Effect of strain gradient modes on flexoelectric response in monolayer FeSe

Two-dimensional (2D) ferroelectrics and flexoelectric materials have garnered significant interest for their potentialnext-generation nanoelectronics. Herein, we thoroughly investigate the flexoelectric response of monolayer FeSe under two distinct strain gradient modes (i.e., intrinsic constant strain gradient and phenomenological wrinkling-induced nonuniform strain gradient) by using first-principles calculations. Our results unveil a significant out-of-plane flexoelectricity under an intrinsic shear strain gradient η311, with a flexoelectric coefficient f3311 = −0.11 nC/m, rivalingthat of typical Janus monolayers. This notable flexoelectric response is uniquely attributed to η311, as the out-of-plane polarization remains insensitive to other strain and strain gradient components. Further analysis indicates that the flexoelectricity stems from the breaking of inversion symmetry due to asymmetric Fe-Se bond variation, which promotes directional charge transfer and leads to the formation of dipole moments. For the case of experimentally feasible wrinkling deformation in sinusoidal and parabolic modes, the strain gradient is non-uniform and thus the intrinsic flexoelectric coefficient cannot be determined, but a notable nonlinear flexoelectric response can be achieved with an out-of-plane polarization up to 10 mC/m2. The nonlinear increase of polarization magnitude stems from the cell-rotation-induced deviation of the polarization vector from the x3 direction. This study highlights the strong flexoelectric potential of monolayer FeSe and demonstrates its versatile responses to distinct strain gradient modes. These findings provide fundamental guidance and key parameters for the design of future flexoelectric devices.