Optics on Graphene

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Optics on Graphene
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Gate-Variable Optical Transitions in
Graphene Feng Wang, Yuanbo Zhang, Chuanshan Tian,
Caglar Girit, Alex Zettl, Michael Crommie, and Y.
Ron Shen, Science 320, 206 (2008).
Direct Observation of a Widely Tunable Bandgap in
Bilayer Graphene Yuanbo Zhang, Tsung-Ta Tang,
Caglar Girit1, Zhao Hao, Michael C. Martin, Alex
Zettl1, Michael F. Crommie, Y. Ron Shen and Feng
Wang (2009)
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Graphene(A Monolayer of Graphite)
2D Hexagonal lattice
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Properties of Graphene
Electrically High mobility at room
temperature, Large current carrying
capability Mechanically Large Youngs
modulus. Thermally High thermal
conductance.
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Exotic Behaviors
Quantum Hall effect, Barry Phase Ballistic
transport, Klein paradox Others
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Quantum Hall Effect
Y. Zhang et al, Nature 438, 201(2005)
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Optical Studies of Graphene
Optical microscopy contrast Raman
spectroscopy Landau level spectroscopy.
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Crystalline Structure of Graphite
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Graphene
2D Hexagonal lattice
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Band Structure of Graphene Monolayer
P.R.Wallace, Phys.Rev.71,622-634(1947)
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Band Structure of Monolayer Graphere
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p-Electron Bands of Graphene Monolayer
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Band Structure in Extended BZ
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Band Structure near K Points
10 eV
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Band Structures of Graphene Monolayer and Bilayer
near K
Bilayer
Monolayer
x
K
K
x
Vertical optical transition
Van Hove Singularity
EF is adjustable
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Exfoliated Graphene Monolayers and Bilayers
Reflecting microscope images.
20 ?m
Monolayer
Bilayer
K. S. Novoselov et al., Science 306, 666 (2004).
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Raman Spectroscopy of Graphene
(Allowing ID of monolayer and bilayer)
A.S.Ferrari, et al, PRL 97, 187401 (2006)
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Reflection Spectroscopy on Graphene
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Experimental Arrangement
Det
OPA
Graphene
Gold
290-nm Silica
Doped Si
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Infrared Reflection Spectroscopyto Deduce
Absorption Spectrum
Differential reflection spectroscopy Difference
between bare substrate and graphene on substrate
RA bare substrate reflectivity RB substrate
graphene reflectivity
A
20 ?m
B
-dR/R ? (RA-RB)/RA versus w
dR/R -Reh(w)s(w)
h(w) from substrate s(w) from graphene
interband transitons free carrier
absorption
Re s(w)/w Absorption spectrum
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Spectroscopy on Monolayer Graphene
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Monolayer Spectrum
x
C capacitance
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Experimental Arrangement
Det
OPA
Graphene
Gold
290-nm Silica
Vg
Doped Si
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Gate Effect on Monolayer Graphene
X
X
X
Small density of states close to Dirac point E
0 Carrier injection by applying gate voltage can
lead to large Fermi energy shift .
EF can be shifted by 0.5 eV with Vg 50 v
Shifting threshold of transitions by 1 eV
If Vg Vg0 Vmod, then
should be a maximum at
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Vary Optical Transitions by Gating
Vary gate voltage Vg.
Laser beam
Measure modulated reflectivity due to Vmod at V
( Analogous to dI/dV measurement in transport)
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Results in Graphene Monolayer
The maximum determines Vg for the given EF.
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Mapping Band Structure near K
For different w, the gate voltage Vg determined
from maximum is different, following
the relation ,
Slope of the line allows deduction of slope of
the band structure (Dirac cone) ??
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2D Plot of Monolayer Spectrum
Experiment
Theory
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Strength of Gate Modulation
Vg 0
D(dR/R) ? (dR/R) 60V -(dR/R) -50V
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Bilayer Graphene(Gate-Tunable Bandgap)
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Band Structure of Graphene Bilayer
For symmetric layers, D 0 For asymmetric layer,
D ? 0
E. McCann, V.I.Falko, PRL 96, 086805 (2006)
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Doubly Gated Bilayer
Asymmetry D ? D ? (Db Dt)/2 ? 0 Carrier
injection to shift EF ??F ? dD ?(Db - Dt)
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Sample Preparation
Effective initial bias due to impurity doping
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Transport Measurement
Maximum resistance appears at EF 0
Lowest peak resistance corresponds to Db Dt 0
?? .
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Optical Transitions in Bilayer
I Direct gap transition (tunable, lt250
meV) II, IV Transition between
conduction/valence bands (400 meV, dominated by
van Hove singularity) III, V Transition
between conduction and valence bands (400 meV,
relatively weak) If dEF0, then II and IV do not
contribute
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Bandstructure Change Induced by
x
IV
x
II
Transitions II IV inactive Transition I active
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Differential Bilayer Spectra (dD 0)(Difference
between spectra of D?0 and D0)
I
I
IV
Larger bandgap ? stronger transition I because
ot higher density of states
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Charge Injection without Change of Bandstructure
(D fixed)
dD ?0
dD 0
x
IV
III
Transition IV becomes active Peak shifts to
lower energy as D increases.. Transition III
becomes weaker and shifts to higher energy as D
increases.
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Difference Spectra for Different D between
dD0.15 v/nm and dD0
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Larger D
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Bandgap versus D
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Strength of Gate Modulation
D(dR/R) ? (dR/R) 60V -(dR/R) -50V
is comparable to dR/R in value
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Summary

• Grahpene exhibits interesting optical behaviors.
  
• Gate bias can significantly modify optical
  transitions over a broad spectral range.
• Single gate bias shifts the Fermi level of
  monolayer graphene.
• Spectra provides information on bandstructure,
  allowing deduction of VF (slope of the Dirac
  cone in the bandstructure).
• Double gate bias tunes the bandgap and shifts
  the Fermi level of bilayer
• graphene.
• Widely gate-tunable bandgap of bilayer graphene
  could be useful in future device applications.
• Strong gating effects on optical properties of
  graphene could be useful in infrared
  optoelectronic devices.

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