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NANOEXPRESSOpenAccessVanderWaalsepitaxyandcharacterizationofhexagonalboronnitridenanosheetsongrapheneYangxiSong1,ChangruiZhang1*,BinLi1,GuqiaoDing2,DaJiang2,HaominWang2andXiaomingXie2*AbstractGrapheneishighlysensitivetoenvironmentalinfluences,andthus,itisworthwhiletodepositprotectivelayersongraphenewithoutimpairingitsexcellentproperties.
Hexagonalboronnitride(h-BN),awell-knowndielectricmaterial,mayaffordthenecessaryprotection.
Inthisresearch,wedemonstratedthevanderWaalsepitaxyofh-BNnanosheetsonmechanicallyexfoliatedgraphenebychemicalvapordeposition,usingborazineastheprecursortoh-BN.
Theh-BNnanosheetshadatriangularmorphologyonanarrowgraphenebeltbutapolygonalmorphologyonalargergraphenefilm.
Theh-BNnanosheetsongraphenewerehighlycrystalline,exceptforvariousin-planelatticeorientations.
Interestingly,theh-BNnanosheetspreferredtogrowongraphenethanonSiO2/Siunderthechosenexperimentalconditions,andthisselectivegrowthspokeofpotentialpromiseforapplicationtothepreparationofgraphene/h-BNsuperlatticestructuresfabricatedonSiO2/Si.
Keywords:Hexagonalboronnitride;Nanosheets;Graphene;vanderWaalsepitaxy;ChemicalvapordepositionBackgroundGraphenehasattractedglobalresearchinterestsacrossawiderangeofapplications[1,2].
However,grapheneishighlysensitivetoextraneousenvironmentalinfluences.
Thus,itwasdeemedworthwhiletodepositprotectivelayersovergraphenewithoutimpairingitsproperties.
Hexagonalboronnitride(h-BN),awell-knowndielectricmaterial,mayaffordthenecessaryprotectionforgraphene[3,4].
Asananalogueofgraphene,h-BNshowsaminimallat-ticemismatchwithgrapheneofabout1.
7%,yethasawidebandgap[5-8]andlowerenvironmentalsensitivity[3,4].
Hence,h-BNprovestobeapromisingdielectricmaterial,orsubstrate,fortwo-dimensionalelectronicdevicesandespeciallyforthosebasedupontheuseofgraphene[9-13].
Graphene,partiallycoveredbyh-BNprotectivelayers,maydisplaypromisingelectroniccharacteristicsofgraphenewithmuchlowerenvironmentalsensitivity.
Recently,chemicalvapordeposition(CVD)synthesisofh-BNonNi[14-16]orCu[13,17-19]substrateshasbeenfurtherinvestigated.
Forthefollowingapplicationsingrapheneelectronicdevices,h-BNcanbeacquiredbyetchingofthecatalystsubstratesandatransfertechnique.
Nevertheless,thetransferprocessbringsinevitablecontam-inationorevendestruction,anditisdifficulttodeterminethepositionandthecoverageratioofh-BNongraphene.
Consideringthisproblem,wepayattentiontothecatalyst-freeCVDgrowthofh-BNongraphene,whichpromisesdirectapplicationingrapheneelectronicdevicesandmayobviatetheneedforatransferprocess.
IthasbeendemonstratedthatvanderWaalsepitaxybycatalyst-freeCVDcanbeapromisingrouteforthegrowthoftopologicalheterostructures[20-22].
More-over,thesurfaceofgrapheneisatomicallyflatandwith-outdanglingbonds,whichmakesgrapheneapromisingtemplateforthevanderWaalsepitaxyofothertwo-dimensionalmaterials.
Compoundswith1:1B/Nstoichi-ometryareoftenselectedash-BNprecursorsforCVD,andborazine(B3N3H6)couldbeapromisingchoiceasitwouldproduceBNandhydrogen,whicharebothenvir-onmentallyfriendly.
Inthisresearch,thevanderWaalsepitaxyofh-BNnano-sheetsonmechanicallyexfoliatedgraphenebycatalyst-freelow-pressureCVD,usingborazineastheprecursortoh-BN,wasdemonstrated.
