The TEM and Its Optics
This chapter is an introduction to the transmission electron microscope (TEM), using ray diagrams to discuss its essential optical design and its component subsystems. Bright- and dark-field imaging are explained, as is selected area diffraction, converge
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The TEM and Its Optics
2.1 Introduction to the Transmission Electron Microscope The transmission electron microscope (TEM) has become the premier tool for the microstructural characterization of materials. In practice, the diffraction patterns B. Fultz, J. Howe, Transmission Electron Microscopy and Diffractometry of Materials, Graduate Texts in Physics, DOI 10.1007/978-3-642-29761-8_2, © Springer-Verlag Berlin Heidelberg 2013
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The TEM and Its Optics
measured by x-ray methods are more quantitative than electron diffraction patterns, but electrons have an important advantage over x-rays; electrons can be focused easily. By focusing the electron beam, diffraction patterns as discussed in Chap. 1 can be measured from microscopic regions, and it is often possible to select a single microcrystal for a diffraction measurement. The optics of electron microscopes can be used to make images of the electron intensity emerging from the sample. For example, variations in the intensity of electron diffraction across a thin specimen, called “diffraction contrast,” is useful for making images of defects such as dislocations, interfaces, and second phase particles. Beyond diffraction contrast microscopy, which measures the intensity of diffracted waves, in “highresolution” transmission electron microscopy (HRTEM or HREM) the phase of the diffracted electron wave is preserved and interferes constructively or destructively with the phase of the transmitted wave. This technique of “phase-contrast imaging” is used to form images of columns of atoms. Alternatively, high-resolution images of atom columns can be made with electron nanobeams incident on the sample, and with electron scattering at high angles to minimize electron interference behavior (a method called “high-angle annular dark-field imaging”). Besides diffraction and spatial imaging, the high-energy electrons in TEM cause electronic excitations of the atoms in the specimen. “Analytical TEM” uses two types of spectrometries to obtain chemical information from electronic excitations: • In energy-dispersive x-ray spectrometry (EDS), an x-ray spectrum is acquired from small regions of the specimen illuminated with a focused electron beam, usually using a solid-state detector as described in Sect. 1.4.2. Characteristic xrays from the chemical elements are used to determine the concentrations of the different elements in the specimen. • In electron energy-loss spectrometry (EELS), energy losses of the electrons are measured after the high-energy electrons have traversed the specimen. Information on local chemistry and structure is obtained from features in EELS spectra caused by plasmon excitations and core electron excitations. A block diagram of a TEM is shown in Fig. 2.1. A modern TEM may have the capability of imaging the variations in diffraction across the specimen (diffraction contrast imaging), imaging the phase contrast of the specimen (high-resolution imaging), obtaining diffraction patterns from selected areas of the specimen, and performing EELS and EDS s
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