Probing oxygen vacancy concentration and homogeneity in solid-oxide fuel-cell cathode materials on the subunit-cell level

Modulating Lattice Oxygen Activity of Iron-Based Triple-Conducting Nanoheterostructure Air Electrode via Sc-Substitution Strategy for Protonic Ceramic Cells

Iron-based perovskite air electrodes for protonic ceramic cells (PCCs) offer broad application prospects owing to their reasonable thermomechanical compatibility and steam tolerance. However, their insufficient electrocatalytic activity has considerably limited further development. Herein, oxygen-vacancy-rich BaFe0.6 Ce0.2 Sc0.2 O3-δ (BFCS) perovskite is rationally designed by a facile Sc-substitution strategy for BaFe0.6 Ce0.4 O3-δ (BFC) as efficient and stable air electrode for PCCs. The BFCS electrode with an optimized Fe 3d-eg orbital occupancy and more oxygen vacancies exhibits a polarization resistance of ≈ 0.175 Ω cm2 at 600 °C, ≈ 1/3 of the BFC electrode (≈0.64 Ω cm2 ). Simultaneously, BFCS shows favorable proton uptake with a low proton defect formation enthalpy (- 81 kJ mol-1 ). By combining soft X-ray absorption spectroscopy and electrical conductivity relaxation studies, it is revealed that the enhancement of Fe4+ -O2- interactions in BFCS promotes the activation and mobility of lattice oxygen, triggering the activity of BFCS in both oxygen reduction and evolution reactions (ORR/OER). The single cell achieves encouraging output performance in both fuel cell (1.55 W cm-2 ) and electrolysis cell (-2.96 A cm-2 at 1.3 V) modes at 700 °C. These results highlight the importance of activating lattice oxygen in air electrodes of PCCs.

Valence-Ordered Thin-Film Nickelate with Tri-component Nickel Coordination Prepared by Topochemical Reduction

The metal-hydride-based "topochemical reduction" process has produced several thermodynamically unstable phases across various transition metal oxide series with unusual crystal structures and nontrivial ground states. Here, by such an oxygen (de-)intercalation method we synthesis a samarium nickelate with ordered nickel valences associated with tri-component coordination configurations. This structure, with a formula of Sm9Ni9O22 as revealed by four-dimensional scanning transmission electron microscopy (4D-STEM), emerges from the intricate planes of {303}pc ordered apical oxygen vacancies. X-ray spectroscopy measurements and ab initio calculations show the coexistence of square planar, pyramidal, and octahedral Ni sites with mono-, bi-, and tri-valences. It leads to an intense orbital polarization, charge-ordering, and a ground state with a strong electron localization marked by the disappearance of ligand-hole configuration at low temperature. This nickelate compound provides another example of previously inaccessible materials enabled by topotactic transformations and presents an interesting platform where mixed Ni valence can give rise to exotic phenomena.

A-Site Nonstoichiometric BaxCo0.4Fe0.4Zr0.1Y0.1O3-δ Cathode for Protonic Ceramics Fuel Cells

Highly active triple (proton, oxygen-ion, and electron) conducting materials BaxCo0.4Fe0.4Zr0.1Y0.1O3-δ (BxCFZY, x = 0.9-1.1) were prepared and characterized as potential cathodes for protonic ceramic fuel cells (PCFCs) in this work. The crystal structure, oxygen vacancy concentration, electrical conductivity, oxygen ion transfer properties, and electrochemical performance of BxCFZY oxides were systematically evaluated. The electrical conductivity of BxCFZY decreases but oxygen vacancies increase with increasing Ba content, indicating that the charge compensation was mainly achieved by the production of oxygen vacancy rather than the increase in the valence of transition metal cations. The power density of 1170 mW cm-2 and the polarization resistance of 0.05 Ω cm2 were achieved at 700 °C for the anode-supported single cells with B1.1CFZY cathode, suggesting that the excess A site on the BxCFZY had a positive effect on the catalytic activity for the oxygen reduction reaction. Furthermore, the distribution of relaxation time (DRT) analysis method was adopted to determine the electrochemical processes of the cells with BxCFZY cathodes. The calculated results confirmed that the cell with B1.1CFZY cathode exhibited the optimum performance due to the best oxygen ion transfer properties in BxCFZY cathodes.

Guided anisotropic oxygen transport in vacancy ordered oxides

Anisotropic and efficient transport of ions under external stimuli governs the operation and failure mechanisms of energy-conversion systems and microelectronics devices. However, fundamental understanding of ion hopping processes is impeded by the lack of atomically precise materials and probes that allow for the monitoring and control at the appropriate time- and length- scales. In this work, using in-situ transmission electron microscopy, we directly show that oxygen ion migration in vacancy ordered, semiconducting SrFeO2.5 epitaxial thin films can be guided to proceed through two distinctly different diffusion pathways, each resulting in different polymorphs of SrFeO2.75 with different ground electronic properties before reaching a fully oxidized, metallic SrFeO3 phase. The diffusion steps and reaction intermediates are revealed by means of ab-initio calculations. The principles of controlling oxygen diffusion pathways and reaction intermediates demonstrated here may advance the rational design of structurally ordered oxides for tailored applications and provide insights for developing devices with multiple states of regulation.

Defect graph neural networks for materials discovery in high-temperature clean-energy applications

We present a graph neural network approach that fully automates the prediction of defect formation enthalpies for any crystallographic site from the ideal crystal structure, without the need to create defected atomic structure models as input. Here we used density functional theory reference data for vacancy defects in oxides, to train a defect graph neural network (dGNN) model that replaces the density functional theory supercell relaxations otherwise required for each symmetrically unique crystal site. Interfaced with thermodynamic calculations of reduction entropies and associated free energies, the dGNN model is applied to the screening of oxides in the Materials Project database, connecting the zero-kelvin defect enthalpies to high-temperature process conditions relevant for solar thermochemical hydrogen production and other energy applications. The dGNN approach is applicable to arbitrary structures with an accuracy limited principally by the amount and diversity of the training data, and it is generalizable to other defect types and advanced graph convolution architectures. It will help to tackle future materials discovery problems in clean energy and beyond.

