Bond Charge Research Articles

The performance of Y2O3 as interface layer between La2O3 and p-type silicon substrate

In this study, the performance of Y2O3 as interface layer between La2O3 and p-type silicon substrate is studied with the help of atomic layer deposition (ALD) and magnetron sputtering technology. The surface morphology of the bilayer films with different structures are observed after rapid thermal annealing (RTA) by atomic force microscopy (AFM). The results show that Y2O3/Al2O3/Si structure has a larger number of small spikes on the surface and its surface roughness is worse than Al2O3/Y2O3/Si structure. The reason is that the density of Si substrate surface is much higher than that of ALD growth Al2O3. With the help of high-frequency capacitance-voltage(C-V) measurement and conductivity method, the density of interface traps can be calculated. After a high temperature annealing, the metal silicate will generate at the substrate interface and result in silicon dangling bond and interface trap charge, which has been improved by X-ray photoelectron spectroscopy (XPS) and interface trap charge density calculation. The interface trapped charge density of La2O3/Al2O3/Si stacked gate structure is lower than that of La2O3/Y2O3/Si gate structure. If Y2O3 is used to replace Al2O3 as the interfacial layer, the accumulation capacitance will increase obviously, which means lower equivalent oxide thickness (EOT). Our results show that interface layer Y2O3 grown by magnetron sputtering can effectively ensure the interface traps near the substrate at relative small level while maintain a relative higher dielectric constant than Al2O3.

Quantum Phase Transition and Local Entanglement in Extended Hubbard Model on Anisotropic Triangular Lattices**Supported by National Natural Science Foundation of China under Grant Nos.11274255, 11475027 and 11305132, Specialized Research Fund for the Doctoral Program of Higher Education of China under Grant No. 20136203110001, and Technology of Northwest Normal University, China under Grants No. NWNU-LKQN-11-26

Using a mean-field theory based upon Hartree—Fock approximation, we theoretically investigate the competition between the metallic conductivity, spin order and charge order phases in a two-dimensional half-filled extended Hubbard model on anisotropic triangular lattice. Bond order, double occupancy, spin and charge structure factor are calculated, and the phase diagram of the extended Hubbard model is presented. It is found that the interplay of strong interaction and geometric frustration leads to exotic phases, the charge fluctuation is enhanced and three kinds of charge orders appear with the introduction of the nearest-neighbor interaction. Moreover, for different frustrations, it is also found that the antiferromagnetic insulating phase and nonmagnetic insulating phase are rapidly suppressed, and eventually disappeared as the ratio between the nearest-neighbor interaction and on-site interaction increases. This indicates that spin order is also sensitive to the nearest-neighbor interaction. Finally, the single-site entanglement is calculated and it is found that a clear discontinuous of the single-site entanglement appears at the critical points of the phase transition.

Theoretical Study on the Effects of Hydrogen-Bonding and Molecule-Cation Interactions on the Sensitivity of HMX.

To assess the effects of weak interactions on the sensitivity of HMX, 11 complexes of HMX (where six of them are hydrogen-bonding complexes, and the other five are molecular-cation complexes) have been studied via quantum chemical treatment. The geometric and electronic structures were determined using DFT-B3LYP and MP2(full) methods with the 6-311++G(2df, 2p) and aug-cc-pVTZ basis sets. The changes of the bond dissociation energy (BDE) of the trigger bond (N-NO2 in HMX) and nitro group charge have been computed on the detailed consideration to access the sensitivity changes of HMX. The results indicate that upon complex forming, the BDE increases and the charge of nitro group turns more negative in complexes, suggesting that the strength of the N-NO2 trigger bond is enhanced and then the sensitivity of HMX is reduced. Atom-in-molecules analyses have also been carried out to understand the nature of intermolecular interactions and the strength of trigger bonds.

A novel Fe(III) dependent bioflocculant from Klebsiella oxytoca GS-4-08: culture conditions optimization and flocculation mechanism.

