VO Defects Research Articles

Facile Synthesis of Ni-Doped ZnO Nanoparticles Using Cashew Gum: Investigation of the Structural, Optical, and Photocatalytic Properties

This work adopted a green synthesis route using cashew tree gum as a mediating agent to obtain Ni-doped ZnO nanoparticles through the sol–gel method. Structural analysis confirmed the formation of the hexagonal wurtzite phase and distortions in the crystal lattice due to the inclusion of Ni cations, which increased the average crystallite size from 61.9 nm to 81.6 nm. These distortions resulted in the growth of point defects in the structure, which influenced the samples’ optical properties, causing slight reductions in the band gaps and significant increases in the Urbach energy. The fitting of the photoluminescence spectra confirmed an increase in the concentration of zinc vacancy defects (VZn) and monovacancies (Vo) as Zn cations were replaced by Ni cations in the ZnO structure. The percentage of VZn defects for the pure compound was 11%, increasing to 40% and 47% for the samples doped with 1% and 3% of Ni cations, respectively. In contrast, the highest percentage of VO defects is recorded for the material with the lowest Ni ions concentration, comprising about 60%. The influence of dopant concentration was also reflected in the photocatalytic performance. Among the samples tested, the Zn0.99Ni0.01O compound presented the best result in MB degradation, reaching an efficiency of 98.4%. Thus, the recovered material underwent reuse tests, revealing an efficiency of 98.2% in dye degradation, confirming the stability of the photocatalyst. Furthermore, the use of different inhibitors indicated that •OH radicals are the main ones involved in removing the pollutant. This work is valuable because it presents an ecological synthesis using cashew gum, a natural polysaccharide that has been little explored in the literature.

Heterostructured MoO3@CoWO4 nanobelts towards high electrochemical performances via oxygen vacancies generation

Heterostructured nanomaterials tend to have a high proportion of oxygen vacancies (VO) due to the presence of heterogeneous interfaces. Herein, a new kind of heterostructured MoO3@CoWO4 nanobelts was successfully evaluated as fascinating cathode material. SEM and TEM analysis indicated that the MoO3 nanobelts were fully blanketed with CoWO4 nanodots. The generation of Vo species was confirmed by XPS and EPR data. By profiting both synergistic and Vo generation effects, MoO3@CoWO4 electrode displayed an excellent capacitance of 246 mAh·g−1 (1966 F·g-1at 0.5 A·g−1) with high-rate capability of 174 mAh·g−1 (1394 F·g−1 at 30 A·g−1) as well as superb stability of 94 % (over 15,000 cycles). Notably, all-solid-state device delivered a good energy value of 63.1 Wh·kg−1 at 375 W·kg−1. Interestingly, the supercapacitor device showed super-low self-discharge comportment of only 12.1 % during 24 h. Importantly, the generation of the VO defects could control the ions diffusion process and lead a sharp decrease in the self-discharge process.

Concentration characterization of underlying intrinsic defects accompany with surface structural defects and their effect on laser damage resistance

During ultra-precision processing of potassium dihydrogen phosphate (KDP) crystals, various macro-structural surface defects are inevitably introduced, but there is no monotonic relationship between different macro-defects and laser-induced damage threshold (LIDT). Therefore, a single macro-perspective cannot fundamentally reveal the causation for low LIDT. In this work, energy, photoelectron spectra characterization and photoluminescence characteristics of surface were adopted to determine the type and concentration of intrinsic defects accompany with macro-structural defects. First-principles calculation was applied to explore the influence of oxygen vacancy (VO) defect concentration on performance in terms of crystal structure, electronic structure, and optical properties. The results show that the causation for low LIDT of cracks is VO defects. With the increase of VO defect concentration, PO43- group would collapse, leading to a decrease in the stability of crystal structure. High concentration of VO defects would introduce defective-energy level, causing irreversible alteration in energy level. Free electrons are susceptible to absorb photons and transition due to defective-energy level, severely reducing the laser damage resistance of components. This work reveals the influence of surface defects on laser damage from intrinsic defects at atomic-level, which is significance for the suppression of surface defects during ultra-precision processing and effective improvement of laser damage resistance of KDP.

