ABSTRACT
Double perovskite films have been extensively studied for ferroelectric order, ferromagnetic order, and photovoltaic effects. The customized ion combinations and ordered ionic arrangements provide unique opportunities for bandgap engineering. Here, a synergistic strategy to induce chemical strain and charge compensation through inequivalent element substitution is proposed. A-site substitution of the barium ion is used to modify the chemical valence and defect density of the two B-site elements in Bi2FeMnO6 double perovskite epitaxial thin films. We dramatically increased the ferroelectric photovoltaic effect to â¼135.67 µA/cm2 from 30.62 µA/cm2, which is the highest in ferroelectric thin films with a thickness of less than 100 nm under white-light LED irradiation. More importantly, the ferroelectric polarization can effectively improve the photovoltaic efficiency of more than 5 times. High-resolution HAADF-STEM, synchrotron-based X-ray diffraction and absorption spectroscopy, and DFT calculations collectively demonstrate that inequivalent ion plays a dual role of chemical strain (+1.92 and -1.04 GPa) and charge balance, thereby introducing lattice distortion effects. The reduction of the oxygen vacancy density and the competing Jahn-Teller distortion of the oxygen octahedron are the main phenomena of the change in electron-orbital hybridization, which also leads to enhanced ferroelectric polarization values and optical absorption. The inequivalent strategy can be extended to other double perovskite systems and applied to other functional materials, such as photocatalysis for efficient defect control.
ABSTRACT
Polarization rotation caused by various strains, such as substrate and/or chemical strain, is essential to control the electronic structure and properties of ferroelectric materials. This study proposes anion-induced polarization rotation with chemical strain, which effectively improves ferroelectricity. A method for the sulfurization of BiFeO3 thin films by introducing sulfur anions is presented. The sulfurized films exhibited substantial enhancement in room-temperature ferroelectric polarization through polarization rotation and distortion, with a 170% increase in the remnant polarization from 58 to 100.7 µC cm-2. According to first-principles calculations and the results of X-ray absorption spectroscopy and high-angle annular dark-field scanning transmission electron microscopy, this enhancement arose from the introduction of S atoms driving the re-distribution of the lone-pair electrons of Bi, resulting in the rotation of the polarization state from the [001] direction to the [110] or [111] one. The presented method of anion-driven polarization rotation might enable the improvement of the properties of oxide materials.
ABSTRACT
With the increasing global electromagnetic pollution, it is more and more important to develop lightweight, flexible, and high electromagnetic shielding materials. Two-dimensional (2D) transition metal material MXenes have good conductivity and excellent electromagnetic shielding performance. Herein, a facile and effective method is reported to synthesize lightweight and flexible MXene/CNF/silver (MCS) composite membranes with a brick-like structure and high-performance electromagnetic interference shielding. MCS composite membranes have an electromagnetic shielding performance of ≈50.7 dB due to MXene self-reduction of silver nanoparticles and the brick-like structure, compared with that of MXene/CNF (MC) membranes (≈14.98 dB). In addition, the MCS composite membranes exhibit super-thin thickness (46 µm) and good tensile strength (up to 32.1 MPa), and their good mechanical properties are attributed to the addition of CNFs. Moreover, the MCS composite membranes show good electrical conductivity (588.2 S m-1). Therefore, MCS composite membranes that are lightweight and flexible and have high electromagnetic shielding performance can replace other electromagnetic shielding materials and be used in aerospace, weapon equipment, and wearable smart materials.
