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  • ‘Magic Size’ Crystals Prepared for LED Lighting

Photonics Spectra
Dec 2005
Anne L. Fischer

According to the US Department of Energy’s Technology Roadmap, half of the money spent on electricity in the country goes to lighting. With the goal of cutting electricity use in half by 2025, more attention is turning to the use of LEDs as sources of solid-state lighting. However, the design and manufacture of the devices have faced many challenges.

"Magic size" quantum dots, shown here
in a glass flow tube, produce white light when illuminated with an ultraviolet laser
beam. Courtesy of Daniel Dubois, Vanderbilt University.

One is self-absorption, which occurs when emitters of multiple colors are employed to make a white LED. Thus, using a single material to achieve white-light emission would have many advantages.

Now a group at Vanderbilt University in Nashville, Tenn., has demonstrated white light from CdSe nanocrystals. The tiny crystals display broadband emission of 420 to 710 nm and are not subject to absorption problems. The scientists used pyrolytically grown “magic size” nanocrystals produced at high temperatures, rather than ones etched from larger nanocrystals.

The emission colors of these nanocrystals can be tuned by controlling their size. One advantage of the crystals, explained researcher Sandra J. Rosenthal, is that they have a Stokes shift of about 50 nm, which means that they peak farther into the red.

Her colleague James R. McBride said that the Stokes shift, which is the degree of separation between the absorption of the material and its emission, reduces the chance of one nanocrystal absorbing the light emitted from a neighboring crystal, which would reduce the brightness.

Another advantage is the broad emission over the visible spectrum, which is the result of the crystals’ extreme surface-to-volume ratio. This ratio forces an electron and a hole to interact at the surface of the nanocrystals.

Although these nanocrystals seem to be an excellent material for use in solid-state lighting, the researchers intend to conduct further studies to determine why they are white when lighted with a laser. By understanding this, they can work to enhance this process to increase the crystals’ brightness and efficiency.

The group also is especially interested in the nanocrystal/molecule transition, McBride said, because it is not well-known at what point a cluster of atoms or a molecule begins to behave as a nanocrystal.

Journal of the American Chemical Society, online Oct. 18, 2005, doi:10.1021/ja055470d.

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