Nanogap Electrodes
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Nanogap Electrodes
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REVIEWNanogapElectrodesByTaoLi,WenpingHu,*andDaobenZhu*
switches[6,7]andtransistors,[8–10]havebeen
accordinglydesignedandreported.AlltheNanogapelectrodes(namely,apairofelectrodeswithananometergap)areaboveaspectsrendermoleculesasidealfundamentalbuildingblocksforthefabricationofnanometer-sizeddevicescandidatesforthenextgenerationofandcircuits.Theyarealsoimportanttoolsfortheexaminationofmaterialelectronics.propertiesatthenanometerscale,evenatthemolecularscale.Inthisreview,Despitealltheseadvantages,therethetechniquesforthefabricationofnanogapelectrodes,thepreparationofremainmanychallengesbeforemolecular
computerscometoreality.Themainassembleddevicesbasedonthenanogapelectrodes,andthepotentialobstacleliesinhowtointegratenanometer-applicationofthesenanodevicesforanalysisofmaterialpropertiesaresizedmoleculestomacroscopicelectronicintroduced.Thehistory,theresearchstatus,andtheprospectsofnanogapcircuits.Therearemainlytwoapproacheselectrodesarealsodiscussed.forwiringmoleculesbetweenelectrodes.
Onemethodistomaketop-contactjunc-
tions,whichincludesscanningprobe1.Introductionmicroscopy(scanningtunnelingmicroscopy(STM)andcon-
ductingatomicforcemicroscopy(AFM)),[11–22]crosswire
Motivatedbytherapiddevelopmentofsemiconductortechnol-junctions,[23–25]mercurydropelectrodes[26,27]andthermallyogies,andguidedbythefamousMoore’sLaw,[1]thepaceatwhichdepositedmetal?lms.[6]Alldevicesmanufacturedbythiskindofthenumberoftransistorspersquareinchonintegratedcircuitmethodcanbecategorizedas‘prototypedevices’,whichareveryboardswoulddoubleabouteveryoneandahalfyearshasbeenusefulforfundamentalinvestigationsandhavealreadyprovidedkeptformorethan40years.Nowadays,thefeaturesizeofthemanyimportantresults.[4–7,11–27]However,thesedevicesarefartransistorshasbeenreducedtobelow50nmandthescalinghasfrompracticalapplications,aswecannotimagineananometerledtoanunprecedentedlevelofintegrationwithapromisinglydevicecarryingahugescanningprobemicroscopy(SPM)systemhigherperformanceandlowerpowerconsumption.However,orothersystems.Theotherwayutilizesnanogapelectrodes[28–32]thislevelofscalingisgraduallyapproachingtheminiaturizationtoformmetal/molecule/http://wendang.chazidian.comparedwiththelimitsofsilicontechnology,whichisforeseentoface‘prototypedevices’,devicesbasedonnanogapelectrodeswithfundamentallimitationswithinadecade.conductivemetalnanowirecircuitsandfunctionalmoleculesAsinglemolecule,evenanatom,representstheultimatelimitinsertedinthedesiredpositionhavethepotentialabilitytorealize[2]thatwecanhandlesofar.Asearlyas1974,AviramandRatnersuperintegratedcircuits,soaremorelikelyusedinpracticalhadpredictedthatindividualmoleculessomedaywouldbeusedapplications.Moreover,becausethenanogapelectrodesareascircuitelementsindevicesandthedreamofutilizingfabricatedbeforethemolecularcomponentsaresubsequentlymoleculesasfunctionalunitsinelectroniccircuitshasmotivatedinserted,thejunctioncanbecharacterizedwithandwithoutresearchersforyears.Ifthiscomestrue,itwouldbepossibletomoleculesinplace,whichfacilitatesthedistinctionoftheintegratetransistorsinthebillions.Themolecular-baseddevicesintrinsicmoleculeproperties.Furthermore,asmostnanogap[3]possessuniqueadvantagesforelectronicapplications,suchaselectrodespresentaplanarcon?guration,itwouldbeeasierforlowercost,lowerpowerdissipation,higheref?ciency,abilityofhigherdensityintegrationandtotaketheunderlyingsubstrateasself-assemblyandrecognition,distinctopticalandelectronicagatecontacttotunetheelectricalpropertiesofthemolecularproperties,andsynthetictailoringabilitybyelaboratechoiceofcomponents.Becauseofthesuperiorcharacteristicsandgeometryandcomposition.Avarietyofspeci?celectronicpromisingfuture,nanogapelectrodeshaveattractedworldwide[4,5]functionsperformedbysinglemolecules,includingrecti?ers,attentionfornearly20years.Manystudieshavebeenreported