Engineering intrinsic defects in CuO NWs through laser irradiation: Oxygen vs copper vacancies

Defect engineering in CuO nanomaterials has been extensively investigated because this technology can greatly improve the performance of CuO components in a variety of applications. While many previous studies have focused on the role of oxygen vacancies, (VO), in CuO, little attention has been paid to that of copper vacancy centers, (VCu), since the presence of these defects is often difficult to quantify. As a result, the specific roles played by these different vacancy centers on the measured properties of CuO have not been studied systematically. In this article, we show that the concentration of both VO and VCu centers can be engineered in CuO NWs through nanosecond (ns) laser irradiation. The identification of these vacancies was achieved through X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Absorption spectra and diffuse reflectance spectra of laser-processed samples show that increasing the concentration of VO centers enhances the optical absorption of CuO in the visible region. Data obtained from Mott-Schottky and Nyquist plots, together with measured I-V characteristics, show that laser-induced VCu centers can enhance the carrier concentration in CuO NWs. These data, in addition to O 1s XPS spectra on laser-processed and unprocessed samples as well as cyclic voltammetry, also indicate that laser irradiation can significantly enhance surface adsorption for applications in photo-electrochemistry and electrochemistry. To demonstrate the efficacy of this laser irradiation technique for potential electrochemistry applications, the role of laser-induced VCu and VO defect centers in CuO NWs on the non-enzymatic sensing of glucose has also been investigated. We find that the introduction of VO centers in CuO NWs enhances the current response for glucose sensing while the presence of VCu centers inhibits this response.

Defects Healing of the ZnO Surface by Filling with Au Atom Catalysts for Efficient Photocatalytic H2 Production.

Healed defects on photocatalysts surface and their interaction with plasmonic nanoparticles (NPs) have attracted attention in H2 production process. In this study, surface oxygen vacancy (Vo ) defects are created on ZnO (Vo -ZnO) NPs by directly pyrolyzing zeolitic imidazolate framework. The surface defects on Vo -ZnO provide active sites for the diffusion of single Au atoms and as nucleation sites for the formation of Au NPs by the in situ photodeposition process. The electronically healed surface defects by single Au atoms help in the formation of a heterojunction between the ZnO and plasmonic Au NPs. The formed Au/Vo -Au:ZnO-4 heterojunction prolongs photoelectron lifetimes and increases donor charge density. Therefore, the optimized photocatalysts of Au/Vo -Au:ZnO-4 has 21.28 times higher H2 production rate than the pristine Vo -ZnO under UV-visible light in 0.35m Na2 SO4 and 0.25m Na2 SO3 . However in 0.35m Na2 S and 0.25m Na2 SO3 , the H2 production rate is 25.84 mmole h-1 g-1 . Furthermore, Au/Vo -Au:ZnO-4 shows visible light activity by generating hot carries via induced surface plasmonic effects. It has 48.58 times higher H2 production rate than pristine Vo -ZnO. Therefore, this study infers new insight for defect healing mediated preparation of Au/Vo -Au:ZnO heterojunction for efficient photocatalytic H2 production.

Fabrication and investigation of the electrical performance of nitrogenated bilayer ZnO:N/ZnO thin-film transistors

In this study, a bilayer ZnO:N/ZnO thin-film transistors (TFTs) was designed to improve both the performance and stability of single-layer ZnO TFTs, using the effects of N-treatment on oxygen vacancy (VO) and the interface trap density. Bilayer ZnO:N/ZnO films and related TFTs were fabricated via radiofrequency magnetron sputtering in a mixture of Ar and N2 gas at 150 °C. The electrical performance and bias-stress stability were systematically investigated. The bilayer ZnO:N/ZnO TFT exhibited a linear field-effect mobility of 13.6 cm2/Vs, high current on-to-off ratio (Ion/Ioff) of 1 × 107, small threshold voltage (Vth) of 2.6 V, and low subthreshold swing of 0.32 V/decade compared with single-layer undoped ZnO and ZnO:N TFTs. Moreover, ZnO:N/ZnO TFT exhibited small threshold voltage shifts (∆Vth) of 0.8 and –0.9 V according to positive bias illumination stress and negative bias illumination stress measurements, respectively. X-ray photoelectron spectroscopy analysis showed that the N-treatment passivated the VO defects, reduced the density of interface traps, and controlled the carrier concentration. Thus, the performance and stability of the ZnO:N/ZnO TFTs were improved. Therefore, ZnO:N/ZnO TFTs have good application prospects in transparent thin-film electronics and flat-panel displays.

