Target Geometry Dependence of Electron Energy Loss Spectra in Scanning Transmission Electron Microscopy (STEM)

In the frame of the Self-Energy formalism, we study the interaction between STEM electrons and small particles in the range of the valence electron excitations. We first calculate the energy loss probability for an isolated sphere and study the loss spectrum dependence on the size of the particle and on the relative impact parameter. Then we analyze the loss spectra in more realistic situations: (a) the effect of the coupling between the particle and supporting surface is studied in a simple geometrical model; and (b) we analyze the dependence of the losses on the geometrical shape of the target by considering hemispherical particle. Our results are in a good qualitative (and in simple cases, quantitative too) agreement with several experimental results which show anomalous excitations. We restate the suitability of the dielectric theory to study the surface excitations of these systems