  A single-layer graphene is synthesized on Cu foil in the absence of H2 flow
by plasma enhanced chemical vapor deposition (PECVD). In lieu of an explicit H2
flow, hydrogen species are produced during methane decomposition process into
their active species (CHx<4), assisted by the plasma. Notably, the early stage
of growth depends strongly on the plasma power. The resulting grain size (the
nucleation density) has a maximum (minimum) at 50 W and saturates when the
plasma power is higher than 120 W because hydrogen partial pressures are
effectively tuned by a simple control of the plasma power. Raman spectroscopy
and transport measurements show that decomposed methane alone can provide
sufficient amount of hydrogen species for high-quality graphene synthesis by
PECVD.