Improvement of Luminescence and Photocatalytic Performance of ZnO:Eu3+ Nanocrystals Activated by Na+ Ions

Undoped and codoped (Eu/Na) ZnO nanocrystals (NCs) were successfully manufactured through an economical sol-gel method. X-ray diffraction (XRD) analysis demonstrated pure hexagonal wurtzite structure without secondary phases for all the samples. The size of the NCs was found to decrease with codoping by Eu3+/Na+ which is related to the existence of strain and stress in the lattice. The dominance of the E2(high) mode in Raman spectra indicates the good crystallinity of the samples. The study from the X-ray photoelectron spectroscopy (XPS) shows the successful insertion of both Eu3+ and Na+ ions into the ZnO lattice and the generation of the zinc and oxygen vacancies (Vo) defects. The band gap energy was reduced and the Urbach energy increased with Na+ content, proving the distortion of the lattice. From the photoluminescence (PL) study, the activation of the Eu3+ ions by Na+ ones was evidenced. Longer PL lifetimes were obtained from Eu3+ ions when they were sensitized by Na+, which may be beneficial to several applications. A process of excitation transfer from both the ZnO host and Na+ sensitizers to the Eu3+ ions was evidenced and discussed. As an application, we tested the performances of the prepared NCs as photocatalysts for Rhodamine B photodegradation under sunlight irradiation. The ZnO NCs codoped with 1% Eu/4% Na displayed the best photodegradation rate with a good stability and a high kinetic rate constant k of 0.021 min−1. The photocatalytic mechanism is discussed in terms of longer radiative recombination from Eu3+ and the generated oxygen vacancies.

Near-IR unidirectional absorption in a tunable asymmetric one-dimensional photonic crystal with VO2 defect layers

In this paper, a dynamically modulated Near-IR asymmetric composite photonic crystal (PC) is proposed, which constitutes by a one-dimensional PC (1-D PC) with vanadium dioxide (VO2) phase transition defect layers. By combining asymmetric composite PC with VO2 phase material, which will undergo the semiconductor-metal transition (SMT) under thermal stimulation, to realize the controllable unidirectional multi-channel absorber under temperature control. Based on a relatively simple 1-D stacked thin film model, the model is investigated and optimized in terms of the structure, number of periods, and the thickness of defect layers, with the result of 20 nm for VO2 defect layers and seven circles for the post-defect period. By using the pre-defect period number of 3, an average absorbance of 0.19 can be achieved when VO2 is in the semiconductor phase at low temperature. With the rise in temperature, VO2 transitions to metal phase, where the structure absorption reaches 0.99. In addition, changing the per-defect period number to 5, the average absorption at semiconductor and metal VO2 is 0.73 and 0.10, respectively. The differential absorption around the SMT enables the tunability of single photonic devices. During the simulation, the effects of electric field and incidence angle on the structure are also analyzed. Meanwhile, the Bruggeman approximation effective medium theory is introduced in this work, and the changes of the absorption during the phase transition from semiconductor to metal in the VO2 defect layers are also given. These characteristics are applicable to controllable multispectral absorbers, infrared detectors, limiter, and optical switchers.

A step toward correct interpretation of XPS results in metal oxides: A case study aided by first-principles method in ZnO.

