Stefan Heun

NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore

Pisa, Italy

Hydrogenated Graphene

Outline

• Epitaxial Graphene

• Hydrogen-Intercalated Graphene

• Hydrogen Chemisorbed on Graphene

Outline

• Epitaxial Graphene

• Hydrogen-Intercalated Graphene

• Hydrogen Chemisorbed on Graphene

Graphene growth on SiC(0001)

Buffer Layer

SiC

ML

SiC

Buffer Layer

Buffer Layer Topologically identical atomic carbon structure as graphene. Does not have the electronic band structure of graphene due to periodic sp3 C-Si bonds.

F. Varchon, et al., PRB 77, 235412 (2008). F. Varchon, et al., PRB 77, 235412 (2008).

Superstructure of both the buffer layer and monolayer graphene on the Si face from the periodic interaction with the substrate.

6√3 x 6√3

Theoretical Calculations

Buffer Layer

6 3 × 6 3

quasi-(6x6)

S. Goler et al.: Carbon 51, 249 (2013).

Bias: +1.7V Current: 0.3nA

F. Varchon, et al., PRB 77, 235412 (2008).

2.00

0.00 2.0Å 0.0Å

6√3x6√3-Superstructure

30 nm, 1V, 100 pA E= 75 eV

Graphene

SiC

Monolayer Graphene

STM

S. Goler et al.: J. Phys. Chem. C 117, 11506 (2013).

Hydrogen Intercalation

Buffer Layer

SiC

ML

SiC

Buffer Layer (BL) Quasi-free standing monolayer graphene

H2

Quartz tube P ~ atmospheric pressure T ~ 800°C

C. Riedl, C. Coletti et al., PRL 103, 246804 (2009)

Hydrogen Intercalation

Buffer Layer

SiC

ML

SiC

Buffer Layer (BL) Quasi-free standing monolayer graphene

p=2.6·1012 cm-2

k (Å-1)

Ener

gy (e

V)

EF=

k (Å-1)

Ener

gy (e

V)

EF=

S. Forti, et al., PRB 84, 125449 (2011).

BL vs. QFMLG

S. Goler et al.: Carbon 51, 249 (2013).

BL vs. QFMLG

S. Goler et al.: Carbon 51, 249 (2013).

Outline

• Epitaxial Graphene

• Hydrogen-Intercalated Graphene

• Hydrogen Chemisorbed on Graphene

quasi-free-standing monolayer

graphene (QFMLG)

epitaxial monolayer

graphene (EMLG)

annealing H intercalation

buffer layer

Graphene on silicon carbide (SiC) (0001)

C. Riedl, C. Coletti, T. Iwasaki, A. A. Zakharov, and U. Starke, Phys. Rev. Lett. 103, 246804 (2009)

Introduction

F. Speck, J.Jobst, F. Fromm, M. Ostler, D. Waldmann, M. Hundhausen, H. B. Weber, and Th. Seyller,

Appl. Phys. Lett. 99, 122106 (2011)

The carrier mobility of QFMLG shows less temperature dependence than EMLG,

indicating less interaction between QFMLG and the SiC substrate.

However, the mobility of QFMLG (-3000 cm2V-1s-1) is still lower than exfoliated

graphene on SiO2 or free standing graphene.

10

3

EMLG

The QFMLG mobility depends on TH,

the substrate temperature during H intercalation

highest mobility by TH = 700ºC

S. Tanabe, M. Takamura, Y. Harada, H. Kageshima, and H. Hibino, Jpn. J. Appl. Phys. 53, 04EN01 (2014).

conductivity – carrier density

• linear for TH = 600-800ºC

- charged impurity

• sublinear for TH = 950ºC

- additional scattering by defect Purpose :

to observe the morphology of QFMLG formed at different TH and

investigate the relationship with transport property

hole density 3×1012 cm-2

TH = 800ºC

50 nm 8 nm

• width: 1.5 nm

• depth: 15-25 pm

Intercalation at 600-800ºC

Y. Murata et al., Appl. Phys. Lett. 105, 221604 (2014).

TH = 800ºC

50 nm 8 nm

• width: 1.5 nm

• depth: 15-25 pm

• honeycomb inside, no defect

Intercalation at 600-800ºC

Y. Murata et al., Appl. Phys. Lett. 105, 221604 (2014).

TH = 800ºC

50 nm 8 nm

• width: 1.5 nm

• depth: 15-25 pm

• honeycomb inside, no defect

• align along SiC<1120>

• periodicity: 1.8 nm = SiC-6×6

Intercalation at 600-800ºC

<1120>

1.8 nm STM of a buffer layer

corrugation – 60 pm

Goler, Carbon 51, 249 (2013)

SiC substrate

H

incomplete H intercalation

– Si dangling bond at interface

Y. Murata et al., Appl. Phys. Lett. 105, 221604 (2014).

Si dangling bond

A150424.161732.dat Ch: 1

Biasvoltage: 1.50568V

Current: 1.0E-11A

Temperature: 5.52998 [K]

0 5 10 15 200

5

10

15

20 -0.2619 nm

-0.1619 nm

-0.06188 nm

0.03812 nm

0.1381 nm

0.2381 nm

-1500 -1000 -500 0 500 1000 1500 2000

Voltage [mV]

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Voltage [

mV

]

Aux channels

Collaboration with Gerd Meyer, IBM Rueschlikon

STS at 4K provides further evidence for dangling bonds,

cf. F. Hiebel et al., PRB 86, 205421 (2012).

