Estudo ab initio por DFT+U de monocamadas de Sn(1-x)SeAg(x), SnSe(1-x)Ag(x) e ZnX(X=O, S, Se, Te)

Abstract

The present thesis addresses two types of theoretical studies for two-dimensional crystalline systems. Initially, the Hamiltonian model approach has been used to study a system composed of graphene deposited on a hexagonal lattice substrate taking into account the Hubbard interaction. The prinicipal objective is to understand the mechanisms that could induce a gap in the dispersion relation of the system. A comparison between the results of the above model and the ab initio approach, via Density Functional Theory - DFT, shows a good agreement, inspite od having different philosophies. The gap in this system could be induced by the inhomogeneity of Hubbard's interaction in the substrate sublattices or by the strong hybridization between the monolayers. The second part of our work involves the use ab initio approach via DFT with Hubbard's interaction, a method known as DFT+U to study the electronic and thermoelectric properties of SnSe monolayer and its derivatives. Pristine structure (M-SnSe) along with that modified by vacancy (M-SnSe(1-x), M-Sn(1-x)Se) and also doped by substitution of Ag atoms (M-SnSe(1-x)Ag(x), M-Sn(1-x)SeAg(x)) were considered. In this work, x corresponds to the proportions of 25% and 12.5%. The effects of vacancies and Ag doping on the thermoelectric performance of the materials are considered. The results show that the Sn[Se] vacancies and the substitutional doping by Ag, reduce the thermoelectric performance when compared to that of the pure structure. Sn vacancies were more detrimental to thermoelectric performance than Se vacancies. Using the same method, the elastic and dynamic stability, as well as the electronic, thermoelectric and optical properties of the ZnX monolayers (X = O, S, Se, Te) are also studied. The results show that the structures are dynamically stable, have a direct gap at the Gamma point symmetry. They have low lattice thermal conductivity and their thermoelectric performance is approximately 1. ZnO and ZnS are transparent to visible light where ZnSe and ZnTe have orange and blue colors, respectively. The observed shift in the energy from the absorption threshold to the ultraviolet region governed by selection rules appears when the polarization is perpendicular to the plane of the monolayers. The absorption spectra are displaced to the ultraviolet region by 1.96 eV (ZnO), 1.46 eV (ZnS), 1.51 eV (ZnSe) and 1.88 eV (ZnTe).