arXiv subjects

Alexandre F. Fonseca

Publications and source records attributed to Alexandre F. Fonseca.

At least 37 records · Page 2Linked to original sources

arXiv2022

Investigating thermal transport in knotted graphene nanoribbons using non-equilibrium molecular dynamics

In this work, we investigated the effect of knots in the thermal transport of graphene nanoribbons through non-equilibrium molecular dynamics simulations. We considered the cases of one, two, and three knots are present. Temperature jumps appear in the temperature profile where the knots are located, which indicates that they introduce thermal resistances in the system, similar to interfacial Kapitza resistance present between two different materials and/or single materials with defects and/or lattice distortions. We found that the thermal resistance introduced by each individual knot is essentially the same as the overall resistance increase linearly with the number of knots, as they behave as thermal resistances associated in series. Also, the relative position between each knot in the arrangement does not strongly affect the thermal current produced by the temperature gradient, showing a weak thermal rectification effect.

cond-mat.mes-hall↗

arXiv2022

Fully Atomistic Molecular Dynamics Simulations of Elastic Properties of Tetragraphene Monolayer

A quasi-2D semiconductor carbon allotrope called tetrahexcarbon, also named tetragraphene, was recently proposed featuring an unusual structure combining squared and hexagonal rings. Mechanical and electronic properties of tetragraphene have been predicted based on first-principles Density Functional Theory (DFT) calculations. However, a comprehensive study of its mechanical behavior under different temperatures is still lacking. In this work, using fully atomistic reactive molecular dynamics (MD) simulations, we investigate the mechanical properties of monolayer tetragraphene under tensile strain from the linear regime up to the complete structural failure (fracture). Different temperatures were considered and the results were compared to that of two other known planar carbon allotropes: graphene and penta-graphene. One interesting result is that tetragraphene experiences a transition from crystalline to an amorphous structure by either temperature or tension application. At room temperature, the critical strains along the two orthogonal unit-cell directions of tetragraphene are 38\% and 30\%, which is higher than that for graphene and penta-graphene. Tetragraphene Young's modulus values along its directions are from three to six times smaller than that of graphene and about 57\% that of penta-graphene at room temperature. Ultimate tensile strength values along the two directions of tetragraphene were obtained and also shown to be smaller than that of graphene and penta-graphene.

cond-mat.mtrl-sci↗

arXiv2022

High performance of carbon nanotube refrigerators over a large temperature span

Compression of greenhouse gases still dominates the market of refrigeration devices. Although well stablished and efficient, this technology is neither safe for the environment nor able to be scaled down to nanoscale. Solid-state cooling technologies are being developed to overcome these limitations, including studies at nanoscale. Among them, the so-called elastocaloric effect (eC) consists of the thermal response, $ΔT$, of a material under strain deformation. In this work, fully atomistic molecular dynamics simulations of the eC in carbon nanotubes (CNTs) are presented over a large temperature span. The efficiency of the CNTs as solid refrigerators is investigated by simulating their eC in a model of refrigerator machine running under Otto-like thermodynamic cycles (two adiabatic expansion/contraction plus two isochoric heat exchange processes) operating at temperatures, $T_\mbox{O}$, ranging from 300 to 2000 K. The coefficient-of-performance (COP), defined as the ratio of heat removed from the cold region to the total work performed by the system per thermodynamic cycle, is calculated for each value of $T_\mbox{O}$. Our results show a non-linear dependence of $ΔT$ on $T_\mbox{O}$, reaching a minimum value of about 30 K for $T_\mbox{O}$ between 500 and 600 K, then growing and converging to a linear dependence on $T_\mbox{O}$ for large temperatures. The COP of CNTs is shown to remain about the same and approximately equal to 8. These results are shown to be weakly depend on CNT diameter and chirality but not on length. The isothermal entropy change of the CNTs due to the eC is also estimated and shown to depend non-linearly on $T_\mbox{O}$ values. These results predict that CNTs can be considered versatile nanoscale solid refrigerators able to efficiently work over a large temperature span.