内容需要下载文档才能查看 内容需要下载文档才能查看andsigni?cantprogresshasbeenmadefromfabricating
techniquestodeviceapplications.Inthisreview,wediscuss[*]T.Li,Prof.W.Hu,Prof.D.Zhuthemanufacturemethodsofnanogapelectrodes,theintegrationBeijingNationalLaboratoryforMolecularSciences
KeyLaboratoryofOrganicSolidsofmolecularcomponentsfornanodevices,andthepotentialInstituteofChemistryapplicationsofnanogapelectrodesformaterialanalysis.ChineseAcademyofSciences
Beijing100190(PRChina)
E-mail:huwp@http://wendang.chazidian.com;zhudb@http://wendang.chazidian.com
T.Li
GraduateSchoolofChineseAcademyofSciences
Beijing100039(PRChina)2.MethodsofFabricatingNanogapElectrodesTypicaldimensionsoftargetmoleculesarewellbelow5nm,so
fabricatingelectrodeswithseparationsthataresuitableforDOI:10.1002/adma.200900864
286ß2010WILEY-VCHVerlagGmbH&Co.KGaA,WeinheimAdv.Mater.2010,22,286–300
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speci?cmoleculesisaverychallengingmissionbecauseitgoesbeyondthecapabilityoftraditionalmicrofabricationtechnologies.Also,ifthegapistoosmall,itwouldbeverylikelytopositthemoleculeinatenseanddistortedstate,resultinginunexpectedperformances.So,theprecisecontrolofthespacingmakesitREVIEW
内容需要下载文档才能查看TaoLigrewupinShandongprovince,China.HereceivedhisB.Sc.degree(2005)intheSchoolofMaterialsandEngi-moredif?culttopreparesuchkindsofdevices.DespitetheneeringfromBeiHangUniver-obstacles,severaleffectiveandcreativemethodsoffabricatingsity.HethenjoinedtheInstitutenanogapelectrodeswithcontrolledspacinghavebeenreportedinofChemistry,ChineseAcademythelastfewyears,includingmechanicalbreakjunctions,[33]ofSciencesasaPh.D.candi-electron-beamlithography,[34]electrochemicalplating,[35]electro-date.Hisresearchfocusesonmigration,[36]focusedionbeamlithography,[32]shadowmaskelectrochemistryandmolecularevaporation,[9]scanningprobeandatomicforcemicroscopyelectronics.
lithography,[37]on-wirelithography,[38]molecularrulers[39]etc.Alltheabovemethodshaveprovidedpromisingresultsandhavetheirowncharacteristics.Differentmethodsarefrequentlycombinedtoobtainadesiredcon?guration.Forexample,WenpingHuisaProfessorofelectron-beamlithographyisoftenusedto?rstlyrealizenanogaptheInstituteofChemistry,electrodeswithspaceat10–20nm,andthenothertechniques,ChineseAcademyofSciences.suchasshadowmaskevaporation,areimplementedtofurtherHereceivedhisPh.D.fromthenarrowthegapwidthtoa1!5nmscale.Institutein1999.HethenjoinedOsakaUniversityasaresearchfellowofJapanSociety2.1.MechanicalControllableBreakJunctions
forthePromotionofSciencesandStuttgartUniversityasanAmechanicalcontrollablebreak(MCB)junctionwas?rstAlexandervonHumboldt.InintroducedbyMorelandandhisco-workersfromtheUS2003heworkedforNipponNationalBureauofStandardstoformanelectrontunnelingTelephoneandTelegraph,and
内容需要下载文档才能查看junction.[40]ThismethodwasthenadoptedcreativelybyReedthen
内容需要下载文档才能查看returnedto
内容需要下载文档才能查看the
内容需要下载文档才能查看institute.Hisresearchfocusesonetal.fromYaleUniversitytofabricatenanogapelectrodes,whichmolecularelectronicsandhehasmorethan120refereedyieldedelectrodeswithaseparationofseveralnanometers.[33,41]Apublications.
schematicdrawingofthesamplemountingoftheMCBtechniqueispresentedinFigure1.Anotchedmetallicwireis
DaobenZhuisaprofessoroftheInstituteofChemistry,ChineseAcademyofSciences.Hewasselectedasan
academicianoftheChineseAcademyofSciencesin1997.Hegraduatedin1968fromtheEastChinaUniversityofScienceandTechnology.AsavisitingscientistheperformedresearchwithProf.HeinzStaabintheMax–PlanckInstituteforMedi-calResearchinHeidelbergduring1977–1979and1985–1986.Heservedasvice-director(1988–1992)anddirector(1992–2000)oftheInstituteofChemistry,andasvice-presidentofNationalNaturalScienceFoundationofChina(2000–2008).He
内容需要下载文档才能查看isthevice-presidentoftheChinese
ChemicalSociety.Hisresearchinterestsincludemolecularmaterialsanddevices.
