4 Pages

On the high resolution content of elemental maps

Two-dimensional elemental maps of materials can now be formed routinely from electron spectroscopic images acquired in an energy-filtering transmission electron microscope. Recently, several authors (e.g. Hashimoto et al., 1997; Freitag and Mader, 1999) demonstrated that elemental maps of crystalline samples can show lattice fringes. However, it is not readily appreciated that the contrast in such fringes may not always represent the true chemical distribution of the element of interest. Instead, it may result from elastic scattering in the sample following the inelastic scattering event. In this paper, we propose a series of experiments that can be used to distinguish between chemical and “elastic” lattice fringe contrast in an elemental map. (We note however that there is a continuous gradation from fine microstructure to lattice fringes; our discussion is applicable to any form of elemental map). We illustrate our approach through the examination of a sample of a tungsten sulphide, in which adjacent WS2 {002} layers have a spacing of 0.618 nm and consist of double atomic layers of S sandwiched between single layers of W.