A UHV STM/STS and ab initio investigation of covellite {001} surfaces

Abstract

Covellite (CuS) {001} surfaces are examined in ultra-high vacuum using atomically resolved scanning tunneling microscopy and spectroscopy (STM/STS), ultraviolet photoelectron spectroscopy (UPS), and low energy electron diffraction (LEED). The layered structure of covellite is predicted to cleave along two possible basal planes based on a bond critical point analysis of the electron density from ab initio periodic calculations. The electronic structures of the two surfaces are expected to differ primarily based on the presence of copper dangling bond states, which have an impact on the interpretation of STM/STS data. The calculated total density of states for the three-dimensional crystal are in good agreement with He I valence band spectra and the calculations indicate that the highest occupied states are of S 3 p character. Atomically resolved STM images show a periodic hexagonal array of surface sites consistent with the measured surface lattice constant using LEED. Atomically resolved STS spectra show n-type rectifying behavior due to the presence of a Schottky barrier and tip modification of the bending of the bulk bands in the sample. This behavior is indicative of a band gap surface which is due to the lack of copper dangling bonds. In this case, high tunneling current sites are assigned to S sites.