Tightening the nanobelt

Researchers at the Georgia Institute of technology believe they 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 – called ‘nanobelts’ – are said to offer significant advantages over the nanowires and carbon nanotubes that have been extensively studied.

The ribbon-like nanobelts are chemically pure, structurally uniform with clean surfaces not requiring protection against oxidation. Each is made up of a single crystal with specific surface planes and shape.

Zhong Lin Wang, professor of Materials Science and Engineering and director of the Centre for Nanoscience and Nanotechnology at the Georgia Institute of Technology and his team have produced nanobelts from oxides of zinc, tin, indium, cadmium and gallium.

This group of materials was chosen because they are transparent semiconductive oxides, which are the basis for many functional and smart devices currently under development. But Wang believes other semiconducting oxides may also be used to make the nanoscale 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,’ said Wang. ‘In comparison to the cylindrical symmetric nanowires and nanotubes, 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 uniformity, which 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, Wang believes the nanobelts can be controlled efficiently, allowing production of large quantities of pure structures that are mostly defect-free.

Nanowires made of silicon and other materials have also generated interest, but these structures oxidise 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 are said to 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.

Wang said production of the nanobelts is simple and should scale up easily for high-volume production.

Researchers begin by placing metal oxide powders in the centre 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.