In this work, the effect of cultivation factors on the flocculation efficiency (FE) of bioflocculant P-GS408 from Klebsiella oxytoca was optimized by the response surface methodology. The most significant factor, i.e. culture time, was determined by gray relational analysis. A total of 240 mg of purified P-GS408 was prepared from 1 liter of culture solution under the optimal conditions. GC-MS analysis results indicated that the polysaccharide of P-GS408 mainly contains Rhamnose and Galactose, and the existence of abundant hydroxyl, carboxyl and amino groups was evidenced by FTIR and XPS analyses. With the aid of Fe3+, the FE of kaolin solution by P-GS408 could achieve 99.48% in ten minutes. Functional groups of polysaccharide were involved in the first adsorption step and the zeta potential of kaolin solution changed from −39.0 mV to 43.4 mV in the presence of Fe3+ and P-GS408. Three-dimensional excitation-emission (EEM) fluorescence spectra demonstrates that the trivalent Fe3+ and Al3+ can bind efficiently with P-GS408, while those univalent and divalent cations cannot. With the help of SEM images, FTIR, zeta potential and EEM spectra, we proposed the P-GS408 flocculation mechanism, which consists of coordination bond combination, charge neutrality, adsorption and bridging, and net catching.

Equilibrium zinc isotope fractionation in Zn-bearing minerals from first-principles calculations

Isotopic composition analysis contributes significantly to the investigation of the biogeochemical cycle of zinc with its economical, environmental and health implications. Interpretation of isotopic measurements is however hindered by the lack of a set of equilibrium isotopic fractionation factors between Zn-bearing minerals. In this study, equilibrium mass-dependent Zn isotope fractionation factors in Zn-bearing minerals are determined from first-principles calculations within the density functional theory (DFT) scheme. A wide range of minerals belonging to sulfide, carbonate, oxide, silicate, sulfate and arsenate mineral groups are modelled to account for the natural diversity of Zn crystal-chemical environment. Calculated reduced partition function ratios (β-factors) span a range smaller than 2‰ at 22°C. All studied secondary minerals (adamite, gahnite, gunningite, hemimorphite, hydrozincite, zincite) but zinc carbonate (smithsonite) are isotopically heavier than zinc sulfide minerals (sphalerite and wurtzite) from which they could form through supergene processes. Zinc–aluminium spinel and zinc silicate are the isotopically heaviest minerals. The investigation of the crystal-chemical parameters at the origin of differences in isotopic properties shows an excellent linear correlation between lnβ and Zn interatomic force constants. β-factors are also observed to increase when the Zn-first neighbour bond lengths decrease and charges on atoms involved in the bonding increase and vice versa. These findings are in line with the observation of heavy isotope enrichment in systems having the largest bond strength.

Second-order nonlinear optical susceptibilities of AIIBVI and AIIIBV semiconductors

The second-order nonlinear optical (NLO) susceptibilities χ123(2) of AIIBVI and AIIIBV groups of semiconductors with zincblende (ZB) structure have been studied. Two relations have been proposed for the calculation of χ123(2)(0) at zero frequency. One is based on bond charge model of Levine and the other is based on plasma oscillations theory of solids. Calculated values of χ123(2)(0) for all compounds are in fair agreement with the available experimental and reported values.

Photoinduced ultrafast charge-order melting: Charge-order inversion and nonthermal effects

The effect of photoexcitation is studied for a system with checkerboard charge order induced by displacements of ligands around a metal site. The motion of the ligands is treated classically and the electronic charges are simplified to two-level molecular bond charges. The calculations are done for a checkerboard charge-ordered system with about 100 000 ligand oscillators coupled to a fixed-temperature bath. The initial photoexcitation is followed by a rapid decrease in the charge-order parameter within 50–100 femtoseconds while leaving the correlation length almost unchanged. Depending on the fluence, a complete melting of the charge order occurs in less than a picosecond. While for low fluences, the system returns to its original state, for full melting, it recovers to its broken-symmetry state leading to an inversion of the charge order. Finally, for small long-range interactions, recovery can be slow due to domain formation.

Effect of Polar Environments on the Aluminum Oxide Shell Surrounding Aluminum Particles: Simulations of Surface Hydroxyl Bonding and Charge.

