Relevant Charge States Research Articles

First-principles studies on the oxygen vacancy formation in α-Na2FePO4F and β-Na2FePO4F

The orthorhombic β-Na2FePO4F has been extensively studied, however, a novel monoclinic α-Na2FePO4F has not been explored sufficiently. Oxygen vacancies (VO) can affect the physical and chemical properties of materials, and oxygen vacancy formation energy reflects the difficulty of VO formation. Using first-principles methods, we have calculated the formation energies of VO and their relevant charge states in both α-Na2FePO4F and β-Na2FePO4F. Our results reveal that charged vacancies (VO+1 or VO+2) are easier to form than neutral vacancies (VO0). The reduction of the Fermi level, the decrease in the oxygen partial pressure, or the increase of the temperature are found to decrease the formation energy of VO. The formation energy of VO in α-Na2FePO4F is slightly lower than that in β-Na2FePO4F. The electronic density of states of α-Na2FePO4F and β-Na2FePO4F suggest that more defect states are introduced into the band gap as the charge states increase. These defect states are mainly the 3d orbitals of the Fe atoms. The introduction of oxygen vacancies distorts the [FeO4F2] octahedra, which affect the charge density distribution and the defect states introduced in the band gap. Significantly, oxygen vacancies have a local impact on the charge density redistribution in both α-Na2FePO4F and β-Na2FePO4F. These findings provide theoretical insights into the formation of oxygen vacancies and the control of oxygen release.

Oxygen-related defects in 4H-SiC from first principles

We investigated the abundance, structures, energy levels, and spin states of oxygen-related defects in 4H-SiC on the basis of first-principles calculations. We applied a hybrid functional in the overall calculations, which gives reliable defect properties, and also considered relevant defect charge states. We identified the oxygen interstitial (O i,1), substitutional oxygen (OC), and oxygen-vacancy (OC V Si) complex as prominent defects in n-type conditions. Among them, OC V Si was predicted as a spin-1 defect with NIR emission in a previous study. On the basis of the obtained results, we discuss the possible spin decoherence sources when employing OC V Si as a spin-to-photon interface.

Molecular properties and tautomeric equilibria of isolated flavins.

Flavins are employed as redox cofactors and chromophores in a plethora of flavoenzymes. Their versatility is an outcome of intrinsic molecular properties of the isoalloxazine ring modulated by the protein scaffold and surrounding solvent. Thus, an investigation of isolated flavins with high-level electronic-structure methods and with error assessment of the calculated properties will contribute to building better models of flavin reactivity. Here, we benchmarked ground-state properties such as electron affinity, gas-phase basicity, dipole moment, torsion energy, and tautomer stability for lumiflavins in all biologically relevant oxidation and charge states. Overall, multiconfigurational effects are small and chemical accuracy is achieved by coupled-cluster treatments of energetic properties. Augmented basis sets and extrapolations to the complete basis-set limit are necessary for consistent agreement with experimental energetics. Among DFT functionals tested, M06-2X shows the best performance for most properties, except gas-phase basicity, in which M06 and CAM-B3LYP perform better. Moreover, dipole moments of radical flavins show large deviations for all functionals studied. Tautomers with noncanonical protonation states are significantly populated at normal temperatures, adding to the complexity of modeling flavins. These results will guide future computational studies of flavoproteins and flavin chemistry by indicating the limitations of electronic-structure methodologies and the contributions of multiple tautomeric states.

Charge state modulation on boron site by carbon and nitrogen localized bonding microenvironment for two-electron electrocatalytic H2O2 production

Design of electrochemical active boron (B) site at solid materials to understand the relationships between the localized structure, charge state at the B site and electrocatalytic activity plays a crucial role in boosting the green electrochemical synthesis of hydrogen peroxide (H2O2) via two-electron oxygen reduction (2eORR) pathway. Herein, we demonstrate a carbon (C) and nitrogen (N) localized bonding microenvironment to modulate the charge state of B site at the boron-carbon nitride solid (BCNs) to realize the efficient selective electrocatalytic H2O2 production. The localized chemical structure of N-B-N, N-B-C and C-B-C bonds at B site can be regulated through solid-state reaction between boron nitride (BN) and porous carbon (C) at variable temperatures. The optimized BCN-1100 achieves an outstanding H2O2 selectivity of 89% and electron transfer number of 2.2 (at 0.55 V vs. RHE), with the production of 10.55 mmol/L during 2.5 h and the catalytic stability duration for 15000 cycles. Further first-principles calculations identified the dependency of localized bonding microenvironment on the OOH* adsorption energies and relevant charge states at the boron site. The localized structure of B site with BNC2-Gr configuration is predicted to be the highest 2eORR activity.

