The Equilibrium Shape of Graphene Domains on Ni(111)

We have determined the equilibrium shape of graphene domains grown on Ni(111) via carbon segregation at 925{\deg}C. In situ, spatially-resolved electron diffraction measurements were used to determine the crystallographic orientation of the edges of the graphene domains. In contrast to recent theoretical predictions of a nearly-circular shape, we show that graphene domains, which nucleate with random shapes, all evolve toward a triangular equilibrium shape with 'zig-zag' edges. Only one of the two possible zig-zag edge orientations is observed.