No abstract

Carbon monoxide (CO) adsorbed on Pt(111) has been extensively studied as a model catalyst. However, there remain some ambiguities in overlayer structures, particularly regarding bridge-site occupation. Here, we report real-space observations of CO on Pt(111) using scanning tunneling microscopy (STM) under ultrahigh vacuum at a cryogenic temperature, from a single CO adsorbed on the atop site to the gradual development of overlayer structures including atop-dominant (√3 × √3)R30°islands, c(4 × 2) domains, and 1 × 1 boundaries in c(4 × 2)-2CO domains. Bridge CO appear at the edge of (√3 × √3)R30°islands, significantly in the center of local c(√3 × 2)rect geometry which is equivalent to c(4 × 2)-2CO. In the c(4 × 2) domain including bridge vacancies, the height of the atop CO in the STM image is modulated according to the number of adjacent bridge CO, which implies the interadsorbate interaction between two different adsorption species. The real space observation presented here not only resolves ambiguities about overlayer structures but also describes an atop-bridge interadsorbate interaction.

Using a combination of scanning tunneling microscopy and density functional theory (DFT) calculations, we have identified a set of related Au-S complexes that form on Au(100), when sulfur adsorbs and lifts the hexagonal surface reconstruction. The predominant complex is diamond-shaped with stoichiometry Au4S5. All of the complexes can be regarded as combinations of S-Au-S subunits. The complexes exist within, or at the edges of, p(2 × 2) sulfur islands that cover the unreconstructed Au regions, and are observed throughout the range of S coverage examined in this study, 0.009 to 0.12 monolayers. A qualitative model is developed which incorporates competitive formation of complexes, Au rafts, and p(2 × 2) sulfur islands, as Au atoms are released by the surface structure transformation.