At #1 is a truly remarkable piece of work from a group headed by Cees Dekker (read an interview with Cess Dekker in the Special Topic of Nanotechnology in the ISI Essential Science Indicators Web based product editorial: ESI Special Topics) of the Delft University of Technology, in The Netherlands. With the help of Leon Balents of the Bell Laboratories, New Jersey, Dekker was able to show that a "kink" in a nanotube created a metal-semiconductor junction in these electrically conducting wires. This was the first demonstration of a molecular scale junction of this type.

Normally all the carbon rings of a nanotube are fused hexagons; this gives a tube its rigidity and provides its electrical conductivity. However, if something goes wrong as the nanotube grows, so that a 5- and a 7-membered ring form, it will cause the tube to bend, and the result is such a junction. Only about 1% of nanotubes are deformed in this way, but the Dutch researchers were able to extract and examine one and, by attaching electrodes to it, they proved that it behaved as a metal-semiconductor junction, and hence paper #1.

More recently, Dekker has developed a method for bending individual nanotubes mechanically, thereby creating electronic junctions to order (see H.W.C. Postma, et al., Phys. Rev. B, 62[16]:10653, 2000; and H.W.C. Postma, et al., Science, 293[5527]:76-9, 2001), and this has enabled him to demonstrate a room temperature single-electron transistor. His group has built on this technology to the extent of making a digital logic circuit based on carbon nanotubes (see A. Bachtold, et al., Science, 294[5545]:1317-20, 2001).

"SWNT is really wonderful material, with extraordinary properties," says Dekker. "It has both desirable mechanical qualities, as shown by its tensile strength and resilient bending, and electrical versatility—witness its metallic and semiconducting abilities. It is a model for fundamental studies, as well as having potential applications." He admits, however, that commercial applications are likely to be some way off.

Nevertheless, other uses of carbon nanotubes may soon be forthcoming, judging by paper #4, the work of a group led by Hongjie Dai and based at Stanford University, California. This paper is about using SWNTs as gas sensors, and reports that their electrical resistance is very sensitive to traces of certain gases. Dai studied two gases: nitrogen dioxide, a pollutant caused by combustion, and ammonia, which is not only an industrial gas but a component of biologically active environments. The Stanford workers showed that when 0.02% of nitrogen dioxide is present, the conductance increases by three orders of magnitude, and when 1% of ammonia is present it immediately decreases by two orders of magnitude.

Moreover, they claim that their nanotube sensors exhibit a faster response and greater sensitivity to these gases at room temperature than existing solid-state sensors, which operate only at high temperatures. But what has been gained on the swings, is lost on the roundabouts. The recovery time for the carbon sensor is much slower. For example, with nitrogen dioxide it took 12 hours to return to normal conductance, although if it was heated to 200° C then this dropped to one hour.