Structure Optimization Calculations Research Articles

Proposal to improve Ni-based superconductors via enhanced charge transfer

Recently discovered superconductivity in hole-doped nickelate Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ has attracted intensive attention in the field. An immediate question is how to improve its superconducting properties. Guided by the key characteristics of electronic structures of the cuprates and the nickelates, we propose that nickel chalcogenides with a similar lattice structure should be a promising family of materials. Using NdNiS$_2$ as an example, through first-principle structural optimization and phonon calculation, we find this particular crystal structure a stable one. We justify our proposal by comparing with CaCuO$_2$ and NdNiO$_2$ with regard to strength of the charge-transfer characteristics and the trend in their low-energy many-body effective Hamiltonians of doped hole carriers. This analysis indicates that nickel chalcogenides host low-energy physics closer to that of the cuprates, with stronger magnetic interaction than the nickelates, and thus they deserve further experimental exploration. Our proposal also opens up the possibility of a wide range of parameter tuning through ligand substitution among chalcogenides, to further improve superconducting properties.

Exploring the Physical Properties of Cu2WSe4 for Optoelectronic and Thermoelectric Applications: A DFT Study

Full-potential linearized augmented-plane wave method and Boltzmann transport theory are used to study the electrical, thermoelectric and optoelectronic response of the Cu2WSe4 compound. The calculations are performed with the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and modified Becke–Johnson (mBJ) potential. Structural optimization and formation energy calculations justify the stability of the studied compound. An indirect bandgap (N-Γ) of 1.36 eV with GGA and 1.52 eV with mBJ is achieved. The density of states also exposes the bandgap and confirms the semiconducting nature of Cu2WSe4. The optical properties such as dielectric function, refractive index, absorption coefficient and energy loss are also calculated. The thermoelectric response is calculated through the figure of merit (ZT), Seebeck coefficient, electrical and thermal conductivity and power factor. At high temperatures, a moderate ZT value of 0.65 is achieved with mBJ potential, which shows the possibility to use Cu2WSe4 in the renewable energy device.

First principles study of Li adsorption properties of a Borophene based hybrid 2D material B5Se

In this work we investigate the Li adsorption properties of a hybrid 2D material, namely monolayer B5Se with first principles calculations. The density functional theory (DFT) calculations are performed with the generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhoff (PBE) exchange correlation functional, inclusive of van der Waals corrections with Grimme's DFT-D3 scheme. Structural optimization calculations show 2 dimensional B5Se to have a distorted hexagonal structure with five B atoms and one Se atom at the vertices of each hexagon, with a favourable cohesive energy of formation of 0.648 eV/atom. A metallic nature was observed for the hybrid material from the band structure and density of states calculations. Electrode performance metrics, as the most preferred adsorption sites and adsorption energies, open circuit anode potentials, charge density differences and specific capacities for varying adatom coverage were evaluated with the DFT calculations. The adatom diffusion barriers were evaluated with a nudged elastic band (NEB) method. The ab-initio calculations predict a maximum theoretical specific capacity of 1486.87 mAhg−1 for Li adsorption on 2D B5Se, which is over four times that of conventional Li ion battery anode materials. This coupled with an open circuit anode potential of 0.291–0.179 V for different degrees of Li coverage, and a small Li diffusion barrier of 0.15 eV, metallic nature of the sheet under pure and lithiated conditions and good charge density variations, make monolayer B5Se a potent anode material for Li-ion battery applications.

Study on Structural Performance of Horizontal Axis Wind Turbine with Air Duct for Coal Mine