Double-Bilayer polar nanoregions and Mn antisites in (Ca, Sr)3Mn2O7

The layered perovskite Ca₃Mn₂O₇ (CMO) is a hybrid improper ferroelectric candidate proposed for room temperature multiferroicity, which also displays negative thermal expansion behavior due to a competition between coexisting polar and nonpolar phases. However, little is known about the atomic-scale structure of the polar/nonpolar phase coexistence or the underlying physics of its formation and transition. In this work, we report the direct observation of double bilayer polar nanoregions (db-PNRs) in Ca_2.9Sr_0.1Mn₂O₇ using aberration-corrected scanning transmission electron microscopy (S/TEM). In-situ TEM heating experiments show that the db-PNRs can exist up to 650 °C. Electron energy loss spectroscopy (EELS) studies coupled with first-principles calculations demonstrate that the stabilization mechanism of the db-PNRs is directly related to an Mn oxidation state change (from 4+ to 2+), which is linked to the presence of Mn antisite defects. These findings open the door to manipulating phase coexistence and achieving exotic properties in hybrid improper ferroelectric.

The competition between the polar and nonpolar phase in the prototypical hybrid improper ferroelectric crystal Ca₃Mn₂O₇ leads to exotic properties. Here, the authors directly imaged the crystal at atomic resolution to understand its nanostructure and discovered the double bilayer polar nanoregion.

Control of Oxygen Vacancy Ordering in Brownmillerite Thin Films via Ionic Liquid Gating

Oxygen defects and their atomic arrangements play a significant role in the physical properties of many transition metal oxides. The exemplary perovskite SrCoO_3-δ (P-SCO) is metallic and ferromagnetic. However, its daughter phase, the brownmillerite SrCoO_2.5 (BM-SCO), is insulating and an antiferromagnet. Moreover, BM-SCO exhibits oxygen vacancy channels (OVCs) that in thin films can be oriented either horizontally (H-SCO) or vertically (V-SCO) to the film's surface. To date, the orientation of these OVCs has been manipulated by control of the thin film deposition parameters or by using a substrate-induced strain. Here, we present a method to electrically control the OVC ordering in thin layers via ionic liquid gating (ILG). We show that H-SCO (antiferromagnetic insulator, AFI) can be converted to P-SCO (ferromagnetic metal, FM) and subsequently to V-SCO (AFI) by the insertion and subtraction of oxygen throughout thick films via ILG. Moreover, these processes are independent of substrate-induced strain which favors formation of H-SCO in the as-deposited film. The electric-field control of the OVC channels is a path toward the creation of oxitronic devices.

Mesoporous poorly crystalline α-Fe2O3 with abundant oxygen vacancies and acid sites for ozone decomposition

In this work, mesoporous poorly crystalline hematite (α-Fe₂O₃) was prepared using mesoporous silica (KIT-6) functionalized with 3-[(2-aminoethyl)amino]propyltrimethoxysilane as a hard template (SMPC-α-Fe₂O₃). The disordered atomic arrangement structure of SMPC-α-Fe₂O₃ promoted the formation of oxygen vacancies, which was confirmed using X-ray photoelectron spectroscopy (XPS), O₂-temperature-programmed desorption (TPD), H₂-temperature-programmed reduction (TPR), and in situ diffuse reflectance infrared Fourier transform (DRIFT) analyses. Density functional theory calculations (DFT) also proved that reducing the crystallinity of α-Fe₂O₃ decreased the formation energy of oxygen vacancies. TPD and in situ DRIFT analyses of NH₃ adsorption suggested that the surface acidity of SMPC-α-Fe₂O₃ was considerably higher than those of mesoporous poorly crystalline α-Fe₂O₃ (MPC-α-Fe₂O₃) and highly crystalline α-Fe₂O₃ (HC-α-Fe₂O₃). The oxygen vacancies and acid sites formed on α-Fe₂O₃ surface are beneficial for ozone (O₃) decomposition. Compared with MPC-α-Fe₂O₃ and HC-α-Fe₂O₃, SMPC-α-Fe₂O₃ exhibited a higher removal efficiency for 200-ppm O₃ at a space velocity of 720 L g^-1 h^-1 at 25 ± 2 °C under dry conditions. Additionally, in situ DRIFT and XPS results suggested that the accumulation of peroxide (O₂^2-) and the conversion of O₂^2- to lattice oxygen over the oxygen vacancies caused catalyst deactivation. However, O₂^2- could be desorbed completely by continuous N₂ purging at approximately 350 °C. This study provides significant insights for developing highly active α-Fe₂O₃ catalysts for O₃ decomposition.

Seeing Structural Mechanisms of Optimized Piezoelectric and Thermoelectric Bulk Materials through Structural Defect Engineering

Aberration-corrected scanning transmission electron microscopy (AC-STEM) has evolved into the most powerful characterization and manufacturing platform for all materials, especially functional materials with complex structural characteristics that respond dynamically to external fields. It has become possible to directly observe and tune all kinds of defects, including those at the crucial atomic scale. In-depth understanding and technically tailoring structural defects will be of great significance for revealing the structure-performance relation of existing high-property materials, as well as for foreseeing paths to the design of high-performance materials. Insights would be gained from piezoelectrics and thermoelectrics, two representative functional materials. A general strategy is highlighted for optimizing these functional materials’ properties, namely defect engineering at the atomic scale.

Engineering Multidefects on Cex Si1- x O2- δ Nanocomposites for the Catalytic Ozonation Reaction

Herein, it is shown that by engineering defects on Ce_x Si_{1- x} O_{2- δ} nanocomposites synthesized via flame spray pyrolysis, oxygen vacancies can be created with an increased density of trapped electrons, enhancing the formation of reactive oxygen species (ROSs) and hydroxyl radicals in an ozone-filled environment. Spectroscopic analysis and density functional theory calculations indicate that two-electron oxygen vacancies (O_V ⁰ ) or peroxide species, and their degree of clustering, play a critical role in forming reactive radicals. It is also found that a higher Si content in the binary oxide imposes a high O_V ⁰ ratio and, consequently, higher catalytic activity. Si inclusion in the nanocomposite appears to stabilize the surface oxygen vacancies as well as increase the reactive electron density at these sites. A mechanistic study on effective ROSs generated during catalytic ozonation reveals that the hydroxyl radical is the most effective ROS for organic degradation and is formed primarily through H₂ O₂ generation in the presence of the O_V ⁰ . Examining the binary oxides offers insights on the contribution of oxygen vacancies and their state of charge to catalytic reactions, in this instance for the catalytic ozonation of organic compounds.

