Graphitization of diamond (111) studied by first principles molecular dynamics

Large scale first principles numerical simulations, performed on modern massively parallel computers, can be usefully applied to study the physics of semiconductor surface and interface systems. We report on a recent study of the surface-initiated diamond to graphite structural transition of crystalline carbon. Our investigation consisted of a series of fully ab initio molecular dynamic simulations of the diamond C(111)-(2 sx 1) surface, with cells containing from 200 to 300 atoms. We observed a spontaneous graphitization of the surface, followed by a fast graphitization of the entire diamond slab, at temperatures above 2500 K. We find that the transition starts at the reconstructed surface layer and rapidly proceeds into the bulk region by highly correlated breaking of z-oriented diamond bonds. We identify a precursor seed to the structural transformation, and in particular we obtain a non abrupt graphite-diamond interface forming prior to the transition. This interface is characterised by a regular alternation of three- and four-fold coordinated atoms along the [110] direction at the convex corner of the phase boundary. Local density of states (LDOS) analysis reveals the presence of chemically active sites at the interface region. Our results are in agreement with experiments on the thermal behaviour of diamond (111), confirm early measurements about surface induced graphitization of diamond, and bear important implications to the formation process of graphite islands in chemical vapor deposited (CVD) diamond films. In particular, we discuss the role of surface dangling bonds as chemisorption sites for atomic hydrogen, in relation to the stabilisation of CVD-grown diamond films by selective etching.