Nanotube LEDs make progress

24 Nov 2009

Researchers at IBM have succeeded in controlling the light emitted from carbon nanotubes with unprecedented precision. The result means that it could now be possible to make narrow-band nanotube light-emitting diodes and single-photon sources for applications in quantum optics.

Carbon nanotubes emit light when a current is passed through them, an effect that could be used to make future electro-optical devices. However, the problem is that the light emitted tends to be spectrally broad, spatially unconfined and has a low radiative yield.

The device

Light-emitting devices rely on charge carriers – electrons and holes – being brought together so that they can recombine to emit photons. Now, Phaedon Avouris and colleagues at IBM's TJ Watson Research Center have shown that they can control charge carrier injection and light generation in a carbon nanotube more precisely than ever before thanks to a novel smart device layout.

When compared to previous nanotube light emitters, the researchers succeeded in producing light from a predefined segment in a nanotube diode 1000 times more efficiently. The spectral bandwidth of the light generated is also much narrower – at around just 10% of earlier devices. That's not all: the new device has a much lower current threshold too and dissipates much less electrical power, which means that it hardly heats up at all.

The IBM team made its nanotube diode by placing an isolated carbon nanotube on a dielectric substrate and contacting both ends by metal electrodes. Both nanotube and metal contacts are covered by another dielectric layer and topped with two metal gate electrodes that define different spatial domains along the nanotube. If voltages of opposite signs are applied to these top gate electrodes, the two nanotube segments underneath become doped with charge carriers having opposite polarities (electrons and holes). When the diode is then switched on, radiative charge carrier recombination occurs and light is emitted from the segments.

The scientists now plan to improve the device layout. "Using nanotube arrays could also be promising for further optimization and tailoring device performance," Avouris told nanotechweb.org.

The results were reported in Nature Nanotechnology.