Considering the characteristics of narrow underground space and energy distribution, based on blade element momentum theory, Wilson optimization model and MATLAB programming calculation results, the torsion angle and chord length of wind turbine blade under the optimized conditions were obtained. Through coordinate transformation, the data were transformed into three-dimensional form. The three-dimensional model of the blade was constructed, and the horizontal axis wind turbine blade under the underground low wind speed environment was designed. The static structural analysis and modal analysis were carried out. Structural design, optimization calculation and aerodynamic analysis were carried out for three kinds of air ducts: external convex, internal concave and linear. The results show that the velocity distribution in the throat of linear air duct is relatively uniform and the growth rate is large, so it should be preferred. When the tunnel wind speed is 4.3 m/s and the rated speed is 224 rad/s, the maximum displacement of the blade is in the blade tip area and the maximum stress is at the blade root, which is not easy to resonate. The change rate of displacement, stress and strain of blade is positively correlated with speed. The energy of blade vibration is mainly concentrated in the swing vibration of the first and second modes. With the increase in vibration mode order, the amplitude and shape of the blade gradually transition to the coupling vibration of swing, swing and torsion. The stress and strain of the blade are lower than the allowable stress and strain of glass fiber reinforced plastics (FRP), and resonance is not easy to occur in the first two steps. The blade is generally safe and meets the design requirements.

Electric field and strain induced gap modifications in multilayered GaN

Two-dimensional materials with a wide bandgap characteristic are critical to developing novel nanoscaled electronic devices. This study investigates the layer, in-plane biaxial strain, and vertical external electric field effects on the stability and electronic properties of multilayered GaN employing the density functional theory. Using structural optimization and phonon-mode calculations, the GaN structures with one to five layers are found to be stable and refer to the Born–Oppenheimer local minima. Under dimensionality effect, GaN unveils a semiconducting feature with an indirect bandgap at the HSE06 level that changes from 3.37eV for a monolayer to 2.79eV for a five-layered structure as the quantum confinement effect is reduced. The calculations demonstrate that the indirect bandgap of multilayered GaN decreases linearly when increasing the tensile strain. However, compressive strain engineering would induce an indirect-to-direct gap transition. Additionally, when the strength of the applied perpendicular external electric field to the multilayered GaN surface is increased, the GaN bandgap rapidly disappears due to the field-induced near free electronic gas. Calculated carrier effective mass indicate that both external effects impact charge carrier mobility as well. Such significant changes in the electronic properties under external excitations show that multilayered GaN shows promise in configurable nanoelectronic devices.

Adsorption based on weak interaction between phenolic hydroxyl, carboxyl groups and silver nanoparticles in aqueous environment: Experimental and DFT-D3 exploration

Numerous studies have shown that there are complex interactions between silver nanoparticles (AgNPs) and humic acid (HA) in the natural ecosystem. However, little is known about the interaction between AgNPs and HA in terms of molecular mechanisms. It is well-accepted that aromatics substituted by phenolic hydroxyl (Ar−OH) and carboxyl (−COOH) groups are primary constituents of HA. In this research, we proved that five benzene series (phenol, BA, OHBA, MHBA and PHBA) have different adsorption capacity sequences with AgNPs by experimental adsorption coefficient in UV–vis. Subsequently, FTIR analysis indicated the weak interaction in molecules is the main adsorption mechanism. Based on these facts, the weak interaction of Ar−OH and −COOH with silver nanoparticles (AgNPs) was systematically investigated by the first-principles calculations using density functional theory (DFT). The structure optimization and energy calculation demonstrated that the Ar−OH and −COOH groups on the benzene ring had the greatest influence on the AgNPs adsorption site, energy and deformation when they are in adjacent positions. Furthermore, the calculations of electronic structure, Density of states (DOS) and crystal orbital Hamilton population (COHP) provided evidence that the charges in C and H atoms were transferred to O atoms after the adsorption. Whereas, there was little charge transfer from AgNPs to benzene series. This study is hoped to provide a useful theoretical reference for the evaluation of HA and AgNPs in the water environment.

Computational exploration of gallium-doped neutral and anionic magnesium nanocluster materials: Ga2Mgnq (n = 1–11; q=0, −1) nanocluster’s properties based on DFT

In this study, using CALYPSO software to search initial structures of the gas-phase Ga2Mgnq (n = 1–11; q=0, −1) nanoclusters in vacuum, both neutral and anionic magnesium nanoclusters doped with two gallium atoms based on DFT were systematically investigated. According to the structural optimization calculations, the ground state structures of Ga2Mgnq nanoclusters evolve from 2D plane structures to 3D tetrahedron-based geometries before stabilizing at 3D triangular prism-based structures. Stability analysis indicates that Ga2Mg8 and Ga2Mg7- are robust and, therefore, could be categorized as "magic" clusters. Based on MOs, DOS and AdNDP computations, it is found that the σ-type chemical bond formed by the hybridization of the s- and p-orbitals of Ga and Mg atoms are responsible for the high stability of Ga2Mg8 and Ga2Mg7-. Furthermore, to provide further support for future experiments, the PES of Ga2Mgnq=−1 (n = 1–11) and IR and Raman spectra of Ga2Mg8 and Ga2Mg7- nanoclusters were also reported.

