New electronic devices and nanogenerators created from zinc oxide
nanowires
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
the body. 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.”
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| 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. The nano-piezotronic 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
power sources.
- The large amount or deformation permits a large volume density of
power output.
- 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
characteristics. 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
applied. “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. Nanogenerators,
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. Piezotronic
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
flow. 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. To top
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