DFT+U and quantum Monte Carlo study of electronic and optical properties of AgNiO2 and AgNi1-xCoxO2 delafossite.

As the only semimetallic d10-based delafossite, AgNiO2 has received a great deal of attention due to both its unique semimetallicity and its antiferromagnetism in the NiO2 layer that is coupled with a lattice distortion. In contrast, other delafossites such as AgCoO2 are insulating. Here we study how the electronic structure of AgNi1-xCoxO2 alloys vary with Ni/Co concentration, in order to investigate the electronic properties and phase stability of the intermetallics. While the electronic and magnetic structure of delafossites have been studied using density functional theory (DFT), earlier studies have not included corrections for strong on-site Coulomb interactions. In order to treat these interactions accurately, in this study we use Quantum Monte Carlo (QMC) simulations to obtain accurate estimates for the electronic and magnetic properties of AgNiO2. By comparison to DFT results we show that these electron correlations are critical to account for. We show that Co doping on the magnetic Ni sites results in a metal-insulator transition near x ∼0.33, and reentrant behavior near x ∼ 0.66.

Stable Supercapacity of Binder-Free TiO2(B) Epitaxial Electrodes for All-Solid-State Nanobatteries.

Owing to its pseudocapacitive, unidimensional, rapid ion channels, TiO2(B) is a promising material for application to battery electrodes. In this study, we align these channels by epitaxially growing TiO2(B) films with the assistance of an isostructural VO2(B) template layer. In a liquid electrolyte, binder-free TiO2(B) epitaxial electrodes exhibit a supercapacity near the theoretical value of 335 mA h g-1 and an excellent charge-discharge reproducibility for ≥200 cycles, which outperform those of other TiO2(B) nanostructures. For the all-solid-state configuration employing the LiPON solid electrolyte, excellent stability persists. Our findings suggest excellent potential for miniaturizing all-solid-state nanobatteries in self-powered integrated circuits.

Phase Transition Dynamics in a Complex Oxide Heterostructure.

Understanding the behavior of defects in the complex oxides is key to controlling myriad ionic and electronic properties in these multifunctional materials. The observation of defect dynamics, however, requires a unique probe-one sensitive to the configuration of defects as well as its time evolution. Here, we present measurements of oxygen vacancy ordering in epitaxial thin films of SrCoO_{x} and the brownmillerite-perovskite phase transition employing x-ray photon correlation spectroscopy. These and associated synchrotron measurements and theory calculations reveal the close interaction between the kinetics and the dynamics of the phase transition, showing how spatial and temporal fluctuations of heterointerface evolve during the transformation process. The energetics of the transition are correlated with the behavior of oxygen vacancies, and the dimensionality of the transformation is shown to depend strongly on whether the phase is undergoing oxidation or reduction. The experimental and theoretical methods described here are broadly applicable to in situ measurements of dynamic phase behavior and demonstrate how coherence may be employed for novel studies of the complex oxides as enabled by the arrival of fourth-generation hard x-ray coherent light sources.

Strain-Induced Atomic-Scale Building Blocks for Ferromagnetism in Epitaxial LaCoO3.

The origin of strain-induced ferromagnetism, which is robust regardless of the type and degree of strain in LaCoO3 (LCO) thin films, is enigmatic despite intensive research efforts over the past decade. Here, by combining scanning transmission electron microscopy with ab initio density functional theory plus U calculations, we report that the ferromagnetism does not emerge directly from the strain itself but rather from the creation of compressed structural units within ferroelastically formed twin-wall domains. The compressed structural units are magnetically active with the rocksalt-type high-spin/low-spin order. Our study highlights that the ferroelastic nature of ferromagnetic structural units is important for understanding the intriguing ferromagnetic properties in LCO thin films.

Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure.

In the bulk, LaCoO_{3} (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film's interior (i.e., its film bulk) shows the opposite trend. We attribute the nonuniformity to the symmetry mismatch at the interface, which induces a structural distortion related to the ferroelasticity of LCO. This assertion is tested by systematic application of hydrostatic pressure during the PNR experiments. The magnetization can be controlled at a rate of -20.4% per GPa. These results provide unique insights into mechanisms driving FM in strained LCO films while offering a tantalizing observation that tunable deformation of the CoO_{6} octahedra in combination with the ferroelastic order parameter.

