Flexoelectricity in self-rolling freestanding heterogeneous films

•Analytical model for investigating the flexoelectricity in self-rolling films.•Utilizing self-rolling films to produce huge and large-area strain gradients.•Enhanced flexoelectric polarization can reach six times of piezoelectric one.•Stretchable self-rolled springs are promising in flexoelectricity-based devices. Large strain gradients are urgently required to fully exploit flexoelectricity in dielectrics. 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, 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. 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.