Canadian researchers have developed an inexpensive and highly sensitive infrared chip that could improve night-vision goggles and medical imaging. Made by spin-coating a glass slide or silicon chip with a solution of conducting nanoparticles called quantum dots, the detector is 10 times more sensitive than traditional infrared detectors.
The team that designed the chip is led by Edward Sargent, who holds the Canada research chair in nanotechnology at the University of Toronto. The chip picks up the near and short-wave infrared (SWIR) bands. SWIR light is abundant at night, even when it’s cloudy or moonless. In such conditions, conventional night-vision goggles, which work by amplifying star light from the redder near-infrared band, are ineffectual.
SWIR light detection might also be integrated into medical imaging technologies, Sargent says, because SWIR light passes easily through tissue. But silicon cannot absorb SWIR light, which has a wavelength of just one to two microns, so SWIR-detecting technologies have been too expensive to come into wider use.
Existing high-precision SWIR detectors are made up of two chips bonded together. One is a SWIR absorber made of three atoms in a combination called InGaAs (indium gallium arsenide). The other chip is made of silicon crystals. These detectors are expensive because the two chips are joined with about 100,000 metal connections and it’s difficult to align the silicon crystals in one chip with the InGaAs crystals in the other. “You pay for a low yield,” says Sargent.
Sargent’s chip is what’s called a solution-processed electronic device. A drop of solution containing semiconductors, whether quantum dots or larger organic molecules, is placed on a conductive surface. Quantum dots are semiconducting crystals only a few nanometers across. The chip is spun to distribute the solution, then dried and chemically treated, leaving an even layer of quantum dots. Sargent’s solution includes lead sulfide nanoparticles (measuring four nanometers) and an oily molecule to keep them from clumping together. In a recent Nature paper, Sargent described work in which he demonstrated the detector using a glass slide with strips of gold electrodes as a substrate. It can also be made on a silicon chip.
John Joannopoulos, director of MIT’s Institute for Soldier Nanotechnologies, says “Sargent’s approach bridges the gap between costly, high-precision detectors” and detectors that are cheap but whose “performance isn’t up to par.”
Sargent’s chip has a sensitivity ten times that of InGaAs chips and a strong signal. Quantum dots can be fine-tuned to determine the wavelengths of light with which they interact; Sargent’s are very good at absorbing the infrared, which is why his chip outperformed other solution-processed devices. He also took advantage of what are called photoconductive gains: while each photon striking the chip can excite only one electron, Sargent was able to make each excited electron flow through the device several times before it lost energy, boosting the current.
Sargent says infrared cameras based on InGaAs chips now cost $40,000 to $60,000, whereas his technology could lead to much cheaper cameras. The cost of coating a square meter with the quantum dot solution is $17, he says, and speculates that infrared cameras might one day cost as little as today’s digital cameras.
Such a dramatic cost reduction could be a boon for those developing infrared imaging for medical diagnosis. Infrared imaging should be able to penetrate centimeters into tissue. Like X-ray imaging, it could be used for medical diagnosis. Researchers at MIT and Harvard University, for example, have developed an infrared imaging technology to detect cancer during surgery. The infrared rays, which have lower energy than visible light, pose no risk to surgeons and nurses and could help ensure that all of a tumor is removed. This technology uses the near-infrared; Sargent says tissue has even greater transparency to SWIR light. (The Massachusetts researchers are led by John Frangioni, associate professor of medicine at Boston’s Beth Israel Deaconess Medical Center and Moungi Bawendi, MIT chemistry professor and a pioneer in quantum dot research.)
The chips could also be integrated into night-vision goggles, likely at the same price as existing near-infrared detectors. According to Sargent, they shouldn’t come with higher manufacturing costs or require more power. In contrast, InGaAs chips are too expensive for widespread use and require extra battery power to keep them cool.
But whatever its future uses, says Peter Peumans, assistant professor of electrical engineering at Stanford University, Sargent’s device “confirms that nanostructuring is a way to improve device performance.”
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