  Electrically-driven light emission from carbon nanotubes could be exploited
in nano-scale lasers and single-photon sources, and has therefore been the
focus of much research. However, to date, high electric fields and currents
have been either required for electroluminescence, or have been an undesired
side effect, leading to high power requirements and low efficiencies. In
addition, electroluminescent linewidths have been broad enough to obscure the
contributions of individual optical transitions. Here, we report
electrically-induced light emission from individual carbon nanotube p-n diodes.
A new level of control over electrical carrier injection is achieved, reducing
power dissipation by a factor of up to 1000, and resulting in zero threshold
current, negligible self-heating, and high carrier-to- photon conversion
efficiencies. Moreover, the electroluminescent spectra are significantly
narrower (ca. 35 meV) than in previous studies, allowing the identification of
emission from free and localized excitons.