Unraveling the molecular mechanisms of color expression in anthocyanins.

Anthocyanins are a broad family of natural dyes, increasingly finding application as substitutes for artificial colorants in the food industry. In spite of their importance and ubiquity, the molecular principles responsible for their extreme color variability are poorly known. We address these mechanisms by computer simulations and photoabsorption experiments of cyanidin-3-O-glucoside in water solution, as a proxy for more complex members of the family. Experimental results are presented in the range of pH 1-9, accompanied by a comprehensive systematic computational study across relevant charge states and tautomers. The computed spectra are in excellent agreement with the experiments, providing unprecedented insight into the complex behavior underlying color expression in these molecules. Besides confirming the importance of the molecule's charge state, we also unveil the hitherto unrecognized role of internal distortions in the chromophore, which affect its degree of conjugation, modulating the optical gap and in turn the color. This entanglement of structural and electronic traits is also shared by other members of the anthocyanin family (e.g. pelargonidin and delphinidin) highlighting a common mechanism for color expression across this important family of natural dyes.

The first-principle study on the formation energies of Be, Mg and Mn doped CuInO<sub>2</sub>

Exploring new type of optoelectronic materials has fundamental scientific and practical significance in the development of society and economy. Recently, intense research has focused on the use of the wide band-gap bipolarity semiconductor material CuInO<sub>2</sub> which will allow to the fabrication of that total transparent optoelectronic materials. However, the conductivity of CuInO<sub>2</sub> is significantly lower than other n-type conductivity of other TCOs. As a result, one of the key question is how to improve the electric properties of CuInO<sub>2</sub> by doping method. Motivated by this observation, in this paper, using the first-principles methods, the formation energetics properties of dopant (Be, Mg, Mn) in transparent conducting oxides CuInO<sub>2</sub> were studied within the local-density approximation. Substituting dopant (Be, Mg, Mn) for In, substituting dopant (Be, Mg, Mn) for Cu and dopant as interstitial in their relevant charge state are considered. By systematically calculating formation energies and transition energy level of defect, the calculated results show that, substituting Mg for In does not induce the large structural relaxation. in CuInO<sub>2</sub>. One can expect that substituting the Mg and Mn for In introduces acceptor because the relative lower formation energies, furthermore, Be atoms would be substitute for In atoms when the <i>E</i><sub>f</sub> move to CBM. In addition, the donor-type extrinsic defects(such as substituting dopant for Cu and dopant as interstitial) have difficulty in inducing n-conductivity in CuInO<sub>2</sub> because of their deep transition energy level or the higher formation energies. Considering the transition energy level position, Be<sub>In</sub>, Mg<sub>In</sub>, and Mn<sub>In</sub> have transition energy levels at 0.06, 0.05, and 0.40 eV above the VBM, respectively. Thus, for all the acceptor-type extrinsic defects, substituting Mg for In is the most prominent doping acceptor with relative shallow transition energy levels in CuInO<sub>2</sub> under O-rich condition. Based on our calculated results and discussion mentioned above, in order to increase p-type conductivity in CuInO<sub>2</sub>, we could substitute Mg atoms for In atoms by the sit-selective doping method through atomic layer epitaxy growth or controlling the oxygen partial pressure in the molecular beam epitaxy or metal-organic chemical vapor deposition crystal growth process. The calculation results will not only provide the guide for design of new type In-based optoelectronic materials, but will also further understand the potential properties in CuInO<sub>2</sub>.

