Browsing by Author "Dias, Rafael Freitas"

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    Investigação por métodos de primeiros princípios do metanol.
    (Programa de Pós-Graduação em Ciências – Física de Materiais. Departamento de Física, Instituto de Ciências Exatas e Biológicas, Universidade Federal de Ouro Preto., 2013) Dias, Rafael Freitas; Batista, Ronaldo Junio Campos
    Neste trabalho, aplicamos métodos de primeiros princípios, baseado na teoria do funcional da densidade para investigar as propriedades estruturais e a estabilidade do metanol líquido sob efeito de campo elétrico externo. Utilizando o potencial ReaxFF para gerar os líquidos usados na dinâmica de primeiros princípios conseguimos reduzir o tempo computacional para obter a convergência da energia (600 ps usando ReaxFF mais 1 ps utilizando primeiros princípios). Observamos que o funcional PBE é capaz de descrever com boa precisão (erro de 4%) a densidade correta do metanol em condições normais de temperatura e pressão, que no caso da água somente é obtido com a inclusão da interação de Van der Waals. Vimos que o campo elétrico externo alinha as moléculas formando cadeias unidimensionais. Observamos que a estabilidade aumenta com o número de moléculas na cadeia. A presença de reservatórios de cargas facilita a quebra do metanol sendo que, é energeticamente favorável que a quebra ocorra no reservatório de elétrons. Vimos também que se a quebra ocorre no ligação C - H é energeticamente favorável que a quebra ocorre na molécula da cadeia mais próxima ao eletrodo. Entretanto, se a quebra ocorrer na ligação O - H é energeticamente favorável que a quebra aconteça na molécula da cadeia mais distante ao eletrodo.
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    Nanomechanics of few-layer materials : do individual layers slide upon folding?
    (2020) Batista, Ronaldo Junio Campos; Dias, Rafael Freitas; Barboza, Ana Paula Moreira; Oliveira, Alan Barros de; Manhabosco, Taíse Matte; Silva, Thiago R. Gomes; Matos, Matheus Josué de Souza; Gadelha, Andreij de Carvalho; Rabelo, Cassiano; Cançado, Luiz Gustavo de Oliveira Lopes; Jorio, Ado; Chacham, Helio; Neves, Bernardo Ruegger Almeida
    Folds naturally appear on nanometrically thin materials, also called “2D materials”, after exfoliation, eventually creating folded edges across the resulting flakes. We investigate the adhesion and flexural properties of single-layered and multilayered 2D materials upon folding in the present work. This is accomplished by measuring and modeling mechanical properties of folded edges, which allows for the experimental determination of the bending stiffness (κ) of multilayered 2D materials as a function of the number of layers (n). In the case of talc, we obtain κ ∝ n 3 for n ≥ 5, indicating no interlayer sliding upon folding, at least in this thickness range. In contrast, tip-enhanced Raman spectroscopy measurements on edges in folded graphene flakes, 14 layers thick, show no significant strain. This indicates that layers in graphene flakes, up to 5 nm thick, can still slip to relieve stress, showing the richness of the effect in 2D systems. The obtained interlayer adhesion energy for graphene (0.25 N/m) and talc (0.62 N/m) is in good agreement with recent experimental results and theoretical predictions. The obtained value for the adhesion energy of graphene on a silicon substrate is also in agreement with previous results.
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    Nanoporous graphene and H‑BN from BCN precursors : first-principles calculations.
    (2018) Dias, Rafael Freitas; Martins, Jonathan da Rocha; Chacham, Helio; Oliveira, Alan Barros de; Manhabosco, Taíse Matte; Batista, Ronaldo Junio Campos
    We propose, based on results of first-principles calculations, that nanoporous graphene and h-BN might be efficiently produced from B–C–N layers as precursors. In our calculations, we find that the removal of the h-BN islands that naturally occur in BN-doped graphene, forming nanoporous graphene, requires less energy than if pristine graphene is used as a precursor. The same reduction ΔEf in pore formation energy is found for nanoporous h-BN obtained from graphene-doped BN as a precursor. ΔEf is found to increase linearly as a function of the number of B–C and N–C bonds at the island boundary, with the slope being nearly the same for either porous graphene or porous h-BN. This is explained by an analytical bond-energy model. In the case of porous graphene, we find that the pore formation energy would be further reduced by passivation by pyridinic and quaternary remnant nitrogen atoms at the pore edges, a mechanism that is found to be more effective than the passivation by hydrogen atoms. Both mechanisms for pore formation energy reduction should lead to a possibly efficient method for nanoporous graphene production.