The Structure of Atomic Sulfur Phases on Au(111)

The structural phases formed by atomic sulfur on Au(111) due to reaction with molecular S 2 have been investigated by qualitative low-energy electron diffraction (LEED), scanning tunneling microscopy, and normal incidence X-ray standing wavefield absorption (NIXSW) combined with X-ray photoelectron spectroscopy (XPS). Three phases are identified with increasing coverage, namely, a newly identified (5 × 5) phase, a ( × )R30° phase, and a “complex” phase. The (5 × 5) phase, with a LEED pattern having the appearance of a “split-spot” ( × )R30° pattern, is interpreted in terms of local ( × )R30° ordering within a (5 × 5) ordered domain structure. The S atoms in the (5 × 5) phase occupy fcc hollow sites 1.56 Å above the outermost extended Au(111) bulk atomic scatterer plane. A specific model of the ordering in this phase is proposed that, together with the observed marginal stability of the true, long-range-ordered, ( × )R30° phase, indicates significant short-range S−S repulsion and probably compressive surface stress. The complex phase, that coexists in a poorly ordered state with the lower coverage atomic chemisorption phases, is interpreted in terms of an incommensurate long-range periodicity, but the NIXSW data shows clear evidence of local commensuration, with the S atoms mainly close to atop sites relative to the underlying Au(111) substrate; these data provide strong support for a previously proposed model based on a sulfide layer of stoichiometry AuS.