Density functional theory (DFT) calculations were performed to understand molecular variations on an alumina surface due to exposure to a polar environment. The analysis has strong implications for the reactivity of aluminum (Al) particles passivated by an alumina shell. Recent studies have shown a link between the carrier fluid used for Al powder intermixing and the reactivity of Al with fluorine containing reactive mixtures. Specifically, flame speeds show a threefold increase when polar liquids are used to intermix aluminum and fluoropolymer powder mixtures. It was hypothesized that the alumina lattice structure could be transformed due to hydrogen bonding forces exerted by the environment that induce modified bond distances and charges and influence reactivity. In this study, the alumina surface was analyzed using DFT calculations and model clusters as isolated systems embedded in polar environments (acetone and water). The conductor-like screening model (COSMO) was used to mimic environmental effects on the alumina surface. Five defect models for specific active -OH sites were investigated in terms of structures and vibrational -OH stretching frequencies. The observed changes of the surface OH sites invoked by the polar environment were compared to the bare surface. The calculations revealed a strong connection between the impact of carrier fluid polarity on the hydrogen bonding forces between the surface OH sites and surrounding species. Changes were observed in the OH characteristic properties such as OH distances (increase), atomic charges (increase), and OH stretching frequencies (decrease); these consequently improve OH surface reactivity. The difference between medium (acetone) and strong (water) polar environments was minimal in the COSMO approximation.

Griffiths like robust ferromagnetism in Co3-xMnxTeO6; (x = 0.5, 1, 2)

We report near room temperature ferromagnetic (FM) as well as low temperature antiferromagnetic (AFM) correlations in Co3-xMnxTeO6 (CMTO); (x=0.5, 1, 2) solid solutions, using thorough DC magnetization studies. For all x, CMTO shows not only short-range robust FM order ∼185K, but also long range enhanced AFM order ≤45K. Griffiths-like FM interactions show up in magnetization data; it exists over an extended temperature range and remains unsuppressed in magnetic field as large as 1T, indicating its robustness. Variations in both FM and AFM phases as a function of Mn concentration also support our observation of anomalous behavior in the average bond distances and charge states (JAP 116: 074904 (2014)). Further, an attempt towards the structural insight into the observed complex magnetic behavior by using network like structural analysis has been drawn. These observations make CMTO an interesting magnetic system from fundamental and application perspectives.

Ab Initio and DFT Studies on CO2 Interacting with Zn(q+)-Imidazole (q=0, 1, 2) Complexes: Prediction of Charge Transfer through σ- or π-Type Models.

Using first-principles methodologies, the equilibrium structures and the relative stability of CO2 @[Zn(q+) Im] (where q=0, 1, 2; Im=imidazole) complexes are studied to understand the nature of the interactions between the CO2 and Zn(q+) -imidazole entities. These complexes are considered as prototype models mimicking the interactions of CO2 with these subunits of zeolitic imidazolate frameworks or Zn enzymes. These computations are performed using both ab initio calculations and density functional theory. Dispersion effects accounting for long-range interactions are considered. Solvent (water) effects were also considered using a polarizable continuum model approach. Natural bond orbital, charge, frontier orbital and vibrational analyses clearly reveal the occurrence of charge transfer through covalent and noncovalent interactions. Moreover, it is found that CO2 can adsorb through more favorable π-type stacking as well as σ-type hydrogen-bonding interactions. The inter-monomer interaction potentials show a significant anisotropy that might induce CO2 orientation and site-selectivity effects in porous materials and in active sites of Zn enzymes. Hence, this study provides valuable information about how CO2 adsorption takes place at the microscopic level within zeolitic imidazolate frameworks and biomolecules. These findings might help in understanding the role of such complexes in chemistry, biology and material science for further development of new materials and industrial applications.

