For Immediate Release
Researchers at the Georgia Institute of Technology have developed a new
class of nanometer-scale structures that spontaneously form helical shapes
from long ribbon-like single crystals of zinc oxide (ZnO). Dubbed "nanosprings,"
the new structures have piezoelectric and electrostatic polarization properties
that could make them useful in small-scale sensing and micro-system applications.
Just 10 to 60 nanometers wide and 5-20 nanometers thick but up to several millimeters long the new structures are similar to but smaller than the "nanobelts" first reported by Georgia Tech scientists two years ago in the journal Science. The new helical structures and their potential applications were described in the journal Nano Letters. The research was supported by the National Science Foundation and NASA.
"These structures are very different from our original nanobelts
and are a major step toward a new system of nanostructures," said
L. Wang, director of Georgia
Tech's Center for Nanoscience and Nanotechnology and a professor in
the School of Materials Science and
Engineering. "Piezoelectric and polar-surface dominated smart
materials based on zinc oxide are important because they could be the
transducers and actuators for future generations of nanoscale devices."
The piezoelectric properties of the new structures could make them useful
in detecting and measuring very small fluid flows, tiny strain/stress
forces, high-frequency acoustical waves and even air flows that would
otherwise be imperceptible. When deflected by the flow of air or fluids,
the nanosprings would produce small but measurable electrical voltages.
"They could be used to measure pressure in a bio-fluid or in other
biomedical sensing applications," said Wang. "You could use
them to measure nano- or pico-newton forces."
The piezoelectric properties could also make the structures useful as
actuators in micro-systems and nanosystems, where applying voltage would
induce strains. "In micromechanical systems, these structures could
provide the coupling between an electrical signal and a mechanical motion,"
Semiconductor-based nanostructures that rely on electrostatic forces
have gained widespread research interest, but Wang said development of
nanomaterials for piezoelectric actuators have lagged. The new nanosprings
could therefore give designers of future nanoscale systems more options.
Beyond their piezoelectric properties, the new structures also display
unusual electrostatic polarization, with positively and negatively charged
surfaces across the thickness of the nanoribbon. This electrical charge
could be used to attract specific molecules, potentially allowing the
nanosprings to be used as biosensors to detect single molecules or cells.
"The polarized surfaces will attract different molecules with different
charges, which would permit selectivity," Wang said. "The nanosprings
have the promise of being able to do single-molecule detection because
they are so small."
Ultimately, he hopes the new structures could prove useful in biomedical
monitoring applications, their small size allowing development of systems
small enough to be implanted in the body. "We would like to use these
materials for in-situ, real-time, non-destructive monitoring within the
body with high levels of sensitivity," Wang said.
Beyond their potential applications, the new structures could give scientists
a way to study the piezoelectric effect and polarization-induced ferroelectricity
at the nanoscale. The helical structure could also provide a new way to
study nanometer-scale electromechanical coupling.
Wang and collaborator Xiang Yang Kong fabricate the structures using
a solid-vapor process, evaporating high-purity zinc oxide powder in a
horizontal tube furnace at temperatures of approximately 1,350 degrees
Celsius under vacuum. After an argon gas flow is introduced into the furnace,
the structures form on a cooler alumina substrate. Most of the structures
formed are nanobelts up to several hundred microns long, but the researchers
also observe nanobelts in ring shapes, and helical structures consisting
of coiled nanobelts. These nanosprings have diameters ranging from 500
to 800 nanometers.
To induce spontaneous polarization in the structures one edge
positively charged and the other negatively charged the researchers
use a proprietary process and precise control over production conditions.
Wang believes the polarization is what causes the nanosprings to spontaneously
form helical structures.
"Because of the way the charge is located, it tends to fold the
structures over and form a ring," he noted. "When that happens,
the center of the positive charge and the center of the negative charge
overlap, reducing the dipole moment or electrostatic energy."
Many challenges remain before the nanosprings can find application, however.
"We can cut this material into specific lengths and manipulate it,
but that's only the first step," Wang noted. "We need to know
how to integrate this into existing technology. We can generate voltages,
but how can we measure them? We must learn to calibrate a system, and
quantify the data to know what force is being applied."
In March 2001, Wang's research team announced in the journal 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
were made of semiconducting metal oxides such as zinc oxide.
Wang predicted at that time that the extremely thin and flat structures
would offer significant advantages over the nanowires and carbon nanotubes
that had been extensively studied at that time. He reported that the ribbon-like
nanobelts were chemically pure, structurally uniform and largely defect-free,
with clean surfaces not requiring protection against oxidation. Each was
made up of a single crystal with specific surface planes and shape.
Sponsorship from the National Science Foundation came through contract
ECS-0210332. Additional support came through the NASA Vehicle Systems
Program and the Department of Defense Research and Engineering (DDR&E)
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