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Electrically induced resistive switching (RS) effects have been proposed as the basis for future non-volatile memories. In this work, 9 nm-thick BiFeO3 (BFO) epitaxial thin films were deposited on (001)-oriented SrTiO3 substrates by pulsed laser deposition and their resistive switching (RS) behaviors were investigated. A large resistive switching with ON/OFF ratio of ∼106 is observed, surpassing the performance of most resistive random access memories ever reported. The conducting filament is proposed to dominate the RS behavior in the positive voltage region, while the modulation of ferroelectric polarization is suggested to play a significant role in the negative voltage region. Our study significantly deepens the understanding of the physical origin of RS and could provide a reference for designing high-performance memories and memristors based on ultrathin ferroelectric films. [ABSTRACT FROM AUTHOR]

In ferroic materials, giant susceptibilities can be realized at artificially constructed phase boundaries through deterministic manipulation of the order parameter. Here, emergent ferroelectric structural phase evolution behavior is demonstrated through a synergistic combination of A‐site doping and strain engineering. Using chemical solution deposition derived (001)‐oriented Sm‐substituted bismuth ferrite (Bi1‐xSmxFeO3) films as a prototypical system, a morphotropic phase boundary comprising a coexistence of four distinct crystallographic phases is uncovered. These ferroelectric, polar, and nonpolar phases form a nanoscale mixture without the presence of crystallographically hard boundaries. The system thus possesses the ability to show both polarization rotation and extension, effectively releasing the polarization from its crystallographic constraint. Consequently, both robust ferroelectric properties and giant electromechanical responses are obtained. For instance, the optimized composition with x = 0.14 has a remnant polarization of 2Pr = 103 µC cm−2 and electromechanical response 175% that of undoped BFO. These findings showcase the tremendous potential of synthetic phase boundaries, particularly in the context of lead‐free functional multiferroics. [ABSTRACT FROM AUTHOR]

The use of ferroelectric materials in memory device applications is held back by low retention times. Here, the authors demonstrate that by intentionally introducing defective nanoregions which increase the activation field for domain wall motion, retention times larger than a year can be achieved.

Artificial perception technologies capable of sensing and feeling mechanical stimuli like human skins are critical enablers for electronic skins (E‐Skins) needed to achieve artificial intelligence. However, most of the reported electronic skin systems lack the capability to process and interpret the sensor data. Herein, a new design of artificial perceptual system integrating ZnO‐based synaptic devices with Pt/carbon nanofibers‐based strain sensors for stimuli detection and information processing is presented. Benefiting from the controllable ion migration after indium doping, the device can emulate various essential functions, such as short‐term/long‐term plasticity, paired‐pulse facilitation, excitatory post‐synaptic current, and synaptic plasticity depending on the number, frequency, amplitude, and width of the applied pulses. The Pt/carbon nanofibers‐based strain sensors can detect subtle human motion and convert mechanical stimuli into electrical signals, which are further processed by the ZnO devices. By attaching the integrated devices to finger joints, it is demonstrated that they can recognize handwriting and gestures with a high accuracy. This work offers new insights in designing artificial synapses and sensors to process and recognize information for neuromorphic computing and artificial intelligence applications. [ABSTRACT FROM AUTHOR]

The process of photoelectrochemical (PEC) water splitting using semiconductor materials offers a remarkable opportunity to convert incident solar energy into chemical energy in hydrogen in a single step. This review chronicles the significant efforts made in developing non-oxide semiconductor materials to enable efficient hydrogen production using this process. We highlight some of these spectacular developments, ranging from the design of entirely new materials to innovative engineering of existing and known non-oxide semiconductors materials in terms of their morphology, composition and band structure such they can act as viable photoelectrodes. We start with a primer on fundamentals of the PEC process, followed by an overview of the materials development efforts. Then the review focuses on a key challenge in the large-scale uptake of these promising non-oxide materials, namely that of stability when in contact with harsh acidic or alkaline electrolytes. The review ends by outlining some of the next generation photoactive materials that are currently being pursued. We anticipate this review will spur on further research and development of non-oxide materials and devices, through both theoretical and experimental works. It is hoped ultimately to realize breakthrough non-oxide semiconductors that will enable improved performance, longer lifetimes and reduced production costs of hydrogen. [ABSTRACT FROM AUTHOR]

