Contribution of thermally scattered electrons to atomic resolution elemental maps

TLDR

EELS and EDX in STEM can produce atomic‑resolution elemental maps, yet the observed oxygen signal contradicts stoichiometry because oxygen density is identical in both column types. The study presents oxygen‑K EELS and EDX maps of <001> strontium titanate and uses theory to show that thermally scattered electrons explain the anomalous signal. The authors model the contribution of thermally scattered electrons and compare the <001> maps with <110> maps to elucidate the signal differences. The experiments show unexpectedly higher oxygen signal over Ti+O columns, yet the data agree with first‑principles simulations that account for thermally scattered electrons.

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.