Molecular Dynamics Study of Nanoribbon Formation by Encapsulating Cyclic Hydrocarbon Molecules inside Single-Walled Carbon Nanotube

Carbon nanotubes filled with organic molecules can serve as chemical nanoreactors. Recent experimental results show that, by introducing cyclic hydrocarbon molecules inside carbon nanotubes, they can be transformed into nanoribbons or inner tubes, depending on the experimental conditions. In this paper, we present our results obtained as a continuation of our previous molecular dynamics simulation work. In our previous work, the initial geometry consisted of independent carbon atoms. Now, as an initial condition, we have placed different molecules inside a carbon nanotube (18,0): C5H5 (fragment of ferrocene), C5, C5+H2; C6H6 (benzene), C6, C6+H2; C20H12 (perylene); and C24H12 (coronene). The simulations were performed using the REBO-II potential of the LAMMPS software package, supplemented with a Lennard-Jones potential between the nanotube wall atoms and the inner atoms. The simulation proved difficult due to the slow dynamics of the H abstraction. However, with a slight modification of the parameterization, it was possible to model the formation of carbon nanoribbons inside the carbon nanotube.

Atomistic modeling of electromechanical properties of piezoelectric zinc oxide nanowires

Currently, numerous articles are devoted to examining the influence of geometry and charge distribution on the mechanical properties and structural stability of piezoelectric nanowires (NWs). The varied modeling techniques adopted in earlier molecular dynamics (MD) works dictated the outcome of the different efforts. In this article, comprehensive MD studies are conducted to determine the influence of varied interatomic potentials (partially charged rigid ion model, [PCRIM] ReaxFF, charged optimized many-body [COMB], and Buckingham), geometrical parameters (cross-section geometry, wire diameter, and length), and charge distribution (uniform full charges versus partially charged surface atoms) on the resulting mechanical properties and structural stability of zinc oxide (ZnO) NWs. Our optimized parameters for the Buckingham interatomic potential are in good agreement with the existing experimental results. Furthermore, we found that the incorrect selection of interatomic potentials could lead to excessive overestimate (61%) of the elastic modulus of the NW. While NW length was found to dictate the strain distribution along the wire, impacting its predicted properties, the cross-section shape did not play a major role. Assigning uniform charges for both the core and surface atoms of ZnO NWs leads to a drastic decrease in fracture properties.

Comparison and Assessment of Different Interatomic Potentials for Simulation of Silicon Carbide

Interatomic potentials play a crucial role in the molecular dynamics (MD) simulation of silicon carbide (SiC). However, the ability of interatomic potentials to accurately describe certain physical properties of SiC has yet to be confirmed, particularly for hexagonal SiC. In this study, the mechanical, thermal, and defect properties of four SiC structures (3C-, 2H-, 4H-, and 6H-SiC) have been calculated with multiple interatomic potentials using the MD method, and then compared with the results obtained from density functional theory and experiments to assess the descriptive capabilities of these interatomic potentials. The results indicate that the T05 potential is suitable for describing the elastic constant and modulus of SiC. Thermal calculations show that the Vashishta, environment-dependent interatomic potential (EDIP), and modified embedded atom method (MEAM) potentials effectively describe the vibrational properties of SiC, and the T90 potential provides a better description of the thermal conductivity of SiC. The EDIP potential has a significant advantage in describing point defect formation energy in hexagonal SiC, and the GW potential is suitable for describing vacancy migration in hexagonal SiC. Furthermore, the T90 and T94 potentials can effectively predict the surface energies of the three low-index surfaces of 3C-SiC, and the Vashishta potential exhibits excellent capabilities in describing stacking fault properties in SiC. This work will be helpful for selecting a potential for SiC simulations.

