Review on ZnO-based piezotronics and piezoelectric nanogenerators: Aspects of piezopotential and screening effect

Among various piezoelectric materials, ZnO has attracted a great deal of attention due to facile preparations and exceptional semiconductor characteristics compared to other conventional piezoceramics or organic piezoelectric materials. One of the issues hindering ZnO from progressing into applications is the screening effect, where the intrinsic piezopotential generated upon mechanical deformations is screened and becomes waned or even diminished by the presence of intrinsic free carriers in ZnO. Consequently, ZnO-based piezoelectric devices often suffer from low output voltages, resulting in low total output power generation even though the output current could be larger than those made of insulating piezoelectric materials, such as PZT, polyvinylidene fluoride, and barium titanate. It is therefore vital to fully understand the impact of the screening effect and produce strategies to handle this issue in the context of piezotronics and piezoelectric nanogenerators (PENG). Therefore, this article presents a comprehensive review of growth methodologies for various ZnO nanostructures, structure modifications, effects of free carriers on the screening effect and strategies for device applications, including strain-gated transistors, PENG and piezotronic sensors for gas, humidity and bio-molecules etc.