First-Principles Study on the Graphene Adatom and Its Aggregation

The stabilities and atomic geometries of the adatom and its aggregates on graphene are studied using first-principles calculations. We determine the most stable atomic structures of ail adatom and its dimer, trimer, and tetramer. In the most stable structure of the dimer, two pentagons and two heptagons are formed. The amount of energy required for the dissociation of the dimer into two adatoms is found to be very high (5.5 eV), indicating that the dimer is very stable. Therefore. it is expected that the dimer will be observed under certain experimental conditions. The line defect where the dimer is aligned in a straight line is found to be very stable, i.e., the formation energy per adatom is 1.3 eV, which is lower than that of the isolated dimer (3.3 eV). Based on our calculated results on dimers, trimers, and tetramers, we conclude that this line defect is not spontaneously formed by adatom diffusion, but might be formed by dimer diffusion.