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 m onolayers (C10) on a Au(111) surface using the technique of X-ray stan ding waves (XSWs). The XSW results for full-coverage monolayers are in consistent with any model incorporating a single sulfur adsorption sit e, such as the widely assumed threefold hollow site on the Au(111) sur face. Instead, the XSW results reveal two distinct sulfur head group s ites, 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 t he structure versus coverage with XSW and X-ray photoelectron spectros copy (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 x 10(14) molecules cm(-2)) is indistinguishable from that of the ' 'standing up'' c(4 x 2) phase at saturation (1 ML). Some important imp lications concerning the structure acid growth of these monolayers are discussed. (C) 1998 Published by Elsevier Science B.V. All rights res erved.