Depiction of chromophores attaching to a transistor made from a single carbon nanotube. Credit SNL.
Research being conducted at Sandia National Lab might eventually be applied to an optical detector with nanometer-scale resolution, ultra-tiny digital cameras, solar cells with more light absorption capability and a better device for genome sequencing. However, the near-term purpose of the research is basic science.
The Sandia researchers report they have created the first carbon nanotube device that can detect the entire visible spectrum of light. This might allow them to study single-molecule transformations, how the molecules respond to light and change shape as well as other fundamental interactions between molecules and nanotubes.
As with many other recent studies, the researchers went back to nature, in this case the human eye, and they improved on the model. A cascade of chemical and electrical events that ultimately trigger nerve impulses occur when light strikes a chromophore on the molecules in the eye’s retina. Likewise, when light strikes a chromophore in the nanoscale color detector, it causes a conformational change in the molecule. This, 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 Sandia researcher Xinjian Zhou. “We placed the nanotube transistor behind the molecule – a more efficient design.”
That carbon nanotubes are light sensitive has been known for a long time, but earlier efforts using an individual nanotube were only able to detect light in narrow wavelength ranges, and then only at laser intensities. The Sandia team nanodetector is orders of magnitude more sensitive, down to about 40 W/m2, which is 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,” says Zhou.
Zhou and his colleagues François Léonard, Andy Vance, Karen Krafcik, Tom Zifer and Bryan Wong created the device, which they described in a paper published in Nano Letters. Zhou and Krafcik created a tiny transistor made from a single carbon nanotube. They deposited carbon nanotubes on a silicon wafer and used photolithography to define electrical patterns to make contacts. Meanwhile, Vance and Zifer synthesized molecules to create three types of chromophores that respond to either red, green or orange bands of the visible spectrum. Zhou immersed the wafer in the dye solution until the chromosphores attached themselves to the nanotubes.
“Detection is now limited to about 3 percent of sunlight, which isn’t bad compared with a commercially available digital camera,” says Zhou. “I hope to add some antennas to increase light absorption.”
The team is now working 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 Vance. “So we’re in the process of looking for dyes that work in infrared.”
“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,” says Léonard, author of The Physics of Carbon Nanotubes, published September 2008.
The next step 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 Vance. “We have an idea of how to do it, but it will be a more challenging fabrication process.”