A Contact-Mode Interface Strategy for Improving the Piezoelectric Performance of ZnO Nanowire-Based Vertically Integrated Nanogenerators

A contact-mode interface strategy is proposed to improve the piezoelectric performance of ZnO nanowire (NW)-based vertically integrated nanogenerator (VING). In conventional full-coverage mode, direct spin-coating of polymethyl methacrylate (PMMA) onto ZnO NW arrays often leads to polymer infiltration, nonuniform surface morphology, and inefficient stress transfer, all of which reduce output performance. In this work, PMMA was first spin-coated onto an Al foil and then laminated onto the ZnO NWs, forming a controlled contact interface that also served as the top electrode. Finite element analysis (FEA) was used to investigate the effects of ZnO NW aspect ratio, PMMA thickness, and interface configuration on piezoelectric output, followed by device fabrication and experimental characterisation under compressive loading. The results show that interface configuration and ZnO NW aspect ratio have a much greater impact on device performance than PMMA thickness. Among the tested devices, the contact-mode device based on 40 mM ZnO NWs and 1 µm PMMA delivered the best overall performance, achieving a power density of 220 µW/cm³. This study demonstrates that contact-mode interface is an effective approach for enhancing ZnO NWs-based VING and highlights their potential for self-powered flexible and wearable electronics.