Theoretical insights on the antioxidant activity of dihydrocaffeic acid

In this work, quantum chemical analysis have been done to study the structure and antioxidant activity (AA) of dihydrocaffeic acid (DHCA) based on the density functional theory (DFT). Structural optimization and thermochemical calculations of the neutral, radical and ionic forms of DHCA have been carried out by DFT-B3LYP/6-311G(d,p) approach. Systematic study of reaction enthalpies related with the hydrogen atom transfer (HAT), single electron transfer proton transfer (SET-PT) and sequential proton loss and electron transfer (SPLET) have also been calculated in gas and water. Global Molecular Descriptors (GMD) and Non-Linear Optical (NLO) properties have been calculated by using the same theory. The interaction between donors and acceptors and stabilization energy have been determined with the help of Natural Bond Orbital (NBO) tool.

Structure optimization and optoelectronical property calculation for organic lead iodine perovskite solar cells

Methylamine lead iodide (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> MAPbI<sub>3</sub>) and formamidine lead iodide (CH(NH<sub>2</sub>)<sub>2</sub>PbI<sub>3</sub> FAPbI<sub>3</sub>) are the most commonly used organic lead iodine perovskite materials for solar cell research. For the perovskite solar cell with a layered structure, the optical properties and thickness of each layer affect the photoelectric conversion efficiency of the cell. In this paper, the optical admittance method and rigorous coupled wave analysis method are used to calculate the absorptivities and transmittances of metal oxide transparent conductive films for tin-doped indium oxide (In<sub>2</sub>O<sub>3</sub>:Sn), fluorine-doped tin oxide (SnO<sub>2</sub>:F), TiO<sub>2</sub>, MAPbI<sub>3</sub> and FAPbI<sub>3</sub>. The influence of each layer thickness and device structure on the short-circuit current density of the cell are analyzed. It is shown that for the FTO(ITO)/TiO<sub>2</sub>/MAPbI<sub>3</sub> structure, when the thickness of the FTO film is 50–450 nm and the thickness of the ITO film is 10–150 nm, the average transmittance for the 360–800 nm wavelength light is 85%. For the FTO(ITO)/TiO<sub>2</sub>/FAPbI<sub>3</sub> structure, when the thickness of the FTO film and ITO film are 50–250 nm and 10–150 nm, respectively, the average values of the transmittance for the 360-840 nm wavelength light are 81.6% and 78%, respectively. Under the optimal thickness of FTO and TiO<sub>2</sub>, and the thickness of MAPbI<sub>3</sub> and FAPbI<sub>3</sub> are 300–1000 nm, the corresponding short-circuit current densities are in a range of 21.9–23.7 and 23.0–24.4 mA·cm<sup>–2</sup>, respectively. The band gap of MAPbI<sub>3</sub> and FAPbI<sub>3</sub> are 1.56 and 1.48 eV, for which the corresponding absorption cut-off wavelengths are 796 and 840 nm, respectively, indicating that FAPbI<sub>3</sub> has a wider absorption spectrum than MAPbI<sub>3</sub>. In order to maximize the Jsc value of the organic lead iodine perovskite solar cell, the thickness range of each layer for MAPbI<sub>3</sub> perovskite solar cell (FTO thickness is (80 ± 50) nm, ITO thickness is less than 120 nm, MAPbI<sub>3</sub> thicknessis 300–600 nm) and for FAPbI<sub>3</sub> perovskite solar cell (FTO thickness is (120 ± 50) nm, ITO thickness is less than 180 nm, FAPbI<sub>3</sub> thickness is 300–600 nm) are given. The research results of this article have guiding significance in designing and preparing the perovskite solar cells with high conversion efficiency.

