Multiferroic properties and giant piezoelectric effect of a 2D Janus WO₃F monolayer

Materials possessing both ferroelectricity and ferromagnetism are regarded as ideal candidates for electronic devices, such as nonvolatile memories. Based on first-principles calculations, we systematically studied the crystal structure, electronic structure as well as magnetic, piezoelectric and ferroelectric properties of a two-dimensional van der Waals WO₃F monolayer material. The WO₃F monolayer was found to possess a robust square crystal structure, exhibiting exceptional stability and mechanical resilience. Magnetic characterization revealed that the material displayed a ferromagnetic state with a magnetic moment of 1μ_B with negligible magnetic anisotropy. In terms of ferroelectric properties, the WO₃F monolayer demonstrated pronounced in-plane polarization, which is in stark contrast to its relatively weak out-of-plane polarization and indicative of anisotropic polarization behavior. Additionally, the material's piezoelectric response could be modulated through strain engineering, with its piezoelectric coefficient (d₁₁) at 4% tensile strain, which exceeds that of the vast majority of known 2D piezoelectric materials, thus underscoring its potential for versatile multifunctional applications in diverse fields, including sensing, energy harvesting, and actuator technologies.