Figure1.Thesamplemountinginathreepointbendingcon?guration.gluedtoanelasticsubstrate,whichservesasthebendingbeam.Thebendingbeamconsistsof?exiblephosphorusbronzecoveredwithaninsulatinglayerofcapton.ThejunctionisformedbybreakingtheelectrodeThesubstrateisbentbypushingitscenterwithadrivingrodandmaterial.Thisisachievedbybendingthebeam.Theelongationofconsequentlythenotchedwireisfractured,afterwhichantheungluedsection,u,isconcentratedonthenotchandwillresultinaadjustabletunnelinggapcanbeestablished.Thesurfacescanbefractureofthematerial.Avoltageonthepiezoelementisusedfor?nebroughttogetheragain,andthedistancecanbecontrolledbyaadjustmentofthecouplingbetweenthetwoelectrodes.Reproducedwithpiezoelectricelement.Thebreakingprocessismostlyconductedpermissionfrom[41].Copyright1996IOPPublishingLimited.underlowtemperatureandhighvacuumconditions,andthis
Adv.Mater.2010,22,286–300ß2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
287
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guaranteestwoatomicallycleansurfaces.Any
elasticsubstratecanbeusedinprinciple,
whilethatmadeofphosphorbronze[28,42,43]is
oftenchosenbecauseitismore?exible,
compatiblewith?neelectronbeamlithogra-
phy,andallowsamuchwiderrangeofmetals
tobestudied.Aninsulatinglayerisdeposited
onthesubstratebeforethemetallicnanos-
tructureistransferredusingstandardelectron
beamlithographytechniques.Thenotched
sectionisusuallymadewithascalpelby
rollingitoverthe?lament.Othermethods
mayalsobeinvolveddependingoncertain
propertiesofdifferentmetals.Forexample,a
sparkmethodwasusedwhendealingwith
thesemimetalSb.[44]Toensurethatthe
notchedsectionwasunglued,selectiveetching
(e.g.,isotropicreactiveionetching)[28]of
theinsulatinglayerwasoftenperformedto
produceametallicbridgesuspendedbetween
anchoringpads.TheMCBtechniqueisvery
stable(downto0.2pmhÀ1).[43]Moreover,the
contactsizecanbecontinuouslyadjusted
underthecontrolofanultra?nepiezoelectric
componentwithoutpollutingthejunction.In
addition,afterthetargetmoleculeisanchoredFigure2.a)Schematicofthemeasurementprocess.A)Thegoldwireofthebreakjunctionbeforebetweenthegap,themechanicalstressisbreakingandtipformation.B)Afteradditionofbenzene-1,4-dithiol,SAMsformonthegoldwire
surfaces.C)Mechanicalbreakageofthewireinsolutionproducestwoopposinggoldcontactsthatcontrolledaswell.
MCBjunctionsarewidelyusedforsingle-arecoveredbyaSAM.D)Afterthesolventisevaporated,thegoldcontactsareslowlymoved
togetheruntiltheonsetofconductanceisachieved.Steps(C)and(D)(withoutsolution)canbemolecule,evensingle-atomdevicesincludingrepeatednumeroustimestotestforreproducibility.b)Aschematicofabenzene-1,4-dithiolateone-atommetallicpointcontacts[42–50]andSAMbetweenproximalgoldelectrodesformedinaMCB.ThethiolateisnormallyH-terminatedmetal–molecule–metaljunctions.[28,33,41,51–57]afterdeposition;endgroupsdenotedasXcanbeeitherHorAu,withtheAupotentiallyarisingvanRuitenbeeketal.[42–49]workdistinguish-fromapreviouscontact/retractionevent.ThesemoleculesremainnearlyperpendiculartotheAu
ablyinthis?eld,especiallyonmakingatomicsurface,makingothermolecularorientationsunlikely.Reproducedwithpermissionfrom[33].sizecontactsandtunneljunctions.TheCopyright1997Science.
