Navegando por Autor "Oliveira, Alan Barros de"

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    Ab initio molecular dynamics simulation of methanol and acetonitrile : the effect of van der Waals interactions.
    (2019) Resende, Rafael Fanni Dias; Costa, Cleidinéia Cavalcante da; Manhabosco, Taíse Matte; Oliveira, Alan Barros de; Matos, Matheus Josué de Souza; Soares, Jaqueline dos Santos; Batista, Ronaldo Junio Campos
    We employed PBE and BLYP semi-local functionals and the van der Waals density functional of Dion et al. (2004) (vdW-DF) to investigate structural properties of liquid acetonitrile and methanol. Among those functionals the vdW-DF is the only one that correctly predicts energy minima in inter-molecular interactions between acetonitrile molecules. We found that van der Waals interactions have a negligible effect on H-bonds in methanol chains. However, it significantly increases chain packing resulting in a more dense liquid in comparison to the other two functionals. The overall trend is that the vdW-DF tends to overestimate density and bulk modulus, meanwhile the semi-local functionals tend to underestimate density. Thus, van der Waals interactions play an important role in the properties of liquids in which much stronger dipole-dipole interactions are present.
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    Accurate calculation of near-critical heat capacities CP and CV of argon using molecular dynamics.
    (2017) Nichele, Jakler; Oliveira, Alan Barros de; Alves, Leonardo Santos de Brito; Borges Júnior, Itamar
    Molecular dynamics (MD) employing the Lennard-Jones (LJ) interaction potentialwas used to compute the heat capacities of argon at constant volume CV and constant pressure CP near the critical point very close to the asymptotic region. The accurateMD calculation of critical divergenceswas shown to be related to a careful choice of the cutoff radius rc and the inclusion of long-range corrections in the LJ potential. The computed CP and CV values have very good agreement as compared to available NIST data. Furthermore, values of CV in a range of temperatures for which NIST data is not available could be computed. In the investigated range of temperatures, both CP and CVMD results were fitted to a simple mathematical expression based on an empirical model that describes the critical effectswhen the asymptotic models are not appropriate. The present approach is of general applicability and robust to compute thermophysical properties of fluids in the near-critical region.
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    Apparent softening of wet graphene membranes on a microfluidic platfor.
    (2018) Ferrari, Gustavo Arrighi; Oliveira, Alan Barros de; Almeida, Ive Silvestre de; Matos, Matheus Josué de Souza; Batista, Ronaldo Junio Campos; Fernandes, Thales Fernando Damasceno; Meireles, Leonel Muniz; Silva Neto, Eliel Gomes da; Chacham, Helio; Neves, Bernardo Ruegger Almeida; Lacerda, Rodrigo Gribel
    Graphene is regarded as the toughest two-dimensional material (highest in-plane elastic properties) and, as a consequence, it has been employed/proposed as an ultrathin membrane in a myriad of microfluidic devices. Yet, an experimental investigation of eventual variations on the apparent elastic properties of a suspended graphene membrane in contact with air or water is still missing. In this work, the mechanical response of suspended monolayer graphene membranes on a microfluidic platform is investigated via scanning probe microscopy experiments. A high elastic modulus is measured for the membrane when the platform is filled with air, as expected. However, a significant apparent softening of graphene is observed when water fills the microfluidic system. Through molecular dynamics simulations and a phenomenological model, we associate such softening to a water-induced uncrumpling process of the suspended graphene membrane. This result may bring substantial modifications on the design and operation of microfluidic devices which exploit pressure application on graphene membranes.
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    Charge and spin current rectification through functionalized boron nitride bilayers.
