Fermi resonance in the Raman spectrum of graphene

TL;DR

This paper reports the discovery of a new Raman peak in graphene caused by Fermi resonance involving the 2ZO mode, influenced by chromium nanoparticles, supported by experimental and first-principles calculations.

Contribution

It demonstrates the observation of a Fermi resonance-induced Raman peak in graphene due to chromium contact, a novel insight into phonon interactions in graphene.

Findings

Identification of a new non-dispersive Raman peak at 1608 cm$^{-1}$

Attribution of the peak to the 2ZO mode activated by Fermi resonance

Support from first-principles calculations confirming the mechanism

Abstract

We report the observation of an intense anomalous peak at 1608 cm$^{-1}$ in the Raman spectrum of graphene associated to 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$'$ peak appearing as defects are created in graphene; the new peak is found non dispersive. 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 $2\omega_{\rm ZO}\sim\omega_{\rm G}$, where $\omega_{\rm G}$=1585~cm$^{-1}$ is the frequency of the Raman-active E$_{\rm 2g}$ mode. Thus, the observed new peak is attributed to the 2ZO mode which becomes Raman-active following a mechanism known as Fermi resonance. First-principles calculations on vibrational and anharmonic properties of the graphene/Cr interface support this scenario.