Theh-BNnanosheetspreferredtogrowongrapheneratherthanonSiO2/Siandtendedtoexhibitatriangularmorphologywhengrownonanarrow*Correspondence:crzhang12@gmail.
com;xmxie@mail.
sim.
ac.
cn1StateKeyLaboratoryofAdvancedCeramicFibersandComposites,CollegeofAerospaceScienceandEngineering,NationalUniversityofDefenseTechnology,109DeyaRoad,Changsha410073,People'sRepublicofChina2StateKeyLaboratoryofFunctionalMaterialsforInformatics,ShanghaiInstituteofMicrosystemandInformationTechnology,ChineseAcademyofSciences,865ChangningRoad,Shanghai200050,People'sRepublicofChina2014Songetal.
;licenseeSpringer.
ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttributionLicense(http://creativecommons.
org/licenses/by/4.
0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycredited.
Songetal.
NanoscaleResearchLetters2014,9:367http://www.
nanoscalereslett.
com/content/9/1/367graphenebelt.
Theh-BNnanosheetsgrownongraphenewerehighlycrystalline,albeitwithvariousin-planelatticeorientations.
Methodsh-BNnanosheetsweresynthesizedinafusedquartztubewithadiameterof50mm.
Graphenewastransferredontosiliconoxide/silicon(SiO2/Si)wafersbymechanicalexfoli-ationfromhighlyorientedpyrolyticgraphite(HOPG,AlfaAsear,WardHill,MA,USA).
Theh-BNprecursor(bora-zine)wassynthesizedbythereactionbetweenNaBH4and(NH4)2SO4andpurifiedaccordingtoourpreviousreports[23,24].
ThetemperaturefortheCVDgrowthofh-BNnanosheetswassetto900°C.
Beforethegrowthofh-BN,withthetubeheatedto900°C,graphenegrownonSiO2/Siwasfirstannealedfor60mininanargon/hydrogenflow(Ar/H2,5:1byvolume,bothgaseswereof99.
999%purityfromPujiangCo.
,Ltd,Shanghai,China)of180sccmtore-movepollutantsremainingonthegrapheneaftermechan-icalexfoliation.
Duringthegrowthprocess,borazine,inahomemadebubbler,wasintroducedtothegrowthcham-berbyanotherArflowof2sccm,whiletheAr/H2flowremainedunchanged.
Thetypicalgrowthtimewas5min,whilethepressurewas10to100Pa.
Afterthegrowthprocess,thetubewasrapidlycooledtoroomtemperature.
RamanspectroscopywasperformedinaThermoDXRwith532-nmlaserexcitation(ThermoFisherScientific,Waltham,MA,USA).
Atomicforcemicroscopy(AFM)(DimensionIcon,Bruker,Karlsruhe,Germany)andscan-ningelectronmicroscopy(SEM)(NovaNanoSEM320,FEICo.
,Hillsboro,OR,USA)wereusedtoobservethethicknessandmorphologyoftheh-BNnanosheets.
X-rayphotoelectronspectroscopy(XPS)(AXISUltra,Kra-tosAnalytical,Ltd,Manchester,UK)wasconductedtoanalyzethechemicalcompositionofthefilms.
Theh-BNnanosheetswiththegraphenesubstrateweretrans-ferredtotransmissionelectronmicroscopy(TEM)gridsforfurthercharacterization.
Bothmorphologyimagesandselectedareaelectrondiffraction(SAED)patternsoftheh-BNnanosheetswereobtainedbyfieldemissionhigh-resolutiontransmissionelectronmicroscopy(HRTEM)(TecnaiG220,FEICo.
).
ResultsanddiscussionAFMimages(Figure1)showthemorphologyandthick-nessoftheh-BNnanosheets.
Figure1ashowsthebound-aryregionofSiO2/Siandgraphenewithitsassociatedh-BNnanosheets.
Figure1bdisplaysthepolygonalmorphologyoftheh-BNnanosheets.
Itwasinterestingtonotethath-BNnanosheetspreferredtogrowongrapheneratherthanonSiO2/Si.
Thisresultpossiblyoriginatedfromtheminimallatticemismatchbetweenh-BNandgraphene,andthesmallamountofdefectsremaininginthegrapheneaftermech-anicalexfoliationandhightemperatureannealing,andthesewouldenabletheh-BNtonucleateongrapheneandgrowthereafter.
Thisselectivegrowthphenomenonprom-isespotentialapplicationsforgraphene/h-BNsuperlatticestructuresfabricatedonSiO2/Si.
ThissamephenomenonwasalsoseeninSEMimagesasshowninFigure2.
Figure2ashowsgrapheneonSiO2/SibeforeCVD,whileFigure2b,cshowsh-BN/grapheneonSiO2/SiafterCVD.
IttooktimetodistinguishgraphenefromSiO2/SiduetotheirlowcontrastundertheSEMasshowninFigure2a,bwheretheboundariesofgraphenezonesontheSiO2/Sisubstrateareindicatedbyarrows.
