Vacancy Model Research Articles

Simulation of defect evolution in tungsten during annealing by developing a vacancy and interstitial-type defect evolution model

In this work, a Vacancy and Interstitial-Type Defect Evolution (VITDE) model is developed to study the annealing and clustering behaviors of the irradiation-induced defect in tungsten (W). The post annealing of W-ion damaging at 300 K (called post-damaging annealing) and simultaneous annealing of W-ion damaging (called dynamic annealing) are simulated. In the post-damaging annealing case, the W is damaged by 20 MeV W-ion irradiation for 1h at 300 K, then the damaged W is annealed for 1h via increasing the temperature to 1000 K. In the dynamic annealing case, the defect evolution under simultaneous damaging and annealing is simulated using the same parameters as Case 1. The simulation results indicate the dynamic annealing case has lower induced defects concentration than the post-irradiation annealing case, which is in qualitative agreement with the experiment. The diffusion of interstitial, vacancy, di-interstitial and di-vacancy, and the cluster properties in the defect annealing are investigated and larger size Vn clusters are observed in dynamic annealing case than in post-damaging annealing case. The dependences of the defect evolution on material temperature (T) and grain size are revealed. By increasing the T, the defect concentration is decreased, which reproduces and explains the experimental result. The Vn trap amount with the reduction of grain size for two cases show a larger difference at T = 400 K, observing the Vn trap amount is unchanged for post-damaging annealing, whereas it is significantly increased by dynamic annealing.

Critical role of lattice vacancies in pressure-induced phase transitions of baroplastic diblock copolymers.

Baroplastic diblock copolymers exhibit order-disorder transitions and melt upon compression at low temperatures, in some cases even at ambient temperatures. Their unique low-temperature processability makes them promising candidates for sustainable polymeric materials. Despite their potential, however, the molecular mechanisms governing the pressure-induced phase transitions of these copolymers remain largely unexplored. This study develops a compressible self-consistent field theory for baroplastic copolymers based on a simple lattice vacancy model that explicitly incorporates voids to account for compressibility. The theory shows that the selective presence of voids in compressible domains stabilizes the ordered phase, while a reduction of voids under compression leads to the order-disorder transition. In addition, this work demonstrates for the first time the critical role of gas absorption rates in each segment in the pressure-induced order-disorder transition of baroplastic diblock copolymers. These findings have significant implications for the rational design of baroplastic polymers with tailored low-temperature processability.

Effect of zirconium content on defect structure and optical uniformity of dysprosium-doped lithium niobate crystals

Four Zr: Dy: LiNbO3 crystals doped with different concentrations of Zr4+ (0, 1, 2, 4, mol%) have been grown by Czochraski technique. The effect of doping concentration of Zr4+ ions on the actual concentration of Zr4+ and Dy3+ ions in doped crystal was studied by the inductively coupled plasma-atomic emission spectrometry (ICP-AES). The results of X-ray diffraction and ultraviolet absorption spectroscopy showed that the Zr: Dy: LiNbO3 crystal still maintained the original lattice structure of the pure lithium niobate crystal, and the doped ions entered the crystal by preferential substitution of Nb Li 4 + , and doped ions entered the crystal When Nb Li 4 + is completely replaced, ions are doped to replace the normal Nb and Li positions into the crystal. The lattice constant with the increase of Zr content shows a trend of first becoming larger and then smaller. Finally, the optical uniformity of Zr4+ doped concentration on Zr: Dy: LiNbO3 crystals was studied by birefringence gradient method. The results showed that with the increase of Zr4+ ion doping concentration, the crystal optical uniformity of Zr: Dy: LiNbO3 gradually increased, and the optical uniformity reached the highest when doped at a concentration of 4 mol%. Based on the Li-site vacancy model, the experimental results are discussed in detail.

Influence of the Cu doping on the physical and H2 gas sensing properties of TiO2

Due to poor selectivity, existing H2 sensors based on pure TiO2 nanostructures do not exhibit good efficiency at low gas concentrations. Therefore, many studies are carried out to improve the gas sensor properties of TiO2s. In this study, the structural and H2 gas sensing parameters of Cu doped TiO2 thin films obtained by using sputtering method were investigated as a function of Cu doping. In the co-sputtering system, direct current (DC) and radio frequency (RF) sources were used to simultaneously deposit of the Ti (fixed at 100 W in DC) and Cu (25, 35 and 45 W in RF). The structural, optical and morphological properties of the films were discussed in the light of XRD, XPS, UV-VIS, SEM and AFM results. XRD results revealed that the produced thin films have anatase TiO2 structure and the crystallite sizes in the most dominant A(110) plane are in the range of 14–18 nm. According to XPS results, copper ratios in Cu25, Cu35 and Cu45 samples were calculated as 0.433%, 1.598% and 2.233%, respectively. It was also found that CuO and Cu2O formations were formed in addition to the anatase phase only in Cu45 sample from these films. It was determined from the 2D and 3D AFM images that the surface roughness values of the films decreased from 63.92 nm to 12.99 nm with increasing Cu doping. In addition to the structural properties of the produced films, hydrogen gas detection performances at 1000 ppm gas concentration and 200 °C temperature were investigated with the current sensitive gas sensing system. The results reveal that Cu doping significantly improves the H2 gas responses and response times of TiO2s. Among the films produced, the highest gas response (2284) and shortest times (τres:128s, τrec:129s) was obtained in Cu45 thin film. This increase in gas responses was explained by the oxygen vacancy model and the results are presented by comparing pristine TiO2 and other 3d transition metal doped TiO2s in the literature.

In-situ investigation of dynamic precipitation in pre-aged Al-Zn-Mg-Cu alloy AA7075

High strength Al-Zn-Mg-Cu alloys rely for their properties on nano-scaled precipitates formed during artificial ageing. Deformation profoundly affects the evolution of these precipitates. This study investigates the effect of warm deformation on precipitation in a pre-aged AA7075 aluminium alloy. Warm deformation was performed using an electro-thermomechanical testing machine for rapid resistance heating. Transmission electron microscopy showed slightly larger, yet evenly distributed, precipitates in the warm deformed sample compared to the thermally treated condition without deformation. Synchrotron small angle X-ray scattering revealed accelerated growth rates of precipitates due to deformation. At low plastic strains, the growth rate increased linearly with applied strain, while at high plastic strains, a slight deviation from linear behaviour was observed. Temperature and strain rate effects were also examined, with a stronger enhancement effect at low temperature and fast strain rate for a given strain. The deformation enhancement effect disappeared rapidly upon finishing plastic deformation (within 400 s for the studied condition). The enhanced growth rate during and after deformation can be reasonably explained by the excess vacancy concentration, indicating the influence of strain-induced excess vacancies. However, the simple excess vacancy model used fails to explain the observed strain rate effect, and possible explanations for this discrepancy are discussed.

Ab initio study of water anchored in graphene pristine and vacancy-type defects.

In this paper, we have addressed two issues that are relevant to the interaction of water in pristine and vacant graphene through first-principles calculations based on the Density Functional Theory (DFT). The results showed that for the interaction of pristine graphene with water, the DOWN configuration (with the hydrogen atoms facing downwards) was the most stable, presenting binding energies in the order of -13.62 kJ/mol at a distance of 2.375 Å in the TOP position. We also evaluated the interaction of water with two vacancy models, removing one carbon atom (Vac-1C) and four atoms (Vac-4C). In the Vac-1C system, the most favourable system was the DOWN configuration, with binding energies ranging from -20.60 kJ/mol to -18.41 kJ/mol in the TOP and UP positions, respectively. A different behaviour was observed for the interaction of water with Vac-4C; regardless of the configuration of the water, it is always more favourable for the interaction to occur through the vacancy centre, with binding energies ​​between -13.28 kJ/mol and -20.49 kJ/mol. Thus, the results presented open perspectives for the technological development of nanomembranes as well as providing a better understanding of the wettability effects of graphene sheets, whether pristine or with defects. We evaluated the interaction of pristine and vacant graphene with the water molecule, through calculations based on Density Functional Theory (DFT); implemented by the SIESTA program. The electronic, energetic, and structural properties were analyzed by solving self-consistent Kohn-Sham equations. In all calculations, a double ζ plus a polarized function (DZP) was used for the numerical baise set. Local Density Approximation (LDA) with the Perdew and Zunger (PZ) parameterisation along with a basis set superposition error (BSSE) correction were used to describe the exchange and correlation potential (Vxc). The water and isolated graphene structures were relaxed until the residual forces were less than 0.05 eV/Å-1 in all atomic coordinates.

Effect of Oxygen Exchange between Two Oxide Layers of a Memristive Bilayer Valence-Change Memory Cell on the Switching Polarity

Valence-change memory (VCM) cells are promising candidates for future nonvolatile memory devices. A special setup of VCM devices consists of bilayer cells where two thin oxide layers are placed in between two metal electrodes. One oxide layer serves as a tunnel barrier, whereas the second oxide layer is a highly doped conductive semiconductor. Experiments show that an exchange of oxygen between the two layers changes the resistance of the cell. However, the exchange process and how it influences the resistance is not well understood yet. With a drift-diffusion model for electrons and oxygen vacancies, we investigate the movement and exchange of oxygen vacancies and their influence on the band structure as well as on the shape of the tunnel barrier. The simulation results show that a high oxygen-vacancy concentration lowers the height of the tunnel barrier; thus it increases the conductivity of the bilayer cell. The effect of the band lowering is stronger in materials with low permittivity. Hence, two different resistance states evolve if there is an exchange of oxygen between the two oxide layers with different permittivities. Thereby, the switching polarity depends on the relation of the permittivities of the two oxide layers. Furthermore, it is revealed that resistance switching can be induced by the movement of vacancies only inside the conductive oxide, without any oxygen exchange between the layers.

Effect of vacancy defects and hydrogen, oxygen and nitrogen atomic occupation on the mechanical and electronic properties of C40-type VSi2

To improve the inherent brittle and mechanical properties of VSi2, the effect of vacancy defects and Hydrogen, Oxygen and Nitrogen atomic occupation on the physical properties of C40-type VSi2 silicide have been explored using first-principles methods. Two vacancy models (V-Va, Si-Va) and six atom occupation models (H-V, H-Si, O-V, O-Si, N-V, N-Si) are selected for assessment. According to the results of thermodynamic parameters, C40-type VSi2 with different models exhibits the structural stability. Due to vacancy defects and atomic occupations, the deformation resistance and elastic hardness of the original C40-VSi2 decreased. B/G (1.845) and Poisson's ratio v (0.27) of H occupation for V-Va site are larger than the critical points. The presence of vacancy defects and atomic occupations causes VSi2 to achieve the brittle-to-ductile transition. The Young's modulus anisotropy of these silicides was described by observing the three-dimensional surface structure and two-dimensional projection maps. The density of state graphs demonstrated that these binary silicides display metallic properties. Moreover, the phase stability of different VSi2 can be further explained by the electronic structure.

Influence of vacancy on the elastic, thermodynamic and electronic properties of LaMgNi4 alloys from first-principles calculations

In this work, the structural stability, mechanical, electronic, and thermodynamic characteristics of LaMgNi4 alloys with different vacancies are discussed in detail. The calculated results of formation enthalpy (ΔH) of LaMgNi4 alloys with Mg-Va, Ni-Va2, La-Va1 and La-Va2 vacancies are −0.294eV/atom, −0.273eV/atom, −0.241eV/atom and −0.241eV/atom, respectively, and the results of phonon dispersion curves show that these vacancy models do not exhibit imaginary frequency in any high symmetry direction, which indicates that these models have stable structure. In addition, different vacancies have different effects on the mechanical properties of LaMgNi4 alloys. For example, the existence of Mg-Va, La-Va1, and La-Va2 vacancies enhances the mechanical properties of LaMgNi4 alloys, while the existence of Ni-Va2 vacancy weakens its mechanical properties. From the perspective of electronic structure, the change of mechanical properties of LaMgNi4 alloys is probably caused by the change of electronic state density at the Fermi level due to the existence of different vacancies. The thermodynamic properties such as the specific heat (Cv) and Debye temperature (ΘD) of the LaMgNi4 alloys with different vacancies are calculated by phonon calculations for wide temperature range from 0 K to 1000 K, and further discussed in detail. Finally, the adsorption of O atom on LaMgNi4 (110) surface and the elastic constants and the elastic modulus of random solid solution alloys of La4Mg4Ni14Cu2 are also investigated.

Application of the convergent education model in the development of a smart learning environment

The article presents a scientific and practical approach to the development and implementation of the concept of convergent learning in the framework of building an open educational platform to manage the process of training specialists. The presented concept of convergence education is based on the integration of educational programs, learning content, competency matrices, methods and technologies for training specialists and personalized learning trajectories in smart learning environment. To implement the model and the convergent approach, it is vital to make use of an information and educational environment with mechanisms to support convergent learning based on regional labor market analysis, actualizing of educational programs and resources, harmonization and synchronization life-cycle models thereof, adaptive customization and personalization of specialist training trajectories. The article presents the developed convergence model of educational programs and employer vacancies as a weighted graph to illustrate the relationship between competence requirement vectors of educational programs and sought-after employment. To isolate the convergence model components and evaluate the convergence degree thereof, a graph clustering algorithm based on the Louvain method has been developed. A method for synthesizing a convergence model of educational content organized as a fuzzy weighted graph has been presented, and sets of topics and sections of electronic learning resources being its vertices. Search and extraction of convergence educational content sections have been implemented by means of an algorithm for extracting fuzzy isomorphic subgraphs, and assessing the convergence degree thereof. The results of the study are used to synthesize an information and educational environment with intelligent mechanisms for supporting learning processes based on an analysis of the requirements of labor markets and actualizing educational content and training programs, as well as adaptive customization and personalization of specialist training trajectories. The architecture of the educational environment includes a set of software and tools for managing the components of the information space of the university with support for mobile, cloud and blended learning technologies.

Oxo‐Bridged Zr Dimers as Well‐defined Models of Oxygen Vacancies on ZrO2

AbstractWhile ZrO2 is known to have a large effect on the activity and selectivity of the Cu/ZrO2 catalyst for methanol synthesis, its role in this process is poorly understood. Surface defects such as oxygen vacancies could play a role in the strong metal‐support interaction (SMSI) between Cu and ZrO2. However, due to the complexity of the surfaces, the exact molecular nature of this interaction is not at present known. Here, we make well‐defined models of both reduced and coordinatively unsaturated surface oxygen vacancies on ZrO2 using the molecular precursor [Cp2ZrCl]2(μ2‐O) (1). Complex 1 can be reduced to form a complex (2) containing one Zr(III) center and a bridging hydride ligand (according to EPR and IR spectroscopy) derived from C−H activation of either thf or the Cp ring. Complex 2 reacts with CO2 to largely produce CO, suggesting that surface defects with similar structures probably do not play a role in the industrial catalyst. Halide abstraction from complex 1 results in the Lewis acidic species 3, which has similar Lewis acid properties to acidic defects on the ZrO2 surface. Similarities of both of these model species to real surface oxygen vacancies and their role in the catalytic reaction are discussed.

Multi-peak emission of In2O3 induced by oxygen vacancy aggregation

Oxygen vacancy is crucial to the optical properties in In2O3, however, the single oxygen vacancy model fails to explain the observed multi-peak emission in the experiment. Herein, we have theoretically investigated the diversity of oxygen vacancy distribution, revealing the relationship between the defect configurations and the optical properties. Combining the first-principles calculations and bayesian regularized artificial neural networks, we demonstrate that the structural stability can be remarkably enhanced by multi-oxygen vacancy aggregation, which will evolve with the defect concentration and temperature. Notably, our results indicate that the single oxygen vacancy will induce the emission peaks centered at 1.35 eV, while multi-peak emission near 2.35 eV will be attributed to the distribution of aggregated double oxygen vacancies. Our findings provide a comprehensive understanding of multi-peak emission observed in In2O3, and the rules of the vacancy distribution may be extended for other metal oxides to modulate the optical properties in practice.

Reply to the Comment on "Understanding the Increasing Trend of Sensor Signal with Decreasing Oxygen Partial Pressure by a Sensing-Reaction Model Based on O2- Species".

In our recent work (ACS Sens.2022, 7, 1095-1104), Mirabella et al. provided comments on our publication, mainly focusing on the controversy between the oxygen vacancy model and the ionosorbed model and the related derivation based on the law of mass action. Herein we explain the correlation between the ionosorption model and the oxygen vacancy model and provide a brief introduction of our view on these two models. Moreover, a more detailed derivation about the law of mass action is provided to explain the relationship between surface electron concentration, oxygen partial pressure, adsorbed oxygen density, and oxygen vacancy density.

The impact of the simultaneous presence of Li and Nb(Ta) vacancies in the defect structure of pure LiBO3 (B=Nb, Ta) on the curie temperature

The lithium niobate (LiNbO3, "LN") and lithium tantalate (LiTaO3, "LT") crystals are piezoelectric materials that exhibit intrinsic or extrinsic defect structures, which immediately impact their optical and electrical properties. In this study, we examine the influence of point defects on the Curie temperature (Tc) for different samples of pure LN and LT, using three vacancy models: the tantalum (niobium) vacancy model, the lithium vacancy model and the mixed lithium and tantalum (niobium) vacancy model. Based on the Safaryan approach, it was found that the calculated values for the tantalum (niobium) vacancy model did not agree with the experimental data. However, the lithium vacancy model and the mixed vacancy model showed good agreement between experimental and theoretical results. In conclusion, the proposed mixed vacancy model provides the best description of the defect structure in LN and LT materials, and the Tc, is influenced by the structural defects present in the LN and LT compounds.

Cation-defect-induced self-reduction towards efficient mechanoluminescence in Mn2+-activated perovskites.

Mechanoluminescent (ML) materials have shown promising prospects for various applications, e.g. in stress sensing, information anti-counterfeiting and bio stress imaging fields. However, the development of trap-controlled ML materials is still limited, because the trap formation mechanism is not always clear. Here, inspired by a defect-induced Mn4+ → Mn2+ self-reduction process in suitable host crystal structures, a cation vacancy model is creatively proposed to determine the potential trap-controlled ML mechanism. Combined with the theoretical prediction and experimental results, both the self-reduction process and ML mechanism are clarified in detail, where the contribution of and defects dominates the ML luminescent process. Electrons/holes are mainly captured by the anionic/cationic defects, followed by the combination of electrons and holes to transfer energy to the Mn2+ 3d states under mechanical stimuli. Based on the multi-mode luminescent features excited by X-ray, 980 nm laser and 254 nm UV lamp, together with the excellent persistent luminescence and ML, a potential application in advanced anti-counterfeiting is demonstrated. These results will deepen the understanding of the defect-controlled ML mechanism, and inspire more defect-engineering strategies to develop more high-performance ML phosphors for practical application.

First-principles calculation of the optical properties of the YBa2Cu3O7-δ oxygen vacancies model.

We used first-principles methods to investigate how oxygen vacancy defects affect the optical properties of YBa2Cu3O7-δ (0 < δ < 1), a high-temperature superconductor with potential applications in optical detectors. We calculated the electronic structure of YBa2Cu3O7-δ with different amounts of oxygen vacancies at three different sites: Cu-O chains, CuO2 planes, and apical oxygens. The formation energy calculations support the formation of oxygen vacancies in the Cu-O chain at higher concentrations of vacancy defects, with a preference for alignment in the same chain. The presence of oxygen vacancies affects the optical absorption peak of YBa2Cu3O7-δ in different ways depending on their location and concentration. The optical absorption peaks in the visible range (1.6-3.2 eV) decrease in intensity and shift towards the infrared spectrum as oxygen vacancies increase. We demonstrate that oxygen vacancies can be used as a powerful tool to manipulate the optical response of YBa2Cu3O7-δ to different wavelengths in optical detector devices.

Molecular traps and exposed Mo sites induced by oxygen knockout enhanced H2S sensing of MoO3 nanoflowers

Limited by the dense arrangement of oxygen atoms in the outer layer, the active Mo sites of MoO3 ultrathin nanoflowers are shielded by oxygen, making it difficult for efficient gas adsorption. Here, oxygen atoms on the surface of MoO3 nanoflowers (MoO3 NFs) were knocked out based on a simple hydrogenation strategy to obtain MoO3 defective nanoflowers (MoO3 DNFs) with abundant molecular traps and exposed Mo sites. Notably, the response of MoO3 DNFs-10 to 2 ppm H2S is 3 times that of MoO3 NFs. Oxygen atom knockout was verified based on Laser Raman Spectroscopy, Photoluminescence (PL), UV–Visible Spectroscopy (UV–Vis), and X-ray photoelectron energy Spectroscopy (XPS), while calculations based on density functional theory (DFT) were used to reveal the sensing enhancement of molecular traps and exposed Mo sites. On the one hand, molecular traps can effectively trap H2S molecules to facilitate further adsorption. On the other hand, exposed Mo sites have higher charge activity and exhibit higher adsorption capacity for H2S gas. This work breaks through the limitations of the traditional oxygen vacancy model and innovatively discusses the synergistic effect of molecular traps and exposed Mo sites on enhanced sensing, which can provide a reference for the gas sensing by metal oxide semiconductors.

Joule heating induced non-melting phase transition and multi-level conductance in MoTe2 based phase change memory

Phase change memory (PCM) is considered as a leading candidate for next generation data storage as well as emerging computing device, but the advancement has been hampered by high switching energy due to the melting process and amorphous relaxation induced large resistance drift. Polymorphic crystal-crystal transition without amorphization in metal dichalcogenides (TMDs) could be employed to solve these issues. Yet, the mechanism is still controversy. A melting-free PCM made of two dimensional (2D) MoTe2, which exhibits unipolar resistive switching (RS) and multi-level states with substantially reduced resistance drift via joule heating, is reported in this work. The device is first prepared based on the temperature dependence of Raman spectrum and electrical transport investigations on MoTe2 films. Significantly improved device performances on energy efficiency, switching speed, and memory window are further achieved by electrode size scaling down, indicating the key role of localized heating. Then, device scale transmission electron microscopy images reveal that the resistive switching stems from the transition between semiconducting 2H phase and metallic 1T′ phase. An entropy induced Te vacancies model is proposed to explain the reversible phase change mechanism in the MoTe2 based device. This study paves the way for further development of PCM based on atomically thin 2D TMDs, aiming for high density storage-class memory and high-precision neuromorphic computing.

Conductometric H2 sensors at room temperature based on palladium nanoparticles on ceria

The rapid detection of hydrogen (H2) at 25 ℃ was of important. Here, A strategy was used to construct a hydroxyl-oxygen vacancy model by functionalizing palladium nanoparticles (Pd NPs) on cubic, rod and spherical CeO2, through a simple reduction process to obtain the Ce4+/Ce3+ transited and the generated of Pd. The results have shown that the 5.0 wt% Pd NPs /CeO2-C sensor exhibited sensitive H2 sensing performance (response value Ra/Rg of 1322 for 1% H2), and fast sensing speed (response time of only 3 s for 1% H2 and 2 s for 3% H2) in the air at 25 °C, the actual detection limit of the instrument is only 50 ppm. Probably due to the reduction of ascorbic acid, the chemisorbed oxygen and Ce3+ were raised by a factor of 2.95 and 1.62. The synergistic effect of the hydroxyl-oxygen vacancy model and Pd presents a new idea for the detection of hydrogen at 25 ℃.

Tuning phononic and electronic contributions of thermoelectric in defected S-shape graphene nanoribbons

Thermoelectrics as a way to use waste heat, is essential in electronic industries, but its low performance at operational temperatures makes it inappropriate in practical applications. Tailoring graphene can change its properties. In this work, we are interested in studying the transport properties of S-shape graphene structures with the single vacancy (SV) and double vacancy (DV) models. The structures are composed of a chiral part, which is an armchair graphene nanoribbon, and two zigzag graphene ribbons. We investigate the changes in the figure of merit by means of the Seebeck coefficient, electronic conductance, and electronic and phononic conductances with the vacancies in different device sizes. The transport properties of the system are studied by using the non-equilibrium Green’s function method, so that the related Hamiltonians (dynamical matrices) are obtained from the tight-binding (force constant) model. The maximum figure of merit (ZT) obtains for the DVs in all lengths. Physical properties of such a system can be tuned by controlling various parameters such as the location and the type of the defects, and the device size. Our findings show that lengthening the structure can reduce phononic contribution, and single vacancies than double vacancies can better distinguish between electronic thermal conductance behavior and electronic conductance one. Namely, vacancy engineering can significantly increase thermoelectric performance. In the large devices, the SVs can increase the ZT up to 2.5 times.