Nature of the lowest electron transitions in styryl bases benzothiazole derivatives and analogues bearing para-methoxy and -trifluoromethyl substituents in phenylyne moiety

Combined quantum-chemical and spectral investigation of cyanine bases derivatives of thiastyryls as well as their analogues with dimethylamino, metoxy and trifluorine-methyl substituents has been fulfilled. The calculations have shown that going from cationic styryl/metoxystyryl to the corresponding neutral bases is accompanied by substantial change of the equilibrium molecular geometry and charge distribution at atoms, while the experimental absorption band undergoes the essential hypsochromic shift. It is established that introducing on the donor groups in the bases causes negligible change of the carbon–carbon bond and atomic charges in the main chromophore, in contrast to the substantial changes of the magnitude and direction state dipole moments in both ground and excited states. It is found that the bases with the donor groups in benzthiazole moiety and with acceptor CF3 substituent demonstrate the inversion of the direction of the dipole moment. Based on the spectral and quantum-chemical study, one has proposed that some widening of the spectral bands is connected with the vibronic interaction, not with the second electron transition.

Experimental and theoretical charge-density analysis of 1,4-bis(5-hexyl-2-thienyl)butane-1,4-dione: applications of a virtual-atom model.

The experimental and theoretical charge densities of 1,4-bis(5-hexyl-2-thienyl)butane-1,4-dione, a precursor in the synthesis of thiophene-based semiconductors and organic solar cells, are presented. A dummy bond charges spherical atom model is applied besides the multipolar atom model. The results show that the dummy bond charges model is accurate enough to calculate electrostatic-derived properties which are comparable with those obtained by the multipolar atom model. The refinement statistics and the residual electron density values are found to be intermediate between the independent atom and the multipolar formalisms.

Defects and thermoelectric performance of ternary chalcopyrite CuInTe2-based semiconductors doped with Mn

In thermoelectric (TE) semiconductors, there are three physical parameters that govern the TE performance (i.e. Seebeck coefficient (), electrical conductivity (), and thermal conductivity ()); they are interrelated, hence it is hard to optimize them simultaneously. In order to improve the TE performance, we need to further explore new materials. Ternary chalcopyrite (diamond-like) I-III-VI2 semiconductors (Eg = 1:02 eV) are new materials of the TE family, which have potential in conversion between heat and electricity. Since in the ternary chalcopyrite structure, such as Cu(Ag) MTe2, there is an inherent Coulomb attraction between charged defects MCu(Ag)2+ and 2VCu(Ag)- (a native defect pair, i.e., metal M-on-Cu or Ag antisites and two Cu or Ag vacancies), hence the electronic and structural properties can easily be tailored if these two defects, along with the creation of other defects, are modified through the introduciton of foreign elements. Besides, the ternary I-III-VI2 compounds often show tetragonal distortion because 0.25, = c/2a 1 (here and are the anion position displacement parameters, and a and c are the lattice parameters), and the cationanion distances are not equal (dCuTedInTe). Any occupation by foreign elements in the cation sites of I-III-VI2 will cause the redistribution of bond charges between I-VI and III-VI, thus leading to a tiny adjustment of the crystal structure and altering the phonon scattering behavior. In this work, we substitute Mn for Cu in the chalcopyrite CuInTe2 and prepare the Cu-poor Cu1-xInMnxTe2 semiconductors. Investigations of Z-ray patterns after Rietveld refinement reveal that Mn prefers In to Cu lattice sites for low Mn content (x 0.1), thus creating MnIn- as an active acceptor, and improving the carrier concentration (n) and electrical conductivity as Mn content increases. However, Mn can either occupy In or Cu sites simultaneously when x 0.1, and generate both the donor defect MnCu+ and the acceptor defect MnIn-. In this case, annihilation may occur between these two defects, allowing the reduction in both the defect and carrier concentrations. Because of the annihilation between the two defects, two values (|| = |-0.25| and ||= |-1.0|) reduce, this only yields a subtle change in the difference between mean cation-anion distance (RInTe-RCuTe), indicating a small distortion tendency in lattice structure as Mn content increases. Because of this, there is a limited enhancement in lattice thermal conductivity (L) at high temperatures. As a consequence, we attain an optimal TE performance at a certain Mn content (x = 0.05) with the dimensionless figure of merit (ZT) ZT = 0.84 at 810.0 K, which is about twice as much as that of Mn-free CuInTe2.

