In-situ stretching strain-driven high piezoelectricity and enhanced electromechanical energy-harvesting performance of a ZnO nanorod-array structure

Although strain engineering has been extensively recognized as a critical pathway in controlling the properties of inorganic materials, there have been very limited reports on the external strain-dependent modulation of piezoelectricity in flexible systems. Herein, we introduce a technical way of imposing extra stress during the deposition of the ZnO nanorods by using the stretching mode of a polymer substrate, specifically for the purpose of enhancing piezoelectricity and bending-driven energy harvesting performance. Depending on the level of stretching up to 4.87% strain, the induced stress of the nanorod structure was modulated after the substrate-releasing step. The 4.87%-stretching mode resulted in an effective piezoelectric coefficient of 33.3 p.m./V corresponding to an enhancement by ~270% compared to the unstrained case. The resultant piezoelectric energy harvester demonstrated ~3.43 V output voltage and ~226 nA output current for the 4.87%-strained sample, which means respective increments by ~90% and ~85% with the application of in-situ strain. The origin of the improvements is chased by estimating the changes in lattice constants and spontaneous polarization, which are dependent on the level of in-situ strain.