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

Covellite (CuS) {001} surfaces are examined in ultra-high vacuum using atom ically resolved scanning tunneling microscopy and spectroscopy (STM/STS), u ltraviolet photoelectron spectroscopy (UPS), and low energy electron diffra ction (LEED). The layered structure of covellite is predicted to cleave alo ng two possible basal planes based on a bond critical point analysis of the electron density from ab initio periodic calculations. The electronic stru ctures of the two surfaces are expected to differ primarily based on the pr esence of copper dangling bond states, which have an impact on the interpre tation of STM/STS data. The calculated total density of states for the thre e-dimensional crystal are in good agreement with He I valence band spectra and the calculations indicate that the highest occupied states are of S 3p character. Atomically resolved STM images show a periodic hexagonal array o f 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 o f the bulk bands in the sample. This behavior is indicative of a band gap s urface which is due to the lack of copper dangling bonds. In this case, hig h tunneling current sites are assigned to S sites. (C) 1999 Elsevier Scienc e B.V. All rights reserved.