Transmission Electron Microscopy

Abstract

Abstract Transmission electron microscopy (TEM) is the premier tool for understanding the internal microstructure of materials at the nanometer level. Although x‐ray diffraction techniques generally provide better quantitative information than electron diffraction techniques, electrons have an important advantage over x rays in that they can be focused using electromagnetic lenses. This allows one to obtain real‐space images of materials with resolutions on the order of a few tenths to a few nanometers, depending on the imaging conditions, and simultaneously obtain diffraction information from specific regions in the images (e.g., small precipitates) as small as 1 nm. Variations in the intensity of electron scattering across a thin specimen can be used to image strain fields, defects such as dislocations and second‐phase particles, and even atomic columns in materials under certain imaging conditions. Transmission electron microscopy is such a powerful tool for the characterization of materials that some microstructural features are defined in terms of their visibility in TEM images. In addition to diffraction and imaging, the high‐energy electrons (usually in the range of 100 to 400 keV of kinetic energy) in TEM cause electronic excitations of the atoms in the specimen. Two important spectroscopic techniques make use of these excitations by incorporating suitable detectors into the transmission electron microscope, energy‐dispersive x‐ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS). A fully equipped transmission electron microscope has the capability to record the variations in image intensity across the specimen using mass thickness or diffraction contrast techniques, to reveal the atomic structure of materials using high‐resolution (phase‐contrast) imaging or Z ‐contrast (incoherent) imaging, to obtain electron diffraction patterns from small areas of the specimen using a selected‐area aperture or a focused electron probe, and to perform EELS and EDS measurements with a small probe. Additional lenses can be installed in conjunction with an EELS spectrometer to create an energy filter, enabling one to form energy‐filtered TEM images. These images enable mapping of the chemical composition of a specimen with nanometer spatial resolution. In addition to the main techniques of (1) conventional imaging, (2) phase‐contrast imaging, (3) Z ‐contrast imaging, (4) electron diffraction, (5) EDS, and (6) EELS, in TEM many other analyses are possible.