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Sandia researchers use quantum dots as a new approach to solid-state lighting...

Sandia researchers use quantum dots as a new approach to solid-state lighting... - Physics Forum

Sandia researchers use quantum dots as a new approach to solid-state lighting... - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

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Old 07-16-2003, 12:18 PM
Do Wah Ditty
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Default Sandia researchers use quantum dots as a new approach to solid-state lighting...

Contact: Chris Burroughs
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DOE/Sandia National Laboratories

Sandia researchers use quantum dots as a new approach to solid-state

ALBUQUERQUE, N.M. -- In a different approach to creating white light several
researchers at the Department of Energy's (DOE) Sandia National Laboratories
have developed the first solid-state white light-emitting device using
quantum dots. In the future, the use of quantum dots as light-emitting
phosphors may represent a major application of nanotechnology.
"Understanding the physics of luminescence at the nanoscale and applying
this knowledge to develop quantum dot-based light sources is the focus of
this work," says Lauren Rohwer, principal investigator. "Highly efficient,
low-cost quantum dot-based lighting would represent a revolution in lighting
technology through nanoscience."

The approach is based on encapsulating semiconductor quantum dots -
nanoparticles approximately one billionth of a meter in size -- and
engineering their surfaces so they efficiently emit visible light when
excited by near-ultraviolet (UV) light-emitting diodes (LEDs). The quantum
dots strongly absorb light in the near UV range and re-emit visible light
that has its color determined by both their size and surface chemistry.

This nanophosphor-based device is quite different from an alternative
approach based upon growth of blue, green, and red emitting semiconductor
materials that requires careful mixing of the those primary colors to
produce white illumination. Efficiently extracting all three colors in such
a device requires costly chip designs, which likely cannot compete with
conventional fluorescent lighting but can be attractive for more specialized
lighting applications.

Rohwer and the quantum dot team -- Jess Wilcoxon, Stephen Woessner, Billie
Abrams, Steven Thoma, and Arturo Sanchez -- started on the project
two-and-a-half years ago. Subsequently, their research has advanced
significantly, including recently reaching a major milestone of creating
white and blue lighting devices using encapsulated quantum dots.

"This accomplishment brings quantum dot technology from the laboratory
demonstration phase to a packaged component," Rohwer says.

LEDs for solid-state lighting typically emit in the near UV to the blue part
of the spectrum, around 380-420 nanometers. Conventional phosphors used in
fluorescent lighting are not ideal for solid state lighting because they
have poor absorption for these energies. So researchers worldwide have been
investigating other chemical compounds for their suitability as phosphors
for solid state lighting.

Quantum dots represent a new approach. The nanometer-size quantum dots are
synthesized in a solvent containing soap-like molecules called surfactants
as stabilizers. The small size of the quantum dots - much smaller than the
wavelength of visible light - eliminates all light scattering and the
associated optical losses. Optical backscattering losses using larger
conventional phosphors reduce the package efficiency by as much as 50

Nanophosphors based upon quantum dots have two significant advantages over
the use of conventional bulk phosphor powders. First, while the optical
properties of conventional bulk phosphor powders are determined solely by
the phosphor's chemical composition, in quantum dots the optical properties
such as light absorbance are determined by the size of the dot. Changing the
size produces dramatic changes in color. The small dot size also means that,
typically, over 70 percent of the atoms are at surface sites so that
chemical changes at these sites allow tuning of the light-emitting
properties of the dots, permitting the emission of multiple colors from a
single size dot.

"This provides two additional ways to tune the optical properties in
addition to chemical composition of the quantum dot material itself,"
Wilcoxon says.

For the quantum dots to be used for lighting, they need to be encapsulated,
usually in epoxy or silicone.

"Doing this we had to take care not to alter the surface chemistry of the
quantum dots in transition from solvent to encapsulant," says Thoma, who
worked on the encapsulation portion of the project.

Quantum dot phosphors are integrated with a commercial LED chip that emits
in the near ultraviolet at 400 nanometers by encapsulating the chip with a
dot-filled epoxy, creating a dome. The quantum dots in the dome absorb the
invisible 400 nanometer light from the LED and reemit it in the visible
region - a principle similar to that used in fluorescent lighting.

However, a key technical issue in the encapsulation process had to be solved
first. When altering the environment of the dots from a solvent to an
encapsulant, the quantum dots would "clump up" or agglomerate, causing them
to lose their light-emitting properties. By attaching the quantum dots to
the "backbone" of the encapsulating polymer they are close, but not
touching. This allows for an increase in efficiency from 10-20 percent to an
amazing 60 percent, Thoma says.

The team notes that other people working in the field of quantum dots have
reported conversion efficiencies of nearly 50 percent in dilute solutions.
However, to their knowledge, Sandia's team is the first to make an
encapsulated quantum dot device with such high efficiencies.

To date, the Sandia's quantum dot devices have largely been composed of the
semiconductor material cadmium sulfide. Cadmium is a toxic heavy metal
similar to lead so alternative nanophosphor materials are desired.
Fortunately, quantum dot phosphors can also be made from other types of
materials, including nontoxic nanosize silicon or germanium semiconductors
with light-emitting ions like mangenese on the quantum dot surface.

"Silicon, which is abundant, cheap, and non-toxic, would be an ideal
material," says Woessner. "The scientific insights gained through the team's
success with cadmium sulfide quantum dots will enable this next step in
nanophosphor development."

In the next year the researchers will increase the concentration of the
quantum dots in the encapsulant to obtain further increases in light output
while extending the understanding of quantum dot electronic interactions at
high concentrations.

While the researchers investigate the use of quantum dots as phosphors as
part of an internally funded research project, they also have a grant from
the DOE Office of Building Technologies for a collaborative project with
Lumileds Lighting, a joint venture between Agilent Technologies and Philips
Lighting. In this project they are helping Lumileds measure quantum
efficiency of light emission from various types of dots.

Jerry Simmons, who with James Gee, heads up the Sandia's Solid State
Lighting grand challenge, says the quantum dot research is an integral part
of the work at Sandia.

"We are very proud of these accomplishments," Simmons says. "The team has
come a long way in a short time."

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Sandia National Laboratories
A Department of Energy National Laboratory
Managed and Operated by Sandia Corporation
ALBUQUERQUE, NM 87185-0167
PHONE: 505-844-8066 FAX: 505-844-6367

Sandia National Laboratories' World Wide Web home page is located at
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can be found at the News and Events button.
Do Wah Ditty

"Now, there are some who would like to rewrite history - Revisionist
Historians is what I like to call them."

- Elizabeth, N.J., June 16, 2003 - President Bushisms

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