Studying of quantum-size effects origination in semiconducting lead sulfide nanocrystals

Lead sulfide (PbS) crystals with sizes from 20 to 500 nm were deposited in chemical bath from an alkaline solution (CBD method). The morphology of specimens was studied using high resolution scanning electron microscopy (HRSEM). Influence of crystallite sizes on the electronic structure was studied with X-ray photoelectron spectroscopy (XPS) and high resolution electrons energy losses spectroscopy (HREELS). The work function was measured with a Kelvin probe microscopy in air. The photoelectron doublet peaks at spectra of Pb 4f donor and S 2p acceptor were found to be shifted toward the higher binding energies comparing to the corresponding lines positions in the reference PbS compound. This shift increases with decreasing of the crystals size. The effect of size shift in lead sulfide could be noticed when size is smaller than 300 nm. HREELS showed that dispersion of nanoparticles causes smoothing of the PbS band gap in different directions of reciprocal lattice, and the minimal transition energy increases from 0.39 to 3.62 eV. The work function of the material is shown to be in inverse proportion to the semiconductor crystal size. This data correlates well with the electron spectroscopy results.