The combination of topological defect mobility and exploitation of the boundary conditions in bilayer heterostructures presents an enormous opportunity for enhancing actuation responses from modest external stimuli in electromechanical applications. Tetragonal PbZr0.3Ti0.7O3 and underlying rhombohedral PbZr0.54Ti0.46O3 ferroelectric bilayers are epitaxially grown on silicon substrates. The topmost layer shows crystallographic symmetry lower than a typical tetragonal structure but is arranged in‐plane with a mosaic pattern of superdomains and ferroelastic a1/a2 nanodomains within. The domain pattern also shows a series of vortex/anti‐vortex pairs induced in the pseudo‐tetragonal film, which can be manipulated to either migrate across the film or annihilate one another in the presence of an external electrical bias. The mobility of defect pairs and domain walls, through reduced substrate clamping by the underlying rhombohedral film, is the driving force for endowing the bilayer with a ≈45% increased effective d33 response compared to a rhombohedral single layer on silicon. Furthermore, a square in‐plane polarization loop (≈32 µC cm–2) provides further evidence of the mobility and large‐scale polarization switching. An improvement of this magnitude demonstrates the opportunity to implement bilayer technology while exploiting a functional silicon substrate for enhanced, industry‐ready, smart material applications. [ABSTRACT FROM AUTHOR]

Artificial synapses are memristor-based devices mimicking biological synapses, and they are used in neuromorphic computing systems that process information in a parallel, energy efficient way and store information in an analog, non-volatile form. The next generation of computing systems are anticipated to use memristive circuits, as they can overcome the shortcomings of the von Neumann computer architecture in which the levels of memory and the CPU are separated, creating a bottleneck that causes energy-loss during information transfer. Memristors are utilized to build Resistive Random Access Memory (RRAM) that allows for multi-level data storage and construction of self-correcting, autonomous learning systems that can solve complex computational tasks that have historically required super-computing hardware. Artificial synapses have received attention since HP Labs fabricated the first practical memristor device. In this review we summarize the working principles, device architectures, fabrication and processing techniques, as well as the strategies for materials selection including binary metal oxide, perovskite, polymer, and organic materials. We also discuss the applications and challenges of using artificial synapses in artificial intelligence tasks such as image recognition, tactile sensing and speech recognition. [ABSTRACT FROM AUTHOR]

Domain nucleation in epitaxial (001)-oriented Pb(Zr0.2TiO0.8)O3 ultrathin ferroelectric films under a sub-critical field regime is investigated by means of piezoresponse force microscopy (PFM). Analytical fits to the domain radius and velocity as a function of time indicate that 180 domain nucleation and growth under a biased PFM tip exhibit a thermally activated, creep behavior. It is also found that an electric field of less than half of the local coercive (or critical) field Ecloc detected by PFM can create stable domains under prolonged bias application. Under these sub-critical bias conditions, it is the temporal evolution of the local electric-field profile due to the slow drift of screening charges or defects (e.g., ionic vacancies) that dictates domain nucleation and growth. [ABSTRACT FROM AUTHOR]

Phase pure bismuth ferrite (BiFeO3) thin films with (001)-oriented epitaxial structure are realized on lanthanum strontium manganite (La0.67Sr0.33MnO3) buffered (001)-SrTiO3 substrates by chemical solution deposition. The annealing process is optimized such that a stoichiometric precursor can be used to accurately control the Bi:Fe ratio. Ferroelectric, dielectric, and resistive switching behaviours are investigated for 40 nm, 70 nm, and 150 nm BFO thin films. While the thinnest film (40 nm) shows very leaky loops, square and fully saturated polarization hysteresis loops are shown for the thicker films. The highest remanent polarization (2Pr=100 μC/cm²) and relative dielectric constant (ϵr=613) are obtained in the 150 nm BFO thin film. High cycle fatigue tests show that the thick films are resistant to polarization fatigue. Piezoresponse force microscopy results show that the domain structure varies with thickness. Resistive switching and polarization mediated diode effects are also observed. These robust properties suggest that chemical solution deposition derived BiFeO3 thin films can offer a viable low cost alternative. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including the use of piezoresponse force microscopy (PFM) to prove electromechanically active materials' functional properties, epitaxial multiferroics composites, and robust properties of sol-gel.

