Nudged Elastic Band Calculations Research Articles

A highly efficient semi-finishing approach for polycrystalline diamond film via plasma-based anisotropic etching

Plasma anisotropic etching polishing (plasma-AEP), a non-contact polishing method, is proposed to achieve highly efficient planarization of polycrystalline diamond (PCD) films. Inductively coupled plasma, with a high concentration of reactive radicals, serves as the source of plasma-AEP. In-situ observation confirms that the planarization effect of plasma-AEP is realized through the preferential removal of the top areas of the pyramid-shaped protrusions, despite the entire surface being uniformly irradiated by the plasma. The material removal rate in plasma-AEP for PCD achieves 127 μm/min. Plasma-AEP is proven effective for PCD films with thicknesses of 0.5, 1, and 2 mm, demonstrating a generic semi-finishing approach for PCD regardless of thickness. Atomic-scale nudged elastic band calculations revealed that the energy barriers for CO and CO2 desorption from 1- and 2-coordinated C atoms are significantly lower than those for 3- and 4-coordinated ones. ReaxFF molecular dynamics simulations showed that at the top areas of the pyramid-shaped protrusions, 1- and 2-coordinated C atoms with a higher etching priority remained dominant during plasma-AEP, leading to the preferential removal of C atoms forming these protrusions. Furthermore, contact polishing was added to complete the finishing of the PCD film, followed by plasma-AEP, resulting in a nanoscale smooth surface with a roughness of 3.4 nm. Transmission electron microscopy confirmed that the crystal structures on the surface and subsurface of the PCD film were well ordered. Overall, this paper displays that plasma-AEP is a promising approach for highly efficient semi-finishing of PCD films.

Force-Free Identification of Minimum-Energy Pathways and Transition States for Stochastic Electronic Structure Theories.

The accurate mapping of potential energy surfaces (PESs) is crucial to our understanding of the numerous physical and chemical processes mediated by atomic rearrangements, such as conformational changes and chemical reactions, and the thermodynamic and kinetic feasibility of these processes. Stochastic electronic structure theories, e.g., Quantum Monte Carlo (QMC) methods, enable highly accurate total energy calculations that in principle can be used to construct the PES. However, their stochastic nature poses a challenge to the computation and use of forces and Hessians, which are typically required in algorithms for minimum-energy pathway (MEP) and transition state (TS) identification, such as the nudged elastic band (NEB) algorithm and its climbing image formulation. Here, we present strategies that utilize the surrogate Hessian line-search method, previously developed for QMC structural optimization, to efficiently identify MEP and TS structures without requiring force calculations at the level of the stochastic electronic structure theory. By modifying the surrogate Hessian algorithm to operate in path-orthogonal subspaces and at saddle points, we show that it is possible to identify MEPs and TSs by using a force-free QMC approach. We demonstrate these strategies via two examples, the inversion of the ammonia (NH3) molecule and the nucleophilic substitution (SN2) reaction F- + CH3F → FCH3 + F-. We validate our results using Density Functional Theory (DFT)- and Coupled Cluster (CCSD, CCSD(T))-based NEB calculations. We then introduce a hybrid DFT-QMC approach to compute thermodynamic and kinetic quantities, free energy differences, rate constants, and equilibrium constants that incorporates stochastically optimized structures and their energies, and show that this scheme improves upon DFT accuracy. Our methods generalize straightforwardly to other systems and other high-accuracy theories that similarly face challenges computing energy gradients, paving the way for highly accurate PES mapping, transition state determination, and thermodynamic and kinetic calculations at significantly reduced computational expense.

