Piezoelectric ZnO nanostructures, synthesis and application for energy harvesting

Abstract

Zinc oxide (ZnO) is a II-VI compound semiconductor with its unique optical, electrical, mechanical, thermal and chemical properties. The morphology of ZnO nanostructures has been proven to be the richest one among inorganic semiconductors. Based on the remarkable properties of ZnO and the motivation to miniaturize devices, large efforts have been recently focused on the synthesis, characterization and applications of nanostructures of ZnO in nanoscale technology and devices. This work in one part focuses on the synthesis of well aligned piezoelectric ZnO nanowires and their application as nanogenerator for harvesting energy from the environment. Here it is indicated that a Schottky contact at the ZnO-metal interface is not a necessity for the operation of a ZnO nanogenerator, as believed by now. However a Schottky contact leads to a more effective energy harvesting. In the other part of this work a new recipe for the synthesis of ZnO comblike-nanostructures by manipulating the Vapor-liquid-solid method is reported. These nanostructures are recently applied as nanocantilevers, UV nanolaser arrays, optical nanogratings and nanopolarizers. Moreover, a theoretical model for the growth of these structures is developed that explains for the first time the formation of these interesting nanostructures on the base of the piezoelectric property of ZnO. Applying the perturbation theory, the elasticity theory and using the Fourier expansion of mechanical stress exerted in the material under the growth kinetics, the induced piezoelectric charge in the nanostructure is approximated. The periodic distribution of the induced piezoelectric charge explains the periodic growth of nanowire branches of ZnO nanocombs on the polar +(0001) surface as a consequence of a self catalytic growth process. The Si mulations show a good agreement between theoretical and experimental results.