Nanotubes beam out bright light

18 Nov 2005

Researchers unveil how they made single-walled carbon nanotubes emit beams of infrared light.

From NanotechWeb

Researchers from IBM, US, and Duke University, US, have generated extra-bright beams of infrared light from single-walled carbon nanotubes. Suspending a portion of the nanotube and applying unipolar operation caused light emission at the junction between the suspended and supported parts of the tube. (Science 310 1171)

"The main point of our work is the discovery of a mechanism for the electrical stimulation of light emission from a single nanotube molecule," Phaedon Avouris of IBM told nanotechweb.org. "This mechanism involves a single type of electrical carrier, i.e. electrons or holes, and the emission is localized in a small, nanometre scale area of the nanotube. Because of the smallness of the emitting area, a very bright light source is generated."

Avouris and colleagues laid down nanotubes with a diameter of 2-3 nm by chemical vapor deposition. The nanotubes spanned trenches in a silica coating on a silicon substrate. The researchers added palladium source and drain electrodes. Under unipolar transport conditions, i.e. a gate voltage of less than -3.1 V for hole transport or a gate voltage of more than -2.1 V for electron transport, the nanotube emitted infrared light at the supported/suspended carbon nanotube interface.

"A 3 µA current in our device generates about 10⁷ photons nm^-2 s^-1, 10⁵ times more photon flux than large area LEDs," said Jia Chen of IBM.

The scientists believe the location of the emission was due to band-bending at the interface between the suspended and supported regions. This accelerated carriers, which then created excitons (bound electron-hole pairs) that recombined to emit light. According to the researchers, this excitation mechanism is around 1000 times more efficient than recombination of independently injected electrons and holes.

"[Our study] demonstrates the very strong attraction between electrons and holes in these [low-dimensionality nanostructure] systems and the weakness of the coupling of the electrical carriers to the vibrations of the atoms," said Avouris. "It is the unique combination of these two factors that allows the phenomenon to be observed. Our study also shows for the first time the importance of 'hot' (energetic) carrier phenomena, i.e. intra-molecular impact excitation, in these one-dimensional systems."

Avouris says that carbon nanotubes emit light with a wavelength of 1-2 µm, which is particularly valuable because these wavelengths are used in optical communications. What's more, it's possible to tune the emission wavelength by using nanotubes with different diameters, producing either infrared or visible light.

"These ultra-small light emitters open up the possibility of performing optical studies and inducing interactions at the single molecule level," said Chen. "They could also be built into arrays or integrated with carbon nanotube or silicon electronic components on the same chip, offering new opportunities in electronics and optoelectronics."

Now the researchers plan to investigate the factors that determine the quantum yield of the emission process. The aim is to increase the efficiency of emission. They will also work on integrating electronic and optoelectronic nanotube devices.