Vacancy defects and the formation of local haeckelite structures in graphene from tight-binding molecular dynamics

The dynamics of multivacancy defects in a graphene layer is investigated by tight-binding molecular dynamics simulations and by first principles calculation. The simulations show that four single vacancies in the graphene layer first coalesce into two double vacancies, each consisting of a pentagon-heptagon-pentagon (5-8-5) defective structure. While one of the 5-8-5 defects further reconstructs into a 555-777 defect, which is composed of three pentagonal rings and three heptagonal rings, another 5-8-5 defect diffuses toward the reconstructed 555-777 defect. During the 5-8-5 defect diffusion process, three interesting mechanisms, i.e., ``dimer diffusion,'' ``chain diffusion,'' and ``single atom diffusion,'' are observed. Finally, the four single vacancies reconstruct into two adjacent 555-777 defects, which is a local haeckelite structure.