XPS studies and factoranalysis of PbS nanocrystal-doped SiO2 thin films

PbS nanocrystal-doped SiO2 glass thin films and a single evaporated PbS film were studied by X-ray photoelectron spectroscopy (XPS) and factor analysis (FA). The composite films contained PbS crystallites with mean dimensions as small ELS for vibrational as 1-2 nm dielectrically embedded in the SiO2 host. Carrying out XPS measurements, film compositions, charging, chemical bonding and size-related effects were investigated. Electrostatic charging was observed in the composite films decreasing opposite to the PbS content and was corrected using the Si2p peak energy of SiO2 as an internal standard. With decreasing mean crystallite size the valence band edge is shifted in correspondence to the optical bandgap energies of the samples. In addition, peak shifts of sulphur and lead core levels are observed and evaluated by FA. From the analysis of the S2s spectra, a spectrum of a surface like species is extracted being 1.8 eV shifted in peak position to the PbS bulk line. Although some oxygen is bonded to the lead atoms Pb-S-0 compounds can be excluded by FA of concatenated spectra. As the samples were ex situ prepared their surfaces had to be sputter cleaned before the XPS investigations. Studying sputtering effects in XPS profile spectra we found out that Pb-S bonding can be considered as resisting sputtering with the crystallites while the Pb-0 bonding is less stable under our sputtering conditions.