单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,h,*,第四章 透射电镜(TEM),Transmission Electron Microscopy,刘炳泗,Department of Chemistry Tianjin University,11/16/2024,1,h,第四章 透射电镜(TEM)刘炳泗10/5/20231h,一、Interactions of Electrons with Matter,1.,Elastic Interactions,No energy is transferred from the electron to the sample(direct beam or is scattered).TEM,SAED,2.,Inelastic Interactions,Energy is transferred from the incident electrons to the sample:,(EDX analysis),11/16/2024,2,h,一、Interactions of Electrons wi,1.,Elastic Interactions,An electron penetrating into the electron cloud of an atom is attracted by the positive potential of the nucleus(Coulombic interaction),and its path is deflected towards the core as a result.The Coulombic force F is defined as:,with r being the distance between the charges Q,1,and Q,2,and,0,the dielectric constant.,The closer the electron comes to the nucleus,i.e.the smaller r,the larger is F and consequently the scattering angle.,In rare cases,even complete backscattering can occur(back scattered electrons,BSE,).,11/16/2024,3,h,1.Elastic Interactions An ele,2.,Inelastic Interactions,Ionization:,The high-energy electrons of the incident beam can transfer a critical amount of energy to an inner-shell electron of an atom,leading to the ejection of this electron.The ionization energy is provided by the incident electron,reducing its energy.This leads to an ionization edge in the,electron energy loss spectrum,(,EELS,).Subsequently,the hole in the inner-shell is filled up by an electron with higher energy from an outer shell.This electron gives away a part of its energy,leading to the emission of,characteristic X-rays,or,Auger electrons,.,Secondary electrons:,Electrons in the conduction or valence band do not need much energy(low work function)to be transferred into vacuum.Thus,the energy of secondary electrons(SE)is low(50 eV).The SEs are mainly exploited in,SEM,.,Phonons:,Phonons are lattice vibrations,which are equal to heating the specimen.This effect may lead to a damage of the sample.,Plasmons:,Plasmons are longitudinal oscillations of free electrons,which decay either in photons or phonons.,Cathodoluminescence:,If semiconductors are hit by high-energy electrons,electron-hole pairs can be formed by promoting an valence electron into the conduction band.Filling this hole with an electron from the conduction band(recombination)leads to the emission of light with a frequency that corresponds to the band gap.,11/16/2024,4,h,2.Inelastic InteractionsIoniz,Basic contrast mechanisms in TEM and STEM.,Electrons,which come from the condenser system of the TEM,are scattered by the sample,situated in the object plane of the objective lens.Electrons scattered in the same direction are focused in the back focal plane,and as a result,a diffraction pattern is formed there.,Electrons coming from the same point of the object are focused in the image plane.In the,TEM,the first intermediate image,is magnified by further lenses(projective system).,11/16/2024,5,h,Basic contrast mechanisms in T,basic contrast mechanisms in TEM and STEM.,Bright field(BF)mode:,Mass-thickness and diffraction,contrast,contribute to image formation:thick areas,areas in which heavy atoms are enriched,and crystalline areas appear with dark contrast.,In dark field(DF)images:,Since diffracted beams have strongly interacted with the specimen,very useful information is present in DF images,e.g.,about planar defects,stacking faults or particle size.,11/16/2024,6,h,basic contrast mechanisms in T,BF and DF TEM images of ZrO2,Electron diffraction pattern:the spots indicate the presence of single microcrystals.The apertures(red circles)are localized around the direct beam for recording the bright field(,BF,)image and around a few diffracted beams for the dark field(,DF,)image.The intense direct beam is blocked by a metal rod(black shadow on the left center)to avoid overexposure.,BF,DF,11/16/2024,7,h,BF and DF TEM images of ZrO2 E,HRTEM mode,If the point resolution of the microscope is sufficiently high and,a suitable sample oriented along a zone axis,then(HRTEM)images are obtained.In many cases,the atomic structure of a specimen can directly be investigated by HRTEM.,11/16/2024,8,h,HRTEM modeIf the point resolut,HRTEM mode,single crystals of(Ce,0.5,Zr,0.5,)O,2,Ag particle supported on ZnO,ReO3 Structure,11/16/2024,9,h,HRTEM modesingle crystals of(,Scanning Electron Microscopy(SEM),compact samples can thus be investigated by SEM.A valuable information about morphology,surface topology and composition can be obtained.SEM microscopes achieving resolutions below 1 nm are available now.,11/16/2024,10,h,Scanning Electron Microscopy(,SEM:Imaging with Back-scattered Electrons,SEM images of Fe particles in carbon recorded with the secondary electron(left)and the back-scattered(right)electron detector.The BSE image shows the Fe particles with bright contrast.,11/16/2024,11,h,SEM:Imaging with Back-scatter,二、,Imaging and Diffraction,1.,objective lens,forms a diffr