Diffusion Of Oxygen Vacancies Research Articles

Optimisation of oxide-ion conductivity in acceptor-doped Na0.5Bi0.5TiO3 perovskite: approaching the limit?

σb of NBT is optimised by acceptor-doping, which is restrained by oxygen-vacancy diffusivity limit in perovskites.

The Enhancement of Surface Reactivity on CeO2 (111) Mediated by Subsurface Oxygen Vacancies

Surface reactivity on metal oxide surfaces and its enhancement play important roles in heterogeneous catalytic reactions. In this work, the interactions of O2 and H2O with reduced CeO2 (111) surface are studied by density-functional theory calculations. The corresponding adsorption geometries, adsorption energies, and reaction barriers are reported. It is found that the diffusion of subsurface oxygen vacancies toward surface can be promoted by the adsorption of O2 on the CeO2 (111) surface. Then those oxygen vacancies diffused onto surface sites will be healed by the adsorbed O2, leaving behind an O adatom on the surface. Interestingly, at moderate temperatures, the surface O adatom will swap positions with surface lattice O dynamically. The adsorption of H2O may also induce the diffusion of oxygen vacancies from subsurface to surface, leading to the formation of two hydroxyls on the CeO2 (111) surface. In addition, the interaction between the paired hydroxyl groups and O2 will result in the formation of ...

Effect of cooling rate on phase transitions and ferroelectric properties in 0.75BiFeO3-0.25BaTiO3 ceramics

The effect of the cooling rate on the electrical properties was investigated in the 0.75BiFeO3-0.25BaTiO3 ceramics. The air-quenched samples had superior ferroelectric and piezoelectric properties to the slowly cooled samples. The quenching effect weakened when the quenching temperature was less than 700 °C and eventually disappeared at 500 °C and below. The X-ray diffraction and transmission electron microscopy showed that the cooling rate had a significant effect on the crystal structure and domain structure. The slowly cooled sample showed a very small rhombohedral distortion and a poorly developed domain structure, which leads to weak ferroelectric and piezoelectric properties at room temperature. The quenched and slowly cooled samples had a ferroelectric rhombohedral structure (R3c) at room temperature and a paraelectric cubic structure (Pm-3m) at temperatures above 650 °C. On the other hand, the slowly cooled sample had a centro-symmetric orthorhombic (Pbnm) structure at intermediate temperatures, while the quenched sample had a noncentrosymmetric orthorhombic structure (Amm2). The diffusion of oxygen vacancies in the slowly cooled sample is believed to lead to a more symmetric orthorhombic structure at intermediate temperatures between 500 °C and 650 °C during the slow-cooling process and consequently very small rhombohedral distortion at room temperature.

A multistep model for the kinetic analysis of the impedance spectra of a novel mixed ionic and electronic conducting cathode

A one-dimensional, heterogeneous and dynamic model is applied to kinetically analyze impedance experiments performed on a novel NdBa0.9Co2O5.6 (NBC) MIEC cathode. The model simulates the spectra in the time domain by accounting for the gas diffusion inside the electrode pores, and for the solid state diffusion of oxygen vacancies inside the bulk of the cathodic material. A detailed kinetic scheme is applied to describe the oxygen reduction mechanism, which includes steps for adsorption and desorption, first and second electronation at the gas/electrode interface, and ion transfer at the electrode/electrolyte interface. The kinetic investigation is based on impedance spectra collected on symmetric NBC/GDC/NBC cells, at open circuit voltage, between 550 and 700°C, and 5–100% O2 molar fraction. The vacancies diffusion coefficient and the kinetic parameters of the reaction steps are fitted to describe the data. At the highest temperatures, a sensitivity analysis reveals that the rate determining step is the first electronation of the oxygen adatom, while the second electronation and the interfacial ion transport are kinetically irrelevant. Overall, the model allows to individuate the key parameters for capturing the kinetics of a MIEC cathode.

