Location Of Oxygen Vacancies Research Articles

Atomic Scale Characterization of Fluxional Cation Behavior on Nanoparticle Surfaces: Probing Oxygen Vacancy Creation/Annihilation at Surface Sites.

Oxygen vacancy creation and annihilation are key processes in nonstoichiometric oxides such as CeO2. The oxygen vacancy creation and annihilation rates on an oxide's surface partly govern its ability to exchange oxygen with the ambient environment, which is critical for a number of applications including energy technologies, environmental pollutant remediation, and chemical synthesis. Experimental methods to probe and correlate local oxygen vacancy reaction rates with atomic-level structural heterogeneities would provide significant information for the rational design and control of surface functionality; however, such methods have been unavailable to date. Here, we characterize picoscale fluxional behavior in cations using time-resolved in situ aberration-corrected transmission electron microscopy to locate atomic-level variations in oxygen vacancy creation and annihilation rates on oxide nanoparticle surfaces. Low coordination number sites such as steps and edges, as well as locally strained sites, exhibited the greatest number of cation displacements, implying enhanced surface oxygen vacancy activity at these sites. The approach has potential applications to a much wider class of materials and catalysis problems involving surface and interfacial transport functionalities.

Drain-Bias Transient Instability of Amorphous Indium–Gallium–Zinc Oxide Thin-Film Transistors

High-voltage amorphous indium–gallium–zinc oxide thin-film transistors can be monolithically integrated into the system-on-chip platform as input–output bridges. However, a transient instability showing substantial ON-current degradation under high drain bias is discovered. This drain-bias transient instability depends on the stress time of less than 10 s and both drain and gate stress voltages. It is attributed to the charge trapping in local oxygen vacancies with shallow energy levels and the migration of oxygen ions near the drain. We found that oxygen vacancies are induced by metal contacts. An elevated-metal structure suppresses the transient instability by separating the metal contact region farther away from the channel compared with the conventional top-contact structure.

Polar Rectification Effect in Electro-Fatigued SrTiO3-Based Junctions.

Rectifying semiconductor junctions are crucial to electronic devices. They convert alternating current into a direct one by allowing unidirectional charge flows. Analogous to the current-flow rectification for itinerary electrons, here, a polar rectification that is based on the localized oxygen vacancies (OVs) in a Ti/fatigued-SrTiO3 (fSTO) Schottky junction is first demonstrated. The fSTO with OVs is produced by an electrodegradation process. The different movabilities of localized OVs and itinerary electrons in the fSTO yield a unidirectional electric polarization at the interface of the junction under the coaction of external and built-in electric fields. Moreover, the fSTO displays a pre-ferroelectric state located between paraelectric and ferroelectric phases. The pre-ferroelectric state has three sub-states and can be easily driven into a ferroelectric state by an external electric field. These observations open up opportunities for potential polar devices and may underpin many useful polar-triggered electronic phenomena.

Ultralow power switching of Ta2O5/AlOX bilayer synergistic resistive random access memory

In this work, an ultra low power switching is achieved in Ta2O5-based resistive random access memory (RRAM) device through inserting AlOx film as tunneling layer. After optimizing the thickness of the AlOX layer, the operating current of the device with 15 nm AlOX is reduced to lower than 100 nA with a switching window about 151, and the ultra-low power consumptions of 586 pW and 40.2 nW are achieved for set and reset process, respectively. According to the local conductive filament formation characterization from high-resolution transmission electron microscope and the calculation from Schottky emission formula, a switching mechanism based on the formation of local oxygen vacancies conductive filament in AlOX layer is proposed to explain the ultralow power switching of the Ta2O5/AlOX bilayer synergistic RRAM device.

Quantitative nanoscale tracking of oxygen vacancy diffusion inside single ceria grains by in situ transmission electron microscopy