    (2022) Moraes, Elizane Efigenia de; Pinto, Alysson Alves; Batista, Ronaldo Junio Campos; Oliveira, Alan Barros de; Chacham, Helio
    Recent experiments have reported the transformation of few-layer hexagonal boron nitride (h-BN) into sp3 -bonded c-BN layers through the application of pressure. A proposed mechanism, based on calculations and experiments, is that the phenomenon is facilitated by BN surface functionalization with OH or H radicals. In the present work, we perform ab initio calculations of ballistic electron transport, between Au electrodes, across such functionalized structures. We find that the stabilization of sp3 binding at zero pressure occurs for partial H coverage (≈58%), whereas large OH coverages (>75%) are necessary. Regarding transport properties, we find that all functionalized BN bilayer films present current rectification, consistent with the experimental findings. Maximum rectification occurs for partial OH and H coverages. The films also show spin-dependent transport, where, for a window of values of applied bias, a single spin component contributes to the total current, characterizing a spin filter behavior. Our results indicate that functionalized BN bilayer films are promising materials for the development of electronic devices where both charge and spin degrees of freedom might be manipulated.
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    A coarse-grained model based on core-oftened potentials for anomalous polymers.
    (2017) Batista, Ronaldo Junio Campos; Torres, Evy Augusto Salcedo; Oliveira, Alan Barros de; Barbosa, Marcia Cristina Bernardes
    Starting from an anomalous monomeric system, where particles interact via a two-scale coresoftened potential, we investigate how the system properties evolve inasmuch as particles are put together to form polymers whose chain size varies from 4 up to 32 monomers. We observed that the density and diffusion anomaly regions in the pressure versus temperature phase diagram of the monomeric system is smaller in the monomeric system when compared with the polymers. We also found that the polymers do not fold into themselves to form solid spheres instead they tend to maximize the chain-fluid contact.Also, Rouse and Reptation models can be employed to describe the polymers diffusive behaviour. But, in contrast to results of simulations where mere interacts via Lennard-Jones potentials, our results shown a much shorter entanglement length of atmost 8 monomers.
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    Comparison between hydroxyapatite/soapstone and hydroxyapatite/ reduced graphene oxide composite coatings : synthesis and property improvement.
    (2021) Nicomedes, Daniel Nilson Nunes; Mota, Laureana Moreira; Vasconcellos, Rebecca; Medrado, Nathanael Vieira; Oliveira, Michelle de; Alvarenga, Érika Lorena Fonseca Costa de; Juste, Karyne Ramos de Campos; Righi, Ariete; Manhabosco, Sara Matte; Silva, Guilherme Jorge Brigolini; Araújo, Fernando Gabriel da Silva; Oliveira, Alan Barros de; Batista, Ronaldo Junio Campos; Soares, Jaqueline dos Santos; Manhabosco, Taíse Matte
    Economic viability and eco-friendliness are important characteristics that make implants available to the pop- ulation in a sustainable way. In this work, we evaluate the performance of a low-cost, widely available, and eco- friendly material (talc from soapstone) relative to reduced graphene oxide as reinforcement to brittle hy- droxyapatite coatings. We employ a low-cost and straightforward technique, electrodeposition, to deposit the composite coatings on the titanium substrate. Corrosion, wear, and biocompatibility tests indicate that the reduced graphene oxide can be effectively replaced by talc without reducing the mechanical, anticorrosion, and biocompatible composite coatings properties. Our results indicate that talc from soapstone is a promising ma- terial for biomedical applications.
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    Compression induced modification of boron nitride layers : a conductive two-dimensional BN compound.
    (2019) Barboza, Ana Paula Moreira; Matos, Matheus Josué de Souza; Chacham, Helio; Batista, Ronaldo Junio Campos; Oliveira, Alan Barros de; Mazzoni, Mario Sergio de Carvalho; Neves, Bernardo Ruegger Almeida
    The ability of creating materials with improved properties upon transformation processes applied to conventional materials is the keystone of materials science. Here, hexagonal boron nitride (h-BN), a large bandgap insulator, is transformed into a conductive two-dimensional (2D) material – bonitrol – that is stable at ambient conditions. The process, which requires compression of at least two h-BN layers and hydroxyl ions, is characterized via scanning probe microscopy experiments and ab initio calculations. This material and its creation mechanism represent an additional strategy on the transformation of known 2D materials into artificial advanced materials with exceptional properties
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    Core-softened fluids, water-like anomalies, and the liquid-liquid critical points.
