Atomistic Approach Research Articles

Effects of an atomistic modeling approach on predicted mechanical properties of glassy polymers via molecular dynamics

Glassy polymers are utilized in numerous applications ranging from light-weight structural materials to membranes for industrial gas separation. In this study, we quantify the ability of non-equilibrium molecular dynamics (NEMD) simulations to predict mechanical properties of glassy polymers based on different modeling approaches: force field selection, number of polymer chains in the simulation cell, and polymer builder algorithm. The polymers analyzed in this work are poly(methyl methacrylate) (PMMA), poly(propylene) (PP), and a polymer of intrinsic microporosity (PIM-1). PMMA samples were synthesized in silico using three different methods: continuous configurational biased Monte Carlo, pseudo-self-avoiding random walk, and a generalized simulated polymerization approach. Following the application of a consistent equilibration approach for each PMMA sample, stress–strain data from simulated tensile testing revealed that the choice of polymer builder algorithm or number of chains comprising the simulation cell did not have significant impact on the predicted elastic modulus. Force field selection effects have been analyzed by applying generalized force fields (GAFF and DREIDING) to each PMMA and PP sample and were found to be the most influencing factor studied. For these polymers, it was found that the DREIDING force field provides excellent agreement with experimental tensile moduli, ∼3.25 GPa and ∼1.53 GPa for PMMA and PP, respectively, while GAFF provides a systematic overestimation of the modulus by approximately 1.0 GPa. However, in the case of the PIM-1 model, with previously validated bonded interactions described by GAFF and non-bonded parameters from the TraPPE force field, the tensile modulus was predicted to be 1.23 GPa, which is well within the range of measured experimental values. Altogether, the simulations performed in this study illustrate the capabilities of atomistic MD simulations to predict the elastic modulus of glassy polymers and highlight energetic potential terms to consider for force field validation.

Size-dependent dynamic characteristics of graphene based multi-layer nano hetero-structures

Carbon-based nano hetero-structures are receiving increasing attention due their ability in multi-synchronous modulation of a range of mechanical and other critically desirable properties. In this paper, the vibration characteristics of two different graphene based heterostructures, graphene-hexagonal boron nitride (hBN) and graphene-molybdenum disulfide (MoS2), are explored based on atomistic finite element approach. Such vibrational characteristics of nanostructures are of utmost importance in order to access their suitability as structural members for adoption in various nano-scale devices and systems. In the current analysis, the developed atomistic finite element model for nano-heterostructures is extensively validated first with the results available in literature considering elastic responses and natural frequencies. Thereafter a range of insightful new results are presented for the dynamic behaviour of various configurations of graphene-hBN and graphene-MoS2 heterostructures including their size, chirality and boundary dependence. The investigation of tunable vibrational properties along with simultaneous modulation of other mechanical, electronic, optical, thermal and chemical attributes of such nano-heterostructures would accelerate their application as prospective candidates for manufacturing nanosensors, electromechanical resonators, and a wide range of other devices and systems across the length-scales.

Atomistic modeling and dynamic analysis of boron nitride nanotube in the presence of hexagonal defect

The feasibility of defective Single-Walled Boron Nitride Nanotube (SW-BNNT) has been explored in the present paper employing finite element based three-dimensional atomistic modeling approach. The effect of missing of three conjugate BN pairs in SW-BNNT resulting in formation of hexagonal defect in SW-BNNT on resonance frequency has been analyzed in the present paper. Two different types of SW-BNNT i.e., armchair (5, 5) and Zigzag (5, 0) BNNT of 5 nm length have been analyzed for cantilever and bridged configuration.Further, present work explores the effect of position of defect on resonance frequency in SW-BNNT. The simulation results revels that resonance frequency decreases due to presence of defect and this reduction in frequency increases if the position of defect moves towards free end in the case of cantilever configuration. On the contrary for bridge configuration the resonance frequency decreases continuously if position of defect moves from fixed end to midpoint.Thus it can be concluded that due to presence of defect in nanotube, the resonance frequency decreases and suggests that stiffness is more dominating than mass of nanotube.