cond-mat.mes-hall↗

arXiv2021

Air filtration from sarin/air mixture by porous graphene-oxide membranes: a molecular dynamics study

Sarin is a very lethal synthetic organophosphorated compound that inhibits the nervous system muscle control. Although not used as a chemical weapon anymore, it still worries the authorities regarding possible use by terrorists. Most of the studies about sarin are theoretical/computational due to its high lethality and are concentrated in its detection and degradation. Few studies are about air filtration from sarin gas. Here, the potential of graphene oxide-based membranes to filter air from sarin/air mixtures is investigated by classical molecular dynamics simulations. Membranes formed by one and two nanosheets of porous reduced graphene oxide (rGO) were considered. The passage of sarin and air molecules through these membranes from a highly concentrated region to an empty one, is evaluated as a function of temperature and sarin/air relative concentration. Sarin molecules are shown to be trapped by hydroxyl and carboxyl chemical groups in the nanosheet, while a considerable passage of air molecules (N$_2$, O$_2$ and Ar) through the membranes was verified. The results show the capacity of the rGO membranes to retain sarin from passing through, even at high temperatures, thus indicating their potential to be used as a filter for sarin gas.

cond-mat.mtrl-sci↗

arXiv2021

Effective acetylene length dependence of the elastic properties of different kinds of graphynes

Graphyne is a planar network of connected carbon chains, each formed by $n$ acetylene linkages. Uncountable ways to make these connections lead to uncountable structural graphyne families (GFs). As the synthesis of graphynes with $n > 1$ has been reported in literature, it is of interest to find out how their physical properties depend on $n$ for each possible GF. Although literature already present specific models to describe the dependence on $n$ of the elastic properties of specific GFs, there is not yet enough amount of data for the physical properties of different graphynes with different values of $n$. Based on fully atomistic molecular dynamics simulations, the Young's modulus, shear modulus, linear compressibility and Poisson's ratio of 10 graphyne members of 7 different GFs are calculated. A simple elastic model consisting of a serial combination of $n$ springs is proposed to describe the dependence on $n$ of the elastic properties of these 7 GFs. We show that except for the Poisson's ratio, this simple unique elastic model is able to numerically describe, with good precision, the Young's modulus, shear modulus and linear compressibility of all different graphynes, including anisotropy and negative values of linear compressibility of some GFs.

cond-mat.mtrl-sci↗

arXiv2021

Auxetic properties of a newly proposed $γ$-graphyne-like material

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 A$γ$G structural/mechanical and electronic properties, as well as its thermal stability, were investigated using classical reactive and quantum molecular dynamics simulations. We found that A$γ$G 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 A$γ$G 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.

cond-mat.mtrl-sci↗

arXiv2021

Twisting or untwisting graphene twisted nanoribbons without rotation

The common sense regarding twisting or untwisting a ribbon is that it requires the application of an external rotation to happen. However, at nanoscale, the application of precise amounts of rotation on a nanoribbon is not a trivial task. Here, the concept of an alternative method to add twist to or remove twist from a twisted graphene nanoribbon (TGNR) without rotation is presented. The method consists of suspending a TGNR on two separate substrates and by changing only their distance, the total amount of twist of the TGNR is shown to change. The possibility to fine-tuning the amount of twist of a TGNR is also shown. The concept is demonstrated through fully atomistic molecular dynamics simulations and numerical calculations of the topological parameters twist and writhe of a TGNR. It is shown that the above process satisfies the so-called linking number theorem of space curves. Besides being experimentally feasible, this concept reveals a new kind of twist to writhe transition phenomenon that is tension-free and does not require controlling neither the nanoribbon end-to-end distance nor its critical twist density.

