Nanowires for Piezoelectric Nanogenerators

The nanogenerator (NG), first proposed for the purpose of self-powered nanotechnology in 2006, converts random mechanical energy into electric energy using piezoelectric zinc oxide nanowire (NW) arrays. The mechanism of the NG relies on the piezoelectric potential created in the NWs by an external strain: a dynamic straining of the NWs results in a transient flow of the electrons in the external load because of the driving force of the piezoelectric potential. The advantage of using NWs is that they can be triggered by tiny-scale physical motions. Further, the excitation frequency can range from one Hz to thousands of Hz, which makes NGs ideal for harvesting random energy in the environment. In our living environment, there exists an abundant amount of mechanical energy otherwise lost, such as light wind, body movement, muscle stretching, acoustic/ultrasonic waves, noises, mechanical vibration, and blood flow. Hence, the NW based piezoelectric NGs can provide a promising potential as energy harvester for solving the energy crisis and preserving environmental health. The objective of this chapter is to introduce the fundamentals of piezoelectric NW based NGs. We start from the synthesis of piezoelectric NWs, followed by a description of the fundamental principle of a NG based on a single NW. We then demonstrate the engineering approaches for achieving high output power by laterally and vertically integrating the contribution from thousands of NWs. Lastly, we show the prototype for self-powered systems and self-powered active sensors.