Zinc oxide nanogenerators allow self-powered nanoscale medical devices
27 April 2006
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 into electricity, these “nanogenerators” could
make possible a new class of self-powered nanodevices.
|A scanning electron
microscope image (top) shows an array of zinc oxide nanowires.
Middle image shows a schematic of how an AFM tip was used to bend
nanowires to produce current. Bottom image depicts output voltages
produce by the array as it is scanned by the AFM tip. Image courtesy
Zhong Lin Wang
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
Described in the April 14th issue of the journal Science, the
nanogenerators produce current by bending and then releasing zinc oxide
nanowires — which are both piezoelectric and semiconducting. The research
was sponsored by the National Science Foundation (NSF), the NASA Vehicle
Systems Program and the Defense Advanced Research Projects Agency (DARPA).
“There is a lot of mechanical energy available in our environment,” said
Zhong Lin Wang, a Regents Professor in the School of Materials Science and
Engineering at the Georgia Institute of Technology. “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.
Other potential power sources also suffer from significant drawbacks.
“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, who also holds affiliated faculty positions at Peking
University and the National Center for Nanoscience and Technology of China.
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
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 a 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
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
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
Current could also be produced by placing the nanowire arrays into fields
of acoustic or ultrasonic energy. 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.
“Our bodies are good at converting chemical energy from glucose into the
mechanical energy of our muscles,” Wang noted. “These nanogenerators can
take that mechanical energy and convert it to electrical energy for powering
devices inside the body. This could open up tremendous possibilities for
self-powered implantable medical devices.”