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 struct ure 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 h ave performed a theoretical study of the molecular and dissociative adsorpt ion of dimethyl disulfide on Au(111) as a function of coverage (0.25 less t han or equal to Theta less than or equal to 1), using gradient-corrected de nsity functional (DFT) calculations with a slab geometry. For the dissociat ed state, our results indicate that the hcp hollow site is much less favora ble than the fee site. For the latter site, we find that, because of surfac e gold atom relaxation, the adsorption energy depends strongly on Theta, ch anging from similar to 18 kcal/mol at Theta = 0.25 to similar to3 kcal/mol at Theta = 1. For the bridge site, instead, the adsorption energy is a weak function of Theta, 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 Theta = 1, of simi lar to 13 kcal/mol with respect to the adsorbed molecular state. We also fi nd, however, that the energy of c(4 x 2) structures containing at least two inequivalent CH3S groups per unit cell (with a minimum S-S distance of sim ilar to3.7 Angstrom) is, within the accuracy of our approach, indistinguish able from the pure (root3 x root3) hexagonal structure. Our results suggest that the full solution of this thorny problem will require, also for the s hortest chains, an estimate of the energetic contribution of dispersion for ces that are not included in the DFT calculations.