Honeycomb Carbon A Review of Graphene 石墨烯综述(11)

纳米材料在生物医学中的应用

138ChemicalReviews,2010,Vol.110,No.1Figure10.Schematicdiagramshowingthebandstructureandresultingambipolar eldeffectingraphene.ConductionandvalencebandsmeetattheDiracpointwithoutanexternal eld.Undergatebias,theFermilevelmovesaboveorbelowtheDiracpointtointroduceasigni cantnumberoffreecarriers.[ReprintedwithpermissionfromNature(),ref10.Copyright2007NaturePublishingGroup.]

hasmajorimplicationsforthesemiconductorindustrybecauseitenables,inprinciple,fabricationofall-ballisticdevicesevenattoday’sintegratedcircuit(IC)channellengths(currentlydownto45nm).

Thesecondimportantpointaboutchargetransportingrapheneisambipolarity.Inthe eld-effectcon guration,thisimpliesthatcarrierscanbetunedcontinuouslybetweenholesandelectronsbysupplyingtherequisitegatebias.Thiscanbeeasilyvisualizedgiventheuniquebandstructureofgraphene(seeFigure10).10Undernegativegatebias,theFermileveldropsbelowtheDiracpoint,introducingasigni cantpopulationofholesintothevalenceband.Underpositivegatebias,theFermilevelrisesabovetheDiracpoint,promotingasigni cantpopulationofelectronsintotheconductionband.

Besidesmotivatingacademicinterest,accesstoatrulyambipolarsemiconductorenablesanumberofnoveldevicestructures.Thesearefundamentallydifferentfromsilicon-basedlogicbecausedopinglevelscanbedynamicallycontrolledentirelybygating.Momentarilyprovidinglocalgatebiasestodifferentpartsofthesame akecanformjunctionsorevenmorecomplicatedlogic.Subsequentlyrearrangingthebiasescanthencompletelyrede nethedevicewithoutmakinganyphysicalchangestothechannelmaterial.

4.2.SingleMoleculeDetection

Thesecondexcitingproof-of-conceptimplementationofmechanicallyexfoliatedgraphenewasinchemicalsensors.Severalimportantfeaturesofgraphenemadeitanexcellentcandidateforthesensingactivearea.Firstandforemost,the2Dstructureofgrapheneconstitutesanabsolutemaximumofthesurfaceareatovolumeratioinalayeredmaterial,whichisessentialforhighsensitivity.Infact,thishasbeenthemajormotivationbehindimplementationofothernano-structuredmaterialsinsensors.Inthecaseoftraditionalmaterials,bulkpropertiessuchasresistivityarenotsubstan-tiallyin uencedbysingleadsorptioneventsontheirsurface.Ingraphene,however,thereisnosuchdistinctionbetweensurfacesitesandthebulkmaterial,soeveryadsorptioneventissigni cant.

Versatilityofgrapheneasthebasisofasensorresultsfromitsuniqueelectronicstructure.Theambipolaritymeansthat

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Figure11.Thelackofsurfacestatesingraphenemakespossiblethedetectionofevensingleadsorbate.Thedirectionofchangeinthe gureindicatesthesignofinducedcarriers(holesforH2OandNO2;electronsforCOandNH(),3).[ReprintedwithpermissionfromNatureref21.Copyright2007NaturePublishingGroup.]

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