"The observed electroluminescence is localized at specific spots along the nanotube and it is intense," Phaedon Avouris of IBM told nanotechweb.org. "We found that this new emission is the result of local acceleration of carriers due to voltage drops at nanotube or substrate defects. The accelerated ('hot') carriers then excite electron-hole pairs whose recombination produces the light."

According to Avouris, the process is enhanced many orders of magnitude over corresponding processes in bulk materials because of the one-dimensional character of the nanotubes and of the resulting strong electron-hole interaction.

"This 'unipolar' electroluminescence provides a novel diagnostic tool for the identification of defects, trapped charges and dielectric environment changes that lead to the scattering of carriers," he said.

Avouris and colleagues made carbon nanotube field-effect transistors on oxidized silicon wafers. The substrate was heavily doped and acted as the gate for the device. The source and drain electrodes, meanwhile, consisted of 20 nm of palladium on a 0.5 nm adhesion layer of titanium. The team coated some devices with a poly(methyl methacrylate) (PMMA) layer to look at the effects of a change in local dielectric environment on the electroluminescence.

The nanotubes also emitted in ambipolar mode, when both electrons and holes were flowing. "Electrons and holes independently injected from the opposite ends of an ambipolar nanotube transistor recombine and emit light," explained Avouris. "This is the same mechanism as that operating in macroscopic light emitting diodes."

But there's one important difference: there are no dopants introduced in the ambipolar nanotube transistor. "This allows the translation of the light emission spot at will along the length of a long nanotube by simply changing the voltage applied to the gate of the transistor," said Avouris.

The researchers say they hope to couple the excellent electrical and optical properties of nanotubes to develop electrically-excited nano-light emission sources and light detection devices. "We are aiming at applications that require nano-sized light sources," said Avouris. "These may include optical interconnects in nano- and molecular electronics, cryptography, or individual molecule spectroscopy."

Now the scientists are working to enhance the electroluminescence yield of carbon nanotubes, as well as evaluating other one-dimensional nanostructures.

The researchers reported their work in Nano Letters.