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
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 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
"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
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
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
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