Electric modulation of conduction in multiferroic Ca-doped BiFeO3 films

Dual-role ion dynamics in ferroionic CuInP2S6: revealing the transition from ferroelectric to ionic switching mechanisms

Due to its "ferroionic" nature, CuInP2S6 combines switchable ferroelectric polarization with highly mobile Cu ions, allowing for multiple resistance states. Its conductive mechanism involves ferroelectric switching, ion migration, and corresponding intercoupling, which are highly sensitive to external electric field. Distinguishing the dominant contribution of either ferroelectric switching or ion migration to dynamic conductivity remains a challenge and the conductive mechanism is not clear yet. Here, based on polarization switching analyses and first-principles calculations, this work demonstrates that the Cu ion migration pathways enable the formation of a quadruple-well state, determining the conductive mechanism. Accordingly, it favors the manipulation of Cu ion transport in the intralayer and interlayer in a controlled manner, and makes a transition from ferroelectric-dominated to ion-migration-dominated conductivity, by tailoring the electric fields. This work deepens the understanding of ion migration dynamics and conductive switching in ferroionic systems, which is critical for the advancement of memristor-based neuromorphic computing.

Antiferroelectric Heterostructures Memristors with Unique Resistive Switching Mechanisms and Properties

A novel antiferroelectric material, PbSnO3 (PSO), was introduced into a resistive random access memory (RRAM) to reveal its resistive switching (RS) properties. It exhibits outstanding electrical performance with a large memory window (>104), narrow switching voltage distribution (±2 V), and low power consumption. Using high-resolution transmission electron microscopy, we observed the antiferroelectric properties and remanent polarization of the PSO thin films. The in-plane shear strains in the monoclinic PSO layer are attributed to oxygen octahedral tilts, resulting in misfit dislocations and grain boundaries at the PSO/SRO interface. Furthermore, the incoherent grain boundaries between the orthorhombic and monoclinic phases are assumed to be the primary paths of Ag+ filaments. Therefore, the RS behavior is primarily dominated by antiferroelectric polarization and defect mechanisms for the PSO structures. The RS behavior of antiferroelectric heterostructures controlled by switching spontaneous polarization and strain, defects, and surface chemistry reactions can facilitate the development of new antiferroelectric device systems.

Three-Dimensional Visualization of Oxygen-Vacancy Migration and Redistribution in Ca-Substituted BiFeO3

Ionic movement has received renewed attention in recent years, particularly in the field of ferroelectric oxides, since it is intrinsically linked to chemical reaction kinetics and ferroelectric phase stability. The associated surface electrochemical processes coupled local ionic transport with an applied electric bias, exhibiting very high ionic mobility at room temperature based on a simple electrostatics scenario. However, few studies have focused on the applied-polarity dependence of ionic migration with directly visualized maps. Here, we use incorporated experiments of conductive scanning probe microscopy and time-of-flight secondary ion mass spectrometry to investigate oxygen ionic migration and cation redistribution in ionic oxides. The local concentrations of oxygen vacancies and other cation species are visualized by three-dimensional mappings, indicating that oxygen vacancies tend to be ejected toward the surface. An accumulation of oxygen vacancies and ionic redistribution strongly depend on tip polarity, thus corroborating their role in the electrochemical process. This work illustrates the interplay between ionic kinetics and electric switching.

Ferroelectric Resistance Switching in Epitaxial BiFeO3/La0.7Sr0.3MnO3 Heterostructures

BiFeO3/La0.7Sr0.3MnO3 (BFO/LSMO) epitaxial heterostructures were successfully synthesized by pulsed laser deposition on (001)-oriented SrTiO3 single-crystal substrates with Au top electrodes. Stable bipolar resistive switching characteristics regulated by ferroelectric polarization reversal was observed in the Au/BFO/LSMO heterostructures. The conduction mechanism was revealed to follow the Schottky emission model, and the Schottky barriers in high-resistance and low-resistance states were estimated based on temperature-dependent current-voltage curves. Further, the observed memristive behavior was interpreted via the modulation effect on the depletion region width and the Schottky barrier height caused by ferroelectric polarization reversal, combining with the oxygen vacancies migration near the BFO/LSMO interface.

Achieving adjustable digital-to-analog conversion in memristors with embedded Cs2AgSbBr6 nanoparticles

In this work, the proportions of Cs₂AgSbBr₆ nanoparticles (NPs) mixed in a PMMA film are adjusted to the digital and analog types of resistive switching (RS) behaviors in Ag/PMMA&Cs₂AgSbBr₆-NPs/ITO memristor devices. It is confirmed that when the concentration of NPs doped in the PMMA film is about 5 wt%, the memristor devices demonstrate bipolar digital RS behaviors with excellent electrical characteristics such as low operating voltage, high ON/OFF ratio (>500), good endurance (>800 cycles), and stable retention ability (>10⁴ s). However, the devices showed a transition to analog-type memristive behavior when the concentration of NPs doped in the PMMA film is around 10 wt%, and several artificial synapse behaviors are successfully simulated. The device model simulation is also used to explore the effect of the NPs on the local electric field and growing filaments. Our work provides an opportunity to explore next-generation artificial synapse devices based on lead-free halide perovskites.

Review on Resistive Switching Devices Based on Multiferroic BiFeO3

This review provides a comprehensive examination of the state-of-the-art research on resistive switching (RS) in BiFeO₃ (BFO)-based memristive devices. By exploring possible fabrication techniques for preparing the functional BFO layers in memristive devices, the constructed lattice systems and corresponding crystal types responsible for RS behaviors in BFO-based memristive devices are analyzed. The physical mechanisms underlying RS in BFO-based memristive devices, i.e., ferroelectricity and valence change memory, are thoroughly reviewed, and the impact of various effects such as the doping effect, especially in the BFO layer, is evaluated. Finally, this review provides the applications of BFO devices and discusses the valid criteria for evaluating the energy consumption in RS and potential optimization techniques for memristive devices.

Configurable Crack Wall Conduction in a Complex Oxide

Mobile defects in solid-state materials play a significant role in memristive switching and energy-efficient neuromorphic computation. Techniques for confining and manipulating point defects may have great promise for low-dimensional memories. Here, we report the spontaneous gathering of oxygen vacancies at strain-relaxed crack walls in SrTiO₃ thin films grown on DyScO₃ substrates as a result of flexoelectricity. We found that electronic conductance at the crack walls was enhanced compared to the crack-free region, by a factor of 10⁴. A switchable asymmetric diode-like feature was also observed, and the mechanism is discussed, based on the electrical migration of oxygen vacancy donors in the background of Sr-deficient acceptors forming n⁺-n or n-n⁺ junctions. By tracing the temporal relaxations of surface potential and lattice expansion of a formed region, we determine the diffusivity of mobile defects in crack walls to be 1.4 × 10^-16 cm²/s, which is consistent with oxygen vacancy kinetics.

Strain induced structural phase transition and compositional dependent magnetic phase transition in Ti doped Bi0.80Ba0.20FeO3 ceramics

Bi_0.80Ba_0.20Fe_1-xTi_xO₃ ( 0 ≤ x ≤ 0.10 ) samples are prepared using solid state reaction technique. Bi³⁺ site is replaced with 20 % Ba²⁺ which induced structural modification from rhombohedral to pseudo cubic accompanied by the creation of oxygen vacancies owing to the charge reimbursement. Fe³⁺ site is replaced with different concentrations of Ti⁴⁺ keeping Ba content fixed. All the samples exhibited similar morphology and no significant variation in grain size is observed by substituting Ti at Fe site. All of the samples exhibited ferromagnetic behavior, which is ascribed to the destruction of spiral spin structures and changes in super-exchange interaction strength caused by variations in bond lengths of Fe-O and Fe-O-Fe. The decrease in magnetization with increasing Ti concentration is due to magnetic moment dilution caused by non-magnetic Ti⁴⁺. An anomalous trend in magnetization is observed for magnetic measurements at low temperature (77 K) where structural transformation from ferromagnetic to diamagnetic behavior was noted for 10% Ti content. Further, because of the incorporation of Ti⁴⁺, an improved dielectric property was observed due to increase in resistivity and decrease in the defect concentration (oxygen vacancies). In the present study, it was concluded that optimum concentration of Ba²⁺ (20%) and Ti⁴⁺ co-doped BiFeO₃ systems have shown enhanced multiferroic properties at room temperature.

Critical ionic transport across an oxygen-vacancy ordering transition

Phase transition points can be used to critically reduce the ionic migration activation energy, which is important for realizing high-performance electrolytes at low temperatures. Here, we demonstrate a route toward low-temperature thermionic conduction in solids, by exploiting the critically lowered activation energy associated with oxygen transport in Ca-substituted bismuth ferrite (Bi_1-xCa_xFeO_3-δ) films. Our demonstration relies on the finding that a compositional phase transition occurs by varying Ca doping ratio across x_Ca ≃ 0.45 between two structural phases with oxygen-vacancy channel ordering along <100> or <110> crystal axis, respectively. Regardless of the atomic-scale irregularity in defect distribution at the doping ratio, the activation energy is largely suppressed to 0.43 eV, compared with ~0.9 eV measured in otherwise rigid phases. From first-principles calculations, we propose that the effective short-range attraction between two positively charged oxygen vacancies sharing lattice deformation not only forms the defect orders but also suppresses the activation energy through concerted hopping.

Phase transition points can be used to reduce the ionic migration activation energy. Here, the authors find a lowered activation energy associated with oxygen transport at a compositional phase transition point in Ca-doped bismuth ferrite films.

On-chip integrated process-programmable sub-10 nm thick molecular devices switching between photomultiplication and memristive behaviour

Molecular devices constructed by sub-10 nm thick molecular layers are promising candidates for a new generation of integratable nanoelectronic applications. Here, we report integrated molecular devices based on ultrathin copper phthalocyanine/fullerene hybrid layers with microtubular soft-contacts, which exhibit process-programmable functionality switching between photomultiplication and memristive behaviour. The local electric field at the interface between the polymer bottom electrode and the enclosed molecular channels modulates the ionic-electronic charge interaction and hence determines the transition of the device function. When ions are not driven into the molecular channels at a low interface electric field, photogenerated holes are trapped as electronic space charges, resulting in photomultiplication with a high external quantum efficiency. Once mobile ions are polarized and accumulated as ionic space charges in the molecular channels at a high interface electric field, the molecular devices show ferroelectric-like memristive switching with remarkable resistive ON/OFF and rectification ratios.

Ca Solubility in a BiFeO3-Based System with a Secondary Bi2O3 Phase on a Nanoscale

In BiFeO₃ (BFO), Bi₂O₃ (BO) is a known secondary phase, which can appear under certain growth conditions. However, BO is not just an unwanted parasitic phase but can be used to create the super-tetragonal BFO phase in films on substrates, which would otherwise grow in the regular rhombohedral phase (R-phase). The super-tetragonal BFO phase has the advantage of a much larger ferroelectric polarization of 130-150 μC/cm², which is around 1.5 times the value of the rhombohedral phase with 80-100 μC/cm². Here, we report that the solubility of Ca, which is a common dopant of bismuth ferrite materials to tune their properties, is significantly lower in the secondary BO phase than in the observed R-phase BFO. Starting from the film growth, this leads to completely different Ca concentrations in the two phases. We show this with advanced analytical transmission electron microscopy techniques and confirm the experimental results with density functional theory (DFT) calculations. At the film's fabrication temperature, caused by different solubilities, about 50 times higher Ca concentration is expected in the BFO phase than in the secondary one. Depending on the cooling rate after fabrication, this can further increase since a larger Ca concentration difference is expected at lower temperatures. When fabricating functional devices using Ca doping and the secondary BO phase, the difference in solubility must be considered because, depending on the ratio of the BO phase, the Ca concentration in the BFO phase can become much higher than intended. This can be critical for the intended device functionality because the Ca concentration strongly influences and modifies the BFO properties.

Materials for a Sustainable Microelectronics Future: Electric Field Control of Magnetism with Multiferroics

This article is written on behalf of many colleagues, collaborators, and researchers in the field of complex oxides as well as current and former students and postdocs who continue to enable and undertake cutting-edge research in the field of multiferroics, magnetoelectrics, and the pursuit of electric-field control of magnetism. What I present is something that is extremely exciting from both a fundamental science and applications perspective and has the potential to revolutionize our world, particularly from a sustainability perspective. To realize this potential will require numerous new innovations, both in the fundamental science arena as well as translating these scientific discoveries into real applications. Thus, this article will attempt to bridge the gap between fundamental materials physics and the actual manifestations of the physical concepts into real-life applications. I hope this article will help spur more translational research within the broad materials community.

Integrating Band Engineering and the Flexoelectric Effect Induced by a Composition Gradient for High Photocurrent Density in Bismuth Ferrite Films

Photovoltaic energy as one of the important alternatives to traditional fossil fuels has always been a research hot spot in the field of renewable and clean solar energy. Very recently, the anomalous ferroelectric photovoltaic effect in multiferroic bismuth ferrite (BiFeO₃) has attracted much attention due to the above-bandgap photovoltage and switchable photocurrent. However, its photocurrent density mostly in the magnitudes of μA/cm² resulted in a poor power conversion efficiency, which severely hampered its practical application as a photovoltaic device. In this case, a novel approach was designed to improve the photocurrent density of BiFeO₃ through the cooperative effect of the gradient distribution of oxygen vacancies and consequently induced the flexoelectric effect realized in the (La, Co) gradient-doped BiFeO₃ multilayers. Subsequent results and analysis indicated that the photocurrent density of the gradient-doped multilayer BiFeO₃ sample was nearly 3 times as much as that of the conventional doped single-layer sample. Furthermore, a possible mechanism was proposed herein to demonstrate roles of band engineering and the flexoelectric effect on the photovoltaic performance of the gradient-doped BiFeO₃ film.

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.

Critical Role of Sc Substitution in Modulating Ferroelectricity in Multiferroic LuFeO3

Understanding how individual dopants or substitutional atoms interact with host lattices enables us to manipulate, control, and improve the functionality of materials. However, because of the intimate coupling among various degrees of freedom in multiferroics, the atomic-scale influence of individual foreign atoms has remained elusive. Here, we unravel the critical roles of individual Sc substitutional atoms in modulating ferroelectricity at the atomic scale of typical multiferroics, Lu_1-xSc_xFeO₃, by combining advanced microscopy and theoretical studies. Atomic variations in polar displacement of intriguing topological vortex domains stabilized by Sc substitution are directly correlated with Sc atom-mediated local chemical and electronic fluctuations. The local FeO₅ trimerization magnitude and Lu/Sc-O hybridization strength are found to be significantly reinforced by Sc, clarifying the origin of the strong dependence of improper ferroelectricity on Sc content. This study could pave the way for correlating dopant-regulated atomic-scale local structures with global properties to engineer emergent functionalities of numerous chemically doped functional materials.

A Microstructural Analysis of 2D Halide Perovskites: Stability and Functionality

Recent observations have demonstrated that the photoelectric conversion properties of perovskite materials are intimately related to the presence of superlattice structures and other unusual nanoscale features in them. The low-dimensional or mixed-dimensional halide perovskite families are found to be more efficient materials for device application than three-dimensional halide perovskites. The emergence of perovskite solar cells has revolutionized the solar cell industry because of their flexible architecture and rapidly increased efficiency. Tuning the dielectric constant and charge separation are the main objectives in designing a photovoltaic device that can be explored using the two-dimensional perovskite family. Thus, revisiting the fundamental properties of perovskite crystals could reveal further possibilities for recognizing these improvements toward device functionality. In this context, this review discusses the material properties of two-dimensional halide perovskites and related optoelectronic devices, aiming particularly for solar cell applications.

Strain Engineering in Electrochemical Activity and Stability of BiFeO3 Perovskites

Strain engineering is widely employed to manipulate the intrinsic relationship of activity and the crystal structure, while the mechanism and rational strategy toward high-performance devices are still under investigation. Here straining engineering is utilized to manipulate a series of a typical perovskite structures via introducing different types of heteroions (Bi_1-xM_xFeO₃, M = Ca²⁺ or Y³⁺ ion). The space group R3c in BiFeO₃ perovskites is found to be maintained with substituting a certain amount of heteroions at Bi³⁺ sites (<5%), while it would shift into either space groups P4mm (with Ca²⁺ substitute) or Pnma (with Y³⁺ substitute) beyond some critical doping amounts (>5%). Such a transformation is linked with the mismatched crystal strain induced by the heteroions substituted at Bi³⁺ sites, while the activity, stability, and energy storage capability of Bi_1-xM_xFeO₃ have been essentially varied. The results offer a strategy for manipulating stability and activity of perovskites in electrochemical energy conversion and storage.

Magnetoelectric Composites: Applications, Coupling Mechanisms, and Future Directions

Multiferroic (MF)-magnetoelectric (ME) composites, which integrate magnetic and ferroelectric materials, exhibit a higher operational temperature (above room temperature) and superior (several orders of magnitude) ME coupling when compared to single-phase multiferroic materials. Room temperature control and the switching of magnetic properties via an electric field and electrical properties by a magnetic field has motivated research towards the goal of realizing ultralow power and multifunctional nano (micro) electronic devices. Here, some of the leading applications for magnetoelectric composites are reviewed, and the mechanisms and nature of ME coupling in artificial composite systems are discussed. Ways to enhance the ME coupling and other physical properties are also demonstrated. Finally, emphasis is given to the important open questions and future directions in this field, where new breakthroughs could have a significant impact in transforming scientific discoveries to practical device applications, which can be well-controlled both magnetically and electrically.

Harnessing the topotactic transition in oxide heterostructures for fast and high-efficiency electrochromic applications

Mobile oxygen vacancies offer a substantial potential to broaden the range of optical functionalities of complex transition metal oxides due to their high mobility and the interplay with correlated electrons. Here, we report a large electro-absorptive optical variation induced by a topotactic transition via oxygen vacancy fluidic motion in calcium ferrite with large-scale uniformity. The coloration efficiency reaches ~80 cm² C^-1, which means that a 300-nm-thick layer blocks 99% of transmitted visible light by the electrical switching. By tracking the color propagation, oxygen vacancy mobility can be estimated to be 10^-8 cm² s^-1 V^-1 near 300°C, which is a giant value attained due to the mosaic pseudomonoclinic film stabilized on LaAlO₃ substrate. First-principles calculations reveal that the defect density modulation associated with hole charge injection causes a prominent change in electron correlation, resulting in the light absorption modulation. Our findings will pave the pathway for practical topotactic electrochromic applications.

Large Switchable Photoconduction within 2D Potential Well of a Layered Ferroelectric Heterostructure

The coexistence of large conductivity and robust ferroelectricity is promising for high-performance ferroelectric devices based on polarization-controllable highly efficient carrier transport. Distinct from traditional perovskite ferroelectrics, Bi₂ WO₆ with a layered structure shows a great potential to preserve its ferroelectricity under substantial electron doping. Herein, by artificial design of photosensitive heterostructures with desired band alignment, three orders of magnitude enhancement of the short-circuit photocurrent is achieved in Bi₂ WO₆ /SrTiO₃ at room temperature. The microscopic mechanism of this large photocurrent originates from separated transport of electrons and holes in [WO₄ ]^-2 and [Bi₂ O₂ ]⁺² layers respectively with a large in-plane conductivity, which is understood by a combination of ab initio calculations and spectroscopic measurements. The layered electronic structure and appropriately designed band alignment in this layered ferroelectric heterostructure provide an opportunity to achieve high-performance and nonvolatile switchable electronic devices.

Investigation of Local Conduction Mechanisms in Ca and Ti-Doped BiFeO3 Using Scanning Probe Microscopy Approach

In this work we demonstrate the role of grain boundaries and domain walls in the local transport properties of n- and p-doped bismuth ferrites, including the influence of these singularities on the space charge imbalance of the energy band structure. This is mainly due to the charge accumulation at domain walls, which is recognized as the main mechanism responsible for the electrical conductivity in polar thin films and single crystals, while there is an obvious gap in the understanding of the precise mechanism of conductivity in ferroelectric ceramics. The conductivity of the Bi_0.95Ca_0.05Fe_1−xTi_xO_3−δ (x = 0, 0.05, 0.1; δ = (0.05 − x)/2) samples was studied using a scanning probe microscopy approach at the nanoscale level as a function of bias voltage and chemical composition. The obtained results reveal a distinct correlation between electrical properties and the type of charged defects when the anion-deficient (x = 0) compound exhibits a three order of magnitude increase in conductivity as compared with the charge-balanced (x = 0.05) and cation-deficient (x = 0.1) samples, which is well described within the band diagram representation. The data provide an approach to control the transport properties of multiferroic bismuth ferrites through aliovalent chemical substitution.

Structure, Performance, and Application of BiFeO3 Nanomaterials

Multiferroic nanomaterials have attracted great interest due to simultaneous two or more properties such as ferroelectricity, ferromagnetism, and ferroelasticity, which can promise a broad application in multifunctional, low-power consumption, environmentally friendly devices. Bismuth ferrite (BiFeO3, BFO) exhibits both (anti)ferromagnetic and ferroelectric properties at room temperature. Thus, it has played an increasingly important role in multiferroic system. In this review, we systematically discussed the developments of BFO nanomaterials including morphology, structures, properties, and potential applications in multiferroic devices with novel functions. Even the opportunities and challenges were all analyzed and summarized. We hope this review can act as an updating and encourage more researchers to push on the development of BFO nanomaterials in the future.

Flexible 3D Electrodes of Free-Standing TiN Nanotube Arrays Grown by Atomic Layer Deposition with a Ti Interlayer as an Adhesion Promoter

Nanostructured electrodes and their flexible integrated systems have great potential for many applications, including electrochemical energy storage, electrocatalysis and solid-state memory devices, given their ability to improve faradaic reaction sites by large surface area. Although many processing techniques have been employed to fabricate nanostructured electrodes onto flexible substrates, these present limitations in terms of achieving flexible electrodes with high mechanical stability. In this study, the adhesion, mechanical properties and flexibility of TiN nanotube arrays on a Pt substrate were improved using a Ti interlayer. Highly ordered and well-aligned TiN nanotube arrays were fabricated on a Pt substrate using a template-assisted method with an anodic aluminum oxide (AAO) template and atomic layer deposition (ALD) system. We show that with the use of a Ti interlayer between the TiN nanotube arrays and Pt substrate, the TiN nanotube arrays could perfectly attach to the Pt substrate without delamination and faceted phenomena. Furthermore, the I-V curve measurements confirmed that the electric contact between the TiN nanotube arrays and substrate for use as an electrode was excellent, and its flexibility was also good for use in flexible electronic devices. Future efforts will be directed toward the fabrication of embedded electrodes in flexible plastic substrates by employing the concepts demonstrated in this study.

Quasiparticle tunnel electroresistance in superconducting junctions

The term tunnel electroresistance (TER) denotes a fast, non-volatile, reversible resistance switching triggered by voltage pulses in ferroelectric tunnel junctions. It is explained by subtle mechanisms connected to the voltage-induced reversal of the ferroelectric polarization. Here we demonstrate that effects functionally indistinguishable from the TER can be produced in a simpler junction scheme—a direct contact between a metal and an oxide—through a different mechanism: a reversible redox reaction that modifies the oxide’s ground-state. This is shown in junctions based on a cuprate superconductor, whose ground-state is sensitive to the oxygen stoichiometry and can be tracked in operando via changes in the conductance spectra. Furthermore, we find that electrochemistry is the governing mechanism even if a ferroelectric is placed between the metal and the oxide. Finally, we extend the concept of electroresistance to the tunnelling of superconducting quasiparticles, for which the switching effects are much stronger than for normal electrons. Besides providing crucial understanding, our results provide a basis for non-volatile Josephson memory devices.

The non-volatile switching of tunnel electroresistance in ferroelectric junctions provides the basis for memory and neuromorphic computing devices. Rouco et al. show tunnel electroresistance in superconductor-based junctions that arises from a redox rather than ferroelectric mechanism and is enhanced by superconductivity.

Toward Electrochemical Studies on the Nanometer and Atomic Scales: Progress, Challenges, and Opportunities

Electrochemical reactions and ionic transport underpin the operation of a broad range of devices and applications, from energy storage and conversion to information technologies, as well as biochemical processes, artificial muscles, and soft actuators. Understanding the mechanisms governing function of these applications requires probing local electrochemical phenomena on the relevant time and length scales. Here, we discuss the challenges and opportunities for extending electrochemical characterization probes to the nanometer and ultimately atomic scales, including challenges in down-scaling classical methods, the emergence of novel probes enabled by nanotechnology and based on emergent physics and chemistry of nanoscale systems, and the integration of local data into macroscopic models. Scanning probe microscopy (SPM) methods based on strain detection, potential detection, and hysteretic current measurements are discussed. We further compare SPM to electron beam probes and discuss the applicability of electron beam methods to probe local electrochemical behavior on the mesoscopic and atomic levels. Similar to a SPM tip, the electron beam can be used both for observing behavior and as an active electrode to induce reactions. We briefly discuss new challenges and opportunities for conducting fundamental scientific studies, matter patterning, and atomic manipulation arising in this context.

The growth and improved magnetoelectric response of strain-modified Aurivillius SrBi4.25La0.75Ti4FeO18 thin films

In this study, we grew 5-layered SrBi4.25La0.75Ti4FeO18 (SBLFT) polycrystalline thin films (80-330 nm thick) via pulsed-laser deposition to study their ferroelectric and magnetoelectric response. Structural/microstructural analysis confirmed the formation of orthorhombic SBLFT with good crystallinity and randomly oriented Aurivillius phases. Detailed scanning transmission electron microscopy analysis of 120 nm film revealed a predominantly five-layered structure with the coexistence of four-layer stacking. Such stacking defects are found to be pertinent to the high structural flexibility of Bi-rich Aurivillius phases, alleviated by lattice strain. Raman spectral features at ambient temperatures depict the signature of the orthorhombic-tetragonal phase transition. SBLFT films have a strong ferroelectric nature (remanent polarization 2Pr of 35 μC cm-2) with a fatigue endurance up to 1010 cycles and strongly improved, switchable magnetization as opposed to its antiferromagnetic bulk counterpart. The scaling behavior of dynamic hysteresis reveals that ferroelectric domain reversal has good stability and low energy consumption. We observed the presence of SBLFT nanoregions (1-5 nm), distributed across the film, with Bi and Fe-rich compositions and oxygen vacancies that contribute to the weak ferromagnetic behavior mediated by the Dzyaloshinskii-Moriya interactions. Subtle changes in the structural strain and lattice distortions of thin films with varied thicknesses led to distinct ferroic properties. Stronger ferroelectric polarization of 80 nm and 120 nm films compared to that of thicker ones can be due to structural strain and the possible rearrangement of BO6 octahedra. The observation of the improved magnetoelectric coefficient of 50 mV cm-1 Oe-1 for 120 nm film, as compared to that of several Aurivillius oxides, indicates that the structural strain modification in SBLFT is beneficial for the fatigue-free magnetic field switching of ferroelectric polarization. The structural strain of the unit cell as well as the presence of Bi- and ferromagnetic Fe-rich nanoregions was found to be responsible for the improved multiferroic behaviour of the SBLFT films.

Gate-Tunable In-Plane Ferroelectricity in Few-Layer SnS

Ultrathin ferroelectrics hold great promise for modern miniaturized sensors, memories, and optoelectronic devices. However, in most ferroelectric materials, polarization is destabilized in ultrathin films by the intrinsic depolarization field. Here we report robust in-plane ferroelectricity in few-layer tin sulfide (SnS) 2D crystals that is coupled anisotropically to lattice strain. Specifically, the intrinsic polarization of SnS manifests as nanoripples along the armchair direction due to a converse piezoelectric effect. Most interestingly, such nanoripples show an odd-and-even effect in terms of its layer dependence, indicating that it is highly sensitive to changes in inversion symmetry. Ferroelectric switching is demonstrated in field-effect transistor devices fabricated on ultrathin SnS films, in which a stronger ferroelectric response is achieved at negative gate voltages. Our work shows the promise of 2D SnS in ultrathin ferroelectric field-effect transistors as well as nanoscale electromechanical systems.

Resistive Switching Behavior in Ferroelectric Heterostructures

Resistive random-access memory (RRAM) is a promising candidate for next-generation nonvolatile random-access memory protocols. The information storage in RRAM is realized by the resistive switching (RS) effect. The RS behavior of ferroelectric heterostructures is mainly controlled by polarization-dominated and defect-dominated mechanisms. Under certain conditions, these two mechanisms can have synergistic effects on RS behavior. Therefore, RS performance can be effectively improved by optimizing ferroelectricity, conductivity, and interfacial structures. Many methods have been studied to improve the RS performance of ferroelectric heterostructures. Typical approaches include doping elements into the ferroelectric layer, controlling the oxygen vacancy concentration and optimizing the thickness of the ferroelectric layer, and constructing an insertion layer at the interface. Here, the mechanism of RS behavior in ferroelectric heterostructures is briefly introduced, and the methods used to improve RS performance in recent years are summarized. Finally, existing problems in this field are identified, and future development trends are highlighted.

Ferroelectrics with a controlled oxygen-vacancy distribution by design

Controlling and manipulating defects in materials provides an extra degree of freedom not only for enhancing physical properties but also for introducing additional functionalities. In ferroelectric oxides, an accumulation of point defects at specific boundaries often deteriorates a polarization-switching capability, but on the one hand, delivers interface-driven phenomena. At present, it remains challenging to control oxygen vacancies at will to achieve a desirable defect structure. Here, we report a practical route to designing oxygen-vacancy distributions by exploiting the interaction with transition-metal dopants. Our thin-film experiments combined with ab-initio theoretical calculations for BiFeO₃ demonstrate that isovalent dopants such as Mn³⁺ with a partly or fully electron-occupied e_g state can trap oxygen vacancies, leading to a robust polarization switching. Our approach to controlling oxygen vacancy distributions by harnessing the vacancy-trapping capability of isovalent transition-metal cations will realize the full potential of switchable polarization in ferroelectric perovskite oxides.

Thickness scaling of ferroelectricity in BiFeO3 by tomographic atomic force microscopy

Intrinsic and extrinsic properties of ferroelectric materials are known to have strong dependencies on electrical and mechanical boundary conditions, resulting in finite size effects at length scales below several hundred nanometers. In ferroelectric thin films, equilibrium domain size is proportional to the square root of film thickness, which precludes the use of present tomographic microscopies to accurately resolve complex domain morphologies in submicrometer films. We report a subtractive experimental technique with volumetric resolution below 315 nm³, that allows for three-dimensional, tomographic imaging of materials properties using only an atomic force microscope. Multiferroic BiFeO₃ was chosen as a model system for illustrating the capabilities of tomographic atomic force microscopy due to its technological relevance in low-power, electrically switchable magnetic logic.

Nanometer-scale 3D imaging of materials properties is critical for understanding equilibrium states in electronic materials, as well as for optimization of device performance and reliability, even though such capabilities remain a substantial experimental challenge. Tomographic atomic force microscopy (TAFM) is presented as a subtractive scanning probe technique for high-resolution, 3D ferroelectric property measurements. Volumetric property resolution below 315 nm³, as well as unit-cell-scale vertical material removal, are demonstrated. Specifically, TAFM is applied to investigate the size dependence of ferroelectricity in the room-temperature multiferroic BiFeO₃ across two decades of thickness to below 1 nm. TAFM enables volumetric imaging of ferroelectric domains in BiFeO₃ with a significant improvement in spatial resolution compared with existing domain tomography techniques. We additionally employ TAFM for direct, thickness-dependent measurements of the local spontaneous polarization and ferroelectric coercive field in BiFeO₃. The thickness-resolved ferroelectric properties strongly correlate with cross-sectional transmission electron microscopy (TEM), Landau–Ginzburg–Devonshire phenomenological theory, and the semiempirical Kay–Dunn scaling law for ferroelectric coercive fields. These results provide an unambiguous determination of a stable and switchable polar state in BiFeO₃ to thicknesses below 5 nm. The accuracy and utility of these findings on finite size effects in ferroelectric and multiferroic materials more broadly exemplifies the potential for novel insight into nanoscale 3D property measurements via other variations of TAFM.

Controllable conductive readout in self-assembled, topologically confined ferroelectric domain walls

Charged domain walls in ferroelectrics exhibit a quasi-two-dimensional conduction path coupled to the surrounding polarization. They have been proposed for use as non-volatile memory with non-destructive operation and ultralow energy consumption. Yet the evolution of domain walls during polarization switching makes it challenging to control their location and conductance precisely, a prerequisite for controlled read-write schemes and for integration in scalable memory devices. Here, we explore and reversibly switch the polarization of square BiFeO₃ nanoislands in a self-assembled array. Each island confines cross-shaped, charged domain walls in a centre-type domain. Electrostatic and geometric boundary conditions induce two stable domain configurations: centre-convergent and centre-divergent. We switch the polarization deterministically back and forth between these two states, which alters the domain wall conductance by three orders of magnitude, while the position of the domain wall remains static because of its confinement within the BiFeO₃ islands.

Effect of Mn doping on electroforming and threshold voltages of bipolar resistive switching in Al/Mn : NiO/ITO

We investigated the bipolar resistive switching (BRS) properties of Mn-doped NiO thin films by sol-gel spin-coating. As the Mn doping concentration increased, lattice constant, grain size and band gap were found to decrease simultaneously. Moreover, the electroforming voltages and threshold voltages were gradually reduced. It can be ascribed to the increase in the density of grain boundaries, and the defects caused by doping Mn and lower formation energy of Mn-O. They would be helpful for the formation of oxygen vacancies and conductive filaments. It is worth mentioning that excellent BRS behaviors can be obtained at a low Mn-doped concentration including enlarged ON/OFF ratio, good uniformity and stability. Compared with other samples, the 1% Mn-doped NiO showed the highest ON/OFF ratio (>106), stable endurance of >100 cycles and a retention time of >104 s. The mechanism should be determined by bulk properties rather than the dual-oxygen reservoir structure. These results indicate that appropriate Mn doping can be applied to improve the BRS characteristics of NiO thin films, and provide stable, low-power-consumption memory devices.

Strong magnetoelectric coupling effect in BaTiO3@CoFe2O4 magnetoelectric multiferroic fluids

Magnetoelectric multiferroic fluids composed of BaTiO3@CoFe2O4 composite nanoparticles dispersed in a highly insulating nonpolar oleic acid/silicone oil mixture have been developed. The effects of the particle volume fraction and a magnetic field, as well as an electric field, on the ferroelectric and magnetic properties, as well as the magnetoelectric coupling effect, have been systematically studied and discussed in this paper. Magnetic characterization shows an approximation to superparamagnetism, and both the remanent magnetization (Mr) and the coercive field (Hc) increase with increases in the volume fraction and applied electric field. Similarly, a superparaelectric state has been observed in the multiferroic fluids, in which both the remanent polarization (Pr) and the coercive field (Ec) are near zero, whereas they increase with increases in the applied magnetic field and volume fraction. High converse and direct magnetoelectric coupling coefficients are estimated to be αH = 8.16 × 10-4 (Oe cm) V-1 and αE = 1.58 × 104 V (cm Oe)-1, respectively. Further analysis indicates that the composite particles can be aligned under an external magnetic/electric field so that their magnetic/electric moments can be parallel to the external field, which in turn results in changes in the magnetization/polarization directions. These results imply that besides magnetoelectric fluids that consist of core/shell-structured nanoparticles, conventional multiferroic fluids based on composite particles may provide an opportunity to gain electrical control of magnetization and vice versa, which implies potential applications.

Periodic Giant Polarization Gradients in Doped BiFeO3 Thin Films

The ultimate challenge for the development of new multiferroics with enhanced properties lies in achieving nanoscale control of the coupling between different ordering parameters. In oxide-based multiferroics, substitutional cation dopants offer the unparalleled possibility to modify both the electric and magnetic properties at a local scale. Herein it is demonstrated the formation of a dopant-controlled polar pattern in BiFeO₃ leading to the spontaneous instauration of periodic polarization waves. In particular, nonpolar Ca-doped rich regions act as spacers between consecutive dopant-depleted regions displaying coupled ferroelectric states. This alternation of layers with different ferroelectric state creates a novel vertical polar structure exhibiting giant polarization gradients as large as 70 μC cm^-2 across 30 Å thick domains. The drastic change in the polar state of the film is visualized using high-resolution differential phase-contrast imaging able to map changes in ferroelectric polarization at atomic scale. Furthermore, a periodic distortion in the Fe-O-Fe bonding angle suggests a local variation in the magnetic ordering. The findings provide a new insight into the role of doping and reveal hitherto unexplored means to tailor the functional properties of multiferroics by doping engineering.

Impact of Isovalent and Aliovalent Doping on Mechanical Properties of Mixed Phase BiFeO3

In this study, we report the effect of doping in morphotropic BiFeO₃ (BFO) thin films on mechanical properties, revealing variations in the elasticity across the competing phases and their boundaries. Spectroscopic force-distance (F-D) curves and force mapping images by AFM are used to characterize the structure and elastic properties of three BFO thin-film candidates (pure-BFO, Ca-doped BFO, La-doped BFO). We show that softening behavior is observed in isovalent La-doped BFO, whereas hardening is seen in aliovalent Ca-doped BFO. Furthermore, quantitative F-D measurements are extended to show threshold strengths for phase transitions, revealing their dependence on doping in the system. First-principles simulation methods are also employed to understand the observed mechanical properties in pure and doped BFO thin films and to provide microscopic insight on them. These results provide key insight into doping as an effective control parameter to tune nanomechanical properties and suggest an alternative framework to control coupled ferroic functionalities at the nanoscale.

Comparison of benzene and toluene photodegradation under visible light irradiation by Ba-doped BiFeO3 magnetic nanoparticles with fast sonochemical synthesis

Benzen and toluene can be successfully removed by the combination of photo-Fenton reactions and photocatalytic reactions occurring on the photocatalyst surface.

Impact of interfacial effects on ferroelectric resistance switching of Au/BiFeO3/Nb:SrTiO3(100) Schottky junctions

Direct evidence of purely interfacial effects on resistance switching is demonstrated in Au/BiFeO₃/Nd:SrTiO₃(001) (Au/BFO/NSTO) Schottky junctions by reducing the thickness of ferroelectric interlayer BFO.

Ferroelectric BiFeO3 as an Oxide Dye in Highly Tunable Mesoporous All-Oxide Photovoltaic Heterojunctions

As potential photovoltaic materials, transition-metal oxides such as BiFeO₃ (BFO) are capable of absorbing a substantial portion of solar light and incorporating ferroic orders into solar cells with enhanced performance. But the photovoltaic application of BFO has been hindered by low energy-conversion efficiency due to poor carrier transport and collection. In this work, a new approach of utilizing BFO as a light-absorbing sensitizer is developed to interface with charge-transporting TiO₂ nanoparticles. This mesoporous all-oxide architecture, similar to that of dye-sensitized solar cells, can effectively facilitate the extraction of photocarriers. Under the standard AM1.5 (100 mW cm^-2 ) irradiation, the optimized cell shows an open-circuit voltage of 0.67 V, which can be enhanced to 1.0 V by tailoring the bias history. A fill factor of 55% is achieved, which is much higher than those in previous reports on BFO-based photovoltaic devices. The results provide here a new viable approach toward developing highly tunable and stable photovoltaic devices based on ferroelectric transition-metal oxides.

Rapid mapping of polarization switching through complete information acquisition

Polarization switching in ferroelectric and multiferroic materials underpins a broad range of current and emergent applications, ranging from random access memories to field-effect transistors, and tunnelling devices. Switching in these materials is exquisitely sensitive to local defects and microstructure on the nanometre scale, necessitating spatially resolved high-resolution studies of these phenomena. Classical piezoresponse force microscopy and spectroscopy, although providing necessary spatial resolution, are fundamentally limited in data acquisition rates and energy resolution. This limitation stems from their two-tiered measurement protocol that combines slow (∼1 s) switching and fast (∼10 kHz-1 MHz) detection waveforms. Here we develop an approach for rapid probing of ferroelectric switching using direct strain detection of material response to probe bias. This approach, facilitated by high-sensitivity electronics and adaptive filtering, enables spectroscopic imaging at a rate 3,504 times faster the current state of the art, achieving high-veracity imaging of polarization dynamics in complex microstructures.

Perovskite BiFeO3 thin film photocathode performance with visible light activity

Perovskite materials are now an important class of materials in the application areas of photovoltaics and photocatalysis. Inorganic perovskites such as BiFeO3 (BFO) are promising photocatalyst materials with visible light activity and inherent stability. Here we report the large area sol-gel synthesis of BFO films for solar stimulated water photo oxidation. By modifying the sol-gel synthesis process we have produced a perovskite material that has p-type behaviour and a flat band potential of ∼1.15 V (versus NHE). The photocathode produces a density of -0.004 mA cm(-2) at 0 V versus NHE under AM1.5 G illumination. We further show that 0.6 μmol h(-1) of O2 was produced at an external bias of -0.5 V versus Ag/AgCl. The addition of a non-percolating conducting network of Ag increases the photocurrent to -0.07 mA cm(-2) at 0 V versus NHE (at 2% Ag loading) with an increase to 2.7 μmol h(-1) for O2 production. We attribute the enhancement in photoelectrochemical performance to increased light absorption due light scattering by the incorporated Ag particles, improved charge transfer kinetics at the Ag/BFO interface and reduced over potential losses. We support these claims by an observed shift in flat band and onset potentials after Ag modification through UV-vis spectroscopy, Mott-Schottky plots and j-v curve analysis.

Tunable photoelectrochemical performance of Au/BiFeO3 heterostructure

Ferroelectric photoelectrodes, other than conventional semiconductors, are alternative photo-absorbers in the process of water splitting. However, the capture of photons and efficient transfer of photo-excited carriers remain as two critical issues in ferroelectric photoelectrodes. In this work, we overcome the aforementioned issues by decorating the ferroelectric BiFeO3 (BFO) surface with Au nanocrystals, and thus improving the photoelectrochemical (PEC) performance of BFO film. We demonstrate that the internal field induced by the spontaneous polarization of BFO can (1) tune the efficiency of the photo-excited carriers' separation and charge transfer characteristics in bare BFO photoelectrodes, and (2) modulate an extra optical absorption within the visible light region, created by the surface plasmon resonance excitation of Au nanocrystals to capture more photons in the Au/BFO heterostructure. This study provides key insights for understanding the tunable features of PEC performance, composed of the heterostructure of noble metals and ferroelectric materials.

Kinetically driven switching and memory phenomena at the interface between a proton-conductive electrolyte and a titanium electrode

Numerous studies have examined the switching properties of semi- or ion-conductors and isolators; however, most of these have focused on the ohmic resistance characteristics. Here, we report a new type of polarity-dependent switching phenomenon obtained for electrical devices with the configuration: metal working electrode│Si0.97Al0.03H0.03P2O7-polytetrafluoroethylene composite electrolyte│Pt/C counter electrode. The counter electrode is reversibly active for the water vapor oxidation and evolution reactions. The composite electrolyte exhibits high withstanding voltage capability in the bias voltage range of ±7 V. When titanium was employed as the working electrode, the anodic polarization resistance was approximately two orders of magnitude greater than the cathodic polarization resistance. The ohmic resistance of the device was almost unchanged, regardless of the bias voltage polarity. Moreover, kinetically induced high-resistance/low-resistance states could be cyclically switched through positive/negative bias voltage pulses, and these states were also confirmed to be memorized at open circuit.

MoS2 memristor with photoresistive switching

A MoS2 nanosphere memristor with lateral gold electrodes was found to show photoresistive switching. The new device can be controlled by the polarization of nanospheres, which causes resistance switching in an electric field in the dark or under white light illumination. The polarization charge allows to change the switching voltage of the photomemristor, providing its multi-level operation. The device, polarized at a voltage 6 V, switches abruptly from a high resistance state (HRSL6) to a low resistance state (LRSL6) with the On/Off resistance ratio of about 10 under white light and smooth in the dark. Analysis of device conductivity in different resistive states indicates that its resistive state could be changed by the modulation of the charge in an electric field in the dark or under light, resulting in the formation/disruption of filaments with high conductivity. A MoS2 photomemristor has great potential as a multifunctional device designed by using cost-effective fabrication techniques.

Two-Dimensional Layered Oxide Structures Tailored by Self-Assembled Layer Stacking via Interfacial Strain

Study of layered complex oxides emerge as one of leading topics in fundamental materials science because of the strong interplay among intrinsic charge, spin, orbital, and lattice. As a fundamental basis of heteroepitaxial thin film growth, interfacial strain can be used to design materials that exhibit new phenomena beyond their conventional forms. Here, we report a strain-driven self-assembly of bismuth-based supercell (SC) with a two-dimensional (2D) layered structure. With combined experimental analysis and first-principles calculations, we investigated the full SC structure and elucidated the fundamental growth mechanism achieved by the strain-enabled self-assembled atomic layer stacking. The unique SC structure exhibits room-temperature ferroelectricity, enhanced magnetic responses, and a distinct optical bandgap from the conventional double perovskite structure. This study reveals the important role of interfacial strain modulation and atomic rearrangement in self-assembling a layered singe-phase multiferroic thin film, which opens up a promising avenue in the search for and design of novel 2D layered complex oxides with enormous promise.

New modalities of strain-control of ferroelectric thin films

Ferroelectrics, with their spontaneous switchable electric polarization and strong coupling between their electrical, mechanical, thermal, and optical responses, provide functionalities crucial for a diverse range of applications. Over the past decade, there has been significant progress in epitaxial strain engineering of oxide ferroelectric thin films to control and enhance the nature of ferroelectric order, alter ferroelectric susceptibilities, and to create new modes of response which can be harnessed for various applications. This review aims to cover some of the most important discoveries in strain engineering over the past decade and highlight some of the new and emerging approaches for strain control of ferroelectrics. We discuss how these new approaches to strain engineering provide promising routes to control and decouple ferroelectric susceptibilities and create new modes of response not possible in the confines of conventional strain engineering. To conclude, we will provide an overview and prospectus of these new and interesting modalities of strain engineering helping to accelerate their widespread development and implementation in future functional devices.

Enhanced photovoltaic properties in bilayer BiFeO3/Bi-Mn-O thin films

We report an external solar power conversion efficiency of ∼1.43% in BiFeO3(BFO)/BiMnO3(BMO) bilayer thin films. Both films are epitaxially grown on (111) oriented niobium doped SrTiO3 (NSTO) single crystal substrates by pulsed laser deposition. By illuminating the BFO/BMO films under 1 Sun (AM 1.5 G), we found a remarkably high fill factor of ∼0.72, much higher than values reported for devices based on BFO or BMO alone. In addition, we demonstrate that the photocurrent density and photovoltage are tunable by changing the polarization direction in the BFO/BMO bilayer, as confirmed by the macroscopic polarization-voltage (P-V) hysteresis loop. This effect is described in terms of a more favorable energy band alignment of the electrode/bilayer/NSTO heterostructure junction, which controls photocarrier separation.

Unusual continuous dual absorption peaks in Ca-doped BiFeO3 nanostructures for broadened microwave absorption

Electromagnetic absorption materials have received increasing attention owing to their wide applications in aerospace, communication and the electronics industry, and multiferroic materials with both polarization and magnetic properties are considered promising ceramics for microwave absorption application. However, the insufficient absorption intensity coupled with the narrow effective absorption bandwidth has limited the development of high-performance multiferroic materials for practical microwave absorption. To address such issues, in the present work, we utilize interfacial engineering in BiFeO3 nanoparticles via Ca doping, with the purpose of tailoring the phase boundary. Upon Ca-substitution, the co-existence of both R3c and P4mm phases has been confirmed to massively enhance both dielectric and magnetic properties via manipulating the phase boundary and the destruction of the spiral spin structure. Unlike the commonly reported magnetic/dielectric hybrid microwave absorption composites, Bi0.95Ca0.05FeO3 has been found to deliver unusual continuous dual absorption peaks at a small thickness (1.56 mm), which has remarkably broadened the effective absorption bandwidth (8.7-12.1 GHz). The fundamental mechanisms based on the phase boundary engineering have been discussed, suggesting a novel platform for designing advanced multiferroic materials with wide applications.