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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.

Rare earth nickelates (RNiO3, where R is rare earth) possess detailed phase diagrams and exhibit a wide variety of physical phenomena such as antiferromagnetism, metal-to-insulator transitions, as well as rich strain-dependent physics. Among them, LaNiO3is unique as it retains its metallic character down to ultralow temperatures, as well as offering the promise of various topological effects and exotic phenomena. Practically speaking, however, the fabrication of LaNiO3in thin-film form is challenging since its various oxygen-deficient phases are close in formation energy. Here, we study a series of epitaxial LaNiO3films fabricated on (001) LaAlO3substrates by pulsed laser deposition under various growth and postannealing conditions. A complex correlation between structure, processing, and conduction properties is found, which is explored using a host of complementary characterization tools including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transport measurements, and scanning transmission electron microscopy. The conclusions of this study are as follows: (i) the XRD patterns of LaNiO3films cannot be used to predict whether films will be conductive or insulating, (ii) the surface XPS of defective and "nondefective" LaNiO3-xfilms can appear identical, despite the bulk having a complex defective structure, and finally, (iii) LaNiO3-xfilms with up to 50% defective phase can exhibit metallic-like transport behavior down to 40 K.

Strain is extensively used to controllably tailor the electronic properties of materials. In the context of indirect band-gap semiconductors such as silicon, strain lifts the valley degeneracy of the six conduction band minima, and by extension the valley states of electrons bound to phosphorus donors. Here, single phosphorus atoms are embedded in an engineered thin layer of silicon strained to 0.8% and their wave function imaged using spatially resolved spectroscopy. A prevalence of the out-of-plane valleys is confirmed from the real-space images, and a combination of theoretical modelling tools is used to assess how this valley repopulation effect can yield isotropic exchange and tunnel interactions in the xy-plane relevant for atomically precise donor qubit devices. Finally, the residual presence of in-plane valleys is evidenced by a Fourier analysis of both experimental and theoretical images, and atomistic calculations highlight the importance of higher orbital excited states to obtain a precise relationship between valley population and strain. Controlling the valley degree of freedom in engineered strained epilayers provides a new competitive asset for the development of donor-based quantum technologies in silicon.

Varying thickness in ferroelectric heterostructures systematically changes both the strain and the electrical boundary conditions and thus the polarization screening. This has a direct result on the observed ferroelectric nanotopologies, from polar vortices, skyrmions, and bubbles to Kittel type stripe/labyrinthine domains. Here, a control of the topological defect transitions is reported in epitaxial (001)-oriented PbZr0.4Ti0.6O3/SrTiO3/PbZr0.4Ti0.6O3 (PZT/STO/PZT) heterostructures. Piezoresponse force microscopy is exploited to capture various topological defect states, such as merons, skyrmions, dislocations, bimerons, and three- or fourfold junctions and hence to understand their transition pathways. The thickness of the dielectric spacer and/or ferroelectric layer is tuned during growth to manipulate the strength of the residual depolarization field; this consequently leads to a range of the abovementioned topological defect structures. This is further corroborated by effective Hamiltonian-based Monte Carlo simulations that provide insight into why and how altering the thickness of ferroelectric or dielectric layers triggers topological phase transitions. This controlled design of nanoscale ferroic topologies opens possibilities of engineering emergent transitions.

Artificial synapses are memristor-based devices mimicing 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.

The nanoscale electrical properties of chemical-solution-deposition (CSD)-derived BiFeO3 grown on pulsed-laser-ablated La0.67Sr033MnO3//SrTiO3 (001) thin film heterostructures are investigated using a host of scanning probe microscopy techniques, including electrostatic force microscopy (EFM), scanning Kelvin probe microscopy (SKPM), piezoresponse force microscopy (PFM), and conductive AFM (CAFM). EFM and SKPM confirm the p-type nature of the CSD-derived BFO thin films as well as charge accumulation at the film surface after electrical bias. For BiFeO3 films of a fixed thickness (∼35 nm), the local current−voltage (I−V) behavior obtained by CAFM is strongly dependent on the La0.67Sr033MnO3 bottom electrode thickness. BiFeO3 films on a 20 nm thick La0.67Sr033MnO3 demonstrate the typical switchable diode behavior governed by polarization orientation. However, when the thickness of La0.67Sr033MnO3 is reduced to less than 5 nm, the BiFeO3 films show only forward diode behaviors regardless of polarization orientation, when the applied bias is up to ±4 V. Higher sweep bias (i.e., ± 8 V) breaks down the diode, following which the BiFeO3 film shows strong resistive switching. The interface band structure for the ultrathin LSMO case, which is very sensitive to accumulation/depletion of carriers at the BFO−LSMO interface, is suggested as the trigger for this resistive switching.

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.

© 2019, The Author(s). Understanding and manipulating complex spin texture in multiferroics can offer new perspectives for electric field-controlled spin manipulation. In BiFeO 3 , a well-known room temperature multiferroic, the competition between various exchange interactions manifests itself as non-collinear spin order, i.e., an incommensurate spin cycloid with period 64 nm. We report on the stability and systematic expansion of the length of the spin cycloid in (110)-oriented epitaxial Co-doped BiFeO 3 thin films. Neutron diffraction shows (i) this cycloid, despite its partly out-of-plane canted propagation vector, can be stabilized in thinnest films; (ii) the cycloid length expands significantly with decreasing film thickness; (iii) theory confirms a unique [11 2 ¯] cycloid propagation direction; and (iv) in the temperature dependence the cycloid length expands significantly close to T N . These observations are supported by Monte Carlo simulations based on a first-principles effective Hamiltonian method. Our results therefore offer new opportunities for nanoscale magnonic devices based on complex spin textures.

Recently, spin spiral type multiferroic materials are widely investigated for their unusual properties and potential applications in memory technology, sensors, and field-effect transistors. It is of interest to investigate surface magnetism in iron vanadate (FeV2O4) due to varying physical properties. Here, the temperature-dependent magnetic domain evolution has been systematically investigated by using cryogenic magnetic force microscopy (MFM). We find irregular and asymmetrical magnetic domain coarsening and shrinking as a function of temperature and also different domain evolution between field cooled and zero field cooled MFM. These findings provide an improved understanding of physical properties related to domain structures.

Theoretical investigations on ferroelectric tunnel junctions (FTJs) with asymmetric electrodes and a composite barrier are presented. A large tunneling electroresistance effect exists for the Pt/SrTiO 3 /BaTiO 3 /SrRuO 3 junction; on the other hand, exchange of the dielectric and ferroelectric layer stacking sequence can seriously degrade the performance. These correlations are rationalized by the proposed concept of an asymmetry factor, defined as the ratio between the average barrier heights of FTJs for two opposite polarization orientations. We show that a large asymmetry factor is beneficial to FTJs. This work may provide a way to enhance the performance of FTJs by structure engineering.

Composition gradients, or dissimilar ferroelectric bilayers, demonstrate colossal electromechanical figures of merit attributed to the motion of ferroelastic domain walls. Yet, mechanistic understanding of polarization switching pathways that drive ferroelastic switching in these systems remains elusive. Here, the crucial roles of strain and electrostatic boundary conditions in ferroelectric bilayer systems are revealed, which underpin their ferroelastic switching dynamics. Using in situ electrical biasing in the transmission electron microscope (TEM), the motion of ferroelastic domain walls is investigated in a tetragonal (T) Pb(Zr,Ti)O3 (PZT)/rhombohedral (R) PZT epitaxial bilayer system. Atomic resolution electron microscopy, in tandem with phase field simulations, indicates that ferroelastic switching is triggered by predominant nucleation at the triple domain junctions located at the interface between the T/R layers. Furthermore, this interfacial nucleation leads to systematic reversable reorientation of ferroelastic domain walls. Deterministic ferroelastic domain switching, driven by the interfacial strain and electrostatic boundary conditions in the ferroelectric bilayer, provides a viable pathway toward novel design of miniaturized energy-efficient electromechanical devices.

The origins of recently reported visible-light photoelectrochemical activity in ZnS–GaP (ZG) multilayer films are investigated using aberration-corrected scanning transmission electron microscopy (STEM). It is revealed that the multilayers carry a large volume fraction of defects, specifically stacking faults and twins, at the interfaces. The defects act as excellent channels for diffusion. For each ZG interface, a ∼5 nm-interdiffused region with an effective chemical composition of a ZnS–GaP solid solution is observed. Previous theoretical calculations have found that ZnS–GaP solid solutions possess a lower band gap than either GaP or ZnS and thus are expected to have better visible-light photo-activity. These findings are thus able to explain the observed commensurate increase in the visible-light photoelectrochemical response with increasing number of ZG layers. This work suggests that interfaces with intentionally designed lattice imperfections and/or intentionally driven interdiffusion leading to local solid solution formation provide a new materials design strategy for achieving efficient visible-light photo-activity.

Here, the deterministic and reversible transformation of nanoscale ferroelectric bubbles into cylindrical domains using a scanning probe microscopy (SPM) approach is demonstrated. The bubble domains—sub-10 nm spheroid topological structures with rotational polarization—can be erased by applying a mechanical force via the SPM tip. Application of an electrical pulse with a specific combination of amplitude and duration can recreate the bubble domain state. This combination of mechanical and electrical passes is essential for realization of reversible transformation as application of only electrical pulses results in complete erasure of the bubble domain state. Effective Hamiltonian-based simulations reproduce phase sequences for both the mechanical and electric passes and confirm the intrinsic nature of these transitions. This simple and effective pathway for switching between various topological defect states may be exploited for emergent devices.

The traditional view of complex oxide heteroepitaxy is that the substrate is merely a passive bystander. Here we show that this is not always the case, in particular for a metal oxide/metal oxide interface during the synthesis of epitaxial nickel oxide (NiO) nanoislands on (001) strontium titanate (SrTiO3-STO) single crystal substrates. We find that the substrate atoms are driven to encapsulate the metal oxide nanocrystal islands - a phenomenon more commonly reported for metal/metal oxide heterogeneous catalysis where the migration of atoms from the support onto the functional nanoparticle system is driven by strong metal-support interactions (SMSI). High-resolution transmission electron microscopy (HRTEM), high angle angular dark field (HAADF) scanning transmission electron microscopy (STEM), and electron tomography (ET) are used to reveal a clear physical displacement of the interface; the atoms from the STO substrate climb and cover the bottom facets of NiO nanoparticles during the synthesis process during which a unique caldera-shaped structure is formed. It is postulated that this phenomenon occurs to lower the work function and surface energy of the system. These results indicate that the SMSI could be important in the metal oxide/metal oxide systems. In this way changes to the substrate-film interface could be exploited by controlling and modifying the properties of nanoscaled functional oxides.

Reversible electric-field induced domain switching in ferroelectric thin films gives rise to a large electromechanical coupling. Despite extensive in situ studies confirming a dominant contribution from domain switching, the speed of the domain wall motion had not been discussed enough. In this study, we performed time-resolved measurement of lattice elongation and non-180° domain switching for an epitaxial rhombohedral (111)/(111)-oriented (Pb(Zr0.65Ti0.35)O3 film under nanosecond electric field pulses by means of synchrotron x-ray diffraction. Both lattice elongation and non-180° domain switching due to a 200-ns electric pulse were directly observed from the shift of the 222 diffraction position toward a lower angle and the change in the integrated intensity ratio of 222 to 222 peaks, respectively. The non-180° domain switching also results in an increase of the switchable polarization. Following the removal of the electric field, it is seen that the non-180° domain back switching from 222 to 222 is sluggish compared to the relaxation of the field-induced lattice strain. This is different from the (100)/(001)-oriented tetragonal epitaxial Pb(Zr,Ti)O3 films, in which no obvious delay was detected. These results show the importance of the direct time-resolved response observation of the crystal structure change with the application of a high-speed electric pulse field to understand the frequency dispersion of the ferroelectric and piezoelectric responses of Pb(Zr,Ti)O3 films.

The misfit dislocations formed at heteroepitaxial interfaces create long-ranging strain fields in addition to the epitaxial strain. For systems with strong lattice coupling, such as ferroic oxides, this results in unpredictable and potentially debilitating functionality and device performance. In this work, we use dark-field X-ray microscopy to map the lattice distortions around misfit dislocations in an epitaxial film of bismuth ferrite (BiFeO 3 ), a well-known multiferroic. We demonstrate the ability to precisely quantify weak, long-ranging strain fields and their associated symmetry lowering without modifying the mechanical state of the film. We isolate the screw and edge components of the individual dislocations and show how they result in weak charge heterogeneities via flexoelectric coupling. We show that even systems with small lattice mismatches and additional mechanisms of stress relief (such as mechanical twinning) may still give rise to measurable charge and strain heterogeneities that extend over mesoscopic length scales. This sets more stringent physical limitations on device size, dislocation density, and the achievable degree of lattice mismatch in epitaxial systems.

In the field of solar water splitting, searching for and modifying bulk compositions have been the conventional approaches to enhancing visible-light activity. In this work, manipulation of heterointerfaces in ZnS-GaP multilayer films is demonstrated as a successful alternative approach to achieving visible-light-active photoelectrodes. The photocurrent measured under visible light increases with the increasing number of interfaces for ZnS-GaP multilayer films with the same total thickness, indicating it to be a predominantly interface-driven effect. The activity extends to long wavelengths (650 nm), much longer than those expected for pure ZnS and also longer than those previously reported for GaP. Density functional theory calculations of ZnS-GaP multilayers predict the presence of electronic states associated with atoms at the interfaces between ZnS and GaP that are different from those found within the layers away from the interfaces; these states, formed due to unique bonding environments found at the interfaces, lead to a lowering of the band gap and hence the observed visible-light activity. The presence of these electronic states attributed to the interfaces is confirmed by depth-resolved X-ray photoelectron spectroscopy. Thus, we show that interface engineering is a promising route for overcoming common deficiencies of individual bulk materials caused by both wide band gaps and indirect band gaps and hence enhancing visible-light absorption and photoelectrochemical performance.

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.

Measuring the diffusion of ions and vacancies at nanometer length scales is crucial to understanding fundamental mechanisms driving technologies as diverse as batteries, fuel cells, and memristors; yet such measurements remain extremely challenging. Here, we employ a multimodal scanning probe microscopy (SPM) technique to explore the interplay between electronic, elastic, and ionic processes via first-order reversal curve I-V measurements in conjunction with electrochemical strain microscopy (ESM). The technique is employed to investigate the diffusion of oxygen vacancies in model epitaxial nickel oxide (NiO) nanocrystals with resistive switching characteristics. Results indicate that opening of the ESM hysteresis loop is strongly correlated with changes to the resonant frequency, hinting that elastic changes stem from the motion of oxygen (or cation) vacancies in the probed volume of the SPM tip. These changes are further correlated to the current measured on each nanostructure, which shows a hysteresis loop opening at larger (∼2.5 V) voltage windows, suggesting the threshold field for vacancy migration. This study highlights the utility of local multimodal SPM in determining functional and chemical changes in nanoscale volumes in nanostructured NiO, with potential use to explore a wide variety of materials including phase-change memories and memristive devices in combination with site-correlated chemical imaging tools.

Observation of a new type of nanoscale ferroelectric domains, termed as “bubble domains”—laterally confined spheroids of sub-10 nm size with local dipoles self-aligned in a direction opposite to the macroscopic polarization of a surrounding ferroelectric matrix—is reported. The bubble domains appear in ultrathin epitaxial PbZr0.2Ti0.8O3/SrTiO3/PbZr0.2Ti0.8O3 ferroelectric sandwich structures due to the interplay between charge and lattice degrees of freedom. The existence of the bubble domains is revealed by high-resolution piezoresponse force microscopy (PFM), and is corroborated by aberration-corrected atomic-resolution scanning transmission electron microscopy mapping of the polarization displacements. An incommensurate phase and symmetry breaking is found within these domains resulting in local polarization rotation and hence impart a mixed Néel–Bloch-like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. The observations are in agreement with ab-initio-based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies possible in ultrathin ferroelectric structures and bring forward the prospect of emergent functionalities due to topological transitions.

The valence band (VB) electronic structure and VB alignments at heterointerfaces of strained epitaxial stannate ASnO3 (A=Ca, Sr, and Ba) thin films are characterized using in situ X-ray and ultraviolet photoelectron spectroscopies, with band gaps evaluated using spectroscopic ellipsometry. Scanning transmission electron microscopy with geometric phase analysis is used to resolve strain at atomic resolution. The VB electronic structure is strain state dependent in a manner that correlated with a directional change in Sn-O bond lengths with strain. However, VB offsets are found not to vary significantly with strain, which resulted in ascribing most of the difference in band alignment, due to a change in the band gaps with strain, to the conduction band edge. Our results reveal significant strain tuning of conduction band offsets using epitaxial buffer layers, with strain-induced offset differences as large as 0.6 eV possible for SrSnO3. Such large conduction band offset tunability through elastic strain control may provide a pathway to minimize the loss of charge confinement in 2-dimensional electron gases and enhance the performance of photoelectrochemical stannate-based devices.

Magnonic devices that utilize electric control of spin waves mediated by complex spin textures are an emerging direction in spintronics research. Room-temperature multiferroic materials, such as bismuth ferrite (BiFeO3), would be ideal candidates for this purpose. To realize magnonic devices, a robust long-range spin cycloid with well-known direction is desired, since it is a prerequisite for the magnetoelectric coupling. Despite extensive investigation, the stabilization of a large-scale uniform spin cycloid in nanoscale (100 nm) thin BiFeO3 films has not been accomplished. Here, we demonstrate cycloidal spin order in 100 nm BiFeO3 thin films through the careful choice of crystallographic orientation, and control of the electrostatic and strain boundary conditions. Neutron diffraction, in conjunction with X-ray diffraction, reveals an incommensurate spin cycloid with a unique [11] propagation direction. While this direction is different from bulk BiFeO3, the cycloid length and Neél temperature remain equivalent to bulk at room temperature.

Nonlinear dynamics underpin a vast array of physical phenomena ranging from interfacial motion to jamming transitions. In many cases, insight into the nonlinear behavior can be gleaned through exploration of higher order harmonics. Here, a method using band excitation scanning probe microscopy (SPM) to investigate higher order harmonics of the electromechanical response, with nanometer scale spatial resolution is presented. The technique is demonstrated by probing the first three harmonics of strain for a Pb(Zr1-xTix)O3 (PZT) ferroelectric capacitor. It is shown that the second order harmonic response is correlated with the first harmonic response, whereas the third harmonic is not. Additionally, measurements of the second harmonic reveal significant deviations from Rayleigh-type models in the form of a much more complicated field dependence than is observed in the spatially averaged data. These results illustrate the versatility of nth order harmonic SPM detection methods in exploring nonlinear phenomena in nanoscale materials.