Molecular alligator clips: a theoretical study of adsorption of S, Se and S–H onAu(111)

Abstract

For the binding of thiols to Au, the Au–S interaction is decisive for the geometry,bonding strength and transmissivity of the metal–molecule interface. Using abinitio methods we investigate the adsorption of sulfur (S) on the Au(111) surfacefor different coverages between 0.25 and 1.0 monolayers (ML). Correspondinggeometries with adsorbed Se are included to establish possible differences betweenS- and Se-based metal–molecule interfaces. We furthermore investigatehydrogenation of sulfur-covered Au(111) surfaces to establish the energeticsand resulting geometry of adsorption of S–H groups on clean Au(111),using it as a simple model system. For the relatively low coverage of 0.25ML the S and Se atoms are found to prefer the in-hollow sites, with Sedisplaying a substantially stronger bond. Increasing the coverage leads todepletion of available free charge in the gold surface, which weakens thebonds to the S (Se). Due to the more extensive hybridization, Se is moreinsensitive to the exact geometry, and the stacking fault position onlycosts 0.04 eV. At even higher coverage (0.75 ML) the adsorbed atomshybridize internally and form triatomic molecules situated on top of theAu surface atoms. In S (Se) rich environments this turns out to be themost stable configuration investigated, while in S (Se) poor conditionsthe surface will adsorb all available S (Se). Forcing the system to adsorbatoms beyond this coverage increases the total energy. For all physicallyrealizable coverages the Au–Se bond is found to be ≥0.25 eVstronger than the corresponding Au–S bond. The Se bond also displays a higherdegree of metallicity and should be expected to make a better head group forthiols, for example; this is relevant for both bonding and conductivity. Turning tothe hydrogenated S systems we find that surfaces with a high coverage of S onlyweakly bind H at low partial hydrogenation, while H adsorption in systems withmedium and low S concentrations is found to be energetically stable by around 0.3eV per H atom. The adsorption geometry is sensitive to the concentration:exposed to free S, the system will increase the S coverage and expel H.