Chemical Imaging of Terrace-Based Active Sites on Gold

Scanning tunneling microscopy data of a mixed monolayer comprised of a 40:60 ratio of H ₈Si ₁₈O ₁₂ and C ₆H ₁₃-H ₇Si ₈O ₁₂ clusters on gold are presented. The images display a composite monolayer surface with well-defined domain regions of the individual components. Holes present at face-centered cubic (fcc) sites of the starting Au/H ₇Si ₈O ₁₂ adsorbate layer indicate the location of active sites for impinging C ₆H ₁₃-H ₇Si ₈O ₁₂ clusters. Adsorption of a C ₆H ₁₃-H ₇Si ₈O ₁₂ cluster likely yields a mobile hydrogen atom available to recombine with and desorb an adjacent H ₈Si ₈O ₁₂ cluster. Hydrogen atom diffusion along substrate directions is the proposed pattern formation mechanism of the mixed monolayer. Imaging of the spherosiloxane cluster domains identifies a novel terrace-based active site located in the fcc regions of the Au(111) 23 × √3 surface reconstruction.