In Z-contrast, not all that glistens is high-Z

Abstract

It is by now a given that high resolution Z-contrast is a useful imaging technique for detecting high atomic number (Z) domains in nanophase materials. However, image intensity also depends on specimen orientation, electron-optical geometry, thickness and local strain fields, as well as local structural ordering. In this paper we will present examples from our materials studies where contrasts from strain and short-range order in hollow-cone dark-field images provide important structural insights. Strain fields were found to be responsible for the unusual "blister' contrasts observed in hollow-cone dark field images of delaminated sheets of calcium sodium niobate layered materials. Wherever two sheets overlap, the average intensity increases and dark disks of diameter ~20 nm, with bright central spots, appear (Fig. 1). For large hollow-cone angles, q1 > 20 nm-1, the blisters vanish, although faint fine-scale features persist. Elasticity analysis and image simulations (Fig. 2) reveal that the blisters are pockets of gas or liquid that became trapped between sheets during the drying stage of the specimen preparation process, when individual sheets come into contact at random mutual angles and reseal (Fig. 3).