Sapphire steps shape nanotubes arrays

Sapphires could become a nanotech engineer's best friend.

Carbon nanotubes -- rolled-up sheets of carbon atoms that can be narrower than a single nanometer -- have the potential to be ready-made electrical components at the molecular scale. A nanometer is one millionth of a millimeter, or the span of 10 hydrogen atoms.

One key to enabling this potential is finding ways to place nanotubes where they are needed.

Researchers from the Weizmann Institute of Science in Israel have found that it is possible to grow carbon nanotubes along atom-size steps on a sapphire surface.

These arrays of carbon nanotubes could eventually be used as electronic circuits, according to the researchers. The combination of aligned nanotubes on a sapphire base could also find use as a transparent conducting material. Sapphire is transparent and resists high temperatures.

Growing the arrays is only the first step in eventually using them as electronic circuits. The researchers must also devise ways of producing the junctions required for the nanotubes to act as transistors, and wiring the nanotubes to electrodes.

Key to the method is the structure of a crystal like sapphire. When the crystal is cut in a plane that is slightly tilted from one of its main faces, the surface contains parallel steps of one-atom height. The researchers were able to grow one-nanometer-wide carbon nanotubes along these steps to produce dense arrays of parallel, separated tubes.

The direction and shape of the atomic steps can be controlled by the crystal cut, according to the researchers. Deliberately introduced defects in the crystal could be used to produce kinks and other useful shapes in the nanotubes, which could lead to nanotube architectures constructed from the bottom up.

The work appeared in the November 15, 2004 issue of Angewandte Chemie International Edition.