内容需要下载文档才能查看metallicpointcontactsweremadebybringing
twofreshlypreparedelectrodesbackinto
contactunderpreciseadjustmentofthepiezo
element.Asforsingle-moleculedevices,moleculesareoftenenhancedRamanscattering(SERS)methodtocharacterizeself-assembledtointegratebetweenmetalleads.Reedandmoleculesinananogapwithacontinuouslyadjustablegapwidth.[33]TheSERSintensitydependedcriticallyonthegapwidthandtheTouretal.successfullyself-assembledbenzene-1,4-dithiol
incidentlightpolarization,whichindicatedsuccessfuldetectionmoleculesonagoldwirebeforetheMCBprocess.Mechanical
ofthesamplemoleculesinsidethegap.breakageofthewireinsolutionproducedtwoopposinggold
AlthoughtheMCBmethodisveryusefulforfundamentalcontactsthatwereself-assembledmonolayer(SAM)-covered.The
investigation,suchaselectronictransportatamolecularscale,ittipswerethenslowlymovedtogetheruntiltheonsetof
isnotfaciletofabricatehighlyintegratedmoleculardevicesconductancewasachieved,whichindicatedthatasingle-[56]becauseoftheconstraintofthepiezoelectriccomponents,andmoleculejunctionwasmade(Fig.2).Rieletal.investigated
itisnoteasytobuilddeviceswiththreeorevenmoreelectrodes.theconductanceswitchingbehaviorofsinglebipyridyl-dinitro
Italsoappearstobedif?culttocontrollablyfabricaterelativelyoligophenylene-ethynylenedithoil(BPDN-DT)moleculescon-
largegaps.tactedbytwosymmetricleadsusingtheMCBtechnique.The
metal–singlemolecule–metalsystemcanbecontrolledand
reversiblyswitchedbetweentwodistinctstatesevenafter2.2.ElectrochemicalandChemicalDepositionforperformingmorethan500positiveandnegativesweeps
http://wendang.chazidian.comparingwiththeexperimentalresultsfrom
bipyridyloligophenylene-ethynylenedithiol(BP-DT)molecules,
theyconcludedthattheswitchingbehaviorwascausedbytheElectrochemicalandchemicaldepositionmethods,combinednitrogroupsofBPDN-DT,whicheliminatedtheaffectofwithstandardlithographytechniques,provideasimple,accurate,electrodepropertiesormolecule–metalinterfaces.Morerecently,andreproduciblewayforthefabricationofnanogapelectrodes.[57]Tianetal.distinguishablycombinedMCBwithasurface-Theinitialelectrodeswitharelativelylargegaparefabricated
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conventionallithographytechniquesonsub-
strates,e.g.,Si/SiO2.Thegapisthennarrowed
downtoananometerorevenatomicscaleby
depositingspeci?catomsontothelithogra-
phicallyde?nedelectrodes.Also,theprocessREVIEW
内容需要下载文档才能查看canbereversedinthedissolutionmodefor
controlledetchingofatomsfromthejunction
tosolution,thuswideningthegapbacktothe
sub-micrometerscale.
Thismethodrequiresnospecialtechnique
orsystem,[58]andcanconvenientlyprepare
gapsthatrangefromseveralangstromsto
10nm,whichisthebestscalefor?tting
desiredmoleculesornanocrystalsfornano-
devices.Thesimplicityandrobustnessofthe
techniqueprovidesapromisingmethodfor
thefabricationoflarge-scaleandhighly
integratednanodevices.Furthermore,the
gapdimensioncanbesimultaneouslymoni-
toredandpreciselycontrolledbyusingaFigure3.SEMimagesofthesamplespreparedat3kHzwithdifferentDtvalues:a)Dt¼9s,feedbacksystem.Thefeedbacksignal,takingd¼26nm;b)Dt¼25s,d¼16nm;c)Dt¼42s,d¼7nm;d)Dt¼62s,d%1nm.Reproducedanelectroplatingmethodasanexample,iswithpermissionfrom[31].Copyright2005Wiley-VCH.
