  Large holes in graphene membranes were recently shown to heal, either at room
temperature during a low energy STEM experiment, or by annealing at high
temperatures. However, the details of the healing mechanism remain unclear. We
carried out fully atomistic reactive molecular dynamics simulations in order to
address these mechanisms under different experimental conditions. Our results
show that, if a carbon atom source is present, high temperatures can provide
enough energy for the carbon atoms to overcome the potential energy barrier and
to produce perfect reconstruction of the graphene hexagonal structure. At room
temperature, this perfect healing is only possible if the heat effects of the
electron beam from STEM experiment are explicitly taken into account. The
reconstruction process of a perfect or near perfect graphene structure involves
the formation of linear carbon chains, as well as rings containing 5, 6, 7 and
8 atoms with planar (Stone-Wales) and non-planar (lump like) structures. These
results shed light on the healing mechanism of graphene when subjected to
different experimental conditions. Additionally, the methodology presented here
can be useful for investigating the tailoring and manipulations of other
nano-structures.
