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Zanfrognini, Matteo, (2023)  - Effetto della deformazione meccanica e di vibrazioni reticolari sulle proprietą eccitoniche di semiconduttori quasi-2D  - , Tesi di dottorato - (, , Universitą degli studi di Modena e Reggio Emilia ) - pagg. -

Abstract: This thesis centers on the effects of lattice dynamics and structural distortions on the excited-state properties of quasi two-dimensional (2D) semiconductors from a theoretical point of view: a new avenue of research which only recently is becoming amenable to predictive computational approaches. We analyse the coupling between excitonic resonances and vibrations, and show its importance for the accurate spectroscopic characterization of prototype systems. The fundamental understanding of the microscopic physical mechanisms governing light-matter interaction, including the effects of strain, is expected to be of key relevance for the design of innovative optoelectronic devices based on such materials. To gain deep insight in these mechanisms, we combine accurate first-principles calculations --based on Density Functional Theory and Many Body Perturbation Theory-- with judicious quantitative models. We then develop simple computational schemes to obtain accurate predictions, while greatly increasing the efficiency with respect to state-of-the-art ab initio methods. In this way, we are able to study complex systems that would otherwise be out of reach. We focus on the effects of strain, stacking geometries, lattice vibrations and pressure in selected relevant systems, and analyse and understand recent cathodoluminescence (CL) and inelastic X-ray scattering (IXS) experiments. First, we investigate graphene-like 2D polyaniline (also known as C3N), focusing on how uniaxial strain can tune its optical properties. We compute excitonic resonances by solving the Bethe-Salpeter equation in a tight-binding model for the electronic bands including a simplified description of the electronic screening. This model retains the accuracy of our fully ab initio calculations at greatly decreased computational cost. We also classify excitons according to the symmetries of the systems, explain the optical anisotropy of the perturbed monolayer and the non-analytic behaviour of the excitonic bands. Our analysis on C3N progresses with the ab-initio characterisation of bilayers in different stacking motifs (namely AB, AB' and AA'), where we explain the anomalous quenching of the optical absorption spectrum as induced by the interlayer coupling. Second, we implement a scheme to predict luminescence spectra, based on calculated exciton-phonon coupling terms. We analyse CL experiments on bulk Boron Nitride (BN) in two stacking motifs: AA' (hBN) and ABC (rhombohedral BN). Our calculations accurately reproduce the fine structure of the observed CL signal and explain the differences in the spectra by revealing the role of out-of-plane phonon branches involved in the photon emission process. Finally, we develop a tool to simulate the dynamical structure factor of IXS, starting from phonon dispersions DFT Density Functional Perturbation Theory. We employ this tool to provide guidance and a sound interpretation of ultra-high pressure IXS experiments and related structural transitions in MoS2.