Adsorption Study of Main Gas Products from Water Radiolysis on 4A Zeolite by Numerical Simulations

This work is motivated by the results obtained during a study on the tritiated water adsorbed in zeolite [L. Frances et al., J. Phys. Chem. C, 119, 28462 (2015)]. The decomposition of water by radiolysis leads to the production of dioxygen and dihydrogen as main stable products. By studying the evolution of their quantities of matter, one can note an increase in a first stage, followed by a decrease after a few hundred days of storage until complete disappearance. This interesting process depends on the water loading ratios, expressed in mass percentage, lying between 4%, and 19%; such a phenomenon is not observed in saturated zeolite. Our goal is to determine, through numerical simulations, how this disappearance, which is associated with the recombination of the radiolysis products, occurs by making a microscopic study on the adsorption of H2O, H2, and O2 molecules on 4A zeolite (Z4A). Computational physics is useful to understand the effects of molecule adsorption on its structure and also to closely examine the molecule-zeolite and molecule-molecule interactions. Indeed, different simulation methods are used from static to dynamic studies employing both quantum and classical tools with the periodical structure of Z4A. To summarize, the adsorption of molecules from the radiolysis of water are studied according to different points of view (quantum and classical) using various numerical simulation tools, such as density functional theory for ab initio structural optimization and energy calculation, Monte Carlo to study the distribution of the adsorbed molecules in the zeolite, and molecular dynamics to follow the evolution of the system (molecule + zeolite) over time depending on the temperature, in order to extract as much information as possible (structurally, statistically, energy, electronically) to understand the main problematic of this work: How do stable molecules issued from radiolysis recombine in Z4A?

First-Principles Study of Defect Levels Caused by Transition Metal Atoms in Silicon Nitride for Non-Volatile Memory Applications

