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Register a new userFermi resonance in the Raman spectrum of graphene
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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 $2omega_{rm ZO}simomega_{rm G}$, 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.
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Massless Dirac fermions in graphene can acquire a mass through different kinds of sublattice-symmetry-breaking perturbations, and there is a growing need to determine this mass using a conventional method. We describe how the mass caused by a staggered sublattice potential is determined using Raman spectroscopy and explain the mechanism in terms of the pseudospin polarization of massive Dirac fermions.
In Raman spectroscopy of graphite and graphene, the $D$ band at $sim 1355$cm$^{-1}$ is used as the indication of the dirtiness of a sample. However, our analysis suggests that the physics behind the $D$ band is closely related to a very clear idea for describing a molecule, namely bonding and antibonding orbitals in graphene. In this paper, we review our recent work on the mechanism for activating the $D$ band at a graphene edge.
By analytically constructing the matrix elements of an electron-phonon interaction for the $D$ band in the Raman spectra of armchair graphene nanoribbons, we show that pseudospin and momentum conservation result in (i) a $D$ band consisting of two components, (ii) a $D$ band Raman intensity that is enhanced only when the polarizations of the incident and scattered light are parallel to the armchair edge, and (iii) the $D$ band softening/hardening behavior caused by the Kohn anomaly effect is correlated with that of the $G$ band. Several experiments are mentioned that are relevant to these results. It is also suggested that pseudospin is independent of the boundary condition for the phonon mode, while momentum conservation depends on it.
The dispersion of phonons and the electronic structure of graphene systems can be obtained experimentally from the double-resonance (DR) Raman features by varying the excitation laser energy. In a previous resonance Raman investigation of graphene, the electronic structure was analyzed in the framework of the Slonczewski-Weiss-McClure (SWM) model, considering the outer DR process. In this work we analyze the data considering the inner DR process, and obtain SWM parameters that are in better agreement with those obtained from other experimental techniques. This result possibly shows that there is still a fundamental open question concerning the double resonance process in graphene systems.
Intermediate frequency range (511 - 514 cm-1) Si phonons in Si-SiO2 nanocomposites are shown to have contribution from both core1 and surface/interface1 Si phonons, where, ratio of contribution of the two depends on the size of a Si nanocrystal. Further, laser heating experiment shows that contribution of the core phonon increases due to increase in size of a nanocrystal. Wavelength dependent Raman mapping reveals that interface phonons are observable due to Resonance Raman scattering. This can well be corroborated with the absorption spectra. This understanding can be gainfully used to manipulate and characterize Si-SiO2 nanocomposite, simultaneously for photovoltaic device applications.
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