Application of scanning tunneling microscopy to study adatom diffusion and lateral interactions: Sulfur on Re(0001) at low coverages

Abstract

Scanning tunneling microscopy(STM) imaging was applied to study coverages of less than 0.3 monolayers of sulfur on a rhenium(0001) surface. At coverages near one quarter monolayer sulfur forms a p(2×2) ordered overlayer. Below this coverage islands of p(2×2) ordered sulfur form, indicating an attractive interaction between sulfur atoms at twice the Re lattice spacing. Between these islands the STM images appear noisy. A correlation technique was applied to these images which showed that this noise was actually due to sulfur atoms diffusing on the same time scale as the STM image was acquired. The pair interaction energy between sulfur atoms at twice the Re lattice distance was determined by fitting the results of this correlation technique to an Ising model. The mobility of sulfur on the surface was further investigated by coadsorbing CO. This was found to compress the dilute sulfur overlayer into an ordered structure usually only seen at higher coverages. The transformation was observed by both low energy electron diffraction and STM. An upper limit for the energy required to compress the overlayer was extracted from the adsorption energy of CO. The energy barrier to diffusion was estimated from the average residence time of the diffusing sulfur atoms. These energies compare well with an extended Hückel calculation.