Rock-salt structure lithium deuteride formation in liquid lithium with high-concentrations of deuterium: a first-principles molecular dynamics study

Because of lithium’s possible use as a first wall material in a fusion reactor, a fundamental understanding of the interactions between liquid lithium (Li) and deuterium (D) is important. We predict structural and dynamical properties of liquid Li samples with high concentrations of D, as derived from first-principles molecular dynamics simulations. Liquid Li samples with four concentrations of inserted D atoms (LiD, , 0.50, 0.75, and 1.00) are studied at temperatures ranging from 470 to 1143 K. Densities, diffusivities, pair distribution functions, bond angle distribution functions, geometries, and charge transfer between Li and D atoms are calculated and analyzed. The analysis suggests liquid–solid phase transitions can occur at some concentrations and temperatures, forming rock-salt LiD within liquid Li. We also observe formation of some D2 molecules at high D concentrations.

Crystal structure and ionic conductivity of the new cobalt polyphosphate NaCo(PO3)3

Polycrystalline sample of sodium cobalt triphosphate NaCo(PO3)3 was obtained by solid-state reaction and characterized by X-ray powder diffraction. The title compound is isostructural to NaZn(PO3)3 and its structure was refined by the Rietveld refinement in the cubic system, space group Pa3̅, with a=14.2484(4)Å. The obtained structural model is supported by bond valence sum (BVS) and charge distribution (CD) methods. The structure is described as a three-dimensional open-anionic framework built up of corner-sharing CoO6 and PO4 polyhedra with interconnecting channels along the 3-axis, in which the Na+ cations are located. The ionic conductivity measurements are performed on pellets with relative density of 84%. The electrical conductivity is 1.01×10−5Scm−1 at 480°C and the activation energy deduced from the slope is 1.1eV. The correlation between crystal structure and ionic conductivity was studied by the means of the bond-valence-site-energy (BVSE) model. The main result is that sodium transport is made mainly via Na2 and Na3 sites.

A theoretical study of the regio- and stereoselectivities of non-polar 1,3-dipolar cycloaddition reaction between C-diethoxyphosphoryl-N-methylnitrone and N-(2-fluorophenyl)acrylamide

ABSTRACTThe 1,3-dipolar cycloadditions (13DC) of C-diethoxyphosphoryl-N-methylnitrone and N-(2-fluorophenyl) acrylamide have been studied using density functional theory (DFT) calculations at B3LYP/6-31G(d) level of theory. Our calculations show that this 13DC reaction takes place with complete ortho regioselectivity with endo stereoselectivity, which favours kinetically the formation of the ortho–endo cycloadduct, in agreement with the experimental observations. The inclusion of solvent effects does not modify the gas-phase selectivities but slightly decreases the reactivity of the reagents. Analysis of the bond order and charge transfer at the transition states indicates that this 13DC reaction takes place via a one-step asynchronous mechanism. Analysis of the DFT global reactivity indices and the Parr functions of the reagents allow us to provide an explanation of the regioselectivity of this 13DC reaction.

Surface elasticity revisited in the context of second strain gradient theory

Surface/interface stresses, when notable, are closely associated with a surface/interface layer in which the interatomic bond lengths and charge density distribution differ remarkably from those of the bulk. The presence of such topographical defects as edges and corners amplifies the noted phenomena by large amounts. If the principal features of interest are such studies as the physics and mechanics of evolving microscopic-/nanoscopic-interfaces and the behavior of nano-sized structures which have a very large surface-to-volume ratio, traditional continuum theories cease to hold. It is for the treatment of such problems that augmented continuum approaches like second strain gradient and surface elasticity theories have been developed by Mindlin (1965) and Gurtin and Murdoch (1975), respectively. In the mathematical framework of the former theory, the surface effect is explicitly revealed through surface characteristic length and modulus of cohesion, whereas within the latter theory, which views the bulk material and its complementary surface as separate interacting entities, the critical role of surfaces/interfaces is directly incorporated through the introduction of the notions of tangential surface strain tensor, surface stress tensor, and surface elastic modulus tensor into the formulation. In the realm of the experimentations, evaluation of the above-mentioned surface parameters poses serious difficulties. One of the objectives of the current study is to provide a remedy as how to calculate, not only these parameters, but also Mindlin’s bulk characteristic lengths as well as Lame constants with the aids of first principles density functional theory (DFT). To this end, surface elasticity is reformulated by maintaining the first and second gradients of the strain tensor for the bulk; as a result two new key equations are obtained. One of these equations is an expression for the net surface stress, needed to relate the surface parameters in surface elasticity to the Mindlin’s second gradient theory parameters. The other equation is for the total elastic energy which is utilized to find an analytical expression for the surface energy. The available data on surface relaxation obtained experimentally and computationally are in good correspondence with the results of the current theory. Moreover, employing the present theory, an estimate for the effective elastic constants of films with infinite extension is provided.

