Stability and Dynamical Process of Graphene Adatom and Its Dimer

The stabilities and atomic geometries of the graphene adatom and its dimer are studied using first-principles calculations based on the spin-polarized density functional theory. In the most stable structure, the adatom is located above the bond-center site. We also find a metastable structure whose symmetry is D-3h and whose formation energy is slightly (0.22 eV) higher than that of the most stable structure. In the most stable structure of the dimer, two pentagons and two heptagons are formed. Since the energy for dissociating the most stable dimer into two isolated adatoms is very large (6.23 eV), we expect that this dimer is observed under certain condition. On the other hand, in the metastable structure whose formation energy is only 0.30 eV higher than that of the most stable structure, the C-2 molecule is bonded with the graphene plane. If the metasable structure is formed instead of the most stable dimer, adatom aggregation is expected to occur because the diffusion barrier of the metastable structure is 0.16 eV, which is much smaller than that of the adatom (0.49 eV).