arXiv subjects

Alexandre F. Fonseca

Publications and source records attributed to Alexandre F. Fonseca.

At least 19 recordsLinked to original sources

arXiv2026

Geometry-Dependent Crack Interaction and Toughening in Graphene

The interaction between neighboring cracks has been shown to strongly influence the fracture behavior of graphene. While previous studies focused primarily on crack spacing, the role of crack width remains poorly understood. Here, computational simulations are performed to investigate the coupled effects of crack width and inter-crack spacing $(W_\text{gap})$ on the tensile response of graphene containing parallel cracks. The results show that increasing crack width amplifies the sensitivity of mechanical properties to crack spacing, leading to significant enhancement of peak stress, fracture strain, and toughness at sufficiently large $W_\text{gap}$. For narrow cracks, crack coalescence dominates and causes brittle failure. In contrast, wider cracks promote delayed ligament rupture, increased energy absorption and ductile-like fracture behavior. The normalized toughness and fracture strain exceed those of equivalent single-crack systems by more than twofold. A crack-geometry design map is proposed to identify regimes of crack coalescence, independent propagation, and enhanced toughness.

cond-mat.mtrl-sci↗

arXiv2026

Theoretical Prediction of Three-Dimensional $sp^2$-free Graphyne-Based Nanomaterials via Density Functional Theory

The search for carbon-based materials with tailored dimensionality and properties remains an important topic in materials science, particularly for applications in electronics, photonics, and nanomechanics. Among the emerging platforms in this context, graphyne (GY) represents a class of two-dimensional (2D) carbon allotropes composed of benzene rings connected by acetylenic linkages, yielding networks containing both $sp$- and $sp^2$-hybridized carbon atoms. By analogy with the transformation of $sp^2$ carbon networks such as graphene into $sp^3$-bonded diamond through interlayer covalent bonding, we construct three-dimensional (3D) GY-derived frameworks (3DGY) by covalently connecting stacked $\alpha$-, $\beta$-, and $\gamma$-GY sheets via out-of-plane acetylene bridges. This approach converts the original $sp^2$ nodes into $sp^3$ centers while preserving the $sp$ character of the acetylenic segments, producing fully $sp$-$sp^3$ carbon networks. Structural relaxation shows that the $\alpha$-derived framework does not converge to a stable configuration within this scheme, whereas the $\beta$- and $\gamma$-3DGY phases form stable architectures. Density functional theory (DFT) calculations, combined with ab initio molecular dynamics (AIMD) simulations, confirm the energetic, thermal, and dynamical stability of these two systems and are further used to investigate their structural, mechanical, electronic, and optical properties. Mechanical analysis reveals anisotropic elastic behavior, whereas electronic structure calculations show indirect band gaps of approximately 0.15 eV for $\beta$-3DGY and 1.65 eV for \gamma-3DGY. Optical calculations further reveal anisotropic responses, with absorption extending from the infrared to the visible. These results identify \beta-3DGY and \gamma-3DGY as new three-dimensional carbon allotropes with distinct mechanical, electronic, and optical properties.

cond-mat.mes-hall↗

arXiv2026

Effect of Interatomic Potential Choice on Fracture Modes of Graphene with Parallel Cracks

Defect engineering via parallel cracks has been proposed as a route to tailor the fracture response of graphene. However, atomistic fracture predictions can be strongly sensitive to the interatomic potential. Here, we quantify the effect of potential choice by revisiting H-passivated graphene containing two parallel cracks separated by a gap $W_{\text{gap}}$ loaded in tension along the armchair (AC) and zigzag (ZZ) directions. Molecular dynamics simulations using the AIREBO potential under the same geometry and loading protocol previously studied with ReaxFF, are employed, so enabling a direct comparison. Stress-strain responses, Young's modulus, an effective mode-I stress intensity factor, and energy absorption are evaluated as functions of $W_{\text{gap}}$. Compared with ReaxFF, AIREBO predicts lower peak stresses and earlier catastrophic softening, leading to reduced post-peak deformation capacity and energy absorption. Ductility and energy absorption are shown to be highly potential-dependent, underscoring the need for careful potential selection in defect-engineered graphene fracture simulations.

