Nanogenerator to power nanoscale devices

Researchers have developed a new technique for powering

nanometer-scale devices without the need for bulky energy sources such as

batteries. By converting mechanical energy from body movement, muscle stretching

or water flow into electricity; these 'nanogenerators' could make possible a

new class of self-powered implantable medical devices, sensors and portable

electronics.

The nanogenerators produce current by bending and then

releasing zinc oxide nanowires, which are both piezoelectric and semiconducting.

The National Science Foundation (NSF), the NASA Vehicle Systems Program and the

Defense Advanced Research Projects Agency (DARPA) sponsored the research.

Zhong Lin Wang, a Regents Professor in the School of Material

Science and Engineering at the Georgia Institute of Technology said, "Our

nanogenerators can convert this mechanical energy to electrical energy. This

could potentially open up a lot of possibilities for the future of

nanotechnology."

Nanotechnology researchers have proposed and developed a

broad range of nanoscale devices, but their use has been limited by the sources

of energy available to power them. Conventional batteries make the nanoscale

systems too large, and the toxic contents of batteries limit their use in the

body.

"We can build nanodevices that are very small, but if

the complete integrated system must include a large power source, that defeats

the purpose," added Wang.

The nanogenerators developed by Wang and graduate student

Jinhui Song use the very small piezoelectric discharges created when zinc oxide

nanowires are bent and then released. By building interconnected arrays

containing millions of such wires, Wang believes he can produce enough current

to power nanoscale devices.

To study the effect, the researchers grew arrays of zinc

oxide nanowires, then used an atomic-force microscope tip to deflect individual

wires. As a wire was contacted and deflected by the tip, stretching on one side

of the structure and compression on the other side created a charge separation

— positive on the stretched side and negative on the compressed side — due

to the piezoelectric effect.

The charges were preserved in the nanowire because an

Schottky barrier was formed between the AFM tip and the nanowire. The coupling

between semiconducting and piezoelectric properties resulted in the charging and

discharging process when the tip scanned across the nanowire, Wang explained.

When the tip lost contact with the wire, the strain was

released and the researchers measured an electrical current. After the strain

release, the nanowire vibrated through many cycles, but the electrical discharge

was measured only at the instant when the strain was released.

To rule out other potential sources of the current, the

researchers conducted similar tests using structures that were not piezoelectric

or semiconducting. "After a variety of tests, we are confident that what we

are seeing is a piezoelectric-induced discharge process," Wang said.

The researchers grew the nanowire arrays using a standard

vapor-liquid-solid process in a small tube furnace. First, gold nanoparticles

were deposited onto a sapphire substrate placed in one end of the furnace. An

argon carrier gas was then flowed into the furnace as zinc oxide powder was

heated. The nanowires grew beneath the gold nanoparticles, which serve as

catalysts.

The resulting arrays contained vertically aligned nanowires

that ranged from 200 to 500 nanometers in length and 20 to 40 nanometers in

diameter. The wires grew approximately 100 nanometers apart, as determined by

the placement of the gold nanoparticles.

A film of zinc oxide also grew between the wires on the

substrate surface, creating an electrical connection between the wires. To that

conductive substrate, the researchers attached an electrode for measuring

current flow.

Though attractive for use inside the body because zinc oxide

is non-toxic, the nanogenerators could also be used wherever mechanical energy

— hydraulic motion of seawater, wind or the motion of a foot inside a shoe —

is available. The nanowires can be grown not only on crystal substrates, but

also on polymer-based films. Use of flexible polymer substrates could one day

allow portable devices to be powered by the movement of their users.

"You could envision having these nanogenerators in your

shoes to produce electricity as you walk," Wang said. "This could be

beneficial to soldiers in the field, who now depend on batteries to power their

electrical equipment. As long as the soldiers were moving, they could generate

electricity."

Placing the nanowire arrays into fields of acoustic or

ultrasonic energy could also produce current. Though they are ceramic materials,

the nanowires can bend as much as 50 degrees without breaking.

The next step in the research will be to maximize the power

produced by an array of the new nanogenerators. Wang estimates that they can

convert as much as 30 percent of the input mechanical energy into electrical

energy for a single cycle of vibration. That could allow a nanowire array just

10 microns square to power a single nanoscale device, if all the power generated

by the nanowire array can be successfully collected.

DQC NEWS BUREAU