Light-emitting devices rely on charge carriers - electrons and holes - being brought together so that they can recombine to emit photons. Single-walled carbon nanotubes have been used as field-effect transistors before and now the IBM team has succeeded in obtaining light from them. Previous nanotubes have only emitted light when excited by another light source such as a laser.

Avouris and colleagues used single nanotubes to make a three-terminal FET device. They randomly dispersed the nanotubes - each about 1.4 nanometres in diameter - onto a silicon substrate that contained a 150 nanometre silicon dioxide layer. 'Source' and 'drain' contacts were then added at either end of the device so that electrons and holes could be injected (figure 1). The electrons and holes recombined in the nanotube to emit infrared radiation at wavelengths longer than about 0.8 microns. This included light at a wavelength of 1.5 micrometres, which is widely used in fibre-optic communications (figure 2).

The device does not rely on doping to create charge carriers, as silicon transistors do, but is ‘biased’ so that one part of the nanotube conducts electrons while the other conducts holes. This is achieved by the formation of Schottky barriers - potential barriers that electrons can tunnel through - at the source and drain.

The wavelength of the emission is determined by the band gap of the nanotube, which depends on the diameter of the nanotube. Changing the thickness of the silicon dioxide layer and using other materials to construct the device might improve the overall efficiency according to the IBM team.