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Flexoelectric effect, which is defined as strain gradient–induced polarization or electric gradient–induced strain in crystalline solids, can be presented as a fourth-rank tensor. The symmetry of the flexoelectric coefficients in matrix form is studied. The results indicate that the direct flexoelectric coefficients should be presented in 3 × 18 form and the converse flexoelectric coefficients in 6 × 9 form, rather than 6 × 6 form, like elastic constants. In addition, non-zero and independent elements in the matrices have been calculated for 32 point groups and 7 Ci groups. These results will provide valuable reference to the theoretical and application studies of flexoelectric effect.

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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.

Commercial Dacron cloth is directly used as a substrate for the growth of Cu(OH)2 nanobelt arrays for application in flexible all-solid-state supercapacitors.

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The ionic liquid, tetramethylammonium hydroxide, was introduced into SnO2 films to enhance the conductivity of both SnO2 and overlying perovskite film for perovskite solar cells with efficiency exceeding 21%.

In this paper, high-performance conducting Al-doped ZnO (AZO) electrodes were deposited on transparent and flexible muscovite mica substrates. The use of mica as a substrate material makes a van der Waals epitaxy possible, which significantly improves the structural, electrical, and optical properties of deposited AZO single-crystal-like films. AZO/mica retains its low electric resistivity, even after continuous bending of up to 1000 times on account of the unique layered structure of mica. When used as a transparent heater, AZO/mica shows an ultrahigh heating rate (200 °C/s) across large areas, which is a record among flexible transparent heaters.