Exploring the Feasibility of Monoclinic‐ZrO2‐Based Memristors as Artificial Olfactory Sensors: An Atomistic Simulation Approach

AbstractMemory devices with sensitivity, selectivity, and operation voltage towards the gases are rarely reported for artificial olfactory sensors. Additionally, there are no reports available on the atomistic aspects of artificial olfactory sensors. This study reports an atomistic simulation of monoclinic‐ZrO2 (m‐ZrO2). The impact of external electric field on the formation of the oxygen vacancies are evaluated by considering the different directions of electric field. Furthermore, it is conducted nudged elastic band calculations which showed a decrease in the migration barrier energy with an increase in the electric field for all considered directions. Moreover, it is simulated the memristor device (Ta/m‐ZrO2/Pt) and investigated the impact of oxygen vacancies on electrical conductivity by considering oxygen vacancies at different locations in m‐ZrO2. Finally, it is evaluated the possibility of using the m‐ZrO2 based memristor device for an artificial olfactory sensor by studying the gas sensing properties of the (111) surface of m‐ZrO2. The pristine structure exhibits low sensitivity towards toxic molecules (CO2, CO, NH3, and NO2), while the sensing performance is significantly enhanced on the oxygen vacancy rich surface. These atomistic simulation results provide an atomic level understanding of the Ta/m‐ZrO2/Pt device and suggest the potential for it to be use as an artificial olfactory sensor.

Sensing behavior of pentagonal two-dimensional noble-metal dichalcogenide PdSeS for NO2 gas: DFT and semiclassical studies

The gas sensing performance of the pentagonal two-dimensional noble-metal dichalcogenide PdSeS for NO2 gas detection has been extensively studied using DFT, the nudged elastic band (NEB) model, and the semiclassical Boltzmann theory. The NEB calculations indicate a highly effective physical NO2 adsorption, implying the potential of the PdSeS monolayer as an excellent material for creating efficient and reusable gas sensors. Additionally, the PdSeS monolayer, characterized as a semiconductor with a band gap of 1.34 eV, demonstrates the emergence of a deep trap in the band structure following the adsorption of NO2 gas. Subsequently, the values of σxx and σyy decrease significantly from 11.41 and 42.82 S/cm to 2.24 and 0.99 S/cm, respectively. The decrease in conductivity can be attributed to several factors, including an increase in effective mass, the formation of deep traps, and a reduction in the number of charge carriers. Furthermore, our calculations demonstrate the potential use of the PdSeS monolayer in work function (WF) type sensors. Consequently, the pentagonal two-dimensional PdSeS monolayer fulfills the demands of portable, affordable, and reusable NO2 gas sensors by exhibiting remarkable sensitivity and short recovery time.

Hydride-Ion Conduction in AELiH3 (AE = Sr, Ba)

