Kinds Of Vacancies Research Articles

Study of native point defects in Al0.5Ga0.5N by first principles calculations

To explore the formation mechanism of native point defects in high Al content AlGaN film, the first principles methods are applied to study the native point defects in Al0.5Ga0.5N. The different kinds of vacancies, interstitials, and antisites are investigated. The formation energies of Al0.5Ga0.5N with native point defects under different charge states and growth conditions are analyzed and compared. Then, the preferable charge state, donor and acceptor properties of Al0.5Ga0.5N with different native point defects are explicitly obtained. The results show that AlN, GaN, Ali, and VN exhibit donor properties in p-type condition while VGa, VAl, VN, and NGa exhibit acceptor properties and can play roles as compensating center in n-type Al0.5Ga0.5N under metal rich condition. For N rich condition,VGa, VAl, NGa, and NAl are favorable acceptors in n-type Al0.5Ga0.5N. Meanwhile, the charge distribution and bonding state of Al0.5Ga0.5N with native point defects are explored. It is found the N atoms in Ni and NGa form covalent bond and ionic bond with atoms in Al0.5Ga0.5N. Moreover, the band calculation reveals that the removal of N atom in Al0.5Ga0.5N makes the conduction band fatter and the effective masses of electrons increase while the introduction of N point defects make the valence band flatter, increasing the effective masses of holes. Furthermore, the corresponding thermodynamic transition energy levels of defects under different charge states are summarized. It is found that the thermodynamic transitions for VN, Ni, and NAl may likely to happen under certain conditions. The above studies yield a detailed and quantitative description of native point defects in Al0.5Ga0.5N, which helps to get a deeper insight to the growth and doping of AlGaN.

Tuning magnetocrystalline anisotropy by Au ion induced defects in NiO thin films

Intercalations and doping effectively induce room-temperature ferromagnetism in antiferromagnetic NiO thin films. This report investigates the defect-controlled ferromagnetic properties of Au ion implanted highly crystalline NiO thin films at different ion fluences. Low energy heavy ion irradiation leads to preferential sputtering of Ni and O atoms that create various kinds of vacancies and interstitial defect states within the matrix. These defect states are responsible for bandgap narrowing and changing the crystallite size and show the defect luminescence in the NiO thin films. It has been observed that the intrinsic Ni and O defects with the non-stoichiometric ratio of these atoms during crystal growth, resulting in uncompensated spins, induce the ferromagnetic properties in the pristine sample and are further modulated with ion fluences. The presence of defects gives rise to energy variations due to the modification of the magnetic environment of Ni ions in the lattice and tune the blocking temperature and magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant (K) reaches its maximum at 70 erg/cm3 with ion fluence and decreases at the highest fluence because of the suppression of exchange interaction by a large number of defect centers. The modulation of anisotropy in NiO thin films is intricately linked to crystallite size, bandgap narrowing, blocking temperature, and the magnetization mechanism, which is a direct result of defect concentrations in the material.

Crime and specific vacancy types in smaller cities and towns

ABSTRACT Prior studies indicate that vacant houses (both derelict and occupiable) are often associated with increased crime. However, the majority of the research on that relationship has been conducted in large cities and urban areas. Thus, the present study aimed to provide additional clarity about vacancy’s association with crime by examining a sample of smaller cities and towns. Additionally, while prior work has focused on various kinds of vacancy, only a few studies have examined the potentially criminogenic effects of different vacancy types simultaneously. The present research contributes to additional clarity on this point by including specific forms of empty occupiable housing as predictors. The analysis also controlled for other structural factors commonly associated with crime. The results indicated that overall vacancy had a significant association with burglary but not with robbery. The results for these smaller municipalities are compared to the findings of studies conducted in larger urban areas, and their policy implications are then discussed.

Hall and bend resistance of a phosphorene Hall bar

The dependence of the Hall and bend resistances on a perpendicular magnetic field and on vacancy defects in a four-terminal phosphorene single layer Hall bar is investigated. A tight-binding model in combination with the Landauer-B\"uttiker formalism is used to calculate the energy spectrum, the lead-to-lead transmissions, and the Hall and bend resistances of the system. It is shown that the terminals with zigzag edge orientation are responsible for the absence of quantized plateaus in the Hall resistance and peaks in the longitudinal resistance. A negative bend resistance in the ballistic regime is found due to the presence of high- and low-energy transport modes in the armchair and zigzag terminals, respectively. The system density of states, with single vacancy defects, shows that the presence of in-gap states is proportional to the number of vacancies. Quantized plateaus in the Hall resistance are only formed in a sufficiently clean system. The effects of different kinds of vacancies where the plateaus are destroyed and a diffusive regime appears in the bend resistance are investigated.

First-principles investigation on the vacancy dependence on the optical and electronic properties of Bi2S3 semiconductor ceramics

Although Bi2S3 is prospective as the semiconductor ceramics, the effects of vacancy on the structural, optical and electronic properties are not clarified. In present work, the first-principles calculations were adopted to study the structural stability, optical and electronic properties of Bi2S3 with five kinds of vacancies. The five vacancy models possess the structural stability by comparing the values of vacancy formation energy. The optical properties of Bi2S3 respond to the ultraviolet light region. And Bi2S3 with V–S3 vacancy increases the absorption coefficient of original Bi2S3. The calculated values of band gap indicate that Bi2S3 and Bi2S3 with different vacancies are all semiconductor materials. All considered vacancies narrow the band gap of original Bi2S3 and make more electron transition become possible.

First-principles study of hydrogen-vacancy complexes in Be12Ti

First-principles calculations have been conducted to gain insight into the vacancy trapping mechanism of H atoms produced by neutron induced transmutation in Be12Ti, which is a promising candidate for the plasma facing materials in fusion reactors. The microscopic mechanisms for H trapping by four kinds of vacancies, namely, Be1, Be2, Be3, and Ti vacancy have been studied by calculating the solution energies and the trapping energies of hydrogen-vacancy complexes. We have included a zero-point energy correction that generally leads to a higher trapping energy, but the energy tendency remains unchanged. Vacancies serve as trapping centers to induce H atoms to segregate on their internal surface by providing an optimal H-embedding isosurface of charge density. The Ti vacancies exhibit a stronger H trapping ability than the Be vacancies. A Ti vacancy can capture up to ten H atoms, while the maximal number of H atoms that a Be vacancy can accommodate is four. But a Be1 vacancy can only capture two H atoms if the zero-point energy correction is included in the simulation. Among all HnVBe1 (n = 1–5) complexes, the configuration of H2 VBe1 has the lowest H solution energy. The H2 molecules are not formed in the most stable hydrogen-vacancy complexes explored. The present results provide a fundamental image of vacancy trapping mechanism for H atoms as well as H bubble formation in solid Be12Ti, which is extremely important for the reduction of radioactive tritium inventory within fusion reactor and appropriate handling of radioactive nuclear wastes during the decommissioning of the fusion blanket.

Understanding the Decomposition Mechanisms of LiNH2, Mg(NH2)2, and NaNH2: A Joint Experimental and Theoretical Study

Metal amides are promising candidates for hydrogen storage, hydrogen production, NH3 synthesis and cracking, and so on. However, the decomposition behaviors and mechanisms of metal amides remain unclear. In this study, the decomposition properties of three metal amides, including LiNH2, Mg(NH2)2, and NaNH2, are studied by thermogravimetry, mass spectroscopy, and in situ X-ray diffraction techniques combined with density functional theory (DFT) calculations. It is found that Mg(NH2)2, LiNH2, and NaNH2 exhibit very different metal–N and N–H bond strengths, which precipitate various formations energies of different kinds of vacancies. As a result, LiNH2 releases a major amount of NH3, with a small amount of N2 at a temperature as high as 350 °C. Mg(NH2)2 releases NH3 and N2 synchronously at a temperature range of 300–400 °C without the emission of H2. NaNH2 synchronously releases H2, NH3, and a small amount of N2, at a narrow temperature range of 275–290 °C. Using DFT calculations, the decomposition behaviors and the corresponding decomposition mechanisms for LiNH2, Mg(NH2)2, and NaNH2 have been well understood.

The effect of growth conditions and vacancies on the electronic and mechanical properties of cubic Mo2N; A DFT study

In this study, the first-principles method was applied to study the effect of growth conditions and vacancies on the properties of cubic Mo2N. The obtained results show that vacancies play an important role in the control of the electronic and mechanical properties of this material. From all kinds of vacancies considered in this work, the molybdenum vacancy has the lowest formation energy under N-rich conditions while the nitrogen vacancy has the lowest formation energy under N-poor conditions. Moreover, we found that the formation of vacancies decreases the elastic constants, the Young's modulus, the bulk modulus, and the Vickers Hardness (Hv) of cubic Mo2N. The most noticeable change is observed in the case of Mo-deficient material.

Origin of enhanced stability in thiocyanate substituted α-FAPbI3 analogues

In the past few years, hybrid perovskites have emerged as the most promising photovoltaic materials due to their excellent optoelectronic properties, and easy fabrication methods. However, the long-term stability is still the main obstacle for their commercial applications. Recently, thiocyanate-doped hybrid perovskites have shown enhanced stability and impressive efficiency, but the reason is still unknown. Herein, we discussed the enhanced stability of SCN-substituted pseudocubic FABX3 (B=Pb2+, Sn2+; X=I−, Br−, and Cl−) based on the density functional theory. Through a series of calculations of Bader charge transfer, vacancy formation energies of different kinds of vacancies, decomposition enthalpy, phonon density of states, and ab initio molecular dynamics simulation, we conclude that the incorporation of SCN− can stabilize pseudocubic FABX3, and attribute the enhanced stability mainly to two factors: (1) the strong interaction between Pb2+/Sn2+ and SCN−, as well as the strong hydrogen bonding between FA+ and X−/SCN−, and (2) the structural tilting induced by the incorporation of SCN−. These findings provide alternative method fortuning the poor stability of pseudocubic FABX3, as well as for obtaining high-performance solar cells.

Atomic-registry-dependent electronic structures of sulfur vacancies in ReS2 studied by scanning tunneling microscopy/spectroscopy

Rhenium disulfide (ReS2) is regarded as a promising candidate for optoelectronic applications (e.g., infrared photodetector), as it maintains a direct bandgap regardless of the number of layers unlike other typical transition metal dichalcogenides. Therefore, it is very important to understand and control the defects of ReS2 for enhancing the performance of photodevices. In this work, we studied the electronic structures of ReS2 affected by sulfur vacancies of different atomic registries at the atomic scale. The atomic and electronic structures of the mechanically exfoliated ReS2 flakes were investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), and were confirmed using density functional theory (DFT) calculations. The atomic structural models indicate four distinguishable atomic registries of sulfur vacancies on one face of ReS2. Energetically, these atomic vacancies prefer to locate on the bottom side of the top monolayer of ReS2 flakes. Only two among four possible kinds of vacancies could be observed using STM and STS, and they were identified using additional DFT calculations. We believe that our results regarding the identification of the defects and understanding the corresponding effects for electronic structures will provide important insights to enhance the performances of ReS2-based optoelectronic devices in the future.

Interfacial oxidation kinetics at SiO2/Si(001) mediated by the generation of point defects: Effect of raising O2 pressure

In the thermal oxidation reaction of Si, point defects (emitted Si atoms and its vacancies) occur owing to oxidation-induced strain. These point defects become not only charge traps that degrade device characteristics but also oxidation reaction sites. However, a reaction rate equation of a model in which point defects are regarded as reaction sites has not yet been proposed. We investigated the O2 pressure dependence of the initial interfacial oxidation rate Ri(0) after the oxidation of an Si(001) surface using real-time Auger electron spectroscopy to establish the reaction rate equation based on the point defect generation model. We found that Ri(0) was proportional to the square root of the O2 pressure, and the initial deceleration of the interfacial oxidation rate was proportional to the O2 pressure. Using the result of the pressure dependence of Ri(0), we derived the reaction rate equation based on the Si oxidation model in which O2 dissociation preferentially occurs at vacancies resulting from Si atom emission at the SiO2/Si(001) interface. The prediction of the O2 pressure dependence of the initial deceleration of the interfacial oxidation rate shows good agreement with the experimental results. In addition, we predicted that there were two kinds of vacancies at the SiO2/Si(001) interface to dissociate the O2 molecules.

How to boost the sluggish lithium-ion hopping dynamic in borophene?

In light of low atomic mass, three types of experimentally synthetized borophene including β12, χ3 and striped t-sheet have been predicted to be promising anode materials for lithium-ion batteries (LIBs) with extremely high capacity. However, the rate performances of β12 and χ3 are quite poor with high diffusion barrier of 0.66–0.81 eV on β12 and 0.60–0.85 eV on χ3 in contrast with that in t-sheet (typically <0.1 eV). Isolation of t-sheet from their blend remains a fundamental challenge in the field of nanotechnology and a mechanistic understanding and control over the hopping dynamic of Li+ therein are thus of extremely important to facilitate the development of borophene-based anode material, but still lacking. In this work, we performed a comprehensive theoretical investigation on the adsorptions and migrations of Li+ on perfect and defective β12 and χ3 based on density functional theory. We determined a new kind of vacancy in β12 that modulates the adsorption and boosts the diffusion of Li+ nearby remarkably. With the aid of charge doping, we uncover a general mechanism (charge-concentration mechanism) involved with the celebrated bonding theory of borophene, where the hopping barrier of Li+ on β12 could be reduced to be 0.06 eV, rationalizing the boosting Li+ hopping as a result of electron deficiency in vacant borophene. By extending our calculations to H functionalized borophene and Ag supported borophene, we further confirm the validity of the “charge-concentration mechanism” under more realistic experimental conditions. The proposed mechanism could be used as a guiding principle to improve or develop new borophene-based electrode materials with high rate performance for LIBs.

Defect formation and electronic structure regulated by strain engineering in ReS2

Abstract By using first-principles calculations, we investigated the monolayer ReS 2 with vacancies under strain engineering, specifically focusing on its energy of formation, band gap, electron density of states, effective mass and optical properties. The calculated results disclose that S4 defect is more likely to form than other kinds of vacancies. Asymmetric deformation induced by strain makes its band structure transformation from direct band gap to indirect band gap. The analysis of the partial density of states indicates that the Re-d and S-p orbitals are the major components of the defect states, being different from MoS 2 , the defect states locate both above and below the Fermi level. Moreover, the effective mass was sensitive and anisotropic under the external strain. The reflection spectrum can be greatly tuned by the external strains, which indicates that the ReS 2 monolayer has promising applications in nanoscale strain sensor and conductance-switch FETs.

Robust insulating La and Ti co-doped BiFeO3 multiferroic ceramics

Low resistivity and high dielectric loss are two major problems associated with the fabrications of BiFeO3 multiferroic ceramics. Adapting co-doping on the A- and B-sites, robust insulating Bi0.98La0.02Fe0.99Ti0.01O3 ceramics with resistivity ~1011 Ω m measured applying electric field 90 kV/cm and dielectric loss tanδ below 0.02 over frequency range of 100 Hz–2 MHz were prepared using a refined solid state reaction electroceramic processing. Intentional La/Ti co-substitution was experimentally illustrated to form ternary solid solution, which enhances thermodynamic stability of BiFeO3 perovskite phase, allowing obtain high resistive low dielectric loss bismuth ferrite ceramics. Meanwhile, passive unknown impure ions from Bi2O3 and TiO2 raw oxides were also found combining synergetic effect available to obtain low dielectric loss ceramics. In contrast to the traditional method introducing vacancies via doping heterovalent ions, various kinds of vacancies were introduced in advance into the ceramic samples through controlling chemical stoichiometry via employing impure raw materials, and they are experimentally demonstrated responsible for dielectric loss relaxation behaviors and high dielectric loss observed in the Bi0.98La0.02Fe0.99Ti0.01O3 ceramics.

Density functional theory calculations of point defects and hydrogen isotopes in Li4SiO4

The Li4SiO4 is a promising breeder material for future fusion reactors. Radiation induced vacancies and hydrogen isotope related impurities are the major types of point defects in this breeder material. In present study, various kinds of vacancies and hydrogen isotopes related point defects in Li4SiO4 are investigated through density functional theory (DFT) calculations. The band gap of Li4SiO4 is determined by UV-Vis diffuse reflectance spectroscopy experiments. Formation energies of all possible charge states of Li, Si and O vacancies are calculated using DFT methods. Formation energies of possible charge states of hydrogen isotopes substitution for Li and O are also calculated. We found that Li-vacancies will dominate among all vacancies in neutral charge state under radiation conditions and the O, Li, and Si vacancies (VO,VLi,VSi) are stable in charge states +2, -1, -4 for most of the range of Fermi level, respectively. The interstitial hydrogen isotopes (Hi) and substitutional HLi are stable in the charge states +1, 0 for most of the range of Fermi level, respectively. Moreover, substitutional HO are stable in +1 charge states. We also investigated the process of tritium recovery by discussing the interaction between interstitial H and Li-vacancy, O-vacancy, and found that HO+ and HLi0 are the most common H related defects during radiation process.

Strain engineering the magnetic states of vacancy-doped monolayer MoSe2

The effect of vacancy defect and strain on the magnetic property of monolayer MoSe2 was investigated by the first-principles calculation. Firstly, the most stable structure of monolayer MoSe2 and four kinds of vacancy, VSe, VSe2, VMoSe3 and VMoSe6 geometries were systematically studied. Our calculations show that 1H-MoSe2, as well as VSe, VSe2 and VMoSe3 vacancy doped MoSe2 monolayer are nonmagnetic, while VMoSe6 vacancy geometry shows magnetism and the magnetic moment mainly comes from the six Mo atoms around the vacancy although it is most difficult to form. Meanwhile, our calculations indicate that MoSe2 monolayer under tensile strain ranging from 0% to 10% does not show any magnetism. However, it is found that the combined tensile strain and VSe, or VSe2 and VMoSe3 vacancy indeed induce magnetism in MoSe2 monolayer by breaking the Mo–Mo metallic bonds. As for VMoSe6 vacancy geometry, the vacancy induced magnetic moment decrease under the imposed tensile strain.

Multiple structural forms of a vacancy in silicon as evidenced by vacancy profiles produced by rapid thermal annealing

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.

Fast and Slow Vacancies in Silicon

Vacancies (and probably also self-interstitials) in silicon appear to exist in several forms (atomic configurations) some of them being fast diffusers and other slow diffusers. The data on enhanced self-diffusivity under proton irradiation, on vacancy and oxide precipitate profiles installed by Rapid Thermal Annealing, and on the self-diffusivity under equilibrium conditions suggest that there are at least two kinds of vacancy: 1) Vw- a fast-diffusing localized vacancy manifested in electron irradiated samples (Watkins vacancy), 2) Vs- a slow-diffusing extended vacancy manifested under hot proton irradiation. In RTA experiments, these two species behave as one equilibrated subsystem of a moderate effective diffusivity intermediate between those of Vwand Vs. There is also strong evidence in favor of a third kind of vacancy: Vfa fast extended species, which controls the grown-in voids in silicon crystals.

Silicon Location in Silicate-Substituted Calcium Phosphate Ceramics Determined by Neutron Diffraction

The silicate incorporation in calcium phosphate bioceramics was studied by X-ray and neutron powder diffraction (joint Rietveld refinement) and micro-Raman spectroscopy. For a phosphate substitution level of 15 mol % multiphase ceramic is obtained passing from biphasic calcium phosphate (BCP) ceramic to ceramic composed of hydroxyapatite (HAp), silicocarnotite, and tricalcium phosphate (both α- and β-TCP polymorphs). The quantity of silicate incorporation increases when passing from β-TCP (Ca3(PO4)2, silicate level not measurable) to α-TCP (Ca3(PO4)1.875(SiO4)0.125 with 6.25 mol % of phosphate substitution or 1.13 wt % of incorporated silicon), to HAp (Ca10(PO4)4.9(2)(SiO4)1.1(2)(OH)1.0(1)O0.66(7) with 18.3 mol % of phosphate substitution or 3.11 wt % of silicon), and to silicocarnotite (Ca5(PO4)1.9(2)(SiO4)1.1(2) with 36.7 mol % of phosphate substitution or 6.40 wt % of silicon). The mechanism of silicate incorporation into HAp structure is multiple and involves the creation of two kinds of vacancies: hydroxyl vacancies and phosphate/silicate vacancies due to a carbonate-involving mechanism leading to the composition Ca10(PO4)6–x−2y(SiO4)x+y(◻T)y(OH)2–xOy◻x–y. The formation of important hydroxyl vacancies in the hexagonal channel of the HAp structure leads to the stabilization of silicocarnotite (that can be considered as a polymorph of the highly Si-substituted HAp phase). The Ca10(PO4)4.9(2)(SiO4)1.1(2)(OH)1.0(1)O0.66(7) composition appears then to be the end-member of the hexagonal Si-HAp solid solution. Silicocarnotite is formed above this silicate insertion level.

Vacancy model of micropipe annihilation in epitaxial silicon carbide layers

Kinetic processes of annihilation (healing) of a micropipe threading into a growing layer from a substrate-seed are considered in terms of the vacancy model of heteropolytype epitaxy of silicon carbide we previously suggested (Fiz. Tekh. Poluprovodn., 39, 296 (2005); 41, 641 (2007)). A relationship is found between the growth rate of an epitaxial film, vacancy lifetime, and defect layer width at which the micropipe is healed. Both kinds of vacancies, of carbon and silicon type, are taken into account. In addition, a simplified linear model of the process of micropipe healing is suggested. The relationship between the micropipe diameter r 0 and the defect layer width l* is determined in terms of this model: l* = r 0(G/g), where G is the layer growth rate and g is the vacancy velocity, which yields l* ≈ 6r 0 for actual growth conditions.