Contribution of thermally scattered electrons to atomic resolution elemental maps

Abstract

Electron energy-loss spectroscopy (EELS) and energy dispersive x-ray (EDX) analysis in scanning transmission electron microscopy (STEM) have the ability to produce elemental maps of a specimen at atomic resolution. In this paper we present EELS and EDX maps for the oxygen K shell in $\ensuremath{\langle}001\ensuremath{\rangle}$ strontium titanate. The results initially seem to be anomalous since substantially more signal is obtained when the STEM probe is above the columns containing both titanium and oxygen than when it is above those containing only oxygen. This is at variance with the stoichiometry: the density of oxygen in both types of columns is the same. Using theory, we show that an understanding of the direct contribution to the recorded signal from electrons which have been thermally scattered is the key to understanding these results. We contrast these results with elemental maps of $\ensuremath{\langle}110\ensuremath{\rangle}$ strontium titanate. While the experimental results are not directly interpretable, they are in concert with simulations from first principles such as those presented in this paper.