Scientists belt out a novel nanostructure

For several years, researchers have been creating a variety of microscopic tubes and wires just tens of atoms wide. They hope someday to use these creations as components in extraordinarily small sensors, electronics, and other microscopic machines. Now, their toolbox has another promising miniature component: the nanobelt.

Scanning electron microscopy shows a mass of zinc oxide nanobelts (above), while transmission electron microscopy reveals a single, flexible belt (below). The dark lines in the single belt indicate strain. Georgia Tech Research Corporation

The ribbonlike structure can be made from various semiconducting metal oxides including zinc oxide and tin oxide, says materials scientist Zhong Lin Wang of the Georgia Institute of Technology in Atlanta. He and his colleagues report their belt-making handiwork in the March 9 Science.

Chemist William E. Buhro of Washington University in St. Louis says he’s surprised by the uniformity of the nanobelts despite their different compositions. Wang’s group “has synthesized a wide variety of compounds having different crystal structures, and they all managed to form with this same kind of a ribbon or belt morphology,” Buhro says.

Each belt, just 30 to 300 nanometers wide and 10 to 15 nanometers thick, is made of a single crystal that has a rectangular cross section. The structures, which grow as much as a few millimeters in length, are flexible enough to bend in half, says Wang.

He and his coworkers made the nanobelts by evaporating metal oxide powders under high temperature in a gas-swept alumina tube. They strictly controlled such conditions as pressure and the duration of the procedure.

Wang says that the process is simple and does not require catalysts, so the belts could be made in industrial quantities.

In the procedure, the metal oxide deposited as a white wool-like clump near one end of the cylinder, the team reports. Microscope observations then revealed that each 5-to-10-gram mass of material contains crystals in the new nanobelt shape, says Wang. The belts formed from oxides of indium, cadmium, gallium, and lead, as well as zinc and tin.

Buhro says that he’s very interested in learning the details of how the nanobelts form from the various metal oxides. “What mechanistic or energetic factors are responsible for this unusual morphology?” he wonders.

The semiconducting belts might have electronic and optical properties handy for the development of a variety of microscopic devices, says Wang. For example, he speculates that the belts eventually could be developed as components of a nanosize sensor for use in medical or industrial settings. These oxides change their conductivity in a predictable way when they encounter specific chemicals or gases, he points out.

However, Buhro stresses that any talk of applications, “at this point, would be purely speculative and far, far from this initial discovery.”

Wang harks back to the 1991 discovery of the first carbon nanotubes, which spawned an ongoing cascade of research and development (SN: 7/29/00, p. 71). He thinks that the belts will be the next big thing in nanoscale materials science. “[They] will spark a lot of research in the next few years because everyone can make [them], and a lot of people already study the properties of these oxides,” says Wang.

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