GRAPHITIZATION OF DIAMOND(111) STUDIED BY FIRST PRINCIPLES MOLECULAR-DYNAMICS

Large scale first principles numerical simulations, performed on moder n massively parallel computers, can be usefully applied to study the p hysics of semiconductor surface and interface systems. We report on a recent study of the surface-initiated diamond to graphite structural t ransition of crystalline carbon. Our investigation consisted of a seri es of fully ab initio molecular dynamic simulations of the diamond C(1 11)-(2 x 1) surface, with cells containing from 200 to 300 atoms. We o bserved a spontaneous graphitization of the surface, followed by a fas t graphitization of the entire diamond slab, at temperatures above 250 0 K. We find that the transition starts at the reconstructed surface l ayer and rapidly proceeds into the bulk region by highly correlated br eaking of z-oriented diamond bonds, We identify a precursor seed to th e structural transformation, and in particular we obtain a non abrupt graphite-diamond interface forming prior to the transition. This inter face is characterised by a regular alternation of three- and four-fold coordinated atoms along the [110] direction at the convex corner of t he phase boundary. Local density of states (LDOS) analysis reveals the presence of chemically active sites at the interface region. Our resu lts are in agreement with experiments on the thermal behaviour of diam ond (111), confirm early measurements about surface induced graphitiza tion of diamond, and bear important implications to the formation proc ess of graphite islands in chemical vapor deposited (CVD) diamond film s. In particular, we discuss the role of surface dangling bonds as che misorption sites for atomic hydrogen, in relation to the stabilisation of CVD-grown diamond films by selective etching.