphene Oxide Based Hybrid Functional Nanomaterials in Vapour Sensors for Human Health Monitoring

SANANDA NAG European University of Brittany France

Cancer kills more than seven million people every year. The current 5-year survival rate for lung cancer is 15 %, but this rate may rise to 49 % if the cancer could be diagnosed when it is still localized [1]. Since the survival of cancer patients depends on early detection of tumour cells, developing technologies applicable for rapid detection of carcinoma is a challenge for the researchers. Breath testing has emerged as a noninvasive technique for anticipated diagnosis of lung cancer, as the breath extract of lung cancer patients are found to display elevated levels of several volatile organic compounds (VOC) such as C4 to C20 monomethylated alkanes, in addition to certain benzene derivatives [2]. Clearly, the invention of a fast, reliable, economic and portable technique is highly required before breath testing become a clinical reality. Nanomaterial based sensor arrays can fulfill all these requirements and can form a solid foundation for identification of disease related VOC patterns in exhaled breath [3]. In the present study, a novel chemo-resistive vapour sensor, comprising of functionalized β cyclodextrin-reduced graphene oxide hybrid transducer has been developed. This hybrid functional material can exploit the combined benefits of high specific surface and good electrical conductivity of graphene as well as host-guest inclusion complex formation ability and variable selective chemical modification of β cyclodextrin. The sensing performance of electronic nose composed of 3 types of functionalized CD wrapped graphene sensor along with one pristine graphene and one pyrene adamantan linked graphene sensor were analysed, after being exposed to 11 selected cancer

biomarker VOC’s. The specific functionality of CD is found to be a predominant influencing parameter to tune the specific molecular selectivity of the sensors used in the electronic nose. The present study therefore opens a novel approach to achieve distinct selectivity of the chemical vapour sensors for specific VOC’s by employing functionalized CD modified reduced graphene oxide hybrid functional nano-material as chemical vapour sensor. Key Words: Cyclodextrin; Graphene; Nanomaterial; Biomarker; Vapour Sensor Sananda Nag1,2, Lisday Duarte4, Emilie

Bertrand4, Véronique Celton4, Mickaël Castro1, Veena Choudhary2, Philippe Guegan3, Jean-

François Feller1

1Smart Plastics Group, European University of Brittany (UEB), LIMATB-UBS, Lorient, France 2Centre for Polymer Science & Engineering, Indian Institute of Technology, Delhi, India 3Pierre & Marie Curie University, Functional Polymers Group, Yvry sur Seine, France 4Université d’Evry Val d’Essonne, Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement équipe Matériaux Polymères aux Interfaces, Evry, France

[email protected]

A


References [1] Y. E. Choi, J. W. Kwak, J. W. Park,

Sensors,10 (2010) 428-455. [2] B. Kumar, Y. T. Park, M. Castro, J. C.

Grunlan, J. F. Feller, Talanta ,88 (2012) 396-402.

[3] S. Chatterjee, M. Castro, J.F. Feller, J. Mater. Chem. B. 1 (2013) 4563.

Figures

Figure 1: (a) Instrumental set up of dynamic vapour sensing (b) Discrimination of mixture of VOC’s by PCA mapping

Figure 2: (a) Chemoelectrical response of a set of 6 sensors in a benzene (b) benzene selectivity of functionalized cyclodextrin wrapped graphene sensor


odulating the properties of MoS2 by plasma thinning and defect engineering

ZHENHUA NI Southeast University China

There is a great need for controlling the properties of two dimensional (2D) materials to fulfill the requirements of various applications.[1,2] Among the mostly investigated 2D layered materials, single and multilayer molybdenum disulphide (MoS2) are semiconductors with bandgap of ~1.2-1.8 eV, which make them promising candidates for optoelectronic applications. Here, we present our results on the modulation of the properties of MoS2 by plasma thinning and defect engineering. The electronic structures of two dimensional materials are strongly dependent on their thicknesses, i.e. single layer MoS2 is a direct bandgap material while bilayer MoS2 has an indirect bandgap structure. A simple, efficient, and nondestructive way to control the thickness of MoS2 is highly important for the study of thickness dependent properties as well as applications. We present layer-by-layer thinning of MoS2 nanosheets down to single layer by using Ar+ plasma. AFM, Raman, and photoluminescence (PL) spectra suggest that the top layer MoS2 is totally removed by plasma while the bottom layer is almost unaffected. We also demonstrate that this method can be used to prepare two dimensional heterostructures with periodical single and bilayer MoS2, by utilizing standard lithographic techniques. The plasma thinning is very reliable (almost 100% success rate), can be easily scaled up, and is compatible with standard semiconductor process to achieve heterostructures/patterns with nanometer sizes, which may bring out interesting properties and new physics.[3]

The PL quantum yield of as-prepared monolayer MoS2 has been found to be quite low, due to theformation of negative charged excitons (also named as negative trions) in the naturally n-doped MoS2. Structural defects have been observed both in pristine/as-grown MoS2 and electron beam/plasma irradiated samples.[4] The proper utilization of these defects to improve the optical properties of MoS2 is highly desirable. We report a strong PL enhancement of monolayer MoS2 through defect engineering and oxygen bonding. High resolution micro- PL and Raman images clearly reveal that the PL enhancement occurs at defects of MoS2. The PL enhancement at defect sites can be as high as thousands of times after considering the laser spot size. First principle calculations reveal a strong binding energy and effective charge transfer for oxygen molecule adsorbed on an S vacancy of MoS2. X-ray photoelectron spectroscopy further confirms the formation of Mo-O bonding. Our results provide a new route for modulating the optical properties of 2D semiconductors.[5 Zhenhua Ni1, Haiyan Nan1, Jinlan Wang1, Xinran

Wang2, Zheng Liang3 1 Department of Physics, Southeast University, Nanjing

211189, China 2 School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China 3 Graphene Research and Characterization Center, Taizhou Sunano New Energy Co., Ltd. Taizhou 225300, China [email protected]

M


References [1] Ni ZH, Ponomarenko LA, Nair RR, Yang

R, Anissimova S, Grigorieva IV, Schedin F, Shen ZX, Hill EH, Novoselov KS, Geim AK On resonant scatterers as a factor limiting carrier mobility in grapheme Nano Letters 10, 3868-3872 (2010)

[2] Zhan D, Yan JX, Lai LF, Ni ZH, Liu L, Shen ZX Engineering the electronic structure of grapheme Advanced Materials 24, 4055-4069 (2012)

[3] Liu YL, Nan HY, Wu X, Pan W, Wang WH, Bai J, Zhao WW, Sun LT, Wang XR, Ni ZH* Layer-by Layer Thinning of MoS2 by Plasma. ACS Nano 7, 4202, (2013)

[4] Qiu H, Xu T, Wang ZL, Ren W, Nan HY, Ni ZH, Chen Q, Yuan SJ, Miao F, Song FQ, Long G, Shi Y, Sun LT, Wang JL, Wang XR Hopping Transport through Defect-induced Localized States in Molybdenum Disulfide Nature Communications 4,2642 (2013)

[5] Nan HY, Wang ZL, Wang WH, Liang Z, Chen Q, He DW, Zhao WW, Miao F, Wang XR, Wang JL, Ni ZH* Strong photoluminescence enhancement of MoS2 through defect engineering and oxygen bonding Submitted.

Figures

Figure 1: Plasma thinning of MoS2: The electron diffraction patterns and high resolution TEM image of plasma-thinned single layer MoS2, and the Raman image of periodical 1/2/1/2/1 … MoS2 heterostructure obtained by plasma thinning and electron beam lithography.

Figure 2: PL enhancement of MoS2 through defect engineering and oxygen bonding. High resolution micro- PL and Raman images, as well as AFM, clearly reveal that the PL enhancement occurs at cracks/defects of MoS2.


uperb Electrocatalytic Activity for the Oxygen Reduction Reaction at N-doped CNT-Graphene Composite Electrodes

NIKAN NOORBEHESHT University of Sydney Australia

An electrocatalyst for the oxygen reduction reaction (ORR) is crucial in fuel cells and vital to the development of advanced electrochemical devices such as metal-air batteries. Replacing the expensive noble metal catalysts, which still only offer limited service life, with cheap and readily available materials for ORR applications is arguably the most important issue facing these technologies. Recently, nitrogen (N) doped carbon nano-materials have shown promise as Pt-free catalysts for ORR[1-4]. Herein, two kinds of N-doped carbon nanotube/graphene composites were developed: (i) N-CNT-N-RGO is synthesized from a pre-doped carbon nanotube and graphene; (ii) N-CNT-RGO is synthesized by first preparing a carbon nanotube/graphene oxide composite and then N-doping applied. X-ray photoelectron spectroscopy (XPS) reveals that although the overall nitrogen content of N-CNT-RGO (~ 8%) surpasses that of N-CNT-N-RGO (~5%), the ratio of graphitic N (nitrogen in graphene basal plane) to pyridinic N in N-CNT-N-RGO (0.87) is higher than that of N-CNT-RGO (0.64). Raman spectroscopy data also confirm the prevalence of nitrogen bonding contributions into the graphitic basal plane in NCNT- N-RGO, which is the most efficient type of doping for ORR enhancement. Electrochemical tests showed that while the N-CNT-RGO exhibits high catalytic activity toward the ORR and favors a close four-electron pathway, the N-CNT-N-RGO operates at significantly higher current density and delivers superior electrocatalytic performance for the ORR with 100% selectivity for complete four electron reduction of oxygen in alkaline aqueous solution compared to a

commercial Pt/C catalyst. Furthermore, the N-CNT-N-RGO demonstrates remarkable tolerance to methanol, thereby avoiding the crossover effect when operated in a direct methanol fuel cell (DMFC). It also shows highly stable performance with 93% relative current retention under 5000 continuous cycling test, which can be compared with the loss of more than 40% of the cathodic current in commercial cells (Pt/C catalyst) operated under the same conditions. The extremely high electrocatalytic activity and durability of N-CNT-N-RGO indicate that this new catalyst opens up new avenues for achieving a wide variety of cheap and commonly available metal-free catalysts for broad applications across the areas of heterogeneous catalysis, sensor, photonic catalysis, hydrogen production, and metal-air batteries. Nikan Noorbehesht1, Anthony F. Hollenkamp 2,

Stephen Hawkins3, Andrew T. Harris1 and

Andrew I. Minett1 1Laboratory for Sustainable Technology, University of Sydney, NSW 2006, Australia 2CSIRO Energy Technology, Clayton, VIC 3168, Australia 3CSIRO Materials Science and Engineering, Clayton, VIC 3168, Australia [email protected]

S


References [1] Gong, K., Du, F., Xia, Z., Durstock, M.

and Dai, L., Science, 323 (2009) 760-764. [2] Lin, Z., Waller, G., Liu, Y., Liu, M. and

Wong, C.-P., Advanced Energy Materials, 2 (2012) 884-888.

[3] Ma, Y., Sun, L., Huang, W., Zhang, L., Zhao, J., Fan, Q. and Huang, W., The Journal of Physical

[4] Chen, P., Xiao, T.-Y., Qian, Y.-H., Li, S.-S. and Yu, S.-H., Advanced Materials, 25 (2013) 3192-3196.

Figures

Figure 1: (a) RDE voltammograms in O2 saturated 0.1 M KOH solution at room temperature (electroderotation speed 1600 rpm, sweep rate 10 mV/s) for the N-CNT, N-RGO, N-CNT-RGO, N-CNT-N-RGOand Pt/C. (b) Linear-scan voltammetry (LSV) of N-CNT-N-RGO and Pt/C in O2 saturated 0.1 M KOH, before (solid line) and after (dash line) a continuous potentiodynamic swept for 5000 cycles at room temperature, scan rate: 10 mV/s, electrode rotating rate: 1600 rpm.


owards integrated atomically smoothed graphene nanoribbons devices

MARC NUÑEZ CEMES CNRS France

The growing interest in graphene stems from the specific properties gathered by this atomically thin sheet of carbon atoms that address a number of technical or scientific locks in fields as diverse as mesoscopic physics, transparent electrodes, electromechanical and optical devices, heat transport, sensors, etc... Graphene offers a direct and stable access to a one-atom thick 2D membrane in which and atomically precise structures can be engineered provided that its lateral confinement is carefully controlled. It appears that the topology and structural quality of this patterning (e.g. nanoribbons) from the mesoscopic to the molecular scale strongly influences the corresponding electronic properties. In spite of numerous approaches and significant progress, graphene patterning at multiple length scales down to a few nanometers while preserving its pristine quality is one of the main challenges.

Indeed, several graphene nanopatterning techniques, yielding for example nanoribbons, have been investigated so far, which rely on e-beam or optical lithography followed by reactive ion etching, [1]direct milling using a focused ion beams, [2] chemical cutting with free or guided catalytic nanoparticles.[3] While well-suited for multiscale patterning of arbitrary designs, these techniques have been shown to introduce contamination (resists, implanted ions, metal adsorbates,....) and to induce severe amorphization along the graphene edges which practically prevents the production of nanometer-scale features in pristine graphene. Alternative top-down approaches to produce graphene ribbons consist in tearing graphene, possibly along preferential crystallographic directions by

extensive ultrasonication [3]or carbon nanotube opening.[4] Recent bottom-up approaches to graphene nanoribbons have reached the ultimate edge precision by polymerizing polyaromatic precursors into ribbons of atomically precise edge structure.[5] However, transport properties can only be studied by scanning tunneling microscopy as device production from these extremely narrow ribbons still face challenges such as contact resistance and multi-scale integration.

In this context, we propose to combine low energy electron beam and chemical etching to design arbitrary graphene patterns of sub-10 nm feature size and crystalline edges. Our approach is compatible with ultra-high vacuum (UHV) environment and applicable to connected device fabrication.

Our graphene nanopatterning technique consists in inducing a localized chemical etching with a focused electron beam. The electron beam activates a chemical reaction between graphene and the injected gas. This allows low energy electrons to produce holes in graphene lattice well below the 86 kV knock-on energy threshold of electron beam etching in vacuum.[6] The programmed raster scanning of the SEM electron beam allows etching an infinite variety of arbitrary patterns. [7, 8]

The presentation will detail the parameters controlling the etching performances such as the dose analysis, influence of the substrate or sample thickness and will show that it easily enables to etch multimicrometer-long cuts as well as sub-10 nm features in monolayered graphene (Fig. 1a). This ability to multi-scale patterning is a key feature to the integration of quantum devices to mesoscopic sizes that allows macroscopic transport measurements.

We will present spherical aberration corrected transmission electron microscopy

T


investigation of cut edges and graphene nanoribbons and show that the edges produced by our technique appear to be crystalline and atomically smooth (Figs. 1b-c). The amorphous carbon pollution induced during the patterning is almost absent and graphene in direct proximity of the pattern edges is free of lattice amorphization.

Our approach has also been adapted to device configuration. To this end, micromechanically exfoliated graphene is deposited on silica substrates perforated with 1x5 µm pools (Figs. 1d-e). The etching procedure has been successfully adapted to this partially suspended graphene and graphene nanoribbons with length of several hundred of nanometers and width below 50 nm are routinely produced (Fig. 1f).

Finally transport measurements performed on suspended graphene nanoribbons with crystalline edgeswill be discussed. Marc Nuñez, Sébastien Linas, Miguel Rubio Roy,

Caterina Soldano, Philippe Salles, Olivier Couturaud and Erik Dujardin

NanoSciences Group, CEMES CNRS UPR 8011 Toulouse, France

[email protected]

References [1] Chen Z et al. Physica E (2007) 40, 228; X.

Wang et al., Phys, Rev. Lett. (2008) 100, 206803; M. Y.Han, J. C. Brant, P. Kim, Phys. Rev. Lett. (2010) 104, 056801

[2] J.F. Dayen et al. Small (2008) 4, 716; M. C. Lemme, et al. ACS Nano (2009) 3, 2674

[3] Campos et al. Nano letters (2009) 9, 2600 [4] H. J. Dai et al., Nature (2009) 458, 877; J. M.

Tour et al., Nature (2009) 458, 872 [5] K. Mullen, R. Fasel, et al. Nature (2010) 466,

470; Koch, M., et al. Nature Nanotech. (2012) 7, 713

[6] Girit et al. Science (2009) 323, 1705 [7] S. Linas, M. Nunez, M. Rubio-Roy, C.

Soldano, R. Arenal, P. Salles, O. Couturaud, A. Miranda, E. Dujardin, in press

[8] M. Nunez, M. Rubio-Roy, R. Arenal, P. Salles, E. Dujardin, in preparation


Figures

Figure 1: (a) Transmission electron microscopy (TEM) image of a 320x8 nm graphene nanoribbons (GNR) etched in monolayer graphene using gas-assisted electron beam etching. (b) Spherical aberration corrected TEM image of a GNR edge showing the crystalline and atomically smooth rim devoid of amorphous carbon adsorbates. (c) Intensity profile measured between the arrows in (b). The distance between successive interference spots (1.5Å) is commensurate with either zig-zag or armchair edge. (d) Schematic representation of a GNR device partially suspended above a micrometric pool etched in the Si/SiO2 substrate. (e) Optical micrograph of micromechanically exfoliated grapheme deposited on top of Si/SiO2 substrates patterned with arrays of 250 nm deep, 1x5 µm pools.(f)Scanning electron micrograph image of a 200x25 nm GNR etched in partially suspended graphene.[7,8]


raphene Hybrid Materials for Energy Storage and Actuator Devices

IL-KWON OH Korea Advanced Institute of Science and Technology (KAIST) Republic of Korea

In this study, we report several novel routes via microwave irradiation to synthesize graphene flakes, metal nanoparticle-decorated graphenes, and graphene-based 3D carbon nanostructures[1] for energy storage and actuation devices. The discovery of mono-layered graphene, achieved through an experiment by Geim to synthesize a free standing 2D lattice material, garnered global attention due to its outstanding mechanical, electrical and thermal properties. These properties have been exploited in a wide range of applications including supercapacitors, actuators, sensors, reinforcing materials in high performance polymer composites and hydrogels, etc. As another approach, researchers have tried integrating carbon nanotubes with graphene to obtain synergetic effects in applications such as actuators, super-capacitors, mechanically compliant films, fuel cell batteries, solar cells, nano-composites and biomedical devices. Herein we report a simple microwave-based technique to synthesize graphene-CNT-M(Fe, Ni, Co, Pd) nano-hybrid structures based on organometallic materials and solvent-based metal catalysts. Our proposed method is not only fast, but can also yield high volume production of the functionalized nano-hybrids at a fraction of the cost of CVD methods. Recently, we succeed in synthesizing bio-inspired hierarchical graphene-nanotube-iron three-dimensional nanostructure[2] as an anode material in lithium-ion batteries. The nanostructure comprises of vertically-aligned carbon nanotubes grown directly on graphene sheets along with shorter branches of carbon nanotubes stemming out from both the graphene sheets and the

vertically-aligned carbon nanotubes. This bio-inspired hierarchical structure provides a three-dimensional conductive network for efficient charge-transfer and prevents the agglomeration and re-stacking of the graphene sheets enabling Li-ions to have greater access to the electrode material. In addition, functional iron-oxide nanoparticles decorated within the three-dimensional hierarchical structure provides outstanding lithium storage characteristics, resulting in very high specific capacities. The anode material delivers a reversible capacity of ~1024 mAhg-1 even after prolonged cycling along with a coulombic efficiency in excess of 99%, which reflects the ability of the hierarchical network to prevent agglomeration of the iron-oxide nanoparticles. Furthermore, the some hybrids are magnetically active and can be used in a wide range of applications including supercapacitors, lithium ion batteries, shape memory and electroactive artificial muscles. We try to produce novel high-performance electroactive polymers or artificial muscles[3-6] based on graphene and graphene-based hybrid materials. We will show some demonstrations of electroactive polymer actuators and discuss the possibility of real applications such haptic and reactive devices, soft robots, energy harvesters and braille display. Il-Kwon Oh

Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro (373-1 Guseong-dong), Yuseong-gu, Daejeon 305-701, Republic of Korea

[email protected]

G


References [1] Vadahanambi Sridhar, Hyun-Jun Kim,

Jung-Hwan Jung, Changgu Lee, Sungjin Park, and Il-Kwon Oh*, Defect-Engineered Three-Dimensional Graphene-Nanotube-Palladium Nanostructures with Ultrahigh Capacitance, ACS Nano, Vol. 6, No. 12, 2012.11, pp. 10562-10570.

