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Nickel oxide (NiO) nanocrystals epitaxially grown on (001) strontium titanate (SrTiO3) single crystal substrates were characterized to investigate interface morphology and chemistry. Aberration corrected high angle annular dark field scanning transmission electron microscopy reveals the interface between the NiO nanocrystals and the underlying SrTiO3 substrate to be rough, irregular, and have a lower average atomic number than the substrate or the nanocrystal. Energy dispersive x-ray spectroscopy and electron energy loss spectroscopy confirm both chemical disorder and a shift of the energy of the Ti L2,3 peaks. Analysis of the O K edge profiles in conjunction with this shift, implies the presence of oxygen vacancies at the interface. This sheds light into the origin of the previously postulated minority carriers' model to explain resistive switching in NiO. [ABSTRACT FROM AUTHOR]

Single-layered Pb(ZrTi)O and Pb(ZrTi)O thin films and heterolayered Pb(ZrTi)O thin films consisting of alternating Pb(ZrTi)O and Pb(ZrTi)O layers were studied for their microstructure and texture development. The texture in the single-layered PZT films is affected by the Zr/Ti ratio as they have different crystallization behavior depending on the Zr/Ti ratio. With increasing film thickness, the average grain size of Pb(ZrTi)O increases. An unusually large grain size of 1-3 μm together with a strong (001)/(100) preferred orientation were observed for the heterolayered PZT, whereby Pb(ZrTi)O was used as the seeding layer, for film as thin as 150 nm. The film microstructure is refined drastically when the stacking sequence is changed, i.e., when Pb(ZrTi)O is employed as the seeding layer. Thermal treatment of the PZT seeding layer also plays an important function in the microstructure development of the heterolayered PZT film. The formation of the large-grained film is correlated to the lowered nucleation energy of crystallizing Pb(ZrTi)O by the top Pb(ZrTi)O. The Pb(ZrTi)O layer facilitated the nucleation and crystallization of the Pb(ZrTi)O amorphous seeding layer, whereby the overall microstructure of the heterolayered thin film was dictated by the Pb(ZrTi)O seeding layer leading to the growth of larger PZT grains. [ABSTRACT FROM AUTHOR]

Incorporation of dilute concentration of dopant having a valence state different than that of the host cation enables controlled incorporation proximity vacancy defects for local charge balance. Since nonvolatile resistive switching is a phenomenon tied to such defects, it can be expected to be influenced by dilute doping. In this work, we demonstrate that enhanced nonvolatile resistive switching is realized in dilutely cobalt doped TiO2 films grown at room temperature. We provide essential characterizations and analyses. We suggest that the oxygen vacancies in the proximity of immobile dopants provide well distributed anchors for the development of systematic filamentary tracks. [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]

Understanding and manipulating complex spin texture in multiferroics can offer new perspectives for electric field-controlled spin manipulation. In BiFeO3, 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 BiFeO3 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 TN. 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. [ABSTRACT FROM AUTHOR]

Highlights from the article: In the initially published version of this article, Nagarajan Valanoor is indicated as the sole corresponding author. In addition to Nagarajan Valanoor, Sergei Prokhorenko should also be listed as a corresponding author for communications related to simulations performed in this study. Qi Zhang, Sergei Prokhorenko,* Yousra Nahas, Lin Xie, Laurent Bellaiche, Alexei Gruverman, and Nagarajan Valanoor*.

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.

Ferroelectric functionalized dye-sensitized solar cells were fabricated by using a positively-poled LiNbO3 substrate coated with ITO (ITO-LiNbO3) as a collector electrode and demonstrated enhanced power conversion efficiency. Surface potential properties of TiO2 nanoparticle film coated on the ITO-LiNbO3 (TiO2/ITO-LiNbO3) examined by Kelvin probe force microscopy (KPFM) confirmed that a large electric field (a few 10 V/µm) generated from LiNbO3 can penetrate through the ITO layer and is applied to TiO2 film. This polarization-induced electric field leads to an increased photocurrent density by attracting and promoting electrons to direct transport through the mesoporous TiO2 network toward the collector electrode and a decreased charge recombination by facilitating electrons to pass through fewer boundaries of nanoparticles, resulting in high power conversion efficiency. The power conversion efficiency was enhanced by more than 40% in comparison with that without polarization-induced electric field. Incorporating functional ferroelectrics into photovoltaic cells would be a good strategy in improving photovoltaic performance and is applicable to other types of photovoltaic devices, such as perovskite solar cells. [ABSTRACT FROM AUTHOR]

