Practical Electron Microscopy and Database

EELS artifacts from ultra-thin TEM specimens can occur, for instance:

i) A high energy incident electron can create surface plasmons at each exterior surface, especially these surface excitations dominate in very thin (e.g. < 20 nm) samples or nanoparticles.

ii) The surfaces of the specimen contribute inelastic scattering. The inelastic scattering originates from the electrons traversing the solid/vacuum interface. The value of the surface loss can be obtained if the dielectric function is known [1]. The intensity of the surface loss is not predominant if the TEM sample is relatively thick, e.g. > 80 nm for some materials.

dσ_T/d(ħω)|_surface presents the surface contribution to the total energy losses of the incident electrons, and in principle is a complicated function of ε, the foil thickness t, and the  incident angle α of the primary electron beam with respect to the surface normal of the foil. These surface excitations are generally induced within a few angstroms at the surface of the foil between the foil and the vacuum. The depth of this “surface region” has a physical extension that increases with the square root of the energy of the incident electrons (E₀).

The surface plasmon-loss (I_s) has dependence on TEM-specimen thickness only when the specimen is extremely thin (e.g. <10 nm in general).

[1] Egerton R F 1986 Electron Energy-Loss Spectroscopy in the Electron Microscope (New York: Plenum).