The combination of ferroelectricity and quantum tunneling enables the tantalizing possibility of next-generation nonvolatile memories based on ferroelectric tunnel junctions (FTJs). In the last two decades, significant progress has been achieved in the understanding of FTJs in terms of the role of the critical thickness for ferroelectricity, interface-related factors that yield an enhanced tunneling electroresistance effect, as well exploiting the combination of magnetism and ferroelectricity to realize multiferroic or magnetoelectric tunnel junctions. One key ingredient in the successful design of FTJs is the type and nature of the electrode used—indeed device performance strongly hinges on the ability to precisely tune and modulate the electrostatic boundary conditions. This perspective presents an overview of the experimental state of the art in electrode engineering for FTJs. We discuss related governing factors and methods for various electrode-FTJ combinations, highlighting and comparing the advantages and weaknesses for each system. Finally, we also reveal the challenges and identify the opportunities for the future development of FTJs. In summary, we aim to provide significant insights into electrode engineering of high-quality FTJs with excellent tunneling electroresistance performance. [ABSTRACT FROM AUTHOR]

Ferroelectric domain walls, topological entities separating domains of uniform polarization, are promising candidates as active elements for nanoscale memories. In such applications, controlled nucleation and stabilization of domain walls are critical. Here, using in situ transmission electron microscopy and phase‐field simulations, a controlled nucleation of vertically oriented 109° domain walls in (110)‐oriented BiFeO3 (BFO) thin films is reported. In the switching experiment, reversed domains that are nucleated preferentially at the nanoscale edges of the "crest and sag" pattern‐like electrode under external bias subsequently grow into a stable stripe configuration. In addition, when triangular pockets (with an in‐plane polarization component) are present, these domain walls are pinned to form stable flux‐closure domains. Phase field simulations show that i) field enhancement at the edges of the electrode causes site‐specific domain nucleation, and ii) the local electrostatics at the domain walls drives the formation of flux closure domains, thus stabilizing the striped pattern, irrespective of the initial configuration. The results demonstrate how flux closure pinning can be exploited in conjunction with electrode patterning and substrate orientation to achieve a desired topological defect configuration. These insights constitute critical advancements in exploiting domain walls in next generation ferroelectronic devices. [ABSTRACT FROM AUTHOR]

Tunneling behaviors of composite ferroelectric tunnel junctions (FTJs) with a no-polar dielectric (DE) layer thickness from 1 to 4 nm were investigated. It is found that the low-resistance state (ON state) current decreases with the DE thickness, while the high-resistance state (OFF state) current decreases first and then increases. The largest tunneling electroresistance (TER) effect is observed for the 3 nm-thick DE layer, which corresponds to the lowest OFF-state current. Studies on the electron transport mechanisms show that direct tunneling dominates the ON-state tunneling behaviors for all FTJs as well as the OFF state for the thinnest DE layer of 1 nm. While Fowler–Nordheim (FN) tunneling plays a significant role in the OFF-state electron transport for thicker DE thicknesses and reinforces its role with the increasing DE thickness, the weak FN tunneling-assisted low OFF-state current for the 3 nm-thick DE layer relative to the DE layer of 4 nm contributes to the largest TER effect. [ABSTRACT FROM AUTHOR]

Memristive switching, a nonlinear current–voltage (I–V) characteristic, has seen a tremendous surge in interest as an approach to achieve implementation of synaptic functions. The memristive switching behavior of self‐assembled NiO nanocrystals is investigated via scanning probe microscopy, based on first‐order reversal curve current–voltage spectroscopy. Synaptic switching is clearly observed as a direct consequence of filament growth (i.e., gradually increased conductance) in the nanocrystals. A spatial dependency of the conduction in the nanocrystals suggests that there is a localization of the switching filament. The current understanding of this localization ignores features related to local lateral variation current, which can generate an excessive local heat and temperature such as electrical dissipation. The observation of low electrical dissipation at the edge of the nanocrystals shows that less energy is wasted as heat such that the bias applied can be utilized more efficiently to assist the nucleation of the filament and thus reduces the power consumption. Electrical power dissipation is also found to scale with nanocrystal height and has spatial dependence within the nanocrystals. The combination of synaptic switching and high density of the nanocrystals demonstrate that it is feasible to exploit them to create a basic architecture for neuromorphic memory devices. [ABSTRACT FROM AUTHOR]