frequentlythecurrent?owingthroughthegap
electrodes,whichismonitoredduringthe
depositionprocess.[35,59–66]Whentheelectrodesareveryclosebutdeposition,thefabricationprocesswasself-terminated.Anothernotyettouching,themonitorcurrentisextremelysensitivetoadvantageoftheelectrochemicaldepositionmethodistheabilityelectrodedistance,soitiseasytocontroltheseparationonantofabricateasymmetricnanogapelectrodes(e.g.,agoldandaatomicscalebystoppingtheelectrodepositionprocessatplatinum?nger)andnanogapelectrodeswiththreeormoreprede?nedconductancevalues.Unlikeacurrent-feedbackmode?ngers,[61]asdemonstratedbyKashimuraetal.,whichwereboththatusesbothfacingelectrodesasaworkingelectrode(WE),Tiandif?culttoachievewithconventionallithographictechniquesandetal.[58]inventedamethodforthecontrollableelectrochemicalsuperiortotheMCBorSPMtechniquesdiscussedabove.fabricationofelectrodeswithananometer/angstrom-sizedgapWhenperformedwithgold,thedrawbackofelectroplatingliesusingthepotentialdistributionintheelectricdoublelayerasinthedif?cultyofobtainingasimplegoldelectrolyteandspecialfeedback,whereintwofacingelectrodesservedastheworkingcareshouldbetakentoavoidgeneratinghighlytoxichydrocyanicelectrodeandreferenceelectrode(RE),respectively.Thepotentialacidgases.Accordingly,UmenoandHirakawa[69]introducedadifferencebetweentheWEandtheRE(Vgap)wasmonitoredsimpleandcost-effectivewaybyusingan‘iodinetincture’asancontinuouslyduringtheelectrodepositionprocess.Thevalueofelectrolyteforgoldelectroplating,whichwasfreeoftoxicVgapremainedconstantuntilthegapwasnarrowedtobelowacompoundsorstrongacids.Kervennicetal.[59]foundthatcertainvalue,andthenitdecreasedwithtimeand?nallycomparedwithgolddeposition,Ptpromisedahigherreprodu-approachedzerotoindicateaconductingstate.Thisresultcibilityandstabilityoftheelectrodes,andpairsofplatinumindicatesthatVgapcouldalsobeusedasafeedbacksignaltoelectrodeswithaseparationbetween20and3.5nmwerecontrolthegapwidth.Liuetal.[31,67]?nelytunedtheelectrodegapobtainednicely.
widthbyutilizinghigh-frequencyimpedanceinfeedback.TheyComparedwiththeelectroplatingmethod,surface-catalyzedstudiedthealternatingcurrent(ac)voltagefrequencydependencychemicaldepositionrepresentsasimplerwaytoobtainnarrowonthe?nalgapsizeandobtainedgapsof30nmbyusingagapelectrodespre-patternedbyconventionallithography,asithigh-frequencyfeedbacksignalwithhighaccuracyandreprodu-doesnotevenhavetocarryanexternalcircuit.Bylocalizingacibility.Thegapsizeisaffectedbythefrequencyandamplitudeofcatalystontheinitialelectrodes,themetalatomscanbeselectivelytheacsignalanditcanbe?nelytunedtoaround1nmbysimplydepositedontotheelectrodesurfaceassistedbyreductiveagents.controllingthesubsequentdepositiontime(Fig.3).Taoetal.[68]Onejustneedtoimmersethechipsintoastocksolutionachievedniceatom-sizegapsandcontactsbetweenelectrodescontainingmetalionsandamildreducingagent,andthegapfabricatedwithaself-terminatedelectrochemicalmethodbaseddistancedependsonthereactiontimeandreactantconcentration,onabuilt-inself-terminationmechanism.Anexternalresistorwhichendowsthismethodwithcontrollabilityandmass(Rext)wasconnectedtooneoftheelectrodesandthe?nalgapproducibility.Guandcoworkers[70]?rstlymodi?edthegoldwidthcouldbepredeterminedbyelaboratelychoosing1/Rextleadswith2-mercaptoethylamine,theaminogroupsofwhichcomparedwiththeconductancequantum(G0¼2e2/h).ThegapbindPdIIparticlestocatalyzethedepositionofmetalliccopper,resistance(Rgap)decreasedwithtime,andresistancesinserieswhichresultedinanelectrodenanogapof45nm.Yunetal.[71](RextandRgap)sharedthetotalappliedbiasvoltage(V0).FinallyreportedthecreativefabricationofintegratednanogapelectrodeswhenRgapwasmuchsmallerthanRext,Vgapapproached0,whichofafewnanometersinseparationwithayieldover90%(Fig.4).meantalmostnoappliedvoltagewasusedforetchingandTheyproposedthattheAudepositionrateofthegapedge
内容需要下载文档才能查看wasAdv.Mater.2010,22,286–300ß2010WILEY-VCHVerlagGmbH&Co.KGaA,
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REVIE
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Figure5.Obliqueangledepositionwithanelevatedmaskfortheprep-arationoftwometalelectrodesspacedtoafewnanometers.Reproducedwithpermissionfrom[9].Copyright2003NaturePublishing
内容需要下载文档才能查看Group.
