Peng Ren

Oxygen vacancy is intrinsically coupled with magnetic, electronic, and transport properties of transition-metal oxide materials and directly determines their multifunctionality. Here, we demonstrate reversible control of oxygen content by postannealing at temperature lower than 300 °C and realize the reversible metal-insulator transition in epitaxial NdNiO₃ films. Importantly, over 6 orders of magnitude in the resistance modulation and a large change in optical bandgap are demonstrated at room temperature without destroying the parent framework and changing the p-type conductive mechanism. Further study revealed that oxygen vacancies stabilized the insulating phase at room temperature is universal for perovskite nickelate films. Acting as electron donors, oxygen vacancies not only stabilize the insulating phase at room temperature, but also induce a large magnetization of ∼50 emu/cm³ due to the formation of strongly correlated Ni²⁺ t(2g)⁶e(g)² states. The bandgap opening is an order of magnitude larger than that of the thermally driven metal-insulator transition and continuously tunable. Potential application of the newly found insulating phase in photovoltaics has been demonstrated in the nickelate-based heterojunctions. Our discovery opens up new possibilities for strongly correlated perovskite nickelates.

Dipeptide self-assembled hydrogels have potential biomedical applications because of their great biocompatibility, bioactivity, and tunable physicochemical properties, which can be modulated in the molecular level by design of amino acid sequences. Herein, a series of dipeptides (Fmoc-FL, -YL, -LL, and -YA) are designed to form shear-thinning hydrogels with self-healing and tunable mechanical properties by adjusting the synergetic effect of hydrophobic interactions (π-π stacking and hydrophobic effect) and hydrogen bonds of peptides through substitution of amino acid residues. The enhancement of hydrophobic interactions is a primary factor to promote mechanical rigidity of hydrogels, and strong hydrogen-bonding interactions between molecules contribute to the instantaneous self-healing property, which is supported by experimental studies (FTIR, CD, SEM, AFM, and rheology) and molecular dynamics simulations. The injectable dipeptide hydrogels were certified as an ideal endoscopic submucosal dissection filler to make operation convenient and secure in mice and living mini-pig's experiments with a longer duration time, higher stiffness, and lower inflammatory response than commercial clinical fillers.

The switchable photovoltaic effect in BiFeO3 thin films capacitors has been studied extensively. However, the origin of the photovoltaic response is still under debate. Both bulk depolarization field and interface effects have been used to explain the observations. In this work, we fabricate BiFeO3 epitaxial films on SrTiO3 substrate with La0.7Sr0.3MnO3 and Pt as electrodes. Much larger switchable photovoltaic response can be observed in the Pt/BiFeO3/La0.7Sr0.3MnO3 samples, as compared with La0.7Sr0.3MnO3/BiFeO3/La0.7Sr0.3MnO3. Moreover, the photovoltaic voltage of the Pt/BiFeO3/La0.7Sr0.3MnO3 samples is nearly independent of the thickness of the La0.7Sr0.3MnO3 bottom electrode. We suggest that the Schottky barrier modulation by ferroelectric polarization at the Pt/BiFeO3 interface is mainly responsible for the photovoltaic effect, with very small contribution from the bulk depolarization field.

Abstract Perovskite manganites are viewed as one of the key building blocks of oxide spintronics devices due to their attractive physical properties. However, cation off‐stoichiometry at epitaxial interfaces between manganites and other materials can lead to interfacial dead layers, severely reducing the device performance. Here, transmission electron microscopy and synchrotron‐based spectroscopy are used to demonstrate that oxygen vacancies during growth serve as a critical factor for modifying the cation stoichiometry in pulsed laser deposited La 0.8 Sr 0.2 MnO 3 films. Near the film/substrate (SrTiO 3 ) interface, A‐site cations (La/Sr) are in excess when oxygen vacancies are induced during film growth, partially substituting Mn. Simultaneously, Sr cations migrate towards the film surface and form a SrO rock‐salt monolayer. Consequentially, a gradient of the Mn nominal valence is observed along the film growth direction, leading to anomalous magnetic properties. The results narrow the selection range of useful oxygen pressures during deposition and demonstrate that accurate cation stoichiometry can only be achieved after oxygen vacancies are eliminated during growth. This finding suggests that the oxygen pressure serves as a tuning parameter for the interfacial dead layers and, hence, for control over device properties.

A noncentrosymmetric crystal structure of CsCdBr(3) has been successfully observed with X-ray single-crystal structure analysis. It crystallizes in the hexagonal space group P6(3)mc, with a = 7.7281(14) A, b = 7.7281(14) A, c = 6.742(2) A, alpha = 90 degrees, beta = 90 degrees, gamma = 120 degrees, Z = 2. It was obtained by a new preparation procedure different from that reported in the literature that gave a centrosymmetric structure. The structure contains Cd-Br octahedrons, which are connected in a plane-sharing way to form one-dimensional long chains. Each octahedron is slightly distorted, as the three Cd-Br bond lengths are 2.774 A, while the other three Cd-Br bond lengths are 2.804 A. The distortion directions of all of the octahedrons are almost parallel and give rise to the accumulation of the microcosmic nonlinear optical (NLO) coefficient. The Kurtz powder technique shows that CsCdBr(3) has a powder second harmonic generation of about 2 times as large as that of potassium dihydrogen phosphate (KDP). It shows excellent transparency in the visible and infrared regions. The thermal stability is also good. Therefore it may be utilized as a potential nonlinear optical crystal for the IR region.