After about ten years' hard work on mesostructural ZnO, now WANG Zhonglin understands this material very well. In the April 14, 2006 issue of the journal Science [ 1 ], he and SONG Jinhui describe a versatile, simple and direct method to harvest piezoelectricity from the field of ZnO nanowires. 

When wind blows throughout the cornfield, the Zhonglin's harvest is not just the wheat.

They fabricate a structure like "cornfield", contained vertically-aligned ZnO nanowires grown on c plane–oriented sapphire substrate with using a standard vapor-liquid-solid process. The nanowires are 200 to 500 nm in length and 20 to 40 nm in diameter. They make ZnO wires with low aspect ratio and packing-density (neighboring wires are spaced about 100 nm apart determined by the packing space of the catalyst gold nanoparticles) for short-circuit protection during the measurements of individual wire.

Profiting from the current astonishing developments in nanoscience, single crystalline nanowires can be fabricated controllably and cost-effectively. Comparing with polycrystalline piezoelectrics, single crystalline piezo-nanowire shows relatively high elasticity than their bulk form and needs not undergo a process of poling (pre-aligning of the dipoles by applying a strong DC electric field).

The energy transfer efficiency of the present wire is about 17 to 30%. Comparing with today's commercial piezoelectrics, this efficiency is somewhat lower than what may be required for practical utility in devices. However, in the application of one device or method to biological systems, whether the design can offer high maneuverability and safety or not should be addressed firstly, but not its efficiency. This route thereby lays an exciting foundation for a potential new battery technology. In the absence of effective solutions, Wang's wire may offer just right alternative to meet the long recognized need for biocompatible nanodevices with self-powered ability, enhanced performance, durability and biosafety. With effective piezo-transformation between electricity and the mechanical energy from the environment as well as our bodies, there are numerous possibilities to make self-adaptive and much compact nanodevices.

Making a review of the piezoelectric applications [ 2 ] should redound to the understanding of the great potential of this work. Piezoelectric crystals can generate a voltage in response to applied mechanical stress. This effect is reversible, that is, when subjected to an externally applied voltage, the crystal size would change a small amount. This ability of transformation between kinetic and electrical energy allows its use for a large variety of applications in daily life as well as laboratory room. The following indicates a selection of the many applications common today. Note that most of the piezo-elements are featured in microscale. 

1) Transducers in acoustics (microphone, alarm buzzer and loudspeaker), hydrophone applications, and ultrasonics for nondestructive imaging and testing;

2) Resonators in bandpass filters, power supplies, delay lines and actuators for active vibration and noise control; sensors for active flow control; 

3) High-voltage source for electric lighter (In a gas lighter/sparker, pressure on a piezoceramic generates an electric potential high enough to create a spark); transformer used in medical treatment, sonochemistry and industrial processing; 

4) As very high voltages correspond to only tiny changes in crystal deformation, this deformation can be changed with sub-micrometer precision, making piezoelectric actuators the most important solution to many positioning tasks that depend on highest accuracy, speed and resolution, for examples, such as piezo-positioners for ultra fine focusing of optical assemblies in auto-focus cameras; acousto-optic modulator for fine-tuning a laser's frequency and laser mirror alignment; stepper motor in scanning systems for controlling the stepwise walk of the tips of AFM and STM with sub-nanometer precision;

5) Microbalance;

6) Ink nozzle of advanced inkjet printers wherein the piezo-displacement is exploited to electronically adjust the velocity of jetted drops; Piezo-driven injection valves in diesel engines;

7) high-value ceramic capacitors and FRAM chips;

8) Frequency reference standard (quartz clocks) and frequency multiplier to generate megahertz or gigahertz pulse.

Need not say much, conventional micro-piezoelectricity gives so many examples to be followed. The clear portraits have been drawn for piezoelectric materials in the past century. As we know, there are 21 non-centrosymmetric among the 32 classes of crystal structures, Except for Cubic (432), the other 20 non-centrosymmetric structures exhibit direct piezoelectricity. There are classes of Cubic/ Isometric (23, -43m), Tetragonal (-4, 4, -42m, 4mm, 422), Orthorhombic (mm2, 222), Hexagonal (-6, 6, 6m2, 6mm, 622)/Trigonal (3, 3m, 32), Monoclinic (m, 2) and Triclinic (1). [ 2 ]

At the present stage, the work of Wang and Song is only the first step towards the aim of bio-battery. Before using ZnO nanowires as direct replacement for conventional piezoelectrics such as lead zirconate titanate (PZT) and piezoelectric polymer of polyvinylidene fluoride (PVDF), practical improvements should be gained through the development of output electrode, working as that of the AFM tip in this study, to make active power supply integrated with nanodevices a reality. Furthermore, incremental steps are to improve the energy transfer efficiency and the piezoelectric properties by investigating different geometries as well as other piezoelectric nanowires. 

* Lin PU is in the Physics Department of Nanjing University, Nanjing 210093, CHINA.

Data

Science 312, 242–246 (2006).

Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays
Zhonglin Wang and Jinhui Song

Science Published online: 14 April 2006

doi:10.1126/science.1124005

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Correspondence to: Zhonglin WANG

Link

Reference

Citation

L. PU

♦ doi: 10.3128/ss20060414a1 | Scidea :: Abs . Full | CrossRef  
♦ Scidea Sketch :: ISSN: 1992 - 8548