Defesa de Tese de Doutorado #399 – Geovani Carvalho de Resende – 30/06/2022

In this thesis, we discuss the power of polarized Raman spectroscopy to study optically anisotropic 2D materials, belonging to the orthorhombic, monoclinic and triclinic crystal families. We start by presenting measurements of angle-resolved polarized Raman spectra of single-layer (1L) and bulk ReSe$_2$ recorded with excitation energies of 1.92 eV (647.1 nm) and 2.34 eV (530.8 nm). The Raman tensors for all modes were obtained by fitting simultaneously the angular dependence of the parallel and crossed polarized intensities. We observed that the tensor elements are, in general, complex numbers and their magnitudes and phases depend on both the dimensionality of the sample (1L or bulk) and the excitation energy. We show that the wavevector dependence of the electron-phonon interaction is essential for explaining the distinct Raman tensor for each phonon mode. Results are discussed by considering the intrinsic contribution of a single layer to the tensor elements and the macroscopic contribution coming from the stacking of several layers. We show that the different behavior of angle-resolved polarized Raman spectra for different excitation energies is due to the resonant Raman effect, which affects both the real and imaginary parts of the Raman tensor elements. In addition, we have studied resonance effects in ReS$_2$ and black phosphorus in distinct polarized configurations. Our results show that each phonon has a unique coupling with the excited states that can differ drastically from the coupling of the other modes. Moreover, we show that double-resonance processes occur for first-order Raman modes of ReS$_2$ for excitation energies in the 1.49-1.59 eV range that are close to the energies of the three excitonic transitions of the compound. Finally, we unveil a resonance in the visible range in black phosphorus by resonant Raman scattering in the parallel polarized configuration along the few explored b-axis, which is perpendicular to the layer plane. In contrast to the results obtained in polarized configurations along the axes in the layer plane, both totally symmetric $A_g^1$ and $A_g^2$ show strong electron-phonon coupling for this resonance. Finally, we close with our concluding remarks and perspectives to be explored by polarized and resonance Raman spectroscopy in optically anisotropic 2D materials.