Protonation-Induced Colossal Chemical Expansion and Property Tuning in NdNiO3 Revealed by Proton Concentration Gradient Thin Films

Quantifying Hydrogen Chemical Diffusivity in NdNiO3 Thin Films through Operando Multimodal Measurements

Nickelate oxides show unique properties that make them highly applicable in electrocatalysis, neuromorphic computing, and superconductors. Proton insertion, which effectively tunes their properties, is critical in advancing these applications. Its dynamics is governed by protonation kinetics, mainly controlled by hydrogen chemical diffusivity in nickelates. However, its precise quantification remains a significant knowledge gap, with reported values showing substantial discrepancies and a lack of comprehensive, rigorous methods. In this study, we propose a new quantitative approach that combines operando multimodal measurements. We provide the precise quantification of hydrogen chemical diffusivity in NdNiO3 (NNO), a prototypical nickelate, using rigorous kinetic modeling and cross-validation across multiple data dimensions. Our results reveal that proton mobility in NNO is inherently limited, challenging the assumption of its rapid transport in nickelates. This finding is critical for optimizing proton-based devices and paves the way for further understandings ion dynamics in correlated oxides.

Compressive Strain-Induced Uphill Hydrogen Distribution in Strontium Ferrite Films

Hydrogen incorporation into metal oxides enhances their electrochemical properties, making them highly suitable for various energy conversion applications. The controlled distribution of hydrogen ions in material systems and their conduction at elevated temperatures have garnered significant attention for various energy storage and environmental monitoring applications, including fuel cells, smart windows, and sensor technologies. In this work, cost-effective, high-concentration hydrogen-doped SrFeO3-δ (HSrFeO3-δ) films were prepared under ambient conditions by treating Al(s)|SrFeO3-δ(s) films with KOH(aq), utilizing electron-proton codoping to investigate hydrogen distribution. The uphill hydrogen distributions in SrFeO3-δ films with compressive strain, in contrast to the density gradient behavior under tensile strain, suggest the fundamental role of the strain states in the hydrogen accommodation. Compressively strained films with a rich Al source follow an anomalous uphill feature of hydrogen distribution, highlighting their potential use as electrolyte for fuel cells. The strain significantly influences the structure, chemical lattice coupling, and consequently the ionic transport in SrFeO3-δ. Ionic conductivity measurements reveal that compressively strained HSrFeO3-δ films with uphill hydrogen distributions exhibit a significant ionic conductivity of 0.189 S/cm at 413 K, with an activation energy of approximately 0.29 eV, making them suitable for low-temperature electrochemical applications. These findings provide a promising approach for tuning material properties and valuable insights for building iontronic devices.

Electrically Controlled Metal-Insulator Heterogeneous Evolution for Infrared Switch and Perfect Absorption

Active switching, which enables multifunctionality within a single optical component, is essential for reconfigurable infrared photonic systems such as radiation engineering, sensing, and communication. Metamaterials offer a solution but involve complex design and fabrication. A simpler approach with a planar layered structure becomes promising for offering economical manufacturing, easier integration, and scalability. However, it requires an active medium with giant tunability and effective modulation mechanisms. Here, an electrically controlled reversible infrared switching is demonstrated via a single layer of perovskite nickelate on an opaque substrate. Driven by the evolution of the refractive index during an electrically triggered proton-mediated metal-to-insulator transition, the device transforms from a high reflective (R ≈0.74) to a low reflective state (R ≈0.09) at λ = 7-10 µm. A temperature-independent perfect absorption (A > 0.99 at λ = 11.6-12.1 µm) emerges in the partially hydrogenated state with the mixture of the metal and insulator phases, which results in a modulation of emissivity ≈0.623 at λ = 7-14 µm. The switching behavior is tunable over a wide temperature and wavelength range, offering a versatile path for adaptive infrared applications.

Kelvin Probe Force Microscopy Imaging of Plasticity in Hydrogenated Perovskite Nickelate Multilevel Neuromorphic Devices

Ion drift in nanoscale electronically inhomogeneous semiconductors is among the most important mechanisms being studied for designing neuromorphic computing hardware. However, nondestructive imaging of the ion drift in operando devices directly responsible for multiresistance states and synaptic memory represents a formidable challenge. Here, we present Kelvin probe force microscopy imaging of hydrogen-doped perovskite nickelate device channels subject to high-speed electric field pulses to directly visualize proton distribution by monitoring surface potential changes spatially, which is also supported with finite element-based electric field distribution studies. First-principles calculations provide mechanistic insights into the origin of surface potential changes as a function of hydrogen donor doping that serves as the contrast mechanism. We demonstrate 128 (7-bit) nonvolatile conductance levels in such devices relevant to in-memory computing applications. The synaptic plasticity measurements are implemented in spiking neural networks and show promising results for classification (SciKit Learn's Iris and Wine data sets) and control (OpenAI's CartPole-v1 and BipedalWalker-v3) simulation tasks.

Establishing Quantitative Understanding of Defect-Tuned Properties in Functional Oxides by an Electrochemically-Induced Gradient of Ionic Defect Concentration

Tuning the physical and chemical properties of functional oxides by controlling the amount of ionic point defects has been recognized as a new paradigm of designing oxides with tailored functionality. In order to enable precise tuning of properties, it is important to construct quantitative relationships between properties of interest and concentration of ionic defects, which are conventionally achieved by synthesizing and measuring a large number of samples with varying defect concentration. Compared with this conventional method, which is labor-intensive and susceptible to sample-to-sample variations, this review focuses on a high-throughput method that utilizes an electrochemically induced gradient of defect concentration in one single oxide sample. Combined with spatially resolved characterizations, this method allows establishing a quantitative property-defect concentration relationship. This review will present working principles and case studies that use this method based on graded concentration of ionic defects. Potentials and future extensions of this method will also be discussed.

Electrically induced insulator-to-metal transition in InP-based ion-gated transistor

With the growing awareness of energy savings and consumption for a sustainable ecosystem, the concept of iontronics, that is, controlling electronic devices with ions, has become critically important. Composite devices made of ions and solid materials have been investigated for diverse applications, ranging from energy storage to power generation, memory, biomimetics, and neuromorphic devices. In these studies, three terminal transistor configurations with liquid electrolytes have often been utilized because of their simple device structures and relatively easy fabrication processes. To date, oxide semiconductors and layered materials have mainly been used as active materials. However, inorganic compound semiconductors, which have a long history of basic and applied research, hardly function as channel materials in ion-gated transistors, partly because of the Schottky barrier at the electrode interface. Herein, we show that a typical group III-V compound semiconductor, InP, is available as a high-performance channel for ion-gated transistors with an on/off current ratio of ≈ 105 and a subthreshold swing as small as 93 mV/dec at room temperature. We fabricated AuGe/Ni contact electrodes via annealing to obtain the Ohmic contacts over a wide temperature range. The electrical resistance of InP was drastically decreased by the ionic liquid gating, which led to an electrically induced insulator-to-metal transition. Bulk compound semiconductors are well characterized and have relatively high carrier mobilities; thus, devices combined with electrolytes should prompt the development of iontronics research for novel device functionalities.

Polar Metallicity Controlled by Epitaxial Strain Engineering

The discovery of polar metal opens the door to incorporating electric polarization into electronics with the potential to invigorate next-generation multifunctional electronic devices. Especially, electric polarization can be induced by geometric design in non-polar perovskite oxides. Here, the epitaxial strain exerted on the deposited single-crystalline NdNiO3 thin films is systematically varied in both sign and amplitude by choosing substrates with different lattice mismatch. The pseudocubic NdNiO3(111) film, which is non-polar in its bulk state, is induced to be polar under both compressive and tensile strain. The fine-tuning of epitaxial strain is realized by continuously varying the film thickness using the "thickness-wedge" growth technique, and from the elucidated thickness dependence, the electric polarization and metallicity can be further optimized. Moreover, transitioning from isotropic to anisotropic epitaxial strain gives rise to an ideal polar metal state in the pseudocubic NdNiO3(102) film on an orthorhombic substrate, achieving a remarkably low resistivity of 173 µΩ cm at room temperature. The metal-insulator transition in NdNiO3 is completely suppressed and the polar metal state becomes the ground state at all temperatures. These results demonstrate alluring possibilities of induction and manipulation of both electric polarization and electric transport properties in functional perovskite oxides by epitaxial strain engineering.

Hydrogen-induced tunable remanent polarization in a perovskite nickelate

Materials with field-tunable polarization are of broad interest to condensed matter sciences and solid-state device technologies. Here, using hydrogen (H) donor doping, we modify the room temperature metallic phase of a perovskite nickelate NdNiO3 into an insulating phase with both metastable dipolar polarization and space-charge polarization. We then demonstrate transient negative differential capacitance in thin film capacitors. The space-charge polarization caused by long-range movement and trapping of protons dominates when the electric field exceeds the threshold value. First-principles calculations suggest the polarization originates from the polar structure created by H doping. We find that polarization decays within ~1 second which is an interesting temporal regime for neuromorphic computing hardware design, and we implement the transient characteristics in a neural network to demonstrate unsupervised learning. These discoveries open new avenues for designing ferroelectric materials and electrets using light-ion doping.

Spatial evolution of the proton-coupled Mott transition in correlated oxides for neuromorphic computing

The proton-electron coupling effect induces rich spectrums of electronic states in correlated oxides, opening tempting opportunities for exploring novel devices with multifunctions. Here, via modest Pt-aided hydrogen spillover at room temperature, amounts of protons are introduced into SmNiO3-based devices. In situ structural characterizations together with first-principles calculation reveal that the local Mott transition is reversibly driven by migration and redistribution of the predoped protons. The accompanying giant resistance change results in excellent memristive behaviors under ultralow electric fields. Hierarchical tree-like memory states, an instinct displayed in bio-synapses, are further realized in the devices by spatially varying the proton concentration with electric pulses, showing great promise in artificial neural networks for solving intricate problems. Our research demonstrates the direct and effective control of proton evolution using extremely low electric field, offering an alternative pathway for modifying the functionalities of correlated oxides and constructing low-power consumption intelligent devices and neural network circuits.

Strain-Induced Uphill Hydrogen Distribution in Perovskite Oxide Films

Incorporating hydrogen into transition-metal oxides (TMOs) provides a facile and powerful way to manipulate the performances of TMOs, and thus numerous efforts have been invested in developing hydrogenation methods and exploring the property modulation via hydrogen doping. However, the distribution of hydrogen ions, which is a key factor in determining the physicochemical properties on a microscopic scale, has not been clearly illustrated. Here, focusing on prototypical perovskite oxide (NdNiO3 and La0.67Sr0.33MnO3) epitaxial films, we find that hydrogen distribution exhibits an anomalous "uphill" feature (against the concentration gradient) under tensile strain, namely, the proton concentration enhances upon getting farther from the hydrogen source. Distinctly, under a compressive strain state, hydrogen shows a normal distribution without uphill features. The epitaxial strain significantly influences the chemical lattice coupling and the energy profile as a function of the hydrogen doping position, thus dominating the hydrogen distribution. Furthermore, the strain-(H+) distribution relationship is maintained in different hydrogenation methods (metal-alkali treatment) which is first applied to perovskite oxides. The discovery of strain-dependent hydrogen distribution in oxides provides insights into tailoring the magnetoelectric and energy-conversion functionalities of TMOs via strain engineering.

Infrared Nanoimaging of Hydrogenated Perovskite Nickelate Memristive Devices

Solid-state devices made from correlated oxides, such as perovskite nickelates, are promising for neuromorphic computing by mimicking biological synaptic function. However, comprehending dopant action at the nanoscale poses a formidable challenge to understanding the elementary mechanisms involved. Here, we perform operando infrared nanoimaging of hydrogen-doped correlated perovskite, neodymium nickel oxide (H-NdNiO3, H-NNO), devices and reveal how an applied field perturbs dopant distribution at the nanoscale. This perturbation leads to stripe phases of varying conductivity perpendicular to the applied field, which define the macroscale electrical characteristics of the devices. Hyperspectral nano-FTIR imaging in conjunction with density functional theory calculations unveils a real-space map of multiple vibrational states of H-NNO associated with OH stretching modes and their dependence on the dopant concentration. Moreover, the localization of excess charges induces an out-of-plane lattice expansion in NNO which was confirmed by in situ X-ray diffraction and creates a strain that acts as a barrier against further diffusion. Our results and the techniques presented here hold great potential for the rapidly growing field of memristors and neuromorphic devices wherein nanoscale ion motion is fundamentally responsible for function.

Protonation-Induced Colossal Lattice Expansion in La2/3Sr1/3MnO3

Ion injection controlled by an electric field is a powerful method to manipulate the diverse physical and chemical properties of metal oxides. However, the dynamic control of ion concentrations and their correlations with lattices in perovskite systems have not been fully understood. In this study, we systematically demonstrate the electric-field-controlled protonation of La2/3Sr1/3MnO3 (LSMO) films. The rapid and room-temperature protonation induces a colossal lattice expansion of 9.35% in tensile-strained LSMO, which is crucial for tailoring material properties and enabling a wide range of applications in advanced electronics, energy storage, and sensing technologies. This large expansion in the lattice is attributed to the higher degree of proton diffusion, resulting in a significant elongation in the Mn-O bond and octahedral tilting, which is supported by results from density functional theory calculations. Interestingly, such a colossal expansion is not observed in LSMO under compressive strain, indicating the close dependence of ion-electron-lattice coupling on strain states. These efficient modulations of the lattice and magnetoelectric functionalities of LSMO via proton diffusion offer a promising avenue for developing multifunctional iontronic devices.

Deeper mechanistic insights into epitaxial nickelate electrocatalysts for the oxygen evolution reaction

Mass production of green hydrogen via water electrolysis requires advancements in the performance of electrocatalysts, especially for the oxygen evolution reaction. In this feature article, we highlight how epitaxial nickelates act as model systems to identify atomic-level composition-structure-property-activity relationships, capture dynamic changes under operating conditions, and reveal reaction and failure mechanisms. These insights guide advanced electrocatalyst design with tailored functionality and superior performance. We conclude with an outlook for future developments via operando characterization and multilayer electrocatalyst design.