Validity of Yubero-Tougaard theory to quantitatively determine the dielectric properties of surface nanofilms

Reflection electron energy loss spectroscopy (REELS) at low energy is very surface sensitive and can be used to characterize the electronic properties of ultra-thin films and surface nano-structures. To extract reliable quantitative information from a REELS experiment it is essential to have accurate theoretical algorithms. In this work, we have studied the validity of a theoretical method proposed by Yubero and Tougaard to determine the dielectric function ε by analysis of an effective experimental REELS cross section determined by the Tougaard-Chorkendorff algorithm. To this end, REELS experiments with electrons incident normal to the surface were carried out for a wide range of exit angles (35°-74° to the surface normal), and energies 200, 500 and 1000 eV for several materials (Cu, Ag, Au, Fe). We find that the theory is in very good agreement with experiment for all geometries and energies studied. It is important to note that for a given element the same ε is used for all geometries and energies and that this ε is determined by the analysis. The fact that the theory applies at energies at least down to 200 eV where the inelastic mean free path of electrons is ~0.5 nm implies that the method can be used to determine the dielectric properties of nano-films and the additional fact that the theory can predict the variation with angle suggests that the method can also be used to determine the dielectric properties of nanostructures.

Physical Review B 77, 155403 (2008).