Atomistic Insights into the Tunable Transition from Cavitation to Crazing in Diamond Nanothread-Reinforced Polymer Composites.

Cavitation and crazing in thermosetting polymers can be sophisticatedly designed for valuable applications in optics, electronics, and biotechnology. It is a great challenge for numerical study to describe the formations of cavity and fibrils in polymer composite due to the complicated interfacial interaction. To explore this challenging task, we exploit a two-phase coarse-grained framework which serves as an efficient atomistic level-consistent approach to expose and predict the transition between cavitation and crazing in a polymeric system. The coarse-grained framework is utilized to transmit the information between single phase and interface in polymer composite, and the learning tasks of force field are fulfilled through parameterization of mechanical performances and structural characterizations. We elaborate on the intrinsic characteristics of the cavitation-crazing transition in diamond nanothread- (DNT-) reinforced polymethyl methacrylate composites, in which DNT plays a specific role of nanomodulator to tune the cavity volume ratio. The transition from cavitation to crazing can be induced through a novel dissipative mechanism of opening an interlocked network, in which case the DNT is stretched to the aligned fibrils and links crazing tightly by interfacial adhesion. The designed computational framework can broaden the scope of theoretical tools for providing better insights into the microstructure design of polymer composites.

The origin and nature of killer defects in 3C-SiC for power electronic applications by a multiscale atomistic approach

Dislocation complexes are identified as killer defects in 3C-SiC/Si(001) by a synergistic approach of molecular dynamics and ab initio simulations.

Ab Initio-Based Structural and Thermodynamic Aspects of the Electrochemical Lithiation of Silicon Nanoparticles

We reported the theoretical understandings of the detailed structural and thermodynamic mechanism of the actual lithiation process of silicon nanoparticle systems based on atomistic simulation approaches. We found that the rearrangement of the Si bonding network is the key mechanism of the lithiation process, and that it is less frequently broken by lithiation in the smaller sizes of Si nanoparticles. The decreased lithiation ability of the Si nanoparticles results in the lithiation potential being significantly lower than that of crystalline silicon phases, which impedes the full usage of the theoretical maximum capacity. Thus, nanosized Si materials could have a negative effect on performance if they become too fine-sized. These findings provide a detailed view of the electrochemical lithiation process of silicon nanoparticles (Si NPs) and engineering guidelines for designing new Si-based nanostructured materials.

An atomistic molecular dynamics simulations approach to the study of H-LDHA inhibition

An atomistic molecular dynamics simulations approach to the study of H-LDHA inhibition

Modeling Energy Bands in Type II Superlattices

We present a rigorous model for the overall band structure calculation using the perturbative k · p approach for arbitrary layered cubic zincblende semiconductor nanostructures. This approach, first pioneered by Kohn and Luttinger, is faster than atomistic ab initio approaches and provides sufficiently accurate information for optoelectronic processes near high symmetry points in semiconductor crystals. k · p Hamiltonians are discretized and diagonalized using a finite element method (FEM) with smoothed mesh near interface edges and different high order Lagrange/Hermite basis functions, hence enabling accurate determination of bound states and related quantities with a small number of elements. Such properties make the model more efficient than other numerical models that are usually used. Moreover, an energy-dependent effective mass non-parabolic model suitable for large gap materials is also included, which offers fast and reasonably accurate results without the need to solve the full multi-band Hamiltonian. Finally, the tools are validated on three semiconductor nanostructures: (1) the bound energies of a finite quantum well using the energy-dependent effective mass non-parabolic model; (2) the InAs bulk band structure; and (3) the electronic band structure for the absorber region of photodetectors based on a type-II InAs/GaSb superlattice at room temperature. The tools are shown to work on simple and sophisticated designs and the results show very good agreement with recently published experimental works.

