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

Reflection electron energy-loss spectroscopy (REELS) at low energies is very surface sensitive and can be used to characterize the electronic properties of ultrathin films and surface nanostructures. To extract reliable quantitative information from a REELS experiment it is essential to have accurate theoretical algorithms. In this paper, we have studied the validity of a theoretical method proposed by Yubero and Tougaard [Phys. Rev. B 46, 2486 (1992); Phys. Rev B 53, 9719 (1996)] to determine the dielectric function epsilon by using an analysis of an effective experimental REELS cross section determined by the Tougaard-Chorkendorff algorithm [Phys Rev B 35, 6570 (1987)]. To this end, REELS experiments with electrons incident normal to the surface were carried out for a wide range of exit angles (35 degrees-74 degrees to the surface normal) and energies 200, 500, and 1000 eV for several materials (Cu, Ag, Au, and 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 epsilon is used for all geometries and energies and that this epsilon 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 (lambda) is similar to 0.5 nm implies that the method can be used to determine the dielectric properties of nanofilms, and the additional fact that the theory can predict the variation with angle suggests that the method might also be used to determine the dielectric properties of nanostructures.