Atomistic Modeling of the Charge Process in Lithium/Air Batteries

We present a combined classical and density functional theory (DFT) based Molecular Dynamics (MD) study of the mechanisms underlying the oxygen evolution reactions during the charging of lithium/air batteries. As models for the Li2O2 material at the cathode we employ small amorphous clusters with a 2:2 Li:O stoichiometry, whose energetically most stable atomic configurations comprise both O atoms and O–O pairs with mixed peroxide/superoxide character, as revealed by their bond lengths, charges, spin moments, and densities of states. The oxidation of Li8O8 clusters is studied in unbiased DFT-based MD simulations upon removal of either one or two electrons, either in vacuo or immersed in dimethyl sulfoxide solvent molecules with a structure previously optimized by means of classical MD. Whereas removal of one electron leads only to an enhancement of the superoxide character of O–O bonds, removal of two electrons leads to the spontaneous dissolution of either an O2 or a LiO2+ molecule. These results are inte...

Interaction-dependent photon-assisted tunneling in optical lattices: a quantum simulator of strongly-correlated electrons and dynamical Gauge fields

We introduce a scheme that combines photon-assisted tunneling (PAT) by a moving optical lattice with strong Hubbard interactions, and allows for the quantum simulation of paradigmatic quantum many-body models. We show that, in a certain regime, this quantum simulator yields an effective Hubbard Hamiltonian with tunable bond–charge interactions, a model studied in the context of strongly-correlated electrons. In a different regime, we show how to exploit a correlated destruction of tunneling to explore Nagaoka ferromagnetism at finite Hubbard repulsion. By changing the photon-assisted tunneling parameters, we can also obtain a t-J model with independently controllable tunneling t, super-exchange interaction J, and even a Heisenberg–Ising anisotropy. Hence, the full phase diagram of this paradigmatic model becomes accessible to cold-atom experiments, departing from the region allowed by standard single-band Hubbard Hamiltonians in the strong-repulsion limit. We finally show that, by generalizing the PAT scheme, the quantum simulator yields models of dynamical Gauge fields, where atoms of a given electronic state dress the tunneling of the atoms with a different internal state, leading to Peierls phases that mimic a dynamical magnetic field.

A facile and novel strategy to synthesize reduced TiO₂ nanotubes photoelectrode for photoelectrocatalytic degradation of diclofenac.

TiO2 nano-materials have been considered as a versatile candidate for the photoelectrochemical (PECH) applications. In this study, we reported a facile and novel strategy to synthesize reduced TiO2 nanotubes (TiO2 NTs) photoelectrode. The microwave reduction could introduce oxygen vacancy in the lattice of TiO2, while the rapid-production of oxygen vacancy facilitated the generation of impurity level between the forbidden band and greatly enhancement of visible light absorption, thereby resulting in an improved separation efficiency of photogenerated charge carriers and photocatalytic (PC) performance. Additionally, the derived valence band X-ray photoelectron spectroscopy (VBXPS) and photoluminescence (PL) spectra confirmed the existence of oxygen vacancy in the lattice of TiO2 NTs photoelectrode, in which the valence bond maximum (VBM) and charge carriers concentration of reduced TiO2 NTs photoelectrode was determined to be 1.75 eV and 5.36 × 10(19) cm(-3), respectively. Furthermore, the scavenging experiments revealed that ·OH radical was the dominated species for the degradation of diclofenac. The enhanced-visible-light PC mechanism could mainly be attributed to the generation of oxygen vacancy, which can provide not only the visible light absorption capacity but also charge separation efficiency.