Vacuum- and etch-free mechanical fabrication of metal wire microtrenches interconnected by ZnO nanowires for flexible bending-sensitive optoelectronic sensors

In recent years, there has been expanding interest in developing optoelectronic transducers using micro- and nanoscale materials and structures in compact, lightweight, and flexible forms. Specifically, optoelectronic micro- and nanoarchitectures composed of micropatterned electrodes connected by semiconductor nanowires (NWs) such as ZnO NWs (ZNWs) have gained considerable attention due to their good sensitivity to incident light. However, traditional methods for fabricating micropatterned electrodes often involve difficult and time-consuming processes such as vacuum deposition, optical lithography, and etching, often preventing scalability and widespread application. Additionally, synthesizing ZNWs, usually dependent on high-temperature seed sintering or chemical vapor deposition, can interrupt their practical use on flexible substrates. One tactful approach to overcoming these challenges is to create a micropatterned electrode via mechanically machining microtrenches and filling the metal wires therein, and then growing ZNWs selectively on the trench-embedded metal wires via a metal-mediated low-temperature hydrothermal process. Working on this novel strategy, we develop a vacuum- and etch-free method for the fabrication of metal-wire-embedded microtrenches interconnected by ZNWs. Our method involves the continuous mechanical inscribing of linear microtrench patterns (microgratings) on a substrate, followed by the doctor-blade-assisted embedding of solution-processable metal wires within those trenches. We then undertake the low-temperature metal-mediated hydrothermal growth of ZNWs selectively onto the metal wires, finalizing the ZNW-interconnected micrograting electrode structure. The entire process can be carried out at a low temperature without resorting to vacuum, lithography, and/or etching steps, thereby enabling the use of flexible polymer substrates of scalable sizes. The resulting flexible device can function as a bending-sensitive optoelectronic sensor with high sensitivity, as the number of ZNWs interconnecting the trench-embedded micrograting electrodes changes upon mechanical bending.<br/><br/>Acknowledgment<br/>This work was supported by the National Research Foundation of Korea (NRF) grants (No. 2021M3H4A3A02099204, and 2022M3C1A3081178 (Ministry of Science and ICT) and No. 2022R1I1A2073224 (Ministry of Education)) funded by the Korean Government.<br/><br/>References<br/>1. D. K. Oh*, W. Lee*, H. Chae, H. Chun, M. Lee, D. H. Kim, J. Kim, J. Choi, S. Hwang, M. Park, G. Yeon, S. Jung, J. Rho, and J. G. Ok (*equal contributions), Burr- and etch-free direct machining of shape-controlled micro- and nanopatterns on polyimide films by continuous nanoinscribing for durable flexible devices, Microelectronic Engineering 257, 111740 (Mar 2022).<br/>2. W. Lee*, H. Chae*, D. K. Oh*, M. Lee, H. Chun, G. Yeon, J. Park, J. Kim, H. Youn, J. Rho, and J. G. Ok (*equal contributions), Solution-processable electrode-material embedding in dynamically inscribed nanopatterns (SPEEDIN) for continuous fabrication of durable flexible devices, Microsystems & Nanoengineering 7 74 (Sep 2021).<br/>3. K. Yoo*, W. Lee*, K. Kang, I. Kim, D. Kang, D. K. Oh, M. C. Kim, H. Choi, K. Kim, M. Kim, J. D. Kim, I. Park, and J. G. Ok (*equal contributions), Low-temperatutre large-area fabrication of ZnO nanowires on flexible plastic substrates by solution-processible metal-seeded hydrothermal growth, Nano Convergence 7, 24 (Jul 2020).