STM Investigation of CO Ordering on Pt(111): From an Isolated Molecule to High-Coverage Superstructures

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.