Layer-number determination in graphene by out-of-plane phonons

We present and discuss a double-resonant Raman mode in few-layer graphene that is able to probe the number of graphene layers. This so-called $N$ mode on the low-frequency side of the $G$ mode results from a double-resonant Stokes--anti-Stokes process combining an longitudinal optical (LO) and an out-of-plane (ZO${}^{\ensuremath{'}}$) phonon. Simulations of the double-resonant Raman spectra in bilayer graphene show very good agreement with the experiments.