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This paper reports the results of atomistic and ab initio simulation of several different tilt grain boundaries in silicon. The boundary structures obtained with genetic algorithm turned out to have no coordination defects, i.e., all silicon atoms restored their tetrahedral coordination during the structure optimisation. That concerns previously known symmetric Σ5 (130), Σ3 (211), and Σ29 (520) boundaries and previously unknown asymmetric Σ9 (̄255)/(̄211), Σ3 (̄255)/(̄211), and Σ13 (790)/(3 11 0) structures. An extensive study has been performed on defect segregation on the boundaries, including neutral vacancy and carbon, phosphorus, and boron impurities. A clear correlation has been revealed between the segregation energy of the defect and local geometry of the boundary site where the defect is segregated. The authors propose a simple purely geometric model for evaluating approximate segregation energies of the listed defects. [ABSTRACT FROM AUTHOR]

Hydrogenation is an effective approach to improve the performance of photocatalysts within defect engineering methods. The mechanism of hydrogenation and synergetic effects between hydrogen atoms and local electronic structures, however, remain unclear due to the limits of available photocatalytic systems and technical barriers to observation and measurement. Here, we utilize oxygen vacancies as residential sites to host hydrogen atoms in a layered bismuth oxychloride material containing defects. It is confirmed theoretically and experimentally that the hydrogen atoms interact with the vacancies and surrounding atoms, which promotes the separati30on and transfer processes of photo-generated carriers via the resulting band structure. The efficiency of catalytic activity and selectivity of defective bismuth oxychloride regarding nitric oxide oxidation has been improved. This work clearly reveals the role of hydrogen atoms in defective crystalline materials and provides a promising way to design catalytic materials with controllable defect engineering. Hydrogenated BiOCl shows high performance as a semiconductor photocatalyst, but the mechanism and synergetic effects of hydrogenation are hard to study. Here interactions of oxygen vacancies and hydrogen atoms in BiOCl nanosheets show a modulation of energy dispersion, which leads to both improvements in charge separation and photocatalytic activity. [ABSTRACT FROM AUTHOR]

The vacancy-type defects and their local chemical environment in different ODS alloys produced in the USA (14YWT), China (K5) and Russia (ODS EP-450) are studied. The Angular Correlation of Annihilation Radiation (ACAR), which is one of the positron annihilation spectroscopy method, was used. It was shown that in all alloys, except 14YWT, the dominant type of positron traps are vacancy-like defects, localised in matrix or associated with dislocations and/or interfaces of the incoherent particles. In the case of 14YWT alloy, which contains Y-Ti-O nanoclusters of a high density, the positrons confine and annihilate at O-vacancy pairs or complexes within nanoclusters. It is testified by enhanced electron density in annihilation sites and neighbourhood of Ti and Y atoms. These results, obtained by the ACAR method, indicate that the vacancies play an essential role in the formation of nanoclusters in ODS 14YWT alloy as it was theoretical predicted by first-principle calculations. [ABSTRACT FROM AUTHOR]

The detailed catalytic oxidation mechanisms of CO over Cr-embedded graphene have been investigated by means of M06-2X density functional. Both models with chromium atom embedded in single and double vacancy (Cr-SV and Cr-DV) in a graphene sheet were considered. It is found that the CO oxidation on Cr-SV-graphene prefers to Langmuir-Hinshelwood (LH) mechanism, while the Cr-DV-graphene shows good catalytic activity toward the CO oxidation via the Eley-Rideal (ER) mechanism. The present results imply that the low-cost Cr-embedded graphene is a prospective catalyst for CO oxidation at room temperature. [ABSTRACT FROM AUTHOR]

In the present work, pure CeO2, neodymium-doped CeO2, and neodymium and transition metal element (Cu, Zn, Cr)-codoped CeO2 nanoparticles were synthesized by an auto-combustion method and annealed at 700 ∘C. The X-ray diffraction (XRD) studies revealed that all the samples exhibited a single-phase cubic fluorite structure with the incorporation of respective dopant ions into the CeO2 lattice. The scanning electron microscopy (SEM) images displayed the prepared nanoparticles which had an irregular flaky structure with large agglomerations. The Fourier transform Raman (FT-Raman) spectroscopy analysis revealed the increased oxygen vacancy defects in CeO2 host after doping and codoping with neodymium and transition metal elements. Ultraviolet-diffuse reflectance spectroscopy (UV-DRS) studies revealed the decrease in bandgap values in doped and codoped samples compared to pure CeO2 sample. The Fourier transform infrared spectroscopy (FTIR) studies revealed the presence of functional groups in the prepared samples. Photoluminescence (PL) spectroscopy analysis reported the decreased luminescence intensities of Nd-doped and neodymium and transition metal-codoped CeO2 nanoparticles. The vibrating sample magnetometer (VSM) results depicted that all the samples exhibited room-temperature ferromagnetism. [ABSTRACT FROM AUTHOR]

The effects of Fe ion vacancy on structural and magnetic properties of NiFeO4 nanoparticles are investigated using X-ray diffraction, Fourier transform infrared, morphological, and magnetic measurements. The introduction of Fe ion vacancy leads to the increase of the lattice parameter and decrease of the particle size. Moreover, vibrating sample magnetometer measurement results revealed that the values of the magnetization decreased and that coercivity had no regular behavior with increasing Fe ion vacancy content. These behaviors could be explained by considering cation distribution in the spinel structure. The estimated cation distribution results show that the Fe ion vacancy mostly occurred at the B sites which is in good agreement with the theoretical calculations. [ABSTRACT FROM AUTHOR]

In this study, a first-principles study using the spin-polarized density functional theory approach within corrected functional was carried out to investigate the electronic features of manganese oxide surfaces under three situations of (a) cation vacancy, (b) intercalation of multi- and univalent ions, and (c) adsorption of a water molecule upon the surface as catalytic performance. The possibility of obtaining the significant absolute magnetic momentum phases from native defects in orthorhombic structures of Mn2O3 and Mn3O4 (001) surface is explored, whereas Mn vacancy provides a transition from the insulating phase into a metal-like nature and modifies the electronic transport. Moreover, bandgap engineering via impurity intercalation has been explored. Ca+2 and Al+3 intercalations have manifested substantial attributes and explain the experimental results as efficient conducting system and catalytic activity. Furthermore, the adsorption of one water molecule and the most stable configuration, adsorption energies and electronic properties were thoroughly discussed. Accordingly, it was explored that H2O: Mn2O3 and Mn3O4 exhibit suitable parameters as efficient catalytic synthesis. [ABSTRACT FROM AUTHOR]

The size of structural vacancies and structural vacancy density of 1/1-Al–Re–Si approximant crystals with different Re compositions were evaluated by positron annihilation lifetime and Doppler broadening measurements. Incident positrons were found to be trapped at the monovacancy-size open space surrounded by Al atoms. From a previous analysis using the maximum entropy method and Rietveld method, such an open space is shown to correspond to the centre of Al icosahedral clusters, which locates at the vertex and body centre. The structural vacancy density of non-metallic Al73Re17Si10was larger than that of metallic Al73Re15Si12. The observed difference in the structural vacancy density reflects that in bonding nature and may explain that in the physical properties of the two samples. [ABSTRACT FROM PUBLISHER]

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We performedin situobservation of one-dimensional (1D) migration of self-interstitial atom (SIA) clusters in iron under electron irradiation at 110–300 K using high-voltage electron microscopy. Most 1D migration was stepwise positional changes of SIA clusters at irregular time intervals at all temperatures. The frequency of 1D migration did not depend on the irradiation temperature. It was directly proportional to the damage rate, suggesting that 1D migration was induced by electron irradiation. In contrast, the 1D migration distance depended on the temperature: distribution of the distance ranged over 100 nm above 250 K, decreased steeply between 250 and 150 K and was less than 20 nm below 150 K. The distance was independent of the damage rate at all temperatures. Next, we examined fluctuation in the interaction energy between an SIA cluster and vacancies of random distribution at concentrations 10−4–10−2, using molecular statics simulations. The fluctuation was found to trap SIA clusters of 4 nm diameter at vacancy concentrations higher than 10−3. We proposed that 1D migration was interrupted by impurity atoms at temperatures higher than 250 K, and by vacancies accumulated at high concentration under electron irradiation at low temperatures where vacancies are not thermally mobile. [ABSTRACT FROM AUTHOR]

Thermal effects are important considerations at the initial stage in spark plasma sintering of non-conductive AlO powders. The generalized thermo-elastic theory is introduced to describe the influence of the heat transport and thermal focusing caused by thermal wave propagation within a constrained space and transient time. Simulations show that low sintering temperature can realize high local temperature because of the superposition effect of heat waves. Thus, vacancy concentration differences between the sink and the cross section of the particles increase relative to that observed during pressure-less and hot-pressure sintering. Results show that vacancy concentration differences are significantly improved during spark plasma sintering, thereby decreasing the time required for sintering. [ABSTRACT FROM AUTHOR]

Vacancy depth profiles installed by rapid thermal annealing can be monitored either by Pt diffusion or through vacancy-assisted oxygen precipitation. The features of these profiles clearly show that the vacancy species manifested in these experiments is a 'slow vacancy', Vs. The evolution of Vs depth profiles is controlled by an exchange with another (mobile) kind of vacancy that is likely to be a 'Watkins vacancy', Vw, first observed at cryogenic temperatures. At low T the conversion of Vs into Vw is slow and practically irreversible. At higher T the two species coexist in an equilibrium ratio and diffuse as one entity with an averaged diffusivity. This model provides a good fit to the RTA-installed depth profiles of Vs. The total vacancy community includes, beside Vs and Vw, also a fast vacancy Vf that is responsible for the vacancy contribution into self-diffusion at high T. In RTA experiments, the Vf species seems to be completely annihilated by self-interstitials which leaves only two other vacancy species, Vs and Vw. [ABSTRACT FROM AUTHOR]

Vacancy clusters have been observed in ZnO by positron-annihilation spectroscopy (PAS), but detailed mechanisms are unclear. This is because the clustering happens in non-equilibrium conditions, for which theoretical method has not been well established. Combining first-principles calculation and kinetic Monte Carlo simulation, we determine the roles of non-equilibrium kinetics on the vacancies clustering. We find that clustering starts with the formation of Zn and O vacancy pairs (VZn-Vo), which further grow by attracting additional mono-vacancies. At this stage, vacancy diffusivity becomes crucial: due to the larger diffusivity of VZn compared to VO, more VZn-abundant clusters are formed than VO-abundant clusters. The large dissociation energy barriers, e.g., over 2.5 eV for (VZn-Vo), suggest that, once formed, it is difficult for the clusters to dissociate. By promoting mono-vacancy diffusion, thermal annealing will increase the size of the clusters. As the PAS is insensitive to VO donor defects, our results suggest an interpretation of the experimental data that could not have been made without the in-depth calculations. [ABSTRACT FROM AUTHOR]

We propose a phase field method to model consistently dislocation climb by vacancy absorption or emission. It automatically incorporates the exact balance between the vacancy flux and the phase field associated with the dislocation evolution, enforced by the conserved character of the total population of vacancies. One of its major advantage is the natural introduction of a dynamic coefficient controlling the kinetics of vacancy emission/absorption by the dislocation. We also derived a closed-form expression of the climb rate valid from the diffusion-limited to the attachment-limited regimes. [ABSTRACT FROM AUTHOR]

On the basis of a vacancy-squashing model for melting that we developed recently, a vacancy formed in the premelting liquid-like-layer is proposed to be squashed at the instance of its formation, which will significantly facilitate vacancy formation. Accordingly, the temperature for surface-induced melting and its size dependence are quantitatively calculated. By comparing the calculated data with the reported experimental results for metals, we show that the present approach may potentially predict the size-dependent melting point of other metals. [ABSTRACT FROM AUTHOR]

Random telegraph noise in resistive switching memory devices is governed by two distinct mechanisms—oxygen vacancy perturbations in the filament as well as the electron trapping–detrapping phenomenon. In this letter, we focus on the dominant role of vacancies in governing the stability of the filament in the high resistance state and characterize the dependence of the read disturb voltage (VDIST) on the depth of the reset level during switching. Our slow voltage ramp read disturb tests at different reset levels indicate the possibility of filamentary instability even for read voltages lower than the standard value of 0.10 V. These experimental trends can be well explained using the quantum point contact model for conduction in the filament, as deeper reset levels induce very steep potential gradients at the two ends of the constriction that make the filaments highly unstable and susceptible to structural modifications due to vacancy generation and/or transport during memory read operation. [ABSTRACT FROM PUBLISHER]

This paper presents the mono-vacancy formation and migration energies of each element Ti, Ga, and C in the MAX phase Ti2GaC, which are obtained by first principles calculations. We also calculate the formation energies of oxygen substituting for Ti, Ga, and C and two formation energies of oxygen interstitial in different sites. The results show that the formation energy of oxygen substituting for Ti is the highest, and the formation energies of the O substitution for Ga atoms decrease as the oxygen concentration increases. The two different formation energies of one oxygen interstitial show that the stable site for the oxygen interstitial is at the center of the triangle composed by three Ga atoms. The effects of vacancy, oxygen substitution, and the interstitial on the electronic properties of Ti2GaC are also discussed in light of the density of states and the electron charge density. [ABSTRACT FROM AUTHOR]

Phosphorene is receiving great research interests because of its peculiar physical properties. Nonetheless, no systematic studies on the transport properties modified due to defects have been performed. Here, we present the electronic band structure, defect formation energy and bias dependent transport property of various defective systems. We found that the defect formation energy is much less than that in graphene. The defect configuration strongly affects the electronic structure. The band gap vanishes in single vacancy layers, but the band gap reappears in divacancy layers. Interestingly, a single vacancy defect behaves like a p-type impurity for transport property. Unlike the common belief, we observe that the vacancy defect can contribute to greatly increasing the current. Along the zigzag direction, the current in the most stable single vacancy structure was significantly increased as compared with that found in the pristine layer. In addition, the current along the armchair direction was always greater than along the zigzag direction and we observed a strong anisotropic current ratio of armchair to zigzag direction. [ABSTRACT FROM AUTHOR]

We describe a one-step, deterministic and scalable scheme for creating macroscopic arbitrary entangled coherent states (ECSs) of separate nitrogen-vacancy center ensembles (NVEs) that couple to a superconducting flux qubit. We discuss how to generate the entangled states between the flux qubit and two NVEs by the resonant driving. Then the ECSs of the NVEs can be obtained by projecting the flux qubit, and the entanglement detection can be realized by transferring the quantum state from the NVEs to the flux qubit. Our numerical simulation shows that even under current experimental parameters the concurrence of the ECSs can approach unity. We emphasize that this method is straightforwardly extendable to the case of many NVEs. [ABSTRACT FROM AUTHOR]

GaAs1 − xBix alloys attract significant interest due to their potentiality for several applications, including solar cells. Recent experiments link the crucial optical properties of these alloys to Bi clustering at certain Bi compositions. Using ab initio calculations, we show that there is no thermodynamical driving force for the formation of small GaBi clusters incorporating As substitutional sites. However, the Ga vacancies should gather Bi atoms leading to small Bi clusters, and the Ga vacancies can act as nucleation centers for phase separation. The formation energy of the GaAs1 − xBix with respect to GaAs and GaBi shows a maximum at intermediate Bi concentrations. Thermodynamics and kinetics of the GaAs1 − xBix film growth is discussed. High Bi solubility is obtained, if the Bi atoms on the energetically favorable atom positions in the subsurface layer are relatively frozen. The Ga vacancy concentration may be increased by the incorporation of Bi. The Bi atoms can also prevent the out diffusion of Ga vacancies. [ABSTRACT FROM AUTHOR]