台北市立阳明高级中学OpticsonGraphene

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1、Optics on GrapheneGate-Variable Optical Transitions in GrapheneFeng 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 GrapheneYuanbo Zhang,Tsung-Ta Tang,Caglar Girit1,Zhao Hao,Michae

2、l C.Martin,Alex Zettl1,Michael F.Crommie,Y.Ron Shen and Feng Wang(2009)Graphene(A Monolayer of Graphite)2D Hexagonal lattice Electrically:High mobility at room temperature,Large current carrying capability Mechanically:Large Youngs modulus.Thermally:High thermal conductance.Properties of GrapheneQua

3、ntum Hall effect,Barry PhaseBallistic transport,Klein paradoxOthersExotic BehaviorsQuantum Hall EffectY.Zhang et al,Nature 438,201(2005)Optical Studies of Graphene Optical microscopy contrast;Raman spectroscopy;Landau level spectroscopy.Other Possibilites Spectroscopic probe of electronic structure.

4、Interlayer coupling effect.Electrical gating effect on optical transitions.OthersCrystalline Structure of GraphiteGraphene2D Hexagonal latticeBand Structure of Graphene MonolayerP.R.Wallace,Phys.Rev.71,622-634(1947)Band Structure of Monolayer Grapherep-Electron Bands of Graphene MonolayerBand Struct

5、ure in Extended BZRelativistic Dirac fermion.Band Structure near K Points10 eVVertical optical transitionVan Hove SingularityK KK KMonolayerBilayerBand Structures of Graphene Monolayer and Bilayer near KEF is adjustablexxExfoliated Graphene Monolayers and BilayersMonolayerBilayerReflecting microscop

6、e images.K.S.Novoselov et al.,Science 306,666(2004).20 mRaman Spectroscopy of GrapheneA.S.Ferrari,et al,PRL 97,187401(2006)(Allowing ID of monolayer and bilayer)Reflection Spectroscopy on GrapheneExperimental ArrangementDoped SiGrapheneGold290-nm SilicaOPADetInfrared Reflection Spectroscopyto Deduce

7、 Absorption SpectrumDifferential reflection spectroscopy:Difference between bare substrate and graphene on substrate AB-d dR/R (RA-RB)/RA versus w wRA:bare substrate reflectivityRB:substrate+graphene reflectivity20 mdR/R=-Reh(w)s(wh(w)s(w)h(wh(w)from substrates(ws(w)from graphene:interband transiton

8、s free carrier absorptionRe s(w)s(w)/w:Absorption spectrumSpectroscopy on Monolayer GrapheneMonolayer Spectrumxd dR/R2 2EFC:capacitanceExperimental ArrangementDoped SiGrapheneGold290-nm SilicaOPADetVgGate Effect on Monolayer Graphene XXXSmall density of states close to Dirac point E=0 Carrier inject

9、ion 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 eVd dR/R2 2EFIf Vg=Vg0+Vmod,then should be a maximum at Vary Optical Transitions by GatingLaser beamVary gate voltage Vg.Measure modulated reflectivity du

10、e to Vmod at V(Analogous to dI/dV measurement in transport)Results in Graphene Monolayer=350 meVThe maximum determines Vg for the given EF.Mapping Band Structure near KFor different w w,the gate voltage Vg determined from maximum is different,following the relation ,d dR/R2 2EFSlope of the line allo

11、ws deduction of slope of the band structure(Dirac cone)2D Plot of Monolayer SpectrumExperimentTheoryD(dD(dR/R)(dRdR/R)/R)60V-(-(dRdR/R)/R)-50VVg=0=0Strength of Gate ModulationBilayer Graphene(Gate-Tunable Bandgap)Band Structure of Graphene BilayerFor symmetric layers,D D=0For asymmetric layer,D D 0

12、0E.McCann,V.I.Falko,PRL 96,086805(2006);Doubly Gated BilayerAsymmetry:D D (Db+Dt)/2 0Carrier injection to shift EF:F D=(Db-Dt)Sample PreparationEffective initial bias due to impurity dopingTransport MeasurementMaximum resistance appears at EF=0Lowest peak resistance corresponds to Db=Dt=0 .Optical T

13、ransitions in BilayerI:Direct gap transition(tunable,250 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 EF=0,then II and IV do not contributeBandstructure Change Induced

14、byTransitions II&IV inactiveTransition I activexxIVIIDifferential Bilayer Spectra(D=0)(Difference between spectra of D0 and D=0)IILarger bandgap stronger transition I because ot higher density of statesIVCharge Injection without Change of Bandstructure(D fixed)xD=0D 0Transition IV becomes activePeak

15、 shifts to lower energy as D increases.Transition III becomes weaker and shifts to higher energy as D increases.IVIIIDifference Spectra for Different D between D=0.15 v/nm and d dD=0Larger DBandgap versus DD D(dR/R)(dR/R)60V-(dR/R)-50Vis comparable to d dR/R in valueStrength of Gate ModulationSummar

16、yGrahpene 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.