By using information in the high-angle annular dark field image and annular bright field images simultaneously, we estimate the specimen thickness and finite source size, and use them to explore in simulation the issue of dark contrast in the vicinity of the grain boundary in the annular dark field image.

High-angle annular dark field scanning transmission electron microscopy (HAADFSTEM) has been established as a direct and robust imaging mode for determining the location of heavy atom columns within grain boundaries. However, light elements are usually not visible in HAADFSTEM. Structural analysis using the observed positions of only the heavy elements as reference points to compare with candidate structure models, perhaps determined by molecular dynamics or first principles calculations, has had some success. For instance, simulations in an alpha_Al2O3 S13 grain boundary show an oxygen-terminated trial structure to be consistent with the experimental HAADF image while an aluminum-terminated structure is not, even though the oxygen itself was not imaged. High voltage electron microscopy, negative Cs imaging conventional TEM and exit surface wave function reconstruction have all been used to image oxygen within grain boundaries and defect structures. However, these approaches are not yet routine, requiring very thin specimens and detailed simulation for interpretation and analysis. By contrast, for bulk crystals, annular bright field (ABF) STEM imaging allows for direct image interpretation with both light and heavy atom columns simultaneously visible over a wide range of thicknesses. In this paper we demonstrate that this property extends to the case of light atoms in a defect structure, specifically a grain boundary.

 

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