The stability of the “T-like” (T′) phase in BiFeO3 films grown on LaAlO3(001) is investigated. We show that the T′ phase can be stabilized for thicknesses >70 nm under ultralow incident flux conditions in pulsed laser ablation growth. This low flux results in a low growth rate; thus, the sample is held at high temperatures (>600 °C) for much longer than is typical. Transmission electron microscopy and X-ray diffraction analysis suggest that such growth conditions favor the formation of nanoscale “defect pockets”, which apply a local compressive strain of ∼1.8%. We propose that the cumulative effect of local stresses induced by such “designer defects” maintains macroscale strain coherence mechanical boundary conditions, which then preserves the T′ phase to thicknesses beyond conventional wisdom. Finally, by intentionally introducing an amorphous phase at the film-substrate interface, it is shown that the mixed-phase proportion can be tuned for a given thickness. [ABSTRACT FROM AUTHOR]

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]

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. [ABSTRACT FROM AUTHOR]

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010. [ABSTRACT FROM PUBLISHER]

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]

Copper sulfides are appealing semiconductors for optoelectronics applications requiring visible-light activity, such as solar water splitting, due to their relatively low band gaps and earth-abundance. This study investigates the effect of deposition conditions, including substrate temperature and background gas pressure, on the characteristics and photoelectrochemical performance of copper sulfide (Cu x S) films grown by pulsed laser deposition (PLD) on Si (100) substrates. The results reveal that varying the deposition parameters influences the crystalline phases, morphology, and optoelectronic properties of the films. For deposition at room temperature, the films exhibited covellite CuS phase, while elevated deposition temperatures (250 °C and 500 °C) led to the formation of Cu 1.8 S and Cu 2 S phases, resulting in narrowing of the band gap, with the increased temperature also enhancing the film crystallinity and surface roughness. As a result of the changes in film morphology and crystal structure, photocurrent density magnitudes increased with higher deposition temperatures, reaching 0.747 mA/cm2 at −0.8 V for films deposited at 500 °C under vacuum, at least an order of magnitude higher than reported in comparable previous studies of the photoelectrochemical performance of Cu x S. Background gas pressure only slightly affected the photocurrent density, with changes in photoactivity also correlating with changes in film roughness and band gap. The results demonstrate the good visible-light activity and behavior of Cu x S as a stand-alone photoelectrode material, with tunability based on the fabrication conditions, suggesting their potential use in photoelectrochemical and other solar energy conversion applications. • The study examines how deposition parameters, such as substrate temperature and background gas pressure, influence the properties and photoelectrochemical performance of Cu x S films grown by PLD. • Increasing deposition temperature (to 250 °C and 500 °C) led to the formation of Cu 1.8 S and Cu 2 S phases and narrowed band gaps, improved crystallinity and increased surface roughness compared with deposition at room temperature. • Photocurrent densities increased with higher deposition temperatures, reaching 0.747 mA/cm2 at −0.2 V RHE for films deposited at 500 °C under vacuum. • The Cu x S films exhibit excellent visible-light activity and tunability, demonstrating their potential for photoelectrochemical and solar energy conversion applications. • This study provides insights into the behavior of Cu x S as an independent photoelectrode and how its performance may be optimized when used in heterostructures to enhance PEC performance. [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]

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]

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]

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. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Efficient hydrogen production from water by photocatalysis under sunlight requires a significant improvement in light-harvesting capability. Zinc sulfide is a promising, inexpensive hydrogen generation photocatalyst, but in its pure, bulk form it is only active under ultra-violet light. Here, we show clear evidence of photoelectrochemical activity of ZnS thin films under visible-light irradiation without any co-catalysts, achieved through defect engineering. Fabrication of nanostructured ZnS under controlled conditions introduces defects, and hence intermediate electronic states within the band gap, which allow significant absorption of light at energies below the band gap energy of pure, bulk ZnS. The measured band gap of the ZnS thin films is ~2.4 eV, while the photocurrent density exceeds 1.5 mA/cm 2 under visible-light irradiation ( λ ≥435 nm). This is the first measurement of such high photocurrents for undoped ZnS under visible light. [ABSTRACT FROM AUTHOR]