[2] Si-Hwa Lee, Vadahanambi Sridhar, Jung-Hwan Jung, Kaliyappan Karthikeyan, Yun-Sung Lee, Rahul Mukherjee, Nikhil Koratkar, and Il-Kwon Oh*, Graphene-Nanotube-Iron Hierarchical Nanostructure as Lithium Ion Battery Anode, ACS Nano, 2013, 7(5), pp 4242-4251.

[3] Jin-Han Jeon, Ravi Kumar Cheedarala, Chang-Doo Kee and Il-Kwon Oh*, Dry-type Artificial Muscles Based on Pendent Sulfonated Chitosan and Functionalized Graphene Oxide for Greatly Enhanced Ionic Interactions and

Mechanical Stiffness, Advanced Functional Materials, Vol. 23, No. 48, 2013.12, pp. 6007-6018.

[4] Mahendran Rajagopalan and Il-Kwon Oh*, Fullerenol-Based Electroactive Artificial Muscles Utilizing Biocompatible Polyetherimide, ACS Nano, Vol. 5, No. 3, 2011.03, pp. 2248-2256.

[5] Choonghee Jo, David Pugal, Il-Kwon Oh*, Kwang J Kim, Kinji Asaka, Recent Advances in Ionic Polymer-Metal Composite Actuators and Their Modeling and Applications, Progress in Polymer Science, Vol. 38, No. 7, 2013.7, pp. 1037-1066.

[6] Jaehwan Kim, Jin-Han Jeon, Hyun-Jun. Kim, Hyuneui Lim, and Il-Kwon Oh*, "Durable and Water-Floatable Ionic Polymer Actuator with Hydrophobic and Asymmetrically Laser-Scribed Reduced Graphene Oxide Paper Electrodes", ACS Nano, Vol. 8, No. 3, 2014, pp. 2986-2997.

Figures

Figure 1: Bio-inspired hierarchical graphene-based 3D carbon nanostructures for anode electrode in

lithium ion batter


new route towards low temperature production of continuous graphene film

HANAKO OKUNO CEA France

One of the tremendous unique materials of our century, graphene, has already proved its excellent optical, mechanical, chemical and electrical properties on the micro scale level [1, 2]. According to the literature database graphene obtained by micro mechanical cleavage has demonstrated the best performances such as highest mobility, strongest half integer hall effect even at room temperature and strongest mechanical properties. Thus, graphene opens up possibilities for new applications (flexible screens and displays, etc.) or promises to be better alternative material for improving existing applications (energy storage, transparent electrodes, super capacitors, etc.) [1,2]. Nevertheless graphene samples produced by scotch tape method suffice only for fundamental studies and cannot be used in industrial application because that technique has no reproducibility and there are difficulties to localize the flakes on the substrate and to control their size and shape [3]. For the large scale graphene production a chemical vapor deposition (CVD) technique is widely used nowadays to grow graphene films samples on different types of metal substrates. Mechanical and electrical characteristics of as-produced CVD graphene are typically lower compared to the properties of exfoliated graphene due to many reasons that are linked with the growth and transfer processes: small domain size, boundaries, wrinkles, scratches, damages etc [4]. Apparently, the main drawback of CVD method lies in necessity of additional graphene transfer process from metal to the insulating surface.

However, the preeminence of that method are its capability of large scale production, low cost and possibility to control the size, location and shape of as-produced graphene material. At the same time, the CVD technique can further be improved as the growth mechanism of graphene on metal is still not completely understood and CVD method has many parameters that are to be optimized, [5]. Therefore, at the present moment the CVD process is the most popular technique for industrial fabrication of graphene, so that to date there are many variations of CVD set-ups, differences in their recipes and also enormous substrate’s choices (metal type, thickness, crystallinity, surface preparation) [6]. In this report, we demonstrate new approach for the graphene film growth on platinum (Pt) thin film via modified CVD technique. Specific configuration of our CVD set-up allows us to perform the carbon deposition without hydrocarbons at the substrate temperature not higher than 700° C. Graphene formation occurs in a single CVD process, without breaking a vacuum, through the conversion step from continuous amorphous carbon film of uniform thickness (about 1-2 atomic layer). From the Fig. 1 one can see significant change of Raman spectrum corresponding to the graphene samples at different stages of our CVD process. Initial stage I corresponds to the formation of amorphous carbon film; for the stage II it is predominantly defective graphene film with small grains of nanometer size; and for the last stage III we obtain low defective uniform graphene film contained of big single crystal domains. Additionally, insets of fig. 1 show HR TEM images, which confirm the structural difference of as-produced graphene on the atomic scale between graphene samples of stage II and stage III. Similar SLG sheets

A


conversion from aromatic monolayers was recently observed on Cu surface [7]. However, that method contains 3 consecutive steps of additional preparation (one of which is electron irradiation) and final long annealing step at 830°C in UHV. Our work on developing a low temperature CVD set-up provides a new route of large-scale graphene film synthesis, which can be performed in the less costly CVD process, and does not required UHV and high temperature. That process guarantees to always form a continuous uniform film of graphene. Detailed investigations of the growth mechanism on thin Pt film for our graphene sample were analyzed by Raman spectroscopy and transmission electron microscopy (Fig. 1). By combination of different characterization techniques it was confirmed that crystallinity, structural quality and also electrical characteristics of our sample are similar to those of typical CVD graphene. Anastasia Tyurnina1, Jean Dijon1, Hanako Okuno2

1 CEA LITEN DTNM, 2 CEA INAC-SP2M 17 rue de Martyrs, Grenoble cedex 9, France

[email protected]

References [1] A.H. Castro Neto, F. Guinea, N.M.R.

Peres, K.N. Novoselov, A.K. Geim, Rev. Mod. Phys., 51 (2009) 109-162.

[2] Y. Zhang, L. Zhang and C. Zhou, Acc. Chem. Res., 46(10) (2013) 2329–2339.

[3] A. K. Geim, Science, 324 (2009) 1530-1534.

[4] H. S. Song, S. L. Li, H. Miyazaki, S. Sato, K. Hayashi, A. Yamada, N. Yokoyama & K. Tsukagoshi, Scientific Reports, 2 (2012) 337:1-6.

[5] N.C. Bartelt and K.F. McCarty, MRS Bulletin, 37(12) (2012) 1158-1165.

[6] C. Mattevi, H. Kim and M. Chhowalla, J. Mater. Chem., 21 (2011) 3324–3334.

[7] D.G. Matei, N.-E. Weber, S.Kurasch, S. Wundrack, M. Woszczyna, M. Grothe, T. Weimann, F. Ahlers, R. Stosch, U. Kaiser, A. Turchanin, Advanced Materials, 25(30) (2013) 4146–415.

Figures

Figure 1: Raman spectra of the carbon films at different stages of the graphene synthesis process. HR TEM images are also shown in the insets for the samples of stages II and III (in black frame is for the stage II; in purple frame is for the stage III). Complex study confirms that graphene sample of the last stage consists of low defective graphene film. Samples were transferred onto the silicon wafer for Raman investigation and onto carbon supported Cu TEM grid for TEM analysis


raphene research at ICN2

PABLO ORDEJÓN ICN2 & CSIC Spain

ICN2 is a research center focused on the discovery and exploration of the properties that arise in matter at the nanometer scale, and on the development of technologies that exploit these properties. These advances have the potential of changing virtually all aspects of our lives, by producing new technologies that will have an enormous economic and social impact.

ICN2 is turning research into marketable technologies with the expertise of over 200 scientists and technicians that put their work in the whole value chain, from fundamental research all the way to design, fabrication, and evaluation of nanotechnology-based devices. The Institute offers a full suite of advanced instruments available for research and innovation in fields such as energy, biosystems (medical and environmental), and information and communication technologies.

Our activities take place in a stimulating environment, close to Barcelona, where over 500 researchers and 200 technicians work on materials science and micro/nanotechnologies within the Barcelona Nanotechnology Cluster - Bellaterra (http://www.bnc-b.net), a cluster comprising several public and private research centers, the Universitat Autónoma de Barcelona, and the ALBA Synchrotron.

Graphene research is one of the cornerstones of the work developed by ICN2’s multidisciplinary groups. The Institute promotes collaboration among scientists from diverse backgrounds (physics, chemistry, biology, engineering) to conduct basic and applied research, always seeking interaction with local and global industries. In this sense, graphene is

one of the basic tools that ICN2 uses to develop its science and technology. The ultimate goal is to produce devices for real life applications, which can be only developed from the discovery and deep knowledge of the most fundamental aspects of the materials and nanostructures.

Graphene research at ICN2 encompasses four major areas:

Computational simulations. Theoretical scientists and experimentalists work together to create, validate, and refine models that predict the behavior of graphene.

Production methods. ICN2 conducts research to achieve higher levels of control over size, shape, and layers and to produce customized graphene-based materials.

Characterization and testing. ICN2 research groups define properties of graphene, measure electronic and quantum phenomena in graphene-based devices, and test how graphene reacts to realistic external forces.

Device design, fabrication and evaluation. Graphene-based biosensors, solar cells, supercapacitors, and information and communication devices are being developed.

More information can be found at www.icn2.cat/graphene

G


Pablo Ordejón1,2 1ICN2 - Institut Català de Nanociència i Nanotecnologia Campus UAB, 08193 Bellaterra, Spain 2CSIC - Consejo Superior de Investigaciones Científicas; Edifici ICN2, Campus UAB, 08193 Bellaterra, Spain

[email protected]

Figures

Figure 1: Simulated charge mobility of polycrystalline graphene. (D.V. Tuan, J. Kotakoski, T. Louvet, F. Ortmann, J. C. Meyer, and S. Roche, Nano Letters, 13(4), 1730–1735 (2013))

Figure 2: Top: Scanning Tunnelling Microscopy image of a triangular graphene nanoisland grown on a Ni(111) surface; Middle: Suspended graphene multiply connected; Bottom: Silicon chip covered with graphene.


nterferometric TEM characterization of graphene materials

LUCA ORTOLANI CNR IMM-Bologna Italy

Graphene-based materials emerged as one of the most important research areas of material science and nano-technology [1]. Knowing the morphology and atomic structure of graphene is essential, since its electronic, optical and chemical properties depend on the three-dimensional folds and buckles of the 2D lattice [2], and upon the stacking geometry of the layers [3]. Transmission electron microscopy (TEM), combining high-resolution imaging and spectroscopy, represents a unique technique to resolve the structure as well as the composition of nano-materials with atomic resolution. In this contribution, we will review the potentialities provided by interferometric TEM techniques, like Geometric Phase Analysis (GPA) [4] and electron holography [5], to resolve the 3D shape of folds and buckles in graphene membranes [6] and to probe interlayer charges in graphene stacks [7]. Combining high-resolution TEM imaging and GPA with continuum elasticity theory and tight-binding atomistic simulations [8,9] allows us for a complete nano-scale geometrical and physical picture on the edge curvature and topography of folded graphene membranes, with different crystalline orientations. Theoretical predictions were validated by experimentally recovering the 3D topography of the folded graphene with sub-nanometre lateral resolution and height precision, analysing apparent strains in the high-resolution TEM images [6]. Using transmission electron holography [5], we further investigate the redistribution of electronic crystal charges in few graphene crystals [7] and individual monolayers,

mapping the effect of the Van der Waals interaction between graphene layers as the distance between the layers varies close to folded edges. We show that electronic density and internal electrostatic potential energy experienced by an electron passing through the crystal can be computed by ab-initio approaches based on Density Functional Theory, with high accuracy on single and multiple graphene layers. The computed phase-shift values are in very good agreement with the ones measured experimentally, strengthening the perspectives of the application of the technique, as well as of the computational approach, to more complicated and interesting systems, like doped and functionalized graphene layers. Luca Ortolani CNR IMM-Bologna, Via Gobetti, 101, Bologna, Italy

[email protected] References [1] C. Soldano et al. Carbon 48, 2127

(2010). [2] J. Feng et al. Phys. Rev. B 80, 165407

(2009). [3] A. H. Castro Neto et al. Rev. Mod.

Phys. 81, 109 (2009). [4] M.J. Hÿtch et al. Ultramicroscopy 74

(1998) 131. [5] H. Lichte et al. Ann. Rev. Mat. Res., 37,

(2007) 539. [6] L. Ortolani et al. Nano Lett. 12, (2012)

5207. [7] L. Ortolani et al. Carbon, 49, (2011)

1423. [8] E. Cadelano et al. Phys. Rev. Lett. 102

(2009) 235502. [9] E. Cadelano et al. Phys. Rev. B 81

(2010) 144105.

I


ntrinsic ferromagnetism induced by hydrogen adsorption on graphite surfaces

JUAN J. PALACIOS Universidad Autónoma de Madrid Spain

A remarkable theoretical prediction for graphene is that, in theory, it can be permanently magnetized by the adsorption of H atoms (see figure). Unfortunately, this will only be possible if the adsorption is selectively realized in such a way that all H atoms occupy the same sublattice so that the contributions of the H-induced local magnetic moments add up due to the expected ferromagnetic coupling in this situation. Motivated by a recent experiment, I will show that such selectivity can be naturally achieved on the graphite surface. Due to the sublattice broken symmetry on the surface, a spontaneous arrangement of the hydrogen atoms where all end up adsorbed on the same sublattice takes place at room temperature in a time scale of minutes. First-principles calculations combined with kinetic Monte Carlo simulations and model Heisenberg-like Hamiltonians derived from them give a complete account of the emergence of this novel ferromagnetism.

Juan J. Palacios¹, Mohammed Moaied¹, Jose V.

Alvarez¹, Maria J. Caturla²

¹ Universidad Autónoma de Madrid, Campus de Cantoblanco, Madrid 28049, Spain

² Universidad de Alicante, Campus de San Vicente del Raspeig, Alicante 03690, Spain [email protected] References [1] Iván Brihuega, Miguel M. Ugeda,

Mohammed Moaied, Héctor González-Herrero, María J. Caturla, José M. Gómez-Rodríguez, Juan J. Palacios, submitted to ACS Nano.

Figures

Figure 1: Intrinsic magnetism induced by H adorption on graphene.

I


New Industrially Relevant Solvent Exfoliation Route to Graphene

KEITH R. PATON Thomas Swan & Co. Ltd.& CRANN UK & Ireland

Due to its ultra-thin, 2-dimensional nature and its unprecedented combination of physical properties, graphene has become the most studied of all nano-materials. In the next decade graphene is likely to find commercial applications in many areas from high-frequency electronics to smart coatings. Some important classes of applications, such as printed electronics, conductive coatings and composite fillers, will require industrial-scale production of defect-free graphene in a processable form. For example, graphene is likely to be used as a low cost electrode material in applications such as solar cells, batteries and sensors. Such electrodes will almost certainly be produced by solution-coating and so will require large quantities of graphene in the form of liquid suspensions, inks or dispersions. Thus, liquid exfoliation of graphene will become a critically important technology in the near future[1]. However, no scalable method exists to give large quantities of graphene that is also defect free. For example, while oxidative exfoliation of graphite can potentially give large quantities of graphene-like nanosheets, such graphene oxide is typically defective. Alternatively, sonication of graphite [2], or indeed other layered compounds [3], in certain stabilising solvents gives defect-free nanosheets. However, the scalability of the latter process is limited by the use of sonication as an energy source. Thus, solution-processing methods tend to display either high production rates or low defect contents, but not both. A detailed literature survey shows that no papers describe production rates above 0.4 g/h coupled with Raman D:G intensity ratios (a

measure of defect content) below 0.65. In fact 80% of the papers surveyed had production rates below 0.04 g/h, far too low for commercial production. One possible solution would be to find a scalable method of exfoliation which, coupled with the use of stabilising solvents, could lead to large scale graphene production. In collaboration with CRANN (Centre for Research on Adaptive Nanostructures and Nanodevices) at Trinity College Dublin we have developed a new method for exfoliating graphite in liquids to give large-volume dispersions of graphene flakes. We will describe the production of 300 L of graphene dispersion at concentration of ~0.1 mg/ml and production rates of ~5g/hr. We have developed a simple model for the exfoliation mechanism and fully characterized the scaling behaviour of the graphene production rate with processing parameters such as batch volume and processing time. TEM, XPS and Raman spectroscopy show the exfoliated flakes to be thin, unoxidised and defect-free. This method produces pristine, high conductivity graphene nanoplatelets and can also be used to exfoliate BN, MoS2 and a range of other layered crystals. Keith R. Paton1,2, Eswaraiah Varrla2, Valeria

Nicolosi2, Jonathan N. Coleman2 1 Thomas Swan & Co. Ltd., Consett, UK 2 CRANN, Trinity College Dublin, Dublin 2, Ireland [email protected]

A


References [1] Nicolosi, V., Chhowalla, M., Kanatzidis,

M. G., Strano, M. S. & Coleman, J. N. Liquid Exfoliation of Layered Materials. Science. 340, 1226419–1226419 (2013).

[2] Hernandez, Y. et al. Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions. J. Am. Chem. Soc. 131, 3611–20 (2009).

[3] Coleman, J. N. et al. Two-dimensional nanosheets produced by liquid exfoliation of layered materials. Science 331, 568–71 (2011).

Figures

Figure 1: Photo showing large volumes of defect-free graphene dispersions able to be produced.

Figure 2: A selection of TEM images (collected from samples prepared with a range of processing parameters) of representative graphene flakes. In all cases, the scalebar is 500 nm. K-M) Evidence for monolayer production. Some of the flakes observed in the survey are clearly monolayers. The flakes observed in K had a diffraction pattern (L) which had more intense inner spots (M). This is a clear fingerprint of a graphene monolayer.


raphene and 2d crystals research @ Istituto Italiano di Tecnologia, Graphene Labs

VITTORIO PELLEGRINI Istituto Italiano di Tecnologia & NEST Italy

The Italian Institute of Technology (IIT) is a Foundation established in 2003 jointly by the Italian Ministry of Education, University and Research and the Ministry of Economy and Finance to promote excellence in basic and applied research. The research plan of the institute focuses on Humanoid technologies and Robotics, Neuroscience and Cognition, Nanotechnology and Materials. The Institute has a staff of about 1200 people, the central research lab being located in Genoa. IIT has a large experience with the management of large research projects and has been involved in more than 96 EU funded projects in the last 7 years.

IIT is organized in 9 Departments located in the IIT headquarter in Genoa and 10 Research Centers scattered in the country. The Departments that are actively involved in graphene and 2d crystals research are Nanochemistry (NACH), Nanobiotechnology (NBT) and Nanostructures (NAST). In addition, the IIT research Center Nanotechnology Innovation in Pisa is involved in bottom-up production of graphene by CVD.

Since September 2013 our graphene research is collected under the umbrella of the IIT Graphene Labs (http://graphene.iit.it), which currently involves 17 senior researchers and 8 post-docs working on different aspects of graphene technology. We are currently strengthening our activities through post-doctoral and PhD recruitments in the field of graphene productions, nanocomposites and energy applications. We expect to reach a total staff of more than 30 people by the end of 2014 with further expansion in 2015.

IIT Central Research Lab, a 30,000m2 facility in Genoa, is equipped with state-of-the-art laboratories for robotics, nanoscience and neuroscience. Relevant to the flagship activities are the colloidal chemistry synthesis for nanoparticle production, the electron microscopy centre, the nanocomposite laboratories, the UHV Low temperature scanning tunnelling microscopy; the optical spectroscopy facility from femtosecond to continues wave (cw), all wavelengths with optical resolution down to 30nm), class 100 clean room for nanofabrication (600m2) and the recently established graphene production and printing lab.

