Vacancy Impurity Research Articles

Quantum gating at a distance

Quantum Systems The processing of quantum information is reliant on the encoding and manipulation of quantum states of a qubit. Superconducting circuits are the most advanced platform at present, but there is an issue with cross-talk between the qubits and the challenge of error correction as the systems are scaled up. Another approach being pursued is a modular platform in which the qubits are spatially separated. Daiss et al. demonstrate the operation of a quantum gate in which one qubit conditionally controls the state of another qubit spatially separated by 60 meters (see the Perspective by Hunger). Because the approach is platform independent, it could be extended from the demonstrated neutral atoms to ions, impurity vacancy centers, or even a combination of these qubits. Science , this issue p. [614][1]; see also p. [576][2] [1]: /lookup/doi/10.1126/science.abe3150 [2]: /lookup/doi/10.1126/science.abg1536

(Invited) Surface Properties of Perovskite Oxynitrides Influence Solar Water Splitting Activity

Photoelectrochemical (PEC) water splitting using semiconductors has been a promising means to directly produce hydrogen gas from water. Semiconductors capable of absorbing a wide wavelength range of visible light are in the limelight at the moment, because the favourable optical property causes thermodynamically high solar-to-hydrogen (STH) conversion efficiency. The n-type perovskite oxynitrides AB(O,N)3 (A=Ca, Sr, Ba and La, B=Ti, Ta and Nb) have intensive absorption bands in visible light above 600 nm so that they are one of the leading candidates for the solar water splitting.1 For instance, LaNbON2 has an Eg of 1.7 eV (wavelength λ = 750 nm) and thus potential for the STH conversion efficiency of 29.5% in reference to an incident photon-to-current efficiency of unity. However, the water splitting activity using the oxynitrides is relatively low and unstable under long-term illumination.1-2 The synthesis of oxynitrides, namely, the nitridation of starting oxides to the corresponding oxynitrides, is performed mostly in harsh conditions including a high temperature, long nitridation time, and a high flow rate of NH3. Such the severe nitridation obviously increases a bulk crystallinity of oxynitrides, which it permits efficient light absorption and charge separation under irradiation. In particular, it is necessary to prepare high-crystalline BaBO2N because the oxynitrides have the absence of stoichiometric, crystalline oxide precursors (with Ba/B ratio of unity) for nitridation.3-4 Unfortunately, the severe nitridation enhances reductions of B-site cations to a lower oxidation state (e.g., Nb5+ to Nb4+ or Nb3+) leading to anion vacancy and/or surface impurity phases such as NbO x N y , owing to the high electronegativity of the B-site cations. The surface defect sites, where the photoreaction takes place, promotes recombination of photoexcited holes and electrons during the PEC water oxidation, thus resulting in low photocurrent density. There were several reports on how to decrease defect density of oxynitrides during the nitridation, including flux-assisted nitridation and etching treatment by an aqua regia.4-5 However, these methods should be positively considered the suitable flux media and unintended doping depending on each oxynitride, and the partial loss of oxynitride particles. There was also a limitation in improving the photocurrent density.Herein we present the active and stable solar water oxidation over oxynitride photoanodes, which was remarkably improved by surface annealing in an inert Ar flow to increase the degree of crystallinity and as well to suppress the generation of surface defects.6 Also, we report the quantitative relationship between the surface defect density of the oxynitride and its photocurrent density, demonstrating that surface properties of oxynitrides greatly influence the solar water splitting performance.7 Oxides (or carbonates) contains A-site cations and B2O5 (B=Ta, Nb) were blended and calcined in air at different temperatures from 1273 to 1573 K for 30 h to obtain crystalline oxides such as Ba5B4O15. The starting oxides were nitrided at different temperatures and durations under a NH3 flow. The products were washed by distilled water and dried naturally. Subsequently, the as-prepared oxynitrides were annealed under an Ar flow at an appropriate temperature depending on oxynitride. For PEC measurements, particulate oxynitride photoanodes were prepared by a particle transfer method or an electrophoretic deposition.The nitridation of Ba5B4O15 caused highly-defective BaBO2N surface due to Lewis base and Ba-rich conditions of the starting oxide. The annealing in Ar both enhances the crystallinity of amorphous surface and effectively suppresses the formation of reduced defects. As a result, the annealed BaNbO2N photoanode exhibited greatly enhanced photocurrent of 5.2 mA cm-2 at 1.23 VRHE during sunlight-driven water oxidation, which is approximately twenty times higher than that of as-prepared BaNbO2N. BaTaO2N is thermally more stable in an Ar flow as compared with BaNbO2N, so that the high-temperature annealing at 1073 K was found to improve both the bulk and surface properties of inactive as-prepared BaTaO2N. Consequently, the particulate BaTaO2N photoanode followed by the Ar annealing produced a half-cell STH energy conversion efficiency of 1.4% at 0.88 VRHE. In addition, the solar water oxidation retained 79% of the initial photocurrent over 24 h. Surface and bulk characterizations of perovskite oxynitrides annealed in Ar and the corresponding photoresponse will be discussed in detail in the presentation.References J. Seo et al., Angew. Chem. Int. Ed., 2018, 57, 2.T. Hisatomi et al., Energy Environ. Sci., 2013, 6, 3595.J. Seo et al., Adv. Energy Mater., 2018, 1800094.S. Jadhav et al., J. Mater. Chem. A, 2020, 8, 1127.M. Matsukawa et al., Nano Lett., 2014, 14, 1038.J. Seo et al., ACS Appl. Energy Mater., 2019, 2, 5777.J. Seo et al., J. Mater. Chem. A, 2019, 7, 493.

Double-Fano resonance in a two-level quantum system coupled to zigzag Phosphorene nanoribbon

Double-level quantum systems are good candidates for revealing coherent quantum transport properties. Here, we consider quantum interference effects due to the formation of a two-level system (TLS) coupled to the edge channel of a zigzag Phosphorene nanoribbon (ZPNR). Using the tight-binding approach, we first demonstrate the formation of a TLS in bulk Phosphorene sheet due to the existence of two nearby vacancy impurities. Then, we show that such a TLS can couple to the quasi-one-dimensional continuum of the edge states in a ZPNR which results in the appearance of two-dip Fano-type line shapes. To this end, we generalize the Lippmann–Schwinger approach to study the scattering of edge electrons in a ZPNR by two coupled impurity defects. We obtain an analytical expression of the transmission coefficient which shows that the positions and widths of the antiresonances can be controlled by changing the intervacancy distance as well as their distance from the edge of the ribbon. This work constitutes a clear example of the multiple Fano resonances in mesoscopic transport.

Effect of Transition Metal Impurities on the Adsorption of Flotation Reagents on Pyrite Surface

Pyrite is one of the most common sulfide minerals on earth, which is usually grown with chalcopyrite and other sulfide minerals. In flotation practice, pyrite from different mines or different sections in a same mine may have different flotation behaviors. This is because the pyrite crystal contains lattice defect, including ionic vacancy and lattice impurity. The present work investigates the effects of three typical 3d transition metal (Co, Ni, and Fe) on the adsorption of CH3OCS2−, OH−, and CN− on pyrite surface by density functional theory (DFT) calculations. Calculated results show that Co impurity reduces the adsorption energies of CH3OCS2− and CN− on the surface of pyrite, while Ni and Cu impurities significantly increase the adsorption energies of CH3OCS2− and CN− on the surface of pyrite. Co impurity has small influence on the adsorption of OH−, but Ni and Cu impurities significantly increase the adsorption energies. Calculations on Mulliken charge population of Co2+, Ni2+, and Cu2+ indicate that the Mulliken charges of Co2+, Ni2+, and Cu2+ after OH− adsorption are more than those after CN− adsorption. This suggests that the ionic property of the M-OH- coordination bond is greater than that of M-CN- coordination bond, while covalent bonding of M-CN- coordination bond is greater than that of M-OH- coordination bond. The adsorption results are discussed based on coordination field theory (CFT).

Ultra-High Damping Capacity of Oxide Reinforced Magnesium Matrix Composites by In Situ Synthesis

In this work, in situ oxidation and particle-dispersed methods are successfully used to prepare novel oxide/Mg composites with ultra-high damping capacity starting from pure Mg as the matrix. Successful incorporation of the MgO particles into the Mg matrix is experimentally demonstrated, and the role of the oxide in enhancing the damping capacities of the composites is examined in detail. The increased damping capacity of the composites with respect to the pure magnesium is attributed to an increased dislocation density and the introduction of interface damping. In the temperature-dependent damping tests, two damping peaks P1 and P2 were found. The P1 peak is considered to be related to the interaction between dislocations and impurity atoms or vacancies. The P2 peak is considered to be caused by the grain boundaries sliding.

First-principles investigations of intrinsic point defects and helium impurities in vanadium monocarbide

Helium (He) trapping at carbide precipitates is significant for understanding the macroscopic effects of He in the normal reduced activation ferritic/martensitic (RAFM) steels and castable nanostructured alloys (CNAs) under irradiation. In this study, the behavior of intrinsic point defects and He impurities in VC is investigated by first-principles calculations. The formation of carbon (C) vacancy is quite easy and vacancy makes covalent interaction of neighboring C and V atoms more obvious. Besides, the diffusion of vacancy in VC is significantly slower than that in Fe matrix. It's worth noting that the interaction between carbon vacancies is repulsive which means that the formation of large voids maybe difficult in VC. Apart from vacancy, He impurities are also investigated. Interstitial He is less stable and diffuses more easily inside VC precipitate than inside TaC and Fe matrix. Besides, vacancy can trap much more He in VC than in Fe matrix. The result means that vacancy-abundant VC particle may have stronger ability to trap He than Fe matrix. Due to the repulsion of vacancies, we suggest that the formation of large He bubbles in VC is difficult. That is to say, little He bubbles have a dispersive distribution in VC. These theoretical results can help to understand the behavior of vacancy and He impurities in VC precipitate and further the bubble formation at precipitate in normal RAFM steels and CNAs.

Anisotropic mechanical strength, negative Poisson's ratio and fracture mechanism of borophene with defects

In this work, we used molecular dynamics simulations to study the effect of vacancy defects and impurity defects on the tensile properties of monolayer borophene under uniaxial tension. The effects of the number of point defects and defect angle on the fracture behavior and the tensile properties were investigated under different tensile strains along the zigzag direction and armchair direction. Furthermore, the impact of temperature on their mechanical properties has been analyzed and discussed. We find that borophene has anisotropic properties, Young's modulus of the borophene nanosheet at 1 K is 165.03 Nm−1 in the zigzag direction and 390.02 Nm−1 in the armchair direction. The borophene has a negative Poisson's ratio in the direction perpendicular to the atomic plane. The mechanical strength and the ultimate strain of borophene decrease as increasing the temperature. Moreover, the stress concentration around the vacancy defect leads to the initial crack formed at the vacancy position and the reduction of the fracture strength. If increasing the dimension of the vacancies projected onto the direction, which is perpendicular to the tensile direction, the fracture strength, fracture strain, and Young's modulus will decrease. Besides, the mechanical strength of borophene is reduced by the substitutional C atoms defects. With the angle defect of θ = 60°, the fracture of nanosheet which has C impurity defects easier occurs than the angle defect of θ = 0° in both zigzag and armchair tensile directions.

Large enhancement of thermoelectric performance in MoS2/h-BN heterostructure due to vacancy-induced band hybridization

Local impurity states arising from atomic vacancies in two-dimensional (2D) nanosheets are predicted to have a profound effect on charge transport due to resonant scattering and can be used to manipulate thermoelectric properties. However, the effects of these impurities are often masked by external fluctuations and turbostratic interfaces; therefore, it is challenging to probe the correlation between vacancy impurities and thermoelectric parameters experimentally. In this work, we demonstrate that n-type molybdenum disulfide (MoS2) supported on hexagonal boron nitride (h-BN) substrate reveals a large anomalous positive Seebeck coefficient with strong band hybridization. The presence of vacancies on MoS2 with a large conduction subband splitting of 50.0 ± 5.0 meV may contribute to Kondo insulator-like properties. Furthermore, by tuning the chemical potential, the thermoelectric power factor can be enhanced by up to two orders of magnitude to 50 mW m-1 K-2 Our work shows that defect engineering in 2D materials provides an effective strategy for controlling band structure and tuning thermoelectric transport.

Transformation of electron density distribution induced by the cation point defects in uranium dioxide

The ab initio calculations on electronic structure of the large fragments of UO2 crystal lattice was performed using Molecular Dynamics and fully relativistic Discrete Variational method. The four types of clusters corresponding to stoichiometric crystal and to the crystals with isolated uranium vacancy and uranium interstitial impurity were considered. We investigated the transformation of valence and vacant bands and the nature of chemical bonding. The estimation of electron density redistribution among atoms of several coordination spheres around defects was performed. In contrast to previous ab initio and semi-empirical calculations, our model included the point symmetry of defects and the possibility of charge transfer between defects and all nearest and next nearest neighbors. We obtained that the deformation of crystal structure and the covalent interactions minimizes (or completely prevent) the electron density transfer between atoms in the vicinity of defects.

Medium Energy Carbon and Nitrogen Ion Beam Induced Modifications in Charge Transport, Structural and Optical Properties of Ni/Pd/n-GaN Schottky Barrier Diodes.

The irradiation effects of carbon and nitrogen medium energy ions (MEI) on charge transport, structural and optical properties of Ni/Pd/n-GaN Schottky barrier diodes are reported. The devices are exposed to 600 keV C2+ and 650 keV N2+ ions in the fluence range of 1 × 1013 to 1 × 1015 ions cm−2. The SRIM/TRIM simulations provide quantitative estimations of damage created along the trajectories of ion beams in the device profile. The electrical parameters like Schottky barrier height, series resistance of the Ni/Pd/n-GaN Schottky barrier diodes decreases for a fluence of 1 × 1013 ions cm−2 and thereafter increases with an increase in fluence of 600 keV C2+ and 650 keV N2+ ions. The charge transport mechanism is influenced by various current transport mechanisms along with thermionic emission. Photoluminescence studies have demonstrated the presence of yellow luminescence in the pristine samples. It disappears at higher fluences due to the possible occupancy of Ga vacancies. The presence of the green luminescence band may be attributed to the dislocation caused by the combination of gallium vacancy clusters and impurities due to MEI irradiation. Furthermore, X-ray diffraction studies reveal that there is a decrease in the intensity and shift in the diffraction peaks towards the lower side of two thetas. The reductions in the intensity of C2+ ion irradiation is more when compared to N2+ ion irradiation, which may be attributed to change in the mean atomic scattering factor on a given site for light C2+ ion as compared to N2+ ion.

Luminescence of F-Type Defects and Their Thermal Stability in Sapphire Irradiated by Pulsed Ion Beams

We studied the luminescence and the thermal stability of defects formed in α-Al2O3 single crystals after pulsed treatment with a beam of C+/H+ ions with an energy of 300 keV and a pulse duration of ~80 ns. By measuring optical absorption, photoluminescence, and pulsed cathodoluminescence, it is found that this type of exposure leads to intense generation of both single F and F+ centers and more complex defects (F2 aggregate centers or vacancy–impurity complexes) in α-Al2O3. The thermal stability of F-type defects formed in α-Al2O3 upon exposure to a pulsed ion beam is comparable to the stability of radiation-induced defects in neutron-irradiated samples.

Study on magnetic properties of Fe-doped 3C-SiC nanowires

With the development of semiconductor spintronics, dilute magnetic semiconductors have become a research hotspot recently. Fe-doped 3C-SiC nanowires (C/Si = 1.4) with ferromagnetism at 300 K were fabricated by LPCVD. The electronic structures and magnetic properties of Fe-doped 3C-SiC nanowires without or with vacancies were studied by first-principle calculation. The experimental results reveal that the samples mainly grew along the [1 1 1] direction and possessed the obvious ferromagnetism at 100 K, 200 K, and 300 K. The theoretical calculations show that Fe-doped nanowires without vacancy are metallic property and non-magnetic, Fe-doped nanowires with C vacancy are metallic property and stable antiferromagnetism, and Fe-doped nanowires with Si vacancy are half-metal materials and stable ferromagnetism. The results of the density of states indicate that the magnetism originates from the hybridization of Fe-3d, C-2p and Si-3p states. The room temperature ferromagnetism of the 3C-SiC nanowires should be attributed to the coexistence of Fe impurity and Si vacancy.

Ab initio Calculation of Impurity–Vacancy Complexes in Diamond at High Pressure

Optical centers in diamond are possible candidates for single-photon emitters for applications in quantum communication, biology, and medicine. The study of the pressure dependence of zero-phonon lines of these centers can provide deeper insight into the electronic and structural properties of optical centers. These studies can also be useful for fine tuning the photon emission of optical centers in diamond by applying mechanical stresses. The results of ab initio calculations obtained in the study show that the pressure dependence of the position of zero-phonon lines is attributed to the electron density redistribution, whereas the effects associated with an increase in the binding energy under pressure turn out to be an order of magnitude weaker.

Oxygen deficiency induced strong electron localization in lanthanum doped transparent perovskite oxide BaSnO3

In this paper, we prepare heavily electron doped $\mathrm{L}{\mathrm{a}}_{0.04}\mathrm{B}{\mathrm{a}}_{0.96}\mathrm{Sn}{\mathrm{O}}_{3}$ (LBSO) thin films on a $\mathrm{SrTi}{\mathrm{O}}_{3}$ (STO) substrate by the pulsed laser deposition method under different oxygen pressures, in which different levels of oxygen vacancy concentrations are present. Normally, oxygen vacancies are implicitly assumed as isolated point defects that dope the materials with electron carriers, whereas oxygen-deficient LBSO films exhibit an apparent metal-to-insulator transition combined with lower carrier concentrations and mobility with increased oxygen vacancies, which are concluded from the transport measurement data. Considering the almost constant density of threading dislocations and the higher formation energies for other intrinsic crystal defects, the observed transport trends are ascribed to the extra oxygen vacancies introduced. Therefore, strong electron localization originating from the interaction between the oxygen vacancy and La impurity is proposed for interpreting the behavior in oxygen-deficient LBSO films. Oxygen-deficient crystal structure models for LBSO have been optimized and the electronic structures are revealed by first-principles calculations based on the density functional theory. The partial density of states results indicate that strong electron localization comes from the deep strongly localized states in the forbidden gap, which are mainly composed of hybridized orbitals of $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ with $\mathrm{Sn}\phantom{\rule{0.16em}{0ex}}5s5p$ and some amount of $\mathrm{Sn}\phantom{\rule{0.16em}{0ex}}4d$. Thus, the control of the oxygen defects and the related electronic states in barium stannate is essential for achieving optimal electrical performance and as potential applications in perovskite heterostructure devices.

Cobalt and Iron Ions in MgO Nanocrystals: Should They Stay or Should They Go

Identification and manipulation of transition-metal ion impurities in oxide nanoparticles require an in-depth understanding of their stability, segregation behavior, and, at the same time, knowledge about their surface reactivity. Powders of magnesium oxide nanoparticles with admixtures of iron or cobalt ions, as two next neighbors in the periodic table, were synthesized in the gas phase via injection of metal–organic precursors into the magnesium combustion flame followed by temperature quenching of resulting nanocrystals in argon. In these model systems of cubic nanocrystals, we explored the distinct stability of these impurities in great detail. While Co2+ ions keep their divalent valence state and substitute the host ions in the cationic sublattice, Fe3+ ions emerge due to the energy gain provided by charge compensation and impurity–vacancy complex formation. The very different behavior of Co and Fe ions in the MgO host lattice, their changes in the local environment, and the different trends in segre...

Space charge control of point defect spin states in AlN

One barrier to developing quantum information systems based on impurity point defects is that the desirable spin states of the defects are often unstable for Fermi levels obtained at increased impurity concentrations. The space charge induced band bending near the interface of Si/Mg aluminum nitride (AlN) homojunction is investigated computationally as a method to control the concentration, spin state, and position of such point defects. This is done by solving Poisson's equation with the charge density described by a grand canonical defect chemistry model informed by hybrid-functional density functional theory (DFT) calculations. Previous experimental works have found unintentional carbon and oxygen impurities pervade AlN homojunctions. First principles calculations have predicted the neutral complex between an aluminum vacancy and oxygen impurity on a neighboring nitrogen site (vAl-1ON)0 has a spin triplet configuration, which is stable in a region when the Fermi level is below midgap. From defect equilibrium simulations considering 602 possible defects, vAl-1ON was found to be unstable on the Mg-doped side of the homojunction and isolated oxygen impurities are preferred. On the Si-doped side, vAl-1ON forms but as (vAl-1ON)–2, not (vAl-1ON)0. This makes vAl-1ON a prototypical test case for the proposed strategy. Simulations of the Si/Mg:AlN homojunction showed (vAl-1ON)0 is stabilized within 6 nm of the interface in the Si-doped portion. This result indicates space charge induced band bending enables control over the concentration, spin state, and position of point defects, which is critical to realizing point defect based quantum information systems.

The Role of Defects on Forward Current in 4H-SiC p-i-n Diodes

We investigated the impact of defect states on the measured forward current–voltage ( ${I}$ – ${V}$ ) curves of ion-implanted planar 4H-SiC p-i-n diodes of a different anode dimension by means of a fine-tuned numerical model. Cross sections and activation energies of defects related to the carbon vacancy (EH6/7 and Z1/2) and Titanium (Ti) impurity used in our model were experimentally identified in the diodes of the same batch. We analyzed the effect of each individual defect on the I–V curves and estimated the unknown hole capture cross sections by ensuring the optimal match between simulated and measured currents. Small discrepancies between measured and simulated forward current–voltage curves of diodes of equal shape but a different perimeter-to-area ratio has been accounted for by considering, in the simulations, the presence of a fixed positive charge at the diode surface. By using this procedure, diodes of every dimension have been simulated without the use of adjustment parameters. These results are valuable in understanding the role of defects in the I–V curves of the ion-implanted SiC diode.

Accumulation of Defects in Austenitic Stainless Steels with Phosphorus and Titanium Additions upon Electron Irradiation at 573 К Investigated Using Positron Annihilation Spectroscopy

Using the positron annihilation spectroscopy method, the accumulation of vacancy-type defects in steels Kh16N15М3 and Kh16N15М3T1 alloyed with phosphorus has been studied at early stages of irradiation at a temperature of 573 К. The obtained data have showed that vacancies in the steels interact with phosphorus atoms at this temperature with the formation of immobile and low-mobile vacancy—impurity complexes, which results in the accumulation of vacancy-type defects. In the steel Kh16N15М3Т1 under irradiation, the formation of nanosized particles of intermetallic Ni3Ti precipitates occurs, which intensify recombination of point defects and reduce their accumulation as compared to the steel Kh16Н15М3. As this occurs, the presence of phosphorus in the steel Х16Н15М3Т1 likewise leads to the intensification of the accumulation of vacancy defects.

Band and Impurity States in Dimond with the (MV)– (M = Si, Ge, Sn) Centers Based on ab Initio Calculations

Ab initio calculations were used to determine the structure, electronic zone dispersion, partial (atomic and orbital) densities of states, and levels of impurity–vacancy cluster (MV)– (M = Si, Ge, Sn) in diamond. Wavefunctions were determined in the density functional theory approximation using hybrid exchange-correlation functional, localized basis set of Gaussians, and a supercell consisting of 192 atoms. Results are compared with data on undoped diamond and with experimental results. The dependence of the direct and indirect energy gap on the cluster type is discussed. Calculations are carried out taking into account the spin–orbit interaction to identify impurity levels in the valence band. The effect of electron–electron interaction on the position of impurity levels is discussed. The dominant contribution of the d orbitals to the density of states at the impurity level in all clusters is detected.

Trotter-Suzuki-time propagation method for calculating the density of states of disordered graphene

We present Trotter-Suzuki-time propagation method to calculate the Density of States (DOS) of disordered (i.e., vacancy and hydrogen adatom impurities) graphene system for the size of up to ≈16,000,000 atoms. The method is based on the numerical solution of Time-Dependent Schrodinger Equation (TDSE). Our calculation is focused on the case whereby the vacancy and hydrogen adatom impurities are mixed into one disordered system systematically. For a considerable high concentration of disorder, at low energy close to Fermi level, we found the band gap opening as the vacancy concentration is reduced, while the localized state remain stay at the Fermi level, which is in accordance with the previous calculation. In addition, the additional peaks occur at E/t=-2.972,-2.704,-2.435,-2.033,-0.959, and E/t=0.978,1.975,2.416,2.684,2.991, which are related to isolated carbon group. As for low concentration of disorder, the result gives a consistent picture of band gap opening and the occurrence of localized state.