Electron Diffraction Images

Abstract

Abstract Several classical imaging processes, such as transmission light and electron microscopy, make use of a lens-the objective lens-to gather a fraction of the radiation scattered by the sample. The sample acts as the object of the lens, which, as shown in Figure 6.1, recreates an image of the sample in the image plane of the lens (CD). Subsequent. lenses magnify this image and project it onto an appropriate imaging surface, such as the retina (for a light microscope), a fluorescent screen (in an electron microscope), or any photographic/recording medium. Between the objective lens and the image plane is the focal plane of the lens (AB). Within this plane the radiation from the sample is focused to positions that depend upon the initial angle of scatter, as also shown in Figure 6.1. Thus the intensity distribution of radiation in the focal plane of the lens constitutes a scatter pattern. In this chapter, we will illustrate the use of such scatter-pattern images taken in the focal plane of the transmission electron microscope (TEM). Although these images are not “images” of the sample in the strict sense of the word, they are important indicators of the nature of the sample. These scatter patterns are also termed diffraction patterns because the phenomenon of diffraction (which means the radiation is scattered coherently, i.e., all the waves are in step) is very dependent on the crystalline nature and orientation of the sample. Thus, these diffraction patterns may be thought of as images that reflect the crystal nature of the sample. In the TEM, the area of the sample contributing to the diffraction pattern is determined by placing an aperture in the image plane of the objective lens, which creates a virtual aperture in the plane of the specimen. The resulting diffraction patterns are usually termed selected area diffraction (SAD) patterns, and typically come from areas of the sample greater than about one micrometer in diameter.