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Title
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Cálculos ab initio de sistemas 2D y de baja dimensionalidad
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Author
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Abstract
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| Two dimensional crystals have been given a large amount of attention since the isolation of one atom thick materials in 2004. Several new two-dimensional compounds have been discovered and extensively studied in the last 15 year. In this thesis, I have studied theoretically some of this new crystals, for example graphene, germanene, germanane, fluorinated germanene, stanene and fluorinated stanene. On the theoretical side, I have extensively used Density Functional Theory (DFT) in these simple materials because the present implementations of DFT have a predictive level of accuracy. We start the discussion of our results in the third chapter with the study of adsorption and diffusion of small fluorine clusters on graphene performed by using Density Functional Theory (DFT). We considered the adsorption of fluorine atoms on one side of the graphene sheet (cis-clusters) or at both sides (trans-clusters). Several possible positions of the fluorine atoms were considered to find the most energetically favorable configuration, and its different stable and metastable magnetic states were investigated. The energy barriers for the diffusion of fluorine atoms were also calculated by using the Nudged Elastic Band (NEB) method for both cis-clusters and trans-clusters [J. Rivera-Julio, and A.D. Hernández-Nieves. Journal of Low Temperature Physics. 179: 3-8, (2015)]. In the fourth chapter, we study the electronic and vibrational properties of germanane and fluorinated germanene within Density Functional Theory (DFT) and Density Functional Perturbation Theory (DFPT) frameworks. Different structural configurations of germanane and fluorinated germanene are investigated. The energy difference between the different configurations are consistently smaller than the energy of thermal fluctuations for all the analyzed DFT functionals LDA, GGA, and hybrid functionals, which implies that, in principle, it is possible to find these different configurations in different regions of the sample as minority phases or local defects. We calculate the Raman and infrared spectra for these configurations by using ab initio calculations and compare it with the available experimental spectra for germanane. Our results show the presence of minority phases compatible with the configurations analyzed in this work [J. Rivera-Julio. et al, Journal of Physics Condensed Matter, 31(7) p.075301 (2018)]. As these low energy configurations are metastable, the present work shows that the synthesis of these energy competing phases is feasible by selectively changing the synthesis conditions, which is an opportunity to expand in this way the availability of new two-dimensional compounds. In the last part of the thesis, we investigated the topological properties of stanine and fluorinated stanene by including spin-orbit coupling effect in DFT calculations. We have studied the electronic structure of armchair stanene and fluorinated stanine nanoribbons. In the case of stanene, we found that the band gap oscillates with the width of the nanoribbon due to the quantization condition. We also studied for small armchair nanoribbons the charge distribution in real space, which allows the observation of edges states. We found that for the small sizes that are treatable by DFT calculations there is no charge localization at the edges in stanene nanoribbons, in both cases with and without including SOC. However, in larger samples and using tight-binding calculations we predict localization of charge at the edges only when spin-orbit coupling is included. Which is a strong indication that stanene is a topological insulator. We found that for fluorinated stanene the band gap decreases monotonically with increasing width and tend to zero if we included the SOC. There is charge localization at the edges in fluorinated stanene nanoribbons when we include the SOC. The results show that fluorinated stanene is a Topological Insulator and we found an interesting dependence of the gap with the size of the nanoribbon. This behavior is different from stanene armchair nanoribbons because the SOC is stronger in fluorinated stanene in comparison with stanene. |
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Language
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Spanish
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Publication
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Bariloche
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Instituto Balseiro
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2021
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Volume/pages
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137 p.
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Note
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Hernández Nieves, Alexander David [Supervisor]
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Peeters, Francois [Supervisor]
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Full text (open access)
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