Fermi resonance in the Raman spectrum of graphene

We report the observation of an intense anomalous peak at 1608 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the Raman spectrum of graphene associated with the presence of chromium nanoparticles in contact with graphene. Bombardment with an electron beam demonstrates that this peak is distinct from the well-studied ${D}^{\ensuremath{'}}$ peak appearing as defects are created in graphene; this peak is found to be nondispersive. We argue that the bonding of chromium atoms with carbon atoms softens the out-of-plane optical (ZO) phonon mode, in such a way that the frequency of its overtone decreases to $2{\ensuremath{\omega}}_{\mathrm{ZO}}\ensuremath{\sim}{\ensuremath{\omega}}_{\mathrm{G}}$, where ${\ensuremath{\omega}}_{\mathrm{G}}=1585$ ${\mathrm{cm}}^{\ensuremath{-}1}$ is the frequency of the Raman-active ${E}_{2\mathrm{g}}$ mode. Thus, the observed peak is attributed to the 2ZO mode which becomes Raman active following a mechanism known as Fermi resonance. First-principles calculations of the vibrational and anharmonic properties of the graphene/Cr interface support this scenario.