Factors influencing graphene growth on metal surfaces

Abstract

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