Reliably counting atomic planes of few-layer graphene (n > 4)

We demonstrate a reliable technique for counting atomic planes (n) of few-layer graphene (FLG) on SiO₂/Si substrates by Raman spectroscopy. Our approach is based on measuring the ratio of the integrated intensity of the G graphene peak and the optical phonon peak of Si, I(G)/I(Si), and is particularly useful in the range n > 4 where few methods exist. We compare our results with atomic force microscopy (AFM) measurements and Fresnel equation calculations. Then, we apply our method to unambiguously identify n of FLG devices on SiO₂ and find that the mobility (μ ≈ 2000 cm ² V^-1 s^-1) is independent of layer thickness for n > 4. Our findings suggest that electrical transport in gated FLG devices is dominated by carriers near the FLG/SiO₂ interface and is thus limited by the environment, even for n > 4.