C−H Activation of Methane to Formaldehyde on Ce1−xZrxO2 Thin Films: A Step to Bridge the Material Gap

AbstractCe1−xZrxO2 (CZ) thin films were prepared by a combination of sol–gel and spin‐coating methods and were evaluated for the C−H activation of methane by using a molecular beam set‐up with the aim to bridge the material gap. The C−H activation of methane begins at 950 K, and a Ce‐rich CZ composition displays a high selectivity (4–12 %) to the partially oxidised product, formaldehyde. A 10–12 % selectivity towards HCHO with 1.6 % methane conversion was observed with methane‐rich CH4/O2 reactant compositions at 1050 K. Short contact times, prevalent under molecular beam conditions, could be a possible reason for HCHO formation. Although combustion products were observed instantly upon shining the mixture of reactants on CZ surfaces, up to 20 s delay was observed before formaldehyde generation, which indicates that the oxygen vacancy migration contributes to the rate‐determining step and the diffusion‐controlled nature of the reaction. A burst in HCHO generation at the point of molecular beam opening, after beam‐closed conditions, suggests that the diffusion of oxygen vacancies to the surface is the reason for HCHO formation. Kinetics results also indicate the necessity of reduction sites for HCHO generation.

Engineering amorphous-crystalline interfaces in TiO2−x/TiO2−y-based bilayer structures for enhanced resistive switching and synaptic properties

The operating principle of resistive random access memories (RRAMs) relies on the distribution of ionic species and their influence on the electron transport. Taking into account that formation and annihilation of conducting filaments (CFs) is the driving mechanism for the switching effect, it is very important to control the regions where these filaments will evolve. Thus, homolayers of titanium oxide with different oxygen contents were fabricated in order to tune the local electrical and thermal properties of the CFs and narrow down the potential percolation paths. We show that the oxygen content in the top layer of the TiO2−x/TiO2−y bilayer memristors can directly influence the morphology of the layers which affect the diffusion barrier and consequently the diffusivity and drift velocity of oxygen vacancies, yielding in important enhancement of switching characteristics, in terms of spatial uniformity (σ/μ < 0.2), enlarged switching ratio (∼104), and synaptic learning. In order to address the experimental data, a physical model was applied, divulging the crucial role of temperature, electric potential and oxygen vacancy density on the switching effect and offering physical insights to the SET/RESET transitions and the analog switching. The forming free nature of all the devices in conjunction with the self-rectifying behavior, should also be regarded as important assets towards RRAM device optimization.

Evaluation of dielectric and piezoelectric behavior of unpoled and poled barium titanate polycrystals with oxygen vacancies using phase field method

ABSTRACTThis article studies the dielectric and piezoelectric behavior of unpoled and poled barium titanate (BaTiO3) polycrystals with oxygen vacancies. A phase field model is employed for BaTiO3 polycrystals, coupled with the time-dependent Ginzburg–Landau theory and the oxygen vacancies diffusion, to demonstrate the interaction between oxygen vacancies and domain evolutions. To generate grain structures, the phase field model for grain growth is also used. The hysteresis loop and butterfly curve are predicted at room and high temperatures. The permittivity, and longitudinal and transverse piezoelectric constants of the BaTiO3 polycrystals are then examined for various grain sizes and oxygen vacancy densities.

Electrochemically Induced Transformations of Vanadium Dioxide Nanocrystals.

Vanadium dioxide (VO2) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. Recently, alternative stimuli have been utilized to trigger insulator-to-metal transformations in VO2, including electrochemical gating. Here, we prepare and electrochemically reduce mesoporous films of VO2 nanocrystals, prepared from colloidally synthesized V2O3 nanocrystals that have been oxidatively annealed, in a three-electrode electrochemical cell. We observe a reversible transition between infrared transparent insulating phases and a darkened metallic phase by in situ visible-near-infrared spectroelectrochemistry and correlate these observations with structural and electronic changes monitored by X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and conductivity measurements. An unexpected reversible transition from conductive, reduced monoclinic VO2 to an infrared-transparent insulating phase upon progressive electrochemical reduction is observed. This insulator-metal-insulator transition has not been reported in previous studies of electrochemically gated epitaxial VO2 films and is attributed to improved oxygen vacancy formation kinetics and diffusion due to the mesoporous nanocrystal film structure.

