Observation of low-wavenumber out-of-plane optical phonon in few-layer graphene

Few-layer graphene grown by chemical vapor deposition has been studied by Raman and ultrafast laser spectroscopy. A low-wavenumber Raman peak of similar to 120 cm-1 and a phonon-induced oscillation in the kinetic curve of electronphonon relaxation process have been observed, respectively. The Raman peak is assigned to the low-wavenumber out-of-plane optical mode in the few-layer graphene. The phonon band shows an asymmetric shape, a consequence of so-called Breit-Wigner-Fano resonance, resulting from the coupling between the low-wavenumber phonon and electron transitions. The obtained oscillation wavenumber from the kinetic curve is consistent with the detected low-wavenumber phonon by Raman scattering. The origin of this oscillation is attributed to the generation of coherent phonons and their interactions with photoinduced electrons. Copyright (C) 2012 John Wiley & Sons, Ltd.