TABLE OF CONTENTS Electron diffraction is a powerful tool for materials research ever since the invention of TEM. A popular experimental setup for electron diffraction is selected area diffraction (SAD). Using parallel illumination the area of interest is selected by placing an aperture in the image plane of the objective lens. Because of the physical size of the aperture, the lens aberration and diffraction, the practical selected area by the aperture is limited to micrometer level. For conventional materials with a grain size of 10 to 100 microns, the diffraction pattern is a spot pattern. However, for nanostructured materials with a grain size of, say 20 nm, the diffraction pattern by the SAD method becomes a ring pattern (also called powder pattern) because so many grains are included in the aperture. Such a ring pattern contains rich information about the structure of the specimen and can be used to identify uniquely a known phase (Liu, et al., 1998). However, it is occasionally necessary to obtain a spot pattern from a single grain of nanometer dimension for such applications as determination of crystal orientation, crystal structure of modulated phase, epitaxy, etc. This is the nanodiffraction technique, e.g., electron diffraction from a nano-sized area of a specimen.
Unlike SAD, nanodiffraction is realized by focusing the electron beam on the specimen. The experimental setup for nanodiffraction is schematically shown in Fig. 2.20(b), in comparison with SAD in Fig. 2.20(a). The area contributing to diffraction is controlled by the probe size.
