http://www.physorg.com/news95001943.html

Nanogenerator provides continuous power by harvesting energy from the
environment

Close-up image shows a prototype direct-current nanogenerator fabricated by
Georgia Tech researchers using an array of zinc oxide nanowires. Credit:
Georgia Tech Photo: Gary Meek

Researchers have demonstrated a prototype nanometer-scale generator that
produces continuous direct-current electricity by harvesting mechanical
energy from such environmental sources as ultrasonic waves, mechanical
vibration or blood flow.
Based on arrays of vertically-aligned zinc oxide nanowires that move inside
a novel "zig-zag" plate electrode, the nanogenerators could provide a new
way to power nanoscale devices without batteries or other external power
sources.

"This is a major step toward a portable, adaptable and cost-effective
technology for powering nanoscale devices," said Zhong Lin Wang, Regents'
Professor in the School of Materials Science and Engineering at the Georgia
Institute of Technology. "There has been a lot of interest in making
nanodevices, but we have tended not to think about how to power them. Our
nanogenerator allows us to harvest or recycle energy from many sources to
power these devices."

      Schematic (top) showing the direct current nanogenerator built using
aligned ZnO nanowire arrays with a zigzag top electrode. The nanogenerator
is driven by an external ultrasonic wave or mechanical vibration and the
output current is continuous. The lower plot is the output from a
nanogenerator when the ultrasonic wave was on and off. Credit: Courtesy of
Zhong Lin Wang, Georgia Tech
Details of the nanogenerator will be reported in the April 6 issue of the
journal Science. The research was sponsored by the Defense Advanced Research
Projects Agency (DARPA), the National Science Foundation (NSF), and the
Emory-Georgia Tech Center of Cancer Nanotechnology Excellence.

The nanogenerators take advantage of the unique coupled piezoelectric and
semiconducting properties of zinc oxide nanostructures, which produce small
electrical charges when they are flexed.

Fabrication begins with growing an array of vertically-aligned nanowires
approximately a half-micron apart on gallium arsenide, sapphire or a
flexible polymer substrate. A layer of zinc oxide is grown on top of
substrate to collect the current. The researchers also fabricate silicon
"zig-zag" electrodes, which contain thousands of nanometer-scale tips made
conductive by a platinum coating.

The electrode is then lowered on top of the nanowire array, leaving just
enough space so that a significant number of the nanowires are free to flex
within the gaps created by the tips. Moved by mechanical energy such as
waves or vibration, the nanowires periodically contact the tips,
transferring their electrical charges. By capturing the tiny amounts of
current produced by hundreds of nanowires kept in motion, the generators
produce a direct current output in the nano-Ampere range.

Wang and his group members Xudong Wang, Jinhui Song and Jin Liu expect that
with optimization, their nanogenerator could produce as much as 4 watts per
cubic centimeter - based on a calculation for a single nanowire. That would
be enough to power a broad range of nanometer-scale defense, environmental
and biomedical applications, including biosensors implanted in the body,
environmental monitors - and even nanoscale robots.

Nearly a year ago, in the April 14, 2006 issue of the journal Science, 
Wang's
research team announced the concept behind the nanogenerators. At that time,
the nanogenerator could harvest power from just one nanowire at a time by
dragging the tip of an atomic force microscope (AFM) over it. Made of
platinum-coated silicon, the tip served as a Schottky barrier, helping
accumulate and preserve the electrical charge as the nanowire flexed - and
ensuring that the current flowed in one direction.

With its multiple conducting tips similar to those of an AFM, the new
zig-zag electrode serves as a Schottky barrier to hundreds or thousands of
wires simultaneously, harvesting energy from the nanowire arrays.

"Producing the top electrode as a single assembly sets the stage for scaling
up this technology," Wang said. "We can now see the steps involved in moving
forward to a device that can power real nanometer-scale applications."

Before that happens, additional development will be needed to optimize
current production. For instance, though nanowires in the arrays can be
grown to approximately the same length - about one micron - there is some
variation. Wires that are too short cannot touch the electrode to produce
current, while wires that are too long cannot flex to produce electrical
charge.

"We need to be able to better control the growth, density and uniformity of
the wires," Wang said. "We believe we can make as many as millions or even
billions of nanowires produce current simultaneously. That will allow us to
optimize operation of the nanogenerator."

In their lab, the researchers aimed an ultrasound source at their
nanogenerator to measure current output over slightly more than an hour.
Though there is some fluctuation in output, the current flow was continuous
as long as the ultrasonic generator was operating, Wang said.

To rule out other sources of the current measured, the researchers
substituted carbon nanotubes - which are not piezoelectric - for the zinc
oxide nanowires, and used a top electrode that was flat. In both cases, the
resulting devices did not produce current.

Providing power for nanometer-scale devices has long been a challenge.
Batteries and other traditional sources are too large, and tend to negate
the size advantages of nanodevices. And since batteries contain toxic
materials such as lithium and cadmium, they cannot be implanted into the
body as part of biomedical applications.

Because zinc oxide is non-toxic and compatible with the body, the new
nanogenerators could be integrated into implantable biomedical devices to
wirelessly measure blood flow and blood pressure within the body. And they
could also find more ordinary applications.

"If you had a device like this in your shoes when you walked, you would be
able to generate your own small current to power small electronics," Wang
noted. "Anything that makes the nanowires move within the generator can be
used for generating power. Very little force is required to move them."

Source: Georgia Institute of Technology
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