Synthesis of Single-Layer Graphene on Nickel Using a Droplet CVD Process

A liquid metal droplet chemical vapor deposition (CVD) process is developed for the synthesis of single-layer graphene flakes utilizing molten nickel droplets (nominal diameter from 0.5 to 1 mu m) as the catalysts. Ultrathin nickel films melt and form nickel droplets at around 1000 degrees C, providing liquid-phase, single-grain catalytic sites for the nucleation of graphene flakes via carbon precipitation process. Experimentally, both single and double-layer graphene flakes with low defects have been synthesized by using nickel thin films of either less or more than 100 nm (up to 130 nm) in thickness, respectively. Furthermore, the synthesized graphene flakes are hybridized by the nickel orbitals as suggested by a density function analysis and experimentally observed blue shifts in Raman spectra tests. When the original nickel thin film is 75 nm in thickness, the resulting nickel droplets are physically and electrically isolated, while the graphene flakes are found to be electrically connected probably due to the outgrowth of graphene from the separated nickel droplets. These electrically connected graphene sheets could be readily available for device applications without the need of transfer processes. This work demonstrates the direct generation of photocurrents (up to 0.53 mu A mm(-2) W-1) due to the photothermal effect based on as-fabricated graphene sheets under a visible light source.