Valence-electron energy loss near edges, truncated slabs, and junctions

Valence losses in electron microscopy can be conveniently computed using classical dielectric excitation theory. In many practical situations, the presence of complex structures gives rise to the possibility of coupling between interface modes, surface modes, and edge modes. In the nonrelativistic case, the losses in such geometries can be calculated by taking a distribution of surface and interface points, each of which is associated with an interface charge. These charges interact self-consistently with each other as well as with the charge of the incident electron. Here, we have applied this boundary charge method to provide scanning transmission electron microscopy energy loss spectra near-edged structures such as truncated slabs and junctions formed by several media. We find the dominant modes associated with such systems and study their contribution to the characteristic energy loss functions. The significance of these excitations compared with the planar interface peaks is discussed in terms of the sample geometry with illustrations for the cases of an MgO cube and an Si/SiO2 thin-film interface. [S0163-1829(99)00939-X].