Four different silica modficatioas-tbermally grown SiOz, quartz glass, silicalite a d silica gel-bave been investigated by XPS. Binding energies for Si 2p, Si 2s and 0 1s were obtained by employing different methods of static charge referencing. In any case, a C 1s reference was used as Isual. In another experiment, deposited 20 nm gold particles were used to provide an Au 4f,,, binding energy (BE) reference. The two different charging correction procedures result in essentially identical BE data. Furthermore, the results confirm that different modifications of silica give substantially the same relevant BEs in XPS. Though the investigated samples are characterized by quite different concentrations of silanol groups, no significant effect on the measured BES has been found. On the other hand, the analysed silica samples are observed with quite different Si 2p and 0 1s peaks widths. This effect is discussed in terms of differential charging.A rather extensive compilation of XPS literature BEs of silica modifications is given.
I N T R O D U C T I O NRecently, we proposed a new method for static charge referencing in photoelectron spectroscopy (XPS).' This method is based on a special procedure to provide a calibrant metal deposit on a sample surface. In brief, the reference metal deposit consists of well-defined metal particles which were prepared as a colloidal metal dispersion. This dispersion is dropped onto the sample surface. Afterwards, the liquid is removed by evaporation, leaving calibrant metal particles on the sample surface.In this paper we present new results obtained by employing the static charge referencing method described above. At first we checked the suitability of the colloidal gold particles used in our laboratory for static charge referencing in XPS analysis of nonconducting samples. This check has been done by employing an electron spectrometer giving a substantially smaller full width at half-maximum (FWHM) for the Au 4f,/, emission compared to the results presented previously. Afterwards, we investigated various modifications of amorphous SiO, and one crystalline modification. All these samples contain silanol groups, which may complicate the interpretation of the XPS data. As a result, we present static charge-corrected binding energies (BE) of Si 2p, Si 2s and 0 1s photopeaks by employing an Au 4f7/, as well as a C 1s reference. We feel that these data are of interest because SiO, is a t Paper presented at ECASlA 91, Budapest, Hungary.Author to whom correspondence should be addressed.technologically very important material which has often been investigated by means of XPS. However, the scatter of the respective BE data is rather too high (e.g. the Si 2p BE scatters over 1.35 eV; cf. Table 1).Finally, we observed an interesting phenomenon with the FWHM of the observed Si photopeaks which can be understood in terms of the differential charging discussion introduced by Barr.'
EXPERIMENTAL Equipment and procedureThree different modifications of amorphous SiO, (silica gel, a thermally oxidized...
