Current-voltage hysteresis and switchable rectifying characteristics have been observed in epitaxial multiferroic BiFeO 3 ͑BFO͒ thin films. The forward direction of the rectifying current can be reversed repeatedly with polarization switching, indicating a switchable diode effect and large ferroelectric resistive switching. With analyzing the potential barriers and their variation with ferroelectric switching at the interfaces between the metallic electrodes and the semiconducting BFO, the switchable diode effect can be explained qualitatively by the polarization-modulated Schottky-like barriers.

OER) are two of the most important electrochemical reactions that limit the efficiencies of fuel cells, metal-air batteries, and electrolytic water-splitting. [4][5][6][7] Although some noble metals and their associated compounds, such as Pt, RuO 2 , and IrO 2 , exhibit high ORR or OER catalytic activity, [8][9][10][11][12] the high cost and scarcity of such precious metals prevent their large-scale use. [13,14] Perovskite-structured (ABO 3 ) transition metal oxides are promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In this paper, a set of epitaxial rare-earth nickelates (RNiO 3 ) thin films is investigated with controlled A-site isovalent substitution to correlate their structure and physical properties with ORR/OER activities, examined by using a three-electrode system in O 2 -saturated 0.1 m KOH electrolyte. The ORR activity decreases monotonically with decreasing the A-site element ionic radius which lowers the conductivity of RNiO 3 (R = La, La 0.5 Nd 0.5 , La 0.2 Nd 0.8 , Nd, Nd 0.5 Sm 0.5 , Sm, and Gd) films, with LaNiO 3 being the most conductive and active. On the other hand, the OER activity initially increases upon substituting La with Nd and is maximal at La 0.2 Nd 0.8 NiO 3 . Moreover, the OER activity remains comparable within error through Sm-doped NdNiO 3 . Beyond that, the activity cannot be measured due to the potential voltage drop across the film. The improved OER activity is ascribed to the partial reduction of Ni 3+ to Ni 2+ as a result of oxygen vacancies, which increases the average occupancy of the e g antibonding orbital to more than one. The work highlights the importance of tuning A-site elements as an effective strategy for balancing ORR and OER activities of bifunctional electrocatalysts.

LaMnO(3) (LMO) films are deposited on SrTiO(3):Nb (0.8 wt%) substrates under various oxygen pressures to obtain different concentrations of oxygen vacancies in the films. The results of X-ray diffraction verify that with a decrease of the oxygen pressure, the c-axis lattice constant of the LMO films becomes larger, owing to an increase of the oxygen vacancies. Aberration-corrected annular-bright-field scanning transmission electron microscopy with atomic resolution and sensitivity for light elements is used, which clearly shows that the number of oxygen vacancies increases with the decrease of oxygen pressure during fabrication. Correspondingly, the resistive switching property becomes more pronounced with more oxygen vacancies in the LMO films. Furthermore, a numerical model based on the modification of the interface property induced by the migration of oxygen vacancies in these structures is proposed to elucidate the underlying physical origins. The calculated results are in good agreement with the experimental data, which reveal from a theoretical point of view that the migration of oxygen vacancies and the variation of the Schottky barrier at the interface with applied bias dominate the resistive switching characteristic. It is promising that the resistive switching property in perovskite oxides can be manipulated by controlling the oxygen vacancies during fabrication or later annealing in an oxygen atmosphere.

Organometal halide perovskite solar cells (PSCs) have emerged as one of the most promising photovoltaic technologies with efficiencies exceeding 20.3%. However, device stability problems including hysteresis in current−voltage scans must be resolved before the commercialization of PSCs. Transient absorption measurements and first-principles calculations indicate that the migration of oxygen vacancies in the TiO 2 electrode under electric field during voltage scans contributes to the anomalous hysteresis in PSCs. The accumulation of oxygen vacancies at the electrode/perovskite interface slows down charge extraction while significantly speeding up charge recombination at the interface. Moreover, nonadiabatic molecular dynamics simulations reveal that the charge recombination rates at the interface depend sensitively (with 1 order of magnitude difference) on the locations of oxygen vacancies. By intentionally reducing oxygen vacancies in the TiO 2 electrode, we substantially suppress unfavorable hysteresis in the PSC devices. This work establishes a firm link between microscopic interfacial structure and macroscopic device performance of PSCs, providing important clues for future device design and optimization.

High-performance flexible pressure sensors have attracted a great deal of attention, owing to its potential applications such as human activity monitoring, man-machine interaction, and robotics. However, most high-performance flexible pressure sensors are complex and costly to manufacture. These sensors cannot be repaired after external mechanical damage and lack of tactile feedback applications. Herein, a high-performance flexible pressure sensor based on MXene/polyurethane (PU)/interdigital electrodes is fabricated by using a low-cost and universal spray method. The sprayed MXene on the spinosum structure PU and other arbitrary flexible substrates (represented by polyimide and membrane filter) act as the sensitive layer and the interdigital electrodes, respectively. The sensor shows an ultrahigh sensitivity (up to 509.8 kPa -1 ), extremely fast response speed (67.3 ms), recovery speed (44.8 ms), and good stability (10 000 cycles) due to the interaction between the sensitive layer and the interdigital electrodes. In addition, the hydrogen bond of PU endows the device with the self-healing function. The sensor can also be integrated with a circuit, which can realize tactile feedback function. This MXene-based high-performance pressure sensor, along with its designing/fabrication, is expected to be widely used in human activity detection, electronic skin, intelligent robots, and many other aspects.

In recent years, inorganic CsPbBr3-based perovskites have accomplished considerable progress owing to their superior stability under harsh humid environment.

Mixed‐valence manganites gain increasing attentions thanks to their extraordinary properties including half‐metallicity and colossal magnetoresistive response, rendering them ideal candidate for oxide spintronics applications. Oxygen vacancies in oxides have been approved to be important functional defects and are effective to manipulating their multifunctional properties. To gain a deep insight into the roles of oxygen vacancies on regulating the atomic structure and electronic properties of the mixed‐valence manganites, two high‐quality epitaxial La2/3Sr1/3MnO3 films around a critical point (without/with oxygen vacancies) were designed and fabricated. From the experiments and theoretical calculations, it was found that the oxygen vacancies induce a weakening of Mn–O–Mn hybridized bond and an increase of concentration of Mn3+ ions, impair the double exchange between Mn3+ and Mn4+, and therefore lead to the transition from metal to insulator and the degraded magnetic properties. Our finding demonstrates a practical approach to tune the magnetic and transport properties of oxide thin films by precisely controlling the oxygen vacancies for high performance spintronics applications.

We present an electro-photo double modulation on the resistive switching behavior in an Au/BiFeO3/La0.7Sr0.3MnO3/SrTiO3 heterostructure, combining the electro-resistance effect and the photo-resistance effect. The pulse voltages can lead to nonvolatile resistance variations in this heterostructure, and the laser illumination can also modulate the high and low resistance states. Consequently, four stable resistance states are achieved. Furthermore, a switchable photoelectric effect-the direction of the photocurrent can be switched by polarization reversal, is also observed in this heterostructure. The present results should have potential applications to develop multi-state memory devices based on perovskite oxides.