The list of useful applications for carbon nanotubes seems to be growing exponentially: Yesterday there was news that the material could strengthen carbon-fiber even further, and now there's news of a breakthrough in making ultra-tiny light sensors.
While it's been possible to craft sensors of a specific wavelength out of nanotubes, a research team at Sandia National Laboratories has now crafted a device that can sense the entire visible spectrum of wavelengths all at once.
To make the device, the team arranged single nanotubes one at a time on top of a silicon wafer—a ridiculously precise task given that a typical nanotube strand is just 1 micron long (100 times smaller than human hair's width) and only 1nm across—and then added electrical connections. The tube array was then "decorated" with chromophores, which are specific molecules that adjust their morphology when exposed to light—adding red, green, and blue-sensitive chemicals makes the whole assembly sensitive to the entire visible spectrum. Cleverly the chromophores were engineered to have the right shape so that they self-assemble by sticking to the outside of the nanotubes in the right place. As light hits the assembled device, it triggers the chomophores to adjust their orientation, and that causes miniscule changes to the electrical conductivity of the nanotubes they're attached to. By measuring the electrical property of the tube, it's possible to work out the color and intensity of inbound light. It's a process pretty close to how the human eye operates.
But there's already a number of technologies for sensing light accurately in the visible spectrum: charge-coupled devices (CCD) and complimentary metal oxide semiconductors (CMOS) and both are used in every digital camera you can buy.
But carbon nanotube sensors have a number of significant advantages. First, the scale: a single "sensor" is a single nanotube—essentially a 1nm scale pixel. And because the array of tubes needed to make a sensor grid would be very small indeed, incoming light could be very tightly focused—that allows the device to be potentially very sensitive to light, much more so than CCD or CMOS. Plus it's possible to "print" the nanotube array onto a flexible polymer.
The team foresees the nanotube sensor being easy to scale up from their experimental setup, and could have applications in a prosthetic retina to restore sight to those with damaged vision. Essentially a flexible nanotube sensor array would be printed onto flexible bio-friendly plastic and implanted at the back of the eye before being connected up to the nervous system.That technology is still far off, but developments like this bring it closer to becoming a reality. Until then the nanotube sensors may find use in miniscule low-light cameras, and in improving the efficiency of solar panels.