    (2011) Salcedo, Evy; Oliveira, Alan Barros de; Barraz Júnior, Ney Marçal; Chakravarty, Charusita; Barbosa, Márcia Cristina Bernardes
    Molecular dynamics simulations are used to examine the relationship between water-like anomalies and the liquid-liquid critical point in a family of model fluids with multi-Gaussian, core-softened pair interactions. The core-softened pair interactions have two length scales, such that the longer length scale associated with a shallow, attractive well is kept constant while the shorter length scale associated with the repulsive shoulder is varied from an inflection point to a minimum of progressively increasing depth. The maximum depth of the shoulder well is chosen so that the resulting potential reproduces the oxygen-oxygen radial distribution function of the ST4 model of water. As the shoulder well depth increases, the pressure required to form the high density liquid decreases and the temperature up to which the high-density liquid is stable increases, resulting in the shift of the liquid-liquid critical point to much lower pressures and higher temperatures. To understand the entropic effects associated with the changes in the interaction potential, the pair correlation entropy is computed to show that the excess entropy anomaly diminishes when the shoulder well depth increases. Excess entropy scaling of diffusivity in this class of fluids is demonstrated, showing that decreasing strength of the excess entropy anomaly with increasing shoulder depth results in the progressive loss of water-like thermodynamic, structural and transport anomalies. Instantaneous normal mode analysis was used to index the overall curvature distribution of the fluid and the fraction of imaginary frequency modes was shown to correlate well with the anomalous behavior of the diffusivity and the pair correlation entropy. The results suggest in the case of core-softened potentials, in addition to the presence of two length scales, energetic, and entropic effects associated with local minima and curvatures of the pair interaction play an important role in determining the presence of water-like anomalies and the liquid-liquid phase transition.
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    Crystal reorientation and plastic deformation of single-layer MoS and MoSe under uniaxial stress.
    (2021) Alencar, Ananias Borges; Oliveira, Alan Barros de; Chacham, Helio
    We investigate theoretically, through of first-principles calculations, the effect of the application of large in-plane uniaxial stress on single-layer of MoS2, MoSe2, and MoSSe alloys. For stress applied along the zigzag (zz) direction, we predict an anomalous behavior near the point fracture. This behavior is characterized by the reorientation of the MoS2 structure along the applied stress from zz to armchair due to the formation of transient square-lattice regions in the crystal, with an apparent crystal rotation of 30 degrees. After reorientation, a large plastic deformation ε = √3 − 1 remains after the stress is removed. This behavior is also observed in MoSe2 and in MoSSe alloys. This phenomenon is observed both in stress-constrained geometry optimizations and in ab initio molecular dynamics simulations at finite temperature and applied stress.
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    Crystal-oriented wrinkles with origami-type junctions in few-layer hexagonal boron nitride.
    (2015) Oliveira, Camilla Karla Brites Queiroz Martins de; Gomes, Egleidson Frederik do Amaral; Prado, Mariana C.; Souza, Thonimar Vieira de Alencar; Nascimento, Regiane do; Moreira, Leandro Malard; Batista, Ronaldo Junio Campos; Oliveira, Alan Barros de; Chacham, Helio; Paula, Ana Maria de; Neves, Bernardo Ruegger Almeida
    Understanding layer interplay is the key to utilizing layered heterostructures formed by the stacking of different two-dimensional materials for device applications. Boron nitride has been demonstrated to be an ideal substrate on which to build graphene devices with improved mobilities. Here we present studies on the morphology and optical response of annealed few-layer hexagonal boron nitride flakes deposited on a silicon substrate that reveal the formation of linear wrinkles along well-defined crystallographic directions. The wrinkles formed a network of primarily threefold and occasionally fourfold origami-type junctions throughout the sample, and all threefold junctions and wrinkles formed along the armchair crystallographic direction. Furthermore, molecular dynamics simulations yielded, through spontaneous symmetry breaking, wrinkle junction morphologies that are consistent with both the experimental results and the proposed origami-folding model. Our findings indicate that this morphology may be a general feature of several two-dimensional materials under proper stress-strain conditions, resulting in direct consequences in device strain engineering.
