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Silicon Changes Nature at the Nanoscale

Model shows that silicon nanotubes lose their semiconductor status

A powerful supercomputer has shown that at small sizes, silicon may conduct electricity, possibly ruling out the use of some nanotubes in future tiny computers.

Researcher Xiao Cheng Zeng from the University of Nebraska-Lincoln used a supercomputer called PrairieFire to create models of silicon tubes less than one nanometer—roughly one billionth of a meter—in diameter.

Researchers are interested in the properties of nanoscale silicon structures because of their potential for creating better disease detectors and biochemical sensors, as well as tiny electronics such as ultra-high density memory chips for super-fast computing.

The unique structural and electronic properties of nanotubes make them attractive for many applications, but their tiny size leaves them prone to the forces of quantum mechanics.

Zeng has since 2001 worked with a Korean researcher named Jai Bai to examine the subatomic behavior of the tiny silicon tubes.

"Bai used the quantum mechanical method to study the problem and from that study we found that these nanotubes are really quite stable—and we found that these tubes are very likely to be metals instead of semiconductors," says Zeng. "Only quantum physics can tell you the correct behavior of electrons in the nanostructure, and it is the electronic structure that determines whether a material is a metal or a semiconductor. So quantum physics was critical to this."

Different properties

By arranging the nanotubes into various shapes, the researchers constructed the thinnest model of a nanotube at a size of less than 0.5 nanometers in diameter.

When they analyzed the tubes, they discovered that they don't appear to have the semiconducting properties that have made silicon the basis for the electronics industry.

"To find that these tubes are very likely to be metals instead of semiconductors is very surprising," says Zeng. "Scientists have studied silicon for more than 50 years and it's the cornerstone material for the modern semiconductor industry."

Hard to reproduce

Zeng is hopeful that researchers will be able to reproduce the computer model in the laboratory, but he acknowledges that this will be difficult.

Creating nanoscale tubes is common in modern labs, he says, but getting silicon into the tubes is challenging because the element is highly reactive at high temperatures, making it difficult to work with.

Zeng also says it's far too early to predict what practical uses the tubes might eventually serve.

"We're just very gratified to find this new nanostructure three years after we entered the field," he says.

The research is reported in the Proceedings of the National Academy of Sciences.

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