  Graphene forms from a relatively dense, tightly-bound C-adatom gas, when
elemental C is deposited on or segregates to the Ru(0001) surface. Nonlinearity
of the graphene growth rate with C adatom density suggests that growth proceeds
by addition of C atom clusters to the graphene edge. The generality of this
picture has now been studied by use of low-energy electron microscopy (LEEM) to
observe graphene formation when Ru(0001) and Ir(111) surfaces are exposed to
ethylene. The finding that graphene growth velocities and nucleation rates on
Ru have precisely the same dependence on adatom concentration as for elemental
C deposition implies that hydrocarbon decomposition only affects graphene
growth through the rate of adatom formation; for ethylene, that rate decreases
with increasing adatom concentration and graphene coverage. Initially, graphene
growth on Ir(111) is like that on Ru: the growth velocity is the same nonlinear
function of adatom concentration (albeit with much smaller equilibrium adatom
concentrations, as we explain with DFT calculations of adatom formation
energies). In the later stages of growth, graphene crystals that are rotated
relative to the initial nuclei nucleate and grow. The rotated nuclei grow much
faster. This difference suggests first, that the edge-orientation of the
graphene sheets relative to the substrate plays an important role in the growth
mechanism, and second, that attachment of the clusters to the graphene is the
slowest step in cluster addition, rather than formation of clusters on the
terraces.
