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Nanowires Promise Battery-free Powering Of Small Devices

The battery has been a great servant for powering devices in situations where mains electricity is inappropriate, but it has its downsides, which include a relatively short life and a toxic composition. ZnO nanowire power generators are free from these weaknesses, and have the potential to drive small devices such as implanted biosensors, says Zhong Lin Wang from Georgia Institute of Technology.
Researchers are developing wireless nanodevices and nanosystems for a variety of applications, such as monitoring changes in cancer cells, providing gas sensing in remote areas, and delivering in vivo measurements of blood pressure and blood sugar levels. All of these applications need a power source. In the case of an implanted wireless biosensor, this power can come directly or indirectly by charging a battery. But the toxic chemicals inside a battery are a potential health hazard, and so a better solution would be to create "self powering" sensors.

Unfortunately, alternative nanoscale power sources are almost non-existent, despite the great need for a non-battery alternative for many miniature sensing devices for biological sensing and defense applications. However, there are many other forms of energy that could drive nanodevices, such as mechanical energy that can come from body movement and muscle stretching, vibration energy from acoustic and ultrasonic waves, and hydraulic energy from the flow of blood and fluids in the body, and the contraction of blood vessels.

At the Georgia Institute of Technology in Atlanta we have invented a technology based on ZnO nanowire (NW) arrays that can harvest energy from its local environment. By combining the coupled semiconducting and piezoelectric properties of ZnO, and the huge elastic deformation that can be produced in NW structures, we can convert mechanical energy into electricity.

We have produced our aligned ZnO NWs, which are the essential building blocks for our nanoscale power generators, by physical techniques and chemical synthesis. The physical vapor–liquid–solid process produces the best quality material, but the chemical synthesis approach is better suited to volume production (see "ZnO nanowire growth and generator design" for details).

Now we have shown that prototypes containing these aligned ZnO NW arrays can convert mechanical energy into electrical power. The power generator couples ZnO s piezoelectric and semiconducting properties, and forms a Schottky barrier between the metal and ZnO contact.

Harnessing the energy

We have evaluated our array s potential performance by using an atomic-force microscope (AFM) tip to deflect individual wires (figure 4a). When the tip comes into contact with a wire, it stretches one side of it and compresses the other. This produces a charge separation within the NW, with positive and negative charges forming on the stretched and compressed sides, respectively. The Schottky barrier that is formed between the AFM tip and the NW preserves these charges. We have measured the electrical voltage produced by each of the NWs in an array just before they are disconnected from the scanning AFM tip (figure 4b).

The charging and discharging process produced by scanning the AFM tip illustrates how the NW array could generate electricity. We have subsequently converted this principal of operation into working nanogenerators built on a polymer substrate that can serve practical applications by overcoming three initial obstacles. The first of these involved no longer using an AFM to produce NW mechanical deformation, and refining our power generator so that it is adaptable, mobile and able to provide a cost-effective approach over a larger scale. In addition, we adapted our system so that all of the NWs generated electricity simultaneously and continuously, and all of that power was collected and used. Finally, we had to start to use a primary energy source that enabled the nanogenerator to operate "independently" and wirelessly.

A vertically aligned ZnO NW nanogenerator that produces electrical energy from ultrasonic waves has recently fulfilled all of these goals. The core of our new and innovative design is a saw-tooth-shaped electrode array that replicates the bending cycle created by the AFM tip. An external ultrasonic wave oscillates this electrode vertically or horizontally and causes the NWs to bend and stretch, leading to simultaneous generation of an electrical signal (figure 5a).

Promising potential

Our array with approximately 500 NWs actively generating electricity has delivered an output of nearly 1 nA (figure 5b). As expected, this current ceases when the ultrasonic wave source is off. The current generated by the design is at the lower limit of the nanoamp to microamp range required to drive a nanodevice, and the power output is insufficient for this task. However, this result proves that our innovative nanogenerator design could be a promising solution to the problem of building a cost-effective, mobile and adaptable power source.

In practice, the energy that is fed into the nano-generators is likely to be mechanical energy from the local environment. This is potentially compatible with our nanogenerator technology, which should produce electrical power by converting mechanical movement or hydraulic energy.

The way forward

We plan to continue to refine and improve the performance of our nanogenerator so that it can produce more power and harvest/recycle energy from its local environment. This further development will initially come from in-house research. However, we are also building a start-up company to commercialize some of our concept and prototype technology that the DC nano-generators have established. These self-powering nanosystems will target a variety of important applications in implantable biosensing, wireless and remote sensing, nanorobotics, MEMS, and sonic-wave detection.

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