Rapid, substrate-independent thickness determination of large area graphene layers

Phase-shifting interferometric imaging is shown to be a powerful analytical tool for studying graphene films, providing quantitative analysis of large area samples with an optical thickness resolution of ≥ 0.05 nm. The technique is readily able to identify single sheets of graphene and to quantitatively distinguish between layers composed of multiple graphene sheets. The thickness resolution of the technique is shown to result from the phase shift produced by a graphene film as incident and reflected light pass through it, rather than from path-length differences produced by surface height variations. This is enhanced by the high refractive index of graphene, estimated in this work to be n _G 2.99 ± 0.18.