Papers by Marilia J Caldas
Estrutura eletrônica das melaninas
As melaninas constituem uma classe de pigmentos biologicos com funcao basica de fotoprotecao e co... more As melaninas constituem uma classe de pigmentos biologicos com funcao basica de fotoprotecao e com caracteristicas de semicondutor amorfo. Embora a estrutura e composicao destes pigmentos nao estejam bem caracterizados, nao restam duvidas de que a 5,6,indolquina (IQ) e o monomero mais abundante. Neste trabalho nos estudamos a estrutura eletronicas da melanina, com base em modelos de polimeros de IQ. Nos utilizamos o metodo de Huckel Simplificado (HS) para a investigacao das propriedades eletronicas de cadeias polimericas, finitas e infinitas (no limite de Bloch), ordenadas e com a presenca de desordem. Os dados obtidos atraves da utilizacao de metodos mais sofisticados (MNDO e INDO) para os monomeros e dinamos de IQ validam as conclusoes obtidas com o HS. O modelo que aqui propomos para a estrutura eletronica das melaninas explica satisfatoriamente a maioria dos dados experimentais disponiveis sobre este material (paramagnetismo, propriedades oticas, etc.), bem como parece confirmar a hipotese da existencia de um mecanismo de defesa celular baseado na captura eletronica de radicais livres (potencialmente citotoxicos) Abstract
New optoeletronic multiblock copolymer: experimental and theoretical study
Program, 2004
Defects in organic semiconductors: polaron states in polyanilines
Geometrical and spectroscopical study on the molecules related to eumelanins
Protonação em oligômeros de polianilinas
arXiv: Mesoscale and Nanoscale Physics, 2016
Vacancy in graphene has been proposed to give rise to magnetism, from experimental and theoretica... more Vacancy in graphene has been proposed to give rise to magnetism, from experimental and theoretical results. Several calculations based on density functional theory (DFT) have been reported in the past decade: these studies yielded widely varying results of magnetic moment in the range of $\mu=1.04-2.0$ $\mu_{B}$. We present a theoretical study of the defect using two approaches, cluster models and periodic boundary conditions. We also apply two different formalisms, pure DFT and hybrid DFT including a fraction of Hartree Fock exchange. We show that the differences in magnetization found for the vacancy are caused by fractional electron occupation of the upper valence bands, that cross the Fermi level when we use DFT, an effect of the self-interaction error. Furthermore, it is demonstrated that the use of the hybrid functional retrieves the correct magnetic properties yielding a final magnetization of $\mu=2\mu_{B}$.
Metodo hartree-fock lcao para sistemas periodicos
Study of 4d and 5d impurities in gallium arsenide
Study of wet oxidations process on Si(100)2x1:H using a first principles methods
Abstract Submitted for the MAR10 Meeting of The American Physical Society
Center nanoStructures and bioSystems at Surfaces (S3), Modena, Italy | Atomisticlevel understandi... more Center nanoStructures and bioSystems at Surfaces (S3), Modena, Italy | Atomisticlevel understanding of photovoltaic processes for hybrid polymer-oxide structures is a great challenge. The mechanisms of charge transfer across the interface are not known, the electronic characteristics of the two components are very difierent, and for the polymer depend strongly on the fllm morphology. The experimental analysis of the interface is di‐cult, so theoretical simulations can be of great value, but the study carries severe di‐culties. We investigated the formation of the interface between thiophene oligomers and the TiO2(101)-anatase surface, starting from classical molecular dynamics simulations for oligomer deposition, and proceeding to ab initio calculations based on Density Functional Theory (DFT) for electronic properties. We observed the formation of rough thiophene fllms, with a majority of molecules oriented almost normal to the surface, also molecules lying-down in the flrst interf...
Anthropogeneous Dark Earth or Terra Preta do Índio: water and stacked graphene nanoflakes
arXiv: Materials Science, 2018
Blends of polymer and C$_{60}$-derived molecules are in the spotlight in recent years for applica... more Blends of polymer and C$_{60}$-derived molecules are in the spotlight in recent years for application in organic photovoltaics, forming what is known as bulk-heterojunction active layers. The character of the heterojunction is determinant, with clear relevance of morphology and phase separation. To better understand the morphology of the systems, we present a classical molecular dynamics (CMD) simulation of polymer/fullerene (P3HT/C$_{60}$) blends, coming from different starting points, using the specifically designed Nanomol Force Field based on the Universal Force Field. We use not-so-short regioregular polymers with 30 hexyl-thiophene units ($\sim$5kg$\cdot$mol$^{-1}$ molecular weight) and, adopting a $\sim$ 1:1 mass proportion for the polymer:molecule blend, we simulate cells of $\sim$ 40 thousand atoms, at room temperature and normal pressure conditions. We find that, independently of the starting-point spatial distribution of C$_{60}$ molecules relative to P3HT chains, segrega...
