Scanning tunneling microscopy studies of sulfur overlayers on the Re(0001) surface

The structure of sulfur adsorbed on the basal plane of rhenium has been studied in ultrahigh vacuum with Auger, low-energy electron diffraction, and scanning tunneling microscopy. At low coverages (<0.25 monolayers) single sulfur adatoms form open structures with p(2×2) and c( 7/8 ×5) rectangular lattices. In these structures both the sulfur adatoms and some of the rhenium substrate atoms are imaged simultaneously, identifying the sulfur adsorption site as a threefold hollow site. At coverages above 0.25 ML sulfur adatoms aggregate into trimers, first disordered and then ordered into a (3 7/8 ×3 7/8 )R30° lattice at a coverage of 0.45 ML. Two types of trimers form that are centered around different substrate sites. Between 0.45 and 0.5 ML diamond-shaped sulfur tetramers appear until an ordered (3113) structure forms. With further exposure to H2S a final structure is formed, consisting of hexagonal sulfur rings in a (2 7/8 ×2 7/8 )R30° lattice. All of the sulfur adatoms in the three high coverage structures occupy the same type of threefold hollow sites as in the low coverage structures.