▎ 摘　　要

We investigated mechanism of the metal-catalyst-free direct growth of graphene films in chemical vapor deposition on sapphire surfaces using three kinds of substrates, randomly stepped (0001), phase-separated (0001) and randomly stepped (11 (2) over bar0). The graphene growth process includes pit formation in the early stage by hydrogen and methane mixed gas flow, appearance of catalytic action inside the pit, nucleation of graphene on the pit surfaces, and expansion and fusion of graphene films. It was found that the quality of graphene films depends on the crystallographic plane. In addition, even on an identical plane, a domain-selective growth mode was observed between domains with different step-terrace structures. This study indicates that morphology of the sapphire surface is crucial in direct growth of graphene films because a certain density of atomic-steps on the surface is favorable to nucleation of graphene films.