Exploring the Spatial Control of Topotactic Phase Transitions Using Vertically Oriented Epitaxial Interfaces

Engineering oxygen vacancy formation and distribution is a powerful route for controlling the oxygen sublattice evolution that affects diverse functional behavior. The controlling of the oxygen vacancy formation process is particularly important for inducing topotactic phase transitions that occur by transformation of the oxygen sublattice. Here we demonstrate an epitaxial nanocomposite approach for exploring the spatial control of topotactic phase transition from a pristine perovskite phase to an oxygen vacancy-ordered brownmillerite (BM) phase in a model oxide La_0.7Sr_0.3MnO₃ (LSMO). Incorporating a minority phase NiO in LSMO films creates ultrahigh density of vertically aligned epitaxial interfaces that strongly influence the oxygen vacancy formation and distribution in LSMO. Combined structural characterizations reveal strong interactions between NiO and LSMO across the epitaxial interfaces leading to a topotactic phase transition in LSMO accompanied by significant morphology evolution in NiO. Using the NiO nominal ratio as a single control parameter, we obtain intermediate topotactic nanostructures with distinct distribution of the transformed LSMO-BM phase, which enables systematic tuning of magnetic and electrical transport properties. The use of self-assembled heterostructure interfaces by the epitaxial nanocomposite platform enables more versatile design of topotactic phase structures and correlated functionalities that are sensitive to oxygen vacancies.

Migration Kinetics of Surface Ions in Oxygen-Deficient Perovskite During Topotactic Transitions

Oxygen diffusivity and surface exchange kinetics underpin the ionic, electronic, and catalytic functionalities of complex multivalent oxides. Towards understanding and controlling the kinetics of oxygen transport in emerging technologies, it is highly desirable to reveal the underlying lattice dynamics and ionic activities related to oxygen variation. In this study, the evolution of oxygen content is identified in real-time during the progress of a topotactic phase transition in La_0.7 Sr_0.3 MnO_3-δ epitaxial thin films, both at the surface and throughout the bulk. Using polarized neutron reflectometry, a quantitative depth profile of the oxygen content gradient is achieved, which, alongside atomic-resolution scanning transmission electron microscopy, uniquely reveals the formation of a novel structural phase near the surface. Surface-sensitive X-ray spectroscopies further confirm a significant change of the electronic structure accompanying the transition. The anisotropic features of this novel phase enable a distinct oxygen diffusion pathway in contrast to conventional observation of oxygen motion at moderate temperatures. The results provide insights furthering the design of solid oxygen ion conductors within the framework of topotactic phase transitions.

Electronic-structure evolution of SrFeO3-xduring topotactic phase transformation

Oxygen-vacancy-induced topotactic phase transformation between the ABO_2.5brownmillerite structure and the ABO₃perovskite structure attracts ever-increasing attention due to the perspective applications in catalysis, clean energy field, and memristors. However, a detailed investigation of the electronic-structure evolution during the topotactic phase transformation for understanding the underlying mechanism is highly desired. In this work, multiple analytical methods were used to explore evolution of the electronic structure of SrFeO_3-xthin films during the topotactic phase transformation. The results indicate that the increase in oxygen content induces a new unoccupied state of O 2pcharacter near the Fermi energy, inducing the insulator-to-metal transition. More importantly, the hole states are more likely constrained to thedx²-y²orbital than to thed3z²-r²orbital. Our results reveal an unambiguous evolution of the electronic structure of SrFeO_3-xfilms during topotactic phase transformation, which is crucial not only for fundamental understanding but also for perspective applications such as solid-state oxide fuel cells, catalysts, and memristor devices.

Negatively Charged In-Plane and Out-Of-Plane Domain Walls with Oxygen-Vacancy Agglomerations in a Ca-Doped Bismuth-Ferrite Thin Film

The interaction of oxygen vacancies and ferroelectric domain walls is of great scientific interest because it leads to different domain-structure behaviors. Here, we use high-resolution scanning transmission electron microscopy to study the ferroelectric domain structure and oxygen-vacancy ordering in a compressively strained Bi_0.9Ca_0.1FeO_3-δ thin film. It was found that atomic plates, in which agglomerated oxygen vacancies are ordered, appear without any periodicity between the plates in out-of-plane and in-plane orientation. The oxygen non-stoichiometry with δ ≈ 1 in FeO_2-δ planes is identical in both orientations and shows no preference. Within the plates, the oxygen vacancies form 1D channels in a pseudocubic [010] direction with a high number of vacancies that alternate with oxygen columns with few vacancies. These plates of oxygen vacancies always coincide with charged domain walls in a tail-to-tail configuration. Defects such as ordered oxygen vacancies are thereby known to lead to a pinning effect of the ferroelectric domain walls (causing application-critical aspects, such as fatigue mechanisms and countering of retention failure) and to have a critical influence on the domain-wall conductivity. Thus, intentional oxygen vacancy defect engineering could be useful for the design of multiferroic devices with advanced functionality.

Electrically Enhanced Exchange Bias via Solid-State Magneto-ionics

Electrically induced ionic motion offers a new way to realize voltage-controlled magnetism, opening the door to a new generation of logic, sensor, and data storage technologies. Here, we demonstrate an effective approach to magneto-ionically and electrically tune the exchange bias in Gd/Ni_1-xCo_xO thin films (x = 0.50 and 0.67), where neither of the layers alone is ferromagnetic at room temperature. The Gd capping layer deposited onto antiferromagnetic Ni_1-xCo_xO initiates a solid-state redox reaction that reduces an interfacial region of the oxide to ferromagnetic NiCo. An exchange bias is established after field cooling (FC), which can be enhanced by up to 35% after a voltage conditioning and subsequently reset with a second FC. These effects are caused by the presence of an interfacial ferromagnetic NiCo layer, which further alloys with the Gd layer upon FC and voltage application, as confirmed by electron microscopy and polarized neutron reflectometry studies. These results highlight the viability of the solid-state magneto-ionic approach to achieve electric control of exchange bias, with potential for energy-efficient magneto-ionic devices.

