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

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Abstract
The structure of sulfur adsorbed on the basal plane of rhenium has been studied in ultrahigh vaccum with Auger, low-energy electron diffraction, and scanning tunneling microscopy. At low coverages ( < 0.25 monolayers) single sulfur adatoms form open structures with p(2X2) and c(square-root 3X5) 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 square-root 3 x 3 square-root 3) R 30-degrees 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 (13(31)) structure forms. With further exposure to H-2S a final structure is formed, consisting of hexagonal sulfur rings in a (2 square-root 3 x 2 square-root 3) R 30-degrees 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.

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