Incorporation of dilute concentration of dopant having a valence state different than that of the host cation enables controlled incorporation proximity vacancy defects for local charge balance. Since nonvolatile resistive switching is a phenomenon tied to such defects, it can be expected to be influenced by dilute doping. In this work, we demonstrate that enhanced nonvolatile resistive switching is realized in dilutely cobalt doped TiO2 films grown at room temperature. We provide essential characterizations and analyses. We suggest that the oxygen vacancies in the proximity of immobile dopants provide well distributed anchors for the development of systematic filamentary tracks. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report the observation of a magnetocapacitance effect at the interface between Ni and epitaxial nonpolar BiInO 3 thin films at room temperature. A detailed surface study using X-ray photoelectron spectroscopy (XPS) reveals the formation of an intermetallic Ni-Bi alloy at the Ni/BiInO 3 interface and a shift in the Bi 4f and In 3d core levels to higher binding energies with increasing Ni thickness. The latter infers band bending in BiInO 3 , corresponding to the formation of a p-type Schottky barrier. The current-voltage characteristics of the Ni/BiInO 3 /(Ba,Sr)RuO 3 /NdScO 3 (110) heterostructure show a significant dependence on the applied magnetic field and voltage cycling, which can be attributed to voltage-controlled band bending and spin-polarized charge accumulation in the vicinity of the Ni/BiInO 3 interface. The magnetocapacitance effect can be realized at room temperature without involving multiferroic materials.

In ferroelectrics, complex interactions among various degrees of freedom enable the condensation of topologically protected polarization textures. Known as ferroelectric solitons, these particle-like structures represent a new class of materials with promise for beyond-CMOS technologies due to their ultrafine size and sensitivity to external stimuli. Such polarization textures have scarcely been demonstrated in multiferroics. Here, we present evidence for ferroelectric solitons in (BiFeO 3 )/(SrTiO 3 ) superlattices. High-resolution piezoresponse force microscopy and Cs-corrected high-angle annular dark-field scanning transmission electron microscopy reveal a zoo of topologies, and polarization displacement mapping of planar specimens reveals center-convergent/divergent topological defects as small as 3 nm. Phase-field simulations verify that some of these structures can be classed as bimerons with a topological charge of ±1, and first-principles-based effective Hamiltonian computations show that the coexistence of such structures can lead to non-integer topological charges, a first observation in a BiFeO 3 -based system. Our results open new opportunities in multiferroic topotronics., (© 2023. The Author(s).)

Spherical ferroelectric domains, such as electrical bubbles, polar skyrmion bubbles and hopfions, share a single and unique feature-their homogeneously polarized cores are surrounded by a vortex ring of polarization whose outer shells form a spherical domain boundary. The resulting polar texture, typical of three-dimensional topological solitons, has an entirely new local symmetry characterized by a high polarization and strain gradients. Consequently, spherical domains represent a different material system of their own with emergent properties drastically different from that of their surrounding medium. Examples of new functionalities inherent to spherical domains include chirality, optical response, negative capacitance and giant electromechanical response. These characteristics, particularly given that the domains naturally have an ultrafine scale, offer new opportunities in high-density and low-energy nanoelectronic technologies. This Perspective gives an insight into the complex polar structure and physical origin of these spherical domains, which facilitates the understanding and development of spherical domains for device applications., (© 2023. Springer Nature Limited.)

The conducting boundary states of topological insulators appear at an interface where the characteristic invariant ℤ 2 switches from 1 to 0. These states offer prospects for quantum electronics; however, a method is needed to spatially-control ℤ 2 to pattern conducting channels. It is shown that modifying Sb 2 Te 3 single-crystal surfaces with an ion beam switches the topological insulator into an amorphous state exhibiting negligible bulk and surface conductivity. This is attributed to a transition from ℤ 2  = 1 → ℤ 2  = 0 at a threshold disorder strength. This observation is supported by density functional theory and model Hamiltonian calculations. Here we show that this ion-beam treatment allows for inverse lithography to pattern arrays of topological surfaces, edges and corners which are the building blocks of topological electronics., (© 2023. Crown.)

