Growth kinetics of decanethiol monolayers self-assembled on Au(111) by molecular beam deposition: An atomic beam diffraction study

A low-energy atom diffraction study of the self-assembly of decanethiol on Au(111) is presented that includes both the growth kinetics and the adsorption energetics for this model system of soft/hard interfaces. Initial dosing on bare gold very rapidly forms the known low-density, lying-down, striped monolayer with a sticking coefficient close to unity. Further dosing results in an intermediate state that lacks long-range order. After an initial ‘induction time’, the final, dense, standing-up c(4×2) phase grows from this intermediate state with a growth rate that is about 500 times slower than that of the striped phase. Our data are consistent with the X-ray results of Eberhardt et al. (A. Eberhardt, P. Fenter, P. Eisenberger, Surf. Sci., 397 (1998) L285) where the growth rate of the c(4×2) phase was studied and found to depend linearly on the pressure at low pressures and high temperatures, changing to a quadratic dependence at higher pressures and lower temperatures. At still higher pressures and lower crystal temperatures, we find that the growth rate pressure dependence weakens substantially, becoming less than linear. In order to understand this behavior, an investigation of the adsorption energetics of thiols on top of the monolayer surface at different points during the growth is made. These results show that in the low- T, high- P regime, the c(4×2) phase grows under a full cover of molecules physisorbed on top of the (decaying) striped phase. This explains the weakening in the pressure dependence mentioned above since, at the limit of growth under a thick overlayer, the growth rate is expected to become pressure-independent.