Oxygen vacancy formation and migration in ceria is critical to its electrochemical and catalytic properties in systems for chemical and energy transformation, but its quantification is rather challenging especially at atomic-scale because of disordered distribution. Here we report a rational approach to track oxygen vacancy diffusion in single grains of pure and Sm-doped ceria at −20 °C to 160 °C using in situ (scanning) transmission electron microscopy ((S)TEM). To create a gradient in oxygen vacancy concentration, a small region (∼30 nm in diameter) inside a ceria grain is reduced to the C-type CeO1.68 phase by the ionization or radiolysis effect of a high-energy electron beam. The evolution in oxygen vacancy concentration is then mapped through lattice expansion measurement using scanning nano-beam diffraction or 4D STEM at a spatial resolution better than 2 nm; this allows direct determination of local oxygen vacancy diffusion coefficients in a very small domain inside pure and Sm-doped ceria at different temperatures. Further, the activation energies for oxygen transportare determined to be 0.59, 0.66, 1.12, and 1.27 eV for pure CeO2, Ce0.94Sm0.06O1.97, Ce0.89Sm0.11O1.945, and Ce0.8Sm0.2O1.9, respectively, implying that activation energy increases due to impurity scattering. The results are qualitatively supported by density functional theory (DFT) calculations. In addition, our in situ TEM investigation reveals that dislocations impede oxygen vacancy diffusion by absorbing oxygen vacancies from the surrounding areas and pinning them locally. With more oxygen vacancies absorbed, dislocations show extended strain fields with local tensile zone sandwiched between the compressed ones. Therefore, dislocation density should be reduced in order to minimize the resistance to oxygen vacancy diffusion at low temperatures.

Highly stable, solution-processed quaternary oxide thin film-based resistive switching random access memory devices via global and local stoichiometric manipulation strategy

Optimization and performance enhancement of a low-cost, solution-processed InGaZnO (IGZO) resistance random access memory (ReRAM) device using the manipulation of global and local oxygen vacancy (Vo) stoichiometry in metal oxide thin films was demonstrated. Control of the overall Ga composition within the IGZO thin film reduced the excessive formation of oxygen vacancies allowing for a reproducible resistance switching mechanism. Furthermore, sophisticated local control of stoichiometric Vo is achieved using a 5 nm Ni layer at the IGZO interface to serve as an oxygen capturing layer through the formation of NiOx, consequently facilitating the formation of conductive filaments (CFs) and preventing abrupt degradation of device performance. Additionally, reducing the cell dimension of the IGZO-based ReRAMs using a cross-bar electrode structure appeared to drastically improve their performances parameters, including operating voltage and resistance distribution due to the suppression of excessive CFs formation. The optimized ReRAM devices exhibited stable unipolar resistive switching behavior with an endurance of >200 cycles, a retention time of 104 s at 85 °C and an on/off ratio greater than about 102. Therefore, our findings address the demand for low-cost memory devices with high stability and endurance for next-generation data storage technology.

The role of oxygen defects on the electro-chemo-mechanical properties of highly defective gadolinium doped ceria

In light of the recent discovery of giant electrostriction in defective fluorites, here we investigate the interplay between mechanical, electrochemical and electromechanical properties of oxygen defective ceria compositions (Ce1−xGdxO2−δ) as the effect of Gd-doping (x = 0.05–0.3) at low temperatures. Highly dense polycrystalline ceramics are prepared as micron-size grains with a minimized grain boundary extent. Electrochemical ionic migration by impedance spectroscopy reveals that dopant content controls the oxygen vacancies association in the samples. Interestingly, we observe that electromechanical activity is strongly controlled by the local oxygen vacancy configuration rather than on its nominal concentration. The primary creep at room temperature indicates a declining viscoelastic trend with increasing oxygen defects.

Exchange-biased nanocomposite ferromagnetic insulator

Magnetic insulating and exchange bias (EB) are two basic magnetic states with actual and potential applications in spintronic and magnonic devices. However, ferromagnetic insulators (FMIs) are rare, and it is even more challenging to obtain a FMI with large EB. In this paper, high performance of FMIs and EB in $\mathrm{LaMn}{\mathrm{O}}_{3}$:NiO nanocomposite epitaxial thin films is reported. The antiferromagnetic NiO nanoparticles, randomly embedded in a ferromagnetic $\mathrm{LaMn}{\mathrm{O}}_{3}$ matrix, enhance the robustness of the insulating state and result in an EB field as large as 1100 Oe. The simultaneously enhanced resistivity and EB are attributed to localized oxygen vacancy, preserved strain state, and increased ferromagnetic-antiferromagnetic interface ratio. This paper provides not only a material candidate for spintronics but also a referential strategy to design artificial materials with multifunctionality.

