Surface charging of insulating samples in X-ray photoelectron spectroscopy

Electrical charging of insulating samples during XPS is of direct concern for referencing the binding energy scale. Results are presented on charging of composite insulating samples that consist of an organic overlayer of polystyrene (PS) or polydimethylsiloxane (PDMS) deposited on NaCl particles. The samples were analysed using small-spot monochromatized radiation, the surface charge being stabilized with an electron flood gun. The charging shift of the NaCl substrate is influenced by three different factors: the total photoelectron flux, the energy of the electrons produced by the flood gun and the flux of electrons originating from the overlayer. The overlayer is acting as an additional electron source, which makes the substrate more negative and reduces inhomogeneities of the local potential affecting the peak full width at half-maximum. Comparison of substrate peak intensities with computed values showed that PS overlayers were less uniform compared to PDMS overlayers. The overlayer thickness and uniformity affect both the total photoelectron flux and the flux of electrons from the overlayer to the substrate. Overlayer charging, like substrate charging, is probably determined by the balance of electron fluxes in the phase considered. The kinetic energy of the photoelectron is determined by the electrical potential in the phase where photoemission occurs and not with respect to the sample surface. This was demonstrated by differential charging between the NaCl substrate and PS overlayer. (C) 1998 John Wiley & Sons, Ltd.