Atomistic approach to simulate kink migration and kink-pair formation in silicon: The kinetic activation-relaxation technique

The energy conversion efficiency of solar cells based on multicrystalline silicon is greatly deteriorated by dislocations. However, an in-depth understanding on the dislocation motion dynamics down to atomic scale is still lacking. In this paper, we propose a novel atomistic approach to simulate the kink migration and kink-pair formation which govern dislocation motion in silicon, namely the kinetic activation-relax technique (k-ART). With this method, long timescale events can be simulated and complex energy landscapes can be explored. Four mechanisms for kink migration are observed, with total activation energy of 0.16, 0.25, 0.32, and 0.25 eV. New nontrivial kink structures that participate in kink migration are identified due to the open-ended search algorithm for saddle points in k-ART. In addition, a new pathway for kink-pair formation, with a minimum activation energy of 1.11 eV is discovered. The effect of shear stress on kink migration is also investigated. It shows that shear stress shifts the energy barriers of available events to lower energies, resulting in a change of the preferred kink-migration mechanism and a reduction of kink-pair formation energy.

India’s Approach to China’s Belt and Road Initiative—Opportunities and Concerns

Abstract China’s Belt and Road Initiative (BRI) as a geoeconomic vision and geopolitical strategy is closely watched and scrutinised by Indian economists, diplomats, and strategists. Perspectives on India’s approach to the BRI can broadly be classified into three—the optimist, the sceptic and the cautionary. Whereas, economists generally appear optimistic, there is a sense of uneasiness within India’s strategic community that the BRI represents much more than China’s ambition to emerge as an economic leader in the region. This article argues that India’s approach to the BRI has largely been pragmatic, cautious and complex. Accordingly, India has taken an atomistic approach to the various components of the BRI depending on its security and economic needs, which explains why on the one hand India has become increasingly receptive of the Bangladesh-China-India-Myanmar Economic Corridor (BCIM EC) and on the other continues to publicly oppose the China-Pakistan Economic Corridor (CPEC).

Importance of second-order deformation potentials in modeling of InAs/GaAs nanostructures

Accurate modeling of electronic properties of nanostructures is a challenging theoretical problem. Methods making use of continuous media approximation, such as k.p, sometimes struggle to reproduce results obtained with more accurate atomistic approaches. On the contrary, atomistic schemes generally come with a substantially larger cost of computation. Here, we bridge between these two approaches by taking 8-band k.p method augmented with non-linear strain terms fit to reproduce sp3d5s* tight-binding results. We illustrate this method on the example of electron and hole states confined in quantum wells and quantum dots of photonics applications relevant InAs/GaAs material system, and demonstrate a good agreement of a non-linear k.p scheme with empirical tight-binding method. We discuss limits of our procedure as well as provide non-linear 8-band k.p parameter sets for InAs and GaAs. Finally, we propose a parameterization for effective term used to improve the accuracy of the standard effective mass method.

Molecular Dynamics Simulation of Heat Transfer at Contact Interface of Hot Forming Die

During metal hot forming process, the billet and the die interact by contact under high temperature, high pressure and severe sliding. In this paper, an atomistic approach is utilized to study the behaviour of heat transfer at the contact interface of hot forming die and the material. Based on non-equilibrium molecular dynamics and phonon scattering theory, the phonon heat transfer process at the billet/die rough interface at nano-scale is analysed. The effects of interfacial lattice defect concentration and average interfacial temperature on the phonon heat transfer coefficient are examined. It is found that when the lattice defect concentration increases the phonon mismatch at the interface deteriorates and the interfacial thermal resistance increases, which leads to a sharp decrease in phonon heat transfer coefficient. A higher temperature of the contact body and a higher average interface temperature lead to a higher probability of inelastic scattering of phonons at the interface. Moreover, the high frequency phonons at high temperature might be decomposed into two or more low frequency phonons during the energy transmission, which increases the propagation of phonon at the interface; thus the phonon heat transfer coefficient increases accordingly.