IIT Graphene Labs is actively involved in realising scientific and technological targets in the field of energy (e.g. charge/energy transfer at the graphene-nanocrystal interface for energy conversion, graphene-based Li batteries and supercapacitors), material production (e.g. CVD and solution processing) and deposition, and bio-nanotechnology (e.g. biocompatibility essays, biomolecule-graphene interaction). We will also have a strong effort in dissemination and technology transfer activities. In particular, the technology transfer program of IIT Graphene Labs is developing through specific agreements with companies. At the moment we have in place agreements with 5 companies on different aspects of graphene exploitation in the energy sector. Francesco Bonaccorso and Vittorio Pellegrini

Istituto Italiano di Tecnologia, IIT Graphene Labs, Genova (Italy)

[email protected]

G


esigning electronic properties of two-dimensional crystals through optimization of deformations

VITOR M. PEREIRA Graphene Research Centre & Department of Physics, National University of Singapore Singapore

One of the enticing features common to most two-dimensional electronic systems that, in the wake of (and in parallel with) graphene, are currently at the forefront of materials science research is the ability to easily introduce a combination of planar deformations and bending in the system. Since the electronic properties are ultimately determined by the details of atomic orbital overlap, such mechanical manipulations translate into modified (or, at least, perturbed) electronic properties. Graphene, in particular, on account of its exceptional range of elastic deformation, and peculiar electron-phonon coupling captured by the concept of a pseudo-magnetic field, has taught us that its intrinsic electronic properties can be molded by many more, and much richer, approaches that can be applied to 3D bulk solids. The ability to manipulate the local strain distribution in graphene opens the enticing prospect of strain-engineering its electronic and optical properties, as well as of enhancing interaction and correlation effects. This presentation will beging with the introduction of examples of how strain-engineered graphene can have richer spectral, transport, and optical properties, and the presentation of a summary of recent experimental work exploring some of these new avenues. At the core, a general-purpose optimization framework for tailoring physical properties of two-dimensional electronic systems by manipulating the state of local strain will be presented and

discussed. This new framework allows a one-step route from the design of specific functionalities and device behavior to their experimental implementation. As one example of its application, it will be shown how it efficiently answers the inverse problem of determining the optimal values of a set of external or control parameters (such as substrate topography, sample shape, load distribution, etc.) that result in a graphene deformation whose associated pseudomagnetic field profile best matches a prescribed target. Another example to be discussed is the typical problem of optimizing external parameters in order to make a graphene nanodevice perform with pre-defined target transport characteristics. The ability to address this inverse problem in an expedited way is one key step for practical implementations of the concept of two-dimensional systems with electronic properties strain-engineered to order. This presentation will try to convey our approach to it. Vitor M. Pereira, Gareth W. Jones, Dario A.

Bahamon Graphene Research Centre & Department of Physics. National University of Singapore

2 Science Drive 3. Singapore [email protected]

D


References [1] Gareth W. Jones, “Designing

electronic properties of two-dimensional crystals through optimization of deformations” (2014, submitted).

[2] G.X. Ni, H.-Z. Yang, W. Ji, et al., Advanced Materials 26, 1081 (2014).

[3] Z. Qi, D. A. Bahamon, V. M. Pereira, et al., Nano Lett. 13, 2692 (2013).

[4] V. M. Pereira and A. H. Castro Neto, Phys. Rev. Lett. 103, 046801 (2009).


igh-mobility h-BN/graphene/h-BN devices: zero-field electrical transport and quantum Hall criticality

SERGIO PEZZINI Universidad de Salamanca & Universitá degli Studi di Pavia Spain & Italy

We performed a wide experimental study on multi-terminal field-effect devices made of exfoliated graphene flakes (both single and double-layer) sandwiched between two thin hexagonal boron nitride (h-BN) flakes. The use of h-BN allowed reaching high mobility (µ ≈ 4 x 104 cm2V-1s-1 for the bilayer sample shown in Fig.1) and provided exceptional stability to the samples, which physical properties are preserved over long time. The samples were prepared at HQGraphene (Groningen) using a dry transfer method [1]. h-BN flakes were first mechanically exfoliated with scotch tape and deposited onto a p-doped Si wafer with a 500 nm-thick SiO2 layer on top. Graphene flakes were exfoliated on top of a transparent mask consisting in a stacking of glass slide, scotch tape and methyl/n-butyl methacrylate copolymer. Suitable single and few-layer graphene flakes were identified with an optical microscope in reflection mode. Using micromanipulators, the selected graphene flake was deposited on top of a thin (10-60 nm) h-BN one by lowering the polymer side of the mask onto the SiO2 wafer, while it was heated up to 75-100° C. The polymer melts and it is released from the mask to the substrate, while the graphene flake adheres to the h-BN by means of van der Waals forces. The polymer is then dissolved in organic solvents and an analogous transfer step is repeated for the uppermost h-BN flake. The first part of the work involved low-temperature (300 mK < T < 50 K) electrical transport measurements, performed with low-frequency ac lock-in technique. The

resistivity (conductivity) of a bilayer graphene device was measured in four-probe configuration, at zero-magnetic field, while varying the carrier density with a back-gate contact connected the underneath p-doped silicon wafer. Our main results are reported in Ref. [2]. We were able to identify a temperature-independent crossing point in the curves of resistivity as a function of back-gate voltage (Vg) (see Fig. 2), corresponding to a carrier density n ≈ 2.5 x 1011 cm-2, indicating a transition from insulating (dρ/dT < 0) to metallic behavior (dρ/dT > 0). By analyzing the temperature-dependence of the resistivity (conductivity) we characterized four different transport regimes, which are realized by tuning T and Vg over easily accessible ranges. Close to the neutrality point the low-T data (T < 10 K) are compatible with variable range hopping (VRH) with a characteristic exponent 1/3, i.e. transport is dominated by localized states. Above 10 K the resistivity grows linearly with T as expected from the parabolic energy-momentum relation, which translates into a constant density of states. At carrier density above the crossing point, the VRH exponent is renormalized to 1/2 as expected if Coulomb interaction is taken into account, while at higher temperature the data are compatible with ballistic transport, both for electrons and holes. The second part of our work involved the use of a superconducting magnet, generating a perpendicular magnetic field (B) of intensity up to 9 Tesla. We centered on the so-called plateau-plateau (p-p) quantum phase transitions in the integer quantum Hall regime, aiming at measuring the critical exponents that govern such phenomenology. Our main results are reported in Ref. [3]. We measured the exponent k by fitting the temperature-

H


dependence of the maximum first derivative of the Hall resistivity (dρxy

max/dB, shown in Fig.3) in correspondence of various plateau-plateau (P-P) quantum phase transitions. The k value we found was not in agreement with previous studies, both on graphene [4] and 2DEGs [5]. We therefore performed an independent determination of the parameters p and γ, that give k via the relation k= p/2γ. p was measured by analyzing the weak localization correction to the longitudinal conductivity in non-quantizing fields, which allowed to determine the T-dependence of the coherence length. Subsequently, we obtained the critical exponent of the localization length γ by studying the temperature dependence of the longitudinal resistivity in correspondence of the tails of the Landau levels. The measured values confirmed that the relation k = p/2γ is valid in our bilayer sample. On the other hand the value of γ indicates that these phase transitions can’t be properly treated by assuming an Anderson-liker disorder: the system seems to approach the classical percolation limit, probably due to smooth long-range potential fluctuation produced in the substrate. C. Cobaleda1, S.Pezzini1,2, V. Clericò1, E. Diez1,

and V. Bellani2 1Laboratorio de Bajas Temperaturas, Universidad de Salamanca, E-37008 Salamanca, Spain 2Dipartimento di Fisica and CNISM, Universitá degli Studi di Pavia, I-27100 Pavia, Italy

[email protected]

References [1] P. J. Zomer, S. P. Dash, N. Tombros,

and B. J. van Wees, A transfer technique for high mobility graphene devices on commercially available hexagonal boron nitride, Appl. Phys. Lett. 99, 232104 (2011).

[2] C. Cobaleda, S. Pezzini, E. Diez, and V. Bellani, Temperature- and density-dependent transport regimes in a h-BN/bilayer graphene/h-BN heterostructure, submitted to Phys. Rev. B (2014).

[3] C. Cobaleda, S. Pezzini, E. Diez, and V. Bellani, Plateau-plateau quantum phase transitions in high-mobility bilayer graphene, in preparation (2014).

[4] A. J. M. Giesbers, U. Zeitler, L. A. Ponomarenko, R. Yang, K. S. Novoselov, A. K. Geim, and J. C. Maan, Scaling of the quantum Hall plateau-plateau transition in graphene, Phys. Rev. B 80, 241411 (2009).

[5] W. Li, C. L. Vicente, J. S. Xia, W. Pan, D. C. Tsui, L. N. Pfeiffer, and K.West, Scaling in Plateau-to-Plateau Transition: A Direct Connection of Quantum Hall Systems with the Anderson Localization Model, Phys. Rev. Lett. 102, 216801 (2009).


Figures

Figure 1: Optical microscopy image of a bilayer graphene device. The uppermost h-BN flake is clearly visible on top of the graphene/lower-h-BN mesa and the metallic contacts. The scale bar is 5 µm.

Figure 2: Solid lines: longitudinal resistivity as a function of the gate voltage Vg, at T between 300 mK and 50 K. Open circles: carrier density as a function of gate voltage, obtained from low-field Hall measurements at T = 0.3 K. (inset) ρxx (Ω) vs Vg (V) in the vicinity of the T-independent crossing point for electrons. Adapted from Ref. [2].

Figure 3: Panels (a) and (b) show the isotherms of the Hall resistivity ρxy as a function of the magnetic field in the electron-like regime at densities n = 10.2 x 1011 cm-2 and n = 6.1 x 1011 cm-2 respectively. The P-P transitions 8-4 and 12-8 are visible in (a), the 8-4 in (b). The insets show the isotherms the longitudinal resistivity ρxx as a function of the magnetic field. Adapted from Ref. [3].


esonance Raman studies of twisted bilayer graphene and 2D transition metal dichalcogenides

MARCOS A. PIMENTA Departamento de Fisica, UFMG Brasil

Raman spectroscopy is a very useful tool to study graphene, since it furnishes information about the atomic structure, presence of disorder, number of layers, charge transfer and strain. However, important information about electrons can be also obtained in a resonance Raman spectroscopy (RRS) investigation, where the energy of the laser excitation can be tuned. An increasing number of studies have been dedicated twisted bilayer graphenes (TBG), since their optical and electronic properties are strongly dependent on the twisting angle. In particular, the Raman G-band can be significantly enhanced for TBG with specific twisting angles, and the enhancement occurs when the incident laser energy is in resonance with van Hove singularities that appear in the crossing region between the two Dirac cones of the top and bottom layers. Moreover, new and sharp extra peaks appear in the spectra both below and above the G-band position, and are ascribed to phonon modes within the interior of the graphene Brillouin zone that become Raman active through an umklapp double resonance (U-DR) Raman process, where momentum conservation involves reciprocal lattice vectors of the Moire patterns. [1]. In this work, we performed a Raman spectroscopy study of 150 samples of TBG with many different twisting angles, using several different laser lines in the visible range. The intensities and FWHM of all Raman features, mainly the G and 2D bands, were analyzed as function of the twisting angle and laser energy. A huge increase in the G band intensity could be observed for samples with intermediate twisting angles (between 9 and 14 degrees) and the results could be explained in terms of resonances with van Hove singularities of TBG. We will also

present resonance Raman results of the sharp and extra peaks ascribed to the umklapp double resonance process. The Raman excitation profile of both the G-band and the new extra peaks was obtained experimentally [2], and our results are compared to the theoretically simulated spectra of TBG considering both resonances with van Hove singularities and the umklapp double resonance processes. Finally, we will present resonance Raman studies of the 2D transition metal dichalcogenides MoS2, WSe2 and WS2 with one and few layers. We will show that the Raman spectra is strongly dependent on the laser energy, and we will present the Raman excitation profile of the first and second-order Raman features, which provide information about the electronic transitions and the electron-phonon interaction in these compounds. Marcos A. Pimenta

Departamento de Fisica, UFMG,

CP 702, 30123-970 Belo Horizonte, Brasil [email protected]

References [1] A. Righi et al, Physical Review B

84.(2011) 241409. [2] A. Righi, et al, Solid State

Communications 175 (2013) 176.

R


hotovoltaic Energy Conversion and Electrically Driven Light Emission in a WSe2 monolayer

ANDREAS POSPISCHi Vienna University of Technology Austria

Recently, a new class of materials has emerged – two-dimensional (2D) atomic crystals. 2D materials, such as grapheme and atomically thin transition metal dichalcogenides (TMDCs), are crystalline, and thus of high material quality and stability. Even so, they can (potentially) be produced in large areas at low cost. Here, we report a 2D p-n junction diode, based on a tungsten diselenide (WSe2) atomic monolayer. The lateral p-n junction is formed by electrostatic doping, using a pair of oppositely biased gate electrodes. We present device applications as photovoltaic solar cell, photodiode, and light emitting diode. Light power conversion and electroluminescence efficiencies are ≈0.5 % and ≈0.1 %, respectively. Two-dimensional atomic crystals such as graphene and atomically thin TMDC’s are currently receiving a lot of attention. These materials can be produced on a large scale, and are bendable, thus making them attractive for a variety of optoelectronic devices. In addition, the possibility of stacking atomically thin layers on top of each other provides the opportunity to create new “artificial” materials. Graphene shows good photodetection properties [1], but does not produce a sizable photovoltage or efficient electrically driven light emission because of its zero bandgap. TMDCs, such as MoS2 and WSe2, are therefore gaining increasing attention for applications in optoelectronics [2,3]. These crystals have an indirect bandgap in bulk. However, if the material is thinned to a monolayer, the band structure changes and they become direct semiconductors. Here, we demonstrate the first p-n junction diode in

a 2D material (WSe2) and applications as photovoltaic solar cell, photodiode and light emitting diode [4]. P-n junction diodes are the basic building block for most optoelectronic devices such as photodetectors, light-emitting diodes and lasers. We use electrostatic doping to form a monolayer WSe2 lateral p-n junction diode (for a schematic drawing see Fig. 1(a)). WSe2 flakes are fabricated by mechanical exfoliation and characterized by photoluminescence (PL) and Raman measurements to assure monolayer thickness. Monolayer flakes show a pronounced PL emission at 1.64 eV whereas for bi- and multilayer flakes a strong reduction of the quantum yield is observed, which is due to the change in the band structure. Split gate electrodes, fabricated by electron beam lithography and covered with a 100 nm thick silicon nitride dielectric layer, couple to two different regions of a WSe2 flake, which is transferred by a dry transfer technique. Electrons and holes can be injected into the channel by applying a positive voltage to one and a negative voltage to the other gate electrode. Figure 1(b) shows the electrical properties of our device in the dark. Applying voltages of opposite polarities to the gate electrodes leads to a rectification of the current (solid curve). By applying voltages with same polarities to both electrodes, our device operates as a resistor (dashed curve). The I-V curve clearly shifts down under optical illumination, meaning a current flow to an external load. Our p-n diode can therefore be utilized as a photovoltaic solar cell (see Figure 2(a)). We extract a maximum electrical power of 9 pW, from which we derive an efficiency of approximately 0.5 % at an incident illumination of 1400 W/m2. When biased in reverse direction our device operates as a photodiode. A photocurrent of

P


29 pA is obtained at a bias voltage of -1 V which translates into an external photoresponsivity of 16 mA/W. Taking into account that only ≈5 % of the light is absorbed in one WSe2 monolayer, we calculate an internal photoresponsivity of ≈0.32 A/W. Finally, by driving a constant current through the device, light emission from the p-n junction could be achieved. Figure 2(b) shows the emission spectrum, which peaks at 1.547 eV. This is 93 meV lower than PL emission, which we assign to a different dielectric environment in both experiments. We estimate the electroluminescence efficiency (ratio of emitted optical power and electrical input power) to be approximately 0.1 %, limited by resistive losses and non-radiative recombination in WSe2. In the future, it can be increased by reducing the contact resistance or by using a crystalline substrate to reduce the density of recombination centers. A. Pospischil, M. M. Furchi and T. Mueller

Vienna University of Technology, Institute of Photonics, Gußhausstraße 27-29, 1040 Vienna, Austria [email protected]

References [1] A. Pospischil et al., CMOS-compatible

graphene photodetector covering all optical communication bands. Nature Photon. 7, (2013) 892–896.

[2] B. Radisavljevic et al., Single-layer MoS2 transistors. Nature Nanotechnol. 6, (2011) 147-150.

[3] Q.H. Wang et al., Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nature Nanotechnol. 7, (2012) 699-712.

[4] A. Pospischil, M.M. Furchi & T. Mueller, Solar energy conversion and light emission in an atomic monolayer p-n diode. To be published in Nature Nanotechnol. (2014).


Figures

Figure 1: (a) Schematic drawing of the device structure. (b) I-V characteristics in the dark for two different biasing conditions. The solid line depicts diode behavior; the dashed line shows operation as a resistor.

Figure 2: (a) I-V characteristics of the device under optical illumination. Electrical power Pel can be extracted when operated as diode. The grey rectangle shows the power area. (b) Electroluminescence emission spectra for constant driving currents. No emission is visible for unipolar device operation. The black lines show Gaussian fits of the measured values (dots).


calable control of graphene growth on 4H-SiC C-face using decomposing silicon nitride masks [1,2]

RENAUD PUYBARET Georgia Tech Lorraine &Georgia Institute of Technology France & USA

Epitaxial graphene on silicon carbide (SiC) has great potential for electronics [3-6] with unique physical attributes leading to half-eV band-gap structures [7], exceptional ballistic transport in sidewall nanoribbons [8] or also high-frequency transistors [9,10] and even highly efficient spintronics [11]. Because SiC is a monocrystalline semiconducting industrial substrate, epitaxial graphene on SiC is directly compatible with established scalable device fabrication techniques, making it attractive for advanced electronic devices [5,12]. Patterning of graphene devices is a key step in the fabrication process. In most cases, 2D graphene is first grown then patterned by oxygen plasma. Selective area growth (SAG) is a more straightforward approach, as it provides shaped structures in a single-step process, during epitaxy itself, without requiring the usual post-growth lithography-and-etching steps. Several techniques have been reported for selective area growth of graphene. These include AlN capping [13], ion implantation of Au or Si [14], and sidewall nanoribbons [15]. We report a method for controlling graphene growth down to the sub-micron level, cf Figure 1. We find that deposition of a 120 nm- to 150 nm-thick silicon nitride (SiN) mask on C-face (000-1) silicon carbide prior to graphitization modifies the relative number of multi-layer epitaxial graphene (MEG) sheets. After preparation of the surface of 4H-SiC wafer dies by a high temperature hydrogen etch [16], low-power plasma-enhanced chemical vapor deposition is used for SiN. We have confirmed by AFM measurements after removing SiN with hydrofluoric acid (HF) that the plasma does not result in detectable damage to the SiC surface. Then the sample

is graphitized using confinement-controlled sublimation (CCS) [16]. The silicon nitride mask decomposes and vanishes before graphitization is complete. Interestingly, the stoichiometry of the silicon nitride layers controls whether the silicon nitride layer enhances or suppresses graphene growth relative to uncovered areas. We find that N-rich silicon nitride masks decrease the number of layers by three compared to uncovered regions while Si-rich silicon nitride masks increase thickness by two to four layers. The graphene layers of samples prepared with nearly stoichiometric silicon nitride show good mobilities, cf Figure 2, up to 7100 cm2.V-1.s-1, with electron concentrations in the 1012 cm-2 range. Raman spectroscopy and AFM measurements, cf Figures 3 and 4, confirm that the graphene grown in areas initially covered by the mask has good structural quality. By tailoring the growth parameters selective graphene growth (Figures 1,3,4) and sub-micron patterns (Figure 1) have been obtained. Renaud Puybaret1,2, John Hankinson3, James

Palmer3, Abdallah Ougazzaden1,2, Paul L Voss1,2, Claire Berger3,4 and Walt A de Heer3 1Georgia Tech Lorraine, GT-CNRS UMI 2958, 57070 Metz, France 2Georgia Institute of Technology, School of ECE, Atlanta, GA 30332, USA 3Georgia Institute of Technology, School of Physics, Atlanta, GA 30332, USA 4CNRS / Institut Néel, BP166, 38042 Grenoble, France [email protected]

S


References [1] R Puybaret et al,

arxiv.org/abs/1307.6197, 2013. [2] R Puybaret et al, submitted to Phys

Rev Appl, 2013. [3] W. A. de Heer,

http://smartech.gatech.edu/xmlui/handle/1853/31270, 2001-2009.

