Facile growth of high aspect ratio c-axis GaN nanowires and their application as flexible p-n NiO/GaN piezoelectric nanogenerators

Piezoelectric nanogenerators (PNGs) have attracted great interest as energy sources to power-up smart clothing, micro/nano systems, and portable electronic gadgets. Due to non-centrosymmetric crystal structure, bio-compatibility, and mechanical robustness of GaN, it is a promising candidate to fabricate PNGs. In this study, c-axis GaN nanowires were grown by MOCVD, then were embedded inside polydimethylsiloxane and flipped on to the flexible substrate, followed by the deposition of p-type NiO to form heterojunction. The fabrication of GaN nanowires based heterojunction PNG on flexible substrate is the first report to the best of our knowledge. The piezoelectric properties of PNGs were investigated as a function of the GaN nanowire length. A maximum piezoelectric output potential of 20.8 V and current of 253 nA were measured. The stability of the device was also evaluated and found stable even after 20,000 cycles. This high piezoelectric output was attributed to the suppression of free carrier screening and junction screening. Moreover, the underlying reasons for the high stability are the malleability of the device and high aspect ratio of the GaN nanowires. The design and stability of our device make it a promising candidate for applications in self-powered systems for environment monitoring and low power electronics.