Semiconducting single-walled carbon nanotubes (CNTs) could be ideal for applications in nano-optoelectronics. This is because they can emit and absorb light and sustain large electrical currents.

Excitons and energy states
Light absorption and emission in CNTs involves bound electron-hole states, or excitons. The exciton states form a ladder of energy states below the so-called single-particle continuum bands (or bandgaps) and the binding energies of these states decrease with increasing index, known as E11, E22, E33 etc.

Phaedon Avouris and colleagues at the IBM Thomas J Watson Research Center in New York have now investigated the effects of an electric field on the optical properties of CNTs. They probed the absorption of visible light into the higher-energy E33 state and monitored the infrared light emission from the lowest-lying E11 state of the same semiconducting CNT. The nanotube was incorporated as the active channel in a field-effect transistor.

Measuring light absorption and emission
The researchers employed a combination of resonant optical laser micro-spectroscopy and electrical transport measurements on a single nanotube. Using these techniques, they were able to measure the light absorption and emission of the nanotube while the gate voltage of the transistor was tuned.

They found a drastic decrease in the light emission of the nanotube as the gate field was turned on. According to the team, the gate field effectively modifies the doping level, or the number of charge carriers, in the CNT. In turn, this change in the doping level modifies the CNT's dielectric properties. As a result, the energies and strengths of the optical transitions in the nanotube are reduced.

In the experiments, the scientists noticed a red shift of the transition energy and loss of absorption strength of the E33 state, as well as a reduced number of single-particle states that are involved in the E11 optical transition.

"All effects induced by the gate field are reversible and allow for efficient switching of light absorption and emission in the CNT," Avouris told nanotechweb.org.

Controlling light absorption and emission in a CNT device in this way is an important step towards nanotube-based light emitters and detectors, says the IBM team. That's not all: demonstrating such control at the single nanotube level paves the way for developing quantum light sources based on isolated CNTs.

The work was reported in Nano Letters.