ThewrinklesinthegrapheneinFigure2a,coriginatedfromthemechanicalexfoliationprocessandcouldalsoactasmarkersindicatingthepresenceofgraphene.
Theh-BNnanosheetsexhibitedapolygonalmorphologywithsomenanosheetsbecomingisolatedislandsonthegraphene,whileotherswithdifferentthicknessesjoinedandbecamestacked,asshowninFigure2c.
Moreover,theh-BNnanosheetstendedtoexhibitatriangularmorph-ologyonthemuchnarrowergraphenebelt,asshowninFigure2b.
ThisresultissimilartovanderWaalsepitaxialgrowthofMoS2ongraphene[21]andperhapsoriginatesfromthehigherboundaryeffectofthenarrowergrapheneFigure1AFMimagesofh-BN/grapheneonSiO2/Si.
(a)Boundaryregionofh-BN/grapheneandSiO2/Si.
(b)h-BNnanosheetsongraphene.
Songetal.
NanoscaleResearchLetters2014,9:367Page2of7http://www.
nanoscalereslett.
com/content/9/1/367beltaftermechanicalexfoliation[25].
Besides,thetriangularh-BNnanosheetsongrapheneshoweddifferentin-planeorientationsfromeachother.
Ramanspectroscopyprovidedausefulmeansofglean-inginformationaboutthelatticevibrationmodesofgra-pheneandh-BN.
AfterbeingtransferredtoSiO2/SibytheScotchtapemechanicalexfoliationmethod,thegra-phenewasgenerallyalignedwiththe(002)latticeplaneparalleltothesurfaceoftheSiO2/Siwafer[1,2].
TheexistenceofgraphenewasshownbyRamanspectrainFigure3,inwhichtheI2D/IGratioofgraphenewaslessthan0.
5,indicatingthemultilayerstructureofthegra-phene.
Moreover,aweakDpeakofgrapheneat1,350cm1wasobservedfromtheRamanspectra(Figure3),indicat-ingasmallnumberofdefectsinthegraphene,whichmayhaveoriginatedfromtheoriginalHOPGorthemechan-icalexfoliationprocess.
ForthesampleexaminedafterCVD,apeakmuchstrongerthantheDpeakofgrapheneappearedat1,367cm1,indicatingtheE2gvibrationmodeofh-BN,whichwasconsistentwiththereportedvalues[5,6,13-19].
Interestingly,the2DandGpeaksforgra-phenediminishedinintensityafterCVD,andthismayhaveoriginatedfromthepartialcoverageofthegraphenebyh-BN.
AsshowninFigure3b,c,theGpeaksofgra-pheneforthegraphenesubstrateandh-BN/graphenewerefittedwithLorentzcurves(solidlines).
Thefittingdatawerewellfittedwiththerawdata,whiletheRamanfrequencyandfullwidthathalfmaximum(FWHMs)forGbandswerealmostequaltoeachother.
Theseresultsarecomparablewiththereportedvaluesofgraphene[26]andgraphite[27,28],showingthehighqualityofgraphenebeforeandafterCVDandindicatingthatthesynthesisofh-BNnanosheetsongrapheneinourmanuscriptdoesnotcauseadegradationofgraphene.
Accordingtopreviousreports[29],thegas-phasenu-cleationforh-BNwasabsentatgrowthtemperatureslowerthan1,000°C;hence,thegrowthofh-BNnano-sheetsongraphenewasdominatedbythesurfacenucle-ationduringourCVDprocessat900°C.
Moreover,thesurfacetopographyofthesubstrateisvitaltothesurfacenucleation[30].
Consequently,thenucleationoftheh-BNnanosheetsonthegraphenesubstratewasregulatedbythesurfacemorphologyofgrapheneinourwork.
Additionally,theatomicscaledefects,dislocations,andFigure2SEMimagesofgrapheneandh-BN/grapheneonSiO2/Si.
(a)MultilayergrapheneonSiO2/SibeforeCVD,withthegrapheneboundary,andwrinkling,indicatedbyarrows.
(b)h-BNnanosheetsonanarrowgraphenebeltonSiO2/Si,withthegrapheneboundaryindicatedbyarrows.
(c)h-BNnanosheetsonalargergraphenefilm,withwrinklesindicatedbyarrows.
Figure3Ramanspectra.
(a)RamanspectraofgraphenebeforeCVD(lowerplot)andh-BN/grapheneafterCVD(upperplot).