cond-mat.mes-hall↗

arXiv2020

Graphene-based Nanoscale version of da Vinci's Reciprocal Structures

A reciprocal structure (RS) is a mechanical resistant structure formed by a set of self-supporting elements satisfying certain conditions of structural reciprocity (SR) . The first condition is that each element of the structure has to support and be supported by the others. The second condition is that these functions cannot occur in the same part of the element. These two properties make beams and two-dimensional materials very much appropriate to build RSs. Commonly seen in floors or roofs, SR is also present in art, religious symbols and decorative objects. Da Vinci has drawn several examples of such RSs. Here, we propose a simple nano version of a da Vinci's RS based on graphene nanoribbons. The stability and resistance against mechanical impacts (ballistic projectile) were investigated through fully atomistic molecular dynamics (MD) simulations. We considered structures with three and four joins with and without RS topologies. Our MD results showed that structures with RS topologies are more impact resistant than those without SR, despite the fact that the used graphene nanoribbons are highly pliable. We discuss these results in terms of the number of joins, energy absorption and stress on the structures. We discuss possible applications in nanoengineering.

cond-mat.mtrl-sci↗

arXiv2020

Sarin and Air Permeation Through a Nanoporous Graphene

Sarin gas is a dangerous chemical warfare agent (CWA). It is a nerve agent capable of bringing a person to death in about 15 minutes. A lethal concentration of sarin molecules in air is about 30 mg/m3. Experimental research on this gas requires very careful safety protocols for handling and storage. Therefore, theoretical and computational studies on sarin gas are very welcome and might provide important safe guides towards the management of this lethal substance. In this work, we investigated the interactions between sarin, air and nanoporous graphene, using tools of classical molecular dynamics simulations. Aiming to cast some light in the possible sarin selective filtration by graphene, we designed a bipartite simulation box with a porous graphene nanosheet placed at the middle. Sarin and air molecules were initially placed only on one side of the box so as to create an initial pressure towards the passage of both to the other side. The box dimensions were chosen so that the hole in the graphene was the only possible way through which sarin and air molecules can get to the other side of the box. The number of molecules that passed through the hole in graphene was monitored during 10 ns of simulation and the results for different temperatures were compared. The results show that, as far as the size of the holes are small, van der Waals forces between graphene and the molecules play a significant role on keeping sarin near graphene, at room temperature.

cond-mat.mes-hall↗

arXiv2019

Elastic and Fracture Properties of Single Walled Pentagraphene Nanotubes

Membranes of carbon allotropes comprised solely of densely packed pentagonal rings, known as pentagraphene, exhibit negative Poisson's ratio (auxetic behavior) and a bandgap of $3.2$ eV. In this work, we investigated the structural stability, mechanical and fracture properties of nanotubes formed by rolling up pentagraphene membranes, the so-called pentagraphene nanotubes (PGNTs). Single-walled PGNT of three distinct configurations: zigzag, $α$-armchair, and $β$-armchair were studied combining first-principles calculations and reactive molecular dynamics simulations. Our results showed Young's modulus values of $680-800 GPa$, critical strain of $18-21\%$, and ultimate tensile stress of $85-110 GPa$. We also observed auxetic behavior. During stretching at room temperature, we observed a transition between $β$-armchair to $α$-armchair PGNT close to the critical strain. With relation to fracture patterns, we observed that mechanical failure starts at bonds mostly aligned to the stretching direction and after tube radial collapse.

cond-mat.mtrl-sci↗

arXiv2019

Torsional refrigeration by twisted, coiled, and supercoiled fibers

Higher efficiency, lower cost refrigeration is needed for both large and small scale cooling. Refrigerators using entropy changes during cycles of stretching or hydrostatically compression of a solid are possible alternatives to the vapor-compression fridges found in homes. We show that high cooling results from twist changes for twisted, coiled, or supercoiled fibers, including those of natural rubber, NiTi, and polyethylene fishing line. By using opposite chiralities of twist and coiling, supercoiled natural rubber fibers and coiled fishing line fibers result that cool when stretched. A demonstrated twist-based device for cooling flowing water provides a high cooling energy and device efficiency. Theory describes the axial and spring index dependencies of twist-enhanced cooling and its origin in a phase transformation for polyethylene fibers.

