CVD-graphene growth on different polycrystalline transition metals

The chemical vapor deposition (CVD) graphene growth on two polycrystalline transition metals (Ni and Cu) was investigated in detail using Raman spectroscopy and optical microscopy as a way to synthesize graphene of the highest quality (i.e. uniform growth of monolayer graphene), which is considered a key issue for electronic devices. Key CVD process parameters (reaction temperature, CH₄/H₂flow rate ratio, total flow of gases (CH₄+H₂), reaction time) were optimized for both metals in order to obtain the highest graphene uniformity and quality. The conclusions previously reported in literature about the performance of low and high carbon solubility metals in the synthesis of graphene and their associated reaction mechanisms, i.e. surface depositionand precipitation on cooling, respectively, was not corroborated by the results obtained in this work. Under the optimal reaction conditions, a large percentage of monolayer graphene was obtained over the Ni foil since the carbon saturation was not complete, allowing carbon atoms to be stored in the bulk metal, which could diffuse forming high quality monolayer graphene at the surface. However, under the optimal reaction conditions, the formation of a non-uniform mixture of few layers and multilayer graphene on the Cu foil was related to the presence of an excess of active carbon atoms on the Cu surface