For Immediate Release
The rings, complete circles formed by a spontaneous self-coiling process,
could serve as nanometer-scale sensors, resonators and transducers - and
provide a unique test bed for studying piezoelectric effects and other
phenomena at the small scale. The nanorings join "nanobelts"
in a family of zinc oxide structures produced by researchers at the Georgia
Institute of Technology using a high-temperature solid vapor process.
"Nanorings are made up of fine nanobelts that are rolled up as coils
layer-by-layer with as many as a hundred loops," said Zhong
L. Wang, director of Georgia Tech's Center
for Nanoscience and Nanotechnology and a professor in the School
of Materials Science and Engineering. "This is a new nanostructure
with a novel growth mechanism. The seamless nanorings, each made of a
uniformly deformed single crystal of zinc oxide, could be the basis for
nanoscale devices and serve as a model system for studying electrical
and mechanical coupling at the nanoscale."
The research has been sponsored by the National Science Foundation and
NASA. Georgia Tech has sought patent protection for the nanoring growth
process, which was developed by Wang and collaborators Xiang Yang Kong,
Yong Ding and Rusen Yang.
The rings, which range in diameter from one to four microns and are 10-30
nanometers thick, form in a horizontal tube furnace when a mixture of
zinc oxide, indium oxide and lithium carbonate - at a ratio of 20:1:1
- is heated to 1,400 degrees Celsius under a flow of argon gas.
The structures form on an alumina substrate in a section of the furnace
maintained at a temperature of between 200 and 400 degrees C. They begin
growing as nanobelts, long ribbon-like structures that were first produced
by Wang and his research team in 2001. The belts have a width and thickness
of about 15 nanometers.
The surfaces of these nanobelts are dominated by polar charges - positive
on one side, negative on the other - created by the terminations of zinc
ions and oxygen ions on opposite sides of the structures. That creates
a spontaneous polarization across the thickness of the nanobelts. If the
nanobelts remain straight, the overall dipole moment diverges as they
grow longer. However, coiling the nanobelts into a ring neutralizes the
polar charges, resulting in a decrease in electrostatic energy. As the
structures grow, therefore, long-range electrostatic forces cause them
to begin folding and coiling upon themselves, likely as a way to minimize
electrostatic energy in the system, Wang explains.
If the nanobelts fold onto themselves, the negatively- and positively-charged
surfaces bind together through charge attraction, and the structure continues
growing parallel to the rim, loop-by-loop. The charge attraction brings
the loops together in perfect alignment.
After about 30 minutes in the furnace, the high temperature causes the
coils to become sintered together, with epitaxial and chemical bonding
forming a single crystal that can no longer be separated into individual
loops. The resulting nanorings vary in width, composed of as few as five
and as many as 100 loops.
Wang says the lithium and indium materials facilitate the unique growth
mechanism by creating a planar defect in the zinc oxide nanobelt. That
defect creates energetic conditions and leads to rapid growth of the nanobelt
along the rim.
Like the earlier structures, nanorings offer new possibilities for fabricating
unique nanoscale electromechanical systems, such as piezoelectric resonators
for detecting single biomolecules, nanoscale elastic bearings and actuators.
"Nanorings offer a combination of many unique and attractive properties
in one system," said Wang. "We want to build some unique devices
that will test different electromechanical properties, particularly electro-mechanical
coupling and applications in bio-detection. We want to fully use the piezoelectric
properties, in addition to the semiconductor properties, because they
will allow us to explore properties no other systems have."
Nanorings and nanosprings are candidates for building implantable sensors
for real-time monitoring of such biomedical measures as blood pressure,
blood flow rate and stress at the level of single cells, Wang said.
"This is a rich family of materials that allows us to make a broad
range of structures with interesting mechanical and electrical properties,"
he explained. "They could be very important because other nanostructures
do not have piezoelectric properties. The discovery of these zinc oxide
structures could open a few field of research in nanoscale piezoelectric
structures and devices. For biomedical applications, zinc oxide would
also have the advantage of being biocompatible."
Snyder, professor and chair of Georgia Tech's School of Materials
Science and Engineering, summarized the importance of the new work being
reported in Science. "The discovery of these nanorings has
three important impacts: a new nanostructure, a new growth mechanism and
new applications - such as piezoelectric-based fluid pumps and switches
for biotechnology," he said.
In March 2001, Wang's research team announced in Science that
they had created a new type of nanometer-scale structure that could be
the basis for inexpensive ultra-small sensors, flat-panel display components
and other electronic nanodevices. Dubbed nanobelts, the structures could
be fabricated from semiconducting metal oxides such as zinc oxide, producing
ribbon-like structures five to 100 nanometers wide and up to a millimeter
or more in length.
The structures offer advantages for making nanoscale devices because
of their well-defined growth direction and regular and well-defined side
surfaces. Their conductivity, bandgap, surface properties and optical
properties can be controlled by introducing oxygen vacancies in the wurtzite
The 2001 Science paper was listed as among the most cited papers
in chemistry by the journal Science Watch, published by the Institute
of Scientific Information. The same journal included Wang on the list
of the 25 "most-cited authors in nanotechnology, 1992-2002."
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