Can Nanotubes Be Engineered to Superconduct?

WASHINGTON, Sept. 25 -- Superconducting nanotubes may lie on the technology horizon, suggests a theoretical study published recently by researchers from the Commerce Department's National Institute of Standards and Technology (NIST), the University of Pennsylvania and Bilkent University in Turkey.

The intriguing possibility is the team's most recent finding in a spate of studies showing how changing the shape of tiny single-walled tubes of carbon may open a potential mother lode of technologically useful properties. The theoretical investigations are pointing out productive paths for other researchers to follow in experiments that pursue opportunities to make new materials and technologies with nanotubes.

Although formidable obstacles remain, nanotubes were discovered only about a decade ago, and initial product offerings are beginning to edge onto the market, according to the NIST.

"Carbon nanotubes are now considered to be building blocks of future electronic and mechanical devices," said Taner Yildirim, a physicist at the NIST Center for Neutron Research. "We'll get there quicker if we have a good understanding of the properties of the materials and the interactions among them."

The new calculations by Yildirim and his colleagues indicate that strategically placing hydrogen on the exterior of so-called zigzag nanotubes leads to dense concentrations of charge-carrying electrons just below the material's conduction band. In fact, the structure of the molecules -- initially resembling cylindrical rolls of chicken wire -- becomes rectangular, with a carbon atom at each corner. During the structural makeover, the nanotubes become diamond-like and are transformed from insulators to metals.

The result, says Yildirim, is a "four-wire nanocable." Because of the high density of electrons in this particular configuration, he added, it may be possible to chemically engineer nanotube wires that are superconducting.

Depending on the initial geometry of the nanotubes and on the pattern of hydrogen coverage on tube walls, electronic structures will vary greatly among the resultant materials, as will their properties. The team's calculations indicate that selective bonding of hydrogen to nanotubes can give rise to a number of potentially useful applications in the emerging field of molecular electronics.

Closing the Band Gap

In an earlier study, team members and another collaborator predicted that, when exposed to external pressure, nanotubes will bind tightly and form stable ropelike networks. Published in late 2000, the prediction was later verified in experiments by other researchers.

Subsequent studies published by the team indicate that the chemical and electrical properties of a single-walled carbon nanotubes can be controlled through a reversible process called mechanical deformation. Flattening the radius of a nanotube so that it becomes elliptical, says Yildirim, alters the arrangement of electrons, suggesting an approach to engineering the gap between different bands of electrons within the materials.

"Our calculations indicate that, with radial deformation, it is possible to close the band gap and make an insulating nanotube metallic and vice versa," Yildirim explains. If verified in experiments, this predicted capability could yield new types of carbon-based materials and a host of novel devices built using nanotubes with properties optimized for specific applications.

The work was partially supported by grants from the National Science Foundation and the Scientific and

"Effects of hydrogen adsorption on single-wall carbon nanotubes: Metallic hydrogen decoration," by O. Gulseren, T. Yildirim and S. Ciraci, was published in Physical Review B, Vol. 66, Article121401. A copy of the paper, in Adobe Acrobat PDF format, is available from Mark Bello at mark.bello@nist.gov

www.ncnr.nist.gov/staff/taner/nanotube/