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理学天津大学胶体与表面化学TEMsurfChem.pptx
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1、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 i
2、s transferredfromtheincidentelectronstothesample:(EDXanalysis)2024/6/25周二31.Elastic InteractionsAnelectronpenetratingintotheelectroncloudofanatomisattractedbythepositivepotentialofthenucleus(Coulombicinteraction),anditspathisdeflectedtowardsthecoreasaresult.TheCoulombicforceFisdefinedas:withrbeingth
3、edistancebetweenthechargesQ1andQ2and0thedielectricconstant.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-ener
4、gy 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,thehol
5、eintheinner-shellisfilledupbyanelectronwithhigherenergyfroman outershell.Thiselectrongivesaway a partofitsenergy,leadingtotheemissionofcharacteristicX-raysorAugerelectrons.Secondaryelectrons:Electronsintheconductionorvalencebanddonotneedmuchenergy(low work function)to be transferred into vacuum.Thus
6、,the energy ofsecondaryelectrons(SE)islow(50eV).TheSEsaremainlyexploitedinSEM.Phonons:Phononsarelatticevibrations,whichareequaltoheatingthespecimen.Thiseffectmayleadtoadamageofthesample.Plasmons:Plasmons are longitudinal oscillations of free electrons,which decay either inphotonsorphonons.Cathodolum
7、inescence:Ifsemiconductorsarehitbyhigh-energyelectrons,electron-holepairscanbeformedbypromotinganvalenceelectronintotheconductionband.Fillingthisholewithanelectronfromtheconductionband(recombination)leadstotheemissionoflightwithafrequencythatcorrespondstothebandgap.2024/6/25周二5Basiccontrastmechanism
8、sinTEMandSTEM.Electrons,which come from thecondensersystemoftheTEM,arescatteredbythesample,situatedinthe object plane of the objectivelens.Electrons scattered in thesamedirectionarefocusedinthebackfocalplane,andasaresult,adiffractionpatternisformedthere.Electrons coming from the samepointoftheobject
9、arefocusedintheimageplane.IntheTEM,thefirstintermediateimageismagnifiedbyfurtherlenses(projectivesystem).2024/6/25周二6basiccontrastmechanismsinTEMandSTEM.Brightfield(BF)mode:Mass-thicknessanddiffractioncontrast contribute to imageformation:thickareas,areasinwhichheavyatomsareenriched,andcrystallinear
10、easappearwithdarkcontrast.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
11、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 mo
12、deIfthepointresolutionofthemicroscopeissufficiently high and asuitable sample orientedalong a zone axis,then(HRTEM)imagesareobtained.Inmanycases,theatomicstructureofaspecimencandirectlybeinvestigatedbyHRTEM.2024/6/25周二9HRTEM modesinglecrystalsof(Ce0.5Zr0.5)O2AgparticlesupportedonZnOReO3 Structure202
13、4/6/25周二10Scanning Electron Microscopy(SEM)compactsamplescanthus be investigatedbySEM.Avaluableinformationaboutmorphology,surfacetopologyandcomposition can beobtained.SEMmicroscopesachievingresolutionsbelow1nmareavailablenow.2024/6/25周二11SEM:Imaging with Back-scattered ElectronsSEMimagesofFeparticle
14、sincarbonrecordedwiththesecondaryelectron(left)andtheback-scattered(right)electrondetector.TheBSEimageshowstheFeparticleswithbrightcontrast.2024/6/25周二12二、Imaging and Diffraction1.objective lens forms adiffractionpatterninthebackfocalplane2.diffraction pattern andimagearesimultaneouslypresent3.real
15、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 sy
16、stem: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 In
17、terference 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 betweeninterplanardistancedanddiff
18、ractionangle:2024/6/25周二16Electron Diffraction(ED)Electron diffraction is a collective elastic scatteringphenomenonwithelectronsbeingscatteringbyatomsinaregulararray(crystal).Theincomingplaneelectronwaveinteractwiththeatoms.Secondarywavesaregeneratedwhichinterferewitheachother.Thisoccurseitherconstr
19、uctively(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 el
20、ectrons 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.Atypicalvaluefo
21、rtheinterplanar 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/
22、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
23、.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.
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