Vacancy Defects Research Articles

LiGa5O8: Fe3+: A novel and super long near-infrared persistent material

The development of near-infrared persistent materials has become a popular research area owing to their diverse applications. However, the efficient emitters for these materials remains limited. Cr3+ is one of the strongest candidates, since it displays better performance with long duration. Fe3+ is a potential nontoxic alternative although its performance is not as good as Cr3+. In this study, a series of novel Fe3+-activated near-infrared long afterglow phosphors of LiGa5O8 have been prepared. Photoluminescence and persistence luminescence spectroscopy have been collected to investigate its optical properties. Furthermore, the trap characteristics of the samples were systematically analyzed by collecting thermoluminescence signals. The results indicate that there is an intense near-infrared emission centered at 675 nm and a long afterglow duration of more than 28 h for the optimized sample. Trap analysis indicates that oxygen vacancy defects are not the primary reasons for the extraordinarily long afterglow in these samples. Instead, the interaction of other trap types, such as antisite defects, probably contribute to the excellent afterglow properties of the material.

Effect of Vacancy Defects on the Properties of CoS2 and FeS2

Effect of Vacancy Defects on the Properties of CoS2 and FeS2

Comprehensive characterization of octahedral Co3O4 doped with Cu induces oxygen vacancies as a battery-type redox active electrode material for supercapacitors

The urgent need to combat global warming has spurred the exploration of renewable energy solutions, among which the supercapacitor has emerged as a promising contender for efficient energy storage in crucial applications. While battery-type electrode materials (BTMs), notably Co3O4, have shown impressive electrochemical performance in recent years, their inherent limitations, such as low conductivity, restricted surface area, and suboptimal electrochemical activity, pose challenges to their utility. Doping has emerged as a strategic solution to address these issues by modifying the crystal structure, surface morphology, specific surface area, electronic conductivity, and chemical stability of the host material, thereby enhancing its electrochemical performance. This study uses a combustion synthesis method to engineer octahedral Co3O4 with Cu-doping. We obtained three different samples of Co3O4: one undoped Co3O4, doped with 3 % Cu (3%Cu–Co3O4), and 6 % Cu (6%Cu–Co3O4). The addition of Cu not only changed the crystal structure and surface morphology but also impacted the surface area and defects (oxygen vacancies) in Co3O4, affecting its electrochemical performance. This doping method effectively controlled the oxygen vacancy defect content, as verified meticulously by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS). Particularly, the 6%Cu–Co3O4 configuration demonstrated significantly improved specific capacity (155.5 mA h g−1/559.8C g−1 at 1 A g−1) and rate performance (67 %), outperforming both 3%Cu–Co3O4 and undoped Co3O4. Additionally, it exhibited superior cycling stability with 91 % retention of the maximum specific capacity after 5000 cycles. Consequently, this study offers valuable insights into designing defect-engineered battery-type electrode materials for hybrid supercapacitors, which could significantly enrich their efficiency.

Enhancement of nitric oxide sensing performance via oxygen vacancy promotion on strontium-doped LaFeO3 perovskites

The potential of Sr-doped LaFeO3 as a nitrogen oxide–sensing material was studied by experimentation and theory calculation. The test results showed a much higher sensitivity and selectivity for a nitric oxide sensor based on Sr-doped LaFeO3 than on pure LaFeO3. The response, which was increased nearly 0.5-fold compared to pure LaFeO3, was attributed to an enhancement of oxygen vacancy and adsorbed oxygen. Further theoretical analysis indicated that Sr doping resulted in the formation of oxygen vacancy defects and altered the adsorption energy of the material, leading to an increase in chemisorbed oxygen species and intermolecular interactions. This work emphasizes the significance of oxygen vacancy defects in gas sensors and provides a feasible approach for enhancing the properties of gas sensors.

