Diffusion of 1,4-butanedithiol on Au(100)-(1×1) : A DFT-based master-equation approach

The functionalization of metal surfaces via thiol-bonded molecules and the assembly of nanodevices on the surfaces should profit from a detailed atomistic understanding of the binding and diffusion properties. These differ substantially from the in-depth investigated situation of single adatoms. We report density-functional calculations for the elementary diffusion steps of 1,4-butanedithiol radicals (BDTRs) adsorbed on a Au(100)-(1×1) surface. The elementary diffusion steps are then combined into a description of the diffusion mechanism on long-time scales by integrating a master equation. The two S-Au bonds cause a multivalley potential-energy surface, which implies a complex diffusion mechanism. We identify the effect of the geometry constraints imposed by the (CH₂)₄ backbone on binding and diffusion. To this purpose we compare to the diffusion of a single SCH₃ radical on the same Au(100)-(1×1) surface. Altogether, the motion of the BDTR is walkinglike with the S-atoms crossing bridge sites of the Au surface one after the other. The lowest density-functional theory-Perdew and Wang 91 energy barrier for translation is 0.35 eV while the energy barrier for rotation comes out larger, 0.43 eV. As a result of this difference there will be correlations between subsequent diffusive displacements of the molecule. The isotropic diffusion constant on long-time scales is computed and the numerical results follow an Arrhenius law.