Flexoelectricity-Induced Novel Effects in Suspended Films

Ferroelectric materials have been attracting wide interest due to their switchable polarization properties and fascinating applications in the areas of information storage, electronics and biology. The flexoelectric field induced from the nano-tip indentation can switch the polarization in ferroelectric films, thus it is highly required for the deterministic control of ferroelectric domain. However, it usually occurs in a very localized area in ultrathin films, with possible permanent surface damage caused by a large tip-force. Here, we report a mechanical domain switching method based on the suspended films. Utilizing the large-scale bending deformation in suspended films subjected to nano-tip force, the transverse flexoelectric effect is significantly enhanced. As a result, the two equivalent low-energy states double-well potential become inequivalent, making it possible to switch the polarization to lower energy state through the flexoelectricity in a mechanical way. Sizable-area domain switching under an ultralow tip-force can be realized in suspended van der Waals ferroelectrics with the surface intact. The film thickness range for domain switching is significantly improved by an order of magnitude to hundreds of nanometres, being far beyond the limited range of the substrate-supported ones. Meanwhile, we also present a series of asymmetric mechanical properties driven by strong flexoelectricity in flexible ferroelectric membranes. The mechanisms underlying these novel effects are revealed by combining the theoretical model and phase-field simulations. These findings provide new opportunities for the flexoelectricity-based domain controls in emerging low-dimensional ferroelectrics and related devices.