Metal oxide semiconductors constitute a vast group of materials whose physical properties are greatly affected by native defects. For decades, x-ray photoelectron spectroscopy (XPS) has been widely used in defect analysis. However, correct interpretation of XPS results remains a difficult task. In this work, we present a detailed first-principles study on the core-level shift of the most stable and commonly cited crystal imperfections in ZnO, including O and -OH species at the surface with different coverages and bulk defects, including O interstitial (Oi), O vacancy in the +2 charge state (Vo2+), and the neutral vacancy (Vo0). The O1s core level spectrum is simulated and compared with experiments to understand the correlation between local atomic structures and features in the O1s spectrum. In particular, our results indicate that the widely adopted assignment in the defect analysis of ZnO, which links the defect peak in XPS to Vo, the most stable defect, is very likely a misinterpretation. Theoretical analysis indicates that there are no distinguishable XPS features arising from the Vo defect. Furthermore, we show that the commonly observed defect-related peak instead arises due to Oi or specific surface configurations. Given the importance of native defects in materials performance, misinterpretation of XPS results may lead to erroneous conclusions regarding materials properties. This work provides a first-principles basis for the analysis of oxide defects through XPS.

Influence of single-ionized oxygen vacancies on the generation of ferromagnetism in SnO2 and SnO2:Cr nanowires

In this work, we report the influence of single-ionized oxygen vacancies (V^{prime}_{{text{O}}}) as a spin ½ system in the ferromagnetic response of undoped and Cr-doped SnO2 nanowires. For this study, undoped and Cr-doped SnO2 nanowires were synthesized by a thermal evaporation method. Raman, Auger, and X-ray photoelectron spectroscopies confirmed the incorporation of Cr3+ ions in the SnO2 lattice. Electron paramagnetic resonance measurements demonstrated the presence of single-ionized oxygen vacancies (V^{prime}_{{text{O}}}) in undoped and Cr-doped nanowires. Complementarily, cathodoluminescence measurements confirmed the presence of VO defects in the samples. Magnetic measurements revealed FM behavior from the undoped SnO2 and Cr-doped SnO2 nanowires, showing magnetization saturation values (MS) of ± 1 × 10–3 and ± 1.6 × 10–3 emu/g, respectively, and magnetic coercivity values (HC) of 180 and 200 Oe. We assign the FM response of nanowires to the presence of single ionized V^{prime}_{{text{O}}} acting as a spin ½ system and to the alignment of magnetic moments of Cr3+ ions, finding that V^{prime}_{{text{O}}} defects dominate in the FM generation.

Effect of Single Spinel Phase Crystallization on Drain-Induced-Barrier-Lowering in Submicron Length IZTO Thin-Film Transistors

This study shows the effect of single spinel phase crystallization on drain-induced barrier lowering (DIBL) of indium-zinc-tin-oxide (IZTO) thin-film transistors (TFTs) with submicron channel length. The 0.9-μm-long amorphous IZTO (a-IZTO) TFT shows a poor DIBL of 318 mV/V. In contrast, a significant improvement in the DIBL is achieved in the single spinel phase IZTO (s-IZTO) TFT, which could be attributed to the suppression of lateral diffusion of oxygen vacancy (VO) and low VO defects through crystallization-induced enforcement of metal-oxygen bonds. Consequently, 0.9-μm-long s-IZTO TFT reveals a small DIBL of 92 mV/V as well as a high field-effect mobility of 90.1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs and a low subthreshold swing of 0.1 V/dec. In addition, reliability against external bias temperature stress is considerably improved through single-phase crystallization, leading to an insignificant threshold voltage shift of +0.4 (–0.4) V under positive (negative) bias stress with electric field of 2 (–2) MV/cm at 60 °C for 10,000 s, respectively, in the 0.9-μm-long s-IZTO TFT.

Enhanced solar-driven water splitting performance using oxygen vacancy rich ZnO photoanodes

This work reports a facile two-step method of producing highly efficient ZnO photoanodes for photoelectrochemical (PEC) water splitting under solar light conditions and describes the role of surface oxygen vacancies (VO) in their enhanced PEC performance. The photoanode fabrication involves post-growth oxidation of a metallic Zn layer, which produces a nanostructured ZnO film consisting of ∼50 nm diameter nanorods containing a high concentration of VO defects. The PEC activity of the ZnO films is investigated by studying water oxidation in an aqueous electrolyte under simulated solar illumination. The relationship of PEC and charge transfer characteristics of the ZnO photoanodes with ionized surface VO defects is established using cathodoluminescence, X-ray photoemission, voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The combined results show that the photoanode fabricated in this work possesses a high surface density of ionized VO states that facilitate the effective transportation of holes for water oxidation. It is found that the photoanode exhibits an exceptional photocurrent density of 1.14 mA/cm2 at 1.23 VRHE, being one of the best performances reported in the literature for ZnO-based photoelectrodes so far. Our results demonstrate a simple, low-cost method for fabricating highly efficient VO rich ZnO-based PEC photoanodes that is suitable for large scale production.