Stabilization of the GeO2/Ge Interface by Nitrogen Incorporation in a One-Step NO Thermal Oxynitridation.

The thermal instability of GeO2/Ge structures lasts as a barrier against the development of Ge-based metal-oxide-semiconductor devices. In the present work, stabilization was achieved through the incorporation of nitrogen into the oxide layer by thermally growing GeOxNy films in NO. With this approach, a stable layer is obtained in a single step as opposed to other nitridation techniques (like plasma immersion) which require additional processing. Significant reduction of GeO desorption from the surface and a strong barrier against additional substrate oxidation were obtained by the insertion of a small amount of nitrogen content (N/O ≈ 10%). Nuclear reaction analysis and profiling showed that nitrogen incorporation and removal occur simultaneously during film growth, yielding N to be distributed throughout the whole film, without accumulation in any particular region. Both the oxidation barrier and the lower GeO desorption rate are explained by a reduction of vacancy diffusivity inside the dielectric. This is not caused by the densification of the oxide, but is a consequence of nitrogen blockage of oxygen vacancy diffusion paths.

Tailoring oxygen vacancies at ZnO(11¯00) surface: An ab initio study

Oxygen vacancies in ZnO crystals have significant impacts on its properties and applications. On the basis of ab initio results, we describe the oxygen vacancy distribution and diffusion paths away from the ZnO(11¯00) surface, aiming to elucidate thermodynamics and kinetic stability of the vacancies and a possible control mechanism. In view of defect engineering and sensor applications, we propose efficient routes to chemically control the equilibrium concentration of the oxygen vacancies at ZnO surfaces by exposure to specific reactive gases: we show that the oxygen vacancy concentration can be increased using sulfur oxide as post-growth treatment, while under exposure to ozone, no significant amount of oxygen vacancies can be sustained on the surface.

Duplex n- and p-Type Chromia Grown on Pure Chromium: A Photoelectrochemical and Microscopic Study

Chromia grown on pure chromium at 900 °C for 30 min at an oxygen partial pressure p(O2) of 10−12 atm has been characterized using photoelectrochemical and electron microscopy techniques. This study reveals a duplex scale: n-chromia with equiaxis morphology in the internal part (~650 nm thick) and p-chromia with columnar morphology in the external part (~900 nm thick). Grain orientation maps also revealed the presence of a c-oriented chromia layer at the interface between the n- and the p-subscales. This 〈 0001 〉 textured layer was identified as the first-grown chromia layer. It means that internal n-equiaxis chromia grew by anionic transport governed by oxygen vacancy diffusion, whereas external p-columnar chromia layer grew outwards and was controlled by chromium vacancy diffusion.

Thermodynamically Introduced Oxygen Vacancies in Epitaxial Pt-CeO2 System and Their Influence on Resistive Switching

