Palestrante: M. A. Pimenta (Departamento de Física, UFMG, 30123-970 Belo Horizonte, Brazil)

Resumo:

In recent years, novel graphene-like two-dimensional (2D) structures have appeared into the scientific scene and different systems are now produced as atomically thin structures. Serious efforts are now focused on studying their unique electronic structure. Raman and Resonance Raman spectroscopy are very useful tools to provide many different information in 2D materials such as defects, charges, strain, edges, excitons and their coupling with phonons. In this talk, I will present Raman results in three different types of 2D materials: (i) the transition metal dichalcogenides  (MoS2, WS2 and WSe2), (ii) twisted bilayer graphene, and (iii) black phosphorus. The Raman spectra of the transition metal dichalcogenides MoS2, WS2 and WSe2 with one, two, three layers and bulk, were measured using more than 30 different laser lines covering the visible range. Results show that all Raman features are enhanced by resonances with excitons, allowing us to obtain the dependence of their energies with the number of atomic layers [1]. In the case of MoS2, we observed that the electron-phonon coupling is symmetry dependent, and our results provide evidence of the C exciton, predicted theoretically [2]. I will then present a Raman study of more than 100 samples of twisted bilayer graphene (TBG) with a continuous distribution of twisting angles from 0 to 30◦. From the Raman maps of the samples, we could observe giant enhancements of the G band for samples with twisting angles between 9◦ and 17◦. Our results allow us to conclude that the enhancement is due to resonances with van Hove singularities associated with the Moiré pattern, which does not necessarily exhibit a translational symmetry. Finally, I will present results in black phosphorus, which has recently emerged as a new two-dimensional crystal with a peculiar and anisotropic crystalline and electronic band structures. Results show that the unusual angular dependence of the polarized spectra can only be explained by taking into account complex values of the Raman tensor elements [3]. Different black phosphorus edges were experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations [4]. Results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements.
[1] E. del Corro et al, ACS Nano 8, 9629 (2014)
[2] B. R. Carvalho, Phys. Rev. Letters 114, 136403 (2015)
[3] H. B. Ribeiro et al, ACS Nano 9, 4270 (2015)
[4] H. B. Ribeiro et al, Nature Comm. vol 7, 12191 (2016)