Semiconducting oxide nanobelts of zinc oxide. The purity and shape of the nanobelts are shown by images from the scanning electron microscope, as shown on the left and transmission electron microscope images, on the right.

Photos by Gary Meek, Courtesy Georgia Tech

Zhong L. Wang with oxide powder sample, high-temperature tube furnace in background

Photos by Gary Meek, Courtesy Georgia Tech

Zheng Wei Pan, Zu Rong Dai and Zhong L. Wang pose with high temperature tube furnace used for producing nanobelts.
Photos by Gary Meek, Courtesy Georgia Tech

Nanotech could affect medicine, industry, and the fabrication of just about everything you buy, because smaller is not just beautiful, but also cheaper, faster and less wasteful of energy.

In fact, we're talking tiny, not small. The thickness of a human hair measures about 50,000 nanometers!!!

You'd need an awful skinny waist to take advantage of the latest nanotech fashion -- nanobelts. If you're a fan o' nano, you know about nanotubes, carbon atoms bonded into honeycomb-like shapes with enormous strength and electrical conductivity.

Nanobelts may turn out to have advantages over tubes in terms of price, flexibility and practicality.

At the very least, belts are easy to make with just $10,000 in basic lab equipment, says Zhang Lin Wang of Georgia Institute of Technology, who reported the discovery in the current issue of Science. (Don't forget that only expensive stuff like transmission electron microscopes can analyze the fluff.)

Just cook 'n cool
Once you read the recipe, you might wonder why nobody thought to make nanobelts before, and in fact, silicon nanowires have already been made. At any rate, Wang and his colleagues scooped some metal oxide from a chemical jar, placed it inside a hot furnace and began a slow flow of the inert gas argon. The temperature was set at roughly 1200 ° Celsius (below the melting point of the particular oxide).

After the oxide evaporated for two hours, what the researchers called "white woollike products" appeared on a plate in a cooler part of the furnace. Using electron microscopes and X-ray diffraction, Wang and his crew analyzed that itty-bitty woolly stuff.

The wool turned out to be pure nanobelts. Whether the oxide contained zinc, tin, cadmium, gallium or indium, the little straps have a rectangular cross-section, with a width of 30 to 300 nanometers and a thickness of 10 to 15 nanometers. Because the material was already an oxide, it did not undergo a chemical reaction, and had a pure, flawless surface.

More important, each belt was a single crystal. Flawlessness is a big advantage, says Wang, professor of material science and engineering and director of Georgia Technology's Center for Nanoscience and Nanotechnology. He attributes the lack of flaws to the minuscule size. "They are so small that no defect can stay in the volume, they just pop out."

Flaws between crystals can cause problems, he adds. "Defects can generate heat" when current flows. If the goal of nano-scale electronics is to increase the density of devices, "Where is the heat going to go?"

That question is especially relevant to the ultra-small world of quantum computing, but nanotech could also find its way into medicine. We've heard predictions that within 10 to 15 years, nanotech will be worth $180 billion per year in the drug industry alone.

Comparison shopping
If you're selling nanostructures, today's competition is the all-carbon, highly-hyped nanotube, which is made in small quantities under a laser. How does the comparison between belts and tubes stack up at this point?

In just four hours, using some dumb-simple lab equipment, you can make 10 grams of nanobelts -- thousands of times greater than the first nanotube production.

While nanotubes are generally only a few millionths of a meter long, the belts are millimeters long.

And while nanotubes are made of pure carbon, belts have already been made from five oxides, a much greater variety for the engineer to play with. "The oxide world is so colorful, so fancy," says Wang, "but carbon is only carbon."

Nanobeltdom, in fact, seems a common result of using evaporate-and-cool technology with the five metal oxides -- and perhaps others. The researchers are also trying to replace the oxygen with the related element sulfur, further increasing the palette of possibilities.

It's still early in the game -- nanobelts were only discovered last summer, but Wang suggests these possible uses:

Sensors: the electrical conductivity of zinc oxide, for example, changes drastically when a gas molecule attaches, so zinc oxide is already used to detect flammable gases. Tiny sensors could replace existing ones, and find new uses as well.

Smart windows: These high-tech pieces of glass respond to temperature changes by, for example, restricting the flow of ultraviolet light. Tin oxide treated with fluoride is currently used for this purpose.

Flat-panel displays. Indium oxide doped with tin is transparent, but electrically conducting, making it a promising material for advanced displays.

Electronic and optical-electronic devices. The tested oxides are semiconductors and are the main members of the "smart and functional materials" family. Thus nanobelts could be used to make tiny devices for electronic or fiber-optics purposes.

Making some real-live products would be a major boost to the field of nanotech, where small may be beautiful, but not necessarily profitable. Maybe it's just a matter of tightening the belt...

-- David Tenenbaum

Bibliography:
Nanobelts of Semiconducting Oxides, Zheng Wei Pan et al, Science, 9 March 2001, pp. 1947-9.