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Topic Name: A prototype nanometer-scale generator
Category: Nanofabrication
Research persons: Dr. Zhong Lin 'ZL' Wang
Location: Georgia Institute of Technology,Materials Science and Engineering,771 Ferst Drive, N.W.,Atlanta, GA 30332-0245, United States
Details
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.”
Details of the nanogenerator are 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
coatingThe 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.”
About Researcher:
Dr.
Zhong Lin 'ZL' Wang
Regents’ Professor and COE Distinguished Professor
Director, Center for Nanostructure Characterization & Fabrication (CNCF)
Georgia Institute of Technology
Materials Science and Engineering
771 Ferst Drive, N.W.
Atlanta, GA 30332-0245
Office: IPST 273A
Phone: 404.894.8008 | Fax: 404.894.9140
zhong.wang@mse.gatech.edu
Website:
www.nanoscience.gatech.edu/zlwang
Funded:
Georgia
Institute of Technology
75 Fifth Street, N.W., Suite 100
Atlanta, Georgia 30308 USA
The Institute offers research opportunities to
both undergraduate and graduate students and is home to more than 100
interdisciplinary units plus the Georgia Tech Research Institute.
In pictures:
1.Schematic shows 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.
2.Zhong Lin Wang, Regents Professor in the
School of Materials Science and Engineering at Georgia Tech, holds a prototype
DC nanogenerator fabricated using an array of zinc oxide nanowires.
3.Close-up image shows a prototype
direct-current nanogenerator fabricated by Georgia Tech researchers using an
array of zinc oxide nanowires.
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