Large-Scale Domain Engineering in Two-Dimensional Ferroelectric CuInP2S6 via Giant Flexoelectric Effect.

Room-temperature ferroelectricity in two-dimensional (2D) materials is a potential for developing atomic-scale functional devices. However, as a key step for the technology implementations of 2D ferroelectrics in electronics, the controllable generation of uniform domains remains challenging at the current stage because domain engineering through an external electric field at the 2D limit inevitably leads to large leakage currents and material breakdown. Here, we demonstrate a voltage-free method, the flexoelectric effect, to artificially generate large-scale stripe domains in 2D ferroelectric CuInP2S6 with single domain lateral size at the scale of several hundred microns. With giant strain gradients (∼106 m-1), we mechanically switch the out-of-plane polarization in ultrathin CuInP2S6. The flexoelectric control of polarization is understood with a distorted Landau-Ginzburg-Devonshire double well model. Through substrate strain engineering, the stripe domain density is controllable. Our results highlight the potential of developing van der Waals ferroelectrics-based flexible electronics.