On the origin of transverse incoherence in Z-contrast STEM

We use a Bloch wave approach to further investigate the origins of the inco herent nature of Z-contrast imaging using an ADF detector in a STEM. We dis cuss how, although at high angles the collected electrons will be mostly th ermally scattered in addition to the elastic scattering, it is not the ther mal scattering that destroys the coherence, rather the combination of the l arge detector with the high-angle elastic scattering. This incoherent natur e of the elastic scattering arises through the filtering of the 1s-type Blo ch states by the detector geometry. We show that it is this filtering that renders an atomic column an independent scatterer insensitive to the config uration of neighbouring columns. It also makes the image contrast insensiti ve to the effects of beam spreading onto neighbouring columns as the probe propagates through the crystal. We also discuss the implications of this fo r previous calculations of the intensity of Z-contrast images.