Proving Peaceful Islam through the Application of Al-'Illat Wa Al-Ma'lûl Approach in Reading Qur’anic Verses of War (Interpretation of Q.2: 190-194 and Q.9: 5-15)

This paper will prove Islam as a religion of peace by using al-illat wa al-ma'lûl approach in reading Qur’anic verses of war. Qur’anic verses of war have been a crucial problem in interfaith relation. Non-Muslims used such verses as a weapon to label Islam as a violent religion. Moreover, some terrorists justified their violent action with verses of war. However, Muslims insist that such verses basically do not express violence especially if they are properly understood. Radical understanding of Qur’anic texts is partly caused by the approach that exegetes use. Apart from the approach is the use of literal translation and atomistic approach. Atomistic approach suggests the readers to split a verse and treat it separately without relating it to other parts of the verse. Moreover, it does not pay a specific attention to the historical context of the texts (asbȃb al-nuzȗl). In addition, it neglects the correlation of the verses (munᾱsaba). Some approaches have contributed to the problem includes: 1. System approach, 2. Esoteric interpretation approach, and 3. Historic-contextual approach. However, these three approaches did not solve the problem. Therefore, the writer introduces al-illat wa al-ma'lûl approach. The research question is how to prove Islam as a religion of peace by implementing al-‘illat wa al-ma‘lȗl approach in understanding Qur’anic verses of war? Methodologically, this approach was adopted from the concept of cause and effect in various studies, specifically: Physics, History, Philosophy, Communication as well as Islamic studies, especially: Uṣȗl al-Fiqh, Ḥadȋth and Tafsȋr. The approach runs through 4 steps: 1. Literal analysis of key words, especially "qᾱtil," in Q.2:190-194 and Q.9:5-15. 2. Analyzing macro context of revelation (asbȃb al-nuzȗl al-‘ȃmm) and the micro context of the texts (asbȃb al-nuzȗl al-khȃṣs). 3. Analyzing the correlation of the verse internally and externally. 4. Determine which part of the verse is the cause (al-‘illat) and which one is the effect (al-ma‘lûl). It also determines which verses function as a cause and which ones function as an effect. The result of study shows that: 1. The order to fight unbelievers has been preceded by a cause that unbelievers have declared war. 3. The order to fight idolaters wherever they found them is issued when the war is raging and in certain places and in certain time. 4. The command to fight is upright on a noble purpose namely justice, liberation and self-defense. This command to fight is based on the real condition of Muslims whom were oppressed, persecuted, tortured and expelled. This war is also upright on the ethics of war, such as not killing old men, women and children, not destroying houses of worship, and plantations. To conclude, all command to fight for Muslim is a consequence of the previous events as a cause. Therefore, Islam is a peaceful religion it is a defensive and not aggressive or offensive religion.

Prediction of Macroscopic Properties of Diamond-like Carbon from Atomic-Scale Structure

The macroscopic mechanical properties of diamond-like carbon (DLC), which is a promising solid lubricant and protective coating, are directly determined by the atomic-scale structure. Understanding the relation between the atomic-scale structure and the macroscopic properties is necessary and helpful for the improvement of DLC; however, experimental investigations of the atomic-scale structure and macroscopic properties require huge time and economic costs, limiting the progress of elucidating their correlation. In this work, the atomistic simulation approach was used to study the atomic-scale structure of DLC and its effect on the Young’s modulus of DLC. Structural analysis of DLC showed that the sp and sp2 hybridizations decrease while the sp3 hybridization increases with increasing density and hydrogen concentration of DLC. The degree of graphitization of DLC was further evaluated and found to be linearly proportional to the sixth power of the sp2 ratio. Then, it was demonstrated that Young’s modulus of DLC can be singly predicted by the effective coordination number (CNeff), which is defined as the average coordination number of carbon atoms without the contributions from hydrogen atoms. Furthermore, this work successfully proposed quantitative relations among CNeff, density, and Young’s modulus, which were verified by comparison to experimental results.

