Resolving the depth coordinate in photoelectron spectroscopy – Comparison of excitation energy variation vs. angular-resolved XPS for the analysis of a self-assembled monolayer model system

Methods for the quantitative XPS analysis of solids with concentration depth profiles in near-surface regions have been validated with reference samples consisting of self-assembled monolayers of n-octadecanethiol (“C 18-SAM”) adsorbed on gold substrates of different surface roughness. In addition, they have been compared with respect to the effect of surface roughness on the results obtained. The roughness of the substrate surfaces has been studied by STM, AFM, and confocal laser scanning microscopy. Depth sensitivity was achieved by variation of the photoemission angle (ARXPS) and of the excitation energy (“ERXPS”). For the latter, a new data treatment algorithm has been developed in which intensity information acquired for different excitation energies is modelled with hypothetical depth distribution functions of the species studied, taking into account possible depth variations of the inelastic photoelectron mean free path. After identification of a suitable function type for the concentration depth profiles, the optimum profile parameters are determined. For a C 18-SAM adsorbed on an atomically flat Au substrate this method allows to correctly identify the nature of the depth profile and to precisely determine the overlayer thickness and the (zero) gold concentration in it. While ARXPS yielded realistic results for the atomically flat Au substrate as well, it proved to be more affected by surface roughness than ERXPS. ARXPS may even completely fail to detect surface enrichment in carbon whereas ERXPS identified the organic surface layer though with exaggerated thickness. In ERXPS, the depth coordinate is extended by surface roughness, but the back-extrapolation to the external surface to describe the properties of the latter is not affected even for extremely rough surfaces. ERXPS offers, therefore, an attractive potential for the analysis of the outmost surface layer of real materials.