The silicon nitride-based charge trap memory architecture has been considered as one of the promising solutions for high-bit-density three-dimensional NAND-type flash memories [1]. In this architecture, electrons and holes are trapped by point defects distributed in silicon nitride. This phenomenon induces a shift of the threshold voltage of memory transistors and is applied to store data. Recently, multi-level cell technology has been introduced in NAND-type flash memories to increase the bit density, which has needed to enlarge the threshold voltage shift [2]. Thus, the increase in the density of the point defects which can generate deep trap levels in silicon nitride is demanded to realize the high-bit-density flash memories. In the present work, we investigate the energy levels caused by transition metal (Ti, V, or Mn) defects in silicon nitride using first-principles calculations.Amorphous silicon nitride film has been used as the charge trap layer in flash memories. The hexagonal β-Si3N4 crystalline phase is believed to have similar local structures as amorphous silicon nitride [3]. Therefore, to discuss the realistic nature of transition metal atoms in silicon nitride, a 168-atom supercell was formed by expanding a unit cell of β-Si3N4 phase in a 2x2x3 matrix on a-axis, b-axis, and c-axis, respectively. The structure optimization and self-consistent field (SCF) calculations were performed using Advance/PHASE ver. 3.5 software. The generalized-gradient approximation with the PBE functional was used for the exchange-correlation energy of electrons. The kinetic energy cutoffs for wavefunctions were 40, 52, 64, 76, and 88 Ry and those for charge density were 400, 520, 640, 760, and 880 Ry in the calculations. 2x2x4 k-points sampling in the Monkhorst-Pack grid was used to perform SCF calculations.The behavior of transition metal atoms in semiconductors and dielectrics has been widely studied. Shibata et al. explained the behavior of transition metal atoms in silicon nitride using density-functional theory (DFT) calculations in terms of formation energies [4]. However, there are few reports on the energy levels generated by transition metal atoms in silicon nitride. In the present work, the density of states (DOS) of electrons in four supercell structures of Si72N96, Si71N96Mn, Si71N96V, and Si71N96Ti have been investigated. The calculations with the conditions mentioned earlier have been performed on the supercells, where a transition metal atom (Ti, V, or Mn) was substituted for a Si atom.Figure 1(a) shows total DOS calculated for the supercell Si71N96Mn. We can see five different defect levels of energy 1.59, 1.91, 2.03, 3.00, and 3.12 eV below the bottom of the conduction band. From Atomic Layer DOS (ALDOS) calculation results, we have found that all five trap levels are mainly localized at the Mn atom. Some of these trap levels would be able to behave as deep electron trap levels.Total DOS calculated for the supercell Si71N96V is shown in Fig. 1(b). Similar to the Si71N96Mn, five different defect levels appeared. However, three levels close to the bottom of the conduction band in Si71N96V are shallower in energy as compared to those in Si71N96Mn. Electrons trapped by these shallow levels might be easily emitted to the conduction band. The emission of electrons from the defect levels would degrade the retention characteristics of flash memories. We also calculated total DOS for the supercell Si71N96Ti. No defect levels appeared in the forbidden gap of Si71N96Ti.In conclusion, we have performed the DFT calculations to investigate the defect levels generated by transition metal atoms (Ti, V, or Mn) in silicon nitride. The substitution of a Si atom with a Mn atom generated deep defect levels in the forbidden gap. Some of the defect levels can be expected to behave as deep trap levels for electrons. The doping of silicon nitride with Mn atoms would have potential for enhancing the carrier trapping properties of the silicon nitride charge trap layer. Further experimental studies would be needed to validate the effectiveness of doping of silicon nitride with Mn atoms to the enhancement of the charge trapping properties in flash memories.We would like to show our gratitude to K. Niisato, Y. Ota and T. Inushima for sharing their pearls of wisdom with us during the course of this research. This work was partly supported by JSPS KAKENHI Grant Number JP18K04244.[1] S. Inaba, 2018 IEEE Inte. Memory Workshop, Kyoto, 1 (2018).[2] K. Parat et al., 2018 IEEE Inter. Electron Devices Meeting, San Francisco, 2.1.1 (2018).[3] L. E. Hintzsche et al., Physical Review B., 86, 235204 (2012).[4] D. Shibata et al., ECS Transactions, 64(11), 229 (2014). Figure 1

Density functional theory study on the catalytic degradation mechanism of polystyrene

The density functional theory method of B3LYP/6-311G(d) was used to study two catalytic degradation (acid-catalyzed and alkali-catalyzed) reaction mechanisms of polystyrene (PS). The geometric structure optimization and frequency calculations of all the molecules involved in the catalytic degradation were performed, and the standard thermodynamic parameters of each catalytic cracking path were obtained. The calculation results show that the energy barrier of the optimal reaction path’s rate control step to form styrene monomer is 68.2 kJ/mol in the alkali-catalyzed degradation reaction paths. In the acid-catalyzed cracking paths, the energy barrier of the optimal path’s rate control step to form styrene is 151.9 kJ/mol. The energy barriers of rate control steps for the formation of styrene monomer in both types of these catalytic cracking reactions are lower than those for other products, so the main degradation product in the two types of catalytic degradation is styrene monomer. Compared with pure thermal degradation, an obvious feature of acid-catalyzed degradation is the formation of benzene, indene, and derivatives of indene. The formation of benzene reduces the phenyl content of the PS main chain, which results in a reduction in the yield of styrene monomer. However, an alkali catalyst shows a positive catalytic effect, which increases the yield of styrene monomer.

Monolayer diboron dinitride: Direct band-gap semiconductor with high absorption in the visible range

We predict a two-dimensional monolayer polymorph of boron nitride in an orthorhombic structure (o-${\mathrm{B}}_{2}{\mathrm{N}}_{2})$ using first-principles calculations. Structural optimization, phonon dispersion, and molecular dynamics calculations show that o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ is thermally and dynamically stable. o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ is a semiconductor with a direct band gap of 1.70 eV according to calculations based on hybrid functionals. The structure has high optical absorption in the visible range in the armchair direction while low absorption in the zigzag direction. This anisotropy is also present in electronic and mechanical properties. The in-plane stiffness of o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ is very close to that of hexagonal boron nitride. The diatomic building blocks of this structure hint at its possible synthesis from precursors having B-B and N-N bonds.

