Surface charging of insulating samples in x‐ray photoelectron spectroscopy

Abstract

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 Ñood gun. The charging shift of the NaCl substrate is inÑuenced by three di †erent factors : the total photoelectron Ñux, the energy of the electrons produced by the Ñood gun and the Ñux 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 a †ecting 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 a †ect both the total photoelectron Ñux and the Ñux of electrons from the overlayer to the substrate. Overlayer charging, like substrate charging, is probably determined by the balance of electron Ñuxes 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 di †erential charging between the NaCl substrate and PS overlayer.

New method to correct for the influence of organic contamination on intensity ratios in quantitative XPS

“…Concerning the samples with PS, no contamination was detected in the overlayers but the overlayer uniformity was shown to be poor (U D 0.1-0.01). 21 Nevertheless, the slope in Fig. 8 is close to the I o Na2s /I o Na1s value determined with the blanks ( Table 2).…”

“…The analysis of blank samples prepared without trichloroethylene and with different amounts of trichloroethylene added ruled out the solvent as the origin of this contamination. 21 The apparent thickness of overlayers was estimated by comparing the experimental I Na1s /I Na2s and I Na1s /I C1s intensity ratios with peak intensities computed by supposing uniform overlayers (U D 1) and taking the influence of surface roughness into account. 21 The values obtained cover the ranges shown in Table 1.…”

“…Overlayers of PDMS and adventitious contamination can be considered reasonably to present the degree of uniformity required by the method proposed (U D 0.4-1) whereas PS overlayers were shown to be significantly less uniform than PDMS overlayers (U D 0.1-0.01). 21 This may be related to the high glass transition temperature of PS (373 K) compared to PDMS (148 K for silicone grease).…”

“…Details on the preparation of samples and XPS analysis can be found in a previous work. 21 Dried NaCl powder (Union Chimique Belge) was freshly crushed and pressed into stainless-steel troughs. The troughs were then placed in an oven at 110°C, where freshly prepared solutions of the overlayer material were added dropwise with a glass pipette.…”

“…Penetration of the solution inside the sample helped to evacuate the excess of overlayer material from the surface and to obtain thin layers on top of the sample. 21 As material for the overlayers, high-molecular-weight molecules were selected to guarantee sample stability under vacuum and during XPS analysis: polydimethylsiloxane (PDMS, high-vacuum grease from Dow Corning) dissolved in trichloroethylene (Merck, p.a. ); and polystyrene (PS; Petri dishes from Falcon) dissolved in boiling cyclohexane (Merck, p.a.).…”