Optical Phonons Of Graphene Research Articles

Unravelling external perturbation effects on the optical phonon response of graphene

Raman spectroscopy is a powerful and nondestructive probe that demonstrates its efficiency in revealing the physical properties of low‐dimensional sp2 carbon systems. It gives access to the number of layers, the quality and the nature of defects of all carbon allotropes, but also to the understanding of the influence of perturbations such as strain and/or doping. In this paper, we review the state of the art regarding the effect of external perturbations on the optical phonons of graphene. We describe how doping can tune the unusual electron–phonon coupling in graphene and thus modify not only the resonance conditions but also the phonon intensities thanks to quantum interferences. We also review the impact of strain on optical phonons and how one can disentangle strain and doping thanks to optical phonons. Last, implementations of this field to strain engineering or to graphene‐based mechanical resonators will be presented. Copyright © 2018 John Wiley & Sons, Ltd.

Spatial coherence in near-field Raman scattering.

Inelastic light scattering in crystals has historically been treated as a spatially incoherent process, giving rise to incoherent optical radiation. Here we demonstrate that Raman scattering can be spatially coherent, in which case it depends on the dimensionality and symmetry of the scatterer. Using near-field spectroscopy, we measure a correlation length of ∼30 nm for the optical phonons in graphene, the results varying with vibrational symmetries and spatial confinement of the phonons.

High-field transport properties of graphene

We present a theoretical investigation on the transport properties of graphene in the presence of high dc driving fields. Considering electron interactions with impurities and acoustic and optical phonons in graphene, we employ the momentum- and energy-balance equations derived from the Boltzmann equation to self-consistently evaluate the drift velocity and temperature of electrons in graphene in the linear and nonlinear response regimes. We find that the current-voltage relation exhibits distinctly nonlinear behavior, especially in the high electric field regime. Under the action of high-fields the large source-drain (sd) current density can be achieved and the current saturation in graphene is incomplete with increasing the sd voltage Vsd up to 3 V. Moreover, for high fields, Vsd>0.1 V, the heating of electrons in graphene occurs. It is shown that the sd current and electron temperature are sensitive to electron density and lattice temperature in the graphene device. This study is relevant to the application of graphene as high-field nano-electronic devices such as graphene field-effect transistors.

High-field transport and optical phonon scattering in graphene

The coupled dynamics of electrons and optical phonons in graphene is investigated. For this purpose, a kinetic model based on space-dependent Boltzmann transport equations for electrons and optical phonons is established. This system is solved by deterministic methods that provide efficiently accurate results by dynamically treating nonequilibrium phenomena. The numerical simulations clearly show the importance of this approach when calculating $I$--$V$ characteristics over a reasonable voltage range. Optical phonon occupations surpassing equilibrium values by factors of thousands were found which affect the charge carrier transport considerably in graphene.

Unconventional plasmon-phonon coupling in graphene

We calculate hybridization of plasmons and intrinsic optical phonons in graphene by using the self-consistent linear response formalism. We find that longitudinal plasmons (transverse magnetic modes) couple exclusively to transverse optical phonons, whereas graphene's transverse plasmons (transverse electric modes) couple only to longitudinal optical phonons. This mixing of polarizations is in contrast to the usual plasmon-phonon coupling in other systems. The resulting change in phonon frequencies increases (decreases) for transverse (longitudinal) phonons by increasing the concentration of charge carriers.

Lifetimes of optical phonons in graphene and graphite by time-resolved incoherent anti-Stokes Raman scattering

We report lifetimes of optical phonons (OPs) in graphene and graphite measured by time-resolved anti-Stokes Raman scattering. Lifetimes in graphite and monolayer graphene are 2.4 and 1.2 ps, respectively. For graphite and graphene with more than five layers, the lifetimes decrease with increasing temperature as $\ensuremath{\sim}1/T$, indicating the dominance of anharmonic processes in the decay of the OP population. The decrease in lifetime with decreasing number of layers suggests an additional decay channel through which excitations in graphene interact directly with lattice vibrations of the ${\text{a-SiO}}_{2}$ substrate.

Ultrafast relaxation dynamics of hot optical phonons in graphene

Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multilayer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5–2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions.

Optical phonons of graphene and nanotubes

We review the optical phonon dispersions of graphene. In particular, we focus on the presence of two Kohn anomalies in the highest optical phonon branch at the \(\bf \Gamma\) and \(\bf K\) points of the Brillouin zone. We then show how graphene can be used as a model for the calculation of phonons in carbon nanotubes. Finally, we present the beyond Born-Oppenheimer corrections to their phonon dispersions. These are experimentally revealed in the Raman spectra of doped samples.