Hydride ions (H–) are expected to conduct fast in solids owing to their monovalent state, appropriate ionic radius for conduction, and large polarizability. Thus, H– is a promising class of the charge carrier with high mobility, and may be applicable to the electrochemical reaction in batteries and fuel cells. The K2NiF4–type structure, wherein the perovskite KNiF3 and the rock-salt KF layers are stacked along the c-axis, has been the most researched material as a hydride-ion conductor, e.g., K2NiF4–type oxyhydrides of La2–x–y Sr x +y LiH1–x+y O3–y [1]. In this study, we focused on perovskite-type hydrides, AELiH3 (AE = Ca, Sr, Ba). A simple perovskite structure is expected to have high ionic conductivity, because it ensures a three-dimensional diffusion pathway for the anion species [2-6].First, the migration energy of H– ions in the AELiH3 materials were evaluated using nudged elastic band (NEB) calculations. The migration pathway between adjacent H– sites via vacancies was considered. First-principles calculations were performed using the projector-augmented wave method and PBEsol functional as implemented in the VASP code. The H– diffusivity of sodium-doped SrLiH3 was investigated using density functional theory molecular dynamics. AELiH3 were experimentally synthesized using a mechanochemical method, and their ionic conductivities were evaluated using electrochemical impedance spectroscopy. To further improve the ionic conductivity of these materials, hydride-ion vacancies were introduced by replacing the A-site cation with the monovalent cation of Na+. Neutron powder diffraction (NPD) data were collected at room temperature using a time-of-flight diffractometer (BL09 SPICA) at the J-PARC facility. The distribution of the nuclear scattering length density was analyzed by the maximum entropy method (MEM) using the Z-MEM code. The effect of the alkali-earth element at A-site on the migration energy of H– was evaluated using NEB. The migration energy are estimated to be 24, 35, and 39 kJ mol–1 for CaLiH3, SrLiH3, and BaLiH3, respectively. These low migration energy (< 40 kJ mol–1) suggest that the perovskite-type AELiH3 are suitable substances for hydride-ion conduction. AELiH3 (AE = Sr, Ba) were synthesized via mechanical milling process. The X-ray diffraction patterns of SrLiH3 and BaLiH3 were attributed to a cubic perovskite structure with the space group Pm–3m. For CaLiH3, the starting materials of CaH2 and LiH remained in the product, and the perovskite-type CaLiH3 could not be synthesized. The ionic conductivities of SrLiH3 and BaLiH3 at 100 °C were 4.6 × 10–7 and 6.7 × 10–9 S cm–1, respectively. This clearly denotes the characteristic of hydride ion; a smaller ionic radius of the A-site cation results in a lower migration energy and a higher ionic conductivity. Solid solutions of Sr1–x Na x LiH3–x were obtained in the range of 0 ≤ x ≤ 0.075. The ionic conductivity was enhanced upon Na-doping. The highest conductivity was observed for Sr0.925Na0.075LiH2.925 (x = 0.075) with a bulk conductivity of 5.0 × 10–6 S cm–1 at 25.6 °C. The figure shows the refined crystal structure of Sr0.925Na0.075LiH2.925 obtained by a Rietveld analysis of the NPD data, in addition to the isosurface plot of the nuclear density obtained by MEM. Rietveld analysis confirmed the presence of H defects and a remarkable anisotropy of the thermal oscillations of H. The negative nuclear density at the H sites showed broadening along the {100} direction. These structural features suggest that hydride ions conduct via vacancies between their adjacent sites. The choice of small cations at the A sites and the presence of hydrogen vacancies in perovskite-type hydrides resulted in high hydride-ion conductivity. [1] Kobayashi et.al., Science, 351, 1314–1317(2016). [2] Bai, Q. et.al., ACS Appl. Energy Mater., 1, 1626–1634(2018). [3] Ishihara, T. et.al., J. Am. Chem. Soc., 116, 3801–3803(1994). [4] Li, M. et.al., Nat. Mater., 13, 31–35(2014). [5] Hull, S. et.al., J. Phys.: Condens. Matter, 11, 5257–5272(1999). [6] Yamane, Y. et.al., Solid State Ionics, 179, 605–610(2008). Figure 1

Finding transition state and minimum energy path of bistable elastic continua through energy landscape explorations

Mechanical bistable structures have two stable equilibria and can transit between them under external stimuli. Due to their unique behaviors such as snap-through and substantial shape changes, bistable structures exhibit unprecedented properties compared to conventional structures and thus have found applications in various fields such as soft robots, morphing wings and logic units. To quantitatively predict the performance of bistable structures in these applications, it is desirable to acquire information about the minimum energy barrier and an energy-efficient transition path between the two stable states. However, there is still a general lack of efficient methodologies to obtain this information, particularly for elastic continua with complicated, unintuitive transition paths. To overcome this challenge, here we integrate energy landscape exploration algorithms into finite element method (FEM). We first utilize the binary image transition state search (BITSS) method to identify the saddle point and then perform nudged elastic band (NEB) calculations with an initial guess based on the BITSS results to find the minimum energy path (MEP). This integrated strategy greatly helps the convergence of MEP calculations, which are highly nonlinear. Two representative cases are studied, including bistable buckled beams and a bistable unit of mechanical metamaterials, and the numerical results agree well with the previous works. Importantly, we numerically predict the complicated MEP of an asymmetric bistable unit of mechanical metamaterials and use experiments to demonstrate that following this MEP leads to successful transition between stable states while intuitive uniaxial compression fails to do so. Our work provides an effective numerical platform for identifying the minimum energy barrier and energy-efficient transition path of a bistable continuum, which can offer valuable guidance to the design of actuators, damping structures, energy harvesters, and mechanical metamaterials.

