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Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,精选ppt,*,Optics on Graphene,1,精选ppt,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),2,精选ppt,Graphene,(A Monolayer of Graphite),2D Hexagonal lattice,3,精选ppt,Electrically:,High mobility at room temperature,Large current carrying capability,Mechanically:,Large Youngs modulus.,Thermally:,High thermal conductance.,Properties of Graphene,4,精选ppt,Quantum Hall effect,Barry Phase,Ballistic transport,Klein paradox,Others,Exotic Behaviors,5,精选ppt,Quantum Hall Effect,Y.Zhang et al,Nature,438,201(2005),6,精选ppt,Optical Studies of Graphene,Optical microscopy contrast;,Raman spectroscopy;,Landau level spectroscopy.,Other Possibilites,Spectroscopic probe of electronic structure.,Interlayer coupling effect.,Electrical gating effect on optical transitions.,Others,7,精选ppt,Crystalline Structure of Graphite,8,精选ppt,Graphene,2D Hexagonal lattice,9,精选ppt,Band Structure of Graphene Monolayer,P.R.Wallace,Phys.Rev.71,622-634(1947),10,精选ppt,Band Structure of Monolayer Graphere,11,精选ppt,p-,Electron Bands of Graphene Monolayer,12,精选ppt,Band Structure in Extended BZ,13,精选ppt,Relativistic Dirac fermion.,Band Structure near K Points,10,eV,14,精选ppt,Vertical optical transition,Van Hove Singularity,K,K,Monolayer,Bilayer,Band Structures of Graphene Monolayer and Bilayer near K,E,F,is adjustable,x,x,15,精选ppt,Exfoliated Graphene Monolayers and Bilayers,Monolayer,Bilayer,Reflecting microscope images.,K.S.Novoselov,et al.,Science,306,666(2004).,20,m,16,精选ppt,Raman Spectroscopy of Graphene,A.S.Ferrari,et al,PRL 97,187401(2006),(Allowing ID of monolayer and bilayer),17,精选ppt,Reflection Spectroscopy on Graphene,18,精选ppt,Experimental Arrangement,Doped Si,Graphene,Gold,290-nm Silica,OPA,Det,19,精选ppt,Infrared Reflection Spectroscopyto Deduce Absorption Spectrum,Differential reflection spectroscopy:,Difference between bare substrate and graphene on substrate,A,B,-,d,R/R,(R,A,-R,B,)/R,A,versus,w,R,A,:bare substrate reflectivity,R,B,:substrate+graphene reflectivity,20,m,dR/R=-Re,h(w)s(w),h(w),from substrate,s(w),from graphene:,interband transitons,free carrier absorption,Re,s(w),/w:,Absorption spectrum,20,精选ppt,Spectroscopy on Monolayer Graphene,21,精选ppt,Monolayer Spectrum,x,d,R/R,2,E,F,C,:capacitance,22,精选ppt,Experimental Arrangement,Doped Si,Graphene,Gold,290-nm Silica,OPA,Det,V,g,23,精选ppt,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.,E,F,can be shifted by 0.5 eV with V,g,50 v;,Shifting threshold of transitions by 1 eV,d,R/R,2,E,F,If V,g,=V,g0,+V,mod,then should be a maximum at,24,精选ppt,Vary Optical Transitions by Gating,Laser beam,Vary gate voltage V,g.,Measure modulated reflectivity due to V,mod,at V,(Analogous to dI/dV measurement in transport),25,精选ppt,Results in Graphene Monolayer,=350 meV,The maximum determines V,g,for the given E,F,.,26,精选ppt,Mapping Band Structure near K,For different,w,the gate voltage V,g,determined from maximum is different,following the relation ,d,R/R,2,E,F,Slope of the line allows deduction of slope of the band structure(Dirac cone),27,精选ppt,2D Plot of Monolayer Spectrum,Experiment,Theory,28,精选ppt,D(d,R/R),(dR/R),60V,-(dR/R),-,50V,Vg,=0,Strength of Gate Modulation,29,精选ppt,Bilayer Graphene,(Gate-Tunable Bandgap),30,精选ppt,Band Structure of Graphene Bilayer,For symmetric layers,D,=0,For asymmetric layer,D,0,E.McCann,V.I.Falko,PRL 96,086805(2006);,31,精选ppt,Doubly Gated Bilayer,Asymmetry:,D,D (D,b,+D,t,)/2,0,Carrier injection to shift E,F,:,F,d,D=(D,b,-D,t,),32,精选ppt,Sample Preparation,Effective initial bias due to impurity doping,33,精选ppt,Transport Measurement,Maximum resistance appears at E,F,=0,Lowest peak resistance corresponds to D,b,=D,t,=0,.,34,精选ppt,Optical Transitions in Bilayer,I:,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,d,E,F,=0,then II and IV do not contribute,35,精选ppt,Bandstructure Change Induced by,Transitions II&IV inactive,Transition I active,x,x,IV,II,36,精选ppt,Differential Bilayer Spectra(,d,D=0),(Difference between spectra of D,0 and D=0),I,I,Larger bandgap,stronger transition I because ot higher density of states,IV,37,精选ppt,38,精选ppt,Charge Injection without Change of Bandstructure,(D fixed),x,d,D=0,d,D,0,Transition IV becomes activePeak shifts to lower energy as D increases.,Transition III becomes weaker and shifts to higher energy as D increases.,IV,III,39,精选ppt,Difference Spectra for Different D,b
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