Identification of nickel-vacancy defects by combining experimental and ab initio simulated photocurrent spectra

There is a continuous search for solid-state spin qubits operating at room temperature with excitation in the IR communication bandwidth. Recently we have introduced the photoelectric detection of magnetic resonance (PDMR) to read the electron spin state of nitrogen-vacancy (NV) center in diamond, a technique which is promising for applications in quantum information technology. By measuring photoionization spectra on a diamond crystal we found two ionization thresholds that were not reported before. On the same sample we also observed absorption and photoluminescence signatures that were identified in literature as Ni associated defects. We performed \emph{ab initio} calculation of the photo-ionization cross-section of the nickel split vacancy complex (NiV) and N-related defects in their relevant charge states and fitted the concentration of these defects to the measured photocurrent spectrum, which led to a surprising match between experimental and calculated spectra. This study enabled to identify the two unknown ionization thresholds with the two acceptor levels of NiV. Because the excitation of NiV is in infrared, the photocurrent detected from the paramagnetic NiV color centers is a promising way towards designing a novel type of electrically readout qubits.

Enhanced pairing susceptibility in a photodoped two-orbital Hubbard model

Local spin fluctuations provide the glue for orbital-singlet spin-triplet pairing in the doped Mott insulating regime of multi-orbital Hubbard models. At large Hubbard repulsion $U$, the pairing susceptibility is nevertheless very low, because the pairing interaction cannot overcome the suppression of charge fluctuations. Using nonequilibrium dynamical mean field simulations of the two-orbital Hubbard model, we show that out of equilibrium the pairing susceptibility in this large-$U$ regime can be strongly enhanced by creating a photo-induced population of the relevant charge states, and that this susceptibility correlates with the local spin susceptibility. Since a strong enhancement of the pairing requires a low kinetic energy of the charge carriers, the phenomenon is supported by the ultra-fast cooling of the photo-doped carriers through the creation of local spin excitations.

Impact of nitrogen and carbon on defect equilibrium in ZrO2

We investigate the electronic properties of nitrogen and carbon impurities in ZrO2 using density functional theory with a hybrid functional. It is commonly accepted that N substitutes on the O site and is stable in the −1 charge state (NO−). The NO− acceptors are then compensated by donor defects such as O vacancies in the +2 charge state. We test the validity of this assumption by determining the formation energies of all relevant charge states of nitrogen and oxygen vacancies as a function of Fermi level. We also examine the effects of carbon, a common unintentional impurity in ZrO2. We find that carbon impurities have relatively low formation energies and would indeed incorporate easily during growth of ZrO2, but they do not affect the equilibrium between nitrogen and oxygen vacancies. Our results show that the Kröger-Vink picture is valid for common growth conditions and the dominant defects are NO− and VO+2.

Environmental Coulomb blockade of topological superconductor-normal metal junctions

We study charge transport of a topological superconductor connected to different electromagnetic environments using a low-energy description where only the Majorana bound states in the superconductor are included. Extending earlier findings who found a crossover between perfect Andreev reflection with conductance $2e^2/h$ to a regime with blocked transport when the resistance of the environment is larger than $2e^2/h$, we consider Majorana bound states coupled to metallic dots. in particular, we study two topological superconducting leads connected by a metallic quantum dot in both the weak tunneling and strong tunneling regimes. For weak tunneling, we project onto the most relevant charge states. For strong tunneling, we start from the Andreev fixed point and integrate out charge fluctuations which gives an effective low-energy model for the non-perturbative gate-voltage modulated cotunneling current. In both regimes and in contrast to cotunneling with normal leads, the conductance is temperature independent because of the resonant Andreev reflections, which are included to all orders.

The Tungsten Project: Dielectronic Recombination data for the International Thermonuclear Experimental Reactor (ITER)

SynopsisThe plasma facing components in ITER will be constructed using Tungsten. Sputtering of plasma against these walls introduces Tungsten impurities into the plasma, possibly causing quenching. As data for Tungsten in the relevant charge states is historically poor, this project aims to calculate this data, providing partial and total Dielectronic/Radiative Recombination rates for use in collisional-radiative modelling of the ITER plasma.

Parity effect in Josephson junction arrays

We study the parity effect and transport due to quasiparticles in circuits comprised of many superconducting islands. We develop a general approach and show that it is equivalent to previous methods for describing the parity effect in their more limited regimes of validity. As an example we study transport through linear arrays of Josephson junctions in the limit of negligible Josephson energy and observe the emergence of the parity effect with decreasing number of non-equilibrium quasiparticles. Due to the exponential increase in the number of relevant charge states with increasing length, in multi-junction arrays the parity effect manifests in qualitatively different ways to the two junction case. The role of charge disorder is also studied as this hides much of the parity physics which would otherwise be observed. Nonetheless, we see that the current through a multi-junction array at low bias is limited by the formation of meta-stable even-parity states.

General method to predict voltage-dependent ionic conduction in a solid electrolyte coating on electrodes

Understanding the ionic conduction in solid electrolytes in contact with electrodes is vitally important to many applications, such as lithium ion batteries. The problem is complex because both the internal properties of the materials (e.g., electronic structure) and the characteristics of the externally contacting phases (e.g., voltage of the electrode) affect defect formation and transport. In this paper, we developed a method based on density functional theory to study the physics of defects in a solid electrolyte in equilibrium with an external environment. This method was then applied to predict the ionic conduction in lithium fluoride (LiF), in contact with different electrodes which serve as reservoirs with adjustable Li chemical potential $({\ensuremath{\mu}}_{\mathrm{Li}})$ for defect formation. LiF was chosen because it is a major component in the solid electrolyte interphase (SEI) formed on lithium ion battery electrodes. Seventeen possible native defects with their relevant charge states in LiF were investigated to determine the dominant defect types on various electrodes. The diffusion barrier of dominant defects was calculated by the climbed nudged elastic band method. The ionic conductivity was then obtained from the concentration and mobility of defects using the Nernst-Einstein relationship. Three regions for defect formation were identified as a function of ${\ensuremath{\mu}}_{\mathrm{Li}}$: (1) intrinsic, (2) transitional, and (3) $p$-type region. In the intrinsic region (high ${\ensuremath{\mu}}_{\mathrm{Li}}$, typical for LiF on the negative electrode), the main defects are Schottky pairs and in the $p$-type region (low ${\ensuremath{\mu}}_{\mathrm{Li}}$, typical for LiF on the positive electrode) are Li ion vacancies. The ionic conductivity is calculated to be approximately ${10}^{\ensuremath{-}31}\phantom{\rule{4pt}{0ex}}\text{S}\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ when LiF is in contact with a negative electrode but it can increase to ${10}^{\ensuremath{-}12}\phantom{\rule{4pt}{0ex}}\text{S}\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ on a positive electrode. This insight suggests that divalent cation (e.g., ${\text{Mg}}^{2+}$) doping is necessary to improve Li ion transport through the engineered LiF coating, especially for LiF on negative electrodes. Our results provide an understanding of the influence of the environment on defect formation and demonstrate a linkage between defect concentration in a solid electrolyte and the voltage of the electrode.

Water driven adsorption of amino acids on the (101) anatase TiO₂ surface: an ab initio study.

Arg, Lys and Asp amino acids are known to play a critical role in the adhesion of the RKLPDA engineered peptide on the (101) surface of the titania anatase phase. To understand their contribution to peptide adhesion, we have considered the relevant charge states due to protonation (Arg and Lys) or deprotonation (Asp) occurring in neutral water solution, and studied their adsorption on the (101) anatase TiO2 surface by ab initio total energy calculations based on density functional theory. The adsorption configurations on the hydrated surface are compared to those on the dry surface considering also the presence of the hydration shell around amino acid side-chains. This study explains how water molecules mediate the adsorption of charged amino acids showing that protonated amino acids are chemically adsorbed much more strongly than de-protonated Asp. Moreover it is shown that the polar screening of the hydration shell reduces the adsorption energy of the protonated amino acids to a small extent, thus evidencing that both Arg and Lys strongly adhere on the (101) anatase TiO2 surface in neutral water solution and that they play a major role in the adhesion of the RKLPDA peptide.

First-principles calculations of the vacancy defects in BiOF as cathode materials for Li-ion batteries

The structural relaxation, formation energies, electronic structure and electrochemical properties of BiOF with vacancy defects were studied by first-principles calculations. Some typical native Bi-related, oxygen-related and fluoride-related vacancy defects in their relevant charge state were discussed, respectively. Calculated vacancy formation energies indicate that Bi3− charged vacancy is easiest to fabricate in BiOF when Fermi level lies closer to the conduction band. Besides, BiOF with Bi3− charged vacancy has the best conductivity and electrochemical properties.

Tautomeric populations of the charged species of 1,12-diamino-3,6,9-triazadodecane (SpmTrien) studied with computer simulations and cluster expansions

Correct net charge and protonation pattern in the polyamine backbone is one of the major factors that define the interactions of this class of compounds. 1,12-diamino-3,6,9-triazadodecane (SpmTrien) is a isosteric charge deficient analogue of naturally occurring spermine (Spm) with different biological features. The tautomeric populations of each SpmTrien charge state were estimated with computer simulations, molecular dynamics (MD) and quantum mechanical calculations, and cluster expansions separately. In the computer simulations, tautomeric populations of each charge state were obtained by constrained least-squares fitting the theoretically calculated (GIAO B3LYP/6-311 + G**) 15 N NMR chemical shieldings of SpmTrien tautomers to the experimentally measured chemical shifts. Theoretical chemical shieldings were calculated for water complexes of SpmTrien obtained from MD simulations in explicit water. Both methods gave highly similar realistic results. SpmTrien has many major populations of tautomers at biologically relevant charge states of three (+3) and four (+4) thus enabling a large variety of structures for specific ionic interactions. Copyright © 2013 John Wiley & Sons, Ltd.

Transition levels of defects in ZnO: Total energy and Janak's theorem methods

Transition levels of defects are commonly calculated using either methods based on total energies of defects in relevant charge states or energy band single particle eigenvalues. The former method requires calculation of total energies of charged, perfect bulk supercells, as well as charged defect supercells, to obtain defect formation energies for various charge states. The latter method depends on Janak's theorem to obtain differences in defect formation energies for various charge states. Transition levels of V(Zn), V(O), and V(ZnO) vacancy defects in ZnO are calculated using both methods. The mean absolute deviation in transition level calculated using either method is 0.3 eV. Relative computational costs and accuracies of the methods are discussed.

Defect properties of CuCrO2: A density functional theory calculation

Using the first-principles methods, we study the formation energetics properties of intrinsic defects, and the charge doping properties of extrinsic defects in transparent conducting oxides CuCrO2. Intrinsic defects, some typical acceptor-type, and donor-type extrinsic defects in their relevant charge state are considered. By systematically calculating the formation energies and transition energy, the results of calculation show that, VCu, Oi, and OCu are the relevant intrinsic defects in CuCrO2; among these intrinsic defects, VCu is the most efficient acceptor in CuCrO2. It is found that all the donor-type extrinsic defects have difficulty in inducing n-conductivity in CuCrO2 because of their deep transition energy level. For all the acceptor-type extrinsic defects, substituting Mg for Cr is the most prominent doping acceptor with relative shallow transition energy levels in CuCrO2. Our calculation results are expected to be a guide for preparing promising n-type and p-type materials in CuCrO2.

First principles study of phosphorus and boron substitutional defects in Si-XII

We present a first principles study of boron and phosphorus substitutional defects in Si-XII. Recent results from nanoindentation experiments reveal that the Si-XII phase is semiconducting and has the interesting property that it can be doped n- and p-type at room temperature without an annealing step. Using the hybrid functional of Heyd, Scuseria and Ernzerhof (HSE), we examine the formation energies of the B and P defects at the two distinct atomic sites in Si-XII to find on which site the substitutional defects are more easily accommodated. We also estimate the thermodynamic transition levels of each defect in its relevant charge states. The hybrid calculations also give an independent prediction that Si-XII is semiconducting, in agreement with recent experimental data.

Vibrational signature of the Si–N defect in Si-doped GaN xAs 1− x

Using first principles density functional theory in a supercell approach, we calculate the full spectrum of localized vibration modes associated with the Si–N defect complex in GaAs. Two recently proposed structures are investigated in detail: (a) substitutional Si on a Ga site with substitutional N on an adjacent As site [ (Si Ga–N As) q , where q is the charge state of the defect complex], and (b) Si and N atoms forming a split-interstitial defect on an As site [ (Si–N) q As]. All the localized mode frequencies for both defect structures are calculated in the relevant charge states. We have also calculated the formation energy of the (Si Ga–N As) q defect as a function of Si–N separation, finding that the configuration with the Si on a second-nearest-neighbor Ga site is more favorable in the positive charge state than the configuration with the Si on a nearest-neighbor Ga site to the N atom.