Nanoparticle Infrared Detector Is Ultrasensitive, Cheap
These chips could lead to highly sensitive night-vision goggles and new medical imaging devices.
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.
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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.”