High-Resolution TEM

Abstract

We will now rethink what we mean by a TEM, in a way that is more suitable for HRTEM, where the purpose is to maximize the useful detail in the image. (Note the word useful here.) You should think of the microscope as an optical device which transfers information from the specimen to the image. The optics consist of a series of lenses and apertures aligned along the optic (symmetry) axis. What we would like to do is to transfer all the information from the specimen to the image, a process known as mapping. There are two problems to overcome and we can never be completely successful in transferring all the information. As you know from Chapter 6, the lens system is not perfect so the image is distorted and you lose some data (Abbe’s theory). The second problem is that we have to interpret the image using an atomistic model for the material. Ideally, this model will include a full description of the atomic potential and the bonding of the atoms, but we don’t know that either. We will also need to know exactly how many atoms the electron encountered on its way through the specimen. So most of our task will be concerned with finding the best compromise and producing models for the real situation. To conclude our discussion of the theory, we will introduce the language of information theory, which is increasingly used in HRTEM. We close the chapter with a review of the experimental applications of HRTEM to include periodic and nonperiodic materials, mixtures of the two, or just single atoms.