Enhanced

Enhancedfieldemissionfromself-assembledZnOnanorodsongraphene/Ni/SisubstratesLinglingWanga,b,MaiphiHunga,GennadyN.Panina,c,TaewonKanga,D.J.Fub,naQSRC,PhysicsDepartment,DonggukUniversity,3-26Pildong,100-715Seoul,RepublicofKoreabAcceleratorLaboratory,SchoolofPhysicsandTechnology,WuhanUniversity,430072Wuhan,ChinacInstituteforMicroelectronicsTechnology,142432Chernogolovka,RussiaarticleinfoArticlehistory:Received28July2013Accepted28August2013Availableonline3September2013Keywords:ZnOnanorodsFieldemissionGrapheneabstractSelf-assembledZnOnanorods/graphenefieldemissionstructureswerefabricatedbyusingahydro-thermalgrowthofnanorodsonfewlayergraphene(FLG)substrates.TheFLGpatternedelectrodewasusedtoimprovethecontactresistanceandtocontrolthedensityandtheareaoffieldemission.Thisapproachallowscreationofhigh-performancefieldemissiondevicesoveralargeareausinganinexpensivelow-temperaturegrowthandFLGpatternedsubstrates.TheobtainedZnONRs/FLGfieldemissionstructuresoperateatalowvacuumof10
5Torrandlowturn-onelectricfieldof0.9V/mm,demonstratingahighfieldenhancementfactorβ¼5556.&2013ElsevierB.V.Allrightsreserved.1.IntroductionZincoxide(ZnO)withawidebandbandgap(Eg¼3.37eV)andlargeexcitionbindingenergy(Eb¼60meV)hasbeenaninterest-ingresearchtopicsinceithasgreatpotentialforvariousapplica-tions,suchasFED,lightemittingdiodes(LED),transparentconductingoxide,sensorsforchemicalsandgases,surfaceacousticwavedevices,ultravioletemittersetc.[1].Thenanorodsofoxidesemiconductorareknowntohaveanadvantageforfieldelectronemitterstoworkatlowvacuumwithoutsignificantdegradationoftheircharacteristics.However,suchemittersusuallyrequirearelativelyhighelectricfieldprimarilybecauseoftheirlargecontactresistance.TherelationshipbetweenFEcurrentdensityandtheappliedelectricfieldisknowntodescribebytheFowler–Nordheim(FN)equation[2]J¼AðβEÞ2Φexp
BΦ3=2βE
ð1ÞwhereAandBareconstantswithvaluesof1.5610
10AV
2eVand6.83103Vμm
1(eV)
3/2,respectively.Thefieldenhance-mentfactorβintheFNequationreflectsthegeometriceffectofthemicroroughnessontheelectricfield,sosharpgeometryandlargeβarecloselyrelated.Onedimensionalnanorods(NRs)naturallyhavehighaspectratio(length/diameter),sotheyarethebestcandidateforfieldemission[3].ZnOhasmanymerits,suchaseasetoformnanorodsandnanostructures,mechanicalandchemicalstability,soitcanoperateatrelativelylowvacuum(10
5Torr)comparedwithCNTs(10
7Torr)[4],andisoflowcost,andcanworkatlowthresholdvoltage.ZnOnanorodscanbesynthetizedbydifferentmethodssuchasCVD,MBE,PLDetc.,whichusuallyneedhightemperatureofgrowth(500–8001C).Hydrothermalgrowthmethod(HTG)usedinthisworkisinexpensiveandworksatlowtemperature(60–1201C),sothecheapglassandtheflexiblepolymersubstratescanbeused.Furthermore,theoxidationofaSiwaferduringtheHTGprocess,unlike,thehightemperatureCVDgrowth,doesnotoccur.Here,wereporttheadvancedfieldemissioncharacteristicsoftheZnOnanorodsgrownbyHTGonasiliconsubstratewithfewlayergraphene(FLG).TheFLGelectrodewasusedtoreducethecontactresistancetonanorodsandtoimprovetheFEDperfor-mance.Seedlithographywasalsoappliedtocontrolthedensityofnanorodsandscreeningeffect.2.ExperimentalFLGsubstrateswereobtainedusingaSi(100)(0.1Ωcm)wafercoveredbyathin(80nm)layerofnickelandaspincoatedphotoresist(PR)layerasacarbonsolidsourceforFLGgrowthasreportedpreviously[5].ZnONRsweregrowninthehydrother-malreactorbyusing50mMzincnitratehexahydrate(Zn(NO3)26H2O)and50mMhexamethylenetetramineat951Cfor4h[6,7].Themorphology,crystalqualityanddefectstructureoftheZnONRsareanalyzedbyafieldemissionscanningelectronmicroscope(FE-SEM)usingaPhillipsXL-30withMonoCLsystem.Thecurrent–voltagemeasurementwasconductedontheNRswithanindiumcontactusingaKeithley4200-SCSanalyzer.ContentslistsavailableatScienceDirectjournalhomepage:://dx.doi.org/10.1016/j.matlet.2013.08.124nCorrespondingauthor.Tel./fax:þ862768753587.E-mailaddress:[e-mail](D.J.Fu).MaterialsLetters112(2013)183–1863.ResultsanddiscussionThetopmorphologyoftheFLGisshownintheFig.1(a),thefilmwrinklesareseenclearly.TheSEMimagesofasgrownZnONRs,4001Cannealinginvacuumfor30and120minareshowninFig.1(b–d),respectively.VerticallygrownZnONRswithperfecthexagonalstructureareseen.Theaspectratioestimatedfromtheseimagesisabout15–50.TherearebrighterpartsontopofalltheNRsaftertheelectronbeamirradiationintheSEMsystem.Thebrightpartsontopofthe30-minannealedZnONRs(Fig.1c)aremuchlargerthanthatoftheNRsbeforeannealing(Fig.1b).ThismeansthatmoredefectswereintroducedtotheNRsafterannealing[8].However,thebrighterpartsontopofNRsdecreaseafter120-minannealing(Fig.1d),comparedwiththatof30minannealing.C-atomsofgraphenedirectlybindtotheZn-atomsofZnO(fromthefirstprinciplescalculation)[9].LowlatticemismatchandthesamehexagonalstructureintheZnO–grapheneinterface,ensurethatZnONRscanbeeasilyverticallygrownongraphenefilms.ToinvestigatetheeffectofannealingZnONRsontheirstructureandcontactresistance,CLandI–Vmeasurementsofthenanorodsannealedfordifferenttimeat4001Cwerecarriedout.Fig.2(a)showsCLspectraofZnONRsinthe350–700n