理学天津大学胶体与表面化学TEMsurfChem.pptx

2024/6/25周二1第四章第四章 透射电镜透射电镜(TEM)Transmission Electron Microscopy刘炳泗DepartmentofChemistryTianjinUniversity2024/6/25周二2一、一、Interactions of Electrons with Matter1.Elastic InteractionsNo energy is transferredfrom the electron to thesample(directbeamorisscattered).TEM,SAED2.Inelastic InteractionsEnergy is transferredfromtheincidentelectronstothesample:(EDXanalysis)2024/6/25周二31.Elastic InteractionsAnelectronpenetratingintotheelectroncloudofanatomisattractedbythepositivepotentialofthenucleus(Coulombicinteraction),anditspathisdeflectedtowardsthecoreasaresult.TheCoulombicforceFisdefinedas:withrbeingthedistancebetweenthechargesQ1andQ2and0thedielectricconstant.Theclosertheelectroncomestothenucleus,i.e.thesmallerr,thelargerisFandconsequentlythe scattering angle.In rare cases,evencomplete backscattering can occur(backscatteredelectronsBSE).2024/6/25周二42.Inelastic InteractionsIonization:The high-energy electrons of the incident beam can transfer a criticalamountofenergytoaninner-shellelectronofanatom,leadingtotheejectionofthiselectron.The ionization energy is provided by the incident electron,reducing itsenergy.Thisleadstoanionizationedgeintheelectronenergylossspectrum(EELS).Subsequently,theholeintheinner-shellisfilledupbyanelectronwithhigherenergyfroman outershell.Thiselectrongivesaway a partofitsenergy,leadingtotheemissionofcharacteristicX-raysorAugerelectrons.Secondaryelectrons:Electronsintheconductionorvalencebanddonotneedmuchenergy(low work function)to be transferred into vacuum.Thus,the energy ofsecondaryelectrons(SE)islow(50eV).TheSEsaremainlyexploitedinSEM.Phonons:Phononsarelatticevibrations,whichareequaltoheatingthespecimen.Thiseffectmayleadtoadamageofthesample.Plasmons:Plasmons are longitudinal oscillations of free electrons,which decay either inphotonsorphonons.Cathodoluminescence:Ifsemiconductorsarehitbyhigh-energyelectrons,electron-holepairscanbeformedbypromotinganvalenceelectronintotheconductionband.Fillingthisholewithanelectronfromtheconductionband(recombination)leadstotheemissionoflightwithafrequencythatcorrespondstothebandgap.2024/6/25周二5BasiccontrastmechanismsinTEMandSTEM.Electrons,which come from thecondensersystemoftheTEM,arescatteredbythesample,situatedinthe object plane of the objectivelens.Electrons scattered in thesamedirectionarefocusedinthebackfocalplane,andasaresult,adiffractionpatternisformedthere.Electrons coming from the samepointoftheobjectarefocusedintheimageplane.IntheTEM,thefirstintermediateimageismagnifiedbyfurtherlenses(projectivesystem).2024/6/25周二6basiccontrastmechanismsinTEMandSTEM.Brightfield(BF)mode:Mass-thicknessanddiffractioncontrast contribute to imageformation:thickareas,areasinwhichheavyatomsareenriched,andcrystallineareasappearwithdarkcontrast.In dark field(DF)images:Since diffracted beams havestrongly interacted with thespecimen,veryusefulinformation is present in DFimages,e.g.,aboutplanardefects,stackingfaultsorparticlesize.2024/6/25周二7BF and DF TEM images of ZrO2Electron diffraction pattern:the spots indicate thepresenceofsinglemicrocrystals.Theapertures(redcircles)are localized around the direct beam forrecordingthebrightfield(BF)imageandaroundafewdiffractedbeamsforthedarkfield(DF)image.Theintensedirectbeamisblockedbyametalrod(black shadow on the left center)to avoidoverexposure.BFDF2024/6/25周二8HRTEM modeIfthepointresolutionofthemicroscopeissufficiently high and asuitable sample orientedalong a zone axis,then(HRTEM)imagesareobtained.Inmanycases,theatomicstructureofaspecimencandirectlybeinvestigatedbyHRTEM.2024/6/25周二9HRTEM modesinglecrystalsof(Ce0.5Zr0.5)O2AgparticlesupportedonZnOReO3 Structure2024/6/25周二10Scanning Electron Microscopy(SEM)compactsamplescanthus be investigatedbySEM.Avaluableinformationaboutmorphology,surfacetopologyandcomposition can beobtained.SEMmicroscopesachievingresolutionsbelow1nmareavailablenow.2024/6/25周二11SEM:Imaging with Back-scattered ElectronsSEMimagesofFeparticlesincarbonrecordedwiththesecondaryelectron(left)andtheback-scattered(right)electrondetector.TheBSEimageshowstheFeparticleswithbrightcontrast.2024/6/25周二12二、Imaging and Diffraction1.objective lens forms adiffractionpatterninthebackfocalplane2.diffraction pattern andimagearesimultaneouslypresent3.real space(image)toreciprocal space(diffractionpattern)is easily achieved bychanging the strength of theintermediatelens.4.Apertureregulation2024/6/25周二13三、SchemeofaCM30TEM1.Electron gun:2.Condenser system3.Objective lens4.Diffraction/inter-mediate lens:5.Projective lenses:6.Image observation:7.Vacuum system:2024/6/25周二14四、四、Bragg Description of DiffractionIf the incident planewave hits the crystalat an arbitrary angle,theinterferenceofthereflected waves canbe either destructiveorconstructive.Destructive interference of reflectedwaves(Max.and Min.of the waveamplitudearesuperimposed).Constructive Interference ofreflectedwaves(maximaaresuperimposed).2024/6/25周二15To obtain constructive interference,the pathdifference between the two incident and thescattered waves,which is 2dsin,has to be amultipleofthewavelength.Forthiscase,theBragg law then gives the relation betweeninterplanardistancedanddiffractionangle:2024/6/25周二16Electron Diffraction(ED)Electron diffraction is a collective elastic scatteringphenomenonwithelectronsbeingscatteringbyatomsinaregulararray(crystal).Theincomingplaneelectronwaveinteractwiththeatoms.Secondarywavesaregeneratedwhichinterferewitheachother.Thisoccurseitherconstructively(reinforcement)ordestructively(extinguishing).As in X-ray diffraction(XRD),the scattering event can be described as areflectionofthebeamsatplanesofatoms(latticeplanes).The Bragg law gives the relation between interplanardistancedanddiffractionangle:2024/6/25周二17Since the wavelength of the electrons isknown,interplanardistancescanbecalculated from ED patterns.Furthermore,information about crystal symmetry can beobtained.Consequently,electron diffractionrepresentsavaluabletoolincrystallography.2024/6/25周二18Estimate of scattering anglesel=0.00197nm(1.97pm)for300kVelectrons.Atypicalvaluefortheinterplanar distance is d=0.2 nm.IfthesevaluesareputintheBragglaw,thenthescatteringangleis:=0.28.Asarule,thescatteringanglesinEDareverysmall:02.2024/6/25周二191.Reflectinglatticeplanesare almost parallel to thedirectbeam.2.Incident electron beam isthezoneaxisofthereflectingsetsoflatticeplanes2024/6/25周二20Comparison of Electron(ED)and XRDBoth,ED and XRD,are caused by positiveinterference of scattered waves,and the samefundamentallaws(e.g.,Bragglaw,extinctionrules)canbeappliedfortheinterpretationoftheresultingdiffractionpatterns.Inbothcases,diffractionpatternsofpowdersandofsinglecrystalsappear.However,EDshowssomeuniquecharacteristics:2024/6/25周二211.The wavelength of electrons(e.g.,1.97 pm for 300keVelectrons)ismuchshorterthanthatofX-rays(about100 pm).Therefore,the radius of the Ewald sphere ismuchlargerandmorereflectionsarise.2.The diffraction angles are very small:ED 0-2(cf.,XRD0-180)3.Electronsarescatteredbythepositivepotentialinsidetheelectroncloud,whileX-raysinteractwiththeelectroncloud.As the result,the interaction of electrons withmatterismuch(106-107)strongerthanthatofX-rays.2024/6/25周二22TheadvantageandthedisadvantageofED1.thediffractedelectronbeamshave a high intensity andexposure times.ED patternscan directly be observed onviewingscreenofTEM.2.diffraction patterns can beobtained from very smallcrystalsselectedwithadiffracted aperture(SelectedAreaElectronDiffractionSAED)1.multiplescatteringplaysanimportantrole,ThismakesstructuredeterminationfromEDmoredifficult and lessreliable than thatfromXRDdata.2024/6/25周二23Ewald Sphere of DiffractionThe diffraction,which mathematically corresponds to aFouriertransform,resultsinspots(reflections)atwell-definedpositions.Eachsetofparallellatticeplanesisrepresentedbyspots（distance of 1/d：d:interplanar spacing)from theorigin and which are perpendicular to the reflecting set oflatticeplane.2024/6/25周二24La2NiO4(4cm/2cm)*51/nm=(101/nm)=(1/10nm)=0.1nmCatal.Today,131(2008):5332024/6/25周二25Carbondeposition overMo2C/ZSM-52024/6/25周二26Calculation:(0.81cm/2cm)*51/nm=(2.0251/nm)=0.494nm1/d=0.493nm1/d=0.294nmSAEDofMo2CUnitCell:Hexagonala=0.3002nmb=0.4724nmAIChEJ57(2011):18522024/6/25周二27The diffraction can be described inreciprocal space by the Ewald sphereconstruction(Figurebelow).Aspherewithradius1/isdrawnthroughtheoriginofthe reciprocal lattice.Now,for eachreciprocal lattice point that is located ontheEwaldsphereofreflection,theBraggconditionissatisfiedanddiffractionarises.2024/6/25周二28point 0:origin ofreciprocallatticek0:wavevectoroftheincidentwavekD:wavevectorofadiffractedwaveZOLZ:ZeroOrderLaueZoneFOLZ(SOLZ):First(Second)OrderLaueZone2024/6/25周二29Duetothesmallwavelengthofelectrons(e.g.,=1.97pmfor300keVelectrons),theradiusoftheEwaldsphereislargerandmanyreflectionsappear.Furthermore,thelatticepointsareelongatedinEDtoformrodssothattheEwaldsphereintersectsmorepoints(seefigure).Becauseofthat,diffractionoccurseventhentheBraggconditionisnotexactlysatisfied.Infact,EDpatternsare2Dcuttingsofreciprocallattice.Therod-shapeisduetothefactthatTEMspecimensareverythinin real space,leading to an elongation in reciprocal space.Iftheinterplanardistanceindirectionofobservationislarge(that means a small distance between ZOLZ and FOLZ inreciprocal space),higher order Laue zones(HOLZ)can beobservedaswell.A general introduction into diffraction is given in aninteractivetutorialbyProvenandNeder.2024/6/25周二30Electron diffraction patterns of Ta97Te60 along twoperpendicular directions.The parameters of thetetragonal unit cell can be determined from theseSAEDpatterns:a=27.6,c=20.6.2024/6/25周二31Electron diffractionpattern of YbSi1.41along001.Thereflectioncondition:h+k=2nforhkl(Cfacecentering)isfulfilled.2024/6/25周二32EDpatternofpolycrystallineplatinum.Theindicesare assigned to thediffraction rings inaccordanceoftheface-centeredcubiclatticeofPt(reflection condition:h,k,lallevenorallodd).2024/6/25周二33In the pseudo-binary system Nb2O5/WO3,Nb8W9O47,crystallizinginathreefoldsuperstructureofthetetragonaltungsten bronze(TTB)type,represents the most stablecompound.The superstructure arises from a systematicoccupation of a part of the pentagonal channels withmetal-oxygenstrings(seefigure).HRTEMimageofNb4W13O47along001.Theinsetsshowthestructuralmodelandasimulation(EMSprogram).2024/6/25周二34HRTEMimageandSAED(inset)ofAl-MCM-41(50).LiuBSetalJPhysChem.C2008,112:15490SBA-152024/6/25周二35SBA-152024/6/25周二36STEM+EDXSPoint Analysis in STEMInSTEM,anelectronbeamisscannedoveradefinedareaofthe sample.The beam can belocalizedon a certainpoint intheimageandusedtomeasurean EDX and/or EEL spectrumat there.Moreover,line scansandmappingscanbeobtainedbythispowerfultechnique.Example:Molybdenum Tungsten Oxide2024/6/25周二37Tosolvethequestionwhetherthenanorodsconsistofamixed Mo-W oxide or of the separated binary oxidesMoO3andWO3,spotanalyseswereperformed.TheEDXspectrum obtained at the position encircled in theHAADF-STEMimageshowsbothmetalsandthusprovesthepresenceofamixedoxideX-ray SpectroscopyX-rayspectroscopyisavaluabletoolforqualitativeand quantitative element analysis.Each element hascharacteristic peak positions corresponding to thepossibletransitionsinitselectronshell.2024/6/25周二38Thepresenceofcopper,forexample,isindicatedbytwoKpeaksatabout8.0and8.9keVandaLpeakat0.85eV.Inheavyelementsliketungsten,alotofdifferenttransitionsarepossibleandmanypeaksaretherefore present.TEMs are almost exclusively equipped withenergy-dispersivespectrometers(energy-dispersiveX-rayspectroscopyEDXS).2024/6/25周二39Generation of X-rays 1.Ionization:Aholeinaninnershell(here:Kshell)isgeneratedbyanincidenthigh-energyelectronthatlosesthecorrespondingenergyEtransferredtotheejectedelectron.*2.X-rayemission:TheholeintheKshellisfilledbyanelectronfromanoutershell(here:L3).ThesuperfluousenergyisemittedasacharacteristicX-rayquantum.InatypicalX-rayspectrum,therearemany peaks caused by such aprocess.TheX-rayenergycorrespondstoacertaindifference in inner-shell energies.Thus,the detection of characteristic X-ray isspecificforaelementinthesample,andX-rayspectroscopycanbeemployedforqualitativeanalysis.2024/6/25周二40Generation of X-rays Another inelastic interaction of the incident electron with matterrepresentsitsdecelerationbytheCoulombfieldofthenucleus.ThisprocesscreatesX-raywithanyenergysmallerthanthebeamenergy.TheseX-raysarecalledbremsstrahlungandformtheuncharacteristicspectrumbackground.If the X-ray spectrum was measured on a TEM,the Cliff-Lorimerratiotechniquecanbeappliedforthequantification:NA/NB=kAB IA/IBNA,NB:atomic%ofelementA,B.IA/IB:measuredintensityofelementA,B.kAB:Cliff-Lorimerfactor.Quantitative X-rays Analysis2024/6/25周二41Generation of X-rays TheCliff-LorimerfactorisNOTaconstantbutdependsontheTEM voltage,on the detector efficiency and several otherparameters.Thus,accurateresultscanonlybeobtainedifastandardcontainingtheelementsAandBinawell-definedratioisusedtodeterminekAB.CalculatedvaluesofkAB,whicharegivenbythevarious programs used for X-ray spectroscopy,are only rawapproximationsandcanonlybeusedforquickandratherinaccurateanalyses.Although the program output for the composition is aseeminglyaccuratevaluewitharathersmallerrorbar,theactualerror might be much higher because of the kAB problem.Inallcases,thebackground(bremsstrahlung)hastobesubtractedfirst.Theaccuracyofthequantificationisadditionallyreducedbystatisticalerrorsofthemeasurement,absorption,fluorescence,andelectronchanneling,justtomentionthemostimportanteffects.2024/6/25周二42Generation of X-rays 2024/6/25周二43DetectorAbsorptionElementWeight%Atomic%Uncertainty%Correctionk-FactorCorrection-O(K)25.08453.3820.3810.4952.0590.952Si(K)23.65028.6710.2520.9781.0000.978S(K)0.7190.7640.0410.9561.0500.985Fe(K)13.1448.0130.2100.9971.4770.996La(K)37.4009.1670.7730.87617.0141.000Correctionmethod:ThicknessQuantificationResultsRef.B.S.LiuetalAppl.Catal.B102(2011):27-362024/6/25周二44STEM+EDXSIntheHAADF-STEMimage(Zcontrast),themetalparticlesappearbright.EDXSspotanalyseswereperformedbyfixingthepositionoftheelectronbeamonmetalparticlesbeingonlyafewnmlarge.BothparticlesinvestigatedcontainPdandPtsimultaneously.122024/6/25周二452024/6/25周二46http:/www.microscopy.ethz.ch/ED-Ewald.htm