A Comprehensive Study of the Bridge Site and Substrate Relaxation Asymmetry for Methanethiol Adsorption on Au(111) at Low Coverage

We
use dispersion-corrected density functional theory to explore
the bridge-site asymmetry for methanethiol adsorbed on Au(111) with
two different S–C bond orientations. We attribute the asymmetry
to the intrinsic character of the Au(111) surface rather than the
adsorbate. The preference for bridge-fcc versus bridge-hcp SCH3 adsorption sites is controlled by the S–C bond orientation.
The system energy difference favors the bridge-fcc site by 8.1 meV
on the unrelaxed Au(111) surface. Relaxing the Au substrate increased
this energy difference to 26.1 meV. This asymmetry is also reflected
in the atomic displacement of the relaxed Au surface. Although in
both cases, the bridge-site Au atoms shift away from the fcc 3-fold
hollow site, the motion is greater for the bridge-fcc allowing a more
favorable geometry for the sulfur atom to bond to the bridging atoms.
We confirm that the adsorption energy is strongly dependent on the
S–C bond orientation and position, which can be understood
in terms of a simple coordination geometry model. This work has important
implications for alkanethiol surface diffusion and the structure of
their self-assembled monolayers.