Honeycomb Carbon A Review of Graphene 石墨烯综述(14)

Theoriginaltechniqueusedbyourgroupfordepositing lmswasspraycoatingfromwaterontoaheatedsubstrate.31Althoughwewereabletoisolateandcharacterizesomesinglesheets,highsurfacetensioncausedsigni cantag-gregationevenifthesubstratewasheatedto ash-drythesuspensionuponcontact.Wehavebeenmoresuccessfulspin-coatingdispersionsmadedirectlyinhydrazine.Themethodallowsforafullrangeofcoveragedensitiesby ne-tuningofspin-speedandapretreatmentappliedtothesurfaceofthesubstrate.

Huang’sgroupatNorthwesternrecentlydemonstratedwonderfulcontroloverdepositionsusingLangmuir-BlodgettassemblyofGO.107Theyshowedthatelectrostaticrepulsionpreventsthesinglelayersfromoverlappingwhencompressedatanair/surfaceinterface.ThisledtodepositionsonSiOthatincludeddilute,close-packed,andoverpacked lms.2Dai’sgroupatStanfordhasdonesimilarworkwithLangmuir-Blodgetttechniquesandalsolayer-by-layeras-semblyusingelectrostaticattractiontobiasedsubstrates.109

5.1.2.DefectDensityinChemicallyDerivedGraphene

Aswithmechanicallyexfoliatedgraphene,itisimportanttocharacterizechemicallyderived akesbeforefabricatingdevices.Thisisespeciallytrueinthechemicalcasebecausethebasalplaneofgrapheneundergoesseriousalterationduringtheprocessofoxidationandreduction.ResidualoxidationwasmadeobviousbyanappreciableDbandnear1350cm-1intheRamanspectrumofchemicallyderivedgraphene.29ThisbandismadeRamanactivebythesigni cantnumberofdefectsandresultingbrokensymmetryofthebasalplane.X-rayphotoelectronspectroscopy(XPS)ofreducedGOindicatesnearlycompleteremovalofoxygen,whichhasledRuoff’sgroupandourowntosurmisethatnonconjugatedsp3carbonconstitutesmostofthedefects.Thesealsolimittheobservationofinterestingphysicsphenomenainchemicallyderivedgrapheneandinhibitmobility.

ThepresenceofalargeDbandalsoprecludesthe ngerprintingtechniquethatisusedtodeterminecrystallitethicknessformechanicallyexfoliatedgraphene.Thepromi-nenceofthebandmakesassigninganyexactpositiontotheGbandnexttoimpossible.Instead,mostgroupshaveturnedtoatomicforcemicroscopyinordertocon rmthethicknessofdeposited akes.Asinothercases,thegraphene-substratestepheightisdif culttoresolveandreportsrangebetween0.4and1.0nmforsinglelayers.30,101,107

5.1.3.Field-EffectDevices

Evenataloweroverallcrystalquality,theavailabilityofchemicallyderivedgraphenehasremovedaseriouslogjamintheengineeringofgraphene-baseddevicesandscientistshavebeeneagertomakeelectricalmeasurements.The rst

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