cond-mat.mtrl-sci↗

arXiv2025

A Simple and Efficient Non-DFT-Based Machine Learning Interatomic Potential to Simulate Titanium MXenes

Titanium MXenes are two-dimensional inorganic structures composed of titanium and carbon or nitrogen elements, with distinctive electronic, thermal and mechanical properties. Despite the extensive experimental investigation, there is a paucity of computational studies at the level of classical molecular dynamics (MD). As demonstrated in a preceding study, known MD potentials are not capable of fully reproducing the structure and elastic properties of every titanium MXene. In this study, we present a simply trained, but yet efficient, non-density functional theory-based machine learning interatomic potential (MLIP) capable of simulating the structure and elastic properties of titanium MXenes and bulk titanium carbide and nitride with precision comparable to DFT calculations. The training process for the MLIP is delineated herein, in conjunction with a series of dynamical tests. Limitations of the MLIP and steps towards improving its efficacy to simulate titanium MXenes are discussed.

cond-mat.mtrl-sci↗

arXiv2025

A Concise Review of Recently Synthesized 2D Carbon Allotropes: Amorphous Carbon, Graphynes, Biphenylene and Fullerene Networks

Two-dimensional (2D) carbon allotropes have received considerable attention due to their unique properties and potential applications in several fields, including electronics, catalysis, energy storage, and sensing. Following the experimental realization of graphene, numerous other 2D carbon structures have been proposed and, in some cases, successfully synthesized. This work presents a concise review of the recently experimentally realized 2D carbon allotropes, including graphynes, biphenylene-based networks, fullerene networks, and monolayer amorphous carbon. For each class, we discuss structural characteristics, theoretical predictions, and synthesis methods, with emphasis on the interplay between theory and experiment. We also highlight instances where experimental studies overlooked relevant theoretical contributions. Finally, we identify theoretically predicted structures that remain unexplored experimentally, suggesting opportunities for synthesis-driven investigations.

cond-mat.mtrl-sci↗

arXiv2025

Graphene Design with Parallel Cracks: Abnormal Crack Coalescence and Its Impact on Mechanical Properties

Graphene is a material with potential applications in electric, thermal, and mechanical fields, and has seen significant advancements in growth methods that facilitate large-scale production. However, defects during growth and transfer to other substrates can compromise the integrity and strength of graphene. Surprisingly, the literature suggests that, in certain cases, defects can enhance or, at most, not affect the mechanical performance of graphene. Further research is necessary to explore how defects interact within graphene structure and affect its properties, especially in large-area samples. In this study, we investigate the interaction between two preexisting cracks and their effect on the mechanical properties of graphene using molecular dynamics simulations. The behavior of zigzag and armchair graphene structures with cracks separated by distances ($W_\text{gap}$) is analyzed under tensile loading. The findings reveal that crack coalescence, defined as the formation of a new crack from two existing crack tips, occurs for lower values of the distance between cracks, $W_\text{gap}$, resulting in a decline in the strength of structures. As $W_\text{gap}$ increases, the stress-strain curves shift upward, with the peak stress rising in the absence of crack coalescence. The effective stress intensity factor formulated in this study exhibits a clear upward trend with increasing $W_\text{gap}$. Furthermore, an increase in $W_\text{gap}$ induces a transition in fracture behavior from crack coalescence to independent propagation with intercrack undulation. This shift in fracture behavior demonstrates a brittle-to-ductile transition, as evidenced by increased energy absorption and delayed failure. A design guideline for the initial crack geometry is suggested by correlating peak stress with the $W_\text{gap}$, within a certain range.

cond-mat.mtrl-sci↗

arXiv2025

Structure and Elastic properties of Titanium MXenes: evaluation of COMB3, REAXFF and MEAM force fields

