Piezoelectric Oxide Nanowire Arrays for Energy Sciences
Growth of Nanowire Arrays: Hydrothermal Synthesis
The search for sustainable micro-/nano-scale power sources for driving wireless and mobile electronics attracts worldwide efforts in today’s energy research. In particular, piezoelectric oxide nanowires can convert the ubiquitously available mechanical energy in the environment into electricity, and can be a potential solution to the energy needs of nanodevices. While uncontrolled nanowire synthesis result in random nucleation and growth, rational synthesis of large-scale ordered nanowire arrays, if done at relevantly low temperatures, is an enabling technology for their applications in electronic, optoelectronic and energy scavenging devices. Hydrothermal decomposition is a solution-based synthesis technique of oxides, and its elimination of metal catalysts ensures integration with silicon-based technology. With proper precursor choice and growth condition, nanowire arrays can be synthesized with this method, in water-based solution, with significantly lower cost and lower growth temperatures compared to vapor-phase synthesis methods.
ZnO Nanowire Arrays
Zinc oxide (ZnO) is interesting because it is both a semiconductor and a piezoelectric material. Hydrothermal growth of ZnO nanowire arrays can be conducted at temperatures as low as 70-95°C. With fine control of the reaction kinetics, tuning of the nucleation density and aspect ratio of ZnO nanowires can be achieved. Combined with electron beam lithography to pattern the growth area, this method can yield highly ordered vertical/horizontal nanowire arrays.
PZT Nanowire Arrays
Lead zirconate Titanate (PZT) is widely used for its remarkable piezoelectric properties, but traditional synthesis methods require temperatures over 650 °C which are incompatible with the microelectronics process. We have reported the first epitaxial growth of PZT nanowire arrays with the hydrothermal method. Growth of single crystalline PbZr0.52Ti0.48O3 nanowire arrays can be achieved at 230 °C, which is suitable for a wide range of electronic applications.
Examples of Applications
Ordered ZnO Nanowire Array Light Emitting Diodes
ZnO is considered as a potential candidate material for the next-generation blue/near-UV light emitting diodes. Blue/near-UV LED arrays were fabricated from position-controlled arrays of n-ZnO nanowires on a p-GaN thin film substrate. Under forward bias, each nanowire functioned a light emitter. Due to the wave guiding properties of these nanowires, these LEDs had a high external quantum efficiency of 2.5%. This approach has considerable potential for applications in high-resolution displays and force sensors, optical interconnects, and high-density data storage.
Piezoelectric Nanowire Energy Harvesting
Under strain, the electric potential created in a piezoelectric nanowire can drive the flow of electrons, which serves as the basis for energy harvesting applications. The power output of a single nanowire is very limited; therefore integration of nanowires in the form of arrays is the major technical challenge. Several integration schemes have been demonstrated for ZnO or PZT nanowire based energy harvesters, and the resulting devices were able to power a variety of components ranging from single nanowire sensors to commercial laser diodes.
S. Xu and Z.L. Wang, Oxide Nanowire Arrays for Light Emitting Diodes And Piezoelectric Energy Harvesters, Pure and Applied Chemistry (invited review), 83, 2171 (2011) [pdf]
S. Xu and Z.L. Wang, One Dimensional ZnO Nanostructures: Solution Growth and Functional Properties, Nano Research (invited review), 4, 1013 (2011) [pdf]