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Ontario Technoblog

Ontario Emperor technology blog.

This blog has been superseded by the mrontemp blog
Location: Ontario, California, United States

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Monday, October 24, 2005

White Light Generator (Michael Bowers, not Ladytron)

From Live Science:

Accidental Invention Points to End of Light Bulbs
By Bjorn Carey

The main light source of the future will almost surely not be a bulb. It might be a table, a wall, or even a fork....

Michael Bowers, a graduate student at Vanderbilt University, was just trying to make really small quantum dots, which are crystals generally only a few nanometers big....

Quantum dots contain anywhere from 100 to 1,000 electrons. They're easily excited bundles of energy, and the smaller they are, the more excited they get. Each dot in Bower's particular batch was exceptionally small, containing only 33 or 34 pairs of atoms.

When you shine a light on quantum dots or apply electricity to them, they react by producing their own light, normally a bright, vibrant color. But when Bowers shined a laser on his batch of dots, something unexpected happened.

"I was surprised when a white glow covered the table," Bowers said. "The quantum dots were supposed to emit blue light, but instead they were giving off a beautiful white glow."

Then Bowers and another student got the idea to stir the dots into polyurethane and coat a blue LED light bulb with the mix. The lumpy bulb wasn't pretty, but it produced white light similar to a regular light bulb....

The new device gives off a warm, yellowish-white light that shines twice as bright and lasts 50 times longer than the standard 60 watt light bulb.

This work is published online in the Oct. 18 edition of the Journal of the American Chemical Society....

If the new process can be developed into commercial production, light won't come just from newfangled bulbs. Quantum dot mixtures could be painted on just about anything and electrically excited to produce a rainbow of colors, including white....

From Vanderbilt University:

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....

[Michael] 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. “The quantum dots were supposed to emit blue light, but instead they were giving 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.

For more information, see

J. Am. Chem. Soc., ASAP Article 10.1021/ja055470d S0002-7863(05)05470-3
Web Release Date: October 18, 2005

Copyright © 2005 American Chemical Society
White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals

Michael J. Bowers II, James R. McBride, and Sandra J. Rosenthal*

Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235

Received August 22, 2005


Magic-sized cadmium selenide (CdSe) nanocrystals have been pyrolytically synthesized. These ultra-small nanocrystals exhibit broadband emission (420-710 nm) that covers most of the visible spectrum while not suffering from self absorption. This behavior is a direct result of the extremely narrow size distribution and unusually large Stokes shift (40-50 nm). The intrinsic properties of these ultra-small nanocrystals make them an ideal material for applications in solid state lighting and also the perfect platform to study the molecule-to-nanocrystal transition.



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