Thin film (15 nm) of LaCoO3 grown on LaAlO3 substrate (not shown here).
LaCoO3 forms horizontal (parallel to the substrate) dark stripes that are in fact layers with dilated vertical (perpendicular to the substrate) unit cell distances. In this way LaCoO3 accomodates the compressive strain imposed by the substrate.
The image taken in a scanning transmission electron microscop (STEM).
In this high angle annular dark field (HAADF) image the heavier atoms (La) are brighter/larger than
the lighter atoms (Co). Very light atoms like oxygen are not visible.
In a tensile strain (LaCoO3 grown on SrTiO3), the dark stripes are perpendicular to the substrate.
Ordering of oxygen vacancies results in a modulation of the Co oxidation state (charge ordering). The increase of Co L23 ratio on dark stripes indicates a reduced Co valence towards Co2+.
EELS: Two-dimensional map of Co L23 ratio (blue rectangle – brighter dots denote higher L23 ratio). Cyan curve in the graph is the Co L23 profile of 2D map.
