  Large-area graphene is a new material with properties that make it desirable
for advanced scaled electronic devices1. Recently, chemical vapor deposition
(CVD) of graphene and few-layer graphene using hydrocarbons on metal substrates
such as Ni and Cu has shown to be a promising technique2-5. It has been
proposed in recent publications that graphene growth on Ni occurs by C
segregation2 or precipitation3, while that on Cu is by surface adsorption5. In
this letter, we used a carbon isotope labeling technique to elucidate the
growth kinetics and unambiguously demonstrate that graphene growth on Cu is by
surface adsorption whereas on Ni is by segregation-precipitation. An
understanding of the evolution of graphene growth and thus growth mechanism(s)
is desired to obtain uniform graphene films. The results presented in this
letter clearly demonstrate that surface adsorption is preferred over
precipitation to grow graphene because it is a self-limiting process and thus
manufacturable.