Identifying the Phonon Contributions to Li+ Hop in La0.5Li0.5TiO3 Solid Electrolyte

Ion diffusion is important in a variety of applications, yet fundamental understanding of the interaction of lattice vibrations (phonons and vibrational motifs) and the mobile species in solids is still missing. Particularly, for Li super ionic conductors (LISICONs), several studies have reported on the important role of the poly-anion octahedral rotations (rocking modes) in facilitating solid-state Li+ ion migration.[1], [2] However, direct calculation of the contribution of these rocking modes to the Li+ hop is missing, and the provided arguments in these reports are mostly based on establishing correlations between different properties of the rocking modes and the migration properties of the hopping Li+ ion without directly quantifying the contribution of the rocking modes to the Li+ hop. For instance, rocking modes have been argued to be able to (i) provide a softer lattice environment for Li+ ion vibration,[3] (ii) increase the bottleneck area of diffusion (4-O square area),[2] (iii) induce additional force on the carrier sublattice,[1] and (iv) maintain a relatively constant coordination number for Li+ ion during its hop[1] – all conducive to Li+ hop in the lattice. Although insightful, we still lack understanding of the exact vibrational frequencies (density of states) of the rocking modes, and, more importantly, the degree of their contribution to Li+ hop with respect to other phonon modes in the structure. In this work, using a combination of ab initio nudged elastic band (NEB) and lattice dynamics calculations based on the recently proposed formalism,[4] we identify the direct contributions of all phonons, including the rocking ones, to Li+ hop in the perovskite solid-state Li+ conductor Li0.5La0.5TiO3 (LLTO).We set up our NEB Li+ hop calculations considering (i) different orderings of the LLTO lattice,[5] and (ii) different Li+ hopping mechanisms (single and concerted (cooperative)) that can occur in the LLTO lattice.2 To sample such a high degree of complexity, we performed 22 independent NEB calculations of Li+ hop in three different LLTO structures with distinct Li|La orderings and based on two different ion hop mechanisms (single and concerted). Our calculations determined that the following two groups of vibrational modes dominate the contributions to the Li+ hop in the LLTO lattice: (i) rocking modes, and (ii) modes that induce large vibrational energies on the hopping Li+ along its hopping direction. Specifically, our results confirmed that the top 5% (10%) contributing modes to the Li+ hop in the LLTO lattice were responsible for 48% (61%) of the total contributions to the Li+ hop, and the two rocking and highly energetic modes comprised >95% (>85%) of these top 5% (10%) contributing modes. Notable from our calculations was that the rocking modes were only present in the < 5.5 THz frequency region, and in this < 5.5 THz frequency region, they comprised 33% of the vibrational modes and contributed 50% to all the possible contributions to the Li+ hop in this low frequency THz region. Moreover, through static modal excitement calculations, we determined that highly contributing rocking modes of vibration were important in solid-state Li+ ion migration because of their ability to (i) increase the O-4 square bottleneck area of conduction[6] and (ii) amplify the force on the hopping Li+ ion. In summary, our observations demonstrated the strong importance of the THz vibrational region to Li+ hop in the lattice, which can be accessible for further explorations using THz spectroscopy techniques to deepen our understanding of the relation between solid-state transport of Li+ and different vibrational motifs in the lattice.

Effect of periodic image interactions on kink pair activation of screw dislocation

Periodic boundary condition along a dislocation line is commonly used in computing activation barriers or formation energies of kink pair of screw dislocation in BCC metals. Although the effect of periodic image interactions on the computation results is obvious, there had been no comprehensive analysis on such effect. In this work, we quantify it through combined nudged elastic band (NEB) simulations and theoretical analysis based on dislocation mechanics. The NEB calculation result demonstrates a non-negligible size dependence on the activation barrier at zero and low stresses. The theoretical analysis offers a practical approach to quantify such size effect without the need of time-consuming NEB simulations. Notably, a simple relationship between kink activation barrier and dislocation line length is derived at zero stress, offering a new approach to compute kink pair formation energy based on NEB simulation results.

