Thermochemistry on the hydrogenated diamond (111) surface

As part of our effort to control the growth of diamond films by chemical vapor deposition, we are studying the chemical mechanism for conversion of gas phase hydrocarbons into diamond. In this work we analyze the thermochemistry of a number of structures on the hydrogenated diamond (111) surface. We use the MM2 molecular mechanics force field to calculate strain energies, which are due to crowding of adsorbed species on the surface, and we use a group additivity scheme to estimate bond enthalpies and entropies. These data allow calculation of equilibrium structures on the surface and, together with estimates for rate constants, will permit a prediction for the kinetics of diamond formation as a function of growth conditions. We find that a straightforward abstraction/addition mechanism using either CH3 or C2H2 to grow on a hydrogenated (111) surface cannot account for experimentally measured growth rates. We suggest that experimental measurements of growth rates on (111) surfaces are strongly influenced by growth at steps, kinks, and edges on those surfaces.