Short nanotubes carry big currents

Researchers from Stanford University have developed a simple way to fabricate carbon nanotube devices whose length is as small as ten nanometers, and have shown that electricity can pass through the nanotubes very efficiently.

Ordinarily electrons traveling through wires and devices bounce off the atoms of materials. In small enough devices, however, electrons can travel ballistically, or straight through, which greatly increases efficiency. Carbon nanotubes are rolled-up sheets of carbon atoms that can be narrower than a single nanometer. A nanometer is one millionth of a millimeter, or the span of 10 hydrogen atoms.

Short carbon nanotubes could eventually be used as ballistic transistor channels and interconnects in tiny circuits, according to the researchers.

The researchers found that 10-nanometer-long, two-nanometer wide carbon nanotubes with metallic properties were capable of carrying four times more current -- about 100 microamps -- than longer nanotubes, and 4,000 times more current by size than copper wire at room temperature. They also found that 50-nanometer-long carbon nanotubes with semiconducting properties were capable of fast, low-power operation -- about 25 microamps at a fraction of a volt.

The researchers grew arrays of carbon nanotubes, used standard chipmaking techniques to place metal electrodes at one end of the array, then deposited metal atoms at an angle to the surface. The electrodes blocked the atoms from covering a narrow area, leaving short lengths of the nanotubes intact between pairs of electrodes.

Existing nanotube fabrication methods tend to use electron beam lithography, which is useful for making a few devices, but is not practical for making the numbers of devices need for commercialization.

The method could be used practically in five to ten years. The work appeared in the September 14, 2004 issue of the Proceedings of the National Academy of Sciences.