[4] W. A. de Heer, C. Berger, and P. N. First, US Patent 7015142, 2006.

[5] C. Berger et al., Journal of Physical Chemistry B, 108(52):19912–19916, Dec 30 2004.

[6] C. Berger et al., Science, 312(5777):1191–1196, May 26 2006.

[7] J. Hicks et al., Nat Phys, 9(1):49–54, JAN 2013.

[8] J. Baringhaus, M. Ruan et al., Nat Phys, arXiv:1301.5354 [cond- mat.mes-hall]., 2013.

[9] Y. M. Lin et al., Science, 327(5966):662, FEB 5 2010.

[10] Z. Guo et al., Nano Letters, 13(3):942–947, MAR 2013.

[11] B. Dlubak, M-B Martin et al., Nat Phys, 8(7):557–561, JUL 2012.

[12] Y.M. Lin et al., Science, 332(6035):1294–1297, JUN 10 2011.

[13] M. Rubio-Roy, F. Zaman et al., Applied Physics Letters, 96(8), Feb 22 2010.

[14] S. Tongay et al., Applied Physics Letters, 100(7), Feb 13 2012.

[15] M. Sprinkle et al., Nat Nano, 5(10):727–731, 10 2010.

[16] W.A de Heer et al., PNAS, 108(41):16900–16905, Oct 11 2011.

Figures

Figure 1: Overview of the process and optical images of patterns, with the SiN initial pattern of top, the resulting graphene pattern on the bottom, with: - left: array of ribbons, which graphitized into 3-layer graphene ribbons on 1-2 layer graphene, 0.87 to 1.2 µm wide, 130 µm long. - right: Proof-of-principle pattern of selectively grown graphene on SiC, representing Buzz, Georgia Tech’s mascot, darker areas are mono-layer graphene, lighter areas SiC.

Figure 2: Transport measurements; (a) (b): Magneto- and Hall resistances at room temperature of IM area of Si-rich sample: n=7.7 e12 cm−2 and µ = 7100 cm2 .V−1 .s−1. Hall bar is 3 µm wide. (c) (d): Mobility and carrier concentrations at room temperature measured on the IM area of N-rich sample, IB area of N-rich sample, and IM area of Si-rich sample.


Figure 3: Si-rich SiN mask, SAG. For this 3.5x4.5 mm2 sample the top half was initially masked (IM) with Si-rich SiN, the bottom half was initially bare (IB) (a) Raman spectra (SiC contribution subtracted), showing the typical MEG spectrum. For both graphene spectra the intensity is normalized to the SiC plateau at 1900 cm−1, indicating that in the IB areas graphene coverage is much reduced. Note the quasi absence of D-peak. (b) AFM image of IM area (scale 20x20 µm2). (c) AFM image of IB area (scale 40x40 µm2).

Figure 4: Selectively-grown two-layer graphene Hall bar using a Si-rich SiN mask, critical dimension 4µm. a: SiN pattern. Scale bar is 8 µm. b: consequent graphene growth. c: Raman 2D intensity mapping. d: Raman 2D/G intensity mapping).


pin Hall Effect Induced by Resonant Skew Scattering in Graphene

TATIANA G. RAPPOPORT Universidade Federal do Rio de Janeiro Brazil

The spin Hall effect is the appearance of a transverse spin current in a non-magnetic conductor by pure electrical control. The extrinsic spin Hall effect originates from the spin-dependent skew scattering of electrons by impurities in the presence of SOI and can be used for an efficient conversion of charge current into spin-polarized currents. Recently, it has been explored for replacing ferromagnetic metals with spin injectors in spintronics applications. We consider a monolayer of graphene decorated by a small density of impurities generating a spin-orbit interaction in their surroundings. We show that large spin Hall effect develops through skew scattering and it is strongly enhanced in the presence of resonant scattering [1]. Unlikein two-dimensional electron gases (2DEG), for which resonant enhancement of skew scattering requires resorting to fine tuning, our proposal takes advantage of graphene being an atomically-thin membrane, whose local density of states easily resonates with several types of adatoms, molecules, or nano-particles. Our single impurity scattering calculations show that impurities with either intrinsic or Rashba spin-orbit coupling in a graphene sheet originate robust spin Hall effect with spin Hall angles comparable to those found in metals. Also, the solution of the transport equations for a random distribution of impurities suggests that the spin Hall effect is robust with respect to thermal fluctuations and disorder averaging.

Tatiana G. Rappoport1, Aires Ferreira 2, Miguel A.

Cazalilla3, Antonio H. Castro Neto2,4 1 Instituto de Física, Universidade Federal do Rio de Janeiro, CP 68.528, 21941-972 Rio de Janeiro, RJ, Brazil 2 Graphene Research Centre and Department

of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117546, Singapore 3 Department of Physics, National Tsing Hua University, and National Center for Theoretical Sciences (NCTS), Hsinchu City, Taiwan 4 Department of Physics, Boston University, 590 Commonwealth Avenue, Boston, MA 02215, USA [email protected] References [1] A. Ferreira, T. G. Rappoport, M. A.

Cazalilla, A. H. Castro Neto, Arxiv.1304.7511 (to be published in Phys. Rev. Lett.)

Figures

Figure 1: Schematic picture of extrinsic spin Hall effect generated by transport skewness. An impurity (sphere) near the graphene sheet causes a local spin-orbit field with range R. The scattering of components with positive (negative) angular momentum is enhanced (suppressed) for charge carriers with spin up (down), resulting in a net spin Hall current

S


uantum magnetotransport phenomena in ultra-narrow graphene ribbons

BERTRAND RAQUET Laboratoire National des Champs Magnétiques Intenses, LNCMI-Toulouse France

Exploring the electronic confinement of Dirac fermions in graphene nanoribbons without degrading the outstanding properties of the mother material remains a delicate task for scientists. We have known, for years, that an atomic cutting of a graphene flake along a crystallographic direction gives rise to a specific electronic band structure driven by the Dirichlet boundary conditions, in close analogy with carbon nanotubes and their rolling vector. However, the often unavoidable presence of edge disorder has been shown to have a detrimental impact on the electronic structure. Only the mastering of the edge quality and orientation of sub-20nm wide graphene ribbons will allow us to fully benefit from their unusual 1D band structures, which is responsible for perfect conducting channels at low energy, spin-polarized edge states or tuneable direct band-gaps suitable for digital electronic applications. Among the already well developed and promising techniques to synthesize high quality graphene nanoribbons (growth on SiC, anisotropic etching, catalytic nanocutting, bottom-up synthesis …), the unzipping of carbon nanotubes constitutes a reliable approach to minimize the disorder-induced transport gap around the charge neutrality point. Here, our strategy to investigate the 1D band structure of unzipped carbon nanotubes consists in using large magnetic fields to make transverse and magnetic electronic confinements compete and to detect the resulting spectrum fragmentation into magneto-electric states. In this presentation, after a brief review of the

Landau magneto-fingerprints already observed in rather wide GNRs, we will focus on recent magnetotransport experiments performed under high magnetic field (62T) on a set of high quality 20nm wide bilayer-GNRs. All the experiments, including a dozen of devices, reveal an exceptionally robust 4e²/h Hall conductance plateau, preceded by large magneto-conductance oscillations, which are signature of 1D sub-bands crossing the Fermi energy. Such a spectroscopy in the open quantum dot regime unveils, in a unique manner, the complex electronic band structure of the bilayer-GNRs, driven by the twisting angle between the layers and the edge chirality inherited from the mother unzipped MWCNT. These experimental fingerprints of the 1D band structure will be directly compared to band structure simulations performed for different bilayer-GNR configurations. To conclude, we will briefly address some comparative remarks on GNRs and MWCNTs, of similar width, in the presence of strong magnetic confinement. Haoliang Shen1,2, Alessandro Cresti3, Walter Escoffier1, Xinran Wang2, Bertrand Raquet1 1 Laboratoire National des Champs

Magnétiques Intenses, LNCMI-Toulouse, UPR 3228, INSA, UPS, UJF, Toulouse, France 2 National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing,

China 3 IMEP-LAHC, UMR 5130, Grenoble INP/UJF/CNRS/Université de Savoie, Grenoble, France

Q


ynthesis and applications of graphene materials

WENCAI REN Shenyang National Laboratory for Materials Science China

Great achievements have been made in graphene research area since 2004, however, it is still remains a great challenge for realizing large-scale controlled synthesis and real applications of graphene materials. First, we have developed an ambient pressure CVD to synthesize millimeter-size high-quality single-crystal graphene on Pt, proposed a novel electrochemical bubbling method to transfer these domains to arbitrary substrates without destroying the metal growth substrates [1], realized the edge and morphology control of single-crystal graphene domains, observed and explained the edge-dependent growth behavior of graphene, and found an efficient way to heal the defects in graphene and fabricate large-size high-quality single-crystal graphene [2]. Second, we have realized the direct synthesis of a three-dimensional (3D) porous graphene macrostructure by template-directed CVD, which we call graphene foam (GF) [3]. GF consists of a 3D interconnected network of graphene, which is flexible and has high electrical conductivity. As a result, it shows many potential applications such as elastic conductors [3], high-sensitivity gas detectors [4], flexible lithium ion batteries (LIBs) with ultrafast charge and discharge rates [5], lightweight and flexible electromagnetic interference shielding materials [6]. Third, we have developed a highly efficient method to produce high-quality graphene materials at a low cost [7]. With a proto-type production line, 5kg/day graphene materials with a high electrical conductivity (1,000 S/cm) can be directly produced, which show a great potential for real applications in LIBs, lithium sulfur batteries [8], as well as various composites and functional coatings.

Wencai Ren

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China

[email protected] References [1] L.B. Gao, W.C. Ren, H.M. Cheng, et al.,

Repeated growth and bubbling transfer of graphene with millimetre-size single-crystal grains using platinum, Nature Communications 3, 699, 2012.

[2] T. Ma, W.C. Ren, H.M. Cheng, et al., Edge-controlled growth and kinetics of single-crystal graphene domains by chemical vapor deposition, PNAS 110 (51), 20386-20391, 2013.

[3] Z.P. Chen, W.C. Ren, H.M. Cheng, Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition, Nature Materials 10 (6), 424-428, 2011.

[4] F. Yavari, W.C. Ren, N. Koratkar, et al., High sensitivity gas detection using a macroscopic three-dimensional graphene foam network, Scientific Reports 1, 166, 2011.

[5] N. Li, W.C. Ren, F. Li, H.M. Cheng, et al., Flexible graphene-based lithium ion batteries with ultrafast charge and discharge rates, PNAS 109 (43), 17360-17365, 2012.

[6] Z. P. Chen, W. C. Ren, H. M. Cheng, et al., Lightweight and flexible graphene foam composites for high-performance electromagnetic interference shielding, Advanced Materials 25 (9), 1296-1300, 2013.

S


[7] S.F. Pei, W.C. Ren, H.M. Cheng et al., A method to synthesize high-quality graphene materials, Chinese Patent 201110282370.5.

[8] G.M. Zhou, S.F. Pei, F. Li, H.M. Cheng, et al., A graphene-pure-sulfur sandwich structure for ultrafast, long-life lithium-sulfur batteries, Advanced Materials 26 (4), 625-631, 2014.


raphene Research Activities at the Institute of Metal Research, Chinese Academy of Sciences

WENCAI REN Shenyang National Laboratory for Materials Science China

The Advanced Carbon Division at the Institute of Metal Research, Chinese Academy of Sciences, is a research pioneer on graphene in China, and mainly focuses on the synthesis of graphene materials and their applications such as energy storage, functional coatings, composites, and flexible optoelectronics. It has developed a novel method to produce high-quality graphene materials in a large quantity and at low cost [1], realized CVD growth of millimeter-size graphene single crystals [2,3], large-area transparent conductive films [4] and highly conductive three-dimensional interconnected graphene networks [5]. It has extensively explored the applications of these materials in lithium ion batteries [6-8], lithium sulfur batteries [9], supercapacitors [10-12], thermal management, conductive inks, anti-corrosion coatings, and composites. In addition to fundamental research, it has also made great efforts to industrialize and commercialize graphene materials in collaboration with companies, and the large-scale production of graphene is now an industrial reality. Wencai Ren

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese

Academy of Sciences, Shenyang 110016, P.R. China

[email protected]

References [1] S.F. Pei, W.C. Ren, H.M. Cheng et al., A

method to synthesize high-quality graphene materials, Chinese Patent 201110282370.5.

[2] L.B. Gao, W.C. Ren, H.M. Cheng, et al., Repeated growth and bubbling transfer of graphene with millimetre-size single-crystal grains using platinum, Nature Communications 3, 699, 2012.

[3] T. Ma, W.C. Ren, H.M. Cheng, et al., Edge-controlled growth and kinetics of single-crystal graphene domains by chemical vapor deposition, PNAS 110 (51), 20386-20391, 2013.

[4] L.B. Gao, W.C. Ren, H.M. Cheng, et al., Efficient growth of high-quality graphene films on Cu foils by ambient pressure chemical vapor deposition, Applied Physics Letters 97 (18), 183109, 2010.

[5] Z.P. Chen, W.C. Ren, H.M. Cheng, Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition, Nature Materials 10 (6), 424-428, 2011.

[6] Z.S. Wu, W.C. Ren, H.M. Cheng, et al., Graphene anchored with Co3O4 nanoparticles as anode of lithium ion batteries with enhanced reversible capacity and cyclic performance. ACS Nano 4 (6), 3187-3194, 2010.

[7] Z.S. Wu, W.C. Ren, H.M. Cheng, et al., Doped graphene sheets as anode materials with superhigh rate and large capacity for lithium ion batteries, ACS Nano 5 (7), 5463-5471, 2011.

[8] N. Li, W.C. Ren, F. Li, H.M. Cheng, et al., Flexible graphene-based lithium ion batteries with ultrafast charge and discharge rates, PNAS 109 (43), 17360-17365, 2012.

G


[9] G.M. Zhou, S.F. Pei, F. Li, H.M. Cheng, et al., A graphene-pure-sulfur sandwich structure for ultrafast, long-life lithium-sulfur batteries, Advanced Materials 26 (4), 625-631, 2014.

[10] Z.S. Wu, W.C. Ren, H.M. Cheng, et al., Anchoring hydrous RuO2 on graphene sheets for high-performance electrochemical capacitors. Advanced Functional Materials 20 (20), 3595-3602, 2010.

[11] Z.S. Wu, W.C. Ren, H.M. Cheng, et al., High-energy MnO2

nanowire/graphene and graphene asymmetric electrochemical capacitors, ACS Nano 4 (10), 5835-5842, 2010.

[12] D.W. Wang, F. Li, W.C. Ren, H.M. Cheng*, et al., Fabrication of graphene/polyaniline composite paper via in situ anodic electropolymerization for high-performance flexible electrode, ACS Nano 3 (7), 1745-1752, 2009.


RACE: how an european research infrastructure supports the graphene community

STÉPHANE REQUENA GENCI/PRACE France

PRACE (Partnership for Advanced Computing in Europe) is a pan European research infrastructure spanning over 25 countries which aims to offers to European scientists access to world-class resources and services in HPC (High Performance Computing) and advanced numerical simulation. Established since April 2010 with a seat in Brussels (Belgium), PRACE is providing in 2014 a unique computing capacity of more than 15 Petaflops across 6 complementary supercomputers based in France, Germany, Italy and Spain. By offering this unique aggregated computing power and services upon a single peer-review based on scientific excellence, PRACE is allowing its scientific and industrial users to have access to similar capacities and services like their competitors in USA, China, Japan or Russia. Since this level of resources and diversity of HPC architectures was clearly unreachable for any single European country, THE rationale of PRACE was to unite efforts from European countries in order to sustain scientific and industrial competitiveness of Europe. Since mid 2010 PRACE has been able to allocate close to 7 billion cpu core hours on 259 research projects, allowing major breakthroughs in climate modelling, astrophysics, chemistry, materials, biology and medicine or combustion to name a few. The study of graphene-based materials and other families of two-dimensional materials crucially demands for advanced simulation techniques to explore realistic models of materials of technology and industrial relevance. To that end, beyond the

development of suitable numerical approaches for studying large-scale models, the access, implementation and use of high-performance computing has become a strategic value for Europe. In this talk, the European High Performance Computing infrastructure (PRACE) will be briefly presented, and illustrated with an undergoing scientific project focused on simulation of Hall Kubo conductivity in graphene-based materials. The scalability and performance of supercomputers will be shown on a concrete study of quantum Hall effect in structurally and chemically disordered graphene, with new physics revealed thanks to such computing resources, and out of reach otherwise. Stéphane Requena1, Nicolas Leconte2

1 Member of the PRACE Board of Directors 2 ICN2, Spain http://www.prace-ri.eu/

P


uture Carbon Materials

RODNEY S. RUOFF Ulsan National Institute of Science & Technology (UNIST) Republic of Korea In addition to discussing some of our recent results on graphene, I offer a personal perspective of what new carbon (and related) materials might be achieved in the future. These include ‘negative curvature carbons’, ‘diamane’ and related ultrathin sp3-bonded carbon films/foils, sp2/sp3-hybrid materials, and others. Rodney S. Ruoff Center for Multidimensional Carbon Materials (CMCM) Institute for Basic Science (IBS) Center on the

UNIST Campus Department of Chemistry and School of Materials Science Ulsan National Institute of Science & Technology (UNIST)

Ulsan 689-798, Republic of Korea [email protected]

References [1] (a) Lu XK, Yu MF, Huang H, and Ruoff

RS, Tailoring graphite with the goal of achieving single sheets, Nanotechnology, 10, 269-272 (1999). (b) Lu XK, Huang H, Nemchuk N, and Ruoff RS, Patterning of highly oriented pyrolytic graphite by oxygen plasma etching, Applied Physics Letters, 75, 193-195 (1999).

[2] Zhu, Yanwu; Murali, Shanthi; Stoller, Meryl D.; Ganesh, K. J.; Cai, Weiwei; Ferreira, Paulo J.; Pirkle, Adam; Wallace, Robert M.; Cychosz, Katie A.; Thommes, Matthias; Su, Dong; Stach, Eric A.; Ruoff, Rodney S. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 332, 1537-1541 (2011).

[3] Odkhuu, Dorj; Shin, Dongbin; Ruoff, Rodney S.; Park, Noejung; Conversion of Multilayer Graphene Into Continuous Ultrathin sp3-bonded Carbon Films on Metal Surfaces Density. Scientific Reports (2013), DOI: 10.1038/srep03276.

[4] Ruoff, Rodney S. Personal perspectives on graphene: New graphene-related materials on the horizon. MRS Bulletin, 37, 1314-1318 (2012).