Enrichment of optical, electrical, and magnetic properties of Li+, La3+ doped BaTiO3 perovskite multifunctional ceramics

Li and La co-doped Ba(1-x)(Lix/2Lax/2)TiO3 (BLLT) ceramics with x = 0.01, 0.02, 0.03, and 0.04 at A—site was synthesized by sol–gel combustion method. The powder X-ray diffraction and Raman analysis showed a good crystalline nature with perovskite tetragonal structure and the grain size of the samples was estimated and compared using a scanning electron microscope. The decreasing trend in the optical band gap upon doping and carrier concentration values was calculated from UV–Vis absorption spectra. Electron paramagnetic resonance g ~ 1.998 for BLLT ceramics confirms that the electrons are localized near oxygen vacancies. The observed signals may be attributed to the reduction of Ti4+/Ti3+ and its related defects. Moreover, the room temperature magnetization versus magnetic field loops showed the mixed weak ferromagnetic and diamagnetic phase. The enhancement of e′ with respect to the doping of Li and La ion and increase in particle size due to the number of grain boundaries in BLLT ceramics were studied. The polarization versus electric field of BLLT samples indicates the lossy capacitor behavior, which is attributed to the relatively high leakage current caused by the existence of a defect or oxygen vacancies.

Enhancing visible light-activated NO2 sensing properties of Au NPs decorated ZnO nanorods by localized surface plasmon resonance and oxygen vacancies

Increasing light absorption is of crucial importance for optimizing light-activated gas detection. However, the relevant research is still far from sufficient. Herein, a high performance visible light-activated NO2 gas sensor is developed relied on the localized surface plasmon resonance (LSPR) and increased surface oxygen vacancies. Au NPs decorated ZnO nanorod array as sensitive materials was synthesized via a two-step low temperature hydrothermal process. The influences of Au decoration and light wavelength on the sensing behaviors were systematically investigated. It is found that the Au NPs decoration can largely promote the visible light-activated gas sensing properties in comparison with pure ZnO film. In addition, the as-prepared sensors demonstrate excellent repeatability and selectivity as well as moisture stability. Moreover, the sensing mechanism based on LSPR was discussed in detail. This work not only sheds some lights on the fundamental understanding for the LSPR enhanced gas sensing mechanism, but also offers an approach in constructing high-performance light-activated gas sensor.

First principles calculations on order and disorder in La2Ce2O7 and Nd2Ce2O7.

In this paper, we highlight the connection between the local structure and collective dynamics of the defective fluorites La2Ce2O7 and Nd2Ce2O7. The local and average structure is explored by investigating a large number of different structural models and snapshots from Born-Oppenheimer Molecular dynamics calculations. Both compounds show a strong preference for local oxygen vacancy order similar to that found in the C-type structure. This suggests that previous studies, where Nd2Ce2O7 and La2Ce2O7 are viewed as disordered defective fluorites, or as a pyrochlore for the latter, did not capture the nature of local order in the disordered phase. We observe more collective chains of migrating oxygen in Nd2Ce2O7- a manifestation of a stronger preference for a dynamic local oxygen vacancy order - than in La2Ce2O7. The stronger preference for 〈210〉 vacancy-vacancy alignments can explain why long range ordering is identified by distinct C-type like superlattice peaks in neutron diffraction patterns for Nd2Ce2O7 whereas they appear to be almost invisible in La2Ce2O7.

The Itinerant 2D Electron Gas of the Indium Oxide (111) Surface: Implications for Carbon- and Energy-Conversion Applications.

Transparent conducting oxides (TCO) have integral and emerging roles in photovoltaic, thermoelectric energy conversion, and more recently, photocatalytic systems. The functional properties of TCOs, and thus their role in these applications, are often mediated by the bulk electronic band structure but are also strongly influenced by the electronic structure of the native surface 2D electron gas (2DEG), particularly under operating conditions. This study investigates the 2DEG, and its response to changes in chemistry, at the (111) surface of the model TCO In2 O3 , through angle resolved and core level X-ray photoemission spectroscopy. It is found that the itinerant charge carriers of the 2DEG reside in two quantum well subbands penetrating up to 65 Å below the surface. The charge carrier concentration of this 2DEG, and thus the high surface n-type conductivity, emerges from donor-type oxygen vacancies of surface character and proves to be remarkably robust against surface absorbents and contamination. The optical transparency, however, may rely on the presence of ubiquitous surface adsorbed oxygen groups and hydrogen defect states that passivate localized oxygen vacancy states in the bandgap of In2 O3 .

Faceting-Controlled Zeeman Splitting in Plasmonic TiO2 Nanocrystals.