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    Density anomaly in a competing interactions lattice gas model.
    (2005) Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes
    Water and other tetrahedral liquids are characterized by a density anomaly whose origin is not well understood. A very simple model of a short-range attraction followed by an outer shell repulsion is proposed as a test potential for the density anomaly. We show that these competing interactions when applied to a two-dimensional lattice gas leads to the formation of two liquid phases and to the appearance of a density anomaly. The coexistence line between the two liquid phases meets a critical line between the fluid and the low-density liquid phase at a tricritical point. The line ofmaximum density emerges in the vicinity of the tricritical point, close to the demixing transition.
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    Dewetting in associating lattice gas model confined by hydrophobic walls.
    (2019) Fonseca, Tássylla Oliveira; Szortyka, Marcia Martins; Dallagnollo, Patricia Ternes; Gavazzoni, Cristina; Oliveira, Alan Barros de; Barbosa, Márcia Cristina Bernardes
    The phase behavior of a two dimensional fluid confined within hydrophobic walls is obtained by Monte Carlo simulations. The fluid is described by the associating lattice gas model which reproduces the density and diffusion anomalous behavior of water. The confined fluid exhibits a liquid-liquid critical temperature which decreases with the decrease of the distance between the confining walls. In contact with the wall a dewetting is observed. The thickness of this interfacial layer is independent of the distance between the two walls. Even for very small distances between the two walls no total depletion is observed and consequently no drying transition is present.
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    Diffusion behaviour of water confined in deformed carbon nanotubes.
    (2019) Mendonça, Bruno Henrique da Silva e; Freitas, Débora Nazaré de; Kohler, Mateus Henrique; Batista, Ronaldo Junio Campos; Barbosa, Márcia Cristina Bernardes; Oliveira, Alan Barros de
    We use molecular dynamics simulations to study the diffusion of water inside deformed carbon nanotubes with different degrees of eccentricity at 300 K. We found a water structural transition between tubular-like to single-file for (7,7) nanotubes associated with change from a high to low mobility regimes. Water is frozen when confined in a perfect (9,9) nanotube and it becomes liquid if such a nanotube is deformed above a certain threshold. Water diffusion enhancement (suppression) is related to a reduction (increase) in the number of hydrogen bonds. This suggests that the shape of the nanotube is an important ingredient when considering the dynamical and structural properties of confined water.
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    Diffusion enhancement in core-softened fluid confined in nanotubes.
    (2012) Bordin, José Rafael; Oliveira, Alan Barros de; Diehl, Alexandre; Barbosa, Márcia Cristina Bernardes
    We study the effect of confinement in the dynamical behavior of a core-softened fluid. The fluid is modeled as a two length scales potential. This potential in the bulk reproduces the anomalous behavior observed in the density and in the diffusion of liquid water. A series of NpT molecular dynamics simulations for this two length scales fluid confined in a nanotube were performed. We obtain that the diffusion coefficient increases with the increase of the nanotube radius for wide channels as expected for normal fluids. However, for narrow channels, the confinement shows an enhancement in the diffusion coefficient when the nanotube radius decreases. This behavior, observed for water, is explained in the framework of the two length scales potential.
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    Difusão da água em nanotubos de carbono sob efeito de campo elétrico.
