A low energy helium atom diffraction study of decanethiol self-assembled on Au(111)

We report the results of low energy helium atom diffraction studies of the surface of decanethiol self-assembled on the (111) face of single crystal gold. Our survey of 13 separate samples indicates that for decanethiol the standard self-assembly methodology results in monolayers of widely varying quality. We observe that surface order is generally enhanced by preparing monolayers in micromolar solution as compared to the usual millimolar preparation [M.D. Porter et al., J. Am. Chem. Sec. 109 (1987) 3559; C.D. Bain et al., J. Am. Chem. Sec. 111 (1989) 321]. This result is in agreement with a parallel study conducted using grazing-incidence X-ray diffraction (GIXD). Particular attention is focused on two exceptionally high-quality samples which show more clearly than ever before the c(4 root 3 x 2 root 3)R30 degrees diffraction pattern first observed for octadecanethiol monolayers [N. Camillone III et at, J. Chem. Phys. 98 (1993) 3503]. The diffraction patterns observed for these samples and their structural implications are discussed in light of recent advances made in other laboratories. We also discuss the results of thermal annealing experiments. (C) 1997 Elsevier Science B.V.