A New Class Of Nanostructure:
Semiconducting "Nanobelts"
Offer Potential For Nanosensors
And Nanoelectronics
Researchers have created a new class of nanometer-scale
structure that could be the basis for inexpensive ultra-small
sensors, flat-panel display components and other electronic
nanodevices.
Made of semiconducting metal oxides, these extremely thin
and flat structures -- dubbed "nanobelts" -- offer significant
advantages over the nanowires and carbon nanotubes that
have been extensively studied. The ribbon-like nanobelts are
chemically pure, structurally uniform and largely
defect-free, with clean surfaces not requiring protection
against oxidation. Each is made up of a single crystal with
specific surface planes and shape.
Described for the first time in the March 9 issue of the
journal Science, nanobelts could provide the kind of
uniform structure needed to make practical the
mass-production of nanoscale electronic and optoelectronic
devices.
"Current research in one-dimensional systems has largely
been dominated by carbon nanotubes," said Zhong Lin
Wang, professor of Materials Science and Engineering and
director of the Center for Nanoscience and Nanotechnology
at the Georgia Institute of Technology. "It is now time to
explore other one-dimensional systems that may have
important applications for nanoscale functional and smart
materials. These nanobelts are the next step in developing
structures that may be useful in wider applications."
Wang and his group members Zhengwei Pan and Zurong
Dai have produced nanobelts from oxides of zinc, tin,
indium, cadmium and gallium. This family of materials was
chosen because they are transparent semiconductive oxides,
which are the basis for many functional and smart devices
being developed today. But Wang believes other
semiconducting oxides may also be used to make the
unique structures.
"The crystallographic structure varies a great deal from one
oxide to another, but they all have a common characteristic
as part of a family of materials that have ribbon-like
structures with a narrow rectangular cross-section" Wang
explained. "In comparison to the cylindrical symmetric
nanowires and nanotubes reported in the literature, these are
really a distinctive group of materials."
Nanobelts may not have the high structural strength of
cylindrical carbon nanotubes, but make up for that with a
uniformity that could make them useful in electronic and
optoelectronic applications. Processes for producing carbon
nanotubes still cannot be controlled well enough to provide
large volumes of high purity, defect-free structures with
uniform properties. However, the nanobelts can be well
controlled, allowing production of large quantities of pure
structures that are mostly defect-free.
"Defects in any nanostructures strongly affect their
electronic and mechanical properties and possibly cause
heating when electrical current passes through them. This
creates problems if you want to integrate them into smaller
and smaller devices at a high density," Wang noted. "More
importantly, defects can destroy quantum mechanical
transport properties in nanowire-like structures, resulting in
the failure of quantum devices fabricated using them."
Nanowires made of silicon and other materials have also
generated interest, but these structures oxidize and require
complex cleaning steps and handling in controlled
environments. As oxides, nanobelts do not have to be
cleaned or handled in special environments and their
surfaces are atomically sharp and clean.
Based on known properties of the oxide nanobelts, Wang
points to at least three significant applications.
Zinc oxide and tin oxide nanobelts could be the basis for
ultra-small sensors because the conductivity of these
materials changes dramatically when gas or liquid
molecules attach to their surfaces. Tin-doped indium oxide
nanobelts provide high electrical conductivity and are
optically transparent, making them candidates for use in
flat-panel displays. And because of their response to
infrared emissions, nanobelts of fluoride-doped tin oxide
could find application in "smart" windows able to adjust
their transmission of light as well as conduction of heat.
"This is a vitally important area of nanotechnology," Wang
said. "If we are successful at these applications, it may lead
to major technological advances in nano-size sensors and
functional devices with low power consumption and high
sensitivity."
Wang says production of the nanobelts is simple and should
scale up easily for high-volume production.
Researchers begin by placing commercially available metal
oxide powders in the center of an alumina tube. As argon or
nitrogen gas is flowed through it, the tube is heated in a
furnace to temperatures just below the melting point of the
powders, approximately 1,100 - 1,400 degrees Celsius,
depending on the material. The powders evaporate, then
form the crystalline nanobelts as they return to solid phase
on an alumina plate in a cooler part of the furnace.
Though the temperature, pressure and processing times
must be kept within bounds, Wang says the growth of the
nanobelts does not appear sensitive to temperature
fluctuations or variations in the processing time.
Finished nanobelts appear as clumps that resemble a wad of
cotton. Under microscopic study, they appear like "shredded
paper," Wang said. Despite their origin in normally brittle
oxide compounds, the nanobelts are flexible and can be bent
180 degrees without breaking.
Typical width of the nanobelts is from 30 to 300
nanometers, with a thickness of 10-15 nanometers. Some
have been produced in lengths of up to a few millimeters,
though most are tens to hundreds of micrometers long.
Georgia Tech researchers have done preliminary studies of
nanobelt properties, though they would still like to learn
more about the optical, electrical and surface
characteristics.
Wang expects the Science paper on nanobelts will spawn a
new area of nanoscience research.
"I believe this area will expand very rapidly. Just like carbon
nanotubes, these nanobelts provide a new nanomaterials
system that allows people to study nano-scale physics and
device fabrication using smart and function oxide
materials," he said. "Anybody can make these. There is
certainly enough to be discovered to occupy researchers for
several years."
Images for this project are available at
http://www.atdc.org/images/nanobelts.html
The research was sponsored by Georgia Tech, and a
provision patent application has been filed on the new
structures.
Note: This story has been adapted from a news release issued by
Georgia Institute Of Technology for journalists and other
members of the public. If you wish to quote from any part of this
story, please credit Georgia Institute Of Technology as the
original source. You may also wish to include the following link in
any citation:
http://www.sciencedaily.com/releases/2001/03/010309080953.htm
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