  The fundamental properties of graphene are making it an attractive material
for a wide variety of applications. Various techniques have been developed to
produce graphene and recently we discovered the synthesis of large area
graphene by chemical vapor deposition (CVD) of methane on Cu foils. We also
showed that graphene growth on Cu is a surface-mediated process and the films
were polycrystalline with domains having an area of tens of square microns. In
this paper we report on the effect of growth parameters such as temperature,
and methane flow rate and partial pressure on the growth rate, domain size, and
surface coverage of graphene as determined by Raman spectroscopy, and
transmission and scanning electron microscopy. Based on the results, we
developed a two-step CVD process to synthesize graphene films with domains
having an area of hundreds of square microns. Scanning electron microscopy and
Raman spectroscopy clearly show an increase in domain size by changing the
growth parameters. Transmission electron microscopy further shows that the
domains are crystallographically rotated with respect to each other with a
range of angles from about 13 degrees to nearly 30 degrees. Electrical
transport measurements performed on back-gated FETs show that overall films
with larger domains tend to have higher carrier mobility, up to about 16,000
cm2 V-1 s-1 at room temperature.