There is a growing need for a new type of nonvolatile memory technologies which would allow further downscale the size of devices and would have lower power consumption. One of the candidates for such technology is resistive random access memory (ReRAM). ReRAM is one of the most promising emerging nonvolatile memories in which both electronic and electrochemical effects play an important role. The principle of ReRAM cell is based on reversible change of resistance states induced by current and/or voltage. Typically, the memory cell consists of a metal-oxide-metal (MOM) structure and needs an initial electroforming step as a current-limited electric breakdown. After this process, the memory cell can be reversibly set-switched between a conductive low resistance state (ON) and a less conductive reset high-resistance state (OFF). The proposed mechanisms for such resistivity change can be divided into two models: the filament type and interface type [1]. The filament model suggests creation of conductive paths between two metal electrodes, which are changing the resistance state of the MOM structure. The interface model implies that the resistance change is caused by a type of electrochemical reaction to form or dissolve a Schottky barrier at the interface and is promoted by the electric field gradient. In our work we investigate CeO2 as a candidate material for ReRAM. Ceria is the high-k dielectric material [2] well known for its oxygen vacancy diffusion [3] and readiness for oxidation state change between Ce3+/Ce4+ [4]. These properties play a major role in resistive switching behavior of ceria as previous investigations suggest that oxygen diffusion is assisting in formation of conductive filaments responsible for such switching. The vacancies can be generated in different ways including applying voltage, incorporation of reducing agent, nonstoichiometric deposition. In this research we performed a set of in situ experiments with a model epitaxial CeO2 thin films on Cu(111) single crystal. Epitaxial films were chosen to avoid grain boundary diffusion which can influence the resistive switching mechanism [5]. The oxygen vacancies were introduced into the thin film trough thermodynamic interaction of Pt with ceria. Pt was deposited onto CeO2 surface, causing reduction of the oxide. Interaction of Pt with ceria was investigated with X-ray photoelectron spectroscopy and conductive atomic force microscopy. We found out that reduction of ceria was occurring not only at the Pt/CeO2 interface, but also on the surface of the ceria film which is not covered by Pt, after Pt deposition and annealing. A different distribution of oxygen vacancies in the film proves to have an influence on the resistance switching process of the film. Finally, the proper balance between the reduced and the unreduced species in order to obtain relatively stable repeatable resistance switch with clear resistance window is discussed. 1. Akinaga H, Shima H (2010) Resistive Random Access Memory (ReRAM) Based on Metal Oxides. Proceedings of the IEEE 98 (12):2237-2251. doi:10.1109/JPROC.2010.2070830 2. Keating PRL, Scanlon DO, Watson GW (2013) Computational testing of trivalent dopants in CeO2 for improved high-k dielectric behaviour. Journal of Materials Chemistry C 1 (6):1093-1098. doi:10.1039/C2TC00385F 3. Nolan M, Fearon JE, Watson GW (2006) Oxygen vacancy formation and migration in ceria. Solid State Ionics 177 (35–36):3069-3074. doi:10.1016/j.ssi.2006.07.045 4. Aneggi E, Boaro M, Leitenburg Cd, Dolcetti G, Trovarelli A (2006) Insights into the redox properties of ceria-based oxides and their implications in catalysis. Journal of Alloys and Compounds 408–412:1096-1102. doi:10.1016/j.jallcom.2004.12.113 5. Shubhakar K, Pey KL, Kushvaha SS, O’Shea SJ, Raghavan N, Bosman M, Kouda M, Kakushima K, Iwai H (2011) Grain boundary assisted degradation and breakdown study in cerium oxide gate dielectric using scanning tunneling microscopy. Applied Physics Letters 98 (7):072902. doi:10.1063/1.3553190 Figure 1

Band gap and oxygen vacancy diffusion of anatase (101) surface: the effect of strain

The effects of strain on the band gap and oxygen vacancy diffusion are investigated for the anatase (101) surface through density functional theory calculations. The results show that biaxial strain can effectively shift the band edge of the surface; for example, tensile strain gently reduces the band gap. With respect to the subsurface-to-surface diffusion of oxygen vacancy, energy barrier has a significant dependence on strain. As strain increases, it facilitates O-vacancy diffusion at the clean surface but hinders this migration for the reduced surface in the presence of water. Analysis based on the water adsorption energy indicates that the interplay between O-vacancy and water is weakened with increasing strain.

Computational Study of Oxygen Diffusion along a[100] Dislocations in the Perovskite Oxide SrTiO3.

