Hard X-ray photoelectron spectra (HXPES) of bulk non-conductor vitreous SiO2: Minimum linewidths and surface chemical shifts

Abstract

Hard X-ray photoelectron spectra (2200eV to 5000eV photon energies) have been obtained for the first time on a bulk non-conductor, vitreous SiO2, on a high resolution (E/ΔE of 10,000) synchrotron beamline at the Canadian Light Source (CLS). To minimize charging and differential charging, the SiO2 was coated with very thin layers (0.5 to 1.5nm) of Cr metal. The O 1s linewidth obtained at 2500eV photon energy was 1.26eV—the minimum linewidth for SiO2—and in good agreement with that obtained at 1486eV on a Kratos Axis Ultra spectrometer equipped with a magnetic charge compensation system. The Si 1s linewidth of 1.5eV, somewhat broader than that previously obtained at 1486eV on the Si 2p3/2 line of 1.16eV, is mainly due to the much larger inherent Si 1s linewidth (0.5eV) compared to the inherent Si 2p linewidth (<0.1eV). Both linewidths are dominated by the large final state vibrational broadening previously described.The Cr coating produces surface monolayers of interfacial Cr “suboxide” (Cr-subox), Cr metal, and a surface Cr oxide (Cr-surfox). Cr-subox (SiOCr) gives rise to the weak near-surface Si 1s peak, while both oxides give rise to both the weak surface O 1s peak and the Cr 2p oxide peak. Both the O 1s and Si 1s surface peaks are shifted by ∼2eV relative to the large bulk Si 1s and O 1s peaks. The weak Si 1s and O 1s surface peaks along with the Cr 2p oxide peak decrease in intensity greatly as the photon energy increases, due to an increase in inelastic mean free path (IMFP) in SiO2 from 27Å at 1486eV to 90Å at 5000eV. The decrease in intensity of these surface peaks is consistent with the theoretical IMFP's in the literature. The intensity of the weak O 1s and Si 1s peaks, along with the Cr 2p oxide peak, shows that the surface structure gives close to monolayer amounts of Cr-subox, Cr metal, and Cr-surfox. These high-energy spectra should be very useful in the future for bulk analysis of the O species in non-conducting silicate glasses, or surface analysis on non-conducting silicate surfaces and bulk as a function of photon energy.