In many cases, each EEL spectrum should be corrected due to dark current and gain variations between the elements of the CCD detector and due to multiple scattering. Most TEM specimens are so thick that plural scattering is usually significant. The plural scattering is generally unwanted since it distorts the shape of the energy-loss spectrum. In general, if the TEM specimen is too thick (t/λ > 0.4), a deconvolution process must be employed to remove the effect of plural scattering. However, if the TEM sample for EELS is very thin, the plural scattering can be negligible impact on the shape of the ionization edges.
Table 4712 shows that electrons interact with 1 electron, many electrons, 1 nucleus, and many nuclei in solids.
Table 4712. Effects of interactions of electrons in solids.
||Interaction with electron(s)
||Interaction with nucleus/nuclei
||Electron Compton effect; electron excitation (from 50 eV to a few keV: EDS and EELS)
||Plasmon excitation (< 50 eV, ~100 nm TEM specimen); Cerenkov effect
||Rutherford scattering; phonon scattering (< 1 eV, heat)
Plural inelastic scattering by plasmon excitation is a special concern when measuring the low energy loss (e.g. Li K-edge) because it is close to the low-energy plasmon region. For instance, double plasmon scattering distorts the pre-edge background and can mask the Li K-edge. On the other hand, plural scattering can originate from the contribution of combined energy losses from core and valence electron
Artifacts due to plural scattering can be reduced by increasing the inelastic mean-free-path with the increase of the accelerating voltage of the electron beam or by restricting analyses to thin regions of the sample. Furthermore, in general, the EELS and EFTEM backgrounds originate from random, plural inelastic scattering events.
In general, the requirements of TEM specimen thickness for EELS and EFTEM measurements are:
i) The specimens should be sufficiently thin to prevent any multiple inelastic scattering, but the degree of single inelastic scattering should be relatively high. For most materials, the optimized specimen thickness is in the range of 25-100 nm, depending on the average atomic number and the beam energy.
ii) To avoid surface effects, the specimen thickness cannot be less than 25 nm if low-loss EELS spectra are measured.