The atoms in a carbon nanotube are arranged in hexagons, just as in a sheet of graphite. If this atomic "chicken wire" is rolled so that rows of eight hexagons go straight around the tube, the tube is a semiconductor. If the chicken wire is rolled on the bias, however, the nanotube can be a freely conducting metal.
Steven Louie and Marvin Cohen discovered that a bent junction between nanotubes of different "twistedness" could be formed by introducing five-member and seven-member rings, making a device in which a metal is joined to a semiconductor.
They predicted another odd electronic device which might be formed from carbon nanotubes. Normally a metal-to-metal junction is conducting, but because the spacing between a nanotube's atoms is comparable to an electron's wavelength, a straight junction between two metal nanotubes can throw electrons out of phase, forming an insulating junction.
It takes much microscopic sifting and poking about in sooty mats of carbon to find joined nanotubes, so Louie studied individual nanotubes deliberately laid across each other instead. Crossed-tube systems built by his experimental colleagues confirmed theoretical predictions and have electronics potential of their own.
An even wilder idea would use mats of nanotubes just as they come. Electronic leads inserted into the mat could find what active functions already exist randomly; these could be programmed, and the power of the system would increase as investigators learn to exploit more of its functions and connections.