Super Tubes

Scientists have turned the world’s smallest nanotubes into superconductors.

Researchers in Hong Kong have made carbon nanotubes superconduct, adding to the electrical repertoire of the multitalented tubes.

The scientists, led by Ping Sheng and Zikang Tang of the Hong Kong University of Science and Technology, demonstrated that the world’s smallest nanotubes appear to reach superconductivity (the ability to transport electricity without resistance) at temperatures below 20 degrees Kelvin. (Zero degrees Kelvin corresponds to absolute zero, or -273 °C.) The findings appeared in the June 30 issue of Science.

Today, superconductors are used in medical imaging, power generation and a handful of magnetic-levitation trains. Superconducting nanotubes could one day be used in nanoscale sensors and nanoelectronic devices. Their lack of electrical resistance could mitigate one of nanoelectronics’ anticipated problems: the buildup of heat from tightly packed components.

“A nanotube is a very natural nanowire,” says Sheng, head of the university’s physics department. “If it can superconduct, so much the better.”

The experiment demonstrated not only the first one-dimensional superconductor, Sheng told, but also the first superconductor made entirely of carbon.

Riding the Curve

For a decade, scientists have known that carbon nanotubes conduct electricity with either low resistance, like a metal, or variable resistance, like a semiconductor. In 1995, researchers at the University of California at Berkeley theorized that, under the right conditions, nanotubes might also superconduct.

The Berkeley team, led by Steven Louie and Marvin Cohen, looked at superconductivity in two other carbon compounds: doped graphite, a sheet-like structure that superconducts at very low temperatures (less than 1 ºK) and doped C-60 (“buckyballs”), a spherical structure that superconducts at a relatively high 40 to 50 ºK.

They concluded that the more the carbon curved, the better it would superconduct. Since small-diameter tubes curve more than big ones, they figured those would be the best superconductors-if nanotubes could be made to superconduct at all.

Smaller and Smaller

In 1999, the Hong Kong researchers succeeded in growing the smallest carbon nanotubes possible-only four angstroms, or less than half a nanometer, in diameter. The researchers grew the tubes inside zeolite, an aluminum phosphate crystal with tiny pores within its crystal lattice. The pores served as a mold for nanotubes so narrow that only six carbon atoms form their circumference, but as long as 300 micrometers-750,000 times their diameter.

“Growing these uniform, well-aligned nanotubes was a great advance,” says Berkeley’s Louie.

The Hong Kong team began to study the electrical conductive properties of the nanotubes. They noticed that as they cooled the tubes, conductivity seemed to increase until suddenly the tubes appeared not to conduct at all.

“I recognized something was happening at low temperatures,” Sheng says. “The conductivity disappeared very abruptly around 15 ºK.”

On a visit to Hong Kong, Louie told Sheng about his superconducting theory. Suddenly, Sheng says, the observations began to make sense. If the nanotubes were behaving like superconductors, then tiny imperfections in the tubes would keep them from conducting at all.

Sheng urged his team to grow shorter nanotubes-only 1,000 angstroms, 1/3,000 the length of the originals. Some of the shorter tubes, he reasoned, would be free of the imperfections-a guess, he says, that was proven right.

“We saw a behavior that was opposite of what we measured in long nanotubes,” he says. “We saw increasing conductivity as temperature decreased.”

The next challenge for his team, Sheng says, is to find ways to dope, or add other atoms to, the nanotubes. If their predictions hold true, he says, doping the nanotubes with lithium or potassium could raise the temperature at which they superconduct and make nanosuperconductors a valuable part of the nanoelectronic toolbox.

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