Enhancing the Performance of Perovskite Light-Emitting Diodes via Synergistic Effect of Defect Passivation and Dielectric Screening

HighlightsSynergistic passivation mechanism of the dual functional compound of potassium bromide for metal halide perovskite films, that is through chemical bonding and physical screening towards the various types of defects.Metal-ion additives can simultaneously passivate the defects of halide vacancies with bromine anions and dangling bond defects with metal cations.Achieve the maximum external quantum efficiency of ~21% and maximum luminance of ~60,000 cd m−2.

Properties of radiation-induced point defects in austenitic steels: a molecular dynamics study

Austenitic steels are recognized as excellent structural materials for pressurized water reactors due to their outstanding mechanical properties and radiation resistance. However, compared to the widely studied FeCrNi series of steels, little is known about the radiation resistance of FeCrNiMn steel. In this study, the generation and evolution of radiation-induced defects in FeCrNiMn steel were investigated by molecular dynamics simulations. The results showed that more defect atoms were produced in the thermal spike stage, but fewer defects survived at the end of the cascades in FeCrNiMn compared to pure Fe. Point defect properties were analyzed by molecular statics, and the formation energies of defects in FeCrNiMn were lower than those of pure Fe, while the migration energies were higher. Compared to FeCrNi, FeCrNiMn had smaller migration energies and a larger overlap of vacancy and interstitial migration energies. The low vacancy formation energies and widely overlapping migration energies suggested that the number of point defects in the thermal spike stage was higher, but the possibility of recombination was greater. Additionally, Mn exhibited the smallest interstitial formation energies and migration energies. The difference in defect migration energies revealed that vacancy and interstitial defects migrate through different alloy constituent elements. This study revealed the underlying mechanism for the excellent irradiation resistance of FeCrNiMn.

Exploring annealing-triggered phase transformation, photoluminescence quenching, and synergistic effects of Co-doped TiO2 nanoparticles

This study investigates the influence of defects on the electronic, optical, and magnetic properties of tetragonal titanium dioxide (TiO2) and its cobalt-doped variant, Ti0.95Co0.05O2. The research findings of diffraction studies confirms the formation of phases such as anatase, rutile, and their combination within TiO2 nano-structures following annealing. The Debye-Scherrer's rings observed in the selected area electron diffraction patterns and transmission electron microscopy images showed an aggregate of randomly oriented nano-structures. Additionally, elemental analysis confirms the presence of Ti and O as primary constituents. The detailed analysis using microscopy and spectroscopy study identified a high-spin D2h state in Co2+, indicative of lattice defects across the TiO2 nano-structures. A blue shifting in the photoluminescence band gap emission peaks suggested the presence of oxygen vacancy defects in the doped nano-structures. Notably, the increased surface defects and reduced catalyst size promote rapid migration of electron-hole pairs to reaction sites i.e.; on the surface, thereby reducing recombination rates. Ultimately, the study concludes that anatase TiO2, due to its exceptional catalytic performance, is more favourable for applications in photo-catalysis and spintronics than its rutile or mixed-phase counterparts.

Vacancy defect activation spin magnetic effect of Ni(OH)2 enhanced oxygen catalysis

The oxygen evolution reaction (OER) process involves the magnetic reversal of oxygen-containing intermediates, and the influence of defect structures on the magnetic behavior of catalysts might play a crucial role in this process. There is scarce research on the intrinsic relationship among defects, magnetism and catalysis. Herein, cation vacancy β-Ni(OH)2 nanosheets were successfully prepared through alkaline selective etching. To further analyze their spin-magnetic behavior, we found that the presence of defects caused the transformation of Ni(OH)2 from antiferromagnetic to ferromagnetic. Furthermore, to further elucidate their intrinsic spin-magnetic effects, it was observed that the initially antiferromagnetic β-Ni(OH)2 exhibited almost no spin-magnetic response upon the introduction of a magnetic field, with the overpotential almost unchanged. Conversely, introducing defect structures showed a noticeable spin-magnetic effect, reducing the overpotential by 20 mV at 20 mA cm−2. The combination of defects and magnetic fields provides new principles for developing high-performance catalysts and understanding catalytic mechanisms at the spin-electronic level.

