Graphene, a single atomic layer of hexagonally packed carbon atoms, has drawn significant attention with its outstanding electrical, optical, and chemical properties. Various promising applications based on graphene have been demonstrated, such as in electronics, optoelectronics, and chemical/bio sensing. To realize the promise, it is critical to synthesize large-scale, high quality graphene films transferable onto arbitrary substrates. In this dissertation, chemical vapor deposition (CVD) synthesis of graphene films and grains on Cu substrates is studied, and the applications of graphene as transparent electrodes and gas sensor are demonstrated.

The effects of Cu surface conditions and CVD parameters such as growth temperature and CH₄ concentration on the nucleation density, grain size, and surface coverage of graphene are discussed. On the basis of the results, a two-step ambient pressure CVD technique is developed to synthesize large-scale uniform single-layer graphene films with grain sizes up to hundreds of square micrometers. The growth mechanism of CVD graphene on Cu is also discussed, and a growth model is proposed. Moreover, graphene grains (either isolated grains or several merged grains) formed during the early stage of CVD growth on Cu are studied. Individual graphene grains typically have a hexagonal shape and are single crystals. The grains show no definite epitaxial relationship with the underlying Cu substrate, and can grow continuously across Cu grain boundaries without any apparent shape distortion, indicating weak graphene-Cu interactions. Edges of these grains are found to be predominantly parallel to zigzag directions. Grain boundaries in graphene are characterized and found to impede electrical transport. An effective approach is proposed to control nucleation of CVD graphene by locally providing high concentrations of carbon, opening a route towards scalable fabrication of single crystal graphene devices without grain boundaries. Finally, graphene films (up to 4 layers) are transferred on cover glass flips for the measurements of optical transmittance and sheet resistance. The transferred graphene films show high electrical conductivity and high optical transmittance that make them suitable as transparent electrodes. Hydrogen sensors are also demonstrated on Pd-decorated graphene films. The sensors show high sensitivity, fast response and recovery, and can be used with multiple cycles.