A new class of nanostructure: semiconducting ‘nanobelts’ offer potential for nanosensors and nanoelectronics

Contact: John Toon

john.toon@edi.gatech.edu

404-894-6986

Georgia Institute of Technology

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.

Research News & Publications Office

Georgia Institute of Technology

430 Tenth Street, N.W., Suite N-116

Atlanta, Georgia 30318 USA

Media Relations Contacts: John Toon (404-894-6986); E-mail: (john.toon@edi.gatech.edu); Fax:
(404-894-4545) or Jane Sanders (404- 894-2214); E-mail: (jane.sanders@edi.gatech.edu);
Fax: (404-894-6983).

Technical Contact: Zhong Lin Wang (404-894-8008); E-mail: (zhong.wang@mse.gatech.edu);
Fax: (404-894-9140).

Visuals Available: Researchers with high-temperature tube furnace,
close-up of Wang with glowing furnace in background, microscope images of
nanobelts under high magnification.