Atomic bonding effects in annular dark field scanning transmission electron microscopy. I. Computational predictions

Annular dark field scanning transmission electron microscopy (ADF-STEM) image simulations
were performed for zone-axis-oriented light-element single crystals, using a multislice method
adapted to include charge redistribution due to chemical bonding. Examination of these image simulations
alongside calculations of the propagation of the focused electron probe reveal that the evolution
of the probe intensity with thickness exhibits significant sensitivity to interatomic charge
transfer, accounting for observed thickness-dependent bonding sensitivity of contrast in all ADFSTEM
imaging conditions. Because changes in image contrast relative to conventional neutral
atom simulations scale directly with the net interatomic charge transfer, the strongest effects are
seen in crystals with highly polar bonding, while no effects are seen for nonpolar bonding.
Although the bonding dependence of ADF-STEM image contrast varies with detector geometry,
imaging parameters, and material temperature, these simulations predict the bonding effects to be
experimentally measureable.