Resistive switching memory devices are an emerging class of nonvolatile memories and have shown that outstanding device performance originates from the migration of oxygen vacancies. Research efforts are made to enhance the resistive switching behavior of metal oxide nanostructures. This work reports resistive switching properties of bismuth doped single-crystalline epitaxial anatase TiO2 nanostructures fabricated on (001) oriented Nb:SrTiO3 substrates. These nanostructures are fabricated with a Bi4Ti3O12 precursor using a "phase separation and evaporation" approach. Resistive switching measurements, performed by conducting atomic force microscopy on the as-fabricated TiO2 nanostructures, reveal a stable ON/OFF ratio (∼5 × 104) (irrespective of the spatial position of the tip) at a read voltage of −0.4 V. Energy dispersive X-ray spectroscopy mapping by scanning transmission electron microscopy confirms presence of trapped bismuth (Bi) near the interface for these as-grown nanostructures. Annealing the nanostructures under high vacuum (10–7 Torr) and temperature (900 °C), i.e., conditions that promote Bi loss and evaporation, results in a marked drop in the ON/OFF ratio to ∼2 × 102. As none of the other morphological or structural parameters change after annealing, it reveals the key role played by the trapped Bi. We propose that these residual Bi ions in the vicinity of the interface act as charge trapping/detrapping sites for carriers under the applied electric field, which enhances the resistance switching behavior. This work demonstrates that purposely introducing defects or dopants during oxide heteroepitaxy can be a very potent method to realize emergent electronic properties in metal oxide nanostructures for the next generation resistive switching application. [ABSTRACT FROM AUTHOR]

Complex oxides with tunable structures have many fascinating properties, though high-quality complex oxide epitaxy with precisely controlled composition is still out of reach. Here we have successfully developed solution-based single-crystalline epitaxy for multiferroic (1- x)BiTi(1- y)/2Fe y Mg(1- y)/2O3–(x)CaTiO3 (BTFM–CTO) solid solution in large area, confirming its ferroelectricity at the atomic scale with strong spontaneous polarization. Careful compositional tuning leads to a bulk magnetization of 0.07 ± 0.035 μB/Fe at room temperature, enabling magnetically induced polarization switching exhibiting a large magnetoelectric coefficient of 2.7–3.0 × 10−7 s/m. This work demonstrates the great potential of solution processing in large-scale complex oxide epitaxy and establishes novel room-temperature magnetoelectric coupling in epitaxial BTFM–CTO film, making it possible to explore a much wider space of composition, phase, and structure that can be easily scaled up for industrial applications. [ABSTRACT FROM AUTHOR]

This report investigates the phase evolution pathway of magnetite nanocrystal synthesis on oxide-supported substrates. A template-free phase separation approach, which exploits the thermodynamic instability of ternary perovskite BiFeO3 and inherent volatility of bismuth oxide in low oxygen pressure and high temperature is presented. The formation of an intermediate hematite nanocrystal phase is found as a key step that controls the eventual size and morphology of the magnetite nanocrystals. X-ray absorption spectra measurements and X-ray magnetic circular dichroism confirm that the spectral fingerprints of the magnetite nanocrystals match with that of bulk crystals. Magnetic measurements show that magnetic anisotropy is directly attributed to the nanocrystal morphology. [ABSTRACT FROM AUTHOR]

We report a study on multiferroic bismuth ferrite (BiFeO3, BFO)-ferromagnetic lanthanum strontium manganese oxide (La0.7Sr0.3MnO3, LSMO) epitaxial interfaces by scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) and energy-filtered transmission electron microscopy (EFTEM). Epitaxial (001) oriented LSMO/BFO heterostructures were fabricated on a (001) strontium titanate (SrTiO3, STO) substrate using pulsed laser deposition (PLD). Different cooling conditions to room temperature (rapid or slow) were used to investigate the effect of fabrication conditions on the structural quality of the interfaces. The combined analysis of bright field transmission electron microscopy imaging, STEM-EDS and EFTEM data reveals that the LSMO-BFO heterostructure interface is free from any defects but the phases are chemically interdiffused over a length scale of ∼4 nm. [ABSTRACT FROM AUTHOR]

Epitaxial mixed-phase bismuth ferrite (BiFeO3, BFO) thin films were successfully synthesized on (001) lanthanum aluminate (LaAlO3, LAO) substrates by a chemical solution deposition (CSD) technique. X-ray diffraction measurements confirm the co-existence of a completely relaxed rhombohedral-like (R′) phase and a strained tetragonal-like (T′) phase. Atomic resolution scanning transmission electron microscopy (STEM) measurements reveal that the T′ and R′ phases in our CSD derived BFO/LAO films are mixed homogeneously at the nanoscale. This is in stark contrast to the typical physical vapor deposition derived mixed-phase BFO thin films, which show R′ phase striations embedded in a T′ phase matrix. This phenomenon is attributed to the specific deposition-nucleation-crystallization-relaxation pathway characteristic of the CSD route. This homogenously mixed-phase still demonstrates the well-known morphotropic phase boundary effect, i.e. superior electromechanical properties compared to either the pure T′ phase or R′ phase constituents themselves. Moreover, the maximum piezoelectric coefficient measured by using nanoscale top electrodes shows surprising insensitivity to the clamping effect from the substrate, thereby offering considerable promise in thin film applications. [ABSTRACT FROM AUTHOR]

Nanoscale localisation of the electroforming-free resistive switching (RS) behaviour in polycrystalline GaP thin films has been observed for the first time. A combination of conductive atomic force microscopy and first-order reversal curve current–voltage measurements indicated that the grain boundaries are the preferred sites for the formation of the conductive switching filaments. It is proposed, based on TEM and XPS results, that local electrochemical migration of Ga ions along the grain boundaries plays a critical role in the switching mechanism. In the low-resistance (ON) state, the conduction mechanism was found to be the space-charge-limited current mechanism, while the high-resistance (OFF) state was governed by the Frenkel–Poole mechanism. A high OFF/ON resistance ratio (∼104) and lower power consumption than current RS devices, in addition to the easy integration of GaP with silicon substrates, make these GaP films promising for future applications in future non-volatile resistive random access memory (RRAM). [ABSTRACT FROM AUTHOR]

The magnetic and magnetodielectric behavior of epitaxial (001) transition metal oxide spinel iron vanadate (Fe1+xV2−xO4, FVO) thin films grown on (001) SrTiO3 substrates by pulsed laser deposition is reported. X‐ray absorption spectroscopy studies confirm the trivalent and nominally divalent nature of V and Fe ions, respectively, while an excess of Fe3+ ions is also found. Aberration‐corrected cross‐sectional scanning transmission electron microscopy analysis suggests that the excess Fe3+ may be accommodated through Fe3O4 stacking faults along the {111} planes. Temperature‐dependent X‐ray magnetic circular dichroism spectra reveal an anti‐parallel alignment of spins between the Fe2+ and V3+ cations. M–H loops acquired at 10 K, well below the noncollinear ferromagnetic phase transition temperature, show magnetic anisotropy and point to the existence of two distinct phases with different coercive fields. Visual evidence for magnetic domains and their switching is given by magnetic force microscopy, which finds a switching field of ≈3 T in agreement with the M–H loops. Magnetodielectric measurements for a Pd/FVO/Nb:STO heterostructure reveal a weak hysteresis in the magneto‐capacitance loops at 10 K, thus confirming magneto‐dielectric coupling. Detailed temperature and frequency‐dependent dielectric studies find that Maxwell‐Wagner‐type relaxation dominates capacitance behavior above ≈40 K due to a FVO/Nb:SrTiO3 Schottky interface. [ABSTRACT FROM AUTHOR]