▎ 摘　　要

Hydrocarbon decomposition on transition metals provides a practical way of producing graphene. Here, ethylene deposition on Rh (111) is taken as an example. In-situ scanning tunneling microscopy measurements, under various growth conditions and at temperatures up to 1100 K, were carried out, revealing the processes of graphene formation at the atomic level. The initial nucleation stage nearly completely determines the phase in which further C is deposited, graphene or rhodium carbide, and the orientation of the growing graphene patches. We demonstrate that by separating the stages of nucleation and further growth and controlling other growth parameters, we obtain graphene of higher quality, while avoiding carbide formation and controlling the dissolved C to form graphene. Based on these observations, a universal physical picture emerges for graphene formation on metal surfaces.