Density functional calculation of a potential energy surface for alkane thiols on Au(111) as function of alkane chain length

Density functional theory calculations of alkane thiols on Au(1 1 1) have been carried out as a function of the alkane chain length from 1 to 12 carbons using a slab geometry with periodic boundary conditions. Geometry optimized structures were obtained and the potential energy was calculated as a function of the binding site and distance from the surface. While the binding site of minimum energy is only subtly different for different chain lengths there is a pronounced difference in the Au-S bonding and side chain interactions of the methyl group of methane thiol compared to ethane thiol or any longer alkane thiol self-assembled monolayer. For example, the barrier to lateral motion along the surface is lowest for motion through fee sites for methane thiol while the twofold bridge sites present the lowest barrier to ethane thiol and all longer alkane thiols. The hexagonal close-packed (hcp) and face-centered cubic (fcc) threefold sites were found to differ in energy by less than 1 kJ/mol and therefore only fee sites were considered in the study. According to the calculated potential energy surfaces the top sites have weaker binding than either the bridge or threefold sites in all cases. The calculations show that ethane thiol is a reasonable model for longer alkane chains. (C) 2003 Published by Elsevier B.V.