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Plasma hydrogenation of graphene

Tobias Völkl Spin-Hall effect in weakly hydrogenated graphene

SOC in graphene and hydrogenation

Castro Neto and Guinea, PRL 103, 026804 Ferreira et al., PRL 112, 066601

• Low SOC in graphene due to its planar structure • Covalently bonded hydrogen atoms can locally increase SOC • Resonant skew scattering is predicted to produce large

x

SHSH

Tobias Völkl Spin-Hall effect in weakly hydrogenated graphene

Spin-Hall Effect

• SHE: A charge current generates a perpendicular spin current • In nonlocal geometry a charge current is converted into a spin current by the SHE and produces a nonlocal voltage, due to the inverse SHE

J. Balakrishnan et al., Nature Physics 9, 284 (2013) H.-A. Engel et al., Phys. Rev. Lett. 95, 166605 (2005)

Tobias Völkl Spin-Hall effect in weakly hydrogenated graphene

Nonlocal measurement of the SHE

J. Balakrishnan et al., Nature Physics 9, 284 (2013)

• Nonlocal voltage increases with hydrogen concentration • Conflicting results on magnetic field dependence of nonlocal voltage

A. Kaverzin and B. van Wees, PRB 91, 16512(2015)

Tobias Völkl Plasma hydrogenation of graphene

Hydrogenation in RIE-chamber

• Plasma hydrogenation in reactive ion etching chamber at lowest ignition power • Recipe: 40mTorr hydrogen pressure, 30sccm gas flow, 2W power, variable time •Hydrogen coverage is studied by Raman-Spectroscopy • At low defect density the amount of defects is given by:

G

DD

D

GD

I

Icmn

I

InmL

4

222

49

108.1)(

108.1)(

M. Wojtaszek et al., J. Appl. Phys. 110, 063715 (2011)

L.G. Cancado et al., Nano Letters 11, 3190 (2011)

Tobias Völkl Plasma hydrogenation of graphene

Annealing

• ID/IG increases with exposure time, but saturates after ≈40s • Annealing temperature is too low for healing lattice defects, confirming that the defects are bonded hydrogen

Tobias Völkl Plasma hydrogenation of graphene

Annealing

• ID/IG increases with exposure time, but saturates after ≈40s • Annealing temperature is too low for healing lattice defects, confirming that the defects are bonded hydrogen

Tobias Völkl Plasma hydrogenation of graphene

Effect of resist residues

• Before hydrogenation, samples were covered with resist, which was removed after 24h • Resist residues prevent efficient hydrogenation • Annealing after resist removal does not remove residues, but increases doping

Tobias Völkl Plasma hydrogenation of graphene

Fabrication of first sample

• Hydrogen plasma exposure for 20s as first step after exfoliation • Design of Hall-bar with L=2μm and W=1μm by RIE with oxygen plasma • Deposition of Au contacts

Tobias Völkl Plasma hydrogenation of graphene

Sample characterization

• ID/IG=0.43 after fabrication is lower than in samples with pristine graphene, hydrogenated for 20s • 4pt measurement shows mobilities of μel=2140cm2/Vs and μh=1650cm2/Vs and CNP of VCNP=28V

Tobias Völkl Plasma hydrogenation of graphene

Nonlocal measurements

• Nonlocal measurements show large nonlocal voltage at the CNP gives λs=0.57 μm, θSH =1.23

sL

s

SHNL eW

R

/2

2

1

Tobias Völkl Plasma hydrogenation of graphene

Magnetic field dependence

• No dependence of nonlocal voltage on an inplane magnetic field could be observed

Tobias Völkl Plasma hydrogenation of graphene

Conclusion and Outlook

Conclusion • Plasma hydrogenation is a viable method for functionalization of graphene • First measurements show good mobility and large nonlocal resistance • Absence of magnetic field dependence of nonlocal signal leaves its origin unclear

Thanks to: • Franz Vilsmeier, Thomas Ebnet • Philipp Nagler, Gerd Plechinger, PD. Tobias Korn • Dr. Jonathan Eroms, Prof. Dr. Dieter Weiss

Tobias Völkl Plasma hydrogenation of graphene

Tobias Völkl Plasma hydrogenation of graphene

Outline

Outline: • Motivation • Method for graphene hydrogenation and investigation by Raman- Spectroscopy • First results of a sample fabricated by the described method