The Littlest Nanotubes

Precious bonds. Nanotubes constructed with diamond-like bonds would be stiffer and smaller than conventional models.

In the world of nanotechnology, the current celebrities are carbon nanotubes--cylinders only billionths of a meter across that might someday play a variety of miniscule roles, from molecular tweezers to logic circuits in future computers. Now physicists have designed blueprints for the world's smallest nanotubes by connecting carbon atoms in a new way that is more compact than ever. What's more, these slender carbon threads--still at the simulation stage--appear to be among the stiffest structures ever devised.

A decade ago, researchers at NEC labs in Tsukuba, Japan, looked at the gunk left over from plasmas used to make the soccer-ball-shaped carbon molecules called buckyballs. In the debris they found microscopic tubules made of carbon. The atoms were arranged like rolled up sheets of graphite, but the tubes were only about one nanometer in diameter. Theorists predicted that these nanotubes would be unusually stiff and strong, and experiments since then have proven that prediction correct.

In the new work, a team of physicists led by Vincent Crespi of Pennsylvania State University, University Park, enlisted top-of-the-line supercomputers to design much more compact nanotube structures. In a standard nanotube, each carbon atom reaches out with three arms to hook up with its neighbors. This is the kind of bonding that occurs in graphite. In harder materials such as diamond, the carbon reaches out with four bonds. The group calculated what would happen if one of the four carbon bonds were capped with a hydrogen atom and the resulting molecules were plugged together to make a nanotube. As they report in the 17 September issue of Physical Review Letters, the theoretical tubes would be about 30% stiffer than conventional nanotubes, and with a diameter of 0.4 nanometers, they'd be half as wide.

If these compact nanowires can be synthesized, says nanotube expert Rodney Ruoff at Northwestern University in Evanston, Illinois, they would add a remarkable new material to nanotechnology's toolbox. And he says that prospects for the new material are good: "We experimentalists in the nanotube field have had a pretty good run in the last 8 years in making things after they were predicted."

Related sites

PRL paper by Stojkovic, Zhang, and Crespi
The Nanotube Site at Michigan State University
Vincent Crespi Group

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