Electric Field Manipulation of Defects and Schottky Barrier Control inside ZnO Nanowires.

We directly measure the three-dimensional movement of intrinsic point defects driven by applied electric fields inside ZnO nano- and micro-wire metal-semiconductor-metal device structures. Using depth- and spatially resolved cathodoluminescence spectroscopy (CLS) in situ to map the spatial distributions of local defect densities with increasing applied bias, we drive the reversible conversion of metal-ZnO contacts from rectifying to Ohmic and back. These results demonstrate how defect movements systematically determine Ohmic and Schottky barriers to ZnO nano- and microwires and how they can account for the widely reported instability in nanowire transport. Exceeding a characteristic threshold voltage, in situ CLS reveals a current-induced thermal runaway that drives the radial diffusion of defects toward the nanowire free surface, causing VO defects to accumulate at the metal-semiconductor interfaces. In situ post- vs pre-breakdown CLS reveal micrometer-scale wire asperities, which X-ray photoelectron spectroscopy (XPS) finds to have highly oxygen-deficient surface layers that can be attributed to the migration of preexisting VO species. These findings show the importance of in-operando intrinsic point-defect migration during nanoscale electric field measurements in general. This work also demonstrates a novel method for ZnO nanowire refinement and processing.

Computational study of native defects and oxygen diffusion in the YTiO3±δ as cathode materials in SOFCs

Solid oxide fuel cells (SOFCs) are a promising technology to produce efficient and sustainable electrical energy as an alternative to those generated by fossil fuels. The perovskites-based ceramics are a viable option for building the cathode of these devices since they have a relatively low cost and remarkable efficiency regarding transport properties. Indeed, perovskites in the presence of dopants or native defects such as oxygen vacancy (VO) play a fundamental role in optimizing the catalysis process of oxygen reduction reaction and enhancing the oxygen ion product diffusion into the cathode of SOFCs. Another native defect found in the perovskites is the interstitial oxygen (Oi) and despite being very common, there are few studies on the effects of this kind of defect on the cathode activities. Based on the Density Functional Theory (DFT) and classical Molecular Dynamics simulations (MD), this work presents a theoretical study of the effects of VO and Oi in YTiO3±δ perovskites. It was observed that the presence of VO and Oi does not significantly change the structural characteristics of the YTiO3±δ. The linear coefficient of thermal expansion (CTE) and the mechanical properties of pristine and defective structures presented satisfactory results and compatibility as compared with the main electrolytes used in SOFCs. From the calculation of the formation energy, Oi−2 is the most stable state in YTiO3. Finally, our findings show a high oxygen self-diffusion coefficient and low activation energy when VO or Oi defects are considered, making the YTiO3±δ a competitive material when compared to other perovskites.

Suppressed polaronic conductivity induced sensor response enhancement in Mo doped V2O5 nanowires

In this paper, we show the direct correlation between the suppression of the polaronic oxygen vacancy defect (Vo) density and gas sensor response of 1 at. % Mo-doped V2O5 nanowires (MVONWs). Doping 1 at. % Mo5+ leads to substitution at the V5+ site in V2O5 nanowires (VONWs) and, therefore, reduction in Vo defects. This, in turn, affects the charge carrier hopping sites and, subsequently, enhances the sensor response at lower temperatures (&amp;lt;320 °C). The Mo5+ dopants lead to the lowering of Fermi energy (EF) toward valence band maxima due to the reduced Vo donor density. The polaron suppression is confirmed with the activation energy of polaron hopping, increasing from 195 to 385 meV in VONWs and MVONWs. As a result, the response to ethanol gas enhanced as the depletion width is widened for the given cross section of the nanowires. This may lead to a large depletion controlled cross-sectional area and, therefore, better sensitivity. At about 350 °C, VONWs show a change in the slope of resistance vs temperature (MIT), which is not observed in the case of MVONWs. This is attributed to the presence of the enhanced non-stoichiometry of V ion resulting in metallic behavior and accompanied by a sudden rise in the sensor response at this temperature. Moreover, the absence of MIT may be attributed to the lack of such a sudden rise in the response in MVONWs.