Transferability of interatomic potentials for silicene

The ability of various interatomic potentials to reproduce the properties of silicene, that is, 2D single-layer silicon, polymorphs was examined. Structural and mechanical properties of flat, low-buckled, trigonal dumbbell, honeycomb dumbbell, and large honeycomb dumbbell silicene phases, were obtained using density functional theory and molecular statics calculations with Tersoff, MEAM, Stillinger-Weber, EDIP, ReaxFF, COMB, and machine-learning-based interatomic potentials. A quantitative systematic comparison and a discussion of the results obtained are reported.

Benchmarking structural evolution methods for training of machine learned interatomic potentials

When creating training data for machine-learned interatomic potentials (MLIPs), it is common to create initial structures and evolve them using molecular dynamics (MD) to sample a larger configuration space. We benchmark two other modalities of evolving structures, contour exploration (CE) and dimer-method (DM) searches against MD for their ability to produce diverse and robust density functional theory training data sets for MLIPs. We also discuss the generation of initial structures which are either from known structures or from random structures in detail to further formalize the structure-sourcing processes in the future. The polymorph-rich zirconium-oxygen composition space is used as a rigorous benchmark system for comparing the performance of MLIPs trained on structures generated from these structural evolution methods. Using Behler-Parrinello neural networks as our MLIP models, we find that CE and the DM searches are generally superior to MD in terms of spatial descriptor diversity and statistical accuracy.

Atomistic Simulation of Structural and Mechanical Properties of the AMgF3 (A = K, Rb, and Cs) Compounds Under Hydrostatic Pressure

Structural, mechanical, elastic, and dielectric properties of the AMgF₃ (A = K, Rb, and Cs) compounds were investigated using classical atomistic simulation. A new set of interatomic potentials was developed for these compounds. Lattice parameters and interatomic distances have shown to accurately reproduce all structures, with very close agreement to the experimental data. In all cases, the relative error is below 0.5%. Effect of hydrostatic pressure in the structural, mechanical, elastic, and dielectric properties of these materials were studied from 0 up to 50 GPa. Compounds behavior and stability under pressure were analyzed. KMgF₃ and RbMgF₃ changed from brittle to ductile at approximately 2 GPa. These calculations play an important role in understanding the properties of the AMgF₃ (A = K, Rb, and Cs) compounds under pressure, and open up a new opportunity to study defects in this class of materials. © 2019 Wiley Periodicals, Inc.

Atomistic Assessments of Lithium-Ion Conduction Behavior in Glass-Ceramic Lithium Thiophosphates

We determined the interatomic potentials of the Li-[PS₄^3-] building block in (Li₂S)_0.75(P₂S₅)_0.25 (LPS) and predicted the Li-ion conductivity (σ_Li) of glass-ceramic LPS from molecular dynamics. The Li-ion conduction characteristics in the crystalline/interfacial/glassy structure were decomposed by considering the structural ordering differences. The superior σ_Li of the glassy LPS could be attributed to the fact that ∼40% of its structure consists of the short-ranged cubic S-sublattice instead of the hexagonally close-packed γ-phase. This glassy LPS has a σ_Li of 4.08 × 10^-1 mS cm^-1, an improvement of ∼100 times relative to that of the γ-phase, which is in agreement with the experiments.

Neutrino Induced Doppler Broadening

When a nucleus undergoes beta decay via the electron capture reaction, it emits an electron neutrino. The neutrino emission gives a small recoil to the atom, which can be experimentally observed as a Doppler broadening on subsequently emitted gamma rays. Using the two-axis flat-crystal spectrometer GAMS4 and the electron capture reaction in (152)Eu, the motion of atoms having an excess kinetic energy of 3 eV in the solid state was studied. It is shown how the motion of the atom during the first hundreds of femtoseconds can be reconstructed. The relevance of this knowledge for a new neutrino helicity experiment is discussed.

The GRID Technique: Current Status and New Trends

In the GRID technique one measures Doppler-broadened line profiles of γ transitions using the high resolution crystal spectrometers GAMS, which are installed at the high flux reactor of the ILL Grenoble. One of the essential applications of this technique is the measurement of nuclear state lifetimes. In the present contribution the precision and the principal limits of the technique are discussed.