Tuesday, July 25, 2006

Nanoparticle Infrared Detector Is Ultrasensitive, Cheap

These chips could lead to highly sensitive night-vision goggles and new medical imaging devices.

By Katherine Bourzac

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.