Nudged elastic stiffness band method: A method to solve kinks problems of reaction paths.

The nudged elastic band (NEB) method has been widely employed for reaction path (RP) finding; however, certain NEB calculations do not converge to the minimum energy paths (MEPs) because of the occurrence of kinks, which are caused by the free bending of bands. Thus, we propose an extension of the NEB method, called the nudged elastic stiffness band (NESB) method, which adds the stress of stiffness using a beam theory. Here we present results from three examples: the NFK potential, the RPs of the Witting reaction, and finding saddle points for a set of five chemical reaction benchmarks. The results showed that the NESB method has three advantages: to decrease the number of iterations, shrink the length of the pathways from reducing unnecessary fluctuations, and find TS structures by converging to paths close to the MEPs for systems that have sharp curves on the MEPs.

Self-Limited Embedding Alternating 585-Ringed Divacancies and Metal Atoms into Graphene Nanoribbons.

Because of their theoretically predicted intriguing properties, it is interesting to embed periodic 585-ringed divacancies into graphene nanoribbons (GNRs), but it remains a great challenge. Here, we develop an on-surface cascade reaction from periodic hydrogenated divacancies to alternating 585-ringed divacancies and Ag atoms via intramolecular cyclodehydrogenation in a seven-carbon-wide armchair GNR on the Ag(111) surface. Combining scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy combined with first-principles calculations, we in-situ-monitor the evolution of the distinct structural and electronic properties in the reaction intermediates. The observation of embedded Ag atoms and further nudged elastic band calculations provide unambiguous evidence for Ag adatom-mediated C-H activation in the intramolecular cyclodehydrogenation pathway, where the strain-induced self-limiting effect contributes to the formation of the GNR superlattice with alternating 585-ringed divacancies and Ag atoms, which shows a band gap of about 1.4 eV. Our findings open an avenue to introducing periodic impurities of single metal atoms and nonhexagonal rings in on-surface synthesis, which may provide a novel route for multifunctional graphene nanostructures.

First principle study on transition metal ammine borohydrides with amphoteric hydrogen for hydrogen storage applications

Usually the ions of a particular element in solids either in positive or in negative oxidation states, which are influenced by their chemical circumstances. Thus, it is quiet interesting for an atom having both positive and negative oxidation states within the same structural framework in a particular compound. Our structural and chemical bonding analyses show that the hydrogen ions in the transition metal ammine borohydrides (TMABHs) with the chemical formula M(BH4)2(NH3)2 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) have both + and − oxidation states. Moreover, our detailed analyses show that the hydrogen present in these compounds have amphoteric behaviour with hydrogen closer to boron is in negative oxidation state and that closer to nitrogen is in the positive oxidation state. The spin-polarised van der Waals interaction included calculation show that all these materials except M = Zn are having finite magnetic moment at the transitions metal site with the anti−ferromagnetic ordering as ground state. The localised magnetic moment similar to those present in molecular magnet with large band gap value indicating that these transparent magnets may find application in novel devices. Our nudged elastic band calculation show that the migration barrier for ammonia diffusion in these materials are more than 1 eV and hence these compounds may be used for energy storage applications since they have high weight percentage of hydrogen. The confirmation of the presence of amphoteric behaviour of hydrogen in TMABHs has implication in designing volume efficient hydrides for hydrogen storage applications.

Curvature-weighted nudged elastic band method using the Riemann curvature.

The nudged elastic band (NEB) method is utilized to find reaction paths (RPs) using discretized intermediate structures called "images" between a reactant and a product. In fact, NEB calculations do not always converge because of the bent of RPs. Mathematically, more images are needed for complex curves, and here, we focused on the curvature. In this study, we propose a new method for calculating the curvature of the RPs as well as a method for weighting the spring constant of the NEB with the curvature, which we named the curvature weighted NEB (CW-NEB) method. In addition, we will propose the CW-NEB method with the climbing image (CI) method (CW-CI-NEB). To show the efficiency of our method, calculations for the ene-reaction of the CW-CI-NEB method were performed and compared with those of the CI-NEB methods. The CW-CI-NEB methods converged in shorter iteration times than the CI-NEB calculation, which was found by gathering the bent of RPs.

