Using inelastic scattering of light to understand the nature of electron-phonon interactions and phonon self-energy renormalizations in graphene materials

Abstract

In the last decade, many theoretical and experimental achievements have been made in the physics of graphene. In particular, Raman spectroscopy has been playing an important role in unraveling the properties of graphene systems. In this thesis we use the Raman spectroscopy to study some effects of the electron-phonon coupling in monolayer and bilayer graphene and to probe the electronic and vibrational structure of bilayer graphene. Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center (q = 0) wavevectors. Here, we combine Raman spectroscopy and gate voltage to study phonons of monolayer graphene for the features originated from a double-resonant Raman (DRR) process with q .= 0 wavevectors. We observe phonon renormalization effects in which there is a softening of the frequency and a broadening of the decay width with increasing the gate voltage, that is opposite from what is observed for the zone-center q = 0 case. We show that this renormalization is a signature for the phonons with q . 2k wavevector that come from both intravalley and intervalley DRR processes. Within this framework, we resolve the identification of the phonon modes contributing to the G. Raman feature, at ¡­ 2450 cm.1, and also forfive second order Raman combination modes in the frequency range of 1700 . 2300 cm.1 of monolayer graphene. By combining the DRR theory with the anomalous phonon renormalization effect, we show a new technique for using Raman spectroscopy to identify the proper phonon mode assignment for each combination mode. We also study the behavior of the optical phonon modes in bilayer graphene devicesby applying top gate voltage, using Raman scattering. We observe the splitting of the Raman G band as we tune the Fermi level of the sample, which is explained in terms of mixing of the Raman (Eg) and infrared (Eu) phonon modes, due to different doping in the two layers. We show that the comparison between the experiment and theoretical model not only gives information about the total charge concentration in the bilayer graphene device, but also allows to separately quantify the amount of unintentional charge coming from the top and the bottom of the system, and therefore to characterize the intrinsic charges of bilayer graphene with its surrounding environment. In the second part of this thesis, the dispersion of electrons and phonons near the K point of bilayer graphene was investigated in a resonant Raman study of the G¡Ç band using different laser excitation energies in the near-infrared and visible range.The electronic structure was analyzed within the tight-binding approximation, and the Slonczewski-Weiss-McClure (SWM) parameters were obtained from the analysis of the dispersive behavior of the G¡Ç band considering both the inner and the outer DRR processes. We show that the SWM parameters obtained considering the inner process are in better agreement with those obtained from other experimental techniques, strongly suggesting that the inner process is the main responsible for the G¡Ç feature in graphene. Additionally, the dependence of the intensity of the four peaks that compose the G¡Ç band of bilayer graphene with laser excitation energy and laser power is explored and explained in terms of the electron-phonon coupling and the relaxation of the photon-excited electron. We show that the carrier relaxation occurs predominantly by emitting a lowenergy acoustic phonon and the different combinations of relaxation processes determine the relative intensities of the four peaks that give rise to the G¡Ç band. Some peaks show an increase of their intensity at the expense of others, thereby making the intensity of the peaks both different from each other and dependent on laser excitation energy and on power level. This effect gives important information about the electron and phonon dynamics and needs to be taken into account for certain applications of bilayer graphene in the field of nanotechnology.