Twist-angle sensitivity of electron correlations in moiré graphene bilayers

Motivated by the recent observation of correlated insulator states and unconventional superconductivity in twisted bilayer graphene, we study the dependence of electron correlations on the twist angle and reveal the existence of strong correlations over a narrow range of twist-angles near the magic angle. Specifically, we determine the on-site and extended Hubbard parameters of the low-energy Wannier states using an atomistic quantum-mechanical approach. The ratio of the on-site Hubbard parameter and the width of the flat bands, which is an indicator of the strength of electron correlations, depends sensitively on the screening by the semiconducting substrate and the metallic gates. Including the effect of long-ranged Coulomb interactions significantly reduces electron correlations and explains the experimentally observed sensitivity of strong correlation phenomena on twist angle.

(Invited) Mesoscale Modeling of Solid-State Hydrogen Storage Mechanisms

Metal hydrides have been considered as a promising class of solid-state hydrogen storage materials owing to their excellent volumetric and gravimetric storage densities, which can overcome critical technological limits of the currently available high-pressure hydrogen gas storage approach. Solid-state hydrogen storage mechanisms involve the complicated coupling of chemical/physical processes over vast ranges of multiple length and time scales. These include surface reactions with hydrogen gas; surface/bulk/interface mass transport processes; structural modifications occurring under operation; and intermediate phase nucleation-and-growth processes associated with hydrogen incorporation. Essentially, these are collective dynamic processes of atomic/molecular species that determine the thermodynamics and kinetics of nano- or micro-level characteristics. The mesoscale continuum modeling framework can provide a unified platform for integrating the necessary atomistic approaches, which typically provide higher accuracy for nanoscale chemical processes, and continuum approaches, which offer far greater flexibility and scalability and can describe materials at the microstructural level. In this presentation, we demonstrate our integrated effort as part of the DOE Hydrogen Storage Materials—Advanced Research Consortium(HyMARC) under the Energy Materials Network(EMN) towards the multiscale modeling of hydrogen storage mechanisms in metal hydrides for hydrogen storage. Specifically, we will report our recent efforts on the development of an integrated mesoscale modeling framework that can directly incorporate parameters derived from predictive atomistic modeling approaches such as ab initiocalculations and molecular dynamics simulations. Representative case studies will be presented, including beyond-ideal thermodynamic modeling of complex hydrides, effective mass transport/mechanical response modeling for complex hydride microstructures, and phase-field modeling of reaction-induced phase transformations of simple or complex metal hydrides during the (re/de)hydrogenation.

Toward an Atomistic Understanding of Deep Eutectics Solvents Electrochemical Interfacial Structure

Green, stable, and wide electrochemical window deep eutectics solvents (DESs) are ideal candidates for electrochemical systems, such as batteries, supercapacitors, catalysis, electrodeposition, and many more. However, since these mixtures are highly dense and composed of only large asymmetric ionic and molecular components, short-range ion-ion, ion-molecule interactions play a dominant role both in the bulk as well as at the interface properties. This results in breakdown of the standard mean-field approximation, which is the basis of dilute solution theory. In addition, due to the hygroscopic nature of DESs, the presence of latent water is unavoidable. Therefore, understanding the DESs-electrode interface (electrical double layer structure, EDLS) together with the role of water at a molecular scale is of great importance for the widespread use of these solvents in electrochemical systems [1-3]. In this presentation, we will first review the benchmark study on conventional dilute solution electrical double layer structure and secondly present our recent findings and theoretical developments on the electrochemical interfacial structure of deep eutectic solvents using a combined experimental and a novel atomistic molecular dynamics approach. In the presentation, we focused on the computational approach. The use of such an atomistic-molecular approach allows investigation of all the possible bulk and interfacial interactions and, consequently, helps to shed light on the nanoscale electrochemical interfacial structure of DESs. Unlike the interfacial structures observed and proposed for other electrolyte-electrode interfaces (compact- diffuse double layer for dilute solution [4]; overscreening or crowding for concentrated solutions [5]), the simulations of DESs-electrode interfacial structure show an unexpected and previously unrecognized phenomenon: the electrochemical interface is composed of a mixed layer structure followed by a mixed charged clustered layer regardless of the surface polarization [1,2]. Some of the findings will be discussed in this presentation.

