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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.
By Katherine Bourzac
Read article at techreview.com