Size shift of XPS lines observed from PbS nanocrystals

Crystals of PbS with different sizes were obtained by chemical deposition from aqueous solution (chemical bath deposition technique) onto undoped Si (100) substrate. The morphology and size of crystals were analyzed by high resolution scanning electron microscopy. The fine structure of Pb 4f and S 2p photoelectron lines was measured by X-ray photoelectron spectroscopy with monochromatic Al Kα source (hv = 1486.6 eV). The phenomenon of size shift, i.e. growth of binding energies for both donor Pb 4f and acceptor S 2p photoelectron lines with decreasing nanocrystals' size, was found and quantified as a function of nanocrystals' size. The size shift values of the Pb 4f _5/2 and 4f_7/2 lines were determined for nanocrystals with a size from 20 to 250 nm. The interband and intraband electronic transitions were investigated by high resolution electron energy losses spectroscopy. It was shown that width and spatial anisotropy of the band gap depend on the size of PbS nanocrystals. The dependence of size shift from nanocrystals' size, the nature of this phenomenon, and a role of changes in fine structure of electronic states near the band gap are discussed in the report.