Honeycomb Carbon A Review of Graphene 石墨烯综述(20)

纳米材料在生物医学中的应用

HoneycombCarbon:AReviewofGrapheneChemicalReviews,2010,Vol.110,No.1

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Figure20.Nanoribbonsofferenhancedtransistorbehaviorduetoquantumcon nement.(a)SEMimageofnanoribbonsde nedbyphotolithographyandO2plasmaetching.(b)Kim’sgroupdemonstratedIon/Ioffratiosashighas104atwidthsof～50nm.(Reprintedwithpermissionfromref124.Copyright2007AmericanPhysicalSociety.)

thicknessandpreventingsecondarycrystalformation.Inanidealcase,bothmethodsrelyonthenucleationandgrowthofasinglecrystalwithouttheformationofaboundaryorseedingofasecondlayer.Currently,thebestspecimenshaveavariationinthicknessofperhaps1-3layersandarepolycrystalline.FieldeffectdevicesfabricatedwithepitaxialandCVDgraphenedisplaycarriermobilitiesinexcessof1000cm2/(Vs).42,118

InthecaseofCVDgraphene,etchingoftheunderlyingmetalallowsthecarbon lmstobetransferredtoothersubstrates.This,combinedwiththelargeareaofdepositions,hasgreatpromisefortransparentconductingapplications.Onesuch lmgrownbyCVDandtransferredviaaPDMSstampontoglassshowedasheetresistanceofjust280 /)at80%opticaltransmittance.42

exfoliatedgraphenetomakethe rstsub-50nmnanoribbons(seeFigure20).123,124AlthoughtheprocessyieldedIon/Ioffratiosofupto104,devicesshowedhighvariabilityduetothelackofcontroloveredgetermination.Stencil-likepatterningwasindiscriminateofcrystallographicdirection,andthusedgeeffectswereessentiallydifferenteverytime.Thechallengeofsynthesizingreproduciblegraphenenanoribbonsisaninterestingoneforchemists,whohavesoughttoexploitthedifferencesinreactivityalonggraphene’stwocrystallographicdirections.64In2008,Dai’sgroupatStanforddevelopedthe rsttechniqueforisolatingnano-ribbonsdirectlyfrombulkgraphite(SeeFigure21).61Itinvolvedsonicationofexpandedgraphiteinthepresenceofapolymerknowntoparticipateinπ-stackingwithconjugatedcarbons.Thepolymeractedtononcovalentlyfunctionalizeandconsequentlystabilizenanoribbonsformedbymechan-icalfracture.Atomicforcemicroscopyofthenanoribbonssuggeststhatfracturefollowsnicelyalongthecrystal-lographicdirectionsofgraphene.Inthepresenceofthepolymer,theribbonscanbesuspendedinorganicsolventsandthendepositedbyspin-coating.

ElectricaltestingofDai’snanoribbonsshowedmuchgreaterconsistencythanthosemadebylithography.Aspredictedbytheory,thebandgap(Eg)ofnanoribbonswasfoundtobeinverselyproportionaltotheirwidth,withanEgof～0.4eVforspecimensfewerthan10nmwide.ThisledtoIon/Ioffratiosofupto106forthethinneststrips.Thenextmajorchallengewillbe ndingawaytoreliablydepositthenanoribbonsinprede nedlocationsforscalabledevicefabrication.

6.GrapheneNanoribbons

Amajorissuewithgraphene-basedlogicdevicesistheirpoorIon/Ioffratios.Conductivityingrapheneisminimizedunderzerogatebias,butdevicesareessentiallyimpossibletoturnoffatanyreasonabletemperaturebecausethermalenergyand uctuationsaremorethansuf cienttoproducelargecarrierpopulations.Thishigh“leakage”currentresultsinIon/Ioffratiosthataretypicallyjust1or2ordersofmagnitude,whichisinsuf cientforimplementationinrealdevices.

Whileanumberofapproacheshavebeensuggested,themoststraightforwardwaytominimizetheoffcurrentingraphene-baseddevicesistointroduceanappreciablebandgap.Thishasmotivatedagreatdealofresearchintographenenanoribbons,whicharenolongersemimetallicduetoquantumcon nement.In2007,Kim’sgroupusede-beampatterningandoxygen(O2)plasmaetchingofmechanically

7.FutureWork

Graphenehasaninterestinghistory,butmanynowwonderaboutitsfuture.Thesubjectofconsiderablescholarlydebate,itdoesseemreasonabletoassertafewthingslookingahead.First,thequalityandavailabilityof“synthetic”graphenewillcontinuetoimprove.Whetherhighqualitymaterialcomesintheformofanalternativechemicalroutetothecompleteexfoliationofgraphiteorfromoptimizationofthethermalprocessesrequiredforsubstrate-basedmethods,thereisnosignthatsynthetictechniquesarenearingtheirupperlimit.Thismeansthatdeviceengineerswillhaveampleaccesstoimprovedmaterialsfordevelopingnovelstructuresand ndingwaystointegrategrapheneintopresent-dayelectronicdevices.

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