ZnO tetrapod materials for functional applications

Low-dimensional piezoelectric semiconductor nanomaterials, such as ZnO and GaN, have superior mechanical properties and can be integrated into flexible devices that can be subjected to large strain. More importantly, the coupling between piezoelectric polarization and semiconductor properties (for example, electronic transport and photoexcitation) in these materials gives rise to unprecedented device characteristics. This has increased research interest in the emerging fields of piezotronics and piezo-phototronics, which offer new means of manipulating charge-carrier transport, generation, recombination or separation in the controlled operation of flexible devices through the application of external mechanical stimuli. We review the recent progress in advancing our fundamental understanding and in realizing practical applications of piezotronics and piezo-phototronics, and provide an in-depth discussion of future research directions. Piezotronics and piezo-phototronics offer new means of implementing adaptive electronics and optoelectronics, taking advantage of the coupling between piezoelectric polarization and semiconductor properties in piezoelectric semiconductor nanomaterials. This Review discusses the recent progress in piezotronics and piezo-phototronics, as well as future research directions.

Piezotronics and piezo-phototronics for adaptive electronics and optoelectronics

ZnO-based nanostructured electrodes for electrochemical sensors and biosensors in biomedical applications

Piezoelectric materials and devices have drawn extensive attention for energy harvesting due to their excellent electromechanical conversion properties. With increasing concerns about environmental problems in traditional lead-based piezoelectric materials, it is imperative to develop lead-free piezoelectric alternatives. This paper is intended to give a brief review on the most recent advances in both inorganic (with an emphasis on piezoelectric ceramics and ZnO nanostructures) and organic (i.e. polyvinylidene difluoride (PVDF) and its copolymers and their composites, and biopolymers) lead-free piezoelectric materials. State-of-the-art piezoelectric devices, namely, nanogenerators, sensors, and transducers, are also introduced with detailed examples. Finally, the challenges and perspectives of lead-free piezoelectric materials and devices are given.

An overview of lead-free piezoelectric materials and devices

Currently, the development of ultraviolet (UV) photodetectors (PDs) has attracted the attention of the research community because of the vast range of applications of photodetectors in modern society A variety of wide-band gap nanomaterials have been utilized for UV detection to achieve higher photosensitivity Specifically, zinc oxide (ZnO) nanomaterials have attracted significant attention primarily due to their additional properties such as piezo-phototronic and pyro-phototronic effects, which allow the fabrication of high-performance and low power consumption-based UV PDs This article primarily focuses on the recent development of ZnO nanostructure-based UV PDs ranging from nanomaterials to architectural device design A brief overview of the photoresponse characteristics of UV PDs and potential ZnO nanostructures is presented Moreover, the recent development in self-powered PDs and implementation of the piezo-phototronic effect, plasmonic effect and pyro-phototronic effect for performance enhancement is highlighted Finally, the research perspectives and future research direction related to ZnO nanostructures for next-generation UV PDs are summarized

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Zinc oxide ultraviolet photodetectors: rapid progress from conventional to self-powered photodetectors

Piezoelectric performance enhancement of ZnO flexible nanogenerator by a NiO–ZnO p–n junction formation

Piezo-phototronic effect enhanced pressure sensor based on ZnO/NiO core/shell nanorods array

Piezoelectric effect enhancing decay time of p-NiO/n-ZnO ultraviolet photodetector

In this study, a nanogenerator based on Cu2O–ZnO p–n junction has been fabricated on Cu wire substrates for harvesting mechanical energy from the environment. The flexible nanogenerator is composed of a Cu substrate, a Cu2O layer, ZnO nanorods and an outer Au-coated paper electrode; the Cu2O layer was obtained by oxidizing Cu wires directly and the ZnO nanorods were grown on the Cu2O layer using a low-temperature hydrothermal method. The existence of the Cu2O–ZnO p–n junction makes a contribution towards reducing the number of excess electrons in the ZnO, which facilitates in improving the output signal and also overcomes short circuits. An Au-coated paper electrode can involve more nanorods in the power generation process. The DC output voltage was up to 42 mV and the maximum output current density was 400 nA, which are approximately a 13-fold higher voltage and a one order of magnitude larger current in comparison to devices without a Cu2O layer, respectively. This study may provide important insight into a facile fabrication method for low-cost and high-performance energy harvesting devices.

Flexible piezoelectric nanogenerator based on Cu2O─ZnO p─n junction for energy harvesting

In this work, a UV photodetector based on a ZnO/NiO core/shell nanorods array is fabricated by hydrothermal growth processes and thermal oxidation. Compressive strain is applied to enhance the Ilight and sensitivity of the UV photodetectors. Under a 1 N compressive strain at −3.5 V, the enhancement in Ilight and sensitivity of the UV photodetector is about 74% and 78.7%, respectively. The possible working principle is that the negative piezo-polarization at the ZnO/NiO interface shifts the depletion region to the ZnO side leading to an increase of the photon-absorption volume. This work may provide a potential approach to enhance the performance of the ZnO/NiO UV photodetector.

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Yue Chang

Papers

Piezoelectric performance enhancement of ZnO flexible nanogenerator by a NiO–ZnO p–n junction formation

Piezo-phototronic effect enhanced pressure sensor based on ZnO/NiO core/shell nanorods array

Piezoelectric effect enhancing decay time of p-NiO/n-ZnO ultraviolet photodetector

Flexible piezoelectric nanogenerator based on Cu2O─ZnO p─n junction for energy harvesting

Enhancing performance of ZnO/NiO UV photodetector by piezo-phototronic effect