Physical Review Materials
is an emerging two-dimensional indirect band gap semiconductor with interesting mechanical, therm... more is an emerging two-dimensional indirect band gap semiconductor with interesting mechanical, thermal, and electronic properties. In this paper we present a description of C 3 N electronic and dielectric properties, focusing on the so-called momentum-resolved exciton band structure. Excitation energies and oscillator strengths are computed in order to characterize bright and dark states, and discussed also with respect to the crystal symmetry. Activation of excitonic states is observed for finite transferred momenta: Indeed, we find an active indirect exciton at ∼ 0.9 eV, significantly lower than the direct optical gap of 1.96 eV, with excitonic binding energies in the range 0.6-0.9 eV for the lowest states. As for other 2D materials, we find a quasilinear excitonic dispersion close to , which however shows a downward convexity related to the indirect band gap of C 3 N as well as to the dark nature of the involved excitons.
Excitonic dispersion in monolayer C 3 N
Bulletin of the American Physical Society, Mar 18, 2021
Electronic Structure, 2022
Thiophene oligomers are an important class of organic materials for photovoltaic applications, ow... more Thiophene oligomers are an important class of organic materials for photovoltaic applications, owing to their unique optoelectronic properties. Recently it has been suggested that incorporation of furan units to the thiophene chains, maintaining the chain structure, namely thienylfuran linear oligomers, can bring improvements to the final material. In this work, we present a theoretical study of thiophene, furan and thienylfuran short chains, up to 4 units. Structural and electronic properties were obtained using Hartree-Fock (HF) and Density Functional Theory (DFT) calculations plus beyond mean-field methodologies, specifically Second-order Möller-Plesset perturbation theory on HF (HF-MP2) and many-body perturbation theory by the G0W0 approximation on DFT (G0W0@DFT). The optical properties were calculated on top of G0W0@DFT data using the Bethe-Salpeter Equation. We investigate properties from the monomers T and F to tetramers with different sequencing of units TT, TF or FF, always...
Chemical Physics Letters, 2021
In this work, we propose a new auxetic (negative Poisson's ratio values) structure, based on a γ-... more In this work, we propose a new auxetic (negative Poisson's ratio values) structure, based on a γ-graphyne structure, here named AγG structure. Graphynes are 2D carbon allotropes with phenylic rings connected by acetylenic groups. The AγG structural/mechanical and electronic properties, as well as its thermal stability, were investigated using classical reactive and quantum molecular dynamics simulations. We found that AγG has a large bandgap of 2.48 eV and is thermally stable at a large range of temperatures. It presents a Young's modulus that is an order of magnitude smaller than that of graphene or γ-graphyne. The classical and quantum results are consistent and validate that the AγG is auxetic, both when isolated (vacuum) and when deposited on a copper substrate. We believe that this is the densest auxetic structure belonging to the graphyne-like families.
Physical Review Materials, 2019
Doping is one of the most common strategies for improving the photocatalytic and solar energy con... more Doping is one of the most common strategies for improving the photocatalytic and solar energy conversion properties of TiO2, hence an accurate theoretical description of the electronic and optical properties of doped TiO2 is of both scientific and practical interest. In this work we use many-body perturbation theory techniques to investigate two typical n-type dopants, Niobium and Hydrogen, in TiO2 rutile. Using the GW approximation to determine band edges and defect energy levels, and the Bethe Salpeter equation for the calculation of the absorption spectra, we find that the defect energy levels form non-dispersive bands lying 2.2 eV above the top of the corresponding valence bands (0.9 eV below the conduction bands of the pristine material). The defect states are also responsible for the appearance of low energy absorption peaks that enhance the solar spectrum absorption of rutile. The spatial distributions of the excitonic wavefunctions associated with these low energy excitations are very different for the two dopants, suggesting a larger mobility of photoexcited electrons in Nb-TiO2.
The Journal of Physical Chemistry C, 2018
Thermally-activated delayed fluorescence has been found in a group of tricarbazolylamines which a... more Thermally-activated delayed fluorescence has been found in a group of tricarbazolylamines which are purely electron-donating, non-charge transfer (CT) molecules. We show that the reverse intersystem crossing step in these materials is mediated through upper triplet states. Reverse internal conversion is shown to be the thermally-activated mechanism behind the triplet harvesting mechanism. The strongly mixed n-π*/π-π* character of the lowest energy optical transitions retains high oscillator strength and gives rise to high ΦPL. OLED devices using these materials were fabricated to show very narrow (FWHM = 38 to 41 nm) electroluminescence spectra, clearly demonstrating the excitonic nature of the excited states. This new combination of physicochemical properties of a non-CT molecule yield TADF, but via a different, physical mechanism, reverse internal conversion delayed fluorescence (rICDF).
Polarized emission from stretched PPV films viewed at the molecular level
Physical Chemistry Chemical Physics, 2015
We present a study on the photoluminescence (PL) of thin films of poly-(p-phenylene vinylene) (PP... more We present a study on the photoluminescence (PL) of thin films of poly-(p-phenylene vinylene) (PPV), non-stretched and uniaxially stretched.
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Papers by Marilia J Caldas