Nanotubes That See Everything

Carbon nanotubes that respond to visible light might mean better solar cells and artificial retinas

Researchers at Sandia National Laboratories, in Livermore, CA, have created the first carbon-nanotube devices that can detect the entire visible spectrum of light. Their work might one day find a range of applications, including in solar cells that absorb more light, tiny cameras that work in very low light, and better artificial retinas.

Other researchers have demonstrated nanotubes that can detect light of specific wavelengths, including ultraviolet light, but never the entire visible spectrum of light. "This is a significant milestone," says George GrĂ¼ner, a professor of physics and head of the Nano-Biophysics Group at the University of California, Los Angeles, who was not involved in the Sandia work.

The light sensor inside a digital camera--known as a charge-coupled device--converts light into an electrical signal because as photons bombard silicon, they create electron holes in the material. In contrast, carbon-nanotube light sensors work in a similar way to biological eyes. The nanotubes are decorated with three kinds of chromophores--molecules that change shape in response to a particular wavelength of light. This change in shape results in a change in the chromophores' orientations with respect to the nanotube that, in turn, changes the electrical conductivity of the nanotube in a way that can be measured to deduce the color and intensity of the light. The Sandia researchers used three different types of chromophores, which respond to either red, green, or blue bands of the visible-light spectrum.

The work is still at an early stage, but nanotube light sensors could have advantages over today's light-sensing chips. Most important, says Sandia researcher Xinjian Zhou, the devices are intrinsically high resolution and small. Their resolution is the same as the diameter of each nanotube--about one nanometer. And because an array of the nanotubes could be very small, light could be focused into a very small area, meaning that future devices would be very sensitive to low light levels. Also, nanotube light sensors could be printed on flexible polymer backings. This could make them cheaper to manufacture and also less irritating to biological tissue--an important consideration for retinal implants.

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