(理学)天津大学胶体与表面化学课件TEM-surf Chem

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1、第四章透射电镜(TEM)TransmissionElectronMicroscopy刘炳泗DepartmentofChemistryTianjinUniversity8/15/20211一、InteractionsofElectronswithMatter1.ElasticInteractionsNoenergyistransferredfromtheelectrontothesample(directbeamorisscattered).TEM,SAED2.InelasticInteractionsEnergyistransferredfr

2、omtheincidentelectronstothesample:(EDXanalysis)8/15/202121.ElasticInteractionsAnelectronpenetratingintotheelectroncloudofanatomisattractedbythepositivepotentialofthenucleus(Coulombicinteraction),anditspathisdeflectedtowardsthecoreasaresult.TheCoulombicforceFisdefinedas:with

3、rbeingthedistancebetweenthechargesQ1andQ2andε0thedielectricconstant.Theclosertheelectroncomestothenucleus,i.e.thesmallerr,thelargerisFandconsequentlythescatteringangle.Inrarecases,evencompletebackscatteringcanoccur(backscatteredelectronsBSE).8/15/202132.InelasticInteraction

4、sIonization:Thehigh-energyelectronsoftheincidentbeamcantransferacriticalamountofenergytoaninner-shellelectronofanatom,leadingtotheejectionofthiselectron.Theionizationenergyisprovidedbytheincidentelectron,reducingitsenergy.Thisleadstoanionizationedgeintheelectronenergylosssp

5、ectrum(EELS).Subsequently,theholeintheinner-shellisfilledupbyanelectronwithhigherenergyfromanoutershell.Thiselectrongivesawayapartofitsenergy,leadingtotheemissionofcharacteristicX-raysorAugerelectrons.Secondaryelectrons:Electronsintheconductionorvalencebanddonotneedmuchener

6、gy(lowworkfunction)tobetransferredintovacuum.Thus,theenergyofsecondaryelectrons(SE)islow(>50eV).TheSEsaremainlyexploitedinSEM.Phonons:Phononsarelatticevibrations,whichareequaltoheatingthespecimen.Thiseffectmayleadtoadamageofthesample.Plasmons:Plasmonsarelongitudinaloscillat

7、ionsoffreeelectrons,whichdecayeitherinphotonsorphonons.Cathodoluminescence:Ifsemiconductorsarehitbyhigh-energyelectrons,electron-holepairscanbeformedbypromotinganvalenceelectronintotheconductionband.Fillingthisholewithanelectronfromtheconductionband(recombination)leadstothe

8、emissionoflightwithafrequencythatcorrespondstothebandgap.8/15/20214Basiccontrastme