Researchers at IBM have overcome an important obstacle to building computers based on carbon nanotubes, by developing a way to selectively arrange transistors that were made using the carbon molecules. The achievement, described in the current issue of Nano Letters, could help make large-scale integrated circuits built out of carbon nanotubes possible, leading to ultrafast, low-power processors.

For decades, the size of silicon-based transistors has decreased steadily while their performance has improved. As the devices approach their physical limits, though, researchers have started looking to less conventional structures and materials. Single-walled carbon nanotubes are one prominent candidate -- already researchers have built carbon nanotube transistors that show promising performance (see The Nanotube Computer). According to estimates, carbon nanotubes have the potential to produce transistors that run 10 times faster than even anticipated future generations of silicon-based devices, while at the same time using less power.

But so far research in the field has hit a roadblock: not being able to control the placement of nanotube transistors, making it impossible to build complex integrated circuits. "The way most [nanotubes transistors] are made now, nanotubes are randomly dispersed on a surface in solution, then source and drain contacts are randomly printed using lithography, and then you search around until you find by chance a tube that goes between a source and a drain," says James Hannon, one of the researchers involved with the work at IBM's T.J. Watson Research Center in Yorktown Heights, NY.

To gain control over the arrangement of transistors, the IBM researchers coated the nanotubes with molecules that bind only to patterns of metal oxide lines on a surface, and not to the areas in-between.

To make working transistors, the researchers laid down lines of aluminum using a lithography technique. These wires serve as the gates that turn the transistors on and off. They then oxidized the aluminum to form a thin aluminum oxide layer on top of the wires, which acts as both a dielectric and the material to which the nanotubes will bind. After applying carbon nanotubes in solution and allowing them to bind to the aluminum oxide, the researchers deposited palladium leads perpendicular to the aluminum/aluminum oxide wires. These leads crossed over the nanotubes, becoming the source and drain of the transistor.