To date, most good piezoelectric materials are ABO 3 structured ferroelectric perovskites. The insensitivity to light is mainly caused by the relatively wide photonic (or optical) band gaps of most piezoelectrics compared to the visible part of the light spectrum. This is because most piezoelectric materials exhibit a negligible response when exposed to light compared to their status in the dark. Recently, ultrahigh piezoelectric coefficients of up to 1,500 pC/N have even been reported for ceramics, comparable to those of single crystals, giving a promising future to cost-effective, high-performance piezoelectrics ( Li et al., 2018).Ĭonventional research of piezoelectric materials and devices does not consider the materials’ interaction with incident light. These applications have supported key functionalities in, to name a few, sonar, medical imaging, fuel injection valves, industrial sensing and battery-free/ubiquitous/autonomous power sources for the IoT (internet of things) and wearable devices ( Bowen et al., 2014 Uchino, 2015 Bai et al., 2018a). Since the first discovery of the piezoelectric effect by the Curie brothers in 1880, studies on piezoelectric materials have rapidly advanced both fundamentally and in applications such as ultrasonic transducers, high-precision actuators, highly sensitive acceleration sensors and high-efficiency/miniaturized kinetic energy harvesters ( Uchino, 2015). This review aims to draw the attention of piezoelectric scientists and device engineers, so that potential applications of photoresponsive piezoelectrics can be comprehensively investigated, as well as more material options that can be offered in future works. Since most of such materials are built on the frame of lead-free perovskite oxides, their band gap (without degrading the piezoelectricity) provides an additional benefit to environmentally friendly lead-free piezoelectrics (compared to lead-based counterparts such as PZT. Pioneer works on the applications of photoresponsive piezoelectrics are also presented. This mini review focuses on the works of simultaneous tuning of piezoelectricity and band gap, which have not previously been discussed as an individual topic in existing reviews. Such photoresponsive piezoelectrics have potential applications in opto-electrical dual-source actuators, single-material multi-sensors and multi-source energy harvesters. As a result, several materials with simultaneously good piezoelectricity and a visible-range band gap have been developed. The Ni-doping strategy for band gap engineering has been successfully applied to other perovskite compositions. This band gap engineered ferroelectric material has also been proved to be piezoelectric. The first narrow band gap (1.1 eV, the same as silicon) ferroelectric material based on the oxide perovskite structure has been achieved by doping Ni on the B-sites of KNbO 3 and paring the Ni 2+ ions with oxygen vacancies to form defect dipoles to ease the band-band transition. Most piezoelectric materials are not interactive with visible light, meaning that their band gaps are beyond the photon energies of the visible part of the light spectrum. Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu, Oulu, Finland.
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