Mobility of thiolates on Au(111) surfaces

Self-assembled monolayers (SAMs), especially those based on thiol containing compounds on gold, are of both practical and fundamental interest. Thiols and thiolates can bind to gold in several ways due to the presence of holes and edges on the surfaces. The variety of binding motifs is increased by the presence of adatoms, i.e. gold atoms present on the surface, that sit between the thiolate and the surface. Although these motifs bind strongly to gold surfaces, they are sufficiently mobile to allow for self-assembly of thiols, either by movement across the surface or by desorption/re-adsorption. The motifs have been investigated primarily in the context of high surface coverage, with some attention given to the mobility of these motifs. Here we focus on the binding in the low-coverage regime, i.e. the initial stage of SAM formation, using theoretical methods. We determine the relative stability of the motifs formed with methane thiolate in the low-coverage regime, and rationalize their relative mobilities. Methane thiolate is used to minimize contributions of intermolecular interactions. Competition between the rates of adsorption, movement, and formation of the motifs can influence the formation of SAMs. In this work we expand the understanding of the early stages of monolayer formation and conclude that the type of motif formed initially depends strongly on the availability of gold adatoms and defects (edges and holes) on the surface at the point of adsorption.