Towards Automatic Detection of Precipitates in Inconel 625 Superalloy Additively Manufactured by the L-PBF Method

In our study, the comparison of the automatically detected precipitates in L-PBF Inconel 625, with experimentally detected phases and with the results of the thermodynamic modeling was used to test their compliance. The combination of the complementary electron microscopy techniques with the microanalysis of chemical composition allowed us to examine the structure and chemical composition of related features. The possibility of automatic detection and identification of precipitated phases based on the STEM-EDS data was presented and discussed. The automatic segmentation of images and identifying of distinguishing regions are based on the processing of STEM-EDS data as multispectral images. Image processing methods and statistical tools are applied to maximize an information gain from data with low signal-to-noise ratio, keeping human interactions on a minimal level. The proposed algorithm allowed for automatic detection of precipitates and identification of interesting regions in the Inconel 625, while significantly reducing the processing time with acceptable quality of results.

Flexoelectric Thin-Film Photodetectors

The flexoelectric effect, which manifests itself as a strain-gradient-induced electrical polarization, has triggered great interest due to its ubiquitous existence in crystalline materials without the limitation of lattice symmetry. Here, we propose a flexoelectric photodetector based on a thin-film heterostructure. This prototypical device is demonstrated by epitaxial LaFeO₃ thin films grown on LaAlO₃ substrates. A giant strain gradient of the order of 10⁶/m is achieved in LaFeO₃ thin films, giving rise to an obvious flexoelectric polarization and generating a significant photovoltaic effect in the LaFeO₃-based heterostructures with nanosecond response under light illumination. This work not only demonstrates a novel self-powered photodetector different from the traditional interface-type structures, such as the p-n and Schottky junctions but also opens an avenue to design practical flexoelectric devices for nanoelectronics applications.

Cooperative evolution of polar distortion and nonpolar rotation of oxygen octahedra in oxide heterostructures

Polarity discontinuity across LaAlO₃/SrTiO₃ (LAO/STO) heterostructures induces electronic reconstruction involving the formation of two-dimensional electron gas (2DEG) and structural distortions characterized by antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. We show that AFD and FE modes are cooperatively coupled in LAO/STO (111) heterostructures; they coexist below the critical thickness (t _c) and disappear simultaneously above t _c with the formation of 2DEG. Electron energy-loss spectroscopy and density functional theory (DFT) calculations provide direct evidence of oxygen vacancy (V _O) formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD rotation and FE distortion of LAO reveals that their evolution is strongly correlated with V _O distribution. The present study demonstrates that AFD and FE modes in oxide heterostructures emerge as a consequence of interplay between misfit strain and polar field, and further that their combination can be tuned to competitive or cooperative coupling by changing the interface orientation.

Overlayer deposition-induced control of oxide ion concentration in SrFe0.5Co0.5O2.5 oxygen sponges

Controlling the oxide ion (O²⁻) concentration in oxides is essential to develop advanced ionic devices, i.e. solid oxide fuel cells, smart windows, memory devices, energy storage devices, and so on. Among many oxides several transition metal (TM)-based perovskite oxides show high oxide ion conductivity, and their physical properties show high sensitivity to the change of the oxide ion concentration. Here, the change in the oxide ion concentration is shown through the overlayer deposition on the SrFe_0.5Co_0.5O_2.5 (SFCO) oxygen sponge film. We grew SFCO films followed by the deposition of two kinds of complex oxide films under exactly the same growth conditions, and observed the changes in the crystal structure, valence states, and magnetic ground states. As the NSMO overlayer grows, strong evidence of oxidation at the O K edge is shown. In addition, the Fe⁴⁺ feature is revealed, and the electron valence state of Co increased from 3 to 3.25. The oxide ion concentration of SFCO changes during layer growth due to oxidation or reduction due to differences in chemical potential. The present results might be useful to develop advanced ionic devices using TM-based perovskite oxides.

We form flat oxide-interfaces, i.e. manganite- and titanate-oxygen sponges, and observe peculiar oxygen transport behaviors relying on chemical potential difference at the interfaces. Oxygen transport into oxygen-sponge leads weak ferromagnetism.

Probing One-Dimensional Oxygen Vacancy Channels Driven by Cation-Anion Double Ordering in Perovskites

Visualizing the oxygen vacancy distributions is highly desirable for understanding the atomistic oxygen diffusion mechanisms in perovskites. In particular, the direct observation of the one-dimensional oxygen vacancy channels has not yet been achieved in perovskites with dual ion (i.e., cation and anion) ordering. Here, we perform atomic-resolution imaging of the one-dimensional oxygen vacancy channels and their structural dynamics in a NdBaCo₂O_5.5 double perovskite oxide. An in situ heating transmission electron microscopy investigation reveals the disordering of oxygen vacancy channels by local rearrangement of oxygen vacancies at the specific temperature. A density functional theory calculation suggests that the possible pathway of oxygen vacancy migration is a multistep route via Co-O and Nd-O_v (oxygen vacancy) sites. These findings could provide robust guidance for understanding the static and dynamic behaviors of oxygen vacancies in perovskite oxides.

Electric field control of ordered oxygen vacancy planes and antiferromagnetic structures in strontium cobaltite

The polarized ionic liquids (ILs) could generate intense electric fields on the surface of solid-state materials and create functional defects by ion migration within them, resulting in phase transitions of metal-insulator or paramagnet-ferromagnet, etc. Such a strong electric field even provides an opportunity for the control of spin ordering in antiferromagnetic (AFM) crystal which is difficult to be manipulated due to the strong exchange coupling between antiparallel spins in the whole bulk. Here we find that the ferromagnetic SrCoO₃of 40 nm could be transformed to AFM SrCoO_2.5with ordered oxygen vacancy planes either vertical (V-SrCoO_2.5) or parallel (P-SrCoO_2.5) to the surface by IL gating. The spin Hall magnetoresistances suggest that the AFM easy axes of V- and P-SrCoO_2.5are along [010] and11¯0, respectively. The orientations of gating induced oxygen vacancy planes are related to the oxygen framework rotation in the parent SrCoO₃and could be controlled by the strain engineering. Our results not only supply a novel way to manipulate the AFM spins by creating functional ordered defects, but also reveal the effect of oxygen framework rotation on the formation of oxygen vacancies under ionic liquid gating.

