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) revealed that the tetrahedrally bonded icosahedral C100 molecule spontaneously decomposed into two concentric fullerenes (C20 and C80). This C100 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 be seen to be effectively a heavily twinned molecular diamond their stability is important in the context of investigating the diamond {111} surface where a twin emerges. We present ab initio self-consistent calculations on a rather small C40H36 molecule representing the core of a twin intersecting two diamond {111} surfaces and compare the results with that obtained with a non-selfconsistent density-functional based tight-binding method. Since the latter is also capable of handling larger and periodic models in a molecular dynamics relaxation we use it to study the graphitization effect in dependence 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 of 128 carbon atoms at elevated temperatures. At 2700 K the top layer of this periodic model completely delaminates.