GpeaksfittingwithLorentzcurves(solidlines)forgraphenesubstrate(b)andh-BN/graphene(c)areshownwiththeirFWHMs,respectively.
Songetal.
NanoscaleResearchLetters2014,9:367Page3of7http://www.
nanoscalereslett.
com/content/9/1/367stepsforthegraphenesubstratewereinevitableduringthemechanicalexfoliationprocessduetothestronginterlayerbindingofgraphite[31],andtheatomic-leveldefects,dislocations,andstepsofthesubstrateswouldserveasthenucleationcentersforCVDgrowth,forthecurvedsp2πbondsinthegraphenedefects,dislocations,andstepsweremorereactivethantheplanargraphenere-gions[21,32].
Inourwork,asmallnumberofdefectsforthegraphenesubstrateswereprovedbytheweakDpeakofRamanspectrainFigure3.
Theatomicdefectsofferadditionalbondsitestothecarbonatoms,makingthemenergeticallypreferredfornucleation.
DuringtheCVDgrowth,theatomic-leveldefectsofgraphenecouldeffect-ivelycausenucleationoftheh-BNonthegraphene.
Subsequently,withanincreasedamountofprecursor,theh-BNnanosheetscouldgrowonthesurfaceofgraphenethroughweakvanderWaalsinteractions.
XPSwasusedtoanalyzethechemicalcompositionoftheh-BN/grapheneonthesurfaceoftheSiO2/Si,asshowninFigure4.
TherawXPSdatawerecorrectedusingthebindingenergyoftheC-Cbondat284.
5eV.
TheSiandOpeaksinFigure4arosefromtheSiO2/Sisubstrate,whiletheCpeakarosefromthepresenceofgra-phene.
ThebindingenergiesofB1sandN1sfromtheXPSspectrawere191.
0and398.
5eV,respectively,whichwereingoodagreementwithreportedvalues[14,16,18,19,33,34]forh-BN.
TheB/Nratioofthesample,astakenfromtheXPSmeasurement,was1.
01,indicatingthenearlystoichio-metriccompositionofthesynthesizedh-BNnanosheetsongraphene.
AsshowninFigure4b,c,d,theXPSpeaksofB1s,N1s,andC1scorelevelswerefittedwithGaussiancurves(redpeaks).
Thefittingdatawerewellfittedwiththerawdata,andnoshoulderpeakscouldbeobservedfromthefittingcurves.
Hence,thesinglepeaksoffittingdataindicatethattheC-BorC-Nbondsdonotexistinourh-BN/graphenesystem,comparedwiththereportedresultsofBCNfilms[35,36].
Theseresultsshowthatthesynthesisofh-BNnanosheetsongrapheneinourmanu-scriptdoesnotcauseadegradationofgraphene.
Wehavepointedoutthereasonforthenucleationoftheh-BNongraphene.
Infact,thedepositionofh-BNnanosheetsongraphenewasperformedasinstantaneousnucleationfollowedbythree-dimensionalgrowthinourcatalyst-freeCVDgrowth.
Similarresultsofthree-dimensionalgrowthincertainsituationshavebeenprovedbypreviousreports[21,32].
Asdiscussedabove,energyoptimizationisofgreatimportancetothenucleationofh-BN,andthede-fects,dislocations,andstepsofgrapheneareenergeticallypreferred.
DuringtheCVDgrowthofh-BNongraphene,theaboveenergeticallypreferredregionsofgraphenewouldbecoveredorremediedbyh-BNlayerswithacertaindo-mainsize.
Asanalternative,theedgesoftheas-grownh-BNlayersandtheregionsnearthedefectsofgrapheneturnedenergeticallypreferredfornucleationofnewh-BNlayers,whichbothfavortheverticalorthree-dimensionalgrowthofh-BNnanosheetsonthegraphene.
Aftertheh-BNnanosheetsongrapheneweretrans-ferredtoTEMgridsaftertheetchingofSiO2/Si,atomicresolutionHRTEMwasusedtostudythecrystallinestructureoftheaforementionedh-BNnanosheetsonFigure4XPSspectraofh-BN/grapheneonSiO2/Si.
(a)Surveyspectrum.
(b-d)XPSspectraofB1s,N1s,andC1scorelevels,respectively.
Thepeaksof(b-d)werefittedwithGaussiancurves(redpeaks),andgoodfitscouldbeobservedfortherawdataandthefittingdata.
Songetal.
NanoscaleResearchLetters2014,9:367Page4of7http://www.
nanoscalereslett.
com/content/9/1/367theirrespectivegraphenesubstrates.
