Adsorption of hydrocarbon species on a stepped diamond (111) surface

The process of adsorption of H, CH₃, CH₂, C₂H, and C₂H₂, on various steps on the diamond (111) surface, has been investigated theoretically. The first-principles density-functional theory was then used in order to calculate the binding energies and equilibrium configurations for these species near or at the different steps. The adsorption energies were found to be in the order C₂H>H>CH₂>CH₃>C₂H₂ for the lower (111) terraces, as well as for the (111) edges with the directions [-2,1,1], [1,-2,1], and [1,1,-2], respectively. This was also the result for the (100) edges with the directions [-1,2,-1], [-1,-1,2], and [2,-1,-1], respectively. The order of adsorption energies were, however, found to be different for the diradicals formed by abstracting two hydrogens from the different type of steps. In particular, the adsorption energy of C₂H₂ was numerically larger than the adsorption energy of CH₃. For these diradical surface sites, the increase in adsorption energy is due to a C-C bonding interaction between the adsorbant and one C atom at the step. This implies that C₂H₂ adsorption at specific sites on the growing diamond (111) surface will be energetically competitive with CH₃ adsorption.