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1、Gate-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,Michael C.Martin,Alex Ze
2、ttl1,Michael F.Crommie,Y.Ron Shen and Feng Wang(2009)第1页/共44页Graphene(A Monolayer of Graphite)2D Hexagonal lattice第2页/共44页 Electrically:High mobility at room temperature,Large current carrying capability Mechanically:Large Youngs modulus.Thermally:High thermal conductance.Properties of Graphene第3页/共
3、44页Quantum Hall effect,Barry PhaseBallistic transport,Klein paradoxOthersExotic Behaviors第4页/共44页Quantum Hall EffectY.Zhang et al,Nature 438,201(2005)第5页/共44页Optical Studies of Graphene Optical microscopy contrast;Raman spectroscopy;Landau level spectroscopy.Other Possibilites Spectroscopic probe of
4、 electronic structure.Interlayer coupling effect.Electrical gating effect on optical transitions.Others第6页/共44页Crystalline Structure of Graphite第7页/共44页Graphene2D Hexagonal lattice第8页/共44页Band Structure of Graphene Monolayer第9页/共44页Band Structure of Monolayer Graphere第10页/共44页p-p-Electron Bands of G
5、raphene Monolayer第11页/共44页Band Structure in Extended BZ第12页/共44页Relativistic Dirac fermion.Band Structure near K Points10 eV第13页/共44页Vertical optical transitionVan Hove SingularityK KK KMonolayerBilayerBand Structures of Graphene Monolayer and Bilayer near KEF is adjustablexx第14页/共44页Exfoliated Grap
6、hene Monolayers and BilayersMonolayerBilayerReflecting microscope images.K.S.Novoselov et al.,Science 306,666(2004).20 m第15页/共44页Raman Spectroscopy of Graphene(Allowing ID of monolayer and bilayer)第16页/共44页Reflection Spectroscopy on Graphene第17页/共44页Experimental ArrangementDoped SiGrapheneGold290-nm
7、 SilicaOPADet第18页/共44页Infrared Reflection Spectroscopyto Deduce 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(w)h(w)s
8、(w)h(w)h(w)from substrates(w)s(w)from graphene:interband transitons free carrier absorptionRe s(w)s(w)/w:Absorption spectrum第19页/共44页Spectroscopy on Monolayer Graphene第20页/共44页Monolayer Spectrumxd dR/R2 2EFC:capacitance第21页/共44页Experimental ArrangementDoped SiGrapheneGold290-nm SilicaOPADetVg第22页/共4
9、4页Gate Effect on Monolayer Graphene XXXSmall 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 eVd dR/R2 2EFIf Vg=Vg0+Vmod,then should be a maximum
10、 at 第23页/共44页Vary Optical Transitions by GatingLaser beamVary gate voltage Vg.Measure modulated reflectivity due to Vmod at V(Analogous to dI/dV measurement in transport)第24页/共44页Results in Graphene Monolayer=350 meVThe maximum determines Vg for the given EF.第25页/共44页Mapping Band Structure near KFor
11、 different w w,the gate voltage Vg determined from maximum is different,following the relation ,d dR/R2 2EFSlope of the line allows deduction of slope of the band structure(Dirac cone)第26页/共44页2D Plot of Monolayer SpectrumExperimentTheory第27页/共44页D(dD(dR/R)(dR/R)(dR/R)60V-(dR/R)-(dR/R)-50VVg=0=0Stre
12、ngth of Gate Modulation第28页/共44页Bilayer Graphene(Gate-Tunable Bandgap)第29页/共44页Band Structure of Graphene BilayerFor symmetric layers,D D=0For asymmetric layer,D D 0 0第30页/共44页Doubly Gated BilayerAsymmetry:D D (Db+Dt)/2 0Carrier injection to shift EF:F D=(Db-Dt)第31页/共44页Sample PreparationEffective i
13、nitial bias due to impurity doping第32页/共44页Transport MeasurementMaximum resistance appears at EF=0Lowest peak resistance corresponds to Db=Dt=0 .第33页/共44页Optical Transitions in BilayerI:Direct gap transition(tunable,250 meV)II,IV:Transition between conduction/valence bands(400 meV,dominated by van H
14、ove singularity)III,V:Transition between conduction and valence bands(400 meV,relatively weak)If EF=0,then II and IV do not contribute第34页/共44页Bandstructure Change Induced byTransitions II&IV inactiveTransition I activexxIVII第35页/共44页Differential Bilayer Spectra(d dD=0)(Difference between spectra of
15、 D 0 and D=0)IILarger bandgap stronger transition I because ot higher density of statesIV第36页/共44页第37页/共44页Charge Injection without Change of Bandstructure(D fixed)xD=0D 0Transition IV becomes activePeak shifts to lower energy as D increases.Transition III becomes weaker and shifts to higher energy
16、as D increases.IVIII第38页/共44页Difference Spectra for Different D between d dD=0.15 v/nm and d dD=0第39页/共44页Larger D第40页/共44页Bandgap versus D第41页/共44页D D(dR/R)(dR/R)60V-(dR/R)-50Vis comparable to d dR/R in valueStrength of Gate Modulation第42页/共44页SummaryGrahpene exhibits interesting optical behaviors:
17、.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.第43页/共44页感谢您的观看！第44页/共44页