ON THE GRAPHITIZATION OF DIAMOND SURFACES - THE IMPORTANCE OF TWINS

Ab initio total energy calculations reported recently (M.I. Heggie, C. D. Latham, R. Jones and P.R. Briddon, Phys. Rev. B, 50 (1994) 5937) re vealed that the tetrahedrally bonded icosahedral C-100 molecule decomp osed spontaneously into two concentric fullerenes (C-20 and C-80). Thi s C-100 molecule belongs to a series of structures that may be viewed as the diamond analogues of fullerenes (L. Zeger and E. Kaxiras, Phys. Rev. Lett., 70 (1993) 2920). Since these molecules can he seen to be effectively a heavily twinned molecular diamond, their stability is im portant in the context of investigating the diamond {111} surface wher e a twin emerges, We present ab initio self-consistent calculations on a rather small C40H36 molecule representing the core of a twin inters ecting two diamond {111} surfaces, and compare the results with those obtained with a non-self-consistent density-functional based tight-bin ding method. Since the latter is also capable of handling larger and p eriodic models in a molecular dynamics relaxation, we use it to study the graphitization effect as a function of temperature. We find nearly the same ground state for the small molecule which is clearly due to a graphitization, and find strong surface graphitization for a model o f 128 carbon atoms at elevated temperatures. At 2700 K the top layer o f this periodic model delaminates completely.