Self-Assembled Monolayers on Au(111): Structure, Energetics, and Mechanism of Reconstruction Lifting

Density-functional theory calculations are used to compare competing structural models for SH molecules on Au(111) and to explain the effect of the adsorbed molecules on the energetics of surface reconstruction. Our results indicate that dumbbells, formed by binding two SH molecules to an adatom, are energetically stable structures at low and intermediate coverages, while the recently proposed c(4 X 2) structure with partial occupancies for adatom and vacancy sites [A. Cossaro et al., Science 2008, 321, 943] is preferred at full coverage. Lifting of the Au(111) herringbone reconstruction is attributed to reduction of surface stress due to outward in-plane relaxations of surface Au atoms under the adsorbed species. The smallest reduction of surface stress per molecule occurs when SH molecules are adsorbed in the form of dumbbells, while isolated adatoms and SH molecules have similar effects on the surface stress and reconstruction energy. SH molecules are predicted to energetically favor the fcc region of the reconstruction. We find quantitative correlation between the directly calculated reconstruction energies and changes in surface stress; this relation covers adatoms, SH molecules, and dumbbells on equal footing.