A Computational Model of Cd2+ Doped Fluorapatite Ca10(PO4)6F2 Biomaterial by Molecular Dynamics Simulations

We have presented a computational model studied with the Cd2+ ions doped hexagonal fluorapatite (FAP), Ca10(PO4)F2 by classic molecular dynamics (MD) simulations using the general utility lattice program (GULP). The potential model has included long-range Coulomb interactions, general three-body, Buckingham and spring (core-shell) interatomic potentials. The Cd2+ ions have substituted into the Ca2+ sublattice of FAP. The structure optimization calculations are carried out for each composition of CdxCa10-x(PO4)F2 (x=1-9) by minimizing the energy of the system. We have predicted lattice parameters, density, bulk modulus and elastic constants of new stoichiometric apatites. The computed results are compared with available experimental data and those obtained by other methods [1]. We have concluded that those interatomic potentials can be applied to Cd2+ ions doped mineral apatites. The developed computing model may provide a useful method in understanding and design of new biomaterials.

Oxidation state and coordination environment of Pb in U-bearing minerals

This study examines the oxidation and coordination of lead (Pb) in the uranium (U) minerals uraninite (UO2), coffinite (USiO4), and brannerite (UTi2O6), and minerals that have U as a minor constituent, zircon (ZrSiO4) and titanite (CaTiSiO5). The characterization of Pb was conducted through a combination of micro-synchrotron X-ray fluorescence mapping (µSXRF) and micro-X-ray absorption spectroscopy (µXAS), collected at the Pb LII and LIII edge on characterized grains (electron microscopy and X-ray diffraction) from a range of geological settings and ages.For all minerals considered in this study, Pb LII and LIII X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data indicate that Pb exists as Pb2+. In the case of uraninite, DFT structure optimization calculations suggest that the substitution of Pb2+ for U4+ is facilitated by the stereochemically active electron lone pair indicative of Pb2+, which effectively takes the place of an oxygen vacancy (coupled substitution Pb2+ + □ = U4+ + O2−). In detail, however, the EXAFS and XANES data, supported by first principle calculations of the XANES associated with Pb2+ replacing U4+ in the crystal lattice of uraninite, indicate that Pb2+ is present in a less distorted coordination than suggested by DFT, which may reflect a yet to be characterized link between interstitial oxygen defects and Pb2+ incorporation, rather than a simple oxygen vacancy. For coffinite, the XANES data demonstrate that Pb exists mainly as Pb2+ in the form of galena, indicating limited stability of the isomorphous substitution in coffinite. In contrast, Pb is homogeneously distributed in the metamict mineral brannerite, which displays a distinct XANES spectrum suggestive of Pb2+ bonded to oxygens in a highly distorted coordination environment. Pb LII XANES indicate that radiogenic Pb exists predominantly as Pb2+ within the crystal lattice of zircon and as distinct mineral PbO nano-scale inclusions or domains. Additionally, Pb LII XANES measurements also indicate that Pb2+ exists within titanite although the coordination environment is unclear.Overall, these data suggest that the previous indications of the presence of Pb4+ in radioactive minerals need to be reconsidered. The nature of radiogenic Pb2+ incorporation in U-bearing minerals controls the mobilization of Pb in hydrothermal fluids and the utility of a mineral as a geochronometer. The Pb valence and coordination data presented in this study provide fundamental constraints imperative for our understanding of the controls governing the mobilization of radiogenic Pb during water-rock interaction and metamorphism.

