Carbon
diffusion barriers are introduced as a general and simple
method to prevent premature carbon dissolution and thereby to significantly
improve graphene formation from the catalytic transformation of solid
carbon sources. A thin Al2O3 barrier inserted
into an amorphous-C/Ni bilayer stack is demonstrated to enable growth
of uniform monolayer graphene at 600 °C with domain sizes exceeding
50 μm, and an average Raman D/G ratio of <0.07. A detailed
growth rationale is established via in situ measurements, relevant
to solid-state growth of a wide range of layered materials, as well
as layer-by-layer control in these systems.