Charge Transfer in Iridate-Manganite Superlattices.

Charge transfer in superlattices consisting of SrIrO3 and SrMnO3 is investigated using density functional theory. Despite the nearly identical work function and nonpolar interfaces between SrIrO3 and SrMnO3, rather large charge transfer was experimentally reported at the interface between them. Here, we report a microscopic model that captures the mechanism behind this phenomenon, providing a qualitative understanding of the experimental observation. This leads to unique strain dependence of such charge transfer in iridate-manganite superlattices. The predicted behavior is consistently verified by experiment with soft X-ray and optical spectroscopy. Our work thus demonstrates a new route to control electronic states in nonpolar oxide heterostructures.

Persistent Electrochemical Performance in Epitaxial VO2(B).

Discovering high-performance energy storage materials is indispensable for renewable energy, electric vehicle performance, and mobile computing. Owing to the open atomic framework and good room temperature conductivity, bronze-phase vanadium dioxide [VO2(B)] has been regarded as a highly promising electrode material for Li ion batteries. However, previous attempts were unsuccessful to show the desired cycling performance and capacity without chemical modification. Here, we show with epitaxial VO2(B) films that one can accomplish the theoretical limit for capacity with persistent charging-discharging cyclability owing to the high structural stability and unique open pathways for Li ion conduction. Atomic-scale characterization by scanning transmission electron microscopy and density functional theory calculations also reveal that the unique open pathways in VO2(B) provide the most stable sites for Li adsorption and diffusion. Thus, this work ultimately demonstrates that VO2(B) is a highly promising energy storage material and has no intrinsic hindrance in achieving superior cyclability with a very high power and capacity in a Li-ion conductor.

Reversible Control of Interfacial Magnetism through Ionic-Liquid-Assisted Polarization Switching.

The ability to control magnetism of materials via electric field enables a myriad of technological innovations in information storage, sensing, and computing. We use ionic-liquid-assisted ferroelectric switching to demonstrate reversible modulation of interfacial magnetism in a multiferroic heterostructure composed of ferromagnetic (FM) La0.8Sr0.2MnO3 and ferroelectric (FE) PbZr0.2Ti0.8O3. It is shown that ionic liquids can be used to persistently and reversibly switch a large area of a FE film. This is a prerequisite for polarized neutron reflectometry (PNR) studies that are conducted to directly probe magnetoelectric coupling of the FE polarization to the interfacial magnetization.

Enhancing Perovskite Electrocatalysis through Strain Tuning of the Oxygen Deficiency.

Oxygen vacancies in transition-metal oxides facilitate catalysis critical for energy storage and generation. However, promoting vacancies at the lower temperatures required for operation in devices such as metal-air batteries and portable fuel cells has proven elusive. Here we used thin films of perovskite-based strontium cobaltite (SrCoOx) to show that epitaxial strain is a powerful tool for manipulating the oxygen content under conditions consistent with the oxygen evolution reaction, yielding increasingly oxygen-deficient states in an environment where the cobaltite would normally be fully oxidized. The additional oxygen vacancies created through tensile strain enhance the cobaltite's catalytic activity toward this important reaction by over an order of magnitude, equaling that of precious-metal catalysts, including IrO2. Our findings demonstrate that strain in these oxides can dictate the oxygen stoichiometry independent of ambient conditions, allowing unprecedented control over oxygen vacancies essential in catalysis near room temperature.

Enhanced Bifunctional Oxygen Catalysis in Strained LaNiO3 Perovskites.

Strain is known to greatly influence low-temperature oxygen electrocatalysis on noble metal films, leading to significant enhancements in bifunctional activity essential for fuel cells and metal-air batteries. However, its catalytic impact on transition-metal oxide thin films, such as perovskites, is not widely understood. Here, we epitaxially strain the conducting perovskite LaNiO3 to systematically determine its influence on both the oxygen reduction and oxygen evolution reaction. Uniquely, we found that compressive strain could significantly enhance both reactions, yielding a bifunctional catalyst that surpasses the performance of noble metals such as Pt. We attribute the improved bifunctionality to strain-induced splitting of the eg orbitals, which can customize orbital asymmetry at the surface. Analogous to strain-induced shifts in the d-band center of noble metals relative to the Fermi level, such splitting can dramatically affect catalytic activity in this perovskite and other potentially more active oxides.