Revealing the Structure and Properties of Polycrystalline Components of the Solid Electrolyte Interface

The formation, structure and composition of the solid-electrolyte interphase (SEI) in lithium batteries have been extensively explored in the prior literature. It is commonly accepted that the SEI consists of two inorganic layer followed by organic layer. However, very little is known about the actual grain boundary (GB) structure of the inorganic layer and the ions transport through it. The present work seeks to understand the structure and stability of the GBs formed between various inorganic components of the SEI and mechanisms of Li diffusion through them. The density functional theory (DFT) calculations supported by the high-resolution transmission electron microscopy (HRTEM) measurements are employed to understand the SEI GB atomic structures and their chemo-mechanical properties.DFT calculations were performed using the Vienna Ab Initio Simulations Package (VASP) code1 employing the generalized-gradient approximation (GGA) using the PBE (Perdew, Burke, and Ernzerhof)2 functional to account for the exchange-correlation effects. The Nudged Elastic Band (NEB) method3 is used for the Li migration barriers calculations as implemented in VASP. To build the GB structures, the Atomistic Tool Kit (ATK)4 “Interface Builder” tool is used, which allows to analyze all possible interphases between two slabs. Cryogenic TEM (Cryo-TEM) images are recorded on a JEOL ARM200CF TEM, equipped with a Gatan Oneview camera operated at 200 kV.The HRTEM measurements revealed the polycrystalline structure of the SEI inorganic layer consisting mainly of LiF, Li2CO3, Li2O and LiOH components. In addition, different surface orientations (e.g., 111, 200, 002, etc.) of the aforementioned components are identified. Based upon the HRTEM results (cf. Fig. 1 left-hand side), the corresponding computational slabs have been built as shown on the right-hand side in Fig. 1. After the optimization of all structures, Li adatom diffusion inside grain and GB and mechanical properties were calculated.For each GB, several Li adatom positions with multi-atom bonding were identified. The activation energy of Li diffusion varies significantly depending upon the slab’s structure and the number of Li adatoms. The NEB calculations also reveal a non-symmetric transition state for all considered structures, which indicates a complex multi-atom hopping mechanism. The calculated elastic properties show the decrease of Young’s modulus with an increase of Li adatoms. Our combined experimental and modeling findings provide valuable insights into developing mitigation strategies towards a better understanding of the SEI components stability and Li migration through the SEI.Figure 1. Identification of grain and GB of the SEI busing HRTEM (left) and the corresponding built DFT slab (right). Lavender spheres denote Li atoms, red spheres are the oxygen atoms, grey spheres represent carbon atoms and green spheres are F atoms. Blue sphere in the GB denotes Li adatom.References G. Kresse, Phys. Rev. B, 59, 1758–1775 (1999).J. P. Perdew, A. Ruzsinszky, G. I. Csonka, O. A. Vydrov, G. E. Scuseria, L. A. Constantin, X. Zhou, K. Burke, Phys. Rev. Lett., 100, 136406 (2007).G. Henkelman, B. P. Uberuaga, H. Jónsson, and G. Henkelman, J. Chem. Phys, 113 9901 (2020). Synopsys QuantumWise A/S, 2017/2. Figure 1

HfOx /AlOy Superlattice-Like Memristive Synapse.