F


arbene Functionalization of Exfoliated Graphene: Towards Scalable Dispersion and Integration of Chemically Modified Graphene

TOBY SAINSBURY National Physical Laboratory United Kingdom

In recent years, graphene in both its pristine and chemically functionalized forms have been demonstrated to have significant economic and societal potential in the areas of electronics, sensing and diagnostics, composites, energy storage and catalysis due to the diverse physical and electronic attributes [1-4]. Graphene in its pristine form has received a great deal of research attention in terms of the fabrication of devices for logic and sensing operation as well as in the area of metrology standards and the demonstration of fundamental physical phenomena [1-3]. While the fabrication of electronic devices and sensor technology based on graphene components may be relatively discreet in terms of total surface area, the emerging utilization of graphene in bulk composites, electronic ink, energy storage and catalytic applications will certainly be dependent on the ability to fabricate, process and integrate graphene on the gram to kilogram scale. To interface graphene with solvent systems, resin matrices, and integrate it with nanoscale components, contacts and substrates there remains tremendous scope for further research directed towards manipulation of the surface chemistry of graphene whilst retaining its fundamentally attractive intrinsic properties [4]. Towards this goal, the development of chemical processing techniques for graphene which are low cost, scalable, and non-destructive are highly attractive [5-8]. Here we report the development of solution-phase radical based functionalization strategies in order chemically modify graphene in a controlled and scalable

manner. This work yields exfoliated graphene nanosheets which retain their wide area whilst integrating additional chemical functionality by the grafting of functional groups to the sp2 hybridized carbon lattice. Specifically, carbene radical species are generated in the presence of solution exfoliated graphene. Reactive radicals covalently graft to the carbon lattice. Characterization of the covalent functionalization of graphene has been conducted in a systematic manner. Evaluation of specific chemical bonding, quantification of adduct chemistry and the modification of key intrinsic properties of the graphene has been performed as follows. The presence of the covalent functionalization was characterized using microscopy including HR-TEM, AFM, and SEM. Spectroscopic techniques; FTIR, Raman and X-ray photoelectron spectroscopy (XPS) were employed to fully elucidate the nature of the covalent bonding. Additional characterization techniques; X-ray diffraction (XRD), Time-of-flight Mass Spectroscopy (ToF SIMS), electron energy loss spectroscopy (EELS), thermo gravimetric analysis (TGA) and energy dispersive x-ray spectroscopy (EDAX) were employed to identify and quantify the fraction of the grafted chemical functionality relative to the bulk graphene. Characterization in the UV-Vis and Terahertz frequencies was employed to investigate modification of the optical band-gap and conductivity as a function of the chemical functionalization. Implications of carbene modified graphene are investigated by its application within a polymer matrix. Graphene-Epoxy nanocomposites are formed and their mechanical properties evaluated. As a result of the chemical modification of the graphene, significant enhancement of

C


mechanical properties are noted relative to pristine unmodified epoxy. In summary, in this work we present scalable strategies for the non-destructive radical based functionalization of graphene using chemically versatile functional groups. These groups form functional adduct groups at the surface of graphene sheets which may be further derivitized in order to disperse and integrate graphene with solvent and polymer systems. A full characterization strategy forms the basis of a model by which alternative functionalization strategies may be evaluated and applied towards technologically relevant modification of graphene. Toby Sainsbury1, Paola Mendiburu Aletti1, Steve

Spencer1, Maud Seraffon1, Mellissa Passarelli1, Ali Rae1, Sophi Shanmuganathan2, Arlene O’Neill3, Jonathan Coleman3 1National Physical Laboratory, Hampton Road,

Teddington, Middlesex, United Kingdom 2Department of Chemistry, University of Oxford, Oxfrod, United Kingdom 3School of Physics, Trinity College Dublin, Ireland [email protected]

References

[1] Novoselov, K. S. Science 2004, 306, 666. [2] Park, S.; Ruoff, R. S. Nat. Nanotechnol.

2009, 4, 217. [3] Blake, P.; Brimicombe, P. D.; Nair, R. R.;

Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; Geim, A. K.; Novoselov, K. S. Nano Lett. 2008, 8, 1704.

[4] Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Won Suk, J.; Potts, J. R.; Ruoff, R. S. Adv. Mater. 2010, 22, 3906-3924.

[5] Sun, Z.; James, D. K.; Tour, J. M. J. Phys. Chem. Lett. 2011, 2, 2425-2432.

[6] He, H.; Gao, C. Chem. Mater. 2010, 22, 5054-5064.

[7] Ramanathan, T.; Abdala, A. A.; Stankovich, S.; Dikin, D. A.; Herrera-Alonso, M.; Piner, R. D.; Adamson, D. H.; Schniepp, H. C.; Chen, X.; Ruoff, R. S.; Nguyen, S. T.; Aksay, I. A.; Prud’Homme, R. K.; Brinson, L. C. Nat. Nanotechnol. 2008, 3, 327-331.

[8] Pan, Y.; Bao, H.; Sahoo, N. G.; Wu, T.; Li, L. Adv. Funct. Mater. 2011, 21, 2754-2763.

Figures

Figure 1: Dihalocarbene functionalized graphene.


onductance mapping of graphene using dual-probe STM

MIKKEL SETTNES Center for Nanostructured Graphene Denmark

We demonstrate anisotropic conductance in graphene using a dual-probe Scanning Tunnelling Microscopy (STM) setup as sketched on Fig. 1. This setup allows us to calculate the local electrical conductance at a designated position in the sample. Electrons injected at one probe are propagating through the sample and collected at another probe. The transmission between different point yields more information than what can be extracted from a standard STM setup measuring topography or local density of states. Using one probe in scanning mode while fixing the other, we are able to compute real space conductance maps showing anisotropic behaviour depending on the underlying crystal direction (Fig. 2). The features of the dual-probe transmission are explained very transparently using analytic expressions for grapheme Green's functions [2]. Fixing both probes and using a gate to vary the Fermi energy, the energy dependent conductance clearly shows different fingerprints depending on armchair and zigzag directions (Fig. 3). Multi- probe spectroscopy also appears to be a promising tool for characterizing individual modifications of the graphene sample such as perforations or grain boundaries. Recent experimental progress in multiple STM probe techniques enables positioning of two STM tips as close as 50-100 nm [1], a length scale within the inelastic mean free path of graphene. Thus we consider ballistic transport and quantum interference effects remain. We investigate

such interference effects around impurities (Fig. 4) and crystalline edges [3]. We show that impurities and their positions relative to the probes can be distinguished by their scattering signature in real and Fourier space. This is seen in analogy to optical diffraction. The theory developed treats point probes on an infinite sheet of graphene, which is in contrast to the conventional Landauer geometry containing semi- infinite contacts connected to a finite sample. This necessitates a reformulation of the conventional recursive Green's function method. We utilize contour integration techniques to evaluate the Green's function for infinite pristine graphene [2]. The approach is valid for a wider range of energies, thus going beyond the linear regime. Defects are added using a Dyson equation approach, which keeps the computational size of the problem proportional to the number of defects and contact sites, rather than the sample size, allowing for an easy investigation of long range features. Mikkel Settnes, Stephen R. Power, Dirch H.

Petersen, and Antti-Pekka Jauho Center for Nanostructured Graphene (CNG), Department of Micro- and Nanotechnology Engineering, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark [email protected]

C


References

[1] An-Ping Li, Kendal W. Clark, X.-G.

Zhang, and Arthur P. Baddorf. Advanced Functional Materials, 23(20):25092524, May 2013.

[2] S. R. Power and M. S. Ferreira. Physical ReviewB, 83(15):155432, April 2011.

[3] M. Settnes, S. R. Power, D. H. Petersen, and A.P.Jauho. Accepted for publication in Physical Review Letters, 2014.

Figures

Figure 1: Schematic of the dual-probe STM setup.

Figure 2: Scanning the output tip over the sample gives the real space conductance map shown with distinct characteristic of the armchair (constant) and zigzag (oscillatory) – directions.

Figure 3: Spectroscopy of two probes separated in thearmchair and zigzag direction, respectively. The probe separation is ~50 nm. A clear difference is observed between the armchair and zigzag directions. The armchair shows the usual linear increase of the density of states characteristic of graphene. However, the zigzag direction exhibits oscillations due to Fermi surface asymmetry.

Figure 4: Real space map of conductance between input (outside the scan area) and scanning probe, around an impurity located at the origin. The figure shows the relative change compared to the pristine case (Τ0). The separation between the impurity and the fixed probe is in the armchair direction. A shadowing effect is clearly visible in the blue regions with low transmission


nteractions inside interlayer spaces of layered materials

YOUNG-WOO SON Korea Institute for Advanced Study Korea

Abstract: Many layered materials such as multilayered graphene, BN, and MoS2 have distinct physical and chemical properties related with interactions between layers. In this talk, I will present our recent theoretical works related with delicate interactions between layers themselves and with inserted liquids in interlayer spaces. First, depending on characteristics of layer interactions, it is shown that the responses of layered materials to external mechanical forces vary greatly ranging from weakly compressible, completely compressible to auxetic spacing variations [1]. Second, when water can immerse into interlayer spaces of graphene oxide, the formation of ice layers and their dislocations are shown to depend on interlayer distances critically [2], which may provide a clue to explain a recent experiment [3] on the peculiar water dynamics in graphene oxide. Young-Woo Son Korea Institute for Advanced Study, Seoul, Korea [email protected]

References

[1] S. Woo and Y.-W. Son, in preparation. [2] D. W. Bouhkvalov, M. I. Katsnelson, Y.-

W. Son, Nano Lett. 13, 3930 (2013). [3] R. R. Nair et al, Science 335, 442

(2012).

I


agnetic functionalities in epitaxial graphene structures by means of molecular deposition

DANIELE STRADI Center for Nanostructured Graphene Denmark

Cryogenic scanning tunneling microscopy (STM) measurement and large-scale density functional theory (DFT) calculations are used to provide evidence that the deposition of electron acceptor molecular species can be employed to add magnetic functionalities to otherwise non-magnetic epitaxial graphene surfaces. DFT calculations predict that the 7,7’,8,8’-tetracyano-p-quinodimethane (TCNQ) molecule develops a finite magnetic moment upon adsorption on epitaxial graphene on Ru(0001) [1,2,3] due to single electron transfer from the substrate, which is subsequently verified by the appearance of a prominent Kondo resonance in the measured STS spectra. At larger coverage, the self-assembled monolayer formed by the charged TCNQ molecules develops spatially extended spin-split electronic bands (see Fig. 1), whose predicted spin alignment is verified by spin-polarized STM measurements [4]. Furthermore, by co-depositing TCNQ and its fluorinated derivative 2,3,5,6-Tetrafluoro-7,7’,8,8’-tetracyano-p-quinodimethane (F4-TCNQ) on the same surface, it is shown that while the charged TCNQ molecules spontaneously self-assemble to form molecular oligomers, F4-TCNQ oligomers are not formed because of the reduced delocalization of the extra electrons transferred from the surface. This makes it possible to control the spatial selectivity of the two distinct radical anions on the surface, thus paving the way for the creation of two-dimensional heteromolecular arrays of purely organic magnetic molecules on epitaxial graphene surfaces [5].

Daniele Stradi1,2,3, Manuela Garnica2,4, Sara

Barja2,4,6, Fabian Calleja2, Cristina Diaz3, ManuelAlcami2,3, Nazario Martin2,5, Amadeo L. Vazquez de Parga2,4, Fernando Martin2,3 and

Rodolfo Miranda2,4 1Center for Nanostructured Graphene (CNG), Department of Micro- and Nanotechnology (DTUNanotech), Technical University of Denmark, Lyngby, Denmark 2IMDEA-Nanociencia, Madrid, Spain 3Departamento de Quimica, Universidad Autónoma de Madrid, Madrid, Spain 4Departamento de Fisica de la Materia Condensada, Universidad Autónoma de Madrid, Madrid, Spain 5Facultad de Quimica, Universidad Complutense de Madrid, 28040, Madrid, Spain 6Lawrence Berkeley National Laboratory, Berkeley, California USA [email protected]

References

[1] Amadeo L. Vazquez de Parga et al.,

Phys. Rev. Lett., 100 (2008) 056807. [2] D. Stradi et al., Phys. Rev. Lett., 106

(2011) 186102. [3] [3] D. Stradi et al., Phys. Rev. B, 85

(2012) 121404. [4] M. Garnica, D. Stradi et al., Nat. Phys.

9 (2013) 368. [5] D. Stradi et al., in preparation.

M


Figures

Figure 1: Intermolecular organic band formed by self-assembled TCNQ molecules on epitaxial graphene on Ru(0001). Panels A and B show an experimental STM topography at 4K (Vb = -0.8 V) and the corresponding simulated STM topography.


aser photochemical synthesis of novel graphene oxide derivatives for organic electronics

EMMANUEL STRATAKIS Foundation for Research & Technology – Hellas Greece

Photochemistry may provide novel ways to covalently modify materials, thus tailoring its electronic and chemical properties. Due to the unique physicochemical processes taking place during the ultrashort pulsed laser-matter interaction, the surface of nanomaterials can be activated, allowing the chemical reaction with different moieties present in the surrounding medium, giving rise to novel materials production. This paper will present our recent work on the application of pulsed laser radiation for the photochemical modification of graphene oxide (GO) nanosheets. In particular we report on a rapid and facile method for the simultaneous reduction, doping and functionalization of GO. This technique is compatible with flexible, temperature sensitive substrates and was initially applied for the efficient production of highly transparent and conductive flexible graphene-based electrodes [1]. It is based on the use of femtosecond laser irradiation for the in-situ, non-thermal, reduction of spin coated GO films on flexible substrates over a large area (Figure 1). Furthermore, we present a fast, non-destructive and roll to roll compatible photochemical method for the simultaneous partial reduction and doping of GO nanosheets through ultraviolet laser irradiation in the presence of reactive Cl2 precursor molecules (Figure 2). By tuning the laser exposure time, it is possible to control the doping and reduction levels and therefore to tailor the work function (WF) of the GO-Cl derivatives from 4.9 eV to a maximum value of 5.23 eV, a WF value that matches the HOMO level of most polymer donors employed in OPV devices. Finally we demonstrate the pulse UV laser-assisted photochemical

functionalization of GO with small molecules as an efficient technique to realize polymer electron acceptors (Figure 3). Potential applications of pulsed laser synthesized and modified materials in organic electronics, particular to bulk heterojunction organic solar cells are demonstrated and discussed. E. Stratakis1,2, K. Savva1,2, M. Stylianakis2,3, M.

Sygletou1,2 C. Petridis3,4, C. Fotakis1,2, E. Kymakis3,4

1Institute of Electronic Structure and Laser, Foundation for Research & Technology Hellas, (IESL-FORTH), Heraklion, Greece 2University of Crete, Heraklion, Greece 3Center of Materials Technology & Photonics,

Technological Educational Institute of Crete, Crete, Greece 4Technological Educational Institute (TEI) of Crete, Greece [email protected]

L


References

[1] E. Kymakis , K. Savva , M. M.

Stylianakis, C. Fotakis , E. Stratakis, Adv. Funct. Mater. 2013, 23, 2742-2749

Figures

Figure 1: In-situ laser photochemical reduction

Figure 2: In-situ laser photochemical chlorination

-0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0

-4

-3

-2

-1

0

1

2

5% GO-EDNB

10% GO-EDNB

15% GO-EDNB

Current density (mA/cm2)

Voltage (V)

Figure 3: In-situ laser photochemical functionalization

Cl2


oing Beyond Graphene: Doped Graphene, Chalcogenide Monolayers and van der Waals Solids

MAURICIO TERRONES The Pennsylvania State University USA

Regarding other 2-Dimensional materials beyond graphene, we will describe various approaches to synthesize WS2 and MoS2 triangular monolayers, as well as large area films using a high temperature sulfurization of WOx clusters deposited on insulating substrates. We will show that depending on the substrate and the sizes of the oxide clusters, various morphologies of layered WS2 could be obtained. In addition, photocurrent measurements on these materials were performed using different laser photon wavelengths. Our results indicate that the electrical response strongly depends on the laser photon energy. The excellent response observed to detect different photon wavelengths in mono- and few-layers of WS2, MoS2, and WSe2, suggest these materials could be used in the fabrication of photo sensors and optoelectronic devices. From the theoretical stand point, using first principles calculations, we found that by alternating individual layers of different metal chalcogenides (e.g. MoS2, WS2, WSe2 and MoSe2) with particular stackings, it is possible to generate direct band gap bi-layers ranging from 0.79 eV to 1.157 eV as well as other novel van der Waals solids with fascinating electronic characteristics. Interestingly, in this direct band gap, electrons and holes are physically separated and localized in different layers. We foresee that the alternation of different chalcogenide layers would result in the fabrication of materials with unprecedented optical and physico-chemical properties. This talk will also discuss the synthesis of large-area, high-quality monolayers of nitrogen- and boron-doped graphene

sheets on Cu foils using ambient-pressure chemical vapor deposition (AP-CVD). Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that the defects in the doped graphene samples arrange in different geometrical configurations exhibiting unique electronic properties. Interestingly, these doped layers could be used as efficient molecular sensors and in the fabrication of electronic devices. In addition, the synthesis of hybrid carbon materials consisting of sandwich layers of graphene layers and carbon nanotubes have been synthesized by a self assembly route. These films are novel, energetically stable and could well find important applications as field emission sources, catalytic supports, gas adsorption materials and super capacitors. Mauricio Terrones Department of Physics, Department of Chemistry, Department of Materials Science and Engineering and Center for 2-Dimensional & Layered Materials. The Pennsylvania State University, University Park, Pennsylvania 16802, USA &

Research Center for Exotic Nanocarbons (JST), Shinshu University, Nagano, Japan [email protected] [email protected]

G


eversible optical doping of graphene [1]

ANTOINE TIBERJ Université Montpellier 2 France

The ultimate surface exposure provided by graphene makes it the ideal sensor platform but also exposes its intrinsic properties to any environmental perturbations. Raman spectroscopy has been used here to demonstrate the influence of the laser power and the surface chemistry of the SiO2/Si substrate on the charge carrier density of exfoliated graphene in air.

Raman spectroscopy is being considered as a high-throughput technique to characterize graphene and to study its fundamental physical properties. This technique is highly sensitive to both the electronic and phononic structure of graphene and can probe the changes of these properties under external parameters such as doping, chemical modifications, or strain. The most intense Raman active bands in graphene are the G and 2D bands. In this work, we studied their evolutions in terms of shifts, widths and intensities as a function of the laser power impinging on graphene.

Results obtained on the same graphene flake with two different environments are compared in figure 1: (i) supported on a hydrophilic SiO2/Si substrate, and (ii) suspended over a trench etched into this substrate. The relative variations of the 2D and G band positions as a function of the laser power are displayed. This specific representation disentangles doping and strain effects [2]. It is demonstrated that the charge carrier density of graphene exfoliated on a SiO2/Si substrate can be finely and reversibly tuned between hole (p-type) and electron (n-type) doping, across neutral state (i-type) with visible photons. This effect is significantly affected by the substrate cleaning method and completely suppressed in suspended

graphene. The observed phenomenon has a subsecond characteristic time and does not involve the chemical modification of graphene. The results were interpreted in terms of a local and reversible perturbation of the chemical equilibrium established between graphene, the substrate and the ambient atmosphere by laser-induced heating. Raman mapping experiments were also performed and allowed to show that these laser-induced doping variations are uniform across the graphene surface.

One technical implication of our study for the entire scientific community using Raman spectroscopy of graphene as a routine characterization technique is that it should be considered as potentially invasive as far as electronic properties are concerned. On the other hand, the ability to tune the charge carrier density with visible photons opens a wide set of opportunities to develop optically gated graphene electronic devices and a new approach to graphene optoelectronics. Finally, this effect should allow studying the interplay between graphene properties and the environment and triggering laser-assisted functionalization of graphene leading to more advanced devices. Antoine Tiberj1,2, Miguel Rubio-Roy3, Matthieu

Paillet1,2, Jean-Roch Huntzinger1,2, Périne Landois1,2, Mirko Mikolasek1,2, Sylvie Contreras1,2, Jean-Louis Sauvajol1,2, Erik Dujardin3, Ahmed-Azmi Zahab1,2

1Université Montpellier 2, Laboratoire Charles Coulomb UMR 5221, Montpellier, France 2CNRS, Laboratoire Charles Coulomb UMR 5221, Montpellier, France 3CEMES-CNRS, Université de Toulouse, Toulouse, France

[email protected]

R


References

[1] A. Tiberj, M. Rubio-Roy, M. Paillet, J.-R.

Huntzinger, P. Landois, M. Mikolasek, S. Contreras, J.-L. Sauvajol, E. Dujardin, A.-A. Zahab, Scientific Reports, 3 (2013) 2355.

[2] J. E. Lee, G. Ahn, J. Shim, Y. S. Lee, S. Ryu, Nature Communications, 3 (2012) 1024.