Dynamic manipulation of discrete states in nanostructured materials is critical for emerging quantum technologies. However, this process often requires a correlation of mutually competing degrees of freedom. Here we report the control of magnetic-field-induced excitonic splitting in colloidal TiO2 nanocrystals by control of their faceting. By changing nanocrystal morphology via reaction conditions, we control the concentration and location of oxygen vacancies, which can generate localized surface plasmon resonance and foster the reduction of lattice cations leading to the emergence of individual or exchange-coupled Ti(III) centers with high net-spin states. These species can all couple with the nanocrystal lattice under different conditions resulting in distinctly patterned excitonic Zeeman splitting and selective control of conduction band states in an external magnetic field. This work demonstrates the concept of using nanocrystal morphology to control carrier polarization in individual nanocrystals using both intrinsic and collective electronic properties, representing a unique approach to multifunctionality in reduced dimensions.

(Invited) Mitigating Oxygen Release in Anionic-Redox-Active Cathode Materials By Cationic Substitution through Rational Design

In classical layered oxide cathodes, when substituting excess amount of lithium in the transition metal layer, oxygen extends its role as a structural framework, exhibiting electrochemical activity and enhancing the reversible capacities of layered oxides through anionic redox mechanisms. Nevertheless, if unoptimized, oxygen activity comes with instability in the form of oxygen loss, which is associated with irreversible voltage decay and capacity fade. To understand this irreversible loss and to increase the stability of lattice oxygen, density functional theory is applied to calculate oxygen vacancy formation energies in lithium rich transition metal layered oxides for a variety of dopants, noting increased stability upon doping with 4d elements Mo and Ru. Driven by these findings, Mo is co-doped with Co into Li[Li0.2Ni0.2Mn0.6]O2, showing notably reduced voltage decay and capacity fade without sacrificing energy density and cycle life, and evidence of Mo incorporation is presented. Calculations suggest this is due to a modified charge density distribution around anions upon incorporation of Mo, altering local band structure and impeding oxygen vacancy formation, while maintaining the anionic activity available for redox. We will demonstrate the other possible strategies to mitigate the oxygen loss in anion redox cathode materials for next generation lithium ion batteries.

Location of oxygen vacancies in Sr2CuO3+δ single crystal determined by polarized EXAFS at Cu K-edge

The location of oxygen vacancies in Sr2CuO3+δ single crystal has been studied by polarized extended x-ray absorption fine structure (PEXAFS) at Cu K-edge. We found that the oxygen vacancies locate both in the CuO2 plane and at the apical site. Among the total 18 ± 8% oxygen vacancies, about 10 ± 5% locates in the CuO2 plane and 8 ± 3% at the apical site.

High-efficiency magnetic modulation in Ti/ZnO/Pt resistive random-access memory devices using amorphous zinc oxide film

Abstract The insertion of a thin amorphous ZnO (a-ZnO) dielectric layer between the Pt electrode and crystallized ZnO (c-ZnO) insulator has been known to enhance magnetic modulation of Ti/c-ZnO/Pt resistive random-access memory (RRAM) devices. This study investigates the relationship between resistivity and magnetization switching within Ti/c-ZnO/a-ZnO/Pt and Ti/c-ZnO/Pt structures. Under an applied electric field, both devices show magnetization changes associated with resistance states, but the Ti/c-ZnO/a-ZnO/Pt RRAM device exhibits substantially enhanced magnetic modulation, improved resistive switching (RS) ratio and reduced operating voltage of the ZnO film. The saturation magnetization of the ZnO film in a low-resistance state is more than ten times higher compared to that in its initial state under a VSET/VRESET ratio of 0.52–0.56 V. Furthermore, the Ti/c-ZnO/a-ZnO/Pt structure shows an improvement in the RS ratio by approximately 880 times, and the coefficients of variation for SET and RESET voltages are only 1.6% and 2.2%, respectively. We propose that these results are related to the stabilized local oxygen vacancy (Vo) migration, metal-like nature of Vo-based filamentary conducting paths and the coupling of Vos with each other among the conductive filaments in the c-ZnO layer. This work presents an efficient means to electrically control the magnetism in metal oxides, i.e., inserting a thin oxide layer (a-ZnO) in electrode–insulator interfaces.