The adjustable conductance of a two‐terminal memristor in a crossbar array can facilitate vector‐matrix multiplication in one step, making the memristor a promising synapse for efficiently implementing neuromorphic computing. To achieve controllable and gradual switching of multi‐level conductance, important for neuromorphic computing, a theoretical design of a superlattice‐like (SLL) structure switching layer for the multi‐level memristor is proposed and validated, refining the growth of conductive filaments (CFs) and preventing CFs from the abrupt formation and rupture. Ti/(HfOx/AlOy)SLL/TiN memristors are shown with transmission electron microscopy , X‐ray photoelectron spectroscopy , and ab initio calculation findings corroborate the SLL structure of HfOx/AlOy film. The optimized SLL memristor achieves outstanding conductance modulation performance with linearly synaptic weight update (nonlinear factor α = 1.06), and the convolutional neural network based on the SLL memristive synapse improves the handwritten digit recognition accuracy to 94.95%. Meanwhile, this improved synaptic device has a fast operating speed (30 ns), a long data retention time (≥ 104 s at 85 ℃), scalability, and CMOS process compatibility. Finally, its physical nature is explored and the CF evolution process is characterized using nudged elastic band calculations and the conduction mechanism fitting. In this work, as an example the HfOx/AlOy SLL memristor provides a design viewpoint and optimization strategy for neuromorphic computing.

Efficient and accurate atomistic modeling of dopant migration using deep neural network

This paper proposes an efficient and accurate method to model atomistic dopant migration, by leveraging the emerging deep neural network (DNN). By performing nudged elastic band (NEB) simulations of three prototype systems (B-doped Si, Li-doped Si, and C-doped GaN), it is shown that the proposed DNN-based method runs about 104-105 times faster than the widely-used atomistic dopant migration modeling method based on density functional theory (DFT), meanwhile keeping DFT-level high accuracy. Active learning is used to reduce training set redundancy, and the DNN model is further optimized for more accurate NEB calculation. As a result, the dopant atomic position in saddle-point and the dopant migration energy barrier in the migration energy path (MEP) predicted by the proposed DNN-based NEB deviate merely about 10−2 Å and 10−2 eV, respectively, from those predicted by the established DFT-based NEB. Given its efficiency and accuracy, the proposed DNN-based method might be useful to develop future-generation atomic-scale technology computer-aided design (TCAD) tools.

DFT Investigation of Ammonia Formation via a Langmuir-Hinshelwood Mechanism on Mo-Terminated δ-MoN(0001).

In this work, we employed density functional theory to elucidate the energetics associated with elementary steps along a Langmuir–Hinshelwood mechanism for the Haber–Bosch synthesis of ammonia from N2 and H2 on a hexagonal, Mo-terminated molybdenum nitride surface. Using nudged elastic band calculations, we determined the energy barriers involved in the reaction processes. An active site consisting of four nearest-neighbor Mo atoms, previously identified as an active site on similar surfaces, was chosen to investigate the reaction processes. Using this approach, we calculate a barrier of ∼0.5 eV for the dissociation of N2. The superior activity of the dissociation of the strong N2 bonds is rationalized based on the unique geometric and electronic configurations present at these active sites. Despite the favorable energetics for nitrogen dissociation, the energy cost for hydrogenation of NHx (0 ≤ x ≤ 2) species is shown to be energetically limiting for the formation of ammonia through the Langmuir–Hinshelwood mechanism at these sites, with elementary step activation barriers calculated to be as large as ∼2 eV. A comparison to Haber–Bosch results derived from a similar γ-Mo2N model system suggests the relative independence of surface chemistry and bulk stoichiometry for rhombic Mo4 active sites present on molybdenum nitrides.

Secondary slip of screw dislocations in zirconium

Plasticity in hexagonal close-packed zirconium is controlled by screw dislocations which easily glide in the prismatic planes where they are dissociated. At high enough temperatures, these dislocations can deviate out of the prism planes to also glide in the first order pyramidal and basal planes. To get a better understanding of these secondary slip systems, we have performed molecular dynamics (MD) simulations of a screw dislocation gliding in a basal plane. The gliding dislocation remains dissociated in the prism plane where it performs a random motion and occasionally cross-slips out of its habit plane by the nucleation and propagation of a kink-pair. Deviation planes are always pyramidal, with an equal probability to cross-slip in the two pyramidal planes on both sides of the basal plane, thus leading to basal slip on average. Basal slip appears therefore as a combination of prismatic and pyramidal slip in the high stress regime explored in MD simulations. This is confirmed by nudged elastic band (NEB) calculations. But NEB calculations also reveal a change of glide mechanism for a decreasing applied stress. At low stress, kinks do not lie anymore in the pyramidal planes. They are now spread in the basal planes, thus fully compatible with a motion of the screw dislocation confined to the basal plane as seen in experiments. Basal slip, which is in competition with pyramidal slip, appears therefore favoured at low stress in pure zirconium.

