  In this report, we demonstrated a simple but efficient strategy to synthesize
millimeter-sized graphene single crystal grains by regulating the supply of
reactants in chemical vapor deposition process. Polystyrene was used as a
carbon source. Pulse heating on the carbon source was utilized to minimize the
nucleation density of graphene on copper foil, while the gradual increase in
the temperature of carbon source and the flow rate of hydrogen is adapted to
drive the continuous growth of graphene grain. As a result, the nucleation
density of graphene grain can be controlled as lower as ~100 nuclei/cm2, and
the dimension of single crystal grain could grow up to ~1.2 mm. Raman
spectroscopy, transmission electron microscopy and electrical transport
measurement show that the graphene grains obtained are in high quality. The
strategy presented here provides very good controllability and enables the
possibility for large graphene single crystals, which is of vital importance
for practical applications.
