Defect Contrast with 4D-STEM, Role of Local Order and Symmetry

Many important material properties are controlled by defects, which break the local order of structures on the nano to atomic scale. Scanning transmission electron microscopy (S/TEM) is an especially apt tool to study structures and their symmetry at relevant length scales. However, there are intrinsic challenges to characterizing these defects with conventional imaging modalities for some materials due to dose, contrast, and probe-size limitations. 4D-STEM experiments, where a diffraction pattern is acquired at each position in real space, can help overcome these obstacles. The combination of the versatility of the available reconstruction techniques and the rich information contained in a diffraction pattern allows for studies of material properties, such as local symmetry, that would not otherwise be possible. Additionally, the modification of the beam through a phase or amplitude plate inserted in the probe forming aperture of a STEM creates another handle to change final image contrast, while reducing dose and highlighting defects. In this presentation, we will show simulations on a variety of structures and illustrate how virtual detectors and beam modification in a 4D-STEM experiment can be used to highlight important changes in local order of materials. We will compare these results to more conventional imaging techniques in light of realistic parameters, such as electron dose and lens aberrations.