Minuscule crystals that glow different colors may be the missing ingredient for white LED lighting that illuminates homes and offices as effectively as natural sunlight.
Light-emitting diodes, better known as LEDs, offer substantial energy savings over incandescent and fluorescent lights and are easily produced in single colors such as red or green commonly used in traffic lights or children's toys. Developing an LED that emits a broad spectrum of warm white light on par with sunlight has proven tricky, however. LEDs, which produce light by passing electrons through a semiconductor material, often are coupled with materials called phosphors that glow when excited by radiation from the LED. Finding a phosphor that can produce the broad range of colors necessary to replicate the sun has been difficult, however.
John Budai, a scientist in Oak Ridge National Laboratory’s (ORNL's) Materials Science and Technology division, has been working with a team of scientists from University of Georgia and Oak Ridge and Argonne national laboratories to understand a new group of crystals that might yield the right blend of colors for white LEDs as well as other uses. Zhengwei Pan's group at UGA grew the nanocrystals using europium oxide and aluminum oxide powders as the source materials because the rare-earth element europium is known to be a dopant, or additive, with good phosphorescent properties.
The new crystals glow in different colors such as orange, green, purple and yellow, which the researchers discovered was due to their atomic structures. Budai studied the atomic structure of the materials using x-rays from Argonne's Advanced Photon Source and discovered that two of the three types of crystal structures in the group of phosphors had never been seen before, which can probably be attributed to the crystals' small size. The green crystals exhibited a known crystal structure; the yellow and blue crystals, however, don’t grow in big crystals. Because they only grow as tiny nanocrystals with different atomic structures, they have different photoluminescent properties.
X-ray diffraction analysis is helping Budai and his collaborators work out how the atoms are arranged in each of the different crystal types. The different-colored phosphors exhibit distinct diffraction patterns when they are hit with xrays, enabling researchers to analyze the crystal structure.
The knowledge gained through their atomic-scale analysis is helping the research team improve the phosphorescent crystals. Different factors in the growth process—temperature, powder composition, and types of gas used— can change the final product. A fundamental understanding of all the parameters could help the team to perfect the recipe and improve the crystals' ability to convert energy into light.
Advancing the material's luminescence efficiency is key to making it useful for commercial LED products and other applications; the new nanocrystals may turn out to have other practical photonic uses beyond phosphors for LEDs. Their ability to act as miniature "light pipes" when the crystal quality is