Credit: © 2009 AAAS

The conversion of light into electricity by a semiconductor is a central process in photovoltaic devices and many photodetectors. For these devices to work the photon must have enough energy to excite an electron from the valence band to the conduction band, leaving a hole behind in the lower band. Researchers have shown that it is possible for a single photon to generate multiple electron–hole pairs in a semiconductor nanocrystal under certain conditions, but the details of this process are not fully understood. Now Nathaniel Gabor of Cornell University and co-workers have demonstrated the generation of multiple pairs in carbon nanotubes with very high efficiency1.

The nanotubes were placed on an insulating substrate between source and drain electrodes, and a further three electrodes were used to electrostatically dope the nanotube and create a p–n junction. The current through the nanotube was then measured as a function of the source–drain voltage as a laser beam illuminated different positions along the nanotubes. At temperatures below 90 K and photon energies above a certain value, Gabor and co-workers observed steps in the current–voltage characteristics that were due to a laser photon exciting a 'hot' electron that proceeded to generate multiple electron–hole pairs.

The multiple pairs were produced when the photon had enough energy to excite electrons into the second electronic sub-band of the nanotube. Previously it was thought that the electrons had to be in the third sub-band or higher for multiple pairs to be generated.