Engineering of Charged Defects at Perovskite Oxide Surfaces for Exceptionally Stable Solid Oxide Fuel Cell Electrodes

Cation segregation, particularly Sr segregation, toward a perovskite surface has a significant effect on the performance degradation of a solid oxide cell (solid oxide electrolysis/fuel cell). Among the number of key reasons generating the instability of perovskite oxide, surface-accumulated positively charged defects (oxygen vacancy, V_o^··) have been considered as the most crucial drivers in strongly attracting negatively charged defects (Sr_A - site^') toward the surface. Herein, we demonstrate the effects of a heterointerface on the redistribution of both positively and negatively charged defects for a reduction of V_o^·· at a perovskite surface. We took Sm_0.5Sr_0.5CoO_3-δ (SSC) as a model perovskite film and coated Gd_0.1Ce_0.9O_2-δ (GDC) additionally onto the SSC film to create a heterointerface (GDC/SSC), resulting in an ∼11-fold reduction in a degradation rate of ∼8% at 650 °C and ∼10-fold higher surface exchange (k^q) than a bare SSC film after 150 h at 650 °C. Using X-ray photoelectron spectroscopy and electron energy loss spectroscopy, we revealed a decrease in positively charged defects of V_o^·· and transferred electrons in an SSC film at the GDC/SSC heterointerface, resulting in a suppression of negatively charged Sr (Sr_Sm^') segregation. Finally, the energetic behavior, including the charge transfer phenomenon, O p-band center, and oxygen vacancy formation energy calculated using the density functional theory, verified the effects of the heterointerface on the redistribution of the charged defects, resulting in a remarkable impact on the stability of perovskite oxide at elevated temperatures.

Nanoscale Phase Mixture and Multifield-Induced Topotactic Phase Transformation in SrFeOx

Nanoscale phase mixtures in transition-metal oxides (TMOs) often render these materials susceptible to external stimuli (electric field, mechanical stress, etc.), which can lead to rich functional properties and device applications. Here, direct observation and multifield manipulation of a nanoscale mixture of brownmillerite SrFeO_2.5 (BM-SFO) and perovskite SrFeO₃ (PV-SFO) phases in SrFeO_x (SFO) epitaxial thin films are reported. The mixed-phase SFO film in its pristine state exhibits a nanoscaffold structure consisting of PV-SFO nanodomains embedded in the BM-SFO matrix. This nanoscaffold structure produces gridlike patterns in the current and electrochemical strain maps, owing to the strikingly different electrical and electrochemical properties of BM-SFO and PV-SFO. Moreover, electric field control of reversible topotactic phase transformation between BM-SFO and PV-SFO is demonstrated by electric-field-induced reversible changes in surface height, conductance, and electrochemical strain response. In addition, it is also shown that the BM-SFO → PV-SFO phase transformation can be enabled by applying mechanical stress. This study therefore not only identifies a strong nanometric structure-property correlation in the mixed-phase SFO but also offers a new paradigm for the multifield control of topotactic phase transformation.

Efficient, stable silicon tandem cells enabled by anion-engineered wide-bandgap perovskites

Maximizing the power conversion efficiency (PCE) of perovskite/silicon tandem solar cells that can exceed the Shockley-Queisser single-cell limit requires a high-performing, stable perovskite top cell with a wide bandgap. We developed a stable perovskite solar cell with a bandgap of ~1.7 electron volts that retained more than 80% of its initial PCE of 20.7% after 1000 hours of continuous illumination. Anion engineering of phenethylammonium-based two-dimensional (2D) additives was critical for controlling the structural and electrical properties of the 2D passivation layers based on a lead iodide framework. The high PCE of 26.7% of a monolithic two-terminal wide-bandgap perovskite/silicon tandem solar cell was made possible by the ideal combination of spectral responses of the top and bottom cells.

Correlation between structural phase transition and surface chemical properties of thin film SrRuO3/SrTiO3 (001)

The correlation between the structural phase transition (SPT) and oxygen vacancy in SrRuO₃ (SRO) thin films was investigated by in situ X-ray diffraction (XRD) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In situ XRD shows that the SPT occurs from a monoclinic SRO phase to a tetragonal SRO phase near ∼200 °C, regardless of the pressure environment. On the other hand, significant core level shifts in both the Ru and Sr photoemission spectra are found under ultrahigh vacuum, but not under the oxygen pressure environment. The directions and behavior of the core level shift of Ru and Sr are attributed to the formation of oxygen vacancy across the SPT temperature of SRO. The analysis of in situ XRD and AP-XPS results provides an evidence for the formation of metastable surface oxide possibly due to the migration of internal oxygen atoms across the SPT temperature, indicating the close relationship between oxygen vacancy and SPT in SRO thin films.

Surface-reaction induced structural oscillations in the subsurface

Surface and subsurface are commonly considered as separate entities because of the difference in the bonding environment and are often investigated separately due to the experimental challenges in differentiating the surface and subsurface effects. Using in-situ atomic-scale transmission electron microscopy to resolve the surface and subsurface at the same time, we show that the hydrogen-CuO surface reaction results in structural oscillations in deeper atomic layers via the cycles of ordering and disordering of oxygen vacancies in the subsurface. Together with atomistic calculations, we show that the structural oscillations in the subsurface are induced by the hydrogen oxidation-induced cyclic loss of oxygen from the oxide surface. These results demonstrate the propagation of the surface reaction dynamics into the deeper layers in inducing nonstoichiometry in the subsurface and have significant implications in modulating various chemical processes involving surface-subsurface mass transport such as heterogeneous catalysis, oxidation, corrosion and carburization.

Emerging Electron Microscopy Techniques for Probing Functional Interfaces in Energy Materials

Interfaces play a fundamental role in many areas of chemistry. However, their localized nature requires characterization techniques with high spatial resolution in order to fully understand their structure and properties. State-of-the-art atomic resolution or in situ scanning transmission electron microscopy and electron energy-loss spectroscopy are indispensable tools for characterizing the local structure and chemistry of materials with single-atom resolution, but they are not able to measure many properties that dictate function, such as vibrational modes or charge transfer, and are limited to room-temperature samples containing no liquids. Here, we outline emerging electron microscopy techniques that are allowing these limitations to be overcome and highlight several recent studies that were enabled by these techniques. We then provide a vision for how these techniques can be paired with each other and with in situ methods to deliver new insights into the static and dynamic behavior of functional interfaces.