Figures

Figure 1: Comparison of the relative evolutions of the 2D band position (ω2D) versus the G band position (ωG) as a function of the laser power (Plaser) for supported and suspended graphene flakes. The color code of each point corresponds to the incident Plaser as displayed on the right hand side color bar. The supported flake is p-doped at low Plaser, it becomes quasi-neutral around 0.5 mW and n-doped for higher Plaser. The suspended graphene flake is neutral and stays neutral with the increasing Plaser. The measured shifts for the suspended flake are only due to laser heating effects. Each plot includes both increasing and decreasing power sweeps demonstrating the reversibility of this photodoping effect.


lectrically controllable strong light-matter interactions with graphene

KLAAS-JAN TIELROOIJ

ICFO

Spain

The nanoscale interaction between light emitters and graphene can lead to a plethora of novel applications in fields such as sensing and telecommunications. Here we demonstrate a hybrid device containing graphene and near infrared light emitters that play a very important role in modern telecommunications: Er3+ ions with stimulated emission at ~1530 nm. By changing the Fermi energy of graphene, we demonstrate, for the first time, the unique capability of in-situ tuning of the optical density of states, experienced by emitters placed nearby the graphene. In particular, we access three distinct regimes of emitter–graphene coupling: i) the non-radiative coupling regime, ii) the reduced coupling regime, and iii) the plasmonic regime. We witness the transition through these regimes by monitoring the lifetime of the emitters and their emission, which are strongly modified by the graphene. In the first regime, non-radiative coupling leads to energy transfer from the emitter to the graphene sheet, where electron-hole pairs are generated with the same energy as the excited state dipole of the emitter. By increasing the Fermi level in graphene, we suppress almost completely this energy transfer process, and thus the emitter–graphene coupling strength. Therefore, in this second regime the emission and the excited state lifetime approach the same value as for uncoupled emitters. Finally, in the third regime, the very high electron density can give rise to collective excitations (plasmons). We will discuss graphene plasmons in the context of light-matter interactions, for both near-infrared and mid-infrared frequencies.

K.J. Tielrooij1, L. Orona2,

M. Badioli1,

L.

Gaudreau1, S. Coop1,

Q. Ma2,

G. Navickaite1,

H.

de Riedmatten1, P. Goldner3,

F.J. Garcia de

Abajo1, A. Ferrier3,

P. Jarillo-Herrero2,

and F.H.L.

Koppens1 1ICFO - Institut de Ciéncies Fotoníques, Mediterranean Technology Park, Castelldefels (Barcelona) 08860, Spain; 2Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; 3Chimie ParisTech, Laboratoire de Chimie de la Matière Condensée de Paris, CNRS-UMR 7574, UPMC Univ Paris 06, 11 rue Pierre et Marie Curie 75005 Paris, France.

[email protected]

E


ulti-Scale Simulations of Graphene for Energy Applications

VALENTINA TOZZINI

Istituto Nanoscienze del CNR

Italy

Graphene is currently considered one of the most promising materials due to its extreme versatility. In its pristine form, it is a high mobility conductor, mechanically resistant and flexible, and was proposed for a number of applications in the field of energy storage and harvesting [1,2]. However, for all of these applications, its interactions with different chemical species and/or with external stimuli must be considered. The enhanced graphene reactivity of rippled graphene towards adatoms has been theoretically studied within the Density Functional Theory (DFT) framework [3] and proposed as a mean to create partially hydrogenated graphene structures [4] with tunable semiconducting properties [5]. Furthermore, we recently showed by means of DFT calculations and Car-Parrinello simulations that this property could be used as the base of a hydrogen storage-release working at room temperature: hydrogen preferentially chemisorbs on convexities, then, provided the curvature can be controlled and inverted, it spontaneously release from concavities [6] (see Fig 1). The first part of this mechanism (chemisorption) was experimentally confirmed using naturally rippled graphene grown on SiC [7]. However, further investigations are necessary to solve a number of problems, for instance (i) the accurate control of the local curvature, statically and dynamically (ii) the evaluation of behavior of the system at the macroscopic level. We proposed different possible solution to (i), namely optical control by functionalization with photosensitive molecules, electro-mechanical control, exploiting piezo or flexo electricity. In both cases

chemisorption [8] or subsitutional doping [9] with different chemical species can be considered to control the electro-mechanical properties of graphene. However both for (i) and for (ii), also the molecular dynamics and thermodynamics of the system at the large size and time scales must be studied. We propose a multi-scale molecular simulation and modeling approach (Fig 2) to address all the different aspects of the problem. DFT calculations and simulations are used to evaluate the quantum chemical properties of the system at the 1-10 nm scale in different conditions. Specifically we evaluated the hydrogen binding properties as a function of the local curvature and on ripples of different sizes and geometry and the effect of the external electric field and of N and B doping on the mechanical and reactive properties. Subsequently, we transfer the quantitative information obtained at the DFT level into a classical empirical force field. This allows molecular dynamics simulations at the 10-100 nm scale, and, especially, to extend the analysis of the mechanical behavior of the system in the time scale, up to hundreds of ns, enabling evaluation of the statistical behavior. Finally, the knowledge get from the two atomistic levels of analysis is transferred to a continuum mechanicistic model [6-9], representing the system as a 2D membrane, onto which all the properties are mapped. This allows the analysis of the statistical properties of the system on the macroscopic scale. This approach combines the accuracy of atomistic and quantum calculations with the extensive and large scale view given by the empirical-continuum models, allowing quantitative evaluation of the feasibility of devices for energy applications.

M


Valentina Tozzini1, Dario Camiola1, Riccardo

Farchioni1,2, Antonio Rossi1,2, Tommaso Cavallucci1,2, Nicola Pugno3, Antonino Favata3,

Stefan Heun1, Vittorio Pellegrini1,4

1Istituto Nanoscienze del Cnr, Lab NEST, Scuola Normale Superiore, Pisa, Italy 2Università di Pisa, Dipartimento di Fisica E Fermi,

Pisa Italy 3Laboratory of Bio-Inspired and Graphene Nanomechanics, Dipartimento di Ingegneria, Università di Trento, and Fondazione Bruno Kessler, Trento Italy 4Istituto Italiano di Tecnologia, Graphene Labs,

Genova, Italy

[email protected]

References

[1] V. Tozzini, V. Pellegrini PCCP, 15, (2013) 80 [2] F Bonaccorso, L Colombo, G Yu, M

Stoller, V Tozzini, A C Ferrari, R S Ruoff, V Pellegrini “Graphene, related two dimensional crystals, and hybrid systems for energy conversion and storage”, review, submitted to Science

[3] Danil W. Boukhvalov and Mikhail I. Katsnelson J. Phys. Chem. C 113, (2009), 14176–14178

[4] Z. F. Wang, Yu Zhang, and Feng Liu Phys Rev B 83, (2011)041403(R)

[5] V. Tozzini, V. Pellegrini Phys Rev B, 81 (2010) 113404

[6] V. Tozzini, V. Pellegrini J Phys Chem C, (2011) 115, 25523

[7] S Goler, C Coletti, V Tozzini, V Piazza, T Mashoff, F Beltram, V Pellegrini, S Heun J Phys Chem C 117 (2013) 11506–11513

[8] MT. Ong, K-A N. Duerloo, and E J. Reed,. Phys. Chem. C (2013) 117, 3615−3620

[9] Z. M. Ao and F. M. Peeters J. Phys. Chem. C (2010), 114, 14503–14509

[10] J. Zang, Q. Wang, Q. Tu, S. Ryu, N. Pugno, M. Buehler, X. Zhao Nat.Mat (2013).

[11] N.Pugno, J. Mech. Phys. Solid., (2010) 58, 1397

[12] X. Shi, Y. Cheng, N. Pugno, H. Gao, Small, (2010) 6, 739

Figures

Figure 1: Left: hydrogen binding energy as a function of the curvature (positive=convex, negative=concave). Right simulation of the hydrogen release upon curvature inversion by a transverse traveling wave.

Figure 2: The three levels of the multi-scale representation of graphene. From left to right, a 2nmx2nm supercell of rippled graphene with a dimer of hydrogen atoms attached. A iso-charge surface of energy states near the Fermi level is represented in orange; a ~1000 atoms supercell of graphene with an hydrogenated ripple; a surface representation of buckled graphene.


onductivity of graphene and rotated graphene bilayers with point defects

GUY TRAMBLY DE LAISSARDIERE Universite de Cergy-Pontoise France

Electronic transport in graphene is sensitive to static defects that are for example frozen ripples, screened charged impurities, or local defects like vacancies or adsorbates (Refs.[1-4] and Refs. therein). Adsorbates, which can be organic groups or adatoms attached to the surface of graphene, are of particular interest in the context of functionalisation which aims at controlling the electronic properties by attaching atoms or molecules to graphene ([5-7] and Refs. therein). Therefore there is a need for a theory of conductivity in the presence of such defects.

We propose a unified description of transport in one and two graphene sheets with adsorbates that fully takes into account localization effects and loss of electronic coherence due to inelastic processes [10,11]. For the monolayer case, we focus on the role of the scattering properties of the adsorbates and analyse in detail cases with resonant or nonresonant scattering. Sufficiently far from the Dirac energy and at sufficiently small concentrations the semi-classical theory can be a good approximation. Near the Dirac energy we identify different quantum regimes, where the conductivity presents universal behaviours. In rotated grapheme bilayers, we analyse in particular the case where defects are in just one layer but affect transport in the other layer due to the interlayer coupling. Numerical results on the role of rotation angle and defects concentration confirm a simple analytical model.

Method: Theoretical studies of transport in the presence of local defects have dealt mainly either with the Bloch- Boltzmann formalism or with self-consistent approximations ([5-10] and Refs. Therein). In these theories a major length scale that characterizes the electron scattering is the elastic mean-free path Le .

These approaches indeed explain some experimental observations but yet these theories have important limitations and can hardly describe in detail the localization phenomena that has been reported in some experiments [4]. Indeed in the presence of a short range potential, such as that produced by local defects the electronic states are localized on the length scale ξ [5-7]. A

sample will be insulating unless some source of scattering, like electron-electron or electron-phonon interaction, leads to a loss of the phase coherence on a length scale Li < ξ.

Therefore, in addition to the elastic mean free path Le the inelastic mean-free path LI and the localization length ξ play also a

fundamental role for the conductivity of graphene with adsorbates. Here we use a numerical approach for the conductivity that treats exactly the tight-binding Hamiltonian and takes fully into account the effect of Anderson localization. The quantum diffusion evaluate numerically using the MKRT approach [12]. It gives access to the characteristic lengths and to the conductivity as a function of the concentration, the Fermi energy EF and the inelastic mean-free path Li [10]. In real samples Li depends on the temperature, or magnetic field, but it is an adjustable parameter in this work.

Results: Our results confirm that sufficiently far from the Dirac energy ED and for sufficiently small adsorbates concentrations, the Bloch-Boltzmann theory and the self-consistent theories are valid. Near the Dirac energy we identify different regimes of transport that depend on whether the adsorbates produce resonant or nonresonant scattering. We show also that a proper tight-binding model of graphene which includes hopping beyond the nearest neighbor leads to sizable modifications of the scattering properties with respect to the mostly used nearest neighbor hopping model.

C


Some universal aspects of the conductivity are present with or without the hoping beyond nearest neighbors. For small inelastic scattering length Li such as Li ≃ Le the conductivity σ is almost equal to the universal minimum (plateau) of the microscopic conductivity (semi-classical conductivity) σM≃ 4 e2 /(πh) except for EF ≃ ED when the model only takes into account nearest neighbour hopping (figure 1). For larger Li, Le<Li, the conductivity follows a linear variation with the logarithm of Li with nearest neighbor hopping only and with hopping beyond nearest neighbours (figure 2). In contrast, the high central peak of the conductivity and the anomalous behaviour at the Dirac energy are not robust and are specific to the model with nearest neighbor hoping only. Therefore, we conclude that a precise comparison of conductivity with experiments requires a detailed description of the electronic structure and in particular of that of graphene. In twisted bilayer graphene, the effective coupling between electronic states of the two layers increases when the angle of rotation decreases, and electronic confinement is obtained for very small angle [13,14,15]. Consequences on transport in twisted bilayer with adsorbates are presented and confirmed by a analytical model. Guy Trambly de Laissardière1, Omid Faizy

Namarvar2, Nhung T. T. Nguyen3, Petrutza Anghel-

Vasilescu2, Laurence Magaud2, Didier Mayou2 1Université de Cergy-Pontoise, Laboratoire de Physique Théorique et Modélisation, Cergy-Pontoise, France 2Institut Néel, CNRS/UJF, Grenoble, France. 3Theoretical and Computational Physics Department, Institute of Physics, VAST, Hanoi, Vietnam [email protected]

References

[1] C. Berger et al., J. Phys. Chem. B 108,

19912 (2004); Science, 312, 1191

(2006).

[2] K. S. Novoselov et al. Nature, 438, 197

(2005).

[3] Y. Zhang et al., Nature, 438, 201

(2005).

[4] X. Wu et al., Phys. Rev. Lett. 98, 136801

(2007); 101, 026801 (2008).

[5] N. M. R. Peres, F. Guinea, and A. H.

Castro Neto, Phys. Rev. B 73, 125411

(2006); N. M. R.Peres, J. Phys.: Cond.

Mat. 21, 323201 (2009).

[6] N. Leconte et al., ACS Nano 4, 4033

(2010).

[7] S. Roche et al., Solid States Comm.

152, 1404 (2012); A. Cresti et al., Phys.

Rev. Lett. 110,196601 (2013).

[8] T. O. Wehling et al., Phys. Rev. Lett.

105, 056802, (2010).

[9] G. Trambly de Laissardière and D.

Mayou, Mod. Phys. Lett. B, 25 1019,

(2011).

[10] G. Trambly de Laissardière, D. Mayou,

Phys. Rev. Lett. 111, 146601 (2013).


Adv. Nat. Sci.: Nanosci. Nanotechnol.

5, 015007 (2014).

[12] D. Mayou, Europhys. Lett.6,549 (1988);

D. Mayou and S. N. Khanna, J. Phys. I

Paris 5, 1199(1995); S. Roche and D.

Mayou, Phys. Rev. Lett. 79, 2518

(1997); F. Triozon et al., Phys. Rev. B 65,

220202 (2002).


L. Magaud, Nano Lett. 10, 804 (2010).


L. Magaud, Phys. Rev. B 86, 125413

(2012).

[15] I. Brihuega et al., Phys. Rev. Lett. 109,

196802 (2012).


Figures

Figure 1: Microscopic conductivity σ versus energy E, for concentration () c = 1%, () c=2% and () c = 3% of resonant scatters. (dashed line) first neighbor coupling only, (continuous line) beyond first neighbor coupling. G0=2 e2/h [11].

Figure 2: Conductivity σ versus the inelastic scattering length Li for concentration c (%) and different energies E (eV) in the plateau of σM (E): (dashed line) first neighbor couplingonly, (continuous line) beyond first neighborcoupling [11].


lectron Interaction and Tunneling in Graphene-Based Heterostructures

EMANUEL TUTUC University of Texas at Austin USA

Recent years have witnessed a tremendous research interest in atomic layer semiconductors, such as graphene and more recently transition metal dichalcogenides. These semiconductors possess unique properties such as linear energy momentum dispersion in grapheme mononolayer, tunable band-gap in graphene bilayers, or large effective mass (~0.5me) in transition metal dichalcogenide layers. Vertical heterostructures, consisting of such atomic layers separated by insulators can enable novel tunneling devices, and also open a window to explore electron interaction effect in these materials, otherwise not accessible in single layer devices. In this presentation we discuss two examples of vertical heterostructures of atomic layer materials, where electron interaction plays a key role. We examine the electron transport in graphene-MoS2 heterostructures, fabricated using a layer-by-layer transfer approach [Fig. 1]. Four point conductivity measurements of the heterostructure as a function of back-gate bias show ambipolar characteristics, with a clear conductivity saturation on the electron branch. Magnetotransport measurements reveal that the conductivity saturation marks the onset of MoS2 being populated with electrons. Most surprisingly, once the MoS2 becomes populated with electrons the carrier density in graphene decreases with increasing gate bias, a finding indicating that the MoS2 electrons have a negative compressibility [1].

A second type of heterostructure examined here are double bilayer heterostructures, consisting of two

bilayer graphene flakes separated by hexagonal boron-nitride [Fig. 2]. Using the top layer as a resistively detected Kelvin probe we map the chemical potential of the bottom bilayer graphene as a function of electron density, perpendicular magnetic field, and transverse electric field. At zero magnetic field the chemical potential reveals a strongly non-linear dependence on density, with an electric field induced energy gap at charge neutrality. The data allow a direct measurement of the electric field-induced bandgap at zero magnetic field, the orbital Landau level (LLs) energies, and the broken symmetry quantum Hall state gaps at high magnetic fields [2]. We observe spin-to-valley polarized transitions for all half-filled LLs, as well as novel phases at filling factors ν = 0 and ν =±2, theoretically expected to have quantum coherence between different LLs. The interlayer transport shows a gate tunable negative differential resistance suggestive of momentum conserving tunneling. Emanuel Tutuc, Kayoung Lee, Stefano Larentis,

Babak Fallahazad and Jiamin Xue Microelectronics Research Centers, The University of Texas at Austin, 10100 Burnet Rd, Austin, TX 78758, U.S.A.

[email protected]

E


References

[1] S. Larentis, et al., “Band Offset and

Negative Compressibility in Graphene-MoS2 Heterostructures”, Nano Letters (2014), dx.doi.org/10.1021/nl500212s.

[2] K. Lee, et al., “Chemical potential and quantum Hall ferromagnetism in bilayer graphene mapped using double bilayer heterostructures”, arXiv:1401.0659 (2014).

Figures

Figure 1: (a) Schematic representation of a back-gated graphene-MoS2 heterostructure on a Si/SiO2 substrate. (b) Optical micrograph of a fabricated graphene-MoS2 Hall bar.

Figure 2: (a) Schematic representation of the double bilayer heterostructure. Back-gate (VBG) andinterlayer (VTL) biases can be applied for the sample characterization. (b) False color optical micrograph of adevice. The dashed yellow (dashed red) contour marks the top (bottom) layer; thedashed green line marks the interlayer hBN perimeter.


VD and applications of standing, dendritic and continuous graphene and their hybrids

YONHUA TZENG National Cheng Kung University Taiwan

Plasma Enhanced Chemical Vapor Deposition (PECVD) and thermal CVD of graphene films, single-domain graphene dendrites, standing graphene structures, hybrid graphene-diamond nanoplatelets, and hybrid graphite-diamond coatings and their properties and applications will be reported. Grain-boundary engineering of CVD thin-film graphene, diamond, and their hybrids allows novel functions of these nanoscale carbon materials to be tailored for practical applications.

Under the influence of electric field in the plasma sheath and the impinging ions and neutral radicals, standing nanocrystalline multi-layer graphene microstructures are grown on graphene, diamond, and non-carbon substrates [1-3]. These standing graphene structures develop into interconnected wall-like microstructures, also known as carbon nanowall, which exhibits a porous surface morphology with a large effective surface area and sharp edges. By properly controlling the synthesis conditions, such standing multi-layer graphene nanowalls can be incorporated with nanocrystalline diamond to form hybrid graphene-diamond films of high electrical conductivity while preserving essential part of diamond’s chemical and electrochemical properties. Incorporated nanodiamond in the hybrid graphene-diamond structure serves as diamond seeds to allow the coating on it a continuous nanodiamond film. This allows a biochemically inert nanodiamond coated electrical circuit to be built as an all-carbon structure such as an electrosurgical tool. The embedded low resistance multi-layer graphene provides a low-resistance and, therefore, allowing low applied voltage for delivering higher power to a resistive heater. Low-voltage and high-

power-density are desirable for special applications such as electrosurgical tools. In the first part of this presentation, the fabrication of an all-carbon and nanodiamond encapsulated resistive heater for biomedical applications will be discussed.

By controlling orientation and shape dependent anisotropic growth rate of graphene by competitive graphene growth and etching processes, single-domain monolayer graphene dendrites of excellent crystalline quality have been achieved [4]. These graphene dendrites exhibit branch aspect ratios higher than 100 and very large ratios of the length of edge lines to their surface areas. Unique electrochemical properties of the long graphene edges and the high electronic quality graphene branches without domain boundaries provide opportunities for special applications of graphene. The atom scale sharp edges allows electron field emission at a low applied voltage. By shifting the competitive growth and etching processes towards a higher growth rate and a lower etch rate, the width of graphene branches is allowed to increase and eventually leading to the merger of neighboring graphene branches. The high growth rate of graphene primary branches, the high crystalline quality of graphene, and the controllable gap spacing between graphene branches provide us a novel route to the growth of high quality graphene films of large domain sizes. In the second part of this presentation, the synthesis process and the mechanisms for the formation of high-aspect-ratio graphene dendrites will be discussed along with their properties and potential applications.