physics.app-ph↗

arXiv2019

Dynamics of Graphene/Al Interfaces using COMB3 Potentials

This work describes the development of a third-generation charge optimized many-body (COMB3) potential for Al-C and its application to the investigation of aluminum/graphene nanostructures. In particular, the new COMB3 potential was used to investigate the interactions of aluminum surfaces with pristine and defective graphene sheets. Classical molecular dynamics simulations were performed at temperatures of 300-900K to investigate the structural evolution of these interfaces. The results indicate that although the interfaces between Al and graphene are mostly weakly bonded, aluminum carbide can form under the right conditions, including the presence of vacancy defects in graphene, undercoordinated Al in surface regions with sharp boundaries, and at high temperatures. COMB3 potentials were further used to examine a new method to transfer graphene between Al surfaces as well as between Al and Cu surfaces by controlling the angle of the graphene between the two surfaces. The findings indicate that by controlling the peeling angles it is possible to transfer graphene without any damage from the surface having greater graphene/surface adhesion to another surface with less adhesion.

cond-mat.mtrl-sci↗

arXiv2019

New zero Poisson's ratio models

Most materials exhibit positive Poisson's ratio (PR) values but special structures can also present negative and, even rarer, zero (or close to zero) PR. Null PR structures have received much attention due to their unusual properties and potential applications in different fields, such as aeronautics and bio-engineering. Here, we present a new and simple near-zero PR 2D topological model based on a structural block composed of two smooth and rigid bars connected by a soft membrane or spring. It is not based on re-entrant or honeycomb-like configurations, which have been the basis of many null or quasi-null PR models. Our topological model was 3D printed and the experimentally obtained PR was$-0.003\,\pm 0.001\,$, which is one the closest to zero value ever reported. This topological model can be easily extended to 3D systems and with compression in any direction. The advantages and disadvantages of these models are also addressed.

cond-mat.mtrl-sci↗

arXiv2019

The Structure of Graphene on Graphene/C60/Cu Interfaces: A Molecular Dynamics Study

Two experimental studies reported the spontaneous formation of amorphous and crystalline structures of C60 intercalated between graphene and a substrate. They observed interesting phenomena ranging from reaction between C60 molecules under graphene to graphene sagging between the molecules and control of strain in graphene. Motivated by these works, we performed fully atomistic reactive molecular dynamics simulations to study the formation and thermal stability of graphene wrinkles as well as graphene attachment to and detachment from the substrate when graphene is laid over a previously distributed array of C60 molecules on a copper substrate at different values of temperature. As graphene compresses the C60 molecules against the substrate, and graphene attachment to the substrate between C60s ("C60s" stands for plural of C60) depends on the height of graphene wrinkles, configurations with both frozen and non-frozen C60s structures were investigated in order to verify the experimental result of stable sagged graphene when the distance between C60s is about 4 nm and height of graphene wrinkles is about 0.8 nm. Below the distance of 4 nm between C60s, graphene becomes locally suspended and less strained. We show that this happens when C60s are allowed to deform under the compressive action of graphene. If we keep the C60s frozen, spontaneous "blanketing" of graphene happens only when the distance between them are equal or above 7 nm. Both above results for the existence of stable sagged graphene for C60 distances of 4 or 7 nm are shown to agree with a mechanical model relating the rigidity of graphene to the energy of graphene-substrate adhesion. In particular, this study might help the development of 2D confined nanoreactors that are considered in literature to be the next advanced step on chemical reactions.

cond-mat.mes-hall↗

arXiv2018

Self-tearing and self-peeling of folded graphene nanoribbons

A recent experimental study showed that an induced folded flap of graphene can spontaneously drive itself its tearing and peeling off a substrate, thus producing long, micrometer sized, regular trapezoidal-shaped folded graphene nanoribbons. As long as the size of the graphene flaps is above a threshold value, the 'tug of war' between the forces of adhesion of graphene-graphene and graphene-substrate, flexural strain of folded region and carbon-carbon (C-C) covalent bonds favor the self-tearing and self-peeling off process. As the detailed information regarding the atomic scale mechanism involved in the process remains not fully understood, we carried out atomistic reactive molecular dynamics simulations to address some features of the process. We show that large thermal fluctuations can prevent the process by increasing the probability of chemical reactions between carbon dangling bonds of adjacent graphene layers. The effects of the strength of attraction between graphene and the substrate on the ribbon growth velocities at the early stages of the phenomenon were also investigated. Structures with initial armchair crack-edges were observed to form more uniform cuts than those having initial zigzag ones. Our results are of importance to help set up new experiments on this phenomenon, especially with samples with nanoscale sized cuts.

cond-mat.mtrl-sci↗

arXiv2018

Self-Driven Graphene Tearing and Peeling: A Fully Atomistic Molecular Dynamics Investigation

In spite of years of intense research, graphene continues to produce surprising results. Recently, it was experimentally observed that under certain conditions graphene can self-drive its tearing and peeling from substrates. This process can generate long, micrometer sized, folded nanoribbons without the action of any external forces. Also, during this cracking-like propagation process, the width of the graphene folded ribbon continuously decreases and the process only stops when the width reaches about few hundreds nanometers in size. It is believed that interplay between the strain energy of folded regions, breaking of carbon-carbon covalent bonds, and adhesion of graphene-graphene and graphene-substrate are the most fundamental features of this process, although the detailed mechanisms at atomic scale remain unclear. In order to gain further insights on these processes we carried out fully atomistic reactive molecular dynamics simulations using the AIREBO potential as available in the LAMMPS computational package. Although the reported tearing/peeling experimental observations were only to micrometer sized structures, our results showed that they could also occur at nanometer scale. Our preliminary results suggest that the graphene tearing/peeling process originates from thermal energy fluctuations that results in broken bonds, followed by strain release that creates a local elastic wave that can either reinforce the process, similar to a whip cracking propagation, or undermine it by producing carbon dangling bonds that evolve to the formation of bonds between the two layers of graphene. As the process continues in time and the folded graphene decreases in width, the carbon-carbon bonds at the ribbon edge and interlayer bonds get less stressed, thermal fluctuations become unable to break them and the process stops.

cond-mat.mes-hall↗

arXiv2018

Mechanical Properties of Pentagraphene-based Nanotubes: A Molecular Dynamics Study

The study of the mechanical properties of nanostructured systems has gained importance in theoretical and experimental research in recent years. Carbon nanotubes (CNTs) are one of the strongest nanomaterials found in nature, with Young's Modulus (YM) in the order 1.25 TPa. One interesting question is about the possibility of generating new nanostructures with 1D symmetry and with similar and/or superior CNT properties. In this work, we present a study on the dynamical, structural, mechanical properties, fracture patterns and YM values for one class of these structures, the so-called pentagraphene nanotubes (PGNTs). These tubes are formed rolling up pentagraphene membranes (which are quasi-bidimensional structures formed by densely compacted pentagons of carbon atoms in sp3 and sp2 hybridized states) in the same form that CNTs are formed from rolling up graphene membranes. We carried out fully atomistic molecular dynamics simulations using the ReaxFF force field. We have considered zigzag-like and armchair-like PGNTs of different diameters. Our results show that PGNTs present YM ~ 800 GPa with distinct elastic behavior in relation to CNTs, mainly associated with mechanical failure, chirality dependent fracture patterns and extensive structural reconstructions.

cond-mat.mes-hall↗

arXiv2018

Mechanical Properties of Phagraphene Membranes: A Fully Atomistic Molecular Dynamics Investigation

Recently, a new 2D carbon allotrope structure, named phagraphene (PG), was proposed. PG has a densely array of penta-hexa-hepta-graphene carbon rings. PG was shown to present low and anisotropic thermal conductivity and it is believed that this anisotropy should be also reflected in its mechanical properties. Although PG mechanical properties have been investigated, a detailed and comprehensive study is still lacking. In the present work we have carried out fully atomistic reactive molecular dynamics simulations using the ReaxFF force field, to investigate the mechanical properties and fracture patterns of PG membranes. The Young's modulus values of the PG membranes were estimated from the stress-strain curves. Our results show that these curves present three distinct regimes: one regime where ripples dominate the structure and mechanical properties of the PG membranes; an elastic regime where the membranes exhibit fully planar configurations; and finally a plastic regime where permanent deformations happened to the PG membrane up to the mechanical failure or fracture.

cond-mat.mes-hall↗