Strain-Inhibited Electromigration of Oxygen Vacancies in LaCoO3

The oxygen vacancy profile in LaCoO₃ exhibits rich phases with distinct structures, symmetries, and magnetic properties. Exploration of the lattice degree of freedom of LaCoO₃ in the transition between these different structural phases may provide a route to enable new functionality in oxide materials with potential applications. To date, the oxygen vacancy profile transition in LaCoO₃ has mainly been induced by transition-metal doping or thermal treatment. Epitaxial strain was proposed to compete with the lattice degree of freedom but has not yet been rationalized. Here, the experimental findings of strain-inhibited structural transition from perovskite to brownmillerite during the electromigration of oxygen vacancies in epitaxial LaCoO₃ thin films are demonstrated. The results indicate that the oxygen vacancy ordering phase induced by the electric field is suppressed locally by both epitaxial strain field and external loads shown by in situ aberration-corrected (scanning)/ transmission electron microscopy. The demonstrated complex interplay between the electric and strain fields in the structural transitions of LaCoO₃ opens up prospects for manipulating new physical properties by external excitations and/or strain engineering of a substrate.

In Situ Method Correlating Raman Vibrational Characteristics to Chemical Expansion via Oxygen Nonstoichiometry of Perovskite Thin Films

Effective integration of perovskite films into devices requires knowledge of their electro-chemomechanical properties. Raman spectroscopy is an excellent tool for probing such properties as the films' vibrational characteristics couple to the lattice volumetric changes during chemical expansion. While lattice volumetric changes are typically accessed by analyzing Raman shifts as a function of pressure, stress, or temperature, such methods can be impractical for thin films and do not capture information on chemical expansion. An in situ Raman spectroscopy technique using an electrochemical titration cell to change the oxygen nonstoichiometry of a model perovskite film, Sr(Ti,Fe)O_{3- y}  , is reported and the lattice vibrational properties are correlated to the material's chemical expansion. How to select an appropriate Raman vibrational mode to track the evolution in oxygen nonstoichiometry is discussed. Subsequently, the frequency of the oxygen stretching mode around Fe⁴⁺ is tracked, as it decreases during reduction as the material expands and increases during reoxidation as the material shrinks. This methodology of oxygen pumping and in situ Raman spectroscopy of oxide films enables future in operando measurements even for small material volumes, as is typical for applications of films as electrodes or electrolytes utilized in electrochemical energy conversion or memory devices.

A Mild in-Situ Method to Construct Fe-Doped Cauliflower-Like Rutile TiO2 Photocatalysts for Degradation of Organic Dye in Wastewater

A mild in situ method was developed to construct an iron doped rutile TiO2 photocatalyst like cauliflower for degradation synthetic textile dye-methyl orange. The synthesized photocatalysts presented distinguished photocatalytic activity. At the optimal Fe concentration (0.5%), the decomposition rate of methyl orange (MO) was about 90% under 40 min of ultraviolet (UV) light irradiation. Whereas, to our knowledge, only 70% of the decomposition rate of MO was achieved by commercial photocatalyst P25 under the similar reaction condition. Additionally, the rutile preparation temperature did not exceed 100 °C, which was much lower than the traditional preparation calcination temperature (e.g., 600 °C). The specific surface area of Fe doped catalysts was bigger than that of the control sample and the catalyst characterization indicated that the doped iron was incorporated into the rutile TiO2 lattice and resulted in the lattice disorder. The lattice disorder would have generated surface defects in the crystal structure, which was in favor of the photocatalytic reaction. The UV-Vis diffuse refection characterization and Density Functional Theory (DFT) calculation suggested that doping a small amount of Fe into the lattice of rutile would lead to a narrower band gap and the formation of a doping energy level between conduction and valence bands of TiO2. This further increased the degradation efficiency of synthetic textile dyes in wastewaters. Our study has provided a relatively easy operation for synthesis Fe doped rutile TiO2, which is a benefit to decrease the cost in wastewater treatment process.

Reversible Control of Physical Properties via an Oxygen-Vacancy-Driven Topotactic Transition in Epitaxial La0.7 Sr0.3 MnO3- δ Thin Films

The vacancy distribution of oxygen and its dynamics directly affect the functional response of complex oxides and their potential applications. Dynamic control of the oxygen composition may provide the possibility to deterministically tune the physical properties and establish a comprehensive understanding of the structure-property relationship in such systems. Here, an oxygen-vacancy-induced topotactic transition from perovskite to brownmillerite and vice versa in epitaxial La_0.7 Sr_0.3 MnO_{3- δ} thin films is identified by real-time X-ray diffraction. A novel intermediate phase with a noncentered crystal structure is observed for the first time during the topotactic phase conversion which indicates a distinctive transition route. Polarized neutron reflectometry confirms an oxygen-deficient interfacial layer with drastically reduced nuclear scattering length density, further enabling a quantitative determination of the oxygen stoichiometry (La_0.7 Sr_0.3 MnO_2.65 ) for the intermediate state. Associated physical properties of distinct topotactic phases (i.e., ferromagnetic metal and antiferromagnetic insulator) can be reversibly switched by an oxygen desorption/absorption cycling process. Importantly, a significant lowering of necessary conditions (temperatures below 100 °C and conversion time less than 30 min) for the oxygen reloading process is found. These results demonstrate the potential applications of defect engineering in the design of perovskite-based functional materials.

Probing vacancy behavior across complex oxide heterointerfaces

Oxygen vacancies ( V O • • ) play a critical role as defects in complex oxides in establishing functionality in systems including memristors, all-oxide electronics, and electrochemical cells that comprise metal-insulator-metal or complex oxide heterostructure configurations. Improving oxide-oxide interfaces necessitates a direct, spatial understanding of vacancy distributions that define electrochemically active regions. We show vacancies deplete over micrometer-level distances in Nb-doped SrTiO₃ (Nb:SrTiO₃) substrates due to deposition and post-annealing processes. We convert the surface potential across a strontium titanate/yttria-stabilized zirconia (STO/YSZ) heterostructured film to spatial (<100 nm) vacancy profiles within STO using (T = 500°C) in situ scanning probes and semiconductor analysis. Oxygen scavenging occurring during pulsed laser deposition reduces Nb:STO substantially, which partially reoxidizes in an oxygen-rich environment upon cooling. These results (i) introduce the means to spatially resolve quantitative vacancy distributions across oxide films and (ii) indicate the mechanisms by which oxide thin films enhance and then deplete vacancies within the underlying substrate.

