On the structure and evolution of the buried S/Au interface in self-assembled monolayers: X-ray standing wave results

We describe a structural study of the S/Au interface for decanethiol monolayers (C10) on a Au(111) surface using the technique of X-ray standing waves (XSWs). The XSW results for full-coverage monolayers are inconsistent with any model incorporating a single sulfur adsorption site, such as the widely assumed threefold hollow site on the Au(111) surface. Instead, the XSW results reveal two distinct sulfur head group sites, each with a distinct lateral and vertical location with respect to the underlying gold lattice. We discuss structural models that are consistent with these results. We have also studied the evolution of the structure versus coverage with XSW and X-ray photoelectron spectroscopy (XPS) and have determined that the local S/Au interface structure of the "lying down" striped phase at low coverages (0.27 ML, 1 ML = 4.62 × 10¹⁴ molecules cm^-2) is indistinguishable from that of the "standing up" c(4 x 2) phase at saturation (1 ML). Some important implications concerning the structure and growth of these monolayers are discussed.