A new class of nanostructure

March 8, 2001–Atlanta, Georgia–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.

“Current research in one-dimensional systems has largely been dominated by carbon nanotubes,” says Zhong Lin Wang, professor of Materials Science and Engineering and director of the Center for Nanoscience and Nanotechnology at the Georgia Tech. “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 colleagues 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 explains. “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,” says Wang. “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.”


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