材料表征教学资料spm.ppt

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1、1ScanningProbeMicroscopenScanningTunnelingMicroscopenAtomicForceMicroscope2ScanningTunnelingMicroscopen nBasicconceptsn nTunnelingcurrentn nLocalbarrierheightn nMorephysicsn nApplicationsIron atoms adsorbed on a copper(111)surface forming a“quantumcorral”inaverylowtemperature(4K).3BasicConceptsn nTh

2、eThebasicbasicprincipleprincipleofofscanningscanningtunnelingtunnelingmicroscopymicroscopy(STM)(STM)isis basedbased onon thethe tunnelingtunneling currentcurrent betweenbetween aametallicmetallictip,tip,whichwhichisissharpenedsharpenedtotoaasinglesingleatomatompointpointandismadeofaconductingmateria

3、l.andismadeofaconductingmaterial.n nAAsmallsmallbiasbiasvoltagevoltage(mV(mVtoto33V)V)isisappliedappliedbetweenbetweenanan atomicallyatomically sharpsharp tiptip andand thethe sample.sample.IfIf thethedistancedistancebetweenbetweenthethetiptipandandthethesamplesampleisislargelargenonocurrentcurrentf

4、lows.flows.However,However,whenwhenthethetiptipisisbroughtbroughtveryverycloseclose(10(10)withoutwithoutphysicalphysicalcontact,contact,aacurrentcurrent(pA(pAtotonA)nA)flowsflowsacrossacrossthethegapgapbetweenbetweenthethetiptipandandthethesample.sample.4BasicConceptsn nSuchSuchcurrentcurrentisiscal

5、ledcalledtunnelingtunnelingcurrentcurrentwhichwhichisisthetheresultresultofofthetheoverlappingoverlappingwavefunctionswavefunctionsbetweenbetweenthethetiptip atomatom andand surfacesurface atom,atom,electronselectrons cancan tunneltunnelacrossacross thethe vacuumvacuum barrierbarrier separatingsepar

6、ating thethe tiptip andandsampleinthepresenceofsmallbiasvoltage.sampleinthepresenceofsmallbiasvoltage.n nTheThe magnitudemagnitude ofof tunnelingtunneling currentcurrent isis extremelyextremelysensitivesensitive toto thethe gapgap distancedistance betweenbetween thethe tiptip andandsample,sample,the

7、the locallocal densitydensity ofof electronicelectronic statesstates ofof thethesamplesample andand thethe locallocal barrierbarrier height.height.TheThe densitydensity ofofelectronicelectronic statesstates isis thetheamountamountofofelectronselectronsexistexist atatspecificspecificenergy.energy.AsA

8、swewemeasuremeasurethethecurrentcurrentwithwiththethetiptipmovingmovingacrossacrossthethesurface,surface,atomicatomicinformationinformationofofthesurfacecanbemappedout.thesurfacecanbemappedout.5HowSTMworks?n nAlthoughAlthoughthetheSTMSTMitselfitselfdoesdoesnotnotneedneedvacuumvacuumtotooperateoperat

9、e(it(it worksworks inin airair asas wellwell asasunderunderliquids),liquids),ultrahighultrahighvacuumvacuum isis requiredrequired toto avoidavoidcontaminationcontaminationofofthethesamplessamplesfromfromthethesurroundingsurroundingmedium.medium.6TunnelingCurrentn nToTo understandunderstand thethe th

10、eorytheory ofof howhow STMSTM works,works,itit isisvitalvital toto knowknow whatwhat isis tunnelingtunneling current,current,andand howhow ititrelatestoalltheexperimentalobservations.relatestoalltheexperimentalobservations.n nTunnelingTunneling currentcurrent isis originatedoriginated fromfrom theth

11、e wavelikewavelikepropertiesproperties ofof particlesparticles(electrons,(electrons,inin thethe case)case)ininquantumquantum mechanics.mechanics.WhenWhen anan electronelectron isis incidentincidentuponupon aa vacuumvacuum barrierbarrier withwith potentialpotential energyenergy largerlargerthanthanth

12、ethekinetickineticenergyenergyofofthetheelectron,electron,therethereisisstillstillaanon-zeronon-zero probabilityprobability thatthat itit maymay traversetraverse thetheforbiddenforbidden regionregion andand reappearreappear onon thethe otherother sideside ofofthebarrier.thebarrier.7TunnelingCurrentn

13、 nItIt isis shownshown byby thethe leakleak outout electronelectron wavewavefunctioninthepicturebelow.functioninthepicturebelow.8TunnelingCurrentn nIfIf twotwo conductorsconductors areare soso closeclose thatthat theirtheir leakleak outoutelectronelectron wavewave functionsfunctions overlap.overlap.

14、TheThe electronelectron wavewavefunctionsfunctions atat thethe FeimiFeimi levellevel havehave aa characteristiccharacteristicexponentialinversedecaylengthKgivenbyexponentialinversedecaylengthKgivenby mm isis thethe massmass ofof electron,electron,isis thethe locallocal tunnelingtunnelingbarrierbarri

15、erheightheightororthetheaverageaverageworkworkfunctionfunctionofofthethetiptipandsample.andsample.9SimpleCalculationsE2E1x0 x0 x0 x0 x0.whenx0.n nWhenWhen x0 x0 isis thethe vacuumvacuum andand x0 xkk-1-1)wavewavefunctionfunctionareareexpandedexpandedinsimilarforminsimilarform wherewhere T T isis the

16、the probeprobe volumevolume andand RR isis thethe radiusradius ofofcurvatureofthetip.curvatureofthetip.n nTheresultingmatrixelementsTheresultingmatrixelementswhererwherer0 0iscenterofthecurvatureofthetip.iscenterofthecurvatureofthetip.19MorePhysicsn nTherefore,thetunnelingcurrentisTherefore,thetunne

17、lingcurrentiswhereDwhereDT T(E(EF F)isthetipdensityofstatesperunitvolume.)isthetipdensityofstatesperunitvolume.n nDivideDivide thethe currentcurrent byby biasbias voltage,voltage,conductanceconductance isisobtainedasobtainedas20TunnelingCurrentn nAtlowvoltageandtemperatureAtlowvoltageandtemperature

18、dd isis thethe distancedistance betweenbetween tiptip andand sample.sample.IfIf thethedistancedistance increasedincreased byby 11 Angstrom,Angstrom,thethe currentcurrent flowflowdecreaseddecreased byby anan orderorder ofof magnitude,magnitude,soso thethesensitivitysensitivitytotoverticalverticaldist

19、ancedistanceisisterriblyterriblyhigh.high.AsAsthethetiptip scansscans acrossacross thethe surface,surface,itit givesgives atomicatomicresolutionimage.resolutionimage.21Remember!n nSTMSTM doesdoes NOTNOT probeprobe thethe nuclearnuclear positionposition directly,directly,butbut ratherrather itit isis

20、 aa probeprobe ofof thethe electronelectron density.density.SoSoSTMSTMimagesimagesdodonotnotalwaysalwaysshowshowthethepositionpositionofofthetheatoms,atoms,andandititdependsdependsononthethenaturenatureofofthethesurfacesurfaceandthemagnitudeandsignofthetunnelingcurrent.andthemagnitudeandsignofthetun

21、nelingcurrent.22LocalBarrierHeightn nItItisisobviousobviousthatthatthethecurrentcurrentisisexponentiallyexponentiallydependsdependsonon bothboth gapgap distancedistance andand thethe locallocal barrierbarrier height,height,changechangeofofcurrentcurrentmightmightbebedueduetotocorrugationcorrugationo

22、fofthethesurfacesurfaceorortotothethelocallylocallyvaryingvaryinglocallocalbarrierbarrierheight.height.Thetwoeffectscanbeseparatedbytherelationship.Thetwoeffectscanbeseparatedbytherelationship.23LocalBarrierHeightn nTheThe tiptip isis vibratingvibrating verticallyvertically andand thethe currentcurr

23、ent isismeasured,measured,inintheory,theory,thethelocallocalbarrierbarrierheightheightcancanbebecalculated.calculated.Again,Again,ififthethetiptiprastersrastersthethesurface,surface,mapmapoflocalbarrierheightcanbeobtained.oflocalbarrierheightcanbeobtained.n nHowever,However,thethe locallocal barrier

24、barrier imageimage alsoalso containcontaintopographictopographicfeatures,features,somesomequestionsquestionsrelatedrelatedtotothethelocallocal barrierbarrier havehave soso farfar remainedremained unexplained.unexplained.So,So,extraextra carecare needsneeds toto bebe takentaken inin performingperform

25、ing suchsuchexperiments.experiments.24LocalDensityofStates(LDOS)n nDensityDensity ofof StatesStates(DOS)(DOS)representsrepresents thethe amountamount ofofelectronselectrons existexist atat specificspecific valuesvalues ofof energy.energy.TheThetunnelingtunnelingconductanceconductance (or(orI/VI/V),)

26、,isisproportionalproportionaltototheLDOS.theLDOS.wherewhere (r(r0 0,E)E)isis thethe locallocal densitydensity ofof statesstates ofof thethesample.sample.25LocalDensityofStates(LDOS)n nKeepingKeeping thethe gapgap distancedistance constant,constant,measuremeasure thethecurrentcurrent changechange wit

27、hwith respectrespect toto thethe biasbias voltagevoltage cancanprobetheLDOSofthesample.probetheLDOSofthesample.n nMoreover,Moreover,changingchanging thethe polaritypolarity ofof biasbias voltagevoltage cancangetlocaloccupiedandunoccupiedstates.getlocaloccupiedandunoccupiedstates.26LocalDensityofStat

28、es(LDOS)n nWhenWhen thethe tiptip isis negativelynegatively biased,biased,electronselectrons tunneltunnelfromfromthetheoccupiedoccupiedstatesstatesofofthethetiptiptotothetheunoccupiedunoccupiedstatesofthesample.statesofthesample.27LocalDensityofStates(LDOS)n nIfIf thethe tiptip isis positivelypositi

29、vely biased,biased,electronselectrons tunneltunnel fromfromthethe occupiedoccupied statesstates ofof samplesample toto thethe unoccupiedunoccupiedstatesofthetip.statesofthetip.28Anexamplen nHereHerearearethethespectraspectraforforSi(111)-7Si(111)-7xx77surface.surface.TheThebottombottomspectrumspectr

30、um isis thethe areaarea averagedaveraged tunnelingtunneling conductanceconductancemeasuredmeasured byby STM,STM,andand thethe toptop spectrumspectrum isis thethe surfacesurfacestatesstates spectrumspectrum measuresmeasures photoemissionphotoemission andand inverseinversephotoemission.photoemission.b

31、othspectrashowsimilarfeatures.bothspectrashowsimilarfeatures.29STMApplicationExampleIn nSinceSincethethebondingbondinggeometrygeometryofofatomsatomsatatthethesurfacesurfacediffersdiffers fromfrom thatthat inin thethe bulk,bulk,onon somesome surfacessurfaces thetheatomsatoms rearrangerearrange andand

32、 formform aa structurestructure energeticallyenergeticallymoremore favorablefavorable thanthan thethe truncatedtruncated bulk.bulk.InIn surfacesurfacephysics,physics,suchsuchaarearrangementrearrangementisiscalledcalledreconstructionreconstruction.surfacereconstructions:therearrangementofsurfaceatoms

33、30STMApplicationExampleIn nAmongAmongthethemetals,metals,thethe5d-elements5d-elementsiridium,iridium,platinumplatinumandandgoldgoldshowshowaastrongstrongtendencytendencytowardstowardsreconstruction,reconstruction,i.e.,i.e.,mostmost oror allall ofof theirtheir low-indexlow-indexfacesfacesarearerecons

34、tructed.reconstructed.TheThesurfacessurfacesofofthethe3d3dandand4d4d elementselements remainremain unreconstructed.unreconstructed.SoSo wewe cancangraduallygraduallyincreaseincreasethethetendencytendencytowardstowardsreconstructionreconstructionbybyalloyingalloyingthethe3d-element3d-elementNiNiandan

35、dthethe5d-element5d-element PtPt andand looklook whatwhat thethe surfacesurface lookslooks likelikewithincreasingPtconcentration.withincreasingPtconcentration.31STMApplicationExampleIn nWhereasWhereas thethe(100)-oriented(100)-oriented surfacesurface ofof NiNi isis aa simplesimple squaresquarelattic

36、elatticeofofatoms,atoms,Pt(100)Pt(100)reconstructs,reconstructs,havinghavingoneoneclose-packedclose-packed(pseudohexagonal,(pseudohexagonal,hex)hex)layerlayer ofof atomsatoms onon toptop ofof thethe squaresquarelatticelatticeofofthetheunreconstructedunreconstructedsecondsecondlayer.layer.AndAndthist

37、hisisiswhatwhatthethe(100)(100)surfacesurface ofof aa PtNiPtNi alloyalloy lookslooks likelike atat approx.approx.68%68%PtPtconcentrationinthefirstmonolayer:concentrationinthefirstmonolayer:32STMApplicationExampleIn nWhatWhat youyou seesee inin mostmost ofof thethe imageimage areare singlesingle brig

38、htbrightrowsrowsofofatomsatomsshiftedshiftedbybyhalfhalfananinteratomicinteratomicdistancedistanceininthethedirectiondirectionofofthetherowsrows(red(redarrow).arrow).YouYouwillwillalsoalsonoticenoticethatthat thesethese atomsatoms havehave aa hexagonalhexagonal environmentenvironment(6(6nearestneare

39、st neighbors)neighbors)inin thethe firstfirst monolayer,monolayer,whichwhich isisalreadyalready thethe samesame locallocal structurestructure asas inin thethe hexhexreconstructionreconstructionofofpurepurePt.Pt.SinceSincethetheshiftedshiftedrowrowatomsatomsdodonotnotsitsitininthethehollow hollow sit

40、essites betweenbetweenfourfouratomsatomsofofthethesecondsecond layer,layer,butbut ratherrather inin bridge bridge sitessites betweenbetween twotwosecond-layersecond-layeratoms,atoms,theytheyarearehigherhigherandandthusthusbrighterbrighterininthetheSTMSTMimage.image.TheTheblackblackpatchespatchesinin

41、thetheimageimagearearedueduetoanimpurity.toanimpurity.33STMApplicationExampleIIn nInInthetheupperupperrightrightcornercorneryouyoucancanalsoalsoseeseetwotwostripesstripesofofthethehexhexreconstruction.reconstruction.There,There,thetheclose-packedclose-packedsurfacesurfacelayerlayerhashasaa higherhig

42、her atomatom densitydensity thanthan thethe squaresquare latticelattice below,below,i.e.,i.e.,88surfacesurfaceatomsatomsperper77substratesubstrateatoms.atoms.SomeSomesurfacesurfaceatomsatomssitsitonontoptopofofsubstratesubstrateatoms,atoms,somesomeininbridgebridgesitessitesandandsomesomeininthetheth

43、ethehollowhollowsitessitesbetweenbetweenfourfouratomsatomsofofthethesecondsecondlayer,layer,andthuslower.andthuslower.n nYouYou cancan seesee thesethese heightheightdifferencesdifferencesasasdifferentdifferentbrightnessbrightnessofofthetheatomsatomsinintheSTMimage.theSTMimage.34STMApplicationExample

44、In nWithWith higherhigher platinumplatinum concentrationconcentration thethe wholewhole surfacesurfacelayerbecomespseudohexagonal:layerbecomespseudohexagonal:35STMApplicationExampleIn nWhereasWhereas thethe aboveabove imageimage showsshows onlyonly thethe geometricalgeometricalstructure,structure,we

45、we cancan alsoalso obtainobtain anan imageimage showingshowing aasuperpositionofgeometryandchemicalcontrast.superpositionofgeometryandchemicalcontrast.n nInIn thisthis imageimage thethe NiNiatomsatomsappearappearasasbrightbrightblobs,blobs,whereaswhereasPtPtatomsatomsininthethesamesamerowrow(same(sa

46、megeometricgeometricheight)aredarker.height)aredarker.36STMApplicationExampleIn nItItisisobviousobviousthatthatmoremoreNiNiatomsatoms(bright(brightblobs)blobs)arearefoundfoundininthethe higherhigher positions,positions,especiallyespecially inin thethe sitessites onon toptop ofofsecond-layeratoms,whe

47、rethecoordinationislowest.second-layeratoms,wherethecoordinationislowest.n nPlatinumPlatinumpreferspreferssitessiteswithwith higherhigher coordination,coordination,asas therethere areare onlyonly aa fewfewbrightbright NiNi atomsatoms inin thethelowerrows.lowerrows.37STMApplicationExampleIn nAnAn imp

48、ortantimportant pointpoint isis thethe driving driving forceforce ofof thethereconstruction,reconstruction,i.e.,i.e.,thethequestionquestion why why does does Pt Pt reconstruct?reconstruct?.n nAA detaileddetailed analysisanalysis ofof thethe reconstructedreconstructed alloyalloy surfacessurfacesshows

49、shows thatthat thethe reconstructionreconstruction cannotcannot bebe duedue toto tensiletensilestressstressininthethesurface,surface,whichwhichwaswasoftenoftenbelievedbelievedtotobebethethedrivingdriving forceforce ofof thethe hexhex reconstruction.reconstruction.SinceSince thethe hexhexreconstructi

50、onreconstructionreducesreducesthethenumbernumberofofsurfacesurfaceatomsatomswithwiththethelowestlowestcoordinationcoordinationnumber,number,ititratherratherseemsseemsplausibleplausiblethatthatthethetendencytendencyofofPtPttotoavoidavoidlow-coordinatedlow-coordinatedsites,sites,asasseenintheimageabov