The role of local chemical environments in the electron energy loss spectra of complex multiferroic oxides was studied using computational and experimental techniques. The evolution of the O K-edge across an interface between bismuth ferrite (BFO) and lanthanum strontium manganate (LSMO) was considered through spectral averaging over crystallographically equivalent positions to capture the periodicity of the local O environments. Computational techniques were used to investigate the contribution of individual atomic environments to the overall spectrum, and the role of doping and strain was considered. Chemical variation, even at the low level, was found to have a major impact on the spectral features, whereas strain only induced a small chemical shift to the edge onset energy. Through a combination of these methods, it was possible to explain experimentally observed effects such as spectral flattening near the interface as the combination of spectral responses from multiple local atomic environments., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Resonant tunneling is a quantum-mechanical effect in which electron transport is controlled by the discrete energy levels within a quantum-well (QW) structure. A ferroelectric resonant tunneling diode (RTD) exploits the switchable electric polarization state of the QW barrier to tune the device resistance. Here, the discovery of robust room-temperature ferroelectric-modulated resonant tunneling and negative differential resistance (NDR) behaviors in all-perovskite-oxide BaTiO 3 /SrRuO 3 /BaTiO 3 QW structures is reported. The resonant current amplitude and voltage are tunable by the switchable polarization of the BaTiO 3 ferroelectric with the NDR ratio modulated by ≈3 orders of magnitude and an OFF/ON resistance ratio exceeding a factor of 2 × 10 4 . The observed NDR effect is explained an energy bandgap between Ru-t 2g and Ru-e g orbitals driven by electron-electron correlations, as follows from density functional theory calculations. This study paves the way for ferroelectric-based quantum-tunneling devices in future oxide electronics., (© 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Bubble-like domains, typically a precursor to the electrical skyrmions, arise in ultrathin complex oxide ferroelectric-dielectric-ferroelectric heterostructures epitaxially clamped with flat substrates. Here, it is reported that these specially ordered electric dipoles can also be retained in a freestanding state despite the presence of inhomogeneously distributed structural ripples. By probing local piezo and capacitive responses and using atomistic simulations, this study analyzes these ripples, sheds light on how the bubbles are stabilized in the modified electromechanical energy landscape, and discusses the difference in morphology between bubbles in freestanding and as-grown states. These results are anticipated to be the starting point of a new paradigm for the exploration of electric skyrmions with arbitrary boundaries and physically flexible topological orders in ferroelectric curvilinear space., (© 2021 Wiley-VCH GmbH.)

The nanoscale resistive switching characteristics of gallium phosphide (GaP) thin films directly grown on Si are investigated as a function of incident light. The formation of conductive channels along the grain boundaries is attributed to the presence of point defects and structural disorder, which provide the ideal environment to enable the filamentary switching process. Both first-principles calculations and UV-vis and photoluminescence spectroscopy strongly point to the possibility of mid-gap electronic states in the polycrystalline GaP film due to such defects. To study the photonic excitation, photoconductive atomic force microscopy (phAFM) measurement is conducted. We observe photocurrents even for incident photon energies lower than the band gap, consistent with the presence of mid-gap electronic states; the photocurrents increase in direct proportion to the incident photon energy with a concomitant decrease in the filament resistance. This demonstrates GaP directly integrated on Si can be a promising photonic resistive switching materials system.

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)O 3 (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 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.

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 PbZr 0.2 Ti 0.8 O 3 /SrTiO 3 /PbZr 0.2 Ti 0.8 O 3 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., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Ferroelastic domain switching significantly affects piezoelectric properties in ferroelectric materials. The ferroelastic domain switching and the lattice deformation of both a-domains and c-domains under an applied electric field were investigated using in-situ synchrotron X-ray diffraction in conjunction with a high-speed pulse generator set up for epitaxial (100)/(001)-oriented tetragonal Pb(Zr 0.4 Ti 0.6 )O 3 (PZT) films grown on (100) c SrRuO 3 //(100)KTaO 3 substrates. The 004 peak (c-domain) position shifts to a lower 2θ angle, which demonstrates the elongation of the c-axis lattice parameter of the c-domain under an applied electric field. In contrast, the 400 peak (a-domain) shifts in the opposite direction (higher angle), thus indicating a decrease in the a-axis lattice parameter of the a-domain. 90° domain switching from (100) to (001) orientations (from a-domain to c-domain) was observed by a change in the intensities of the 400 and 004 diffraction peaks by applying a high-speed pulsed electric field 200 ns in width. This change also accompanied a tilt in the angles of each domain from the substrate surface normal direction. This behaviour proved that the 90° domain switched within 40 ns under a high-speed pulsed electric field. Direct observation of such high-speed switching opens the way to design piezo-MEMS devices for high-frequency operation.

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.

The valence band (VB) electronic structure and VB alignments at heterointerfaces of strained epitaxial stannate ASnO 3 (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 SrSnO 3 . 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., Competing Interests: The authors declare no competing financial interests.