Flexoelectricity in Self-Rolling Freestanding Heterogeneous Films

Large strain gradients are urgently required to fully exploit flexoelectricity in dielectrics. However, it is difficult to introduce large, tunable and high strain gradients simultaneously, which greatly limits the development of flexoelectric devices. The recently developed freestanding oxide films technique opens up new possibilities for self-rolling heterogeneous films, and thus provides additional degrees of freedom for introducing large and tunable strain gradients to enhance the flexoelectricity. In this work, based on extended continuum theory and variational principle, we develop an electromechanical model to investigate flexoelectricity in self-rolling freestanding heterogeneous films. The results show that the strain gradient reaches as large as 10⁷ m⁻¹ and can be tuned directly by changing the interfacial mismatch strain, film thickness and the thickness ratio of membrane to substrate layer. Various self-rolling morphologies (ring, spring and twisted ribbon) with different flexoelectric polarization can be obtained by tuning the anisotropic interfacial mismatch strain and the released direction. Differing from the neutralized piezoelectric response, the flexoelectric polarization in self-rolling films is uniform across the thickness and is enhanced significantly as the film thickness reduces. The effective flexoelectric polarization is six times larger than the piezoelectric one when the thickness reduced to 4 nm. This work offers a deliberate strain gradient engineering of self-rolling freestanding heterogeneous films for utilizing the flexoelectricity in nanoscale and opens up new possibilities for the self-rolling film-based flexoelectric devices and applications.