Theoretical Study of Dispersion Binding of Hydrocarbon Molecules to Hydrogen-Terminated Silicon(100)-2x1

Noncovalent interactions between organic molecules and hydrogen-terminated silicon(100)-2x1 influence surface chemistry and diffusion. To develop our understanding of these interactions, we studied the dispersion binding of two model hydrocarbons, methane and benzene, on hydrogen-terminated silicon(100)-2x1 using a density-functional theory (DFT) technique incorporating dispersion corrections. The corrections were implemented using previously developed carbon-centered potentials (DiLabio, G. A. Chem. Phys. Lett. 2008, 455, 348-353.) and newly developed silicon-centered potentials, designed to correct the well-known erroneous long-range behavior of DFT methods. Our calculations predict that the preferred position for methane binding to the surface occurs over the area between two dimer rows (the gulley) and between two sets of dimers (binding energy (BE) = 2.2 kcal/mol). When the methane is positioned over a dimer row, BE = 1.9 kcal/ mol. Diffusion of methane along the dimer row direction was calculated to be only marginally more facile than diffusion across rows. In the case of benzene, the preferred binding position is over the dimer row with the benzene roughly parallel to the surface and centered over one surface hydrogen (BE = 5.6 kcal/mol). The complex in which the benzene is over the gulley is less strongly bound (BE = 5.3 kcal/mol). Additional higher energy structures were also found. A detailed potential energy surface map reveals that diffusion of benzene in the row direction is more facile than diffusion across dimer rows. The calculated, room-temperature rate constant associated with diffusion along the row is calculated to be 1.7 times larger than that for diffusion in the perpendicular-to-row direction. Our findings show that surface anisotropy and molecule shape contribute to a directional preference for diffusion.