Continuous graphene films synthesized by conventional CVD or by the merger of branches of dendritic graphene possesses excellent properties for electronic and optoelectronic applications. In the third part of this presentation, effects of graphene and

C


hydrogenated graphene on surface enhance Raman scattering of molecules and photo-induced conductivity change in graphene and hydrogenated graphene [5] will be presented. Hydrogenated graphene is among the thinnest possible electrical insulators which is stable in the ambient atmosphere. It, therefore, serves as an excellent encapsulation layer to protect metallic nanoparticles such as silver from undesirable environmental reactions without significantly attenuating the SERS effects for sensor applications. On the other hand, intrinsic graphene is a zero-gap semimetal. Hot carrier effects and carrier multiplication in graphene result in special optoelectronic effects such as reduced conductivity upon light illumination, which will be discussed as negative photoconductivity in this part of presentation. The basic mechanisms and methods of enhancing the photoresponse will be presented. Environmental effects on photoresponse are discussed [6]. Applications of such negative photoconductivity including photodetectors will be discussed. Yonhua Tzeng Institute of Microelectronics Advanced Optoelectronics Technology Center National Cheng Kung University

No. 1 University Road Tainan 70101, Taiwan [email protected]

References

[1] Yonhua Tzeng, Wai Leong Chen,

Chiahao Wu, Jui-Yung Lo, and Chiuan-Yi Li, The synthesis of graphene nanowalls on a diamond film on a silicon substrate by direct-current plasma chemical vapor deposition. Carbon 53 (2013) 120-129.

[2] Chia-hao Tu, Waileong Chen, Hsin-Chiao Fang, Yonhua Tzeng* and Chuan-Pu Liu, Heteroepitaxial nucleation and growth of graphene nanowalls on silicon. Carbon 54 (2013) 234-240.

[3] Yonhua Tzeng, Chia-lung Chen, Young-Yi Chen, Chih-Yi Liu, Carbon Nanowalls on Graphite for Cold Cathode Applications. Diamond and Related Materials 19, 2-3 (2010) 201-204.

[4] Yonhua Tzeng, Snowflake-Like Graphene Dendrites on Polycrystalline Copper. IEEE Nanotechnology Conference, Beijing, China, August 5-9, 2013.

[5] Chih-Yi Liu, Keng-Chih Liang, Waileong Chen, Chia-hao Tu, Chuan-Pu Liu, and Yonhua Tzeng, Plasmonic coupling of silver nanoparticles covered by hydrogen-terminated graphene for surface-enhanced Raman spectroscopy. Optics Express 19, 18, (2011) 17092-17098.

[6] Chih-Yi Liu, Kengchih Liang, Chun-Cheng Chang, and Yonhua Tzeng, Effects of plasmonic coupling and electrical current on persistent photoconductivity of single-layer graphene on pristine and silver-nanoparticle-coated SiO2/Si. Optics Express 20(20) (2012) 22943–22952.


pin-Pseudospin Entanglement and Spin Relaxation in Graphene

DINH VAN TUAN ICN2 Spain

The extremely small intrinsic spin-orbit coupling (SOC) of graphene and the lack of hyper ne interaction with the most abundant carbon isotope have led to intense research into possible applications of this material in spintronic devices due to the possibility of transporting spin information over very long distances[1, 2, 3]. However, the spin relaxation times are found to be orders of magnitude shorter than initially predicted[4, 5, 6, 7, 8], while the major physical process for spin equilibration and its dependence on charge density and disorder remain elusive. Experiments have been analyzed in terms of the conventional Elliot-Yafet and Dyakonov-Perel processes, yielding contradictory results. Recently, a mechanism based on resonant scattering by local magnetic moments has also been proposed[9]. Here, we unravel a spin relaxation mechanism for nonmagnetic samples that follows from an entanglement of spin and pseudospin degrees of freedom driven by random SOC[10], which makes it unique to graphene and is markedly different to conventional processes. We show that the mixing between spin and pseudospin-related Berry's phases results in unexpectedly fast spin dephasing, even when approaching the ballistic limit, and leads to increasing spin relaxation times away from the Dirac point, as observed experimentally. This hitherto unknown phenomenon points towards revisiting the origin of the low spin relaxation times found in graphene, where SOC can be caused by adsorbed adatoms, ripples or even the substrate. It also opens new perspectives for spin manipulation using the pseudospin degree of freedom (or viceversa), a tantalizing quest for the emergence of radically new information storage and processing technologies.

Dinh Van Tuan1,2, Frank Ortmann1,3, David

Soriano1, Sergio O. Valenzuela1,4 and Stephan Roche1,4 1ICN2 - Institut Catala de Nanociencia i Nanotecnologia, (Barcelona), Spain 2Department of Physics, UAB, Spain 3 Institute for Materials Science and Max Bergmann Center of Biomaterials, Technische Universität Dresden, Germany 4ICREA, Institució Catalana de Recerca i Barcelona, Spain [email protected]

References

[1] Huertas-Hernando, D., Guinea, F.,

Brataas, A., Spin-orbit mediated spin relaxation in graphene. Phys.Rev. Lett. 103, 146801 (2009)

[2] Ertler, C., Konshush, S., Gmitra, M., Fabian, J., Electron spin relaxation in graphene: the role of substrate Phys. Rev. B 80, 045405(R) (2009)

[3] Ochoa, H., Castro Neto, A.H., Guinea, F. Elliot-Yafet mechanism in graphene. Phys. Rev. Lett. 108, 206808(2012).

[4] Tombros, N., Jozsa, C., Popinciuc, M., Jonkman, H.T., van Wees, B. J. Electronic spin transport and spin precession in single graphene layers at room temperature, Nature 448, 571-574 (2007)

[5] Avsar, A., Yang, T., Bae, S., Balakrishnan, J., Volmer, F., Jaiswal, M., Yi, Z., Ali, S.R., Guntherodt, G., Hong, B.H., Beschoten, B., Ozyilmaz, B., Towards wafer scale fabrication of graphene based spin valvedevices, Nano letters 11, 2363-2368 (2011).

[6] Han, W. & Kawakami, R. K. Spin Relaxation in Single-Layer and Bilayer Graphene. Phys. Rev. Lett. 107, 047207 (2011)

S


[7] Maassen, T., Berg, J., Fromm, F., Seyller, T., Yakimova, R., Wees, B.J.v. Long spin relaxation times inwafer scale epitaxial graphene on SiC(0001), Nano Letters 12, 1498 (2012).

[8] Dlubak, B., Martin, M.-B., Deranlot, C., Servet, B., Xavier, S., Mattana, R., Sprinkle, M., Berger, C., DeHeer, W.A., Petro, F., Anane, A., Seneor, P., Fert, A. Highly ecient spin transport in epitaxial grapheneon SiC. Nature Physics 8, 557 (2012).

[9] Kochan, D., Gmitra, M., Fabian, J. Spin relaxation mechanism in graphene: resonant scattering by magnetic impurities. arxiv:1306.0230v1

[10] Dinh Van Tuan, Frank Ortmann, David Soriano, Sergio O. Valenzuela and Stephan Roche, Spin-pseudospin entanglement and spin relaxation in graphene. Under consideration at Nature Nanotechnology.

Figures

Figure 1: Spin Dynamics in disordered graphene. (a) Ball-and-stick model of a random distribution of ad-atoms on top of a graphene sample (b) Top view of the gold ad-atom sitting on the center of an hexagon (c),(d) Time-dependent projected spin polarization Sz(E, t) of charge carriers (symbols) initially prepared in an out-of-plane polarization (at Dirac point (red curves) and at E=150meV (blue curves)). Analytical fits are given as solid lines (see text). Parameters are VI=0,007γ0, VR=0,0165 γ0, µ=0,1γ0, ρ = 0,05% (c) and ρ = 8% (d).

Figure 2: Spin relaxation times and transport mechanisms. Spin relaxation times (τs) for ρ = 0.05% (a) and ρ = 8% (b). Black (red) solid symbols indicate τs for µ = 0.1 γ0 (µ = 0.2 γ0). TΩ vs. E is also shown (open symbols). τρ

(dotted line in (b)) is shown over a wider energy range (top x-axis) in order to stress the divergence around E = 0 (µ = 0.2 γ0). We cannot evaluate τρ below 100 meV, since the diffusive regime is not established within our computational reach. Panels (c) and (d): Time dependent diffusion coefficient D(t) for ρ = 0.05% and ρ = 8% with µ = 0.20 γ0.


lectron spin relaxation in bilayer graphene and monolayer MoS2

LIN WANG University of Science and Technology of China China

Electron spin relaxation due to the D’yakonov-Perel’ mechanism is investigated in bilayer graphene with only the lowest conduction band being relevant [1]. We construct the spin-orbit coupling Ωµ(k) from the symmetry group analysis where the coefficients are obtained by fitting to the numerical results following the work by Konschuh et al. [Phys. Rev. B 85 (2012) 115423] from first principles. Specifically = αksinθ + μαsin2θ +

αsin4θ, = −αkcosθ +

μαcos2θ − αcos4θ and =

μβk + βkcos3θ with µ=1(-1) for K(K´). The leading term of the out-of-plane component

serves as a Zeeman-like

term with opposite effective magnetic fields in the two valleys. This provides an intervalley inhomogeneous broadening, which leads to intervalley spin relaxation in the presence of intervalley scattering. The intervalley electron-phonon scattering strongly suppresses the in-plane spin relaxation time at high temperature whereas the intervalley short-range scattering plays an important role in the in-plane spin relaxation especially at low temperature. A marked nonmonotonic temperature dependence of the in-plane spin relaxation time with a minimum of several hundred picoseconds is predicted without short-range scatterers. This minimum is comparable to the experimental data. The nonmonotonic behavior is attributed to the crossover between the weak and strong intervalley electron-phonon scattering. In addition, a peak is predicted in the electron density dependence of the in-plane spin relaxation time at low temperature. We also find a rapid decrease of the in-plane spin relaxation time with increasing initial spin

polarization at low temperature, which is opposite to the situation in semiconductors and single-layer graphene. Moreover, we carry out a detailed comparison with the existing experiments of Han and Kawakami [Phys. Rev. Lett. 107 (2011) 047207], Avsar et al. [Nano Lett. 11 (2011) 2363] and Yang et al. [Phys. Rev. Lett. 107 (2011) 047206] in the temperature and electron-density dependences of the spin relaxation time as shown in Fig. 1. Excitingly, our results are comparable to the experimental data at high temperature without short-range scatterers, indicating the importance of the intervalley spin relaxation channel due to intervalley electron-phonon scattering in the in-plane spin relaxation at high temperature. As for low temperature, with the short-range scatterers included, the spin relaxation time from our calculation shows a fairly good agreement with the experimental data.

Similar to bilayer graphene, the intervalley spin relaxation process due to the D’yakonov-Perel’ mechanism also exists in the new two-dimensional material, i.e., monolayer MoS2 [2]. However, in contrast to bilayer graphene, a monotonic decrease of the in-plane spin relaxation time in the temperature dependence is observed since the intervalley electron-phonon scattering is always in the weak scattering limit in monolayer MoS2. In addition to the intervalley process, the intravalley one due to the D’yakonov-Perel’ mechanism is also investigated [3]. From Fig. 2, we find that the intravalley process plays a more important role at low temperature whereas the intervalley one becomes more important at high temperature. At the temperature in between, the leading process of the in-plane spin relaxation changes from the intervalley to intravalley one as the electron density increases. Moreover, we also find that the intravalley process is dominated by the electron-electron Coulomb scattering

E


even with high impurity density. In addition to the D’yakonov-Perel’ mechanism, we also take into account the Elliot-Yafet one with the intra- and inter-valley processes included [3]. However, the contribution of the Elliot-Yafet mechanism to the in-plane spin relaxation is negligible compared with that of the D’yakonov-Perel’ one due to the marginal interband mixing for in-plane spins.

This work was supported by the National Natural Science Foundation of China under Grant No. 11334014, the National Basic Research Program of China under Grant No. 2012CB922002 and the Strategic Priority Research Program of the Chinese Academy of Sciences under Grant No. XDB01000000. L. Wang and M. W. Wu*

Hefei national Laboratory for Physical Sciences at Microscale and Department of Physics,

University of Science and Technology of China, Hefei, Anhui, 230026, China *[email protected]

References

[1] L. Wang and M. W. Wu, Phys. Rev. B 87

(2013) 205416. [2] L. Wang and M. W. Wu,

arXiv:1305.3361. [3] L. Wang and M. W. Wu,

arXiv:1312.6985.

Figures

Figure 1: Comparison of (a) temperature dependence (b) electron density dependence of the in-plane spin relaxation time with the experiment data of Han and Kawakami (HK), Avsar et al. and Yang et al.. Curves without symbols represent our numerical calculation. (a) Red solid (light blue double-dot-dashed) curve stands for our calculation corresponding to the experiment of HK without (with) short-range scatterers.

Figure 2: Left: total in-plane spin relaxation time (crosses) due to the D’yakonov-Perel’ mechanism and that calculated with only the intravalley (filled squares) or intervalley process (filled dots) included as function of temperature. In addition, in (a), curve with filled triangle up (filled triangle down) stands for the spin relaxation time due to the D’yakonov-Perel’ mechanism with only the electron-electron Coulomb (electron-impurity) scattering whereas the one with plus signs represents the in-plane spin relaxation time due to the Elliot-Yafet mechanism; Right: total in-plane spin relaxation time (crosses) due to the D’yakonov-Perel’ mechanism and that calculated with only the intravalley (filled squares) or intervalley process (filled dots) included as function of the electron density. The impurity density Ni=0.1Ne.


hemical Identification of Topological Defects in Graphene by Carbon Isotope labeling

SHENGNAN WANG NTT Basic Research Laboratories Japan

Recent progress on large scale chemical vapor deposition (CVD)-grown graphene provides a promising opportunity to scale up the fabrication of graphene-based nanoelectronics.[1] However, such large scale graphene samples tend to be polycrystalline, that is, composed of micrometer-size singlecrystalline domains of varying lattice orientation and related structural irregularities. These structural irregularities, including point defect, grain boundary, and dislocations, inevitably affect the chemical and physical properties of graphene.[2] Elucidating the topological structure in CVD graphene is crucial for its potential technological applications in electronics and related fields. Here we demonstrate a method to identify the topological defects in CVD-grown graphene by carbon isotope labeling. In a stepwise CVD process, the introduction of isotopic carbon source induced the surface exchange of 13C-12C atoms in graphene on copper substrate. Taken the advantage of the separation of the 12C and 13C Raman modes, the spatial structure of CVD graphene are observed with confocal Raman spectroscopy (as shown in Figure 1). The 13C-rich regions form a network-like structure, indicating the atom exchange is probably along the grain boundary of graphene. This isotopic labelling method provides an effective way to investigate the topological defects in graphene, and also gives new insight for understanding the growth mechanism of graphene on copper catalyst.

Shengnan Wang, Satoru Suzuki, Hiroki Hibino

NTT Basic Research Laboratories, 3-1,

Morinosato Wakamiya, Atsugi, Kanagawa, Japan [email protected]

References

[1] X. Li et al., Science, 324 (2009) 1312. [2] F. Banhart et al., ACS Nano, 5 (2011) 26.

Figures

Figure 1: Raman evidence for isotope labeling of CVD graphene

C


agnetoresistance of large-area epitaxial graphene : interactions and dislocations

HEIKO B. WEBER University of Erlangen

Germany

We report on charge transport measurements on large-area epitaxial graphene Hall bars, both monolayer and bilayer in magnetic fields below Landau quantization. In contrast to small (µm sized) samples, our mm sized samples do not show any universal conductance fluctuations, or edge effects. Consequently, in monolayers the weak localization and the electron-electron interaction correction can be accurately and consistently resolved by a careful analysis of the magnetoresistance, including interesting crossover phenomena [1]. When impurities are added, a logarithmic R(T) occurs, which should not be confused with Kondo effect [2], but rather is rather a consequence of EEI and inhomogeneities [3, 4]. Unexpectedly, the picture changes completely when bilayer graphene is investigated: a strong linear magnetoresistance occurs. We associate this effect, which is displayed in Fig. 1, as a result of the dislocation patterns we recently discovered in bilayer graphene [5]. We present careful experimental analyses and a theory that describes the linear magnetoresistance. The consideration of dislocations may further shed new light on interesting problems that have been recently discussed for bilayer graphene. Heiko B. Weber, Johannes Jobst, Ferdinand

Kisslinger, Christian Ott Chair for Applied Physics,

University of Erlangen 91058 Erlangen, Germany [email protected]

References

[1] J. Jobst, D. Waldmann, I.V. Gornyi,

A.D. Mirlin, H.B. Weber, Electron-Electron Interaction in the Magnetoresistance of Graphene, Physical Review Letters, 108 (2012).

[2] J.H. Chen, L. Li, W.G. Cullen, E.D. Williams, M.S. Fuhrer, Tunable Kondo effect in graphene with defects, Nature Physics, 7 (2011) 535-538.

[3] J. Jobst, F. Kisslinger, H.B. Weber, Detection of the Kondo effect in the resistivity of graphene: Artifacts and strategies, Physical Review B, 88 (2013) 5412.

[4] J. Jobst, H.B. Weber, Origin of logarithmic resistance correction in graphene, Nature Physics, 8 (2012) 352-352.

[5] B. Butz, C. Dolle, F. Niekiel, K. Weber, D. Waldmann, H.B. Weber, B. Meyer, E. Spiecker, Dislocations in bilayer graphene, Nature, 505 (2014) 533-537.

Figures

Figure 1: Linear magnetoresistance measured in large area devices of bilayer epitaxial graphene on SiC(0001).

M


lasma-assisted electrochemical exfoliation of graphite for rapid production of graphene sheets

KUNG-HWA WEI National Chiao Tung University Taiwan

Several methods have been developed for the preparation of graphene sheets (GSs), including chemical vapor deposition,[1] liquid-phase exfoliation of graphite,[2, 3] chemical reduction of exfoliated graphite oxide,[4, 5] and electrochemical exfoliation.[6-16] Among them, the electrochemical exfoliation of graphite is one of the simplest and most convenient methods for the large-scale production of GSs. Herein, we describe a highly efficient plasma-assisted electrochemical exfoliation method, involving a plasma-generated graphite cathode and a graphite anode, for the production of graphene sheets from electrodes in a basic electrolyte solution in a short reaction time; the production rate of GSs is six times as fast as that from conventional electrochemical methods. AFM image of the samples prepared from a diluted solution, revealing an average lateral dimensions of approximately 2.5 µm and a thickness of approximately 2.5 nm for an individual graphene sheet, corresponding to approximately seven layers of graphene. This method is quite promising because of its simple setup, low cost (graphene sheets can be derived from inexpensive commercial graphite resources), and rapid throughput. The graphene sheets produced through this process should serve as good precursors for the preparation of graphene sheet–based nanocomposites.

Kung-Hwa Wei1, Dang Van Thanh1, Lain-Jong

Li2, Chih-Wei Chu3, Po-Jen Yen1

1Department of Material Science and

Engineering, National Chiao Tung University, Hsinchu 300, Taiwan 2Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 11529, Taiwan 3Research Center for Applied Sciences, Academia Sinica, Taipei, 11529, Taiwan

[email protected]

References

[1] C.-T. Lin, P.T.K. Loan, T.-Y. Chen, K.-K.

Liu, C.-H. Chen, K.-H. Wei, L.-J. Li, Advanced Functional Materials, 23 (2013) 2301-2307.

[2] E.-K. Choi, I.-Y. Jeon, S.-Y. Bae, H.-J. Lee, H.S. Shin, L. Dai, J.-B. Baek, Chemical Communications, 46 (2010) 6320-6322.

[3] J. Geng, B.-S. Kong, S.B. Yang, H.-T. Jung, Chemical Communications, 46 (2010) 5091-5093.

