Resonance raman spectroscopy in twisted bilayer graphene

Abstract

The understanding of fundamental interactions in ultrathin heterostructures is crucial for engineering novel 2D-based devices. Due to these interactions new physical phenomena arise, altering change the response of the composite material compared withits insulated layers. Among many experimental techniques, Raman spectroscopy has played a major role in 2D materials study, since the electrons, the phonons and the interactions between them can be studied together. In this thesis, we used resonant Raman spectroscopy to study these interactions focusing on one system: twisted bilayer graphene, a system composed of two layers of graphene where the crystallographic orientation of the layers is dierent, generating the moiré pattern. Initially we acquired Raman maps of more than 100 samples using visible laser with a continuous distribution of twisting angles from 0 to 30, we observed huge enhancements of the intensity of G band ( 1590 cm1) of graphene for samples in an intermediate range of twisting angles ( between 10and 17). The samples that showed resonance in the Raman maps were selected for a more detailed study of Raman excitation proles (REP). The analysis of the proles using a theoretical expression for the Raman intensities allowed us to obtain the energies of the van Hove singularities and the damping parameters associated with the Raman process. Our results show a good agreement between experimental and calculated energies for van Hove singularities and demonstrate that the damping parameter does not depend on the twisting angle in the range of intermediate angles. We observed that the damping parameter ( 250 meV) is higher than those obtained in carbon nanotubes. They also present van Hove singularities, both for the radial breathing mode and for the G band. The result is similar to the optical absorption, suggesting that electron-electron and electron-hole are the dominant mechanisms. With the results of the measurements of REP in the visible region, we conclude that the processes have very dierent behaviours. In the interlayer process, we observed the enhancement of the G band as well as that of another peak associated to the phonon of the optical transverse branch of the graphene. These two proles have similar behaviour and are associated with the resonance of van Hove singularities. On the other hand, in the intralayer process, we did not observe variation in the intensity of the G band, but a peak associated with the phonon of the optical longitudinal branch which presents a resonant prole. The energy and the damping parameter of this prole are dierent when compared to the interlayer process proles. We developed a theoretical model to explain both the eects, in which we obtained a great agreement between the results of the resonant proles.Additionalmeasurementsinthe infrared and the ultraviolet regions were obtained. When confronting these measurements with the theoretical result, once again there was a good agreement between theory and experiment. In order to prove the description of the intralayer process, we produced samples of monolayer graphene deposited on top of a crystal of h-BN. We got Raman spectra that have conrmed the presence of new modes activated by the intralayer electron-phonon process. The possibility of distinguishing the intralayer and the interlayer electron-phonon interactions by the Raman spectroscopy produces a new tool to engineering heterostructures in any type of devices based on graphene. on a single-layer graphene deposited on the top of h-BN crystal conrmed the presence of new modes activated by the el-ph intralayer process. The possibility of distinguishing intralayer and interlayer el-ph interactions by Raman spectroscopy yields a new tool to engineer electrons and phonons in any kind of graphene-based device. A theoretical model was developed that successfully explains these mechanisms.