Dynamic Scaling and Island Growth Kinetics in Pulsed Laser Deposition of SrTiO_{3}.

We use real-time diffuse surface x-ray diffraction to probe the evolution of island size distributions and its effects on surface smoothing in pulsed laser deposition (PLD) of SrTiO_{3}. We show that the island size evolution obeys dynamic scaling and two distinct regimes of island growth kinetics. Our data show that PLD film growth can persist without roughening despite thermally driven Ostwald ripening, the main mechanism for surface smoothing, being shut down. The absence of roughening is concomitant with decreasing island density, contradicting the prevailing view that increasing island density is the key to surface smoothing in PLD. We also report a previously unobserved crossover from diffusion-limited to attachment-limited island growth that reveals the influence of nonequilibrium atomic level surface transport processes on the growth modes in PLD. We show by direct measurements that attachment-limited island growth is the dominant process in PLD that creates step flowlike behavior or quasistep flow as PLD "self-organizes" local step flow on a length scale consistent with the substrate temperature and PLD parameters.

Reversible redox reactions in an epitaxially stabilized SrCoO(x) oxygen sponge.

Fast, reversible redox reactions in solids at low temperatures without thermomechanical degradation are a promising strategy for enhancing the overall performance and lifetime of many energy materials and devices. However, the robust nature of the cation's oxidation state and the high thermodynamic barrier have hindered the realization of fast catalysis and bulk diffusion at low temperatures. Here, we report a significant lowering of the redox temperature by epitaxial stabilization of strontium cobaltites (SrCoO(x)) grown directly as one of two distinct crystalline phases, either the perovskite SrCoO(3-δ) or the brownmillerite SrCoO(2.5). Importantly, these two phases can be reversibly switched at a remarkably reduced temperature (200-300 °C) in a considerably short time (< 1 min) without destroying the parent framework. The fast, low-temperature redox activity in SrCoO(3-δ) is attributed to a small Gibbs free-energy difference between two topotatic phases. Our findings thus provide useful information for developing highly sensitive electrochemical sensors and low-temperature cathode materials.

Oxygen diffusion pathways in brownmillerite SrCoO2.5: influence of structure and chemical potential.

To design and discover new materials for next-generation energy materials such as solid-oxide fuel cells (SOFCs), a fundamental understanding of their ionic properties and behaviors is essential. The potential applicability of a material for SOFCs is critically determined by the activation energy barrier of oxygen along various diffusion pathways. In this work, we investigate interstitial-oxygen (Oi) diffusion in brownmillerite oxide SrCoO2.5, employing a first-principles approach. Our calculations indicate highly anisotropic ionic diffusion pathways, which result from its anisotropic crystal structure. The one-dimensional-ordered oxygen vacancy channels are found to provide the easiest diffusion pathway with an activation energy barrier height of 0.62 eV. The directions perpendicular to the vacancy channels have higher energy barriers for Oint diffusion. In addition, we have studied migration barriers for oxygen vacancies that could be present as point defects within the material. This in turn could also facilitate the transport of oxygen. Interestingly, for oxygen vacancies, the lowest barrier height was found to occur within the octahedral layer with an energy of 0.82 eV. Our results imply that interstitial migration would be highly one-dimensional in nature. Oxygen vacancy transport, on the other hand, could preferentially occur in the two-dimensional octahedral plane.

Tunneling electroresistance induced by interfacial phase transitions in ultrathin oxide heterostructures.

The ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. Here, we show that in practice the junction current ratios between the two polarization states can be further enhanced by the electrostatic modification in the correlated electron oxide electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. The FE-induced phase modulation at the heterointerface ultimately results in an enhanced electroresistance effect. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics.

Reversal of the lattice structure in SrCoO(x) epitaxial thin films studied by real-time optical spectroscopy and first-principles calculations.

