Vacancy Type Research Articles

A DFT study on the effect of oxygen vacancies and H2O in Mn-MOF-74 on SCR reactions.

As one of the main air pollutants, nitrogen oxides (NOx) have serious effects on human health and the environment. In our previous study, we found that Mn-MOF-74 shows excellent catalytic performance for the selective catalytic reduction (SCR) reaction with NH3 being the reductant (NH3-SCR) at low temperature. To obtain a further understanding of the NH3-SCR mechanism in Mn-MOF-74, in this paper, we investigated two important parts of the NH3-SCR process in Mn-MOF-74 using the density functional theory (DFT) method. On the one hand, the structural characteristics of two types of oxygen vacancies of Mn-MOF-74, namely carboxyl oxygen vacancies and hydroxyl oxygen vacancies, and their adsorption properties to reaction species were calculated. It was found that the oxygen vacancies not only activate the reaction species, but also promote the desorption of NO2 molecules from metal sites for the subsequent rapid SCR reactions. On the other hand, we studied the effect of H2O on the structural stability and catalytic performance of Mn-MOF-74. It was found that the interaction of Mn-O bonds was weakened by H2O. Therefore, the influence of H2O should be considered for the future design of MOF-based catalysts for the SCR process.

Simulation of static and dynamic mechanical characteristics of carbon nanotubes and carbon nano-peapods with defects

The static and dynamic mechanical characteristics of carbon nanotubes with double and multiple vacancy defects are simulated by the molecular dynamics method. Firstly, the effects of diatomic and polyatomic vacancy defects on the quasi-static mechanical properties of carbon nanotubes are discussed. Then, the effects of defects and axial pre-stress on the dynamics of C60 molecular oscillation in carbon nano-peapods are discussed. The results show that the ultimate stress, ultimate strain and elastic modulus of carbon nanotube containing different types of diatomic vacancies are significantly reduced as compared with those of non-defective carbon nanotubes. When the carbon nanotubes have many defective atoms and the defects are connected together to form a crack, the axial compressive properties of the carbon nanotubes are greatly reduced. Compared with the circumferential development of cracks, the cracks along the axis greatly reduce the compressive capacity of carbon nanotubes, which is similar to that of shell models with cracks. The oscillation frequency of C60 molecular in defective carbon nano-peapods is affected by the number of missing atoms. The single vacancy defect increases the oscillation frequency of C60 molecule, while with the further increase of vacancy number, the oscillation frequency of C60 molecule decreases gradually. When the defective carbon nano-peapod has axial tensile or compressive pre-stress, the oscillation of the C60 molecule is affected not only by the defects, but also by the axial pre-stress, which makes the oscillation of C60 molecule more complicated.

High-temperature defect recovery in self-ion irradiated W-5 wt% Ta

A study of high-temperature defect recovery has been carried out for W-5 wt% Ta alloy, irradiated with 2 MeV W+ ions at 500 °C, up to 1.2 dpa. After post-irradiation annealing at 1200 °C for 15 min, the damage microstructure evolved from a random distribution of small loops to a mixture of large loops, dislocation lines and voids. The average size of loops increased by a factor of ∼4, up to 17.7 nm, whereas the number density dropped by an order of magnitude, to ∼5.3 × 1021 m−3. Only loops with b = ½<111> were observed and they were identified to be exclusively interstitial type. This is in sharp contrast with the presence of ∼25% vacancy type ½<111> loops in the as-irradiated condition. Voids were formed as a result of accelerated vacancy/vacancy-cluster migration at 1200 °C, achieving an average size of ∼2.5 nm and a high density of ∼5.1 × 1022 m−3. The role of tantalum is discussed. Furthermore, the high-temperature defect recovery in W-5Ta after self-ion (this work) and proton irradiations (Ipatova et al., 2017) are compared, based on which the possible influence of hydrogen upon defect evolution is discussed.

The Nature of the Oxygen Vacancy in Amorphous Oxide Semiconductors: Shallow Versus Deep

Using first‐principles calculation, we investigate the nature of oxygen vacancy (VO), namely shallow versus deep, in the amorphous oxide semiconductor InGaZnO4 (a‐IGZO), which has not been fully clarified despite its technological importance. Oxygen‐deficient amorphous models are generated through the hybrid functional molecular dynamics (MD) simulations that allow for finding stable VO configurations while minimizing computational approximations. From eight independent models, we consistently find that VO serves as the shallow donor, increasing the Fermi level above the conduction band minimum. For comparison purpose, we also generate deep VO models by charging the system during MD simulations. It is found that deep VO is higher in the formation energy than shallow VO, confirming that shallow VO is the preferred type of oxygen vacancies in a‐IGZO.

Changes in the substructure of tempered martensitic steel during the application of cyclic elastic stress in the presence of hydrogen

Changes in the substructure of tempered martensitic steel during the application of cyclic elastic stress in the presence of hydrogen have been investigated. Three types of specimens were prepared. The first type was subjected to cyclic elastic stress (strain rate: 3.0×10-4 s−1, number of cycles: 70000, and stress level: 0.8±0.1 σB) without hydrogen. The second type was subjected to cyclic elastic stress with hydrogen. The third type was prepared by annealing the second type at 200 °C for 2 h. These specimens are denoted here as [(n)-cycled], [H+(n)-cycled] and [H+(n)-cycled+annealed] specimens, respectively. Tracer hydrogen was used as a probe for clarifying changes in the specimen substructure. After tracer hydrogen charging, hydrogen states present in the specimens were examined using low-temperature thermal desorption spectrometry (L-TDS). Tracer hydrogen desorption in the high-temperature range from the [H+(n)-cycled] specimen increased compared with that from the [(n)-cycled] specimen. In addition, tracer hydrogen desorption in the high-temperature range from the [H+(n)-cycled+annealed] specimen was reduced by annealing at 200 °Cto the level of the [(n)-cycled] specimen. As a result, it is inferred that the formation of vacancy type defects was enhanced under the application of cyclic elastic stress in the presence of hydrogen.

First Principles Study of Tritium Diffusion in Li₂TiO₃ Crystal with Lithium Vacancy.

Li2TiO3 is one of the most significant breeder materials and has potential applications in future fusion reactors. Defect models with three types of lithium vacancies were considered to study the diffusion behavior of tritium in Li2TiO3 by the density functional theory calculations. The possible tritium adsorption sites inside the lithium vacancy were examined and analyzed. The energy barrier of all diffusion paths between different adsorption sites was calculated and the minimum energy barrier is about 0.45 eV, which indicates that the tritium atom diffuses freely inside the lithium vacancy; when a tritium diffuses across the crystal in the typical three directions, our results reveal that the tritium atom prefers to move along the [010] direction. Furthermore, we found that the minimum energy barrier for the tritium atom to escape the trap of Li vacancy is 0.76 eV. After the tritium jumping out of the Li vacancy, the minimum energy barrier is 0.5 eV for the tritium atom diffusing in the crystal. Therefore, we predict that tritium can easily escape from the trap of the Li vacancy and then diffuse across the crystal. Such results are beneficial to the tritium release process in Li2TiO3 and could provide theoretical guidance for the future applications of the Li2TiO3 materials.

Size matching effect between anion vacancies and halide ions in passive film breakdown on copper

Different types of anion vacancies were optimized in passive films on copper via first-principles calculations based on density functional theory, and numerous potentiodynamic polarizations were performed to evaluate the size matching effect between anion vacancies and halide ions in passive film breakdown on copper. The size of the anion vacancies decreased to some extent compared with the corresponding atom due to the surface relaxation effect, and electrochemical experiments showed that the pitting potential for an oxide passive film on copper followed the order F− < Cl− < Br−, while this order was Cl− < F− < Br− for a sulfide passive film, which was attributed to the integrated effect of the energy of anion vacancy expansion and the Gibbs energy of dehydration. The pitting potential orders were the same on pre-oxidized or pre-sulfided copper with that of fresh copper in the same solution. Meanwhile, iodide ions were not compared due to the formation of poorly soluble copper iodides. The ratio of pit depth to pit mouth diameter on copper decreased with increasing pit volume, resulting in shallow pits with large mouth; thus, the catalytic-occluded cell effect was not obvious for copper.

Effects of vacancies and divacancies on the failure of C3N nanosheets

Two-dimensional materials, and especially carbon-based 2D materials, have gained a special reputation and status in many engineering fields and applications recently; because they can be engineered by various methods to precisely yield the properties desired by the designers. One of the most important techniques used for engineering these structures is to create nanopores in them. The varied effects of nanopores in graphene sheets and some other 2D materials have already been studied. In this work, for the first time, the effects of nanopores on the mechanical properties of C3N sheets have been evaluated. Using the molecular dynamics method, various types of achievable vacancy and divacancy pores in C3N sheets have been specified, and the formation energy for each type has been calculated. The formation energy for a vacancy formed by removing a carbon atom or a nitrogen atom is 2.63 eV or 1.31 eV, respectively. In comparing the formation energies for different types of vacancies and divacancies, it was observed that the removal of carbon atom in the structure of C3N requires much more energy, and that the highest amount of energy is needed for removing two carbon atoms in two types of divacancies. Since graphene sheets with a large number of vacancies do not fail easily and remain intact, these sheets can be considered as suitable candidates for ‘straintronic’ applications. By studying the effect of pore density on the mechanical properties of C3N sheets it was observed that at pore densities lower than 5%, C3N sheets exhibit better mechanical properties than graphene, and thus, they can be a better option for straintronic applications.

Morphology evolution and defect distribution in irradiated graphite from molecular dynamics

Molecular dynamics is carried out to study the morphology evolution and point defect distribution in irradiated graphite. Collision cascade processes are simulated in a wide PKA energy range (0.5–2.5 keV) by using the AIREBO potential. The formation process of a hillock (bump) defect that frequently observed in radiation experiments is repeated at atomic level and two formation mechanisms are proposed qualitatively. One light spot of this work is the quantitative analysis of defect distribution involving total vacancy defects and classified vacancy types (monovacancy, divacancy and vacancy loop). Our data is in good agreement with other theoretical calculations and the broadly accepted Norgett-Robinson-Torrens model. An evidence of the transform between different vacancy types is provided from a statistical view, which can be illustrated that monovacancy tends to form vacancy loops by transition of divacancy. The simulations help to provide a deep insight into the cascade procedure in graphite and associate surface features with the generation of defects due to the impact of energetic particles.

Internal defect structure of calcium doped magnesium oxide nanoparticles studied by positron annihilation spectroscopy

Calcium-ion substitution to different concentrations in the normally insulating but wide band gap semiconductor magnesium oxide nanocrystallites was successfully achieved through a sol-gel route and the synthesized samples were characterized by X-ray diffraction and transmission electron microscopy. The substitution of Mg2+ by Ca2+ gave rise to distinct variation in the nanocrystallite sizes and lattice constants. The band gap energies (Eg = 5.04 to 5.50 eV) obtained from ultra-violet and visible absorption spectroscopy hinted to the formation of Positronium Hydrogen (Ps-H) complex within nanocrystallites. Excitonic energy levels reduced the band gap after doping. In the samples of larger nanocrystallites, strong evidences to the presence of very large vacancy clusters were observed. The coincidence Doppler broadening measurements also indirectly supported this interpretation. The lineshape and wings parameters also supported the type of defects as evidenced from lifetime measurements. The results appeared significant from the context of choosing doping as an efficient way of molding the properties of semiconductor oxide nanocrystals as concomitant lattice parameter variations and the evolutions of vacancy type defects can significantly alter the expected physical changes towards other directions.

Mechanical properties of pristine and defective carbon–phosphide monolayers: a density functional tight-binding study

Using density functional tight-binding theory, we investigated the elastic properties and deformation and failure behaviors of pristine and defective carbon–phosphide (CP) monolayers subjected to uniform uniaxial tensile strain along arm-chair (AC) and zig-zag (ZZ) directions. Two variants of CP (α-CP and β-CP) were studied and two types of carbon and phosphorous vacancies (single and double) were considered. It was found that carbon monovacancies have the lowest formation energy, while phosphorous divacancies have the highest one in both CP allotropes. A strong mechanical anisotropy for CP was found with the Young’s modulus and the failure stress along ZZ direction being an order of magnitude larger than those along AC direction. In both allotropes, the Young’s modulus, failure stress and strain are considerably affected by vacancies, especially along AC direction. Fracture of pristine CP monolayer occurred via the rupture of phosphorous–phosphorous bonds when CP monolayer is stretched along AC direction, while via the rupture of carbon–phosphorous bonds when stretched along ZZ direction. Defective α-CP and β-CP monolayers both undergo a brittle-like failure initiated around the hosted vacancies at a lower critical strain. The failure strain and stress along the AC direction are affected only by phosphorous vacancies, while along the ZZ direction, they are almost equally affected by both phosphorous and carbon vacancies. These understandings may provide useful guidelines for potential applications of CP monolayers in nanoelectromechanical systems.

Theoretical calculation and experimental study of the electronic structure and catalysis of defected semiconductor

TiO2 photocatalysts with different types of oxygen vacancies were synthesized via a hydrolysis-precipitation method. The oxygen vacancies types, the charge carriers separation and the band structures of the photocatalysts were investigated by theoretical calculations and experimental characterizations. It is revealed that the energy level of oxygen vancancies with two trapped electrons is higher than that with one trapped electrons; the oxygen vancancies could act as charge carriers recombination centers. Therefore, TiO2 with two-electrons-trapped oxygen vancancies show enhanced visible-light photocatalytic activity, compared with the counterparts with one-electron-trapped oxygen vancancies. The photocatalyst with the lowest oxygen vacancies content shows the best photocatalytic activity under both UV and visible-light irradiation.

Experimental and theoretical approaches for the investigation of proton conductive characteristics of La1-xBaxYbO3-δ

The proton conductive characteristics of La1-xBaxYbO3-δ (x = 0.02, 0.06, 0.10) were examined by an impedance analysis in the temperature range of 873–1473 K. The maximum proton concentrations in La1-xBaxYbO3-δ determined by the weight changes were almost 1/5 of the oxygen vacancies formed by the substitution of Ba. This result indicated that at least two types of oxygen vacancies, a hydratable oxygen vacancy and hydration independent one, exist in the crystal. The bond distances and electronic structure of the Ba-substituted LaYbO3 supercell with or without an oxygen vacancy were investigated by the first-principles molecular dynamics calculations. The substitution of Ba for the La-site of LaYbO3 was found to induce the local structure change of oxygen atoms surrounding the La atom.

Influences of vacancy and doping on electronic and magnetic properties of monolayer SnS

Based on the first-principles calculations, we investigate the structural, electronic, and magnetic properties of defects in monolayer SnS. We study the formation and migration of vacancies at both Sn- and S-sites. In comparison to the S-site vacancy, our calculations show that creating a vacancy at the Sn-site requires lesser energy, indicating that the vacancy at the Sn-site is more likely to be formed in experiments with energetic particle irradiation. For the Sn-rich (S-rich) environment, the vacancy at the S-site (Sn-site) is more likely to be found than the vacancy at the Sn-site (S-site). Reducing the formation of vacancy clusters, the S vacancy remains at the position where it is created because of the high vacancy migration barrier. Both types of vacancies remain nonmagnetic. To induce magnetism in monolayer SnS, we also study the transition metal (TM = Mn, Fe, and Co) doping at the Sn-site and find a significant influence on the electronic and magnetic properties of monolayer SnS. The doping of TM changes non-magnetic monolayer SnS to magnetic one and keeps it semiconducting. Additionally, long-range ferromagnetic behavior is observed for all the doped system. Hence, doping TM atoms in monolayer SnS could be promising to realize a two-dimensional diluted magnetic semiconductor. More interestingly, all the doped TM configurations show a high spin state, which can be used in nanoscale spintronic applications such as spin-filtering devices.

Smeared Lattice Model as a Framework for Order to Disorder Transitions in 2D Systems

Order to disorder transitions are important for two-dimensional (2D) objects such as oxide films with cellular porous structure, honeycomb, graphene, Bénard cells in liquid, and artificial systems consisting of colloid particles on a plane. For instance, solid films of porous alumina represent almost regular crystalline structure. We show that in this case, the radial distribution function is well described by the smeared hexagonal lattice of the two-dimensional ideal crystal by inserting some amount of defects into the lattice.Another example is a system of hard disks in a plane, which illustrates order to disorder transitions. It is shown that the coincidence with the distribution function obtained by the solution of the Percus–Yevick equation is achieved by the smoothing of the square lattice and injecting the defects of the vacancy type into it. However, better approximation is reached when the lattice is a result of a mixture of the smoothed square and hexagonal lattices. Impurity of the hexagonal lattice is considerable at short distances. Dependencies of the lattice constants, smoothing widths, and contributions of the different type of the lattices on the filling parameter are found. The transition to order looks to be an increase of the hexagonal lattice fraction in the superposition of hexagonal and square lattices and a decrease of their smearing.

Defects-driven magnetism in bulk [formula omitted]-Li3N

Ab initio calculations based on density functional theory with local spin density approximation are used to study defects-driven magnetism in bulk α-Li3N. Our calculations show that bulk Li3N is a non-magnetic semiconductor. Two types of Li vacancies (Li-I and Li-II) are considered, and Li-vacancies (either Li-I or Li-II type) can induce magnetism in Li3N with a total magnetic moment of 1.0 μB which arises mainly due to partially occupied N-p-orbitals around the Li vacancies. The defect formation energies dictate that Li-II vacancy, which is in the Li2N plane, is thermodynamically more stable as compared with Li-I vacancy. The electronic structures of Li-vacancies show half-metallic behavior. On the other hand N-vacancy does not induce magnetism and has a larger formation energy than Li-vacancies. N vacancy derived bands at the Fermi energy are mainly contributed by the Li atoms. Carbon is also doped at Li-I and Li-II sites, and it is expected that doping C at Li-I site is thermodynamically more stable as compared with Li-II site. Carbon can induce metallicity with zero magnetic moment when doped at Li-I site, whereas magnetism is observed when Li-II site is occupied by the C impurity atom and C-driven magnetism is spread over the N atoms as well. Carbon can also induce half-metallic magnetism when doped at N site in Li3N, and has a smaller defect formation energy as compared with Li-II site doping. The ferromagnetic (FM) and antiferromagnetic (AFM) coupling between the C atoms is also investigated, and we conclude that FM state is more stable than the AFM state.

The effect of phason defects on the radiation-induced swelling of quasicrystalline materials

ABSTRACTThe radiation-induced vacancy swelling of quasicrystals is considered. In quasicrystals, the dislocation kinetics involves the formation of phasons: vacancy- and interstitial-like localized topological defects. Phasons interact with radiation-induced point defects (PD; vacancies and interstitials) and influence the swelling behavior of quasicrystals. After absorption of vacancies, phasons of the interstitial type transform into phasons of the vacancy type and vice versa. In other words, phasons are the recombination centers of alternating polarity for PD. Assuming that (i) the production rate of phasons is proportional to the dislocation climbing rate and (ii) voids are the sinks for mobile phasons, the set of rate equations for PD and phasons is formulated and analyzed both analytically and numerically. It is shown that the swelling rate is lower in quasicrystals compared with crystals. Growth rates of loops and voids, as well as the swelling rate, strongly depend on the phason concentration which is controlled by phason diffusion to sinks.

Single-atom vacancy in monolayer phosphorene: A comprehensive study of stability and magnetism under applied strain

Using first-principles calculations based on density-functional theory we systematically investigate the effect of applied strain on the stability and on the electronic and magnetic properties of monolayer phosphorene with single-atom vacancy. We consider two types of single vacancies: the symmetric SV-55|66, which has a metallic and non-magnetic ground state, and the asymmetric SV-5|9, which is energetically more favorable and exhibits a semiconducting and magnetic character. Our results show that compressive strain up to 10%, both biaxial and uniaxial along the zigzag direction, reduces the formation energy of both single-atom vacancies with respect to the pristine configuration and can stabilize these defects in phosphorene. We found that the magnetic moment of the SV-5|9 system is robust under uniaxial strain in the range of −10 to +10%, and it is only destroyed under biaxial compressive strain larger than 8%, when the system also suffers a semiconductor-to-metal transition. Additionally, we found that magnetism can be induced in the SV-55|66 system under uniaxial compressive strain larger than 4% along the zigzag direction and under biaxial tensile strain larger than 6%. Our findings of small formation energies and non-zero magnetic moments for both SV-5|9 and SV-55|66 systems under zigzag uniaxial compressive strain larger than 4% strongly suggest that a magnetic configuration in monolayer phosphorene can be easily realized by single-vacancy formation under uniaxial compressive strain.

Education outcomes and the labor market

The quality of education appears to be negatively correlated with both the overeducation of workers at the tasks they perform and the unemployment rate across EU-15 countries, and positively correlated with the wage premium associated to tertiary education. We develop a model of the labor market with frictions to quantitatively investigate the impact of the education outcomes on the labor market. We show that both the ability of educated andnon educated workers have sizable effects on the incentives of firms regarding the type of vacancies they open and also regarding the incentives of educated workers as of where to search for a job. Therefore education outcomes are relevant to understand the overeducation phenomena observed in the labor market. According to our quantitative analysis had the quality of education observed in Spain been similar to the European average then the overeducation rate would have been between 5 and 10 percentage points lower and the unemployment rate of the two types of workers would be reduced by 40%, but the tertiary education wage premium would be slightly smaller than in the benchmark economy.

Formation of modulated structures induced by oxygen vacancies in α-Fe2O3 nanowires

Two modulated structures caused by long-range ordering of oxygen vacancies have been observed in α-Fe2O3 nanowires (NWs) produced after oxidation of Fe, one being ten times (303¯0) interplanar spacing and the other being six times (112¯0) interplanar spacing. Both types of oxygen vacancy ordering structures have a similar modulation periodicity of 1.45 or 1.50 nm with corresponding atomic ratios of Fe and O (Fe/O) of 0.7407 and 0.7273, respectively. The Fe/O ratios in the NWs with oxygen-vacancy ordering are very close to that of Fe3O4 (0.7500). The similar Fe/O ratio between NWs and Fe3O4 may explain the similar modulation periodicity of different oxygen-vacancy orderings. Electron energy-loss spectroscopy studies show that the Fe/O ratio of NWs is close to that of Fe3O4 when oxygen atoms are not sufficient, which makes the NWs energetically favorable. The elucidation of the mechanism governing the formation of the modulated structures in α-Fe2O3 NWs is critical for controlling the microstructure and correspondingly physicochemical properties of NWs.