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Dr. Max Mustermann Referat Kommunikation & Marketing Verwaltung
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