New electronic devices and nanogenerators created from zinc oxide
8 March 2007
Researchers used the unique semiconducting and
piezoelectric properties of zinc oxide nanowires to create a new class of
electronic components and devices that could provide the foundation for a
broad range of new applications, including devices safe for implanting in
So far, the researchers have demonstrated field-effect
transistors, diodes, sensors and current-producing nanogenerators that
operate by bending zinc oxide nanowires and nanobelts. The new components
take advantage of the relationship between the mechanical and electronic
coupled behaviour of piezoelectric nanomaterials, a mechanism the
researchers call “nano-piezotronics.”
|This diagram compares a nanowire/nanobelt based
field effect transistor (FET) with a piezoelectric FET. The role
played by a gate electrode is replaced by the piezoelectric field
produced across the nanowire/nanobelt by an external force (F) so
the transport current is gated by the degree of nanowire bending.
“Nano-piezotronics utilizes the coupling of piezoelectric and
semiconducting properties to fabricate novel electronic components,” said
Zhong Lin Wang, a Regents Professor in the School of Materials Science and
Engineering at the Georgia Institute of Technology. “These devices could
provide the fundamental building blocks that would allow us to create a new
area of electronics.”
For example, in a nano-piezotronic transistor,
bending a one-dimensional zinc oxide nanostructure alters the distribution
of electrical charges, providing control over the current flowing through
it. By measuring changes in current flow through them, piezotronic sensors
can detect forces in the nano- or even pico-Newton range. Other piezotronic
sensors can determine blood pressure within the body by measuring the
current flowing through the nanostructures. And, an electrical connection
made to one side of a bent zinc oxide nanostructure creates a piezotronic
diode that limits current flow to one direction.
mechanism takes advantage of the fundamental property of nanowires or
nanobelts made from piezoelectric materials: bending the structures
separates electrical charges — positive on one side and negative on the
other. The connection between bending and charge creation has also been used
to create nanogenerators that produce measurable electrical currents when an
array of zinc oxide nanowires is bent and then released.
Development of a
piezotronic gated diode based on zinc oxide nanowires was reported February
13 in the online advance issue of the journal Advanced Materials. Other
nano-piezotronic components have been reported in the journals Nano Letters
and Science. The research has been sponsored by the National Science
Foundation (NSF), Defense Advanced Research Projects Agency (DARPA), the
National Institutes of Health (NHI) and NASA.
“The future of
nanotechnology research is in building integrated nanosystems from
individual components,” said Wang. “Piezotronic components based on zinc
oxide nanowires and nanobelts have several important advantages that will
help make such integrated nanosystems possible.”
These advantages include:
- Zinc oxide nanostructures can tolerate large amounts of deformation
without damage, allowing their use in flexible electronics such folding
- The large amount or deformation permits a large volume density of
- Zinc oxide materials are biocompatible, allowing their use in the
body without toxic effects.
- The flexible polymer substrate used in nanogenerators would allow
implanted devices to conform to internal structures in the body.
- Nanogenerators based on the structures could directly produce power
for use in implantable systems.
In comparison to conventional electronic components, the
nano-piezotronic devices operate very differently and exhibit unique
In conventional field-effect transistors, for
instance, an electrical potential — called the gate voltage — is applied to
create an electrical field that controls the flow of current between the
device’s source and its drain. In the piezotronic transistors developed by
Wang and his research team, the current flow is controlled by changing the
conductance of the nanostructure by bending it between the source and drain
electrodes. The bending produces a “gate” potential across the nanowire, and
the resulting conductance is directly related to the degree of bending
“The effect is to reduce the width of the channel to carry the
current, so you can have a 10-fold difference in the conductivity before and
after the bending,” Wang explained.
Diodes, which restrict the flow of
current to one direction, have also been created through nano-piezotronic
mechanisms to take advantage of a potential barrier created at the interface
between the electrode and the tensile (stretched) side of the nanowire by
mechanical bending. The potential barrier created by the piezoelectric
effect limits the follow of current to one direction.
which were announced in the April 14, 2006 issue of the journal Science,
harvest energy from the environment around them, converting mechanical
energy from body movement, muscle stretching, fluid flow or other sources
into electricity. By producing current from the bending and releasing of
zinc oxide nanowires, these devices could eliminate the need for batteries
or other bulky sources for powering nanometer-scale systems.
nanosensors can measure nano-Newton (10 -9) forces by examining the shape of
the structure under pressure. Implantable sensors based on the principle
could continuously measure blood pressure inside the body and relay the
information wirelessly to an external device similar to a watch, Wang said.
The device could be powered by a nanogenerator harvesting energy from blood
Other nanosensors can detect very low levels of specific compounds
by measuring the current change created when molecules of the target are
adsorbed to the nanostructure’s surface. “Utilizing this kind of device, you
could potentially sense a single molecule because the surface area-to-volume
ratio is so high,” Wang said.
The research team was formed from scientists
from Georgia Tech in the USA, the National Tsing Hua University in Taiwan
and Sun Yat-Sen University in China.