Researchers in the US have used carbon nanotubes to build an infra red (IR) light detector with higher sensitivity than existing technology.

The device, known as an IR bolometer, is made by suspending a 0.5mm wide film of single-walled carbon nanotubes (SWNTs) over a 3.5mm gap between two electrical contacts. The film is heated by incident IR radiation, causing its resistance to change.

The researchers found the absorption coefficient for the SWNT film was ten times higher than that of the material mercury cadmium telluride (HgCdTe) typically used in IR bolometers. The film also had a temperature coefficient of resistance (TCR) comparable to that of vanadium dioxide, which is the material typically used for thermistors in MEMS silicon bolometers.

Best results were obtained from a 40nm thick film, which showed a ratio of  resistance at 4.2K to resistance at 300K of 100. It also had a responsivity of up to 1000V/W.

Much of the work, led by Professor Robert Haddon at the University of California and published in the journal Science, focused on explaining the effect of the radiation on the nanotubes’ electronic structure.

For efficient transfer of temperature modulation into an electrical signal the TCR of the film should be high and negative, and Haddon said it could be modified by altering the chemical composition of the nanotubes.

“We have shown that the introduction of various chemical species - such as octadecylamine and polyamino benzene sulphonic acid - greatly affects the electrical resistivity properties of carbon nanotube films,” said Haddon.

Two-dimensional arrays of the devices could be used for applications such as thermal imaging, spectroscopy and astronomy.