Piezoelectric Nanogenerator Platform Integrating Laterally Grown ZnO Nanowires within Topographic Nanograting Electrodes

Portable electronic devices such as personal healthcare sensors and smart wearable devices are of emerging demand, which require low power consumption and/or self-powering function in order for obviating extra batteries. Piezoelectric materials and piezo-driven nanogenerators for converting mechanical motion into electrical energy can provide a practical solution to this end. Among many piezoelectric materials, zinc oxide (ZnO) has been a workhorse for various piezoelectric devices because it is biocompatible, environmentally friendly, and can be engineered into various structural configurations in a relatively easy way. Among various structural forms of ZnO, 1D structures such as nanowire arrays are suitable for piezoelectric-based nanogenerators because they can generate piezoelectric voltage through higher elastic strain upon mechanical deformation. As a process for synthesizing ZnO nanowires, various methods such as chemical vapor deposition (CVD) and hydrothermal reaction can be used [1]. However, the CVD-based synthesis and the high-temperature synthesis of the Zn-salt-based ZnO seed layer, which require a high-temperature environment, introduce limitations in the substrate material, which poses a great obstacle to the development of flexible piezoelectric nanogenerators to maximize device displacement. Addressing this issue, it is highly requested to grow ZnO nanowires through seedless hydrothermal synthesis on metal thin films. This research develops a piezoelectric nanogenerator framework that integrates ZnO nanowires inside a laterally arrayed, topographic electrodes on flexible substrates. In more detail, a nanograting pattern with an aspect ratio of 1:1 was formed on a PET substrate by UV nanoimprint lithography, and glancing angle deposition (GLAD) was performed at an angle of 45° to selectively deposit a metal thin film on the nanograting pattern, especially on the sidewall region. Depositing Ag on one side and depositing Au on the opposite side, another GLAD was performed by using the catalytically inactive Cr layer at an angle of 80° to selectively deposit a Cr layer only on the top of the nanograting pattern. Following our previously developed recipe [2], ZnO nanowires were then grown selectively between the patterned valleys, forming numerous bridges. Bending of the ZnO nanowires laterally connecting the adjacent nanograting electrodes occurs according to the bending of the base substrate, and piezoelectric polarization occurs in the countless cross-linked structures. Considering the electron affinity of ZnO (4.5 eV) and work functions of Au (5.1-5.47 eV) and Ag (4.52-4.74 eV), the Schottky junction and Ohmic junction are formed in between ZnO-Au and ZnO-Ag interfaces, respectively. [3]. Due to the difference in the junction, a rectification action preferentially occurs at the ZnO-Au junction, which suggests that the device can also be used as a DC nanogenerator.<br/><br/>Acknowledgment<br/>This work was supported by the National Research Foundation of Korea (NRF) grants (No. 2021M3H4A3A02099204, and 2022M3C1A3090850 (Ministry of Science and ICT) and No. 2022R1I1A2073224 (Ministry of Education)) funded by the Korean Government.<br/><br/>References<br/>1. D. K. Oh* <i>et al.</i>, Tailoring zinc oxide nanowire architectures collectively by catalytic vapor-liquid-solid growth, catalyst-free vapor-solid growth, and low-temperature hydrothermal growth, <i>Ceramics International</i> <b>47</b>, 2131-2143 ,2021.<br/>2. K. Yoo* <i>et al.</i>, Low-temperatutre large-area fabrication of ZnO nanowires on flexible plastic substrates by solution-processible metal-seeded hydrothermal growth, <i>Nano Convergence</i> <b>7</b>, 24, 2020.<br/>3. X. Wen <i>et al.</i>, Seedless synthesis of patterned ZnO nanowire arrays on metal thin films (Au, Ag, Cu, Sn) and their application for flexible electromechanical sensing, <i>Journal of Materials Chemistry</i> <b>22</b>, 9469-9476, 2012.