The role of hydrogen is not confined to these tasks; it might also influence graphene growth kinetics and crystal quality. On Ni, higher hydrogen flow increases the growth rate by enhancing the surface activity, while in the case of Cu, high hydrogen flow decreases the growth due to surface site blockage . Finally, in some systems, hydrogen acts like a carrier gas and provides transport for the carbon precursor at the substrate surface, hence assuring uniform carbon deposition. In general, hydrogen presence in the CVD process is necessary in order to control both the quality and kinetics of graphene growth; it therefore is a key factor in the synthesis process itself .