In addition to being very strong and flexible, carbon nanotubes also have unusual electronic properties. Nanotubes are described by two numbers, conventionally m and n, and a specific nanotube can be metallic or semiconducting, depending on these numbers.

Nanotubes are made by rolling up two-dimensional sheets of carbon atoms. The atoms are arranged in hexagons in both the sheet and the tube. The values of m and n are related to the direction in which the sheet is rolled: they also determine the diameter of the tube.

By connecting nanotubes with different electronic properties it should be possible to make a range of nanoscale electronic devices. A major challenge will be to make T-shaped junctions between different types of tubes. Now two American physicists -- Madhu Menon of the University of Kentucky and Deepak Srivastava of the NASA Ames Research Center -- have shown that this is indeed possible. By introducing pentagons and heptagons into the junction they showed that a (10, 0) nanotube, which is semiconducting, can be connected at right angles to a (9, 0) tube, which is metallic. And by adding only heptagons, they showed how to connect a (10, 0) nanotube with a metallic (5, 5) nanotube.

"These quasi-2D junctions could be the building blocks of nanoscale tunnel junctions in a 2D network of nanoelectronic devices" the authors write in the December 1 issue of Physical Review Letters.