    (2017) Freitas, Débora Nazaré de; Oliveira, Alan Barros de; Oliveira, Alan Barros de; Silva, Leandro Barros da; Matos, Matheus Josué de Souza
    A água, substância fundamental para a vida no planeta, é formada por apenas três átomos, um de oxigênio e dois de hidrogênio. Apesar da aparente simplicidade, ela possui características incomuns aos líquidos, como sua anomalia na difusão, que é o aumento da mobilidade sob pressão. Atualmente há mais de sessenta anomalias catalogadas para a água bulk. Contudo, a maior parte da água responsável pela vida está em estado confinado, dentro de células e em canais em escala nanométrica. Sendo assim, é importante se saber como ela se comporta em meios confinados. Certo avanço já foi feito nesse sentido, mas com foco em meios confinantes perfeitos e no vácuo. Neste trabalho, utilizando o método de dinâmica molecular para calcular a difusão em sistemas água-nanotubos de carbono, investigamos a ação de um campo elétrico externo atuando no sistema. Apesar de neutra, a molécula de água tem dipolo elétrico, que certamente será influenciado pela ação do campo e, assim, modificando sua difusão. A importância deste estudo se dá pelo fato da maior parte da água em meios biológicos não se encontrar isolada, pelo contrário, está na maior parte das vezes em meio iônico, logo, carregado.
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    Dinâmica ab initio de líquidos polares : acetonitrila e metanol.
    (2014) Costa, Cleidinéia Cavalcante da; Batista, Ronaldo Junio Campos; Manhabosco, Taíse Matte; Oliveira, Alan Barros de; Mota, Vinícius Cândido
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    Dynamics of the contact between a ruthenium surface with a single nanoasperity and a flat ruthenium surface : molecular dynamics simulations.
    (2011) Oliveira, Alan Barros de; Fortini, Andrea; Buldyrev, Sergey V.; Srolovitz, David
    We study the dynamics of the contact between a pair of surfaces (with properties designed to mimic ruthenium) via molecular dynamics simulations. In particular, we study the contact between a ruthenium surface with a single nanoasperity and a flat ruthenium surface. The results of such simulations suggest that contact behavior is highly variable. The goal of this study is to investigate the source and degree of this variability. We find that during compression, the behavior of the contact force displacement curves is reproducible, while during contact separation, the behavior is highly variable. Examination of the contact surfaces suggests that two separation mechanisms are in operation and give rise to this variability. One mechanism corresponds to the formation of a bridge between the two surfaces that plastically stretches as the surfaces are drawn apart and eventually separate in shear. This leads to a morphology after separation in which there are opposing asperities on the two surfaces. This plastic separation/bridge formation mechanism leads to a large work of separation. The other mechanism is a more brittle-like mode in which a crack propagates across the base of the asperity (slightly below the asperity/substrate junction) leading to most of the asperity on one surface or the other after separation and a slight depression facing this asperity on the opposing surface. This failure mode corresponds to a smaller work of separation. This failure mode corresponds to a smaller work of separation. Furthermore, contacts made from materials that exhibit predominantly brittle-like behavior will tend to require lower work of separation than those made from ductile-like contact materials.
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    Edge-reconstructed, few-layered graphene nanoribbons : stability and electronic properties.
    (2017) Gonçalves, Juliana Aparecida; Nascimento, Regiane do; Matos, Matheus Josué de Souza; Oliveira, Alan Barros de; Chacham, Helio; Batista, Ronaldo Junio Campos
    We report a first-principles study of edgereconstructed, few-layered graphene nanoribbons. We find that the nanoribbon stability increases linearly with increasing width and decreases linearly with increasing number of layers (from three to six layers). Specifically, we find that a threelayer 1.3 nm wide ribbon is energetically more stable than the C60 fullerene, and that a 1.8 nm wide ribbon is more stable than a (10,0) carbon nanotube. The morphologies of the reconstructed edges are characterized by the presence of five-, six-, and sevenfold rings, with sp3 and sp2 bonds at the reconstructed edges. The electronic structure of the few-layered nanoribbons with reconstructed edges can be metallic or semiconducting, with band gaps oscillating between 0 and 0.28 eV as a function of ribbon width.