Metal Oxide Nanocomposites: A Perspective from Strain, Defect, and Interface

Vertically aligned nanocomposite thin films with ordered two phases, grown epitaxially on substrates, have attracted tremendous interest in the past decade. These unique nanostructured composite thin films with large vertical interfacial area, controllable vertical lattice strain, and defects provide an intriguing playground, allowing for the manipulation of a variety of functional properties of the materials via the interplay among strain, defect, and interface. This field has evolved from basic growth and characterization to functionality tuning as well as potential applications in energy conversion and information technology. Here, the remarkable progress achieved in vertically aligned nanocomposite thin films from a perspective of tuning functionalities through control of strain, defect, and interface is summarized.

Oxygen Vacancy Ordering Modulation of Magnetic Anisotropy in Strained LaCoO3- x Thin Films

Oxygen vacancy configurations and concentration are coupled with the magnetic, electronic, and transport properties of perovskite oxides, and manipulating the physical properties by tuning the vacancy structures of thin films is crucial for applications in many functional devices. In this study, we report a direct atomic resolution observation of the preferred orientation of vacancy ordering structure in the epitaxial LaCoO_{3- x} (LCO) thin films under various strains from large compressive to large tensile strain utilizing scanning transmission electron microscopy (STEM). Under compressive strains, the oxygen vacancy ordering prefers to be along the planes parallel to the heterointerface. Changing the strains from compressive to tensile, the oxygen vacancy planes turn to be perpendicular to the heterointerface. Aberration-corrected STEM images, electron diffractions, and X-ray diffraction combined with X-ray photoelectron spectroscopy demonstrate that the vacancy concentration increases with increasing misfit strains and vacancy distribution is more ordered and homogeneous. The temperature-dependent magnetization curves show the Curie temperature increases, displaying a positive correlation with the misfit strains. With change in the strain from compressive to tensile, anisotropy fields vary and show large values under tensile strains. It is proposed that oxygen vacancy concentration and preferred ordering planes are responsible for the enhanced magnetic properties of LCO films. Our results have realized a controllable preparation of oxygen vacancy ordering structures via strains and thus provide an effective method to regulate and optimize the physical properties such as magnetic properties by strain engineering.

Direct imaging of structural changes induced by ionic liquid gating leading to engineered three-dimensional meso-structures

The controlled transformation of materials, both their structure and their physical properties, is key to many devices. Ionic liquid gating can induce the transformation of thin-film materials over long distances from the gated surface. Thus, the mechanism underlying this process is of considerable interest. Here we directly image, using in situ, real-time, high-resolution transmission electron microscopy, the reversible transformation between the oxygen vacancy ordered phase brownmillerite SrCoO_2.5 and the oxygen ordered phase perovskite SrCoO₃. We show that the phase transformation boundary moves at a velocity that is highly anisotropic, traveling at speeds ~30 times faster laterally than through the thickness of the film. Taking advantage of this anisotropy, we show that three-dimensional metallic structures such as cylinders and rings can be realized. Our results provide a roadmap to the construction of complex meso-structures from their exterior surfaces.

Local and reversible oxidation is used to exploit the very different properties of oxygen and vacancy ordered oxides. Here the authors directly image and make use of anisotropic migration velocities of oxygen in SrCoO_x to create 3D meso-structures of those two phases by ionic liquid gating.

Atomic-Scale Measurement of Flexoelectric Polarization at SrTiO_{3} Dislocations

Owing to the broken translational symmetry at dislocations, a strain gradient naturally exists around the dislocation cores and can significantly influence the electrical and mechanical properties. We use aberration corrected scanning transmission electron microscopy to directly measure the flexoelectric polarization (∼28  μC cm^{-2}) at dislocation cores in SrTiO_{3}. The polarization charges can interact with the nonstoichiometric dislocation cores and thus impact the electrical activities. Our findings can help us to understand the properties of dislocations in perovskite, providing new insights into the design of new devices via defect engineering such as bicrystal fabrication and thin film growth.

Feasible Defect Engineering by Employing Metal Organic Framework Templates into One-Dimensional Metal Oxides for Battery Applications

Facile synthesis of rationally designed nanostructured electrode materials with high reversible capacity is highly critical to meet ever-increasing demands for lithium-ion batteries. In this work, we employed defect engineering by incorporating metal organic framework (MOF) templates into one-dimensional nanostructures by simple electrospinning and subsequent calcination. The introduction of Co-based zeolite imidazole frameworks (ZIF-67) resulted in abundant oxygen vacancies, which induce not only more active sites for Li storage but also enhanced electrical conductivity. Moreover, abundant mesoporous sites are formed by the decomposition of ZIF-67, which are present both inside and outside the resultant SnO₂-Co₃O₄ nanofibers (NFs). Attributed to the creation of vacancy sites along with the synergistic effects of SnO₂ and Co₃O₄, SnO₂-Co₃O₄ NFs exhibit an excellent reversible capacity for 300 cycles (1287 mA h g^-1 at a current density of 500 mA g^-1) along with superior rate capabilities and improved initial Coulombic efficiency compared with pristine SnO₂ NFs. This is an early report on utilizing MOF structures as the defect formation platform into one-dimensional nanostructures, which is expected to result in superior electrochemical performances required for advanced electrodes.

Creation and Ordering of Oxygen Vacancies at WO3-δ and Perovskite Interfaces

Changes in the structure and composition resulting from oxygen deficiency can strongly impact the physical and chemical properties of transition-metal oxides, which may lead to new functionalities for novel electronic devices. Oxygen vacancies (V_O) can be readily formed to accommodate the lattice mismatch during epitaxial thin film growth. In this paper, the effects of substrate strain and oxidizing power on the creation and distribution of V_O in WO_3-δ thin films are investigated in detail. An ¹⁸O₂ isotope-labeled time-of-flight secondary-ion mass spectrometry study reveals that WO_3-δ films grown on SrTiO₃ substrates display a significantly larger oxygen vacancy gradient along the growth direction compared to those grown on LaAlO₃ substrates. This result is corroborated by scanning transmission electron microscopy imaging, which reveals a large number of defects close to the interface to accommodate interfacial tensile strain, leading to the ordering of V_O and the formation of semi-aligned Magnéli phases. The strain is gradually released and a tetragonal phase with much better crystallinity is observed at the film/vacuum interface. The changes in the structure resulting from oxygen defect creation are shown to have a direct impact on the electronic and optical properties of the films.