Geometry Orbital of Deep Learning (GOODLE): A uniform carbon potential

This work presents a geometry-driven deep learning framework Geometry Orbital of Deep Learning (GOODLE) that accurately predicts all carbon properties in real space and momenta space. We focus on the hybrid carbon orbitals (sp2 and sp3) geometry to establish their deep learning “potential”. We demonstrate its excellent performance with the properties predictions in dual space, including energy, equation of states, phonon structure, electronic band structure, and optical absorption. Capable of obtaining the carbon orbital geometries from small lattice carbon structures, GOODLE can be used for both inorganic carbon allotropes and organic hydrocarbon molecules, including the magic-angle twisted bilayer graphene, with excellent transferability. GOODLE is also available for the nudged-elastic band calculation. Besides, we proposed an analytic geometry condition criterial for the carbon allotropes stability.

Li+ Diffusion in Amorphous and Crystalline Al2O3 for Battery Electrode Coatings

Al2O3 is often applied protectively to lithium-ion battery anode and cathode materials to inhibit surface degradation, suppress dendrite formation, and relieve mechanical stresses. Given the very high intrinsic band gap and diffusion barrier of the material, the mechanism that allows Li diffusion through these coatings is not well understood, and widely varying laboratory results indicate that there may be dependencies on morphology and stoichiometry. Using nudged elastic band calculations and ab initio molecular dynamics, we perform a systematic investigation across Al2O3 structures, both crystalline and amorphous, and at various concentrations of Li+ to uncover the optimal parameters for maximally diffusive coatings. We find a correlation between the low proximity of Li+ to Al3+ and the low Li+ migration barrier. Although barriers are the lowest in the highly diffusive one-dimensional channels of crystalline θ-Al2O3, the system is structurally delicate and subject to detrimental distortion as the Li+ content is increased. The α-Al2O3 lattice is, conversely, highly stable against distortion at all Li+ concentrations but disadvantageous for Li+ migration. In amorphous systems, unscreened Li+–Li+ Coulomb repulsion and pre-emptive occupation of “trapping sites” combine to lower the energy barriers as a function of increasing concentration. One of our most important findings is that Al-deficient materials can sharply increase Li+ movement, and we predict that an amorphous material with a combination of high Li+ concentration and Al deficiency would enable highly Li+-conductive protective coatings for electrodes.

The effect of vacancy defects on the adsorption of methane on calcite 104 surface

Density functional theory calculations are used to study the effect of vacancy defects (ionic Ca and charge neutral CO 3 ) on the adsorption properties of CH 4 molecule on different calcite surfaces. Both types of vacancy defect results in strong adsorption of the gas molecules as compared to defect-free sample. For example, for the 104 surface of the material the methane molecules gets adsorbed stronger by 63.6% and 24.4% for Ca and CO 3 vacancy defects, respectively. Such enhanced adsorption of the CH 4 molecule due to the presence of the vacancy defects is also obtained for the other surface symmetries. Electronic structure calculations show that the latter is due to the orbital-overlap/hybridization between the organic molecules and the substrate. Adsorption capacity of the calcite also increases due to the presence of the defect. Vacancy defects also create energy barriers for the migration of CH 4 on the surface of calcite as revealed in our nudged elastic band calculations. The effect on the CH 4 adsorption and migration becomes more pronounced in the case of Ca vacancy defect. These findings could be of practical importance for, e.g., estimating ultimate gas recovery.