Figure4.a)Opticalandb)?eldemissionscanningelectronmicroscopy(FESEM)imagesofanarrayofnanogapswithsub-5nmseparationsaftersurface-catalyzedchemicaldeposition.FESEMimagesoftwo-?ngerednanogapelectrodeswithmagni?cationsofc)Â40000andd)Â320000.Reproducedwithpermissionfrom[71].Copyright2006AmericanInstituteof
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smallerthanthatoftheotheredgesoftheelectrodesbecauseofthemass-transportlimitationinthenarrowgapandconsequentlytheplatingprocessslowsdownastheelectrodesapproachtunnelingdistances.Multi-?ngerednanogapelectrodescanalsobeconvenientlypreparedbyanelectrolessplatingmethod.[72]Ifthefabricationprocessofchemicaldepositioncanbemonitoredinrealtimeandthemaincontrollingfactors,suchasreactiontimeandreactantconcentration,aremoretheoreticalratherthanempirical,amuchbettercontrollabilityofthismethodcanbeexpected.
2.3.ObliqueAngleShadowEvaporationforNanogapElectrodes
TheobliqueangleshadowevaporationmethodwaspioneeredbyDolan[73]in1977.Thistechniquewasthenadoptedandre?nedforthefabricationofnanogapelectrodes.Shadowevaporationisoftencombinedwithopticalandelectron-beamlithographytode?nemetalleads.[9,74–76]Bysuspendingthemaskabovethesubstrateandcontrollingthedirectionofthedepositionangletobeobliqueagainstthesurfacenormal(Fig.5),featuresizesthataresmallerthanthoseofthemaskscanbeobtainedperfectlyasreportedbyBjørnholmandhiscolleagues.[9]Gaplengthsbetweentheelectrodesoflessthan10nmcanbereproduciblyfabricatedinthisway.Thegapsizecanbeadjustedbychangingstepwisethetiltangleuntiladesiredspaceisobtained,[76]andin-situsampleconductancemeasurementscanhelpto?ndtheproperangle,[77]whichendowsthismethodwithhighcontrollability.
Ifthesubstrateisuneven,itcanprovideasimilareffectasanelevatedmaskandcansimplifythefabricationprocess.Accordingly,double-angleevaporation[78,79]ofthinmetallic?lmswasintroduced.Bysimplychangingthe?lmthickness,theseparationoftheelectrodescanbeadjusted.Furthermore,evenwithoutemployingconventionallithographymethods,nanogap
electrodescanalsobeaccuratelyfabricated.Kawaietal.[80]developedamethodbasedonobliqueangleshadowevaporationtoconstructnanogapelectrodeswithoutawetprocesstomeasuretheelectricalcharacteristicsofmoleculesaftertheywerepositionedonthesubstrate(Fig.6).Thegaplengthbetweentheelectrodeswaseasilycontrolledunder100nmandatop-contactcon?gurationpreventedstructuraldeformationofthemolecules.Moreover,thismethodissuitabletoreliablyfabricatenanogaparraysformolecularelectronics.Forexample,Sunetal.[81]achievedthefabricationofgapelectrodearraysassmallas3nminlargequantities,andappliedthemtotheelectricalstudyofnanocrystalssuccessfully.Frequentlyobliqueangleshadowevaporationmethodisconductedatlowtempera-ture[9](e.g.,inliquidhelium),asundersuchconditionsthegrainsizesofthemetaldepositsarerelativelysmall,whichresultsinthegapwidthremainingclosertothedesignedvalue.Inthissense,otherimprovements,suchasusingmetalswithsmallergranules,decreasingthe?lmthickness,andadjustingtheevaporationrate,canbemadetoenableanarrowergap-widthdistribution.[78]
2.4.ElectromigrationandElectricalBreakdownMethodforNanogapElectrodes
2.4.1.ElectromigrationforNanogapElectrodes
Thephenomenonofelectromigrationhasbeenknownformorethan100years.[82]Inanappliedelectric?eld,themomentum
内容需要下载文档才能查看of
Figure6.Procedureoftheangle-controlledshadow-maskingmethod.Thewidthofthemetalmaskshownin(e)isactuallymuchwiderthanthatshownintheillustration.Reproducedwithpermissionfrom[80].Copyright2004IOPPublishingLimited.
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