  Chemical vapor deposition (CVD) is an important method to synthesis grapheme
on a substract. Recently, Cu becomes the most popular CVD substrate for
graphene growth. Here, we combine electronic structure calculation, molecular
dynamics simulation, and thermodynamics analysis to study the graphene growth
process on Cu surface. As a fundamentally important but previously overlooked
fact, we find that carbon atoms are thermodynamically unfavorable on Cu surface
under typical experimental conditions. The active species for graphene growth
should thus mainly be CHx instead of atomic carbon. Based on this new picture,
the nucleation behavior can be understood, which explains many experimental
observations and also provides us a guide to improve graphene sample quality.