We used classical molecular-dynamics simulations to study the atomistic structure of, and the diffusion of oxygen ions along, the periodic array of edge dislocations comprising a symmetrical 6.0° [100] tilt grain boundary in SrTiO3. The results indicate that, at elevated temperatures, the two types of dislocation core (TiO2-type and SrO-type) that make up the boundary are stable and that oxygen-deficient cores maintain their dissociated structures. They also confirm that oxygen vacancies prefer to reside at the cores rather than in the bulk. Tracer diffusion coefficients of oxygen were obtained for oxygen-deficient bulk and grain-boundary simulation cells at temperatures in the range of 1000 ≤ T/K ≤ 2300. Calculated values of the oxygen-vacancy diffusion coefficient for the bulk phase agree extremely well with published experimental data. Tracer diffusion coefficients obtained for the grain-boundary cell are, in comparison to those for the bulk, lower in magnitude and have a higher activation enthalpy, indicating that, relative to the bulk, the migration of oxygen ions along a[100] dislocation cores in SrTiO3 is hindered. These results provide further support for the decoupled model of filament formation in resistively switching SrTiO3.

Enhanced oxygen vacancy diffusion in Ta2O5 resistive memory devices due to infinitely adaptive crystal structure

Metal oxide resistive memory devices based on Ta2O5 have demonstrated high switching speed, long endurance, and low set voltage. However, the physical origin of this improved performance is still unclear. Ta2O5 is an important archetype of a class of materials that possess an adaptive crystal structure that can respond easily to the presence of defects. Using first principles nudged elastic band calculations, we show that this adaptive crystal structure leads to low energy barriers for in-plane diffusion of oxygen vacancies in λ phase Ta2O5. Identified diffusion paths are associated with collective motion of neighboring atoms. The overall vacancy diffusion is anisotropic with higher diffusion barriers found for oxygen vacancy movement between Ta-O planes. Coupled with the fact that oxygen vacancy formation energy in Ta2O5 is relatively small, our calculated low diffusion barriers can help explain the low set voltage in Ta2O5 based resistive memory devices. Our work shows that other oxides with adaptive crystal structures could serve as potential candidates for resistive random access memory devices. We also discuss some general characteristics for ideal resistive RAM oxides that could be used in future computational material searches.

Encapsulation layer design and scalability in encapsulated vertical 3D RRAM

Here we propose a novel encapsulated vertical 3D RRAM structure with each resistive switching cell encapsulated by dielectric layers, contributing to both the reliability improvement of individual cells and thermal disturbance reduction of adjacent cells due to the effective suppression of unwanted oxygen vacancy diffusion. In contrast to the traditional vertical 3D RRAM, encapsulated bar-electrodes are adopted in the proposed structure substituting the previous plane-electrodes, thus encapsulated resistive switching cells can be naturally formed by simply oxidizing the tip of the metal bar-electrodes. In this work, TaOx-based 3D RRAM devices with SiO2 and Si3N4 as encapsulation layers are demonstrated, both showing significant advantages over traditional unencapsulated vertical 3D RRAM. Furthermore, it was found thermal conductivity and oxygen blocking ability are two key parameters of the encapsulation layer design influencing the scalability of vertical 3D RRAM. Experimental and simulation data show that oxygen blocking ability is more critical for encapsulation layers in the relatively large scale, while thermal conductivity becomes dominant as the stacking layers scale to the sub-10 nm regime. Finally, based on the notable impacts of the encapsulation layer on 3D RRAM scaling, an encapsulation material with both excellent oxygen blocking ability and high thermal conductivity such as AlN is suggested to be highly desirable to maximize the advantages of the proposed encapsulated structure. The findings in this work could pave the way for reliable ultrahigh-density storage applications in the big data era.

Oxygen vacancy diffusion in bulk SrTiO3 from density functional theory calculations

Point defects and their diffusion contribute significantly to the properties of perovskite materials. However, even for the prototypical case of oxygen vacancies in SrTiO3 (STO), the predictions of oxygen vacancy activity vary widely. Here we present a comprehensive and systematic study of the diffusion barriers in bulk STO. Using density functional theory (DFT), we assess the role of different supercell sizes, density functionals, and charge states. Our results show that vacancy-induced octahedral rotations, which are limited by the boundary conditions of the supercell, can significantly affect the computed oxygen vacancy diffusion energy barrier. In addition, we find that the diffusion energy barrier of a charged oxygen vacancy is lower than that of a neutral one. This difference is magnified in small supercells. We demonstrate that with increasing supercell size, the effects of the oxygen vacancy charge state and the type of DFT exchange and correlation functional diminish, and all DFT predicted migration energy barriers asymptote to a range of 0.39–0.49eV, which is smaller than the reported experimental values. This work provides important insights and guidance that should be considered for investigations of point defect diffusion in perovskite materials and in oxide superlattices.

