Quantum crystallography of bonding at defects using advanced electron microscopy

The term “quantum crystallography” refers to using diffraction to determine not only the positions of the atoms in a material but also the movements of electrons due to bonding.  Diffraction is very powerful for crystals but cannot reveal information about local defects which often dictate the properties of a material.  An exciting, currently emerging technique, is four-dimensional scanning transmission electron microscopy (4D STEM).  The principle of this technique is that a focused beam of electrons, which can be as small as an atom, is scanned over a thin sample and the diffraction patterns formed by the transmitted electrons are collected for each probe position.  This data set is very rich and we are exploring ways it can be used.  One approach is called ptychography and we have already shown that it gives information that is sensitive to bonding.  This project will combined DFT modelling and experimental microscopy to explore how bond effects at defects can be detected and explained in terms of the underlying electronic structure.