On the visibility of very thin specimens in annular bright field scanning transmission electron microscopy

Annular bright field (ABF) scanning transmission electron microscopy (STEM) is emerging as an important observation mode for its ability to simultaneously image both heavy and light elements. However, recent results have demonstrated that in the limit of a very thin specimen (a few atomic layers), the ABF and high angle annular dark field (HAADF) signals cease to be intuitively related: a phenomenon which is generally irrelevant when imaging "normal" specimens. ABF/HAADF STEM observations and multislice image simulations of two catalyst samples of differing atomic weights are presented; it is shown that the nature of the ABF signal is specimen dependent