The new method, detailed in an article in the October issue of Nanoletters, involves using silicon rather than metal as a catalyst in the nanotube fabrication process, said Phaedon Avouris, manager of nanoscale science at IBM. The shift could improve the manufacturing yields of single-walled nanotubes, which show the most promise for circuitry.
"We have shown that there are ways of making single-walled nanotubes without the use of metals," Avouris said.
Carbon nanotubes represent one of the two leading candidates to replace wires inside chips and other components in a decade. Not only do these structures conduct electricity well, they are incredibly small, which should allow manufacturers to squeeze billions of transistors onto a single chip.
For now, nanotubes can only be manufactured in small numbers inside labs. Affordable, mass-marketing techniques remain years away.
Under the metal catalyst method, nickel, iron or cobalt is heated with carbon atoms until the metal melts. Single-walled nanotubes then form on the surface of the liquid metal.
Unfortunately, metal particles become attached to the nanotubes, which magnetizes them and makes them unusable as transistors. "For every nanotube there is a particle of metal," Avouris said. "Removing the metal involves boiling them in nitric acid, which damages the nanotube."
In IBM's technique, the nanotubes are not damaged. Researchers take a crystal formed from layers of silicon and carbon and heat it to 1,650 degrees centigrade. The silicon evaporates, leaving an exposed layer of carbon. Because the carbon formerly was bonded to the silicon, it is freed to bond with another material. In this case, it bonds with itself, curling up to form a tube.
The atomic structure that the carbon tubes lay upon then serves as a template for arranging the tubes so they can be used in processors, said Avouris. The structures must be organized in grids of parallel lines to form transistors.
IBM, Avouris added, does not manufacture tubes. Instead, companies like Carbon Nanotechnologies, founded by Nobel laureate and Rice University professor Richard Smalley do. Nonetheless, IBM will encourage these companies to adopt its research.