Coverage-dependent adsorption of CH₃S and (CH₃S)₂ on Au(111): A density functional theory study

The origin of the superlattice present in the commensurate hexagonal structure of self-assembled monolayers of n-alkanethiols on gold and the question of whether the thiols are adsorbed onto the surface as dimers (disulfides) or monomers (thiolates) have been under debate for many years. Looking for a better understanding of the structural properties of these systems, we have performed a theoretical study of the molecular and dissociative adsorption of dimethyl disulfide on Au(111) as a function of coverage (0.25 ≤ Θ ≤ 1), using gradient-corrected density functional (DFT) calculations with a slab geometry. For the dissociated state, our results indicate that the hcp hollow site is much less favorable than the fcc site. For the latter site, we find that, because of surface gold atom relaxation, the adsorption energy depends strongly on Θ, changing from ∼18 kcal/mol at Θ = 0.25 to ∼3 kcal/mol at Θ = 1. For the bridge site, instead, the adsorption energy is a weak function of Θ, and for all investigated coverages, this site is by far the most stable. According to our DFT approach, the adsorption of dimethyl disulfide is dissociative with a thermodynamic gain, at Θ = 1, of ∼13 kcal/mol with respect to the adsorbed molecular state. We also find, however, that the energy of c(4 × 2) structures containing at least two inequivalent CH₃S groups per unit cell (with a minimum S-S distance of ∼3.7 Å) is, within the accuracy of our approach, indistinguishable from the pure (√3 × √3) hexagonal structure. Our results suggest that the full solution of this thorny problem will require, also for the shortest chains, an estimate of the energetic contribution of dispersion forces that are not included in the DFT calculations.