Ab initiomolecular-dynamics studies of the graphitization of flat and stepped diamond (111) surfaces

We present ab initio molecular-dynamics studies of the graphitization of flat and stepped diamond (111) surfaces. Fully self-consistent calculations were performed within the local-density-functional approximation with ultrasoft pseudopotentials and periodic boundary conditions. We investigated the effects of the system size, heating rate, and other computational parameters. If an initially reconstructed flat diamond (111) surface is heated as slowly as possible, graphitization (within some ps) is observed only at temperatures between 3500 and 3900 K, i.e., when the average kinetic energy of the atoms is comparable to the height of the barrier for the phase transition. A step on the surface has no significant influence on the graphitization temperature when the Pandey chains of the $(2\mathrm{}\times 1)$-reconstructed surface are running parallel to the step. But if this is not the case, i.e., if the reconstruction has a rotated domain such that the step cuts across the Pandey chains, the graphitization temperature is lowered to 2500 K.