0.25 nm ~

single layer of SiC

50 nm 200 nm 8 nm

Intercalation at 1000ºC

- width: 4-10 nm, depth: 0.25 nm

- distribute randomly

- honeycomb inside

0.25 nm ~

single layer of SiC - width: 4-10 nm, depth: 0.25 nm

- distribute randomly

- honeycomb inside

SiC substrate

hole in the SiC substrate

etched at high TH

Intercalation at 1000ºC

50 nm 200 nm 8 nm

SiC substrate

H

TH = 600-800ºC

Forti, et.al., Phys. Rev. B 84, 125449 (2011). ARPES

annealing QFMLG in vacuum

-H atoms desorb

-Si dangling bonds donate charge to

graphene (and act as charged scattering

centers).

• small dark spots

incomplete H intercalation

- Si dangling bonds

Morphology and transport

SiC substrate

TH = 1000ºC • dark spot – hole in SiC substrate

• wrinkle of graphene

Scanning tunneling potentiometry Ji, et.al., Nature Materials 11, 114 (2012)

T. Low, V. Perebeinos, J. Tersoff, and Ph. Avouris, Phys. Rev. Lett. 108, 096601 (2012)

• π-σ hybridization by curvature of graphene

• strain of graphene

• reduced doping due to a larger distance at the interface

resistance of EMLG increases over SiC substrate steps

Morphology and transport

• small dark spot decreases.

-more H intercalation

• hole in SiC substrate and

wrinkles of graphene appear

As TH increases from 600-800 to 1000ºC,

The formation of holes and wrinkles has

a larger influence on the QFMLG mobility

than increased H intercalation.

S. Tanabe, Jpn. J. Appl. Phys. 53, 04EN01 (2014).

Morphology and transport

• small dark spot decreases.

-more H intercalation

• hole in SiC substrate and

wrinkles of graphene appear

As TH increases from 600-800 to 1000ºC,

The formation of holes and wrinkles has

a larger influence on the QFMLG mobility

than increased H intercalation.

S. Tanabe, Jpn. J. Appl. Phys. 53, 04EN01 (2014).

Morphology and transport

Results consistent with the work of the Sendai group:

K. Yamasue et al., Phys. Rev. Lett. 114 (2015) 226103.

Outline

• Epitaxial Graphene

• Hydrogen-Intercalated Graphene

• Hydrogen Chemisorbed on Graphene

Band Gap 3.5 eV

Science 323 (2009) 610

Gap vs. H Coverage

𝐸𝑔𝑎𝑝

= 3.8 𝑒𝑉 𝑐𝑜𝑣100%

0.6

A. Rossi et al.,

J. Phys. Chem. C 119, 7900 (2015).

Para-dimer Ortho-dimer Tetramer

H-dimers and tetramers

S. Goler et al.: J. Phys. Chem. C 117, 11506 (2013).

STS after hydrogenation

S. Goler et al.: J. Phys. Chem. C 117, 11506 (2013).

H adsorption and desorption

S. Goler et al.: J. Phys. Chem. C 117, 11506 (2013).

RMS roughness

S. Goler et al.: J. Phys. Chem. C 117, 11506 (2013).

DFT calculations

S. Goler et al.: J. Phys. Chem. C 117, 11506 (2013).

Graphene for hydrogen storage

Durgen et al., PRB 77 (2007) 085405 Lee et al., Nano Lett. 10 (2010) 793

• Graphene is lightweight, inexpensive, robust,

chemically stable

• Large surface area (~ 2600 m2/g)

• Functionalized graphene has been predicted to adsorb

up to 9 wt% of hydrogen

Graphene Curvature

• Exploit graphene curvature for

hydrogen storage at room

temperature and pressure

• The hydrogen binding energy

on graphene is strongly

dependent on local curvature

and it is larger on convex parts

• Atomic hydrogen

spontaneously sticks on

convex parts; inverting

curvature H is expelled

• Hydrogen clusters on

corrugated graphene

V. Tozzini and V. Pellegrini: J. Phys. Chem. C 115, 25523 (2011).

Graphene Curvature

• Exploit graphene curvature for

hydrogen storage at room

temperature and pressure

• The hydrogen binding energy

on graphene is strongly

dependent on local curvature

and it is larger on convex parts

• Atomic hydrogen

spontaneously sticks on

convex parts; inverting

curvature H is expelled

• Hydrogen clusters on

corrugated graphene

V. Tozzini and V. Pellegrini: J. Phys. Chem. C 115, 25523 (2011).

Graphene Curvature

• Exploit graphene curvature for

hydrogen storage at room

temperature and pressure

• The hydrogen binding energy

on graphene is strongly

dependent on local curvature

and it is larger on convex parts

• Atomic hydrogen

spontaneously sticks on

convex parts; inverting

curvature H is expelled

• Estimated GD: ~8 wt%

V. Tozzini and V. Pellegrini: J. Phys. Chem. C 115, 25523 (2011).

C. Coletti V. Piazza F. Beltram

S. Goler

V. Pellegrini P. Pingue F. Colangelo V. Tozzini

T. Mashoff Y. Murata

M. Takamura S. Tanabe H. Hibino K. V. Emtsev, U. Starke, S. Forti

Funding