  Using low-energy electron microscopy, we image in real time the intercalation
of a cobalt monolayer between graphene and the (111) surface of iridium. Our
measurements reveal that the edges of a graphene flake represent an energy
barrier to intercalation. Based on a simple description of the growth kinetics,
we estimate this energy barrier and find small, but substantial, local
variations. These local variations suggest a possible influence of the graphene
orientation with respect to its substrate and of the graphene edge termination
on the energy value of the barrier height. Besides, our measurements show that
intercalated cobalt is energetically more favorable than cobalt on bare
iridium, indicating a surfactant role of graphene.
