Semiconducting single-walled carbon nanotubes (CNTs) could be ideal for applications in nano-optoelectronics because they can emit and absorb light at distinct frequencies. These spectral lines are predicted to shift slightly in applied electric fields, a well known phenomenon in atoms and molecules known as the "DC Stark effect".

Phaedon Avouris and colleagues set out to measure this effect in carbon nanotubes by suspending a CNT field-effect transistor freely in air, to eliminate effects from its surroundings. The researchers found that the shift in spectral lines of the CNT under a source-drain field is much larger, at 30–50 meV, than that expected from Stark effect theory, which predicts a value of less than 1 meV. Moreover, they found that the strength of the optical transition was dramatically reduced (or "quenched").

The IBM team discovered that the emission shift and quenching occur because the applied drain field dopes the nanotube. That is, it increases the density of free charge carriers (electrons or holes) in the nanotube.

"This is interesting because a big electrically induced change in the optical properties of CNTs might prove useful for electro-optics devices," team member Marcus Freitag told nanotechweb.org. "Such devices include absorbers that can be turned on and off by applying a voltage, or optical emitters that can shift their colour."

Electroluminescence and photoluminescence
In their work, Avouris and colleagues measured the electrically induced light emission, or electroluminescence, as well as the optically induced light emission (photoluminescence). Electroluminescence is employed in electro-optical devices. Scientists are unsure how the two should compare if they are emitted from the same CNT, explains Freitag.

"Experimentally, we found that the electroluminescence width is broadened by a factor of five and shifted by 40 meV to lower energies compared with the photoluminescence," he said.

According to the researchers, this is because the CNT self heats when current is applied through it, but drain-induced doping also plays a role.

"In conclusion, our work shows that the effects of source-drain fields on the CNT optical properties are huge, and come about due to doping rather than the traditional DC Stark effects," added Freitag. "These large effects will be useful for future electro-optics devices built out of CNTs."

The results were reported in ACS Nano.