Secondary electrons (SEs) are produced when an electron beam interacts with matter. This signal is widely used in the scanning electron microscope (SEM). The peak intensity of the SE energy distribution occurs at about 2 eV, and such electrons escape from within a few nanometers of the surface. Thus, the SE signal is highly surface-specific, and the image resolution is a function of the imaging probe [1]. Typical field emission gun (FEG) SEMs have imaging resolutions of just a few nm in SE mode. The great strength of SE imaging is in providing high-resolution surface (topographic) information. Scanning transmission electron microscopy (STEM) uses a scanning electron probe, much like in SEM. However, the STEM signals originate from electrons transmitted through a thin specimen. Conventional STEM imaging of crystalline specimens is based on brightfield (BF) (Bragg diffraction contrast) and high-angle annular dark-field (HAADF) signals. These provide crystallographic and mass-thickness information, respectively. The resulting projection images impart little direct information about the surface morphology. In the case of regularly shaped particles (for example, spheres), the morphology can be inferred from the image contrast. However, for irregularly shaped particles of varying composition, this becomes difficult.
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Mitchell et al. (2016) studied this question.