Mechanism and kinetics studies of the atmospheric oxidation of p,p'-Dicofol by OH and NO3 radicals

As an effective organochlorine pesticide, Dicofol has been extensively applied in more than 30 countries for protecting over 60 different crops. Considering its large consumption and potential adverse effect on human health (endocrine disrupting and carcinogenicity), the fate of Dicofol sprayed into the air is of public concern. In this study, we conducted a comprehensive study on the reaction mechanisms of p,p'-Dicofol with OH and NO3 radicals using DFT method. Comparing the abstrations by OH and NO3 radical, OH and NO3 radical addition reactions are predominant due to the lower potential barriers and stronger heat release. The phenolic substances (P1P5), epoxides (P11 and P15), dialdehyde (P13) and other species (P8, P9, P10 and P14) are generated by OH additions and their subsequent reactions while OH abstraction reactions produce DCBP, P7 and chlorphenyl radical. Particularly, NO3 additions and their subsequent reactions yield dialdehydes (P16 and P17) and 2,8-DCDD, which is the first report of the generation of dioxin from atmospheric oxidation of p,p'-Dicofol. Additionally, based on the structure optimization and energy calculation, rate constants and Arrhenius formulas of the elementary reactions of p,p'-Dicofol with OH and NO3 radicals were obtained over the temperature range of 280–380 K and at 1 atm. The rate constants for the reactions of p,p'-Dicofol with OH and NO3 radicals are 1.51 × 10−12 and 8.88 × 10−14 cm3 molecule−1 s−1, respectively. The lifetime (τTotal) of p,p'-Dicofol determined by the reactions of OH and NO3 radical is 5.86 h, indicating a potential long-range transport in the atmosphere.

Comparison of radical scavenging behavior of chromones dihydrogenistein and demethyltexasin—a DFT approach

The present work deals with the validation of radical scavenging behavior of two identical chromones: 4′,5,7 trihydroxy isoflavone dihydrogenistein (DGT) and 4′,6,7 trihydroxy isoflavone demethyltexasin (DMT) through structural activity analysis to study the influence of H atom on the radical scavenging behavior. Structural optimization and thermochemical calculations for the studied chromones is supported by DFT under the correlation functional B3LYP and M062X under 6-311G(d,p) basis set using Gaussian 09 package. Computations are carried out in gas phase and polar environment. Comparative analyses of radical scavenging ability of the two isoflavones are validated with the aid of two different levels of theory in three different environments which facilitates the antioxidant mechanism. Higher binding probability of DMT than DGT towards the selected enzyme target ALOX5 is observed in molecular docking analysis. This work paves a way to the elucidation of the beneficial effects on health of these compounds.

Theoretical Study of the Electronic Structure of Threading Edge Dislocations in GaN

