The TEM and its Optics

Abstract

The transmission electron microscope (TEM) has become the premier tool for the microstructural characterization of materials. In practice, the diffraction patterns 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 Chapter 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 “high-resolution” 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. High-resolution images of atom columns are also possible with electron nanobeams incident on the sample, and electron scattering at high angles to minimize electron interference behavior (a method called “high-angle annular dark-field imaging”).