Credit: © 2009 ACS

Single-walled carbon nanotubes — individual layers of carbon atoms rolled into a seamless cylinder — posses a number of fascinating and potentially useful electronic properties. However, nanotubes can be metallic or semiconducting and current methods for growing them produce mixtures of both, which is a significant obstacle for their use in nanoscale electronics. Previous attempts to get around these synthetic difficulties have predominately involved selectively separating out or etching the metallic nanotubes. Now, Chongwu Zhou and colleagues at the University of Southern California have shown that metallic nanotubes can be converted into semiconducting ones by simply exposing them to light1.

The researchers first synthesized aligned single-walled carbon nanotubes over quartz and sapphire crystals. These were then transferred to Si/SiO2 wafers and fabricated into arrays of field-effect transistors. A collimated white-light beam was used to irradiate the wafer.

Following irradiation in air, an increase in the channel current on/off ratio of up to 5 orders of magnitude was observed in the nanotube devices. This effect was attributed to a diameter-dependent photochemical reaction of the nanotubes, in which ultraviolet light causes oxygen molecules in the air to form oxygen radicals and ozone. These oxygen species can oxidize the sidewalls of the metallic nanotubes, resulting in stronger localization of the π-electrons and an increase in their semiconducting behaviour.

Notably, the approach is relatively straightforward to implement and scalable to complete wafers.