Turning off hydrogen to realize seeded growth of subcentimeter single-crystal graphene grains on copper.

Subcentimeter single-crystalline graphene grains, with diameter up to 5.9 mm, have been successfully synthesized by tuning the nucleation density during atmospheric pressure chemical vapor deposition. Morphology studies show the existence of a single large nanoparticle (>~20 nm in diameter) at the geometric center of those graphene grains. Similar size particles were produced by slightly oxidizing the copper surface to obtain oxide nanoparticles in Ar-only environments, followed by reduction into large copper nanoparticles under H2/Ar environment, and are thus explained to be the main constituent nuclei for graphene growth. On this basis, we were able to control the nanoparticle density by adjusting the degree of oxidation and hydrogen annealing duration, thereby controlling nucleation density and consequently controlling graphene grain sizes. In addition, we found that hydrogen plays dual roles on copper morphology during the whole growth process, that is, removing surface irregularities and, at the same time, etching the copper surface to produce small nanoparticles that have only limited effect on nucleation for graphene growth. Our reported approach provides a highly efficient method for production of graphene film with long-range electronic connectivity and structure coherence.