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    Efeito do tratamento térmico no processo de calcinação da selenita em escala nanométrica.
    (2020) Silva, Thiago Rodrigo Gomes da; Barboza, Ana Paula Moreira; Manhabosco, Taíse Matte; Barboza, Ana Paula Moreira; Oliveira, Alan Barros de; Pinto, Elisângela Silva
    A selenita ou sulfato de cálcio dihidratado (CaSO4 · 2 H2O) é um mineral abundante na natureza. Trata-se de um material lamelar, de aspecto translúcido que pode ser mecanicamente esfoliado de maneira análoga ao que é feito com grafite para obtenção de grafeno. É usada principalmente na fabricação de cimento e gesso. A calcinação é um processo utilizado para remover a água de cristalização da estrutura e o processo tem inicio em temperaturas superiores a 160 ◦C na escala macroscópica. Acima dessa temperatura, há remoção de cerca de 75% da água, o que produz um material hemidrato: o gesso (CaSO ·1/2 H2O). Este projeto tem como objetivo estudar o efeito da temperatura no processo de calcinação de selenita, em escala nanométrica. O material bruto foi esfoliado mecanicamente e depositado em um substrato apropriado. Em seguida as amostras foram submetidas a diferentes temperaturas, durante o processo de aquecimento, em atmosfera ambiente. Medidas de microscopia de força atômica, microscopia ótica, microscopia de potencial de superfície e espectroscopia Raman foram utilizadas pras caracterizar as amostras. Os resultados mostram que flocos, na escala nanométrica, apresentam uma resistência térmica maior que o bulk do material. Resultados preliminares mostram que na escala nanométrica é, aparentemente, mais difícil remover a água que compõe a estrutura da selenita, se compararmos com o material em sua forma bulk. Medidas de SKPM e Raman corroboram com essa conclusão. Este resultado pode representar um passo importante para aplicação nanotecnológica da selenita, tendo em vista que se trata de uma propriedade associada à dimensionalidade da mesma.
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    Effect of TiO2 nanoparticles on polyaniline films electropolymerized at different pH.
    (2016) Leite Filho, Valfrido Furtado; Machado, Giovanna; Batista, Ronaldo Junio Campos; Soares, Jaqueline dos Santos; Oliveira, Alan Barros de; Vasconcelos, Cláudia Karina Barbosa de; Lino, Andre Alves; Manhabosco, Taíse Matte
    In this work hybrids of titanium nanoparticles and polyaniline are obtained by pulsed electrodeposition at different pH (1.5, 3.9 and 5.9) and characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, cyclic voltammetry, ultraviolet−visible, and Raman spectroscopies. We found that films deposited at pH 5.9 with nanoparticles incorporation are composed of emeraldine meanwhile films without nanoparticles are composed of pernigraniline. As a result, films deposited with nanoparticles incorporation present conductivity 6 times higher than that of films deposited without nanoparticles. Films deposited at pH 3.9 with or without nanoparticles incorporation are both made of pernigraniline. Even though films with nanoparticles incorporation still present higher conductivity. To explain such a result, we performed first-principles calculations on polyaniline/ TiO2 interface. The calculations predict a metallic polyaniline/TiO2 interface in spite of polyaniline and TiO2 being semiconductors. At pH 1.5, the presence of nanoparticles has negligible effect on films characteristics. We believe that at low pH (pH 1.5) H atoms tend to bind TiO2 surface resulting in positively charged nanoparticles, which are further screened by SO4 −2 anions. Such a screening layer prevents the physical contact between nanoparticles and polyaniline monomers diminishing the effects of nanoparticle presence.