AScribe Newswi Quantum Dots That Produce White Light Could BeLight Bulb's Successor

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Quantum Dots That Produce White Light Could Be Light Bulb's Successor

NASHVILLE, Tenn., Oct. 20 (AScribe Newswire) -- Take an LED that
produces intense, blue light. Coat it with a thin layer of special
microscopic beads called quantum dots. And you have what could become
the successor to the venerable light bulb.

The resulting hybrid LED gives off a warm white light with a slightly
yellow cast, similar to that of the incandescent lamp.

Until now quantum dots have been known primarily for their ability to
produce a dozen different distinct colors of light simply by varying
the size of the individual nanocrystals: a capability particularly
suited to fluorescent labeling in biomedical applications. But
chemists at Vanderbilt University discovered a way to make quantum
dots spontaneously produce broad-spectrum white light. The report of
their discovery, which happened by accident, appears in the
communication "White-light Emission from Magic-Sized Cadmium Selenide
Nanocrystals" published online October 18 by the Journal of the
American Chemical Society.

In the last few years, LEDs (short for light emitting diodes) have
begun replacing incandescent and fluorescent lights in a number of
niche applications. Although these solid-state lights have been used
for decades in consumer electronics, recent technological advances
have allowed them to spread into areas like architectural lighting,
traffic lights, flashlights and reading lights. Although they are
considerably more expensive than ordinary lights, they are capable of
producing about twice as much light per watt as incandescent bulbs;
they last up to 50,000 hours or 50 times as long as a 60-watt bulb;
and, they are very tough and hard to break. Because they are made in
a fashion similar to computer chips, the cost of LEDs has been
dropping steadily. The Department of Energy has estimated that LED
lighting could reduce U.S. energy consumption for lighting by 29
percent by 2025, saving the nation's households about $125 million in
the process.

Until 1993 LEDs could only produce red, green and yellow light. But
then Nichia Chemical of Japan figured out how to produce blue LEDs.
By combining blue LEDs with red and green LEDs - or adding a yellow
phosphor to blue LEDs - manufacturers were able create white light,
which opened up a number of new applications. However, these LEDs
tend to produce white light with a cool, bluish tinge.

The white-light quantum dots, by contrast, produce a smoother
distribution of wavelengths in the visible spectrum with a slightly
warmer, slightly more yellow tint, reports Michael Bowers, the
graduate student who made the quantum dots and discovered their
unusual property. As a result, the light produced by the quantum dots
looks more nearly like the "full spectrum" reading lights now on the
market which produce a light spectrum closer to that of sunlight than
normal fluorescent tubes or light bulbs. Of course, quantum dots,
like white LEDs, have the advantage of not giving off large amounts
of invisible infrared radiation unlike the light bulb. This invisible
radiation produces large amounts of heat and largely accounts for the
light bulb's low energy efficiency.

Bowers works in the laboratory of Associate Professor of Chemistry
Sandra Rosenthal. The accidental discovery was the result of the
request of one of his coworkers, post-doctoral student and electron
microscopist James McBride, who is interested in the way in which
quantum dots grow. He thought that the structure of small-sized dots
might provide him with new insights into the growth process, so he
asked Bowers to make him a batch of small-sized quantum dots that he
could study.

"I made him a batch and he came back to me and asked if I could make
them any smaller," says Bowers. So he made a second batch of even
smaller nanocrystals. But once again, McBride asked him for something
smaller. So Bowers made a batch of the smallest quantum dots he knew
how to make. It turns out that these were crystals of cadmium and
selenium that contain either 33 or 34 pairs of atoms, which happens
to be a "magic size" that the crystals form preferentially. As a
result, the magic-sized quantum dots were relatively easy to make
even though they are less than half the size of normal quantum dots.

After Bowers cleaned up the batch, he pumped a solution containing
the nanocrystals into a small glass cell and illuminated it with a
laser. "I was surprised when a white glow covered the table," Bowers
says. "I expected the quantum dots to emit blue light, but instead
they gave off a beautiful white glow."

"The exciting thing about this is that it is a nano-nanoscience
phenomenon," Rosenthal comments. In the larger nanocrystals, which
produce light in narrow spectral bands, the light originates in the
center of the crystal. But, as the size of the crystal shrinks down
to the magic size, the light emission region appears to move to the
surface of the crystal and broadens out into a full spectrum.

Another student in the lab got the idea of using polyurethane wood
finish for thin film research while working on his parent's summer
cabin. He had even brought some Minwax into the lab. That gave Bowers
the idea of mixing the magic-sized quantum dots with the polyurethane
and coating an LED. The result was a bit lumpy, but it proved that
the magic-sized quantum dots could be used to make a white light
source.

The Vanderbilt researchers are the first to report making quantum
dots that spontaneously emit white light, but they aren't the first
to report using quantum dots to produce hybrid, white-light LEDs. The
other reports - one by a group at the University of St. Andrews in
Scotland and one by a group at Sandia National Laboratories -
describe achieving this effect by adding additional compounds that
interact with the tiny crystals to produce a white-light spectrum.
The magic-sized quantum dots, by contrast, produce white light
without any extra chemical treatment: The full spectrum emission is
an intrinsic effect.

One difference between the Vanderbilt approach and the others is the
process they used to make the quantum dots, Bowers observes. The
other groups use synthesis methods that take between a week and a
month to complete; whereas, the Vanderbilt method takes less than an
hour.

A second significant difference, according to Rosenthal, is that it
should be considerably easier to use the magic-sized quantum dots to
make an "electroluminescent device" - a light source powered directly
by electricity - because they can be used with a wider selection of
binding compounds without affecting their emissions characteristics.
Other research groups have reported stimulating quantum dots to
produce light by applying an electrical current. Of course, those
produced colored light. So, one of the projects at the top of
Rosenthal's list is to duplicate that feat with magic-sized
nanocrystals to see if they will produce white light when
electrically stimulated.

The light bulb is made out of metal and glass using primarily
mechanical processes. Current LEDs are made using semiconductor
manufacturing techniques developed in the last 50 years. But, if the
quantum dot approach pans out, it could transform lighting production
into a primarily chemical process. Such a fundamental change could
open up a wide range of new possibilities, such as making almost any
object into a light source by coating it with luminescent paint
capable of producing light in a rainbow of different shades,
including white.

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CONTACT: David F. Salisbury, Vanderbilt Science and Research
Communications, 615-343-6803,