Theoretical Studies on Nanocrystalline Diamond: Nucleation by Dicarbon and Electronic Structure of Planar Defects

Density functional theory and ab initio molecular orbital theory have been used to calculate the energetics of C 2 insertion into C 9 H 12 and C 9 H 14 clusters that model unhydrided and monohydrided (100) diamond surfaces, respectively. The reaction of C 2 with either the C 9 H 12 or C 9 H 14 cluster is exothermic by more than 100 kcal/mol, but the lowest energy product is different for the two clusters. The reaction of singlet C 2 with the C C double bond of the C 9 H 12 cluster leads to either carbene structures or a cyclobutyne-like structure, with the former having the lower energy at both the HF/6-31G* and B3LYP/6-31G* levels of theory. No barrier for insertion into the C C double bond of the C 9 H 12 cluster was found at the HF/6-31G* and B3LYP/6-31G* levels of theory. The reaction of singlet C 2 with the HC−CH single bond or C−H bonds of the C 9 H 14 cluster leads to a structure having a cyclobutene-like geometry. We propose that the disparate nucleation rates of diamond crystallites grown in hydrogen-rich vs hydrogen-poor C 60 /Ar microwave plasmas are accounted for qualitatively by these results. The carbon dimer, C 2, is a possible growth or nucleation species produced by fragmentation of C 60 . Periodic density functional calculations of the electronic structure of a simple model of an sp 2 -bonded diamond grain boundary show that π-bonded planar defects introduce new electronic bands into the fundamental band gap of diamond.