Library of intrinsic defects in β-Ga2O3: First-principles studies

Gallium sesquioxide (β-Ga2O3) acts as a prime candidate for next-generation power electronics, of which complex defects largely impact its electrical performances and limit its practical applications. Herein, using the first-principles method, various intrinsic defects (63 in total) are systematically investigated, including point defects and complexes, of which properties involve formation energies, transition levels, binding energies, etc. It is demonstrated that most defects are likely easier to form under Ga-rich chemical conditions, in which VO, Gai, GaO, OGa-GaO, Oi-GaO, and GaO-VGa defects are metastable to be probably detected in the experiment. Combing with deep-level transient spectroscopy (DLTS) experiments, it is preferred that OGa-VGaI(4-/6-) are related to the unknown experimental level EC-0.12–0.28 eV; Gai(+3/+1), GaOII(+3/+1) or Oi-GaOIII(+3/+1) are related to the level EC-0.74–0.82 eV; GaOIII(+3/+1), Oi-GaOI(+3/+1), Oi-GaOII(+3/+1) or OGa-GaOII(+2/0) are related to the level EC-0.95–1.1 eV; VOIII(+2/0), GaO-VGaII(+2/0) or OGa-GaOI(+2/0) are associated with level EC-1.2–1.48 eV; VOI(+2/0), OGa-VOI(1-/3-) or OGa-VOII(1-/3-) are associated with level EC-2.0–2.3 eV; OGa-VOII(+3/+2) is associated with level EC-4.37–4.38 eV. Although the uncertainty of the consistency between our theoretical predictions and experiments always remains due to the complex atomic structure and low symmetry, our findings provide a near-complete database in combination with experimental observations to define and identify defects in β-Ga2O3, which has a profound impact on understanding the radiation damage and degradation of β-Ga2O3 devices.

The Influence of Deep-Level Defects With Various Depths in SiO2 on Ionizing Damage

In this article, we analyzed the interaction mechanism between ionization and displacement defects in SiO2 using experimental and simulation techniques. In the experiment, the gate-controlled lateral p-n-p transistors (GLPNPs) were pretreated by He ions irradiation with different energies, which can create additional oxygen vacancy defects (Vo) at various depth ranges in SiO2. The pretreated GLPNPs were irradiated with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> -rays to analyze their response to ionization damage. It can be found that the introduction of additional deep Vo in SiO2 weakens the ionization damage of GLPNPs, and this weakening effect is heavily related to the depth of additional deep Vo. In addition, the TCAD simulation tool was used to investigate the mechanism by which the ionization damage is weakened with additional deep Vo depth. The simulation results show that as the additional deep Vo defect approaches the SiO2–Si interface, the additional deep Vo defect enhances the capture of the holes (or release of electrons), resulting in a decrease in hydrogenated Vo ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {Vo}\mathbf {H}_{\mathbf {2}}^{\mathbf {+}}$ </tex-math></inline-formula> ) concentration, which affects the release of protons and ultimately reduces <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N_{\mathrm {it}}$ </tex-math></inline-formula> . Based on the above mechanism, it would be helpful to accurately evaluate the electrical characteristics degradation of bipolar transistors in complex space environments.

