Flexoelectric Domain Walls Originated from Structural Phase Transition in Epitaxial BiVO4 Films.

Polar domain walls in centrosymmetric ferroelastics induce inhomogeneity that is the origin of advantageous multifunctionality. In particular, polar domain walls promote charge-carrier separation and hence are promising for energy conversion applications that overcome the hurdles of the rate-limiting step in the traditional photoelectrochemical water splitting processes. Yet, while macroscopic studies investigate the materials at the device scale, the origin of this phenomenon in general and the emergence of polar domain walls during the structural phase transition in particular has remained elusive, encumbering the development of this attractive system. Here, it is demonstrated that twin domain walls arise in centrosymmetric BiVO4 films and they exhibit localized piezoelectricity. It is also shown that during the structural phase transition from the tetragonal to monoclinic, the symmetry reduction is accompanied by an emergence of strain gradient, giving rise to flexoelectric effect and the polar domain walls. These results not only expose the emergence of polar domain walls at centrosymmetric systems by means of direct observation, but they also expand the realm of potential application of ferroelastics, especially in photoelectrochemistry and local piezoelectricity.

Deterministic optical control of room temperature multiferroicity in BiFeO3 thin films.

Controlling ferroic orders (ferroelectricity, ferromagnetism and ferroelasticity) by optical methods is a significant challenge due to the large mismatch in energy scales between the order parameter coupling strengths and the incident photons. Here, we demonstrate an approach to manipulate multiple ferroic orders in an epitaxial mixed-phase BiFeO3 thin film at ambient temperature via laser illumination. Phase-field simulations indicate that a light-driven flexoelectric effect allows the targeted formation of ordered domains. We also achieved precise sequential laser writing and erasure of different domain patterns, which demonstrates a deterministic optical control of multiferroicity at room temperature. As ferroic orders directly influence susceptibility and conductivity in complex materials, our results not only shed light on the optical control of multiple functionalities, but also suggest possible developments for optoelectronics and related applications.

Ultrafast Giant Photostriction of Epitaxial Strontium Iridate Film with Superior Endurance.

Photostriction, optical stimulus driven mechanical deformation in materials, provides a solution toward next-generation technology. Here, the giant photostriction (∼2% change of lattice) of epitaxial strontium iridate (SrIrO3) films under illumination at room temperature is revealed via power-dependent Raman scattering, which is significantly larger as compared to conventional inorganic materials. The time scale and mechanism of this giant photostriction in SrIrO3 are further studied through time-resolved transient reflectivity measurements. The main mechanism is determined to be the electron-phonon coupling. In addition, we find that such an exotic behavior happens within few picoseconds and remains up to 107 cyclic on/off operations. The observation of giant photostriction in SrIrO3 films with superior endurance promises the advance of shape responsive solids that are sensitive to environmental stimuli, which could be widely utilized for multifunctional optoelectronics and optomechanical devices.

A Strain-Driven Antiferroelectric-to-Ferroelectric Phase Transition in La-Doped BiFeO3 Thin Films on Si.

A strain-driven orthorhombic (O) to rhombohedral (R) phase transition is reported in La-doped BiFeO3 thin films on silicon substrates. Biaxial compressive epitaxial strain is found to stabilize the rhombohedral phase at La concentrations beyond the morphotropic phase boundary (MPB). By tailoring the residual strain with film thickness, we demonstrate a mixed O/R phase structure consisting of O phase domains measuring tens of nanometers wide within a predominant R phase matrix. A combination of piezoresponse force microscopy (PFM), transmission electron microscopy (TEM), polarization-electric field hysteresis loop (P-E loop), and polarization maps reveal that the O-R structural change is an antiferroelectric to ferroelectric (AFE-FE) phase transition. Using scanning transmission electron microscopy (STEM), an atomically sharp O/R MPB is observed. Moreover, X-ray absorption spectra (XAS) and X-ray linear dichroism (XLD) measurements reveal a change in the antiferromagnetic axis orientation from out of plane (R-phase) to in plane (O-phase). These findings provide direct evidence of spin-charge-lattice coupling in La-doped BiFeO3 thin films. Furthermore, this study opens a new pathway to drive the AFE-FE O-R phase transition and provides a route to study the O/R MPB in these films.

Self-Assembled Epitaxial Core-Shell Nanocrystals with Tunable Magnetic Anisotropy.

Epitaxial core-shell CoO-CoFe2 O4 nanocrystals are fabricated by using pulsed laser deposition with the aid of melted material (Bi2 O3 ) addition and suitable lattice mismatch provided by substrates (SrTiO3 ). Well aligned orientations among nanocrystals and reversible core-shell sequence reveal tunable magnetic anisotropy. The interfacial coupling between core and shell further engineers the nanocrystal functionality.

Tuning phase stability of complex oxide nanocrystals via conjugation.

Nanocrystals (NCs) attract tremendous research interests because of their unique properties to meet the demands of functionalities. To date, hybrid NCs with multiple components are developed to meet the rising demands that could be very difficult, or even impossible to be achieved by single-component NCs. Tuning properties by strain via conjugation could be an alternative solution. Strain engineering has been discovered and widely applied to many thin-film materials for tuning physical properties. Then, there is a further question to be addressed in this study: can we take the advantages we have learned in heteroepitaxy of thin films and transfer that into the NC conjugation? In order to demonstrate this possibility, we investigated NC conjugation of BiFeO3 and LaAlO3. We found that change in either LaAlO3-NC or BiFeO3-NC size would change the stability of rhombohedral-to-tetragonal phase transition. The present results show that strain engineering is possible to be realized in not only thin film but also NC conjugation. The same concept should be applicable to other complex oxide systems in order to broaden their practical applications for the rising demands of multifunctionalities.

Magnetic mesocrystal-assisted magnetoresistance in manganite.

Mesocrystal, a new class of crystals as compared to conventional and well-known single crystals and polycrystalline systems, has captured significant attention in the past decade. Recent studies have been focused on the advance of synthesis mechanisms as well as the potential on device applications. In order to create further opportunities upon functional mesocrystals, we fabricated a self-assembled nanocomposite composed of magnetic CoFe2O4 mesocrystal in Sr-doped manganites. This combination exhibits intriguing structural and magnetic tunabilities. Furthermore, the antiferromagnetic coupling of the mesocrystal and matrix has induced an additional magnetic perturbation to spin-polarized electrons, resulting in a significantly enhanced magnetoresistance in the nanocomposite. Our work demonstrates a new thought toward the enhancement of intrinsic functionalities assisted by mesocrystals and advanced design of novel mesocrystal-embedded nanocomposites.

Global Compressive Loading from an Ultra-Thin PEEK Button Augment Enhances Fibrocartilage Regeneration of Rotator Cuff Enthesis.

A PEEK button is developed to improve the tendon-to-bone compression area. In total, 18 goats were divided into 12-week, 4-week, and 0-week groups. All underwent bilateral detachment of the infraspinatus tendon. In the 12-week group, 6 were fixed with a 0.8-1 mm-thick PEEK augment (A-12, Augmented), and 6 were fixed with the double-row technique (DR-12). Overall, 6 infraspinatus were fixed with PEEK augment (A-4) and without PEEK augment (DR-4) in the 4-week group. The same condition was performed in the 0-week groups (A-0 and DR-0). Mechanical testing, immunohistochemistry assessment, cell responses, tissue alternation, surgical impact, remodeling, and the expression of type I, II, and III collagen of the native tendon-to-bone insertion and new footprint areas were evaluated. The average maximum load in the A-12 group (393.75 (84.40) N) was significantly larger than in the TOE-12 group (229.17 (43.94) N) (p < 0.001). Cell responses and tissue alternations in the 4-week group were slight. The new footprint area of the A-4 group had better fibrocartilage maturation and more type III collagen expression than in DR-4 group. This result proved the novel device is safe and provides superior load-displacement to the double-row technique. There is a trend toward better fibrocartilage maturation and more collagen III secretions in the PEEK augmentation group.

Self-Enhancement of Water Electrolysis by Electrolyte-Poled Ferroelectric Catalyst.

Efficient and durable electrocatalysts with superior activity are needed for the production of green hydrogen with a high yield and low energy consumption. Electrocatalysts based on transition metal oxides hold dominance due to their abundant natural resources, regulable physical properties, and good adaptation to a solution. In numerous oxide catalyst materials, ferroelectrics, possessing semiconducting characteristics and switchable spontaneous polarization, have been considered promising photoelectrodes for solar water splitting. However, few investigations noted their potential as electrocatalysts. In this study, we report an efficient electrocatalytic electrode made of a BiFeO3/nickel foam heterostructure, which displays a smaller overpotential and higher current density than the blank nickel foam electrode. Moreover, when in contact with an alkaline solution, the bond between hydroxyls and the BiFeO3 surface induces a large area of upward self-polarization, lowering the adsorption energy of subsequent adsorbates and facilitating oxygen and hydrogen evolution reaction. Our work demonstrates an infrequent pathway of using functional semiconducting materials for exploiting highly efficient electrocatalytic electrodes.

High Entropy Nonlinear Dielectrics with Superior Thermally Stable Performance.

A high configurational entropy, achieved through a proper design of compositions, can minimize the Gibbs free energy and stabilize the quasi-equilibrium phases in a solid-solution form. This leads to the development of high-entropy materials with unique structural characteristics and excellent performance, which otherwise could not be achieved through conventional pathways. This work develops a high-entropy nonlinear dielectric system, based on the expansion of lead magnesium niobate-lead titanate. A dense and uniform distribution of nano-polar regions is observed in the samples owing to the addition of Ba, Hf, and Zr ions, which lead to enhanced performance of nonlinear dielectrics. The fact that no structural phase transformation is detected up to 250 °C, and no noticeable change or a steep drop in structural and electrical characteristics is observed at high temperatures suggests a robust thermal stability of the dielectric systems developed. With these advantages, these materials hold vast potential for applications such as dielectric energy storage, dielectric tunability, and electrocaloric effect. Thus, this work offers a new high-entropy configuration with elemental modulation, with enhanced dielectric material features.

Electric-field control of the nucleation and motion of isolated three-fold polar vertices.

Recently various topological polar structures have been discovered in oxide thin films. Despite the increasing evidence of their switchability under electrical and/or mechanical fields, the dynamic property of isolated ones, which is usually required for applications such as data storage, is still absent. Here, we show the controlled nucleation and motion of isolated three-fold vertices under an applied electric field. At the PbTiO3/SrRuO3 interface, a two-unit-cell thick SrTiO3 layer provides electrical boundary conditions for the formation of three-fold vertices. Utilizing the SrTiO3 layer and in situ electrical testing system, we find that isolated three-fold vertices can move in a controllable and reversible manner with a velocity up to ~629 nm s-1. Microstructural evolution of the nucleation and propagation of isolated three-fold vertices is further revealed by phase-field simulations. This work demonstrates the ability to electrically manipulate isolated three-fold vertices, shedding light on the dynamic property of isolated topological polar structures.

Supercritical Ammoniation-Enabled Interfacial Polarization for Function-Mode Transformation and Overall Optimization of Thin-Film Transistors.

Thin-film transistors (TFTs) have drawn widespread applications in the increasingly sophisticated display field. Despite the mature process of fabricating enhancement-mode TFTs, lack of facile methods to realize depletion-mode TFTs restrains the implementation of complementary-type circuits, which in turn leads to relatively high power. Here, the supercritical fluid technique is introduced to elaborately design and tune the interface, providing the opportunity for function-mode transformation of TFTs. By harnessing supercritical-assisted ammoniation (SCA) treatment, interfacial polarization induces negative shift of threshold voltage (from 0.2 to -9.8 V), which allows TFTs to remain normally on-state in the absence of complex capacitor-integrated circuits. This convenient technique, without an additional manufacturing process to achieve function-mode transformation, can thus enable the fabrication of comprehensive-mode TFTs under the same process. Furthermore, comprehensive optimizations in the mobility (increases from 2.08 to 17.12 cm2 V-1 s-1), leakage current (reduces from 1.33 × 10-11 to 2.22 × 10-12 A), hysteresis (reduces from 11.2 to 0.2 V), and on/off current ratio (increases from 9.65 × 104 to 7.98 × 106) are achieved simultaneously. Based on conjoint analysis of electrical and material characterization, a reaction model is established for a clearer understanding of the interfacial polarization process. Overall, this low-temperature SCA treatment offers an environmentally benign strategy to modulate the function mode of electronic devices via interfacial engineering and optimize device performance at the same time, exhibiting promise in promoting the implementation of complementary, low-power circuit.

Achieving complementary resistive switching and multi-bit storage goals by modulating the dual-ion reaction through supercritical fluid-assisted ammoniation.

Complementary resistive switching (CRS) is a core requirement in memristor crossbar array construction for neuromorphic computing in view of its capability to avoid the sneak path current. However, previous approaches for implementing CRS are generally based on a complex device structure design and fabrication process or a meticulous current-limiting measurement procedure. In this study, a supercritical fluid-assisted ammoniation (SFA) process is reported to achieve CRS in a single device by endowing the original ordinary switching materials with dual-ion operation. In addition to self-compliant CRS behavior, a multi-bit storage function has also been achieved through the SFA process accompanied by superior retention and reliability. These substantial evolved resistive phenomena are elucidated attentively by our chemical reaction model and physical mechanism model corroborated by the material analysis and current conduction fitting analysis results. The findings in this research present the most efficient way to achieve CRS through only one chemical procedure with significantly improved device performance. Moreover, the supercritical fluid approach envisions tremendous possibilities for further development of materials and electric devices by a low-temperature process, with semiconductor fabrication compatibility and environmental friendliness.

Bifunctional homologous alkali-metal artificial synapse with regenerative ability and mechanism imitation of voltage-gated ion channels.

As a key component responsible for information processing in the brain, the development of a bionic synapse possessing digital and analog bifunctionality is vital for the hardware implementation of a neuro-system. Here, inspired by the key role of sodium and potassium in synaptic transmission, the alkali metal element lithium (Li) belonging to the same family is adopted in designing a bifunctional artificial synapse. The incorporation of Li endows the electronic devices with versatile synaptic functions. An artificial neural network based on experimental data exhibits a high performance approaching near-ideal accuracy. In addition, the regenerative ability allows synaptic functional recovery through low-frequency stimuli to be emulated, facilitating the prevention of permanent damage due to intensive neural activities and ensuring the long-term stability of the entire neural system. What is more striking for an Li-based bionic synapse is that it can not only emulate a biological synapse at a behavioral level but realize mechanism emulation based on artificial voltage-gated "ion channels". Concurrent digital and analog features lead to versatile synaptic functions in Li-doped artificial synapses, which operate in a mode similar to the human brain with its two hemispheres excelling at processing imaginative and analytical information, respectively.

Low-temperature supercritical dehydroxylation for achieving an ultra-low subthreshold swing of thin-film transistors.

Thin-film transistors (TFTs) have been widely used in the increasingly advanced field of displays. However, it remains a challenge for TFTs to overcome the poor subthreshold swing in the fast switching and high-speed applications. Here, we provide a solution to the above-mentioned challenge via supercritical dehydroxylation, which combines a low temperature, environmentally friendly supercritical fluid technology with a CaCl2 treatment. An embedded structure of amorphous indium gallium zinc oxide (a-IGZO) TFTs with double-layer high-k dielectric containing Ta2O5 and SiO2 layers was first manufactured. The subthreshold swing of the fabricated TFTs treated with supercritical dehydroxylation was optimized to an ultra-low value of 72.7 mV dec-1. Moreover, other key figures of merits including threshold voltage, on/off ratio and field effect mobility all improved after the supercritical dehydroxylation. The bandgap of the gate dielectric material increased due to the supercritical dehydroxylation verified by the current conduction mechanism. Besides, numerous material analyses further confirmed that owing to the supercritical dehydroxylation the dominant dehydration reactions can effectively repair the defects introduced in the device manufacture. The ultra-low subthreshold swing with optimized electrical performances can be achieved via the low-temperature supercritical dehydroxylation treatment, enabling its promising potential in realizing ultra-fast and low power electronics.

Fabrication of Large-Scale High-Mobility Flexible Transparent Zinc Oxide Single Crystal Wafers.

Single crystal wafers, such as silicon, are the fundamental carriers of advanced electronic devices. However, these wafers exhibit rigidity without mechanical flexibility, limiting their applications in flexible electronics. Here, we propose a new approach to fabricate 1.5 in. flexible functional zinc oxide (ZnO) single crystal wafers with high electron mobility (>100 cm2 V-1 s-1) and optical transparency (>80%) by a combination of thin-film deposition, a chemical solution method, and surficial treatment. The uniformity of the flexible single crystal wafers is examined by an advanced scanning X-ray diffraction technique and photoluminescence spectroscopy. The transport properties of ZnO flexible single crystal wafers retain their pristine states under various bending conditions, including cyclability and endurability. This approach demonstrates a breakthrough in the fabrication of the flexible single crystal wafers for future flexible optoelectronic applications.

Unveiling the influence of surrounding materials and realization of multi-level storage in resistive switching memory.

Considerable efforts have been made to obtain better control of the switching behavior of resistive random access memory (RRAM) devices, such as using modified or multilayer switching materials. Although considerable progress has been made, the reliability and stability of the devices greatly deteriorate due to dispersed electric field caused by low permittivity surrounding materials. By introducing surrounding materials with a relatively higher dielectric constant, the RRAM devices become promising for cost-effective applications by achieving multilevel storage functionality and improved scalability. A device designed by this principle exhibits multiple distinct and non-volatile conductance states. Moreover, the issue of the increasing forming voltage during device scaling is also solved, improving the capacity of the chips and reducing the power dissipation in the process of the device miniaturization. The COMSOL simulation helps to reveal that the enhanced performance is correlated with a more concentrated electric field around the conductive filament, which is favorable for controlling the connection and rupture of the resistive filament.

Anisotropic properties of pipe-GaN distributed Bragg reflectors.

We report here a simple and robust process to convert periodic Si-doped GaN/undoped-GaN epitaxial layers into a porous-GaN/u-GaN distributed Bragg reflector (DBR) structure and demonstrate its material properties in a high-reflectance epitaxial reflector. Directional pipe-GaN layers with anisotropic optical properties were formed from n+-GaN : Si layers in a stacked structure through a lateral and doping-selective electrochemical etching process. Central wavelengths of the polarized reflectance spectra were measured to be 473 nm and 457 nm for the pipe-GaN reflector when the direction of the linear polarizer was along and perpendicular to the pipe-GaN structure. The DBR reflector with directional pipe-GaN layers has the potential for a high efficiency polarized light source and vertical cavity surface emitting laser applications.

Manipulating magnetoelectric energy landscape in multiferroics.

Magnetoelectric coupling at room temperature in multiferroic materials, such as BiFeO3, is one of the leading candidates to develop low-power spintronics and emerging memory technologies. Although extensive research activity has been devoted recently to exploring the physical properties, especially focusing on ferroelectricity and antiferromagnetism in chemically modified BiFeO3, a concrete understanding of the magnetoelectric coupling is yet to be fulfilled. We have discovered that La substitutions at the Bi-site lead to a progressive increase in the degeneracy of the potential energy landscape of the BiFeO3 system exemplified by a rotation of the polar axis away from the 〈111〉pc towards the 〈112〉pc discretion. This is accompanied by corresponding rotation of the antiferromagnetic axis as well, thus maintaining the right-handed vectorial relationship between ferroelectric polarization, antiferromagnetic vector and the Dzyaloshinskii-Moriya vector. As a consequence, La-BiFeO3 films exhibit a magnetoelectric coupling that is distinctly different from the undoped BiFeO3 films.

Self-Assembled Ferroelectric Nanoarray.

Self-assembled heteroepitaxial nanostructures have played an important role for miniaturization of electronic devices, e.g., the ultrahigh density ferroelectric memories, and cause for great concern. Our first principle calculations predict that the materials with low formation energy of the interface ( Ef) tend to form matrix structure in self-assembled heteroepitaxial nanostructures, whereas those with high Ef form nanopillars. Under the guidance of the theoretical modeling, perovskite BiFeO3 (BFO) nanopillars are swimmingly grown into CeO2 matrix on single-crystal (001)-SrTiO3 (STO) substrates by pulsed laser deposition, where CeO2 has a lower formation energy of the interface ( Ef) than BFO. This work provides a good paradigm for controlling self-assembled nanostructures as well as the application of self-assembled ferroelectric nanoscale memory.