Using real-time spectroscopic ellipsometry, we directly observed a reversible lattice and electronic structure evolution in SrCoO(x) (x=2.5-3) epitaxial thin films. Drastically different electronic ground states, which are extremely susceptible to the oxygen content x, are found in the two topotactic phases: i.e., the brownmillerite SrCoO2.5 and the perovskite SrCoO3. First-principles calculations confirmed substantial differences in the electronic structure, including a metal-insulator transition, which originate from the modification in the Co valence states and crystallographic structures. More interestingly, the two phases can be reversibly controlled by changing the ambient pressure at greatly reduced temperatures. Our finding provides an important pathway to understanding the novel oxygen-content-dependent phase transition uniquely found in multivalent transition metal oxides.

Room-temperature multiferroic hexagonal LuFeO3 films.

The crystal and magnetic structures of single-crystalline hexagonal LuFeO(3) films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO(3) films are room-temperature multiferroics.

Fractionally δ-doped oxide superlattices for higher carrier mobilities.

A two-dimensional (2D) electron gas system in an oxide heterostructure serves as an important playground for novel phenomena. Here, we show that, by using fractional δ-doping to control the interface's composition in La(x)Sr(1-x)TiO(3)/SrTiO(3) artificial oxide superlattices, the filling-controlled 2D insulator-metal transition can be realized. The atomic-scale control of d-electron band filling, which in turn contributes to the tuning of effective mass and density of the charge carriers, is found to be a fascinating route to substantially enhanced carrier mobilities.

Strain-induced spin states in atomically ordered cobaltites.

Epitaxial strain imposed in complex oxide thin films by heteroepitaxy is recognized as a powerful tool for identifying new properties and exploring the vast potential of materials performance. A particular example is LaCoO(3), a zero spin, nonmagnetic material in the bulk, whose strong ferromagnetism in a thin film remains enigmatic despite a decade of intense research. Here, we use scanning transmission electron microscopy complemented by X-ray and optical spectroscopy to study LaCoO(3) epitaxial thin films under different strain states. We observed an unconventional strain relaxation behavior resulting in stripe-like, lattice modulated patterns, which did not involve uncontrolled misfit dislocations or other defects. The modulation entails the formation of ferromagnetically ordered sheets comprising intermediate or high spin Co(3+), thus offering an unambiguous description for the exotic magnetism found in epitaxially strained LaCoO(3) films. This observation provides a novel route to tailoring the electronic and magnetic properties of functional oxide heterostructures.

Intermittent Defect Fluctuations in Oxide Heterostructures.

The heterogeneous nature, local presence, and dynamic evolution of defects typically govern the ionic and electronic properties of a wide variety of functional materials. While the last 50 years have seen considerable efforts into development of new methods to identify the nature of defects in complex materials, such as the perovskite oxides, very little is known about defect dynamics and their influence on the functionality of a material. Here, the discovery of the intermittent behavior of point defects (oxygen vacancies) in oxide heterostructures employing X-ray photon correlation spectroscopy is reported. Local fluctuations between two ordered phases in strained SrCoOx with different degrees of stability of the oxygen vacancies are observed. Ab-initio-informed phase-field modeling reveals that fluctuations between the competing ordered phases are modulated by the oxygen ion/vacancy interaction energy and epitaxial strain. The results demonstrate how defect dynamics, evidenced by measurement and modeling of their temporal fluctuations, give rise to stochastic properties that now can be fully characterized using coherent X-rays, coupled for the first time to multiscale modeling in functional complex oxide heterostructures. The study and its findings open new avenues for engineering the dynamical response of functional materials used in neuromorphic and electrochemical applications.

High-resolution field effect sensing of ferroelectric charges.

Nanoscale manipulation of surface charges and their imaging are essential for understanding local electronic behaviors of polar materials and advanced electronic devices. Electrostatic force microscopy and Kelvin probe force microscopy have been extensively used to probe and image local surface charges responsible for electrodynamics and transport phenomena. However, they rely on the weak electric force modulation of cantilever that limits both spatial and temporal resolutions. Here we present a field effect transistor embedded probe that can directly image surface charges on a length scale of 25 nm and a time scale of less than 125 µs. On the basis of the calculation of net surface charges in a 25 nm diameter ferroelectric domain, we could estimate the charge density resolution to be as low as 0.08 µC/cm(2), which is equivalent to 1/20 electron per nanometer square at room temperature.