Figure5ashowsaTEMimageoftheh-BNnanosheetsongraphene,withthearrowsindicatingtheedgeofthegraphene.
Thepol-ygonalobjectsonthegrapheneindicatedtheexistenceofh-BNnanosheets.
Thenumbers'1'to'4'indicatetypicalregionsofFigure5a.
Region1referstoaregionofgraphenewithoutanyh-BNnanosheetthereon,whileregions2to4refertoisolatedh-BNnanosheetsonthegraphene.
Figure5b,c,dshowstheatomicimagescorre-spondingtoregions2to4,whilethecorrespondingSAEDpatternsforregions1to4areshowninFigure5e,f,g,h,respectively.
Theregular,periodicSAEDspotsevincedthehighdegreeofcrystallinityofboththegrapheneandh-BNnanosheets.
Figure5bshowsthattheh-BNnanosheetinregion2hadthesamein-planelatticeorientationasthegraphenesubstrate.
However,theh-BNnanosheetsandgrapheneinregions3and4wererotationallydisplaced,accordingtotheirMoirépatterns(seeinsetsofFigure5c,d,re-spectively).
Theh-BNnanosheetsongraphenehadvari-ousin-planelatticeorientations,whichwereconsistentwiththeSAEDpatternsofFigure5f,h.
TheseresultswerealsoevincedbytheSEMimage(Figure2b),asthetriangularh-BNnanosheetsonthenarrowgraphenebeltalsolayinvariousdirections.
ConclusionsInsummary,wehavedemonstratedthevanderWaalsepitaxyofh-BNnanosheetsongraphenebycatalyst-freeCVD,whichmaymaintainthepromisingelectronicchar-acteristicsofgraphene.
Theh-BNnanosheetstendedtohaveatriangularmorphologyonanarrowgraphenebelt,whereastheyhadapolygonalmorphologyonamuchlar-gergraphenefilm.
TheB/Nratiooftheh-BNnanosheetsFigure5Imagesofh-BN/graphenetransferredontoTEMgrids.
(a)Alow-magnificationTEMimageofh-BNnanosheetsongraphene,withthearrowsshowingthegrapheneboundary.
(b-d)HRTEMatomicimagescorrespondingtoregions2,3,and4in(a),withtheinsetsshowingFFT-filteredimages,respectively.
(e-h)SAEDpatternscorrespondingtoregions1to4.
Songetal.
NanoscaleResearchLetters2014,9:367Page5of7http://www.
nanoscalereslett.
com/content/9/1/367ongraphenewas1.
01,indicativeofanalmoststoichiomet-riccompositionofh-BN.
Theh-BNnanosheetspreferredtogrowongrapheneratherthanonSiO2/Si,whichof-feredthepromiseofpotentialapplicationsfortheprepar-ationofgraphene/h-BNsuperlatticestructures.
Theh-BNnanosheetsongraphenehadahighdegreeofcrystallin-ity,exceptforvariousin-planelatticeorientations.
Thesynthesisofh-BNnanosheetsonmultilayergraphenehasbeenstudied,andh-BNnanosheetsonfew-layerandevenmonolayergraphenewillbesynthesizedinfuturework.
Thismaysatisfycertainapplicationrequirementsfortopologicalheterostructuresandgraphene-relatedelec-tronicdevices.
CompetinginterestsTheauthorsdeclarethattheyhavenocompetinginterests.
Authors'contributionsYS,CZ,BL,andXXdesignedtheexperiments,andYScarriedoutmostoftheexperimentalworkandmaterialcharacterizations.
CZandBLsynthesizedtheborazine.
YS,CZ,BL,GD,andXXdiscussedtheresults,andYSdraftedthemanuscript.
Allauthorshavereadandapprovedthefinalmanuscript.
AcknowledgementsThisworkwasfinanciallysupportedbyprojectsfromtheNaturalScienceFoundationofChina(GrantNos.
11104303,11274333,11204339,61136005,and50902150),ChineseAcademyofSciences(GrantNos.
KGZD-EW-303,XDA02040000,andXDB04010500),theOpenFoundationofStateKeyLaboratoryofFunctionalMaterialsforInformatics(GrantNo.
SKL201309),theNationalHigh-techR&DProgramme(GrantNo.
2012AA7024034),andtheNationalScienceandTechnologyMajorProjectsofChina(GrantNo.
2011ZX02707).
Wethanktheanonymousreviewersfortheirhelpfulsuggestionswhichhaveimprovedthemanuscript.
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