Temperature-dependent nucleation kinetics of Guinier-Preston zones in Al–Cu alloys: An atomistic kinetic Monte Carlo and classical nucleation theory approach

Guinier-Preston (GP) zones, which form in Al–Cu alloys, exhibit characteristic patterns of Cu-rich, disk-shaped atomic clusters along the {100} planes. GP zones have received much attention not only for their precipitation-hardening effect on the Al matrix, but also from fundamental interest in the physics and chemistry of the nanoscale organization of solute atoms. In this study, we established an atomistic kinetic Monte Carlo (kMC) modeling technique for exploring the nucleation and formation processes of GP zones in Al–Cu alloys by constructing an effective on-lattice multibody potential for a dilute Al–Cu-vacancy system by density functional theory calculations. Our model can describe the clustering of Cu atoms via successive exchanges with vacancies and reproduce the characteristic planar nanoprecipitates along the {100} planes in a manner consistent with the crystallographic nature of the GP zones in the early-stage aging of Al–Cu alloys. The time evolution and critical nucleus size of Cu clusters were characterized at various temperatures based on the kMC results. Consequently, we predicted the existence of an optimum temperature (i.e., nose temperature) for the formation of Cu clusters at which the cluster growth was maximized, which was attributable to the interplay between the critical nucleation barrier and the diffusion rate. In addition, the critical nucleus size and temperature for cluster formation were examined based on classical nucleation theory along with the developed multibody potential. These provided an insight into the competition between the enthalpic and entropic effects on the formation of GP zones in the Al–Cu system.

Topologically protected wave packets and quantum rings in silicene

We study chiral wave packets moving along the zero-line of a symmetry breaking potential of vertical electric field in buckled silicene using an atomistic tight-binding approach with initial conditions set by an analytical solution of the Dirac equation. We demonstrate that the wave packet moves with a constant untrembling velocity and with a presevered shape along the zero line. Backscattering by the edge of the crystal is observed that appears with the transition of the packet from $K$ to $K'$ valley or vice versa. We propose a potential profile with branching of the flip line that splits the wave packet and produces interference of the split parts that acts as a quantum ring. The transition time exhibits Aharonov-Bohm oscillations in the external magnetic field that are translated to conductance oscillations when the intervalley scattering is present within the ring.

Atomistic modelling of diamond-type SixGeyCzSn1−x−y−z crystals for realistic transmission electron microscopy image simulations

The realistic simulation of transmission electron microscopy (TEM) images requires an accurate definition of the positions of all atoms, which are linked to the mechanical properties of the material. This paper proposes an optimized atomistic modeling approach to model the lattice parameters and elastic properties of Si, Ge, diamond, alpha-tin, and related diamond alloys, with an approach compatible with systems bigger than 50000 atoms. In order to compute precisely the elastically strained SixGeyCzSn1−x−y−z diamond crystals, a dedicated parameterization of the Keating force field is provided. An original periodic boundary strategy is provided. Our tool is successfully used to interpret experimental TEM data with a reasonable accuracy and precision in a time scale about 10000 times faster than ab initio methods. The method predicts the correct lattice parameters and elastic constants of elementary compounds and alloys with a deviation inferior to 8.1%. We show that subsequent Monte-Carlo simulations predict original self-ordering effects in C in good agreement with the theory. An original approach is used to quantify the short-range and long-range order in comparison with high-resolution cross-sectional TEM experiments: the projected radial distribution function (p-RDF) appears to be a universal and very sensitive analytical tool to quantify the matching between our atomistic model and the experimental HR(S)TEM results. For our reference Si-Ge multilayer with 20 millions of atoms, a maximum broadening of 100 pm is obtained for the third-nearest neighbor (3nn) simulated peak of the p-RDF compared to the experimental one. The same value is obtained from a template matching analysis of the maximum local displacements between the projected experimental atomic positions and the corresponding simulation.