Crystal structure of LaSr3Fe3O9 and its phase relation with LaSr3Fe3O10

The Ruddlesden-Popper phase LaSr3Fe3O10 and its reduced derivatives with nominal composition LaSr3Fe3O10–x, with 0 < x < 1.21, are described in detail with respect to crystal structure, magnetic order, thermal stability and likely vacancy mechanisms. The crystal structure of the LaSr3Fe3O9 (x = 1) phase is described. The LaSr3Fe3O10-δ phase prevails for x < 0.51, whereas LaSr3Fe3O9±ε prevails for x > 0.85. The phases are separated by a two-phase region 0.51 < x < 0.85. By combined Rietveld refinements of high-resolution powder synchrotron and neutron diffraction, the crystallographic and magnetic structure of LaSr3Fe3O9 is described. LaSr3Fe3O9 takes a layer like RP3 type structure with features of the brownmillerite structure in the triple perovskite type slabs; unit cell dimensions a = 28.7562(13) Å, b = 5.5280(2) Å, c = 5.4583(2) Å, space group Cmcm. LaSr3Fe3O9 is an antiferromagnet with TN > 350 K and with a G-type magnetic structure, magnetic space group PCbcm. Based on unit cell volume considerations, local oxygen vacancy ordering schemes are suggested.

Effects of electrostatic field strength on grain‐boundary core structures in SrTiO3

AbstractUnderstanding interactions between externally applied electric fields and the interfacial structures of nanoscale ceramics is important for controlling their functional properties. In ceramic oxides, functional properties are determined by oxygen vacancy concentrations near and within grain‐boundary core structures. In this study it is shown that the application of electrostatic fields ranging from 0 to nominally 170 V/cm during diffusion bonding of bicrystals alters the atomic and electronic core structures of (100) twist grain boundaries in SrTiO3. The applied electric field strength affects local oxygen vacancy concentrations and ordering of the oxygen sublattice. Results for this model system indicate that electrostatic fields applied during ceramic manufacturing can be employed as a new processing parameter to tailor defect structure configurations and obtain unprecedented ceramic microstructures. The ability to manipulate interface configurations with electric fields in the absence of any sintering additives may have far reaching implications for tuning polarization and band structures in electroceramics while avoiding effects of often unwanted dopants.

Local structural changes due to the electric field‐induced migration of oxygen vacancies at Fe‐doped SrTiO3 interfaces

AbstractWe report on our study of dc voltage‐induced structural changes at reduced and oxidized Fe‐doped SrTiO3 (Fe:STO) electrode interfaces using second harmonic generation (SHG) together with photoluminescence (PL) method. We show that oxygen vacancy defects play a critical role in determining the local electrical and structural properties of interfacial depletion regions at Schottky junctions. The SHG results show that the dc electric field causes oxygen ions and vacancies to displace toward the anode and cathode in the low field regime, respectively. This process forms electrostrictive distortions within local interfacial depletion regions which are described by Fe:Ti‐O bond stretching and bending. Differences in the EFISHG responses from the oxidized and reduced crystal interfaces are explained according to local oxygen vacancy concentrations and dynamics and their effects on the Schottky barrier heights and depletion region widths at each interface. These results are further supported by our PL measurements. Oxygen ion migration toward the Fe:STO surface leads to enhanced fluorescence intensities from in‐gap acceptor states. We demonstrate that SHG and PL measurements are well‐suited for understanding and resolving the underlying causes of dielectric breakdown processes and device failure brought on by dc electric field and ionic defect migrations in perovskite‐type electroceramics.

Combined Experimental and Theoretical Insights into the Synergistic Effect of Cerium Doping and Oxygen Vacancies in BaZrO3−δ Hollow Nanospheres for Efficient Photocatalytic Hydrogen Production

The long-standing debate over the influence of oxygen vacancies and various dopants has been the center point in perovskite-based compounds for their photocatalytic applications. Hydrothermally synthesized cerium-doped BaZrO3 (BZO) hollow nanospheres have been systematically studied by experimental and theoretical calculations to understand the effect of cerium doping and oxygen vacancies on the photocatalytic properties. Compounds synthesized by a template-free route were composed of hollow nanospheres generated by Ostwald ripening of spherical nanospheres, which were formed by agglomeration of nanoparticles. The high alkaline condition and high temperature during the hydrothermal condition may lead to the formation of local disorders and oxygen vacancies in the compounds, confirmed by ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) analysis, and density functional theoretical (DFT) calculations. Combination of oxyge...