Investigation of structural and electrical properties of ZnMn2O4 synthesized by sol–gel method

In this present study, we have reported the synthesis of ZnMn2O4, transitional metal oxide by sol–gel method. The structural and morphological properties are confirmed using various characterization techniques namely XRD, FT-IR and SEM with EDX. Dielectric studies of ZnMn2O4 are measured at the frequency varying from 50 Hz to 5 MHz for the temperature range of 303–573 K. The temperature dependent electrical parameters like impedance and modulus exhibit a strong correlation with the grains, grain boundaries and space charge effects in the synthesized material. Diffusion of oxygen vacancies in the dipoles and defects in the material due to oxygen vacancy complexes are investigated by the activation energy obtained from Arrhenius plot. It was found that the relaxation process was dominated by the hopping mechanism between the Mn3+ and Mn4+. Nyquist plot of impedance was attributed to the existence of space charge interface, grain boundary and grain conduction mechanism of the material.

The Dynamic Phase Transition Modulation of Ion‐Liquid Gating VO2 Thin Film: Formation, Diffusion, and Recovery of Oxygen Vacancies

Electrolyte gating with ionic liquids (IL) on correlated vanadium dioxide (VO2) nanowires/beams is effective to modulate the metal‐insulator transition (MIT) behavior. While for macrosize VO2 film, the gating treatment shows different phase modulation process and the intrinsic mechanism is still not clear, though the oxygen‐vacancy diffusion channel is always adopted for the explanation. Herein, the dynamic phase modulation of electrolyte gated VO2 films is investigated and the oxygen vacancies formation, diffusion, and recovery at the IL/oxide interface are observed. As a relatively slow electrochemical reaction, the gating effect gradually permeates from surface to the inside of VO2 film, along with an unsynchronized changes of integral electric, optical, and structure properties. First‐principles‐based theoretical calculation reveals that the oxygen vacancies can not only cause the structural deformations in monoclinic VO2, but also account for the MIT transition by inducing polarization charges and thereby adjusting the d‐orbital occupancy. The findings not only clarify the oxygen vacancies statement of electrolyte gated VO2 film, but also can be extended to other ionic liquid/oxide systems for better understanding of the surface electrochemical stability and electronic properties modulation.

A two-step surface exchange mechanism and detailed defect transport to model oxygen permeation through the La0.9Ca0.1FeO3−δmixed-conductor

A framework to model materials experiencing defect mechanism similar to that of La1−xCaxFeO3−δ (LCF) membranes is considered. Using the La0.9Ca0.1FeO3−δ mixed-conductor, we propose a model that incorporates a two-step thermodynamically consistent surface exchange mechanism and charged species transport within the material using the Planck–Nernst–Poisson model. The surface reaction mechanism has been proposed under equilibrium; the current study expands the theory to finite rate kinetics and shows significant kinetic limitations for oxygen incorporation. Existing models describing the diffusion of oxygen vacancies may not be applicable at low oxygen partial pressures. Evidence shows that for the La1−xCaxFeO3−δ membrane, simultaneous presence of multiple Fe states should be considered. As a result, the surface chemistry and charged species transport models developed in this study conserve the Fe sites and the electroneutrality condition both at the feed and sweep sides of the membrane. The role of a strict electroneutrality condition within the material is also examined. Using inert argon experimental data, we demonstrate the fidelity of the model. Predictions reveal that no crossover in the concentration of FeFe• and FeFe′ takes place within our operating conditions; thus, electronic conductivity within the material is of p-type.