  We report graphene films composed mostly of one or two layers of graphene
grown by controlled carbon precipitation on the surface of polycrystalline Ni
thin films during atmospheric chemical vapor deposition(CVD). Controlling both
the methane concentration during CVD and the substrate cooling rate during
graphene growth can significantly improve the thickness uniformity. As a
result, one- or two- layer graphene regions occupy up to 87% of the film area.
Single layer coverage accounts for 5-11% of the overall film. These regions
expand across multiple grain boundaries of the underlying polycrystalline Ni
film. The number density of sites with multilayer graphene/graphite (>2 layers)
is reduced as the cooling rate decreases. These films can also be transferred
to other substrates and their sizes are only limited by the sizes of the Ni
film and the CVD chamber. Here, we demonstrate the formation of films as large
as 1 in2. These findings represent an important step towards the fabrication of
large-scale high-quality graphene samples.
