Potential Energy-Efficient Successor to the Light Bulb
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.
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.
Click through to the Vanderbilt press release. Read a classic tale of a one lab associate asking another to see how small he could make quantum dots, again and again, till they got down to crystals determined by grouping pairs of atoms — and leading to further experiments by mixing the “magic-sized” quantum dots with some Minwax polyurethane [someone else was using in thin film experiments] and coating an LED to produce a usable light source.
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.
------------
One difference between the Vanderbilt approach and the others is the process they used to make the quantum dots, Bowers observes. They 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.
The class of lighting sources these belong to produce twice as much light per watt consumed as an ordinary lightbulb — and should last 50 times longer.
Posted: Sat - October 22, 2005 at 05:20 PM