Although silicon is the material mainly used for power devices, we are now approaching the performance limits of power devices based on silicon. Moreover, it is difficult to improve their performance due to the intrinsic physical properties of silicon. Gallium nitride (GaN), on the other hand, is attracting a lot of attention as an alternative material for future power devices. The bandgap of GaN is three times as wide as that of silicon, and GaN has some superior features, such as its high thermal conductively, large dielectric breakdown field, and high saturation velocity for the electrons. Therefore GaN is expected to become a key material in the next generation of power devices, enabling higher voltages and higher speed operation than current devices. Moreover, the use of GaN will allow miniaturization of the devices and higher temperature operation. During the crystal growth of GaN, however, a high density of threading dislocations appears due to the lattice mismatch with the sapphire (Al2O3), silicon carbide (SiC). Moreover, it has been reported that threading dislocations degrade the performance of GaN-based electronic devices fabricated even on GaN substrates. There have been some experimental studies in which screw or mixed dislocations have been reported as being the primary sources for the leakage current in GaN-based devices [1], whereas threading edge dislocations were found not to contribute to the leakage current [2]. Therefore, we need to fabricate GaN-based power devices to examine the electronic behavior at threading dislocations in GaN and to clarify the origin of the leakage current. As for theoretical studies, the electronic structure of threading dislocations in GaN is not fully understood, although Akiyama et al. reported the electronic structures of threading dislocations by determining the atomistic structures by empirical inter-atomic potential calculations [3]. The purpose of this theoretical study is to discover whether threading edge dislocations contribute to the leakage current or not. To do this, we used first principles calculations based on density functional theory (DFT) to examine the electronic structure at threading edge dislocations with Burghers vectors of 1/3[11-20]. In order to investigate the electronic states of threading edge dislocations in GaN, it was necessary to prepare an initial atomic structure for the dislocations. Our initial atomic structure, which has a supercell containing about 200 atoms, is shown in Fig. 1. Fig. 1(a) and Fig. 1(b) show this model in the [0001] and [1-210] directions, respectively. The model has a periodic boundary condition in the [0001] direction, parallel to the dislocation. The surface of the model shown Fig. 1(a) is terminated by fictitious hydrogen atoms [4] and the model is surrounded by a vacuum region. The shape of the model is like a square pillar. First, the model structure was optimized. It is known that several types of core configurations are proposed for 1/3[11-20] threading dislocations in GaN. In this study we focused on an 8-atom ring core configuration, a symmetric 4-atom ring core configuration, and 5/7-atom ring core configurations. The electronic structures for these relaxed atomic models were calculated. Structural optimization and electronic state calculations were performed using VASP (Vienna Ab-initio Simulation Package) [5], which is a first principles calculation code based on DFT. The electronic densities of states of several core configurations are shown in Fig. 2. Fig. 2(a), Fig. 2(b), Fig. 2(c) and Fig. 2(d) show the electronic densities of states for bulk wurtzite GaN, an 8-atoms ring core, a 5/7-atoms ring core, and a symmetric 4-atoms ring core, respectively. In Fig. 2, the Fermi level is shown by the black lines at 0 eV, and the bandgaps are shown by the shaded blue or green parts. It is known that undoped GaN generally has n-type semiconductor characteristics. Therefore, focusing on the blue parts above the Fermi level, the bandgap of every core configuration is similar to that of bulk GaN. In other words, no defect levels can be seen near the bottom of the conduction band. These results demonstrate that threading edge dislocations in gallium nitride do not contribute to leakage current in GaN-based devices. Moreover, we also discuss the atomic and electronic structures using larger scale systems. Acknowledgement This work was conducted as part of a project entrusted with “research and development for next generation of power devices that contributes to the realization of an energy saving society” (MEXT; Ministry of Education, Culture, Sports, Science and Technology). Figure 1

Structures of aniline(pyrrole)+, aniline(ethanol)+, and aniline-(benzene)+

Molecular structures of aniline(pyrrole)+, aniline(ethanol)+, and aniline(benzene)+ produced via resonance two-photon ionization at 266 nm were analyzed by infrared predissociation spectroscopy coupled with tandem mass spectrometry. Structural optimization and frequency calculation using density functional theory were carried out to suggest the most probable isomers which are in good agreement with the observed infrared absorption spectra. Intermolecular bonds in the cluster ions were formed such that the electronegative oxygen atom of the solvent molecule or the pi electron of the aromatic ring forms a hydrogen bonding to NH of aniline.

Effects of stress and point defect on the physical properties of ZnO:Nd

The magnetism sources and mechanism of Nd doping and point defect are frequently controversial. To solve these controversies, ZnO supercell models with different doping modes of Nd were constructed, and the geometric structure optimization and energy calculation of the established models were calculated based on the generalized gradient approximation method of density functional theory. Calculation results indicated that the formation energy of Nd doped ZnO systems completely followed the sequence Zn15NdO16 > Zn15NdO15 > Zn14NdO16. For systems in which Nd doping and Zn vacancy coexist, a strong hybrid coupling electron exchange effect was present among the unpaired O-2p that was nearest to the Zn vacancy, the Zn-4 s orbit that was second nearest to the Zn vacancy, and the Nd-3d orbit. Therefore, the magnetism originated from the RKKY double-exchange interaction, which was based on the hole carriers in the complexus as the medium. In contrast with the Nd doping system, systems with coexisting Nd doping and O vacancy did not exhibit increased magnetism. Thus, the systems are worthless in designing and preparing dilute magnetic semiconductors. The magnetic moment of the systems with coexisting Nd doping and Zn vacancy was the largest. Thus, the systems are valuable in such design and preparation. The absorption spectra of Zn16NdiO16 and Zn15NdO16 were red shifted under the condition without stress, whereas the absorption spectrum of Zn15NdO16 with stress was blue shifted and the magnetic moment increased. The Curie temperature of Nd double-doped ZnO system was higher than room temperature when the Nd-Nd distance was the shortest.