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.

Light tube: This colorized atomic force microscope image shows a carbon nanotube that is two micrometers long and one nanometer wide. It is covered with light-sensing molecules that respond to visible light and that are not visible in this image. Light-sensitive carbon nanotubes could be used to make artificial retinas, solar cells, and digital cameras. Credit: Xinjian Zhou

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 Grüner, 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.

The Sandia researchers created their proof-of-concept device using a painstaking process. They laid individual carbon nanotubes on top of a silicon wafer, one at a time, and then added electrical contacts at either end. Using this process, it takes a few days to make just one device. Adding the chromophores, though, is straightforward. When the Sandia researchers put a droplet of chromophore solution on the devices, they self-assemble, anchoring themselves to the outsides of the nanotubes. "The chromophores are engineered with grooves that stick to the nanotubes," explains Zhou.

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Scaling up this process should not be an insurmountable obstacle because other researchers have already figured out how to make large networks of nanotubes, says Grüner. "You can spray down a network of nanotubes, then add contacts, then combine them with the [light-sensing] molecules they designed."

Zhou adds that the Sandia team is currently working on making nanotubes that are sensitive to the infrared light and on increasing the devices' sensitivity. "We only have a thin layer of chromophores now, and most of the light is not absorbed," he says. The researchers are also trying to stack the chromophores on the devices in thicker layers and hope to add tiny antennas to the devices to concentrate the light.