[4] H.-J. Shin, K.K. Kim, A. Benayad, S.-M. Yoon, H.K. Park, I.-S. Jung, M.H. Jin, H.-K. Jeong, J.M. Kim, J.-Y. Choi, Y.H. Lee, Advanced Functional Materials, 19 (2009) 1987-1992.

[5] O.C. Compton, B. Jain, D.A. Dikin, A. Abouimrane, K. Amine, S.T. Nguyen, ACS Nano, 5 (2011) 4380-4391.

[6] J. Wang, K.K. Manga, Q. Bao, K.P. Loh, Journal of the American Chemical Society, 133 (2011) 8888-8891.

[7] K. Parvez, R. Li, S.R. Puniredd, Y. Hernandez, F. Hinkel, S. Wang, X. Feng, K. Müllen, ACS Nano, 7 (2013) 3598-3606.

P


[8] D. Wei, L. Grande, V. Chundi, R. White, C. Bower, P. Andrew, T. Ryhanen, Chemical Communications, 48 (2012) 1239-1241.

[9] M. Zhou, J. Tang, Q. Cheng, G. Xu, P. Cui, L.-C. Qin, Chemical Physics Letters, 572 (2013) 61-65.

[10] T. Lin, J. Chen, H. Bi, D. Wan, F. Huang, X. Xie, M. Jiang, Journal of Materials Chemistry A, 1 (2013) 500-504.

[11] C.T.J. Low, F.C. Walsh, M.H. Chakrabarti, M.A. Hashim, M.A. Hussain, Carbon, 54 (2013) 1-21.

[12] D.A.C. Brownson, D.K. Kampouris, C.E. Banks, Chemical Society Reviews, 41 (2012) 6944-6976.

[13] B. Erable, N. Duteanu, S.M.S. Kumar, Y. Feng, M.M. Ghangrekar, K. Scott, Electrochemistry Communications, 11 (2009) 1547-1549.

[14] M. Mao, M. Wang, J. Hu, G. Lei, S. Chen, H. Liu, Chemical Communications, 49 (2013) 5301-5303.

[15] D.V. Thanh, H.-C. Chen, L.-J. Li, C.-W. Chu, K.-H. Wei, RSC Advances, 3 (2013) 17402-17410.

[16] D. Van Thanh, L.-J. Li, C.-W. Chu, P.-J. Yen, K.-H. Wei, RSC Advances, 4 (2014) 6946-6949.

Figures

Figure 1: (a) Schematic representation of the equipment used for plasma-electrochemically exfoliated graphene sheet

Figure 2: plasma-electrochemically exfoliated graphene sheets.


ack-gated Microwave Field-Effect Transistors Based on Transferred CVD-Grown Graphene

WEI WEI Institute of Electronics, Microelectronics and Nanotechnology France

Graphene based transistors have drawn growing interest from both industries and laboratories [1-3]. In this work, we present both fabrication process and characterization of graphene field-effect transistors. Large scale monolayer graphene was grown by chemical vapor deposition (CVD) on Cu foils and transferred over pre-patterned back-gated devices on Si/SiO2 substrate. Scanning electron microscopy, Raman spectroscopy and Hall effect measurement were used for characterizing graphene quality before and after the transfer. It was found that monolayer graphene with a low defect density and hole mobility up to 3180cm2/Vs at n=1.3·1012 cm-2, could be obtained. For device characterization, we report an intrinsic current gain cut-off frequency (ft ) of 13.5 GHz and maximum oscillation frequency of 8 GHz, deduced from the S-parameters measurements. This study demonstrate the potential of CVD-grown graphene for high speed electronics in combination with a technological process compatible with arbitrary substrates [4,5]. W. Wei, M. Belhaj, G. Deokar, D. Mele, E.

Pallecchi, E. Pichonat, D. Vignaud, H. Happy Institute of Electronics, Microelectronics and Nanotechnology, CNRS UMR8520 Villeneuve d’Ascq cedex, France

[email protected]

References

[1] Frank Schwierz, Nature

Nanotechnology, 5 (2010) 487-495. [2] Frank Schwierz, Proceedings of the

IEEE, 7 (2013) 1567-1580. [3] Ji WonSuk, ACSNano, 9 (2011) 6916-

6924. [4] Nicholas Petrone, Nano Letters, 13

(2013) 121-125. [5] Felice Torrisi, ACSNano, 6 (2012) 2292-

3006.

Figures

Figure 1: Schematic of (a) the device fabrication process and (b) the graphene transfer process

B


Figure 2: (a) Raman spectra of graphene and SEM images of (b) overview of the final device and (c) activepart and (d) enlarged channel part, where has the gate length of 300nm.

Figure 3: Transfer characteristics of the device, Dirac Point is at Vgs=0.8V, the maximum Gm is -150uS/um.

Figure 4: Drain current Ids versus drain voltage Vds at different gate voltage.

Figure 5: RF characteristics, as measured (DUT) and after de-embedding (Intrinsic) of a device with gatelength Lg=200nm at Vds=2V. We found an intrinsic cut-off frequency (ft ) of 13.5 GHz and a maximum oscillation frequency of 8 GHz.


ayered assembly of hierarchical graphene/Ni-Al hydroxide composites for supercapacitors

YASODINEE WIMALASIRI University of South Australia Australia

A high performance supercapacitor electrode material was synthesized by a novel route using 2D nanosheets of graphene oxide and delaminated-layers of Ni-Al double hydroxide (LDH) in aqueous dispersion. The composite— G/Ni-Al LDH-based electrodes provided a specific capacitance of 915 F/g at a current density of 2 A/g based on the total mass of active materials in the absence of conductive additives. After 1500 cycles at 10 A/g current density, 95% of the initial capacitance was retained.

Supercapacitors (SCs), also known as electrochemical capacitors (ECs), are recognized as a class of promising electrical energy storage devices that possesses high power performance, long cycle life, short charging time and good safety[1, 2], which are complements to batteries and conventional capacitors. The capacitance of a supercapacitor is largely dependent on the electrode material and thus research advancement in developing high performance electrode materials is of great importance[3]. Graphene based electrodes have been comprehensively studied as supercapacitors due to high theoretical electrical double layer capacitance[4]. Significant improvements in capacitance can be achieved by the controlled structural design of graphene based composites. Utilizing pseudocapacitive materials such as layered double hydroxides (LDH) could further enhance capacitance in such composites [5], and the research on this specific topic is still limited. This study presents an innovative route to prepare a composite material with graphene and Ni/Al LDH and evaluates the performance of the composite as supercapacitors.

Ni-Al LDH was synthesized by urea hydrolysis under hydrothermal conditions and the wet cake was exfoliated in water by simple stirring and mild ultrasonication. A reasonably stable colloidal suspension having a zeta potential of +36.2 mV at pH 3 was formed. This Ni-Al LDH suspension was mixed with an aqueous dispersion of graphene oxide with a zeta potential of –47 mV at pH 5. Subsequently the graphene oxide was reduced to graphene by chemical reduction using hydrazine monohydrate in the presence of Ni-Al LDH. The resultant graphene/Ni–Al LDH (G/Ni-Al LDH) composite had 40% of Ni-Al LDH and revealed a hierarchical nanostructure where graphene and Ni-Al LDH sandwiched each other. The electrical resistivity of the composite was 0.67 Ω.cm. The composite material demonstrated a superior electrochemical performance due to the synergistic effect from the electrical double layer capacitance of graphene and pseudocapacitance of Ni-Al LDH. The high electrochemical performance and facile aqueous-based synthesis route demonstrated that the G/Ni-Al LDH composite can be a promising electrode material for supercapacitor applications. Yasodinee Wimalasiri and Linda Zou

SA Water Centre for Water Management and Reuse, University of South Australia, Mawson Lakes, SA 5095, Australia [email protected]

L


References

[1] Conway BE. Electrochemical

Supercapacitors: Scientific Fundamentals and Technological Applications: Springer; 1999.

[2] Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nat Mater. [10.1038/nmat2297]. 2008;7(11):845-54.

[3] Wang Y, Xia Y. Recent Progress in Supercapacitors: From Materials Design to System Construction. Advanced Materials. 2013;25(37):5336-42.

[4] Xu C, Xu B, Gu Y, Xiong Z, Sun J, Zhao XS. Graphene-based electrodes for electrochemical energy storage. Energy & Environmental Science. [10.1039/C3EE23870A]. 2013;6(5):1388-414.

[5] Gao Z, Wang J, Li Z, Yang W, Wang B, Hou M, et al. Graphene Nanosheet/Ni2+/Al3+ Layered Double-Hydroxide Composite as a Novel Electrode for a Supercapacitor. Chemistry of Materials. 2011;23(15):3509-16.

Figures

Figure 1: Synthesis process steps of the G/Ni-Al LDH composite (a) the schematic representation and (b) the corresponding digital photographs.

Figure 2: The XRD patterns for graphene, Ni-Al LDH and G/Ni-Al LDH composite

Figure 3: TEM images of (a) dry Ni-Al LDH crystals; (b) exfoliated Ni-Al LDH nanosheets; (c) graphene; (d) G/Ni-Al LDH (e) cross sectional SEM image of G/Ni-Al- LDH, scale bar is 5 µm, and (f) the corresponding overlay of elemental distributions of C, Ni and Al by EDX, scale bar is 2 µm, green-C, yellow and brown- Ni-Al LDH

Figure 4: electrochemical characterization (a) Variation of the average specific capacitance of G/Ni-Al LDH, Ni-Al LDH and graphene with scan rate (b) Galvanostatic discharge performance and the specific capacitance vs Cycle number at a current density of 10 A/g; (c) Ragone plot based on the mass of active material for G/Ni-Al LDH composite. The power densities and energy densities were calculated from the galvanostatic discharge curves at various current densities.


raphene in China

XIAOYUE XIAO China Innovation Alliance of the Graphene Industry

China

In the past years, the NNSF has launched more than 400 millions RMB to cultivate R&D projects of graphene in China. Today's graphene-based companies are closely related to the above mentioned R&D projects. In July 13 of 2013, China Innovation Alliance of the Graphene Industry (CGIA) was established by a group of laboratories and companies, under the guidance of China Industry-University Research Institute Collaboration Association. It also receives a variety of supports from the Ministry of Science and Technology, Ministry of Industry and Information Technology, National Development and Reform Commission, and National Natural Science Foundation. Now it covers most of institutes and companies related to graphene. Since then, CGIA has accomplished several significant jobs:

1. The Intellectual Property Committee published a patent report in October, 2013 [1].

2. The Standardization Committee published the first standard document in December, 2013 [2].

3. The Expert Panel and Industrialization Committee supported the establishments of Wuxi Graphene Industry Park and Qingdao Graphene Industry Park in 2013.

4. The International Business Cooperation Committee has initiated the "2014 International Graphene Innovation Conference" in September, 2014 [3], and has proposed to establish "Global Graphene Alliance".

Meanwhile, some significant events have been accomplished:

1. Prof. Chao Gao reported the lightest materials - carbon sponge (density = 0.16 mg/cm3) [4].

2. AWIT INC, a Chinese smartphone maker, sold out its first batch of 2000 units of graphene-smartphone AWIT AT26 [5].

3. The Sixth Element (Changzhou) Materials Technology Co., Ltd announced that it has launched a production line of 100 tons of graphene oxide/graphene per year [6].

4. Ningbo Morsh Technology announced that it had opened the world’s largest graphene production facility in Ningbo, making 300 tons of graphene per year [7].

Xiaoyue Xiao Ph.D., Director, International Business Cooperation Committee China Innovation Alliance of the Graphene Industry

China [email protected]

G


References

[1] "Report on Patenting Activity of

Graphene Technology", CGIA, October, 2013.

[2] "Definitions and Terminalogy of Graphene Materials", Q/LMO1CGS001-2013, CGIA, December, 2013.

[3] http://c-gia.org/ http://www.grapheneconf.com/2014/Scienceconferences_Graphene2014.php

[4] Haiyan Sun, Zhen Xu, Chao Gao (2013), Multifunctional. Ultra-Flyweight, Syringistically Assembled Carbon Aerogels, Advanced Materials, Vol. 25, Issue 18, Page 2554-2560.

[5] http://www.awit.com.cn/mobile.htm [6] http://www.jgri.gov.cn/content.aspx?id=438l [7] http://www.morsh.cn/newsDetail.asp?id=240


ntegrated Graphene Researches at Sungkyunkwan University

HEEJUN YANG Sungkyunkwan University, Korea

Two major organizations for integrated graphene researches are present at Sungkyunkwan University (SKKU); Center for Integrated Nanostructure Physics (CINAP) and SKKU Advanced Institute of Nano Technology (SAINT).

CINAP is cutting edge research institute founded by Institute of Basic Science (IBS) in Korea. The center is to secure creative knowledge and fundamental technology for the future through world-class basic science research in Korea. The CINAP goals are to conduct outstanding research in the fields of fundamental and applied physics of low dimensional structures such as hybrid graphene structure and to make young scientists committed to nanophysics and nanoscience. To establish interdisciplinary research center on nanostructures incorporating condensed matter physics, biophysics, organic and inorganic chemistry, and material science, we established five groups, consisting of 1) synthesis of hybrid nanostructures and their new functional properties group, 2) structure analysis group, 3) photo-thermoelectric group, 4) correlation nanoscopy group, 5) computational modeling group. 11 professors from SKKU are participating in the research of CINAP.

SAINT is an unique graduate school for an interdisciplinary collaboration between SKKU and Samsung electronics. With the strategic alliance of both organizations, fundamental understanding of graphene applications is being investigated. Sumio Iijima is the director of SAINT and 28 professors from SKKU are participating in graphene research. Staff scientists from SAIT are also doing the close collaboration with SAINT.

Heejun Yang

Sungkyunkwan University, Korea

[email protected]

Figures

Figure 1: Free standing iron membrane suspended in graphene pores from CINAP (Science,2014)

Figure 2: Role-to-role production of graphene films for transparent electrodes from SAINT

I


ight-matter interaction in 2D materials: from graphene to TMDs

TING YU Nanyang Technological University Singapore

Graphene and other atomically thin transition metal dichalcogenides (TMDs), as exceptional two dimensional materials, possess extremely promising potential for fundamental studies and practical applications. Here we report our studies on 2D materials such as graphene, MoS2, TaSe2 and WS2. Photons, electrons, phonons and the interaction among them are systematically investigated through various optical probes. The results presented here are highly relevant to the application of 2D materials in nano-electronics and optoelectronics and help in developing a better understanding of the optical and electrical properties of these 2D materials.

Ting Yu (于霆) 1Division of Physics and Applied, School of Physical and Mathematical Sciences, Nanyang

Technological University, Singapore 637371 2Department of Physics, National University of Singapore, Singapore 117542 3Graphene Centre, National University of Singapore, Singapore 117542 [email protected]

L


lasmonic Graphene-Antenna Photodetector and Transistor

YANFENG ZHANG Peking University China

Nanoscale antennas sandwiched between two graphene monolayers yield a photodetector that efficiently converts visible and near-infrared photons into electrons with an 800% enhancement of the photocurrent relative to the antennaless graphene device [1]. The antenna contributes to the photocurrent in two ways: by the transfer of hot electrons generated in the antenna structure upon plasmon decay [2], as well as by direct plasmon-enhanced excitation of intrinsic graphene electrons due to the antenna near field. This results in a graphene-based photodetector achieving up to 20% internal quantum efficiency in the visible and near-infrared regions of the spectrum. This device can serve as a model for merging the light-harvesting characteristics of optical frequency antennas with the highly attractive transport properties of graphene in new optoelectronic devices [3]. Zheyu Fang, Ziwei Li, Xing Zhu ,Pulickel M

Ajayan, Peter Nordlander School of Physics, State Key Lab for Mesoscopic Physics, Peking University, Beijing 100871, China Department of Electrical and Computer

Engineering, Laboratory for Nanophotonics, and Mechanical Engineering and Materials Science Department, Rice University, Houston, Texas 77005, United States [email protected]

References

[1] Z. Fang*, Z. Liu, Y. Wang, P. M. Ajayan,

et. al. Nano Lett. 12, 3808, 2012. [2] Z. Fang*, Y. Wang, Z. Liu, A. Schlather,

P. M. Ajayan, et. al. ACS Nano 6, 10222, 2012.

[3] Z. Fang, S. Thongrattanasiri, A. Schlather, et. al. ACS Nano 7, 2388, 2013.

Figures

Figure 1: Schematic illustration of a single gold heptamer sandwiched between two monolayer graphene sheets.

Figure 2: Schematic illustration of optically induced electronics (OIE) by nanoantenna n-doping and quantum dot p-doping for an n-p-n transistor.

P


pin transport in high mobility graphene devices

PAUL ZOMER University of Groningen The Netherlands

There is a large interest in high mobility graphene devices and the methods to fabricate heterostructures using graphene and hexagonal boron nitride (hBN) continue to improve [1]. Here we present a very recent development that allows for building stacks by picking up the respective crystals one by one from a substrate yielding excellent charge transport without the need for time consuming cleaning steps [2]. We then employ this method to take the next step in high mobility graphene spintronics [3]. In spintronics, an electrons spin degree of freedom is used to carry information. In order to reduce loss of this information and carry it over larger distances, a medium that preserves the spin orientation is required. Graphene promises to be an excellent candidate for this purpose; very large spin relaxation times are expected due to small spin orbit coupling and hyperfine interaction. Indeed, experimental work demonstrated graphene’s potential for room temperature spintronics already in 2007 [4]. Typically spin relaxation times τ in the order of 100 ps and lengths λ of ~2 µm are obtained for graphene on SiO2. Although the potential of graphene for spintronics has been demonstrated, the theoretical expectations were set considerably higher, predicting relaxation times of hundreds of nanoseconds [5]. Two main mechanisms for spin relaxation in graphene are distinguished: the Elliott-Yafet and the D’Yakonov-Perel mechanism, with τ respectively depending linearly and inversely on the momentum scattering time. In order to build a better understanding of the spin relaxation

mechanism in graphene we therefore investigate spin transport in high mobility devices. Two approaches are taken to this end; graphene is either suspended [6] or based on hBN [3,7], increasing the charge carrier mobility by at least one order of magnitude with respect to SiO2 based devices. For hBN based spintronic devices we could achieve spin transport over record lengths of 20 µm, yet τ remained relatively unaffected with values ranging between 50 and 500 ps and λ ≈ 4.5 µm [7]. A drawback for the hBN based geometry is that polymer remains that negatively affect the device quality cannot be removed using a standard annealing step as this strongly degrades the ferromagnetic contacts. Therefore we now take an alternative route where we use our new method to encapsulate graphene with hBN and prevent further contamination during processing. Interestingly, this does lead to a clear increase of τ to 1.3 ns and λ to 12 µm at room temperature [3]. Paul Zomer, Marcos Guimarães, Juliana Brant,

Nikolaos Tombros and Bart van Wees Zernike Institute for Advanced Materials,

University of Groningen, Groningen, The Netherlands [email protected]

S


References

[1] L. Wang, I. Meric, P. Y. Huang, Q. Gao,

Y. Gao, H. Tran, T. Taniguchi, K. Watanabe, L. M. Campos, D. A. Muller, J. Guo, P. Kim, J. Hone, K. L. Shepard, and C. R. Dean, Science 342 (2013) 614

[2] P. J. Zomer, M. H. D. Guimarães, J. C. Brant, N. Tombros, and B. J. van Wees, arXiv:1403.0399 (2014)

[3] M. H. D. Guimarães et al. In preparation

[4] N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman, and B. J. van Wees, Nature 448 (2007) 571

[5] D. Huertas-Hernando, F. Guinea, and A. Brataas, PRL 103 (2009) 146801

[6] M. H. D. Guimarães, A. Veligura, P. J. Zomer, T. Maassen, I. J. Vera-Marun, N. Tombros, and B. J. van Wees, Nanoletters 12 (2012) 3512

[7] P. J. Zomer, M. H. D. Guimarães, N. Tombros, and B. J. van Wees, PRB 86 (2012) 161416(R)

Graphene2014 Conference Book

Documents

Transcript of Graphene2014 Conference Book