Material structure, properties, and dynamics through scanning transmission electron microscopy

Scanning transmission electron microscopy (STEM) has advanced rapidly in the last decade thanks to the ability to correct the major aberrations of the probe-forming lens. Now, atomic-sized beams are routine, even at accelerating voltages as low as 40 kV, allowing knock-on damage to be minimized in beam sensitive materials. The aberration-corrected probes can contain sufficient current for high-quality, simultaneous, imaging and analysis in multiple modes. Atomic positions can be mapped with picometer precision, revealing ferroelectric domain structures, composition can be mapped by energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS), and charge transfer can be tracked unit cell by unit cell using the EELS fine structure. Furthermore, dynamics of point defects can be investigated through rapid acquisition of multiple image scans. Today STEM has become an indispensable tool for analytical science at the atomic level, providing a whole new level of insights into the complex interplays that control material properties.

Defect-Induced Hedgehog Polarization States in Multiferroics

Continuous developments in nanotechnology require new approaches to materials synthesis that can produce novel functional structures. Here, we show that nanoscale defects, such as nonstoichiometric nanoregions (NSNRs), can act as nano-building blocks for creating complex electrical polarization structures in the prototypical multiferroic BiFeO_{3}. An array of charged NSNRs are produced in BiFeO_{3} thin films by tuning the substrate temperature during film growth. Atomic-scale scanning transmission electron microscopy imaging reveals exotic polarization rotation patterns around these NSNRs. These polarization patterns resemble hedgehog or vortex topologies and can cause local changes in lattice symmetries leading to mixed-phase structures resembling the morphotropic phase boundary with high piezoelectricity. Phase-field simulations indicate that the observed polarization configurations are mainly induced by charged states at the NSNRs. Engineering defects thus may provide a new route for developing ferroelectric- or multiferroic-based nanodevices.

Manipulating the Structural and Electronic Properties of Epitaxial SrCoO2.5 Thin Films by Tuning the Epitaxial Strain

Structure determines material's functionality, and strain tunes the structure. Tuning the coherent epitaxial strain by varying the thickness of the films is a precise route to manipulate the functional properties in the low-dimensional oxide materials. Here, to explore the effects of the coherent epitaxial strain on the properties of SrCoO_2.5 thin films, thickness-dependent evolutions of the structural properties and electronic structures were investigated by X-ray diffraction, Raman spectra, optical absorption spectra, scanning transmission electron microscopy (STEM), and first-principles calculations. By increasing the thickness of the SrCoO_2.5 films, the c-axis lattice constant decreases, indicating the relaxation of the coherent epitaxial strain. The energy band gap increases and the Raman spectra undergo a substantial softening with the relaxation of the coherent epitaxial strain. From the STEM results, it can be concluded that the strain causes the variation of the oxygen content in the BM-SCO2.5 films, which results in the variation of band gaps with varying the strain. First-principles calculations show that strain-induced changes in bond lengths and angles of the octahedral CoO₆ and tetrahedral CoO₄ cannot explain the variation band gap. Our findings offer an alternative strategy to manipulate structural and electronic properties by tuning the coherent epitaxial strain in transition-metal oxide thin films.

Surface-screening mechanisms in ferroelectric thin films and their effect on polarization dynamics and domain structures

For over 70 years, ferroelectric materials have been one of the central research topics for condensed matter physics and material science, an interest driven both by fundamental science and applications. However, ferroelectric surfaces, the key component of ferroelectric films and nanostructures, still present a significant theoretical and even conceptual challenge. Indeed, stability of ferroelectric phase per se necessitates screening of polarization charge. At surfaces, this can lead to coupling between ferroelectric and semiconducting properties of material, or with surface (electro) chemistry, going well beyond classical models applicable for ferroelectric interfaces. In this review, we summarize recent studies of surface-screening phenomena in ferroelectrics. We provide a brief overview of the historical understanding of the physics of ferroelectric surfaces, and existing theoretical models that both introduce screening mechanisms and explore the relationship between screening and relevant aspects of ferroelectric functionalities starting from phase stability itself. Given that the majority of ferroelectrics exist in multiple-domain states, we focus on local studies of screening phenomena using scanning probe microscopy techniques. We discuss recent studies of static and dynamic phenomena on ferroelectric surfaces, as well as phenomena observed under lateral transport, light, chemical, and pressure stimuli. We also note that the need for ionic screening renders polarization switching a coupled physical-electrochemical process and discuss the non-trivial phenomena such as chaotic behavior during domain switching that stem from this.

Atomic Resolution Imaging of Nanoscale Chemical Expansion in PrxCe1-xO2-δ during In Situ Heating

Thin film nonstoichiometric oxides enable many high-temperature applications including solid oxide fuel cells, actuators, and catalysis. Large concentrations of point defects (particularly, oxygen vacancies) enable fast ionic conductivity or gas exchange kinetics in these materials but also manifest as coupling between lattice volume and chemical composition. This chemical expansion may be either detrimental or useful, especially in thin film devices that may exhibit enhanced performance through strain engineering or decreased operating temperatures. However, thin film nonstoichiometric oxides can differ from bulk counterparts in terms of operando defect concentrations, transport properties, and mechanical properties. Here, we present an in situ investigation of atomic-scale chemical expansion in Pr_xCe_1-xO_2-δ (PCO), a mixed ionic-electronic conducting oxide relevant to electrochemical energy conversion and high-temperature actuation. Through a combination of electron energy loss spectroscopy and transmission electron microscopy with in situ heating, we characterized chemical strains and changes in oxidation state in cross sections of PCO films grown on yttria-stabilized zirconia (YSZ) at temperatures reaching 650 °C. We quantified, both statically and dynamically, the nanoscale chemical expansion induced by changes in PCO redox state as a function of position and direction relative to the film-substrate interface. Additionally, we observed dislocations at the film-substrate interface, as well as reduced cation localization to threading defects within PCO films. These results illustrate several key aspects of atomic-scale structure and mechanical deformation in nonstoichiometric oxide films that clarify distinctions between films and bulk counterparts and that hold several implications for operando chemical expansion or "breathing" of such oxide films.