Density functional calculations of diffusion paths of CH₃S_ad on c(2 × 2)-Cl and -Br covered Cu(100) surfaces

Identification of the atomic-scale mechanisms of surface diffusion at interfaces covered by co-adsorbates is relevant for understanding electrochemical processes at these interfaces. The surface dynamics of CH₃S_ad on c(2 × 2)-Cl covered Cu(100) surfaces has been studied with video-STM in electrochemical environment by Yang, Taranowski, and Magnussen [Langmuir, 2012, 28, 14143]. We present density functional calculations to predict diffusion paths and energy barriers of CH₃S_ad substitutionally adsorbed on c(2 × 2)-Cl or -Br covered Cu(100) surfaces and compare them to the case of S_ad. Additional vacancies in the halogen adlayer enable further diffusion paths with significantly lower DFT energy barriers (i.e. energy barriers in case of uncharged surfaces vs. vacuum). We argue that at least in case of Cl-covered surfaces this preference for vacancy-assisted diffusion of CH₃S_ad persists when the energy for creating a Cl-vacancy is accounted for. However, we have not yet been able to include the effect of the electric field on the computed energy barriers for this system, which might affect the preferred diffusion mechanism.