Simulations of large multi-atom vacancies in diamond

We generated and evaluated energetically a very large number of vacancy Vn clusters representing nanosize voids or cavities in diamond for n up to 14 using a new generational algorithm. We evaluated the relaxed geometries and energies of these contiguous vacancy clusters using a tight binding density functional theory (TBDFT). For up to n = 7 we generated all possible structures and evaluated their relaxed geometries. For n = 8 through n = 14 we selectively generated a large number of vacancy clusters and obtained highly stable structures and their energies. By analyzing the energy levels and the corresponding orbitals, we identified the surface states of the voids and their symmetries. Significant differences with respect to vacancy clusters in silicon were found. The results were interpreted by finding that certain structures become relatively more stable due to a process we call local graphitization, which can be identified by a tetrahedron of graphitization (TOG), and it can be characterized by elongation of certain carbon–carbon contacts and by the concomitant appearance of new states in the gap. The beginning of graphitization, as indicated by geometrical and energetic descriptors in small stable vacancy clusters, may have a role in the formation mechanisms of various sp2 hybridized structures in carbons.