Passivation of oxygen vacancy defects in conductive ZnO nanoparticles via low-temperature annealing in NF3

Oxygen vacancies (VO) often exist in wide-bandgap metal oxide semiconductors (MOSs) as deep-level defects and undermine the reliability of various optoelectronic devices based on MOSs. Conventional methods to passivate VO defects, such as high-temperature annealing or plasma treatment, can compromise device performance. This work demonstrates that passivation of VO defects in aluminum-doped ZnO (AZO) nanoparticles can be realized through low-temperature annealing (350 °C) in an NF3 atmosphere. After the NF3 annealing, the longitudinal optical phonon scattering mode in Raman spectroscopy, the visible light emission intensity in photoluminescence spectroscopy, and the oxygen deficiency-related peak in x-ray photoelectron spectroscopy decrease simultaneously, indicating the passivation of VO defects in AZO nanoparticles. As a result, AZO nanoparticles show higher visible light reflectance and better stability of electrical conductivity owing to the suppression of deep-level light absorption and gas molecule adsorption. This work also offers insights into the passivation mechanism of VO defects in MOSs.

Modulating the resistive switching stability of HfO2-based RRAM through Gd doping engineering: DFT+U.

Oxide-based resistive random access memory (RRAM) is standing out in both non-volatile memory and the emerging field of neuromorphic computing, with the consequence of increasing performance demands. Rare-earth doping is often used as an effective means for performance modulation. In this work, the modulation mechanism of the resistive switching (RS) behaviors in trivalent rare-earth Gd-doped HfO2-based RRAM has been carefully investigated using first-principles calculations. The results of electronic structure analysis show that Gd doping would lead to a change in the local geometry of the m-HfO2 defect system and would enhance the Coulomb interaction between the atoms around Gd and oxygen vacancy (VO), which may be one of the reasons for the enhanced conductivity of the HfO2-based RRAM after Gd doping. Thermodynamic and kinetic study results indicate that there is a strong interaction between Gd and its surrounding VO defects, and this strong interaction would not only attract more oxygen vacancies (VOs) to be generated near the dopant Gd, but also increase the migration energy barrier of the +2 charged VOs around the Gd doping site, thus suppressing the random generation of VO filaments, which leads to a better uniformity of the switching parameters during the RS process and improves the performance stability of the devices. The results of this work will provide new insights into modulating the RS behaviors and improving the device performance of HfO2-based RRAM through doping engineering.

The influence of strontium deficiency on thermodynamics of defect formation, structural stability and electrical transport of SrFe0.5Ta0.5O3-δ-based solid solutions with an excess tantalum content.

The crystalline and electronic band structures, thermodynamic stability, oxygen non-stoichiometry and high-temperature transport properties of perovskite-like solid solutions with a general formula Sr1-yFe0.5-xTa0.5+xO3-δ, where x, y ≥ 0, are thoroughly studied using a combination of experimental and theoretical methods. It is argued that the basic compound SrFe0.5Ta0.5O3-δ possesses an orthorhombic lattice symmetry, while its tantalum-doped derivatives belong to a tetragonal space group. Importantly, the purposeful addition of a certain deficiency in a strontium sublattice is shown to be a valid method for stabilizing the Sr1-yFe0.5-xTa0.5+xO3-δ oxides with an excess tantalum content. Detailed studies of charge states in an iron sublattice suggest the predominance of Fe3+ ions even in tantalum-enriched materials. Also, the band structure calculations support the semiconducting nature of electrical transport with localized n-type conductivity provided by small polarons represented by Fe2+ ions. The overall defect structure of Sr1-yFe0.5-xTa0.5+xO3-δ compounds is proved to heavily rely on oxygen vacancy (VO) formation processes; in turn, the presence of strontium vacancies is shown to be an important factor that can decrease the respective energy penalties to introduce VO defects in the lattice. As a result, the experimentally measured oxygen non-stoichiometry for Sr0.95Fe0.45Ta0.55O3-δ at elevated temperatures appears to be sufficiently enlarged as compared to pristine SrFe0.5Ta0.5O3-δ. Similar to that, the conductive properties of tantalum-enriched phase Sr0.95Fe0.45Ta0.55O3-δ are shown to be improved. On the basis of the obtained results, it is argued that cation non-stoichiometry is a valuable tool for enhancing thermodynamic and transport characteristics of perovskite-like compounds, which are currently viewed as promising materials for high-temperature applications.