Piezoelectric Nanowires for Energy Harvesting
At the macroscale, energy harvesting using piezoelectric materials is a mature area in which commercial systems have already reached the markets. The conversion from mechanical to electrical energy, and vice versa, allows energy scavenging from sources such as body movements. Recently, energy harvesting schemes using semiconducting nanowires have been proposed in the literature. These schemes hold great promise because it is possible that the enhanced properties of materials at the nanoscale (resulting from surface effects and much higher strains to fracture than bulk) could result in greater conversion efficiency and several orders of magnitude larger amount of energy harvested per unit volume than in bulk. However, as with many other applications using nanomaterials, fundamental understanding of the properties of the individual components is still in its infancy. Such understanding is key to assessing the feasibility and potential of these emerging technologies. In this context, we have recently performed simulations that demonstrate that the piezoelectric coefficients in semiconducting nanowires of GaN and ZnO increase by almost two orders of magnitude at the nanoscale.
Currently, we are interested in experimentally characterizing this piezoelectricity size effect. To this end, we have developed a MEMS device for electromechanical characterization of nanomaterials.
The device has the capability to perform two-point electrical characterization of nanostructures, while exerting simultaneous mechanical deformation. This is ideal to characterize piezoelectric properties, which are the key elements in energy harvesting using nanowires. Coupled with state-of-the-art electrical characterization instruments, this suite of tools provides unprecedented capabilities to investigate fundamental electrical and mechanical properties at the nanoscale.
We have also performed experimental studies to measure piezoelectricity at a single nanowire level. We employed piezoresponse force microcopy (PFM) to characterize the 3D piezoelectric effect in single GaN nanowires. The results revealed enhancement of piezoelectricity in nanowires up to six times of that in the bulk. This finding highlights applications of these nanowires in nanogenerators for energy harvesting.