Titanium carbide and nitride MXenes are two-dimensional inorganic materials that exhibit noteworthy physical and chemical properties. These materials are considered for a variety of technological applications, ranging from energy harvesting to optical and biomedical applications. Given the growing interest in titanium MXenes, there is an expanding demand for computational studies to predict physical properties and behaviors under diverse physical conditions. Complex and large-scale systems necessitate computational methodologies that surpass the constraints imposed by ab initio calculations. In this regard, it is imperative to ascertain the reliability of the computational tools employed to simulate and predict the physical properties of titanium MXenes. In this study, the ability of three known classical molecular dynamics (MD) potentials to provide the structural and elastic properties of titanium carbide and nitride MXenes is evaluated. The MD potentials that were the focus of this study include the Charge-Optimized Many-Body (COMB3), the Reactive Force Field (REAXFF) and the Modified Embedded Atom Method (MEAM). These three potentials possess two or more sets of parameters, herein referred to as force fields, capable of simulating Ti-C and Ti-N systems. The MD results for the lattice parameter and thickness of the MXenes are then compared to those from DFT calculations found in the literature. A total of ten force fields were considered; of these, two REAXFF and two MEAM ones were identified as the most adequate to simulate both the structure and elastic properties of titanium MXenes. Additionally, the values for the linear compressibility of MXenes are presented for the first time. Consequently, researchers can utilize the obtained results to design novel MD-based computational studies of titanium MXenes, leveraging the established relative validity of the available force fields.

cond-mat.mtrl-sci↗

arXiv2024

Enhanced Elastocaloric Effects in {\gamma}-graphyne

The global emphasis on sustainable technologies has become a paramount concern for nations worldwide. Specifically, numerous sustainable methods are being explored as promising alternatives to the well-established vapor-compression technologies in cooling and heating devices. One such avenue gaining traction within the scientific community is the elastocaloric effect (eC). This phenomenon holds promise for efficient cooling and heating processes without causing environmental harm. Studies carried out at the nanoscale have demonstrated the efficiency of the eC, proving to be comparable to that of state-of-the-art macroscopic systems. In this study, we used classical molecular dynamics simulations to investigate the elastocaloric effect for {\gamma}-graphyne. Our analysis goes beyond obtaining changes in eC temperature and the coefficient of performance (COP) for two species of {\gamma}-graphyne nanoribbons (armchair and zigzag). We also explore their dependence on various conditions, including whether they are on deposited on a substrate or pre-strained. Our findings reveal a substantial enhancement in the elastocaloric effect for {\gamma}-graphyne nanoribbons when subjected to pre-strain, amplifying it by at least one order of magnitude. Under certain conditions, the change in the eC temperature and the COP of the structures reach expressive values as high as 224 K and 14, respectively. We discuss the implications of these results by examining the shape and behavior of the carbon-carbon bond lengths within the structures.

cond-mat.mes-hall↗

arXiv2024

Angular Dependence of Four Mechanical Properties of Graphynes

Graphyne is a porous two-dimensional carbon allotrope of graphene that possesses interesting physical properties, including non-null bandgap. It is composed of carbon hexagonal rings or carbon-carbon bonds connected by acetylenic chains. The diverse forms of these connections yield a variety of graphyne structures. In a previous study, we have obtained the elastic properties of seven distinct families of graphyne structures as a function of the number of acetylene chains, from 1 to 10. The Young's modulus, shear modulus, Poisson's ratio and linear compressibility were predicted for the zigzag and armchair directions of all 70 graphyne structures. Here, we present some noteworthy findings regarding the angular dependence of these four elastic properties of asymmetric graphynes. Our results demonstrate that in a single structure, the minimum and maximum Young's modulus can vary by a factor of 10. Additionally, the directions of null linear compressibility in some asymmetric structures were determined.

cond-mat.mtrl-sci↗

arXiv2024

Length and torsion dependence of thermal conductivity in twisted graphene nanoribbons

Research on the physical properties of materials at the nanoscale is crucial for the development of breakthrough nanotechnologies. One of the key properties to consider is the ability to conduct heat, i.e., its thermal conductivity. Graphene is a remarkable nanostructure with exceptional physical properties, including one of the highest thermal conductivities (TC) ever measured. Graphene nanoribbons (GNRs) share most fundamental properties with graphene, with the added benefit of having a controllable electronic bandgap. One method to achieve such control is by twisting the GNR, which can tailor its electronic properties, as well as change their TC. Here, we revisit the dependence of the TC of twisted GNRs (TGNRs) on the number of applied turns to the GNR by calculating more precise and mathematically well defined geometric parameters related to the TGNR shape, namely, its twist and writhe. We show that the dependence of the TC on twist is not a simple function of the number of turns initially applied to a straight GNR. In fact, we show that the TC of TGNRs requires at least two parameters to be properly described. Our conclusions are supported by atomistic molecular dynamics simulations to obtain the TC of suspended TGNRs prepared under different values of initially applied turns and different sizes of their suspended part. Among possible choices of parameter pairs, we show that TC can be appropriately described by the initial number of turns and the initial twist density of the TGNRs.

cond-mat.mes-hall↗

arXiv2023

Elastocaloric Effect in Graphene Kirigami

Kirigami, a traditional Japanese art of paper-cutting, has recently been explored for its elastocaloric effect (ECE) in kirigami-based materials (KMs), where applying strain induces temperature changes. In this study, we investigate the ECE in a nanoscale graphene kirigami (GK) monolayer, representing the thinnest possible KM, to better understand this phenomenon. Through molecular dynamics simulations, we analyze the temperature change and coefficient of performance (COP) of the nanoscale GK architecture. Our findings reveal that while GKs lack the intricate temperature changes observed in macroscopic KMs, they exhibit a substantial temperature change of approximately 9.32 K (23 times higher than that of macroscopic KMs, which is about 0.4K) for heating and -3.50 K for cooling. Additionally, they demonstrate reasonable COP values of approximately 1.57 and 0.62, respectively. It is noteworthy that the one-atom-thick graphene configuration prevents the occurrence of the complex temperature distribution observed in macroscopic KMs.

cond-mat.mtrl-sci↗

arXiv2023

Fully Atomistic Molecular Dynamics Investigation of the Simplest Model of Dry-Draw Fabrication of Carbon Nanotube Fibers

Macroscopic assemblies of carbon nanotubes (CNTs) are desirable materials because of the excellent CNT properties. Amongst the methods of production of these CNT materials, the dry-draw fabrication where CNT fibers (CNTFs) are directly pulled out from a CNT forest is known to provide good physical properties. Although it is known that vertical alignment of CNT bundles within the CNT forest is important, the mechanisms behind the dry-draw fabrication of CNTFs are still not completely understood. The simplest known dry-draw model consists of CNT bundles laterally interacting by only van der Waals forces (vdWf). Here, by fully atomistic classical molecular dynamics simulations, we show that the simplest dry-draw model does not produce CNTFs. We also show one important condition for a pair of adjacent CNT bundles to connect themselves under vdWf only and discuss why it leads to the failure of the simplest model.

cond-mat.mes-hall↗

arXiv2023

Exploring the Elastic Properties and Fracture Patterns of Me-Graphene Monolayers and Nanotubes through Reactive Molecular Dynamics Simulations

Me-graphene (MeG) is a novel two-dimensional (2D) carbon allotrope. Due to its attractive electronic and structural properties, it is important to study the mechanical behavior of MeG in its monolayer and nanotube topologies. In this work, we conducted fully atomistic reactive molecular dynamics simulations using the Tersoff force field to investigate their mechanical properties and fracture patterns. Our results indicate that Young's modulus of MeG monolayers is about 414 GPa and in the range of 421-483 GPa for the nanotubes investigated here. MeG monolayers and MeGNTs directly undergo from elastic to complete fracture under critical strain without a plastic regime.

cond-mat.mtrl-sci↗

arXiv2023

First-Principles and Reactive Molecular Dynamics Study of the Elastic Properties of Pentahexoctite-based Nanotubes

Pentahexoctite (PH) is a pure sp$^2$ hybridized planar carbon allotrope whose structure consists of a symmetric combination of pentagons, hexagons, and octagons. The proposed PH structure was shown to be an intrinsically metallic material exhibiting good mechanical and thermal stability. PH nanotubes (PHNTs) have also been proposed, and their properties were obtained from first principles calculations. Here, we carried out fully-atomistic simulations, combining reactive (ReaxFF) molecular dynamics (MD) and density functional theory (DFT) methods, to study the PHNTs elastic properties and fracture patterns. We have investigated the mechanical properties behavior as a function of the tube diameter and temperature regimes. Our results showed that the PHNTs, when subjected to large tensile strains, undergo abrupt structural transitions exhibiting brittle fracture patterns without a plastic regime.

cond-mat.mtrl-sci↗

arXiv2023

Density dependence of elastic properties of graphynes

Graphyne is a two-dimensional carbon allotrope of graphene. Its structure is composed of aromatic rings and/or carbon-carbon bonds connected by one or more acetylene chains. As some graphynes present the most of the excellent properties of graphene and non-null bandgap, they have been extensively studied. Recently, Kanegae and Fonseca reported calculations of four elastic properties of 70 graphynes, ten members of the seven families of graphynes [Carbon Trends 7, 100152 (2022)]. They showed that the acetylene chain length dependence of these properties can be simply modelled by a serial association of springs. Here, based on those results, we present the density dependence of these properties and show that the elastic moduli, $E$, of graphyne are less dependent on density, $ρ$, than porous cellular materials with an exponent of $E \sim ρ^{n}$, smaller than 2. We discuss the results in terms of the shape of the pores of the graphyne structures.

cond-mat.mtrl-sci↗

arXiv2023

On the Mechanical, Electronic, and Optical Properties of 8-16-4 Graphyne: A 2D Carbon Allotrope with Dirac Cones

Due to the success achieved by graphene, several 2D carbon-based allotropes were theoretically predicted and experimentally synthesized. We used density functional theory and reactive molecular dynamics simulations to investigate the mechanical, structural, electronic, and optical properties of 8-16-4 Graphyne. The results showed that this material exhibits good dynamical and thermal stabilities. Its formation energy and elastic moduli are -8.57 eV/atom and 262.37 GPa, respectively. This graphyne analogue is a semi-metal and presents two Dirac cones in its band structure. Moreover, it is transparent, and its intense optical activity is limited to the infrared region. Remarkably, the band structure of 8-16-4 Graphyne remains practically unchanged at even moderate strain regimes. As far as we know, this is the first 2D carbon allotrope to exhibit this behavior.

cond-mat.mtrl-sci↗

arXiv2023

Scalable synthesis and characterization of multilayer $γ$-graphyne, new carbon crystals with a small direct bandgap

$γ$-Graphyne is the most symmetric sp2/sp1 allotrope of carbon, which can be viewed as graphene uniformly expanded through insertion of two-carbon acetylenic units between all the aromatic rings. To date, synthesis of bulk $γ$-graphyne has remained a challenge. We here report the synthesis of multilayer $γ$-graphyne through crystallization-assisted irreversible cross-coupling polymerization. Comprehensive characterization of this new carbon phase is described, including synchrotron X-ray diffraction, electron diffraction, lateral force microscopy, Raman and infrared spectroscopy, and cyclic voltammetry. Experiments indicate that $γ$-graphyne is a 0.48 eV bandgap semiconductor, with a hexagonal a-axis spacing of 6.88 Å and an interlayer spacing of 3.48 Å, which is consistent with theoretical predictions. The observed crystal structure has an aperiodic sheet stacking. The material is thermally stable up to 240 $^\circ$C but undergoes a transformation at higher temperatures. While conventional 2D polymerizations and reticular chemistry rely on error correction through reversibility, we demonstrate that a periodic covalent lattice can be synthesized under purely kinetic control. The reported methodology is scalable and inspires extension to other allotropes of the graphyne family.

cond-mat.mtrl-sci↗

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↗