(掃描隧道顯微鏡)

ScanningTunnelingMicroscope

(掃描隧道顯微鏡）宋世嬌0410370

HistoryIn1924,DeBroglepredictedthatallmicroparticleshadthewave-particleduality.Andin1927,DavissonandGermerconfirmedit.

Animportantresultofthewave-particledualityisthetunnelingeffectwhichScanningTunnelingMicroscope(STM)basedon.In1982,G.binningandH.RohrerinventedSTM,TheyhadthehonortoreceivetheNobelprize.DoyouwanttoknowwhatisthetunnelingeffectandhowSTMworks?Nowlet'sgo!TheTunnelingEffect

Anexample:

one-dimensionalpotentialbarrierLikethechart(1)showschart(1)TheTunnelingEffectIftheenergyoftheparticleEislowerthanAccordingtoclassicalmechanicsviewpoint,theparticlescannotenterthepotentialbarrier,butallrecoil.But,Wewillhavethecompletelydifferentconclusioninthequantummechanics.(1)TheTunnelingEffectfromtheone-dimensionalstationaryschrodingerequation:(2)Solvingtheequationfromthreeregion.Wecanfindthepenetrationprobabilityoftheparticle:(3)andisthethicknessofthepotentialbarrierTheTunnelingEffectThusitcanbeseenthat:PotentialbarrierthicknessDbigger,thepenetrationprobabilityoftheparticleissmaller.AndtheenergyEbigger,thenthepenetrationprobabilityoftheparticleisbigger,thetwoallpresentexponentialrelation.SothechangeofDorEwillverytremendousaffectsthepenetrationprobabilityoftheparticleP.Wecallthiskindofphenomenon,thatwhentheparticleenergyislowerthanthebarrierheight,itcanstillpenetratesthebarrier,forTunnelingEffect.operatingprincipleofSTM

Asaresultofelectronictunnelingeffect,theelectronsinmetalwillnotcompletelystayunderthesuperficialboundary,that’stosay,theelectronicdensitysuddenlydoesnotreducetozerosuddenlyinthesuperficialboundary,buthastheexponentialdecrease

outsidethesurface,Theattenuationlengthisabout1nm.It’sthemeasurementthattheelectronescapethesurfacepotentialbarrier.ThebasicprincipleofSTMisexactlyTheTunnelingEffect.Weusethedetectingprobe’sneedle-tip,whosediameterhastheatomicscale,andthesurfaceofthesampletobethetwoelectrodes.Speakingoftheelectron,Iftwometalsapproachtoeachotherverynear,theirelectroniccloudswillpossiblyoverlap.Ifweaddsasmallvoltage()betweenthetwometals,thenwecanobservetheelectriccurrent(wecallittunnelingcurrent)betweenthem.

ThebasicprincipleofSTMisexactlyTheTunnelingEffect.Weusethedetectingprobe’sneedle-tip,whosediameterhastheatomicscale,andthesurfaceofthesampletobethetwoelectrodes.Speakingoftheelectron,Thegapbetweentheneedle-tipandthesampleisequaltoapotentialbarrier,Likethechart(1)shows:operatingprincipleofSTMchart(1)Fromtheequation(3),wecanknow:Theelectronicpenetrationprobabilityhastheexponentialrelationwiththepotentialbarrierwidth.Whentheneedle-tipandthesampleextremelyapproach(smallerthan1nm),thepotentialbarrierbecomesverythinandtheelectroniccloudswilloverlap.Iftheelectronichasenoughenergy,itwillprobablepenetratethepotentialbarrier.Addasmallvoltageandtheelectroniccantransfersfromthesampletotheneedle-tip,orfromtheneedle-tiptothesamplebythetunnelingeffect.Andthenthereisthetunnelingelectriccurrent.Thetunnelingelectricisindirectproportiontothevoltage.operatingprincipleofSTM(4)isthevoltage,isthedistancebetweentheneedle-tiptothesample.istheaverageheightofthepotentialbarrier.operatingprincipleofSTMFromtheequation(4),wecansee:when=constant,andhasthenegativeexponentialrelation.Soifchanges0.1nm,thetunnelingelectriccurrentwillchanges7.4times,aboutoneorderofmagnitude.Andsowhentheneedle-tipscanssurfthesurfaceofthesample,thecharacterwhichissmallasatomicscalewillappearsasthechangeofthetunnelingelectriccurrent.STMworkswiththecurrent’ssensibilitytothedistance.Itcandifferentiatethediscreteatom.STM’sbasicstructureSTMisgenerallyconstitutedbyscanningsystem,electronicsystemandcomputersystem.Likechart(2).Scanningsystemistheexecutionpart.Weusethecomputertocontrolthesystemandtocomputethedatawhichisobservedfromthescanningsystem.ThenwecanknowtheInformationofthesample’ssurface.Chart(2)ThemodelsoftheSTM’ssignalmeasurement

First:TheconstantcurrentmodelWhentheneedle-tipscanningalongthesamplesurfaceonthexydirection,asaresultofthefluctuationofthesurface,thetunnelelectriccurrentsizewillchange.Theelectriccurrentsizecomparewiththepre-placedvalue,itsdifferencevaluefeedsbacktotheverticalZdirectioncontrolsystemthroughfeedbackloops.throughtheZdirectionpiezoelectricceramic’sexpansionandcontraction,wecanchangesthedistancebetweentheneedle-tipandthesample,thusmakethemagnitudeofcurrentandthepre-placedvaluemaintenanceisconstant,thenwecanobtaintheappearancepictureofthesurface,thiskindofsurveypatterniscalledtheconstantcurrentmodel.Thismodelissuitableforthesurfacethatisrelativelyrough,thescanningscopethatisrelativelybig.ItisonekindofworkingpatternwhichtheSTMcommonlyuses(chart3).Thedeficiencyisthattheparticleofthesample’ssurfacebetweencertaintrenchescannotaccuratelysurvey,andwhenthesamplesurfacehastheflaw,theneedle-tipisveryeasydamaged.ThemodelsoftheSTM’ssignalmeasurement

chart(3)Second:Theconstantheightmodel

Whentheknownsamplesurfaceisextremelysmoothorthesurveyscopeisverysmall,weusuallyusethemodelofconstantheighttoscanthesurface,That’stosay,whenSTMdotheimagescanning,itmakesurethatthedistancebetweentheneedle-tipandthesamplewillnotchange.Thenwecanobtainthesamplesurfaceappearancethroughthesignalofthetunnelelectriccurrent’schange.Whichisneededtopointoutisthattheabovesuperficialappearancehasalsocontainedthestructureinformationofthesurfaceelectronics.ThemodelsoftheSTM’ssignalmeasurement

ThemodelsoftheSTM’ssignalmeasurement

chart(4)Themodelofconstantheightcanonlybeusedinthesamplewhosesurface’sfluctuationisnotbig.Itsmeritisthatithasafastscanningvelocity,thusitcanreducethenoiseandtheinfluenceoftheheatdriftingtothesignal.Theconstantheightmodelislikethechart(4)ThemodelsoftheconstantcurrentandtheconstantheightaretheSTM’sregularworkpatterns,theneedle-tipdonotcontactwiththesamplesurface,thereistheairpotentialbarrierbetweenthem,thetunnelelectriccurrentsubmitsexponentialdecayalongwiththepotentialbarrierwidth.ThemodelsoftheSTM’ssignalmeasurement

Third:ThemodelofscanningtunnelingspectraIfweaddthetripleshapevoltagebetweentheneedle-tipandthesurface,thenthetunnelelectriccurrentcanprovidealotofinformation,whenneedle-tipscanningonthesurface,itformsthescanningtunnelspectrum,itishelpfultoanalysisthenatureofthesurface.Intheresearchoftheorgan