The calculation of STM images, STS spectra, and XPS peak shifts for galena: New tools for understanding mineral surface chemistry

Abstract

Scanning tunneling microscope (STM) images and spectra of galena (PbS) surfaces were calculated using ab-initio methods, not only to interpret experimentally collected STM data, but also to gain insight into changes of the electronic structure at an atomic level that are associated with surface reactions such as heterogeneous oxidation. STM images of the upper valence band of galena show a periodic array of spots which could be matched to either Pb or S. Our calculations show that these spots mainly stem from electronic states with sulfur 3p character near the Fermi level. In addition, images of the mid-band gap and lower conduction band also show states that we calculate as being due to orbitals with lead 6p character. If the galena surface is exposed to air, more and more of the formerly bright sulfur spots appear as dark spots. This agrees with the calculated depletion of the upper valence band electronic density near sulfur sites after adsorbing oxygen atoms to them. Therefore, tunneling microscopy on semiconducting sulfides can not only be understood as a tool of visualizing single atoms, but also a probe for surface sites at an atomic level that can be most easily oxidized. Modeling galena with infinitely wide slabs as opposed to clusters shows significant differences in the oxidation mechanisms. On slabs, suitable for modeling the electronic structure of terraces, the calculations predict that oxygen will be adsorbed on top of the surface, whereas on clusters, portions of which can be used to represent steps and kinks, oxygen was calculated to migrate underneath the surface. XPS spectra were calculated for the S (2s) and S (2p) peaks before and after oxidation in order to show that the conventional way of analyzing experimental XPS spectra in terms of known bulk structures can lead to misinterpretations if specific surface structures, as calculated in this study, are neglected. These calculations are beginning to explain the reactivity of galena in terms of the electronic surface structure of this mineral. They are helping us to understand the mechanism of early stages of the adsorption/oxidation process, especially when combined with experimental tunneling spectroscopy at specific sites.