Flexoelectricity refers to the mechanical-electro coupling between strain gradient and electric polarization, and conversely, the electro-mechanical coupling between electric field gradient and mechanical stress. This unique effect shows a promising size effect which is usually large as the material dimension is shrunk down. Moreover, it could break the limitation of centrosymmetry, and has been found in numerous kinds of materials which cover insulators, liquid crystals, biological materials, and semiconductors. In this review, we will give a brief report about the recent discoveries in flexoelectricity, focusing on the flexoelectric materials and their applications. The theoretical developments in this field are also addressed. In the end, the perspective of flexoelectricity and some open questions which still remain unsolved are commented upon.

Complex micro-architectured metamaterials exhibit unique elastic and dielectric properties as well as ultrahigh piezoelectric responses.

The resistive switching (RS) mechanism of hybrid organic-inorganic perovskites has not been clearly understood until now. A switchable diode-like RS behavior in MAPbBr3 single crystals using Au (or Pt) symmetric electrodes is reported. Both the high resistance state (HRS) and low resistance state (LRS) are electrode-area dependent and light responsive. We propose an electric-field-driven inner p-n junction accompanied by a trap-controlled space-charge-limited conduction (SCLC) conduction mechanism to explain this switchable diode-like RS behavior in MAPbBr3 single crystals.

The direct measurement of flexoelectric coefficients in epitaxial thin films is an unresolved problem, due to the clamping effect of substrates which induces a net strain (and hence parasitic piezoelectricity) in addition to strain gradients and flexoelectricity. Herein, we propose and demonstrate the use of van der Waals epitaxy as a successful strategy for measuring the intrinsic (clamping-free = flexoelectric coefficients of epitaxial thin films. We have made, measured, and compared BaTiO 3 and SrTiO 3 thin film capacitor heterostructures grown both by conventional oxide-on-oxide epitaxy and by van der Waals oxide-on-mica epitaxy, and found that, whereas the former is dominated by parasitic piezoelectricity, the response of the latter is truly flexoelectric. The results are backed by theoretical calculations of the film-substrate mechanical interaction, as well as by direct measurements that confirm the strain-free state of the films. van der Waals epitaxy thus emerges as powerful new tool in the study of flexoelectricity and, in particular, they finally allow exploring flexoelectric phenomena at the nanoscale (where strain gradients are highest) with direct experimental knowledge of the actual flexoelectric coefficients of thin films.

The resistive switching (RS) mechanism of hybrid organic-inorganic perovskites is an open question until now. Here, a switchable diode-like RS behavior in MAPbBr3 single crystals using Au (or Pt) symmetry electrodes is reported. Both the high resistance state (HRS) and low resistance state (LRS) are electrode-area dependent and light responsive. We propose an electric-field-driven inner p-n junction accompanied by an interface trap-controlled SCLC mechanism to explain this switchable diode-like RS behavior in MAPbBr3 single crystals.