Endre Tóvári International Workshop on Electrical Properties of New Materials (IWEPNM)...

Endre Tóvári

International Workshop on Electrical Properties of New Materials (IWEPNM)

Kirchberg-in-Tirol, 2012. March 3-10.

1

Tóvári Endre: Kirchberg IWEPNM 2012 2

Topics

1 mm

• synthesis of CNT and graphene• CNT sorting and functionalization, nanopore arrays• graphene magnetism, spintronics, optoelectronics, transport in suspended SLG, BLG and TLG, FQHE, graphene on hexagonal BN• optical conductivity, Raman spectroscopy, intrinsic properties,

effect of strain, substrate, ESR• functionalization, doping


A few interesting presentations

Liquid-induced densification of different SWNT forests by soaking the samples with ethanol and drying in air.

NATURE COMMUNICATIONS | 2:309 | DOI: 10.1038/ncomms1313Japanese Journal of Applied Physics 51 (2012) 01AH01

Growth of high-density CNT forests

Sorting of SWCNTs using multi-column gel chomatography

structure-dependent interaction strength of SWCNTs with an allyl dextran-based gel


Repeated growth and bubbling transfer of graphene with millimetre-size single-crystal grains using platinum


Nature Communications, 3:699 | DOI: 10.1038/ncomms1702

1 mm 0,5 mm 100 µm

10 µm 10 µm

10 µm10 µm

• mm-size single-crystal SLG• CVD on polycrystalline Pt• µ > 7000 cm2V-1s-1 • bubbling transfer: nondestructive to

Pt and graphene both

• increasing T or low conc. of CH4: nucleation density decreases, grain size increases (ambient pressure, T>1000°C)

• CH4/H2 flow ratio 4/700 sccm: dominantly hexagonal grains with smooth edges (suppressed nucleation, and low stability edges are etched away by active atomic H)

• most grains: no reflex angle at edges, no visible boundaries under SEM

• no new nuclei with increasing growth time

d,e,fg,h,i

d,e,f: grain boundary

Raman D-band intensity map: showing grain boundary


Repeated growth and bubbling transfer of graphene with millimetre-size single-crystal grains using platinum


Nature Communications, 3:699 | DOI: 10.1038/ncomms1702

Pt+Gr+PMMA cathode (-):

Bubbling transfer: aqueous NaOH electrolysis cellnondestructive to Pt: reusablenondestructive to graphene, transfer to SiO2

•free of metal residues•preserves the original shape•mostly monolayer•small Raman D-band (ID/IG<5%)•µ > 7000 cm2V-1s-1

400 µm

100 µm


A few interesting presentationsSpin-half paramagnetism in graphene induced by point defects

Nature Physics Vol 8, 199, March 2012

graphene laminates: large collections of electronically non-interacting, parallel SLG and BLG crystallites (10-50 nm) before and after fluorination

SQUID magnetometry

fluorination: clustering, only the atoms at cluster edge without pairs on the other C sublattice contribute ~10-3 µB/Fhigh conc. (x≈1): lower M, but still PM (still a large number of defects in the CFx lattice)(F conc.: Raman, XPS)

350-400 keV proton irradiation or 20 MeV C4+ irrad.:~0,1-0,4 µB/defect (defect conc. calc. with simulation!)


Review:Spin transport and relaxation in graphene

Han, McCreary, Pi, Wang, Li, Wen, Chen, Kawakami ; Journal of Magnetism and Magnetic Materials 324 (2012) 369–381

• low intrinsic spin-orbit coupling (SOC) and hyperfine coupling (HFC)

• key words: spin injection, diffusion (spin-polarized currents), precession in magnetic field, gate dependence, relaxation mechanisms

• extrinsic spin relaxation mechanisms (τS~µs expected, ~100 ps – ns measured): Elliot-Yafet, Dyakonov-Perel

spin diffusion is usually described by a spin-dependent chemical potential (µ↑ and µ↓), where a splitting of the chemical potential corresponds to the spin density in the graphene.

local setup nonlocal setup

spin valves


Nature Letters Vol 448, 571 (Aug 2007)

Spin valves: introductionchanging in-plane magnetic field: changing magnetic polarization of Co electrodes (different widths, different coercivities: switching at different fields)

nonlocal

local


Transparent and tunneling contacts: difference in spin injection efficiency (1% or 26-30%)

RG: graphene spin resistanceRF: FM contact’s spin resistanceRJ: contact resistancePF: spin polarization in FM contactPJ: polarization if interfacial currentL: contact spacingλG: spin diffusion lengthtransparent/tunneling: relation of RJ and RG

Spin relaxation: contact spacing

Spin valves: introduction


Hanle spin precession in a ┴ magnetic field: spin precession

sign: P or AP contact polariz.

spin diffusion in the L (contact spacing) length

spin precession spin relaxation

L Bg H S SD ~ 1 4 m

Spin valves: introduction


Tunneling contacts on SLG: 400-1000 ps Spin diffusion from Co into SLG: escape time

Spin transport and relaxation in graphene

𝜏S does not decrease, although D does:Au doping is effective at generating momentum scattering, but in SLG for transparent contacts charged impurity scattering is not the dominant process behind spin relaxation

1 1 1 1S spin flip esc spin flip in good tunnel barriers

For pinhole and transparent contacts (50-200 ps range) the dominant spin relaxation is generated by the contacts (escape time, inhomogeneous fields, interfacial scattering)


Spin relaxation in single-layer graphenetunnel barriers: suppress the contact-induced spin relaxation𝜏S ~ 400-1000 ps300 K: no correlation with D4 K: strong correlation of 𝜏S and D in SLG with tunneling contacts: both increasing with carrier conc.; similar behaviour as a function of T

D (10-2 m2/s)

𝜏S ~ D ~ 𝜏p: Elliott-Yafet spin relaxation dominant at low T in SLG: finite probability of spin-flip during a momentum scattering event (possible sources: long-range and short-range impurity scattering; at RT multiple sources are possible, such as phonons, which ruin the linear relationship; see Ref. 5)


Spin relaxation in bilayer graphene300 K: : 𝜏S ~ 200-400 ps, and no correlation with D4 K: 𝜏S ~ 2-6 ns, strong correlation with D: opposite behaviour with D as a function of gate voltage

𝜏S-1 ~ D ~ 𝜏p dominant spin relaxation mechanism in BLG at low T (with

tunneling contacts): random magnetic Rashba fields of the Dyakonov-Perel-type (can be generated by ripples, adatoms in the graphene sheet): spin relaxation via precession in internal spin-orbit fields. (Elliott-Yafet mechanisms negligible due to enhanced screening of scatterers)


Related articles (among many; mainly from van Wees’ and Kawakami’s group) :1.Electronic spin transport and spin precession in SLG at room temperature, Nature Letters Vol 448, 571 (Aug 2007)2.Tunneling spin injection into SLG, PRL 105, 167202 (2010)3.Comparison between charge and spin transport in FLG, PRB 83, 115410 (2011)4.Observation of long spin-relaxation times in BLG at room temperature, PRL 107, 047206 (2011)5.Spin relaxation in single layer and bilayer graphene, PRL 107, 047207 (2011)

All of the above mainly from:Han, McCreary, Pi, Wang, Li, Wen, Chen, Kawakami: Review - Spin transport and relaxation in graphene, Journal of Magnetism and Magnetic Materials 324 (2012) 369–381

Thank you for your attention!16


pictures from the first slide:•arXiv:1202.3212v1•PRL 107, 217203 (2011)•SCIENCE VOL 334, 648 (2011)•Nature Communications, 3:699 | DOI: 10.1038/ncomms1702

http://www.iwepnm.org/2012/calendar.php

Endre Tóvári International Workshop on Electrical Properties of New Materials (IWEPNM)...

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