Dottorato in FISICA

Titolo della tesi: To the Infrared and Beyond: Resonance Raman Spectroscopy with Infrared Excitation Energy to Study Scattering Processes in Layered Materials

The discovery of 2D materials and their practical realization have opened up one of the most flourishing fields in solid state physics. Recent synthesis capabilities, as well as the possibility to combine different materials together, and reduce their dimensionality, have sparked hope for thorough control and tailoring of material properties. In this thesis I propose a novel experimental approach by leveraging on Raman processes resonant with electronic states in the infrared. Resonance Raman scattering has been proven to be a key asset to study small-gap semiconductors and Dirac systems such as carbon nanotubes and graphene. In particular by studying the resonance Raman response one can gain information on the electron-phonon coupling (EPC), a fundamental interaction in solid state systems. To date Raman spectroscopy is commonly performed using visible excitation energy, mainly due to the higher Raman cross section. This has prevented the study of how lattice vibrations affect low energy conduction electrons in layered semiconductors displaying an infrared bandgap energy or in Dirac materials, such as graphene. The scope of this PhD work is to address the Resonance Raman response of layered materials by means of infrared excitation energies. To this end, different materials will be investigated and several excitation energies will be exploited. In this thesis I critically discuss the feasibility and the experimental advances needed to perform resonance Raman with infrared excitation energy. I subsequently present results on different layered materials: In graphene I have focused on the study of the interplay of electron phonon coupling and its role in determining the intensity and the lineshape of the double resonance Raman peaks. In particular my results show the existence of a momentum-dependent EPC, providing the first experimental validation to theoretical predictions [DM Basko and and I.L. Aleiner. Physical Review B 77.4 (2008): 041409.]. A second interesting class of materials to study with infrared excitation energy are transition metal dichalcogenides, layered materials that can be thinned toward the single atomic layer. I will in particular focus on those materials showing an electronic bandgap energy falling in the infrared, hence addressing those scattering phenomena occurring among carriers at the bottom of the conduction band and lattice vibrations. Among the samples studied I report on the possibility of unambiguously assign the phonon branches involved in high order Raman modes of MoSe2 while exciting close to the lowest indirect optical transition of the material, or study the helicity dependent photoluminescence emission in MoTe2, exciting resonantly with its excitons.