Electronic structure, transport and optical properties of MoS2 monolayers and nanoribbons

• Ano de defesa: 2017
• Autor(a) principal: Ridolfi, Emilia
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Niterói
• Palavras-chave em Português: Modelo de ligação forte; Molibdênio de enxofre; Transporte eletrônico; Defeito; Nanofita; Exciton; Condutividade óptica
• Link de acesso: https://app.uff.br/riuff/handle/1/6375

First, we propose an accurate tight-binding parametrization for the band structure of MoS2 monolayers near the main energy gap. We introduce a generic and straightforward derivation for the band energies equations based on the Slater-Koster model that can be employed for other monolayer dichalcogenides. The parametrization includes spin-orbit coupling. The proposed set of model parameters reproduces both the correct orbital compositions and location of valence and conductance band in comparison with ab initio calculations. The model gives a suitable starting point for realistic large-scale atomistic electronic transport calculations. Next, we study the electronic structure and transport properties of zigzag and armchair monolayer molybdenum disufide nanoribbons using the tight-binding model mentined above, that accurately reproduces their bulk band structure near the band gap. We study the electronic properties of pristine zigzag and armchair nanoribbons, paying particular attention to the edge states that appear within the MoS2 bulk gap. By analyzing both their orbital composition and their local density of states, we find that in zigzag-terminated nanoribbons, in distinction to graphene, edge states can be localized at a single edge for certain energies independent of the nanoribbon width. We also study the effects of disorder in these systems using the recursive Green's function technique. We show that for the zigzag nanoribbons, the conductance due to the edge states is strongly suppressed by short-range disorder such as vacancies. In contrast, the local density of states still shows edge localization. We also show that long-range disorder has a small effect on the transport properties of nanoribbons within the bulk gap energy window. Finally, we study the excitonic spectrum of MoS2 monolayers. Our approach takes into account the anomalous screening in two dimensions and the presence of a substrate by the effective Keldish potential. The Bethe-Salpeter equation is solved for a multi-band tightbinding description of the single particle spectrum. We obtain the main features of the optical conductivity spectrum analyzing the localization in k-space of the excitonic peaks. We study the effect of different TB descriptions on the main features of the experimental optical absorption, namely, the contributions of higher energy bands, and the validity of reduced TB models with and without spin-flipping terms in the spin-orbit contribution to the Hamiltonian. We compare the absolute magnitude of the linear optical conductivity, one-particle optical conductivity and one of the most recent experimental measurements of this quantity. Other related TB and effective-mass calculations fail to reproduce the actual experimental magnitude of the optical conductivity.