Imaging in the TEM

Abstract

We've already mentioned back in Chapters 2--4 that TEM image contrast arises because of the scattering of the incident beam by the specimen. The electron wave can change both its amplitude and its phase as it traverses the specimen and both these kinds of change can give rise to image contrast. Thus a fundamental distinction we make in the TEM is between amplitude contrast and phase contrast. In most situations, both types of contrast actually contribute to an image, although one will tend to dominate. In this chapter we'll discuss only amplitude contrast and we'll see that there are two principal types, namely mass-thickness contrast and diffraction contrast. This kind of contrast is observed in both TEM and STEM BF and DF images and we'll discuss the important differences between the images formed in each of these two modes of operation. We'll then go on to discuss the principles of diffraction contrast, which are sufficiently complex that it takes Chapters 23--26 to show you how this form of contrast is used to identify and distinguish different crystal defects. Diffraction-contrast imaging came into prominence in about 1956, when it was realized that the intensity in a diffracted beam depends strongly on the deviation parameter, s, and that crystal defects distort the diffracting planes. Therefore, the diffraction contrast from regions close to the defect would depend on the properties (in particular, the strain field) of the defect. We'll then consider phase contrast and how it can be used to image atomic level detail in Chapters 27--30. Other forms of TEM imaging and variations on these major types of contrast are gathered in the catch-all Chapter 31.