Sandia LabNews

Seeing the colors of the rainbow, on the nanoscale

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To construct a nanoscale color detector, a team of Sandia researchers took inspiration from the human eye, and in a sense, improved on the model.

When light strikes the retina, it initiates a cascade of chemical and electrical impulses that ultimately trigger nerve impulses. In the nanoscale color detector, light strikes a chromophore and causes a conformational change in the molecule, which in turn causes a threshold shift on a transistor made from a single-walled carbon nanotube.

“In our eyes the neuron is in front of the retinal molecule, so the light has to transmit through the neuron to hit the molecule,” says Xinjian Zhou (8656). “We placed the nanotube transistor behind the molecule — a more efficient design.”

Detecting the entire visible spectrum

Zhou, François Léonard (8656), Andy Vance, Karen Krafcik, Tom Zifer, and Bryan Wong (all 8223) created the first carbon nanotube device that can detect the entire visible spectrum of light. The team recently published a paper, “Color Detection Using Chromophore-Nanotube Hybrid Devices,” in the journal Nano Letters. The research has garnered attention in industry press, with stories appearing in, Technology Review, and Nature Photonics.

The idea of carbon nanotubes being light-sensitive has been around for a long time, but earlier efforts using an individual nanotube were only able to detect light in narrow wavelength ranges at laser intensities. The Sandia team found that their nanodetector was orders of magnitude more sensitive, down to about 40 watts per square meter — about 3 percent of the density of sunshine reaching the ground. “Because the dye is so close to the nanotube, a little change turns into a big signal on the device,” explains Xinjian.

The research is a Laboratory Directed Research and Development (LDRD) project, now in its second year, based on François’ collaboration with the University of Wisconsin to explain the theoretical mechanism of carbon nanotube light detection. If you’re going to work on carbon nanotubes, François is the guy to have on your team because he literally wrote the book on carbon nanotubes — The Physics of Carbon Nanotubes, published September 2008 (Lab News, April 25, 2008).

He points out that a key component of the project was bringing together the right people and equipment, as the LDRD draws upon Sandia’s expertise in both materials physics and materials chemistry. In fact, when asked what the most difficult part of the project was, the different team members pointed to their counterparts in other disciplines; Andy says he felt it was the device fabrication, while Xinjian thought it was the chemical synthesis.

“This is an example of the sum of the parts being more than the individuals,” says Andy.

François and Bryan laid the groundwork for the project with their theoretical research. Bryan did the first-principles calculations that supported the hypothesis of how the chromophores were arranged on the nanotubes and how the chromophore isomerizations affected electronic properties of the devices.

To construct the device, Xinjian and Karen first had to create a tiny transistor made from a single carbon nanotube. They deposited carbon nanotubes on a silicon wafer and then used photolithography to define electrical patterns to make contacts.

The final piece came from Andy and Tom, who synthesized molecules to create three types of chromophores that respond to either the red, green, or orange bands of the visible spectrum. Xinjian immersed the wafer in the dye solution and waited a few minutes while the chromophores attached themselves to the nanotubes.

The team reached their goal of detecting visible light faster than they expected; they thought the entire first year of the LDRD would be spent testing UV light. Now they are looking to increase the efficiency by creating a device with multiple nanotubes.

Larger size more practical for applications

“Detection is now limited to about 3 percent of sunlight, which isn’t bad compared with a commercially available digital camera,” says Xinjian. “I hope to add some antennas to increase light absorption.”

A device made with multiple carbon nanotubes would be easier to construct and the resulting larger area would be more sensitive to light. A larger size is also more practical for applications.

The team is now setting its sights on detecting infrared light. “We think this principle can be applied to infrared light and there is a lot of interest in infrared detection,” says Andy. “So we’re in the process of looking for dyes that work in infrared.”

This research eventually could be used for a number of applications, such as an optical detector with nanometer-scale resolution, ultra-tiny digital cameras, solar cells with more light absorption capability, or even genome sequencing. The near-term purpose, however, is basic science, to understand the fundamental interactions between the molecules and nanotubes.

“A large part of why we are doing this is not to invent a photo detector, but to understand the processes involved in controlling carbon nanotube devices,” explains François. “We can use a nanotube to probe single molecule transformations and study how individual molecules respond to light and change shapes.”

The next step in the LDRD is to create a nanometer-scale photovoltaic device. Such a device on a larger scale could be used as an unpowered photo detector or for solar energy. “Instead of monitoring current changes, we’d actually generate current,” says Andy. “We have an idea of how to do it, but it will be a more challenging fabrication process.

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