Hybrid mode for absorption enhancement in the Ga2O3 nanocavity photodetector with grating electrodes

Gallium oxide (Ga2O3) photodetectors have drawn increased interest for their widespread applications ranging from military to civil. Due to the inherent oxygen vacancy defects, they seriously suffer from trade-offs that make them incompetent for high-responsivity, quick-response detection. Herein, a Ga2O3 nanocavity photodetector assisted with grating electrodes is designed to break the constraint. The proposed structure supports both the plasmonic mode and the Fabry–Perot (F-P) mode. Numerical calculations show that the absorption of 99.8% is realized for ultra-thin Ga2O3 (30 nm), corresponding to a responsivity of 12.35 A/W. Benefiting from optical mechanisms, the external quantum efficiency (EQE) reaches 6040%, which is 466 times higher than that of bare Ga2O3 film. Furthermore, the proposed photodetector achieves a polarization-dependent dichroism ratio of 9.1, enabling polarization photodetection. The grating electrodes also effectively reduce the transit time of the photo-generated carriers. Our work provides a sophisticated platform for developing high-performance Ga2O3 photodetectors with the advantages of simplified fabrication processes and multidimensional detection.

Ce-4f as an electron-modulation reservoir weakening Fe-O bond to induce iron vacancies in CeFevNi hydroxide for enhancing oxygen evolution reaction

Designing novel rare-earth-transition metal composites is at the forefront of electrocatalyst research. However, the modulation of transition metal electronic structures by rare earths to induce vacancy defects and enhance electrochemical performance has rarely been reported. In this study, we systematically investigate the mechanism by which Ce-4f electron modulation weakens the Fe-O bond, thereby altering the electronic structure in CeFevNi hydroxide to improve oxygen evolution reaction (OER) performance. Theoretical calculations and experimental characterizations reveal that Ce-4f orbitals function as electron-modulation reservoirs, capable not only of retaining or donating electrons but also of influencing the material's electronic structure. Moreover, Ce-4f bands optimize the Fe lower Hubbard bands (LHB) and O-2p bands, leading to weakened Fe-O bonds and the formation of cationic vacancies. This change results in the upshift of the d-band center at the active sites, favoring the reaction energy barrier for oxygen intermediates in the OER process. The synthesized catalyst demonstrated an overpotential of 201 mV at 10 mA cm−2 and a lifetime exceeding 200 h at 100 mA cm−2 under alkaline conditions. This work offers a proof-of-concept for the application of the mechanism of rare earth-induced transition metal vacancy defects, providing a general guideline for the design and development of novel highly efficient catalysts.

Effect of temperature on polaronic transport in CeO2 thin-film.

The outstanding catalytic property of cerium oxide (CeO2) strongly depends on the polaron formation due to the oxygen vacancy (V̈O) defect and Ce4+ to Ce3+ transformation. Temperature plays an important role in the case of polaron generation in CeO2 and highly influences its electrical transport properties. Therefore, a much needed attention is required for detailed understanding of the effect of temperature on polaron formation and oxygen vacancy migration to get an idea about the improvement in the redox property of ceria. In this work, we have probed the generation of polarons in CeO2 thin-film deposited on a silicon (Si) substrate using the resonance photoemission spectroscopy (RPES) study. The RPES data show an increase in polaron density at the substrate-film interface of the thermally annealed film, indicating the formation of an interfacial Ce2O3 layer, which is, indeed, a phase change from the cubic to hexagonal structure. This leads to a modified electronic band structure, which has an impact on the capacitance-voltage (C-V) characteristics. This result nicely correlates the microscopic property of polarons and the macroscopic transport property of ceria.

Preparation of SnS2/MoS2 with p–n heterojunction for NO2 sensing

Conventional metal sulfide (SnS2) gas-sensitive sensing materials still have insufficient surface area and slow response/recovery times. To increase its gas-sensing performance, MoS2 nanoflower was produced hydrothermally and mechanically combined with SnS2 nanoplate. Extensive characterization results show that MoS2 was effectively integrated into SnS2. Four different concentrations of SnS2–MoS2 composites were evaluated for their NO2 gas sensitization capabilities. Among them, SnS2–15% MoS2 at 170 °C demonstrated the greatest response values to NO2, 7.3 for 1 ppm NO2, which is about three times greater than the SnS2 sensor at 170 °C (2.58). The creation of pn junctions following compositing with SnS2 was determined to be the primary reason for the composite’s faster recovery time, while the heterojunction allowed for the rapid separation of hole–electron pairs. Because the MoS2 surface has multiple vacancy defects, the adsorption energy of these vacancies is significantly higher than that of other places, resulting in increased NO2 adsorption. Furthermore, MoS2 can serve as active adsorption sites for SnS2 micrometer sheets during gas sensing. This study may help to build new NO2 gas sensors.

Molecular insights into strengthening of biomineral apatite with carbon nanofillers: A simulation study

Biomineral apatite (Ap) has been widely utilized in clinical bone graft procedures for over 25 years. However, its lower tensile strength and fracture toughness compared to natural bone limit its application in large load-bearing systems. This approach aims to enhance the mechanical performance of Ap while preserving its bioactivity, thereby expanding its potential clinical applications. In this study, the creation and processing of carbon nanofiller-based Ap nanocomposites, including pristine and vacancy-defective carbon nanofillers, are investigated using molecular dynamics simulations. The mechanical behaviour of Ap nanocomposites reinforced with three types of carbon nanofillers (CNF) is examined: (i) three-dimensional metallic carbon nanostructure with interlocking hexagons T6B (beam-like), (ii) carbon nanotubes (CNT), and (iii) three-dimensional metallic carbon nanostructure with interlocking hexagons T6P (plate-like). The analysis encompasses detailing the methodology utilized for modelling the equilibrated structures of CNF/Ap nanocomposites, along with the characterization of their elastic parameters. Computational findings reveal the significant impact of CNT chirality, Aspect Ratio (AR) of T6B and orientation of T6P on the mechanical properties of the nanocomposites and observed inclusion of CNF enhances the mechanical properties compared with those of pure Ap. The findings indicate that the lower diameter CNT and smaller cross-sectional area of T6B exhibit higher tensile strength compared to the larger CNT diameter and larger cross-sectional area of T6B of the same length. Further, the analysis highlights the substantial influence of vacancy defects concentration on CNF on the mechanical behaviour of nanocomposites, with an increase in defects correlating to a decrease in the mechanical strength of both CNF and CNF-based nanocomposites.

Preparation of high-entropy nitride ceramics (TiVCrNbZr1-x)Ny by introducing nitrogen vacancies

ABSTRACT High-entropy ceramics have emerged as a research hotspot in the field of ceramics due to their single-crystal structure and excellent physicochemical properties. In this paper, an innovative approach is adopted to synthesize high-entropy nitride ceramics (TiVCrNbZr1-x)Ny. To achieve this, the non-stoichiometric compound TiN0.3 was introduced as a sintering additive and the Spark Plasma Sintering (SPS) technique was employed. By adding TiN0.3, successfully preparing high-entropy nitride ceramics at 1500°C, which introduces vacancy defects that provide additional space for the diffusion and migration of atoms, allowing for more efficient migration and diffusion within the crystal structure, thereby accelerating the sintering process. The (TiVCrNbZr1-x)Ny- xZrO2 samples achieved outstanding mechanical properties when sintered at 1700°C, owing to the mechanisms of solid solution strengthening and second-phase diffusion toughening. Specifically, the samples exhibited a hardness of 25.42 ± 1.58 GPa, a flexural strength of 888 ± 32 MPa, and a fracture toughness of 6.23 ± 0.27 MPa·m1/2. This innovative sintering route provides an efficient method for preparing high-entropy nitride ceramics at relatively low temperatures without sacrificing quality. This approach not only enhances production efficiency but also expands the potential practical applications of these ceramics.

Super-defects in superatomic crystals

Superatoms hold great potential for creating novel crystals, yet the effect of resulting superatomic-based defects on the properties remains unclear. Here, we explore the effects induced by superatom-based defects, including typical vacant, substitutional and co-doped defects in the crystal formed directly from the metal superatom W@Cu12. Surprisingly, crystals with these defects retain the inherent properties of the perfect superatomic crystal in filtering light at approximately 27 eV while simultaneously being able to either reduce or broaden the filtered energy range. Moreover, substitutional and co-doped defects enhance the optical reflectivity and absorption below approximately 20 eV, and vacant defects decrease the specific heat capacity and elastic constants. Given that these defects originate from superatoms, they may be called “super-defects”. It is anticipated that this study of super-defects will not only reveal the potential for further tuning the unique properties of superatomic crystals but also facilitate the development of superatomic-based defect engineering.

Effective prediction of SnO2 conduction band edge potential: The key role of surface oxygen vacancies.

Several theoretical studies at different levels of theory have attempted to calculate the absolute position of the SnO2 conduction band, whose knowledge is key for its effective application in optoelectronic devices such us, for example, perovskite solar cells. However, the predicted band edges fall outside the experimentally measured range. In this work, we introduce a computational scheme designed to calculate the conduction band minimum values of SnO2, yielding results aligned with experiments. Our analysis points out the fundamental role of encompassing surface oxygen vacancies to properly describe the electronic profile of this material. We explore the impact of both bridge and in-plane oxygen vacancy defects on the structural and electronic properties of SnO2, explaining from an atomistic perspective the experimental observables. The results underscore the importance of simulating both types of defects to accurately predict SnO2 features and provide new fundamental insights that can guide future studies concerning design and optimization of SnO2-based materials and functional interfaces.

Oxygen vacancy migration and impact on high voltage DC polarization in 0.8BaTiO3–0.2BiZn0.5Ti0.5O3

AbstractElectrical polarization and defect transport are examined in 0.8BaTiO3–0.2BiZn0.5Ti0.5O3, an attractive capacitor material for high power electronics. Oxygen vacancies are suggested to be the majority charge carrier at or below 250°C with a grain conduction hopping activation energy of 0.97 eV and 0.92 eV for thermally stimulated depolarization current (TSDC) and impedance spectroscopy measurements, respectively. At higher temperature, thermally generated electronic conduction with an activation energy of 1.6 eV is dominant. Significant oxygen vacancy concentration is indicated (up to ∼1%) due to cation vacancy formation (i.e., acceptor defects) from observed Bi (and likely Zn) volatility. Oxygen vacancy diffusivity is estimated to be 10−12.8 cm2/s at 250°C. Low diffusivity and high activation energies are indicative of significant defect interactions. Dipolar oxygen vacancy defects are also indicated, with an activation energy of 0.59 eV from TSDC measurements. The large oxygen vacancy content leads to a short lifetime during high voltage (30 kV/cm), high temperature (250°C) direct current (DC) electrical measurements.

Oxygen vacancies promote the activation of O2 in transition metal oxide doped ε-MnO2 for low-temperature CO oxidation

Carbon monoxide (CO), a toxic pollutant usually formed from incomprehensive combustion especially at low temperature, has harmful effects on human health. Therefore, the catalytic oxidation of CO always receives widespread attention. Herein, we reported transition metal (Cu, Co, Ni, Fe) doped ε-MnO2 as CO catalysts using a hydrothermal-deposition method. Physiochemical properties for CO oxidation were meticulously analyzed employing characterization techniques such as SEM, XRD, BET, XPS, EPR, CO-TPR. The doped ε-MnO2 catalyst showed expanded BET surface area, increased oxygen vacancy defects, more active sites, enhanced adsorption, and faster electron transfer. Catalytic activity tests indicated that the ε-MnO2 catalyst doped with transition metals exhibited exceptionally high catalytic activity, with the Cu-doped catalyst showing the fastest chemical reaction rate and the lowest complete conversion reaction temperature (TOF of 9.92 × 10−3 s−1, at 65℃). Density Functional Theory (DFT) calculations indicated that Cu doping more effectively induced oxygen vacancy defects compared to Co, Ni, Fe, and raw ε-MnO2 catalysts. It identifies the CO oxidation reaction and improved the CO adsorption capacity of the catalyst. Furthermore, a novel potential reaction pathway for the M-vK mechanism in transition metal oxides was proposed.

Construction of carbon and nitride double vacancy defects on ultrathin porous g-C3N4 nanosheets assisted by freeze-drying for enhanced photocatalysis

Graphitic carbon nitride based photocatalysts have been found extensive application in the degradation of antibiotics and organic pollutants. However, the insufficient of surface active sites and the low efficiency of photogenerated charge carriers separation hinder the development. Herein, ultrathin porous polymer carbon nitride nanosheets (PCNS-F) with carbon and nitride double vacancy defects were successfully fabricated. The facile freeze-drying step, following solvent-thermal treatment and preceding the thermal etching process, stands as a critical step for triggering vacancy generation and adjusting the carbon/nitride ratio. The optimized PCNS-F photocatalyst exhibits an ultrathin structure featuring a substantial abundance of defects in a porous state, resulting in exceptional structural characteristics (183.3 m² g-1, 0.84 cm³ g-1). Surface defect sites increase, and the efficient separation and transfer of photo-generated charge carriers are promoted with the introduction of a suitable vacancy density. PCNS-F demonstrated outstanding photocatalytic degradation performances for tetracycline (81.7 %) and methylene blue (97.7 %) under visible light irradiation for 2 h, which are 10 and 7.2 times higher than those of bulk g-C3N4. The active species of ∙O2- was confirmed to play a major role in the photodegradation process. This work provides a mild approach for engineering carbon and nitride double vacancy defects on g-C3N4 nanosheets, accompanied by tuning carbon/nitride ratio, to effectively eliminate pollutants and cleanse the environment.’

Influence of Normal-to-High Anodizing Voltage on AAO Surface Hardness from 1050 Aluminum Alloy in Oxalic Acid.

Anodic aluminum oxide (AAO) has been widely applied for the surface protection of electronic component packaging through a pore-sealing process, with the enhanced hardness value reaching around 400 Vickers hardness (HV). However, the traditional AAO fabrication at 0~10 °C for surface protection takes at least 3-6 h for the reaction or other complicated methods used for the pore-sealing process, including boiling-water sealing, oil sealing, or salt-compound sealing. With the increasing development of nanostructured AAO, there is a growing interest in improving hardness without pore sealing, in order to leverage the characteristics of porous AAO and surface protection properties simultaneously. Here, we investigate the effect of voltage on hardness under the same AAO thickness conditions in oxalic acid at room temperature from a normal level of 40 V to a high level of 100 V and found a positive correlation between surface hardness and voltage. The surface hardness values of AAO formed at 100 V reach about 423 HV without pore sealing in 30 min. By employing a hybrid pulse anodization (HPA) method, we are able to prevent the high-voltage burning effect and complete the anodization process at room temperature. The mechanism behind this can be explained by the porosity and photoluminescence (PL) intensity of AAO. For the same thickness of AAO from 40~100 V, increasing the anodizing voltage decreases both the porosity and PL intensity, indicating a reduction in pores, as well as anion and oxygen vacancy defects, due to rapid AAO growth. This reduction in defects in the AAO film leads to an increase in hardness, allowing us to significantly enhance AAO hardness without a pore-sealing process. This offers an effective hardness enhancement in AAO under economically feasible conditions for the application of hard coatings and protective films.