Recent-advances-in-free-standing-two-dimensional-c

ThisjournaliscTheRoyalSocietyofChemistry2013Chem.Soc.Rev.Citethis:DOI:10.1039/c3cs60138bRecentadvancesinfree-standingtwo-dimensionalcrystalswithatomicthickness:design,assemblyandtransferstrategiesXiaodongZhangandYiXie*Free-standingtwo-dimensional(2D)crystalswithatomicthicknesshaveattractedextensiveattentionbecauseoftheirnovelelectronic,optical,mechanicalandbiocompatibleproperties,andsoon.Inrecentyears,thestudyofatomicallythick2DcrystalshasmainlyfocusedonthelayeredmaterialswithweakvanderWaalsforcesbetweenthelayers.Forthelackofexecutablesyntheticstrategies,preparationofatomicallythick2Dcrystalswithanonlayeredstructureorquasi-layeredstructurewithrelativelystrongbondsbetweenthelayersisstillagreatchallenge.Thisreviewmainlyfocusesonrecentadvancesinsyntheticstrategiesforatomicallythick2Dcrystalswithanonlayeredstructureaswellasthequasi-layeredstructurewithrelativelystrongbondsbetweenthelayers.Furthermore,methodsforthemodulationoftheelectronicstructuresof2Dcrystalsalongwithassemblyandtransfertechniquesofthe2Dcrystalsarediscussed.Thekeypointsofeachstrategyinpreparation,electronicstructuremodulation,assemblyandtransferprocessesarealsopresented.Keylearningpoints(1)Comprehensiveoverviewofrecentadvancesin2Dcrystalswithatomicthickness.(2)Summaryofdirectchemicalsyntheticstrategiesforthepreparationof2Dcrystalswithatomicthickness.(3)Summaryofmodifiedliquidexfoliationstrategiesforthepreparationof2Dcrystalswithatomicthickness.(4)Summaryofmethodsfortuningtheelectronicstructuresof2Dcrystals.(5)Summaryoftheassemblyandtransfertechniquesoffree-standing2Dcrystalsintointegratedfilmsanddesiredpatterns.1.IntroductionFree-standingtwo-dimensional(2D)crystalswithatomicthick-nesshaveattractedextensiveattentionfromscientistsfortheirpromisingpracticalapplicationandtheoreticalvalues.1–7Strictlyspeaking,materialswithfew-layeredatomicplanesand2Dscalabilityshouldbecalled‘‘2Dcrystals’’,whichcanalsobevisuallynamedas‘‘ultrathinnanosheets’’.Theatomicallythick2Dcrystalscannotonlyenhancetheintrinsicpropertiesoftheircorrespondingbulksamples,butalsomaygivebirthtototallynewproperties.8Ontheotherhand,thefree-standingnanosheetswithatomicthicknessareconsideredasthemosteffectivebridgetocombinenovelmicroscopicelectronicstructureswithmacroscopicultrathin,transparentandflexibleelectronicdevices,guaranteeingamaximumfunctionalitywhilekeepingthesizeminimized.9,10Inrecentyears,accom-panyingtheboomingdevelopmentofgraphene,moreandmoreattentionhasbeenfocusedontheinorganicgrapheneanalogues(IGAs)withweakvanderWaalsforcesbetweenthelayers,whichcanbeeasilyexfoliatedinto2Dnanosheetswithatomicthickness.6–15Forexample,singlelayeredSnS2hasbeenfoundtoshowefficientvisiblelightwatersplittingproperties,11ultrathing-C3N4nanosheetscanbeusedforbioimagingapplications,8andsingle-layeredMoS2canbefabricatedintofieldeffecttransistorsandphototransistors,12,13andsoon.Thoughalotofnew2Dcrystalswithatomicthicknesshavebeenfabricatedbymechanicalcleavage,16CVDgrowth17,18ortherecentlydevelopedliquidexfoliationstrategy,19theirspeciesweremainlyrestrictedtolayeredmaterialswithweakvanderWaalsforcesbetweenthelayers,suchas,graphite,layeredtransitionmetaldichalcogenides,etc.Duetothelackofintrinsicdrivingforcefor2Danisotropicgrowth,controllingassemblyofsmallbuildingblockswasonceconsideredastheHefeiNationalLaboratoryforPhysicalScienceatMicroscale,CollaborativeInnovationCenterofChemistryforEnergyMaterials,UniversityofScienceandTechnologyofChina,Hefei,Anhui,230026,P.R.China.E-mail:[e-mail]18thApril2013DOI:10.1039/c3cs60138b:08:59.ViewArticleOnlineViewJournalChem.Soc.Rev.ThisjournaliscTheRoyalSocietyofChemistry2013exclusivewayforthepreparationof2Dcrystalsofnonlayeredstructurematerials.5,20Expandingthestudyareatoatomicallythick2Dcrystalswithanonlayeredstructureorquasi-layeredstructurewithrelativelystrongbondsbetweenthelayerswillnotonlyenrichthefamilyof2Dcrystalswithatomicthickness,butalsobringusunprecedentedsurprises.Inthistutorialreview,thenonlayeredmaterialsdenotethematerialswhichpossessisotropicstructures,suchas,Co9Se8,ZnSewithacubicsymmetrystructure,andunobviousanisotropicstructures.Forexample,VO2(M)withamonoclinicstructuredoesnotshowobviousanisotropyasall[VO6]octahedraarelinkedbysharinganedgealongthea-axistoformchains,andthechainsaretheninterlinkedbysharingcorneroxygenatomstoforma3Dframework.Incomparison,thequasi-layeredmaterialsstandforthematerialswhichpossessanobviouslyanisotropiclayeredstructurebutwithrelativelystrongbondsbetweenthelayers.Forexample,layeredternarycarbidesornitrides(MAXphase,Misatransitionmetal,AisagroupIIIAorIVAelement,andXiscarbonornitrogenatom)consistofalternatelyarrangedMXandAlayers,wherethebondsbetweentheMXlayerandAlayeraremuchstrongerthanthevanderWaalsforcesbutweakerthanthebondsofM–X.Inanothercase,VO2(B)isformedbyedge-sharingandcorner-sharingofdeformed[VO6]octahedraintheac-planeandb-axis,respectively,andthusthepackingdensityofatomsalongtheb-axisislowerthanthatintheac-plane,resultin