Scanning tunneling microscopy observations of butanethiol self-assembled monolayers on Ag underpotential deposition modified Au(111)

The thermal stability of self-assembled monolayers (SAMs) of alkanethiols on gold substrates can be enhanced by underpotential deposition (upd) of a silver interlayer. In this study butanethiol SAMs prepared on Ag upd-modified Au(111) are examined in air by scanning tunneling microscopy (STM). When the upd layer is prepared at a potential of 460 mV (vs EAg+/0), the surface exhibits atomically smooth terraces, distinctly different from pit features of alkanethiol SAMs on bare Au. When the upd layer is prepared at potentials just prior to bulk deposition, butanethiol SAMs show protrusions with ca. 0.09 nm in height. Molecularly resolved STM images show that the nearest-neighbor spacing is 0.50 +/- 0.02 nm, the same as long-chain alkanethiol SAMs on Au(111). However, the film exhibits a hexagonal lattice structure with a height modulation, distinctly different from that of butanethiol SAMs on bare Au(111) but similar to that, of decanethiol monolayers on Ag(111). The similarity in structure demonstrates the effect of Au substrates and Ag interlayers on the formation of butanethiol SAMs.