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 ab
initio methods we investigate the adsorption of sulfur (S) on the Au(111) surface
for different coverages between 0.25 and 1.0 monolayers (ML). Corresponding
geometries with adsorbed Se are included to establish possible differences between
S- and Se-based metal–molecule interfaces. We furthermore investigate
hydrogenation of sulfur-covered Au(111) surfaces to establish the energetics
and 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 0.25
ML the S and Se atoms are found to prefer the in-hollow sites, with Se
displaying a substantially stronger bond. Increasing the coverage leads to
depletion of available free charge in the gold surface, which weakens the
bonds to the S (Se). Due to the more extensive hybridization, Se is more
insensitive to the exact geometry, and the stacking fault position only
costs 0.04 eV. At even higher coverage (0.75 ML) the adsorbed atoms
hybridize internally and form triatomic molecules situated on top of the
Au surface atoms. In S (Se) rich environments this turns out to be the
most stable configuration investigated, while in S (Se) poor conditions
the surface will adsorb all available S (Se). Forcing the system to adsorb
atoms beyond this coverage increases the total energy. For all physically
realizable coverages the Au–Se bond is found to be ≥0.25 eV
stronger than the corresponding Au–S bond. The Se bond also displays a higher
degree of metallicity and should be expected to make a better head group for
thiols, for example; this is relevant for both bonding and conductivity. Turning to
the hydrogenated S systems we find that surfaces with a high coverage of S only
weakly bind H at low partial hydrogenation, while H adsorption in systems with
medium and low S concentrations is found to be energetically stable by around 0.3
eV 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.
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