Abstract
Vacancy clusters in graphite have been investigated using Density Functional Theory (DFT) within B3LYP exchange-functional. The smallest size of vacancy clusters (V-4) has been chosen to study the migration energy and aggregation mechanism. Two main types of V-4 vacancy clusters have been modeled, the disc (V-4(d)) and the line vacancy clusters; including boat vacancy (V-4(b)) and zig-zag vacancy (V-4(z)). The results show that the presence of unstable V-3 vacancy may induce the mono-vacancy to migrate with low energy and vanish through forming stable V-4 vacancy cluster. Also, the calculated energy barriers required to form the boat vacancy cluster (V-4(b)), the zig-zag vacancy cluster (V-4(z)) and disk vacancy cluster (V-4(d)) support that the disc and the boat vacancy clusters co-exist. However the zig-zag type might only exist by knocking-out mechanism for highly irradiated graphite.