纳米材料和纳米结构第十三讲-2-透射电子显微镜及其能谱_精品文档.ppt

纳米材料和纳米结构第十三讲-2-透射电子显微镜及其能谱_精品文档.ppt

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TransmissionElectronMicroscopyandSpectroscopy透射电子显微镜及其能谱,纳米材料和纳米结构,第十三讲,1IntroductiontoTEM,Nanomaterialsaredominatedbyparticleswithsizesinnanometerrange,andthereforetheimagingoftheatomsandthequantitativeanalysisofthesurfacechemistryarevitallyimportantfornanomaterialsresearchTransmissionelectronmicroscopy（TEM）isapowerfulanduniquetechniqueforstructurecharacterization,whichnotonlycanproviderealspaceimagingataresolutionof1-2,butalsoananalyticaltoolforquantitativestructureandchemicalanalysis,

（1）IlluminationSystema.ThermionicEmission;b.FieldEmission

（2）SpecimenStage（3）ObjectiveLensSystem（4）MagnificationSystem（5）DataRecodingSystem（6）ChemicalAnalysisSystema.EnergyDispersiveX-raySpectroscopy（EDS）b.ElectronEnergy-LossSpectroscopy（EELS）c.ChargeCoupledDevice（CCD）,InstrumentStructureofTEM,SchematicstructureofaTEM,2AtomicResolutionLatticeImagingofCrystallineSpecimens,QualitiesofTEMimagesaredecidedbythreetypesofcontrast:

DiffractionContrastduetoalocaldistortionintheorientationofthecrystal,whichwillperturbthediffractionintensityandleadstocontrastoftheimagePhaseContrastduetothephasemodulationoftheincidentelectronwave,whichissensitivetotheatomdistributioninthespecimenandisthebasisofhigh-resolutionTEMMassthicknessoratomicnumberproducedcontrastduetoatomswithdifferentatomicnumbersexhibitingdifferentpowersofscattering,2-1ImageContrast,2.2ImagingTheory,2.3ImagingInterpretationandtheoreticalsimulation,TheoreticallysimulatedimagesforadecahedralAuparticalatvariousorientationsandatdifferentfocuses（f=42nmand70nm）,3FacetedShapesofNanocrystals,AparticleconstitutingafinitenumberofatomscanhaveaspecificgeometricalshapeSurfaceenergiesassociatedwithdifferentcrystallographicplanesareusuallydifferent,andageneralsequencemayholdforface-centeredcubiccrystal,111100110Forasphericalsinglecrystallineparticle,itssurfacemustcontainhighindexcrystallographyplanes,whichpossiblyresultinalowersurfaceenergyFacetstendtoformontheparticlesurfacetoincreasetheportionofthelowindexplanes,3.1PolyhedralShapesofNanocrystales,Forparticlessmallerthan10-20nm,thesurfaceispolyhedron.Inthefollowingfigure,agroupofcubo-octahedral（立方八面体）shapesasafunctionoftheratio（R）ofthegrowthrateintothatofthearedemonstratedThefastestgrowthdirectioninacube（R=0.58）is,thelongestdirectionintheoctahedron（R=0.7）isthediagonal,andthelongestdirectioninthecubo-octahedronis（R=0.87）isthedirection,Geometricalshapesofcubo-octahedralnanocrystalsasafunctionoftheratio,R,ofthegrowthratealongto,Theparticleswith0.87R1.73havethe111and100facets,theothergroupofparticleshasafixed（111）basewithexposed111and100facets.Anincreaseinthearearatioof111to100resultsintheevolutionofparticleshapesfromatriangle-basedpyramidtoatetrahedron（四面体）.,Geometricalshapesofmultiplytwinneddecahedral（十面体）andicosahedral（二十面体）particles,HRTEMimagesofcubicPtnanocrystalsorientedalong（a）001and（b）110,showingsurfacesteps/ledgesandthermodynamicallynonequilibriumshapes,HRTEMimagesofPtnanocrystals（a）withatruncated（去顶的）octahedralshapeandorientedalong110,and（b）withaoctahedralshapeandorientedalong110and001.Theinsetin（a）isamodeloftheparticleshape,HRTEMimagesoftruncatedtetrahedralPtnanocrystalsorientedalong110.Thesurfacestepsandledgesatthetruncatedcornersareclearlyresolved,3.2TwinningStructureandStackingFaults,Twinningisoneofthemostpopularplanardefectsinnanocrystals,anditisfrequentlyobservedforfccstructuredmetallicnanocrystalsTwinningistheresultoftwosubgrainssharingacommoncrystallographicplane,thus,thestructureofonesubgrainisthemirrorreflectionoftheotherbythetwinplaneStackingfaultsaretypicalplanardefect.Theyareproducedbyadistortiononthestackingsequenceofatomplanes,HRTEMimagesof（a）Ptand（b）Aunanocrystalshavingtwin（T）andstackingfault（S）structures,respectively.Thetwinplaneandstackingfaultareparalleltothedirectionoftheelectronbeam,3.3DecahedralandIcosahedralParticles,Thetwomosttypicalexamplesofmultipletwinnedparticlesarethedecahedral（十面体）andicosahedral（二十面体）,andthecorrespondingparticlesarethemostextensivelystudiedtwinnednanocrystals.,Atomicmodelsof（a）decahedral,and（b,c）icosahedralnanocrystals,TEMimagesofdecahedralAunanocrystalswhentheincidentelectronbeamisparallelornearlyparalleltothefive-foldsymmetryaxis.Thestrainfieldintroducedbythefive-foldtwinsisapparentin（a）.Afouriertransformoftheimageclearlyrevealsthefive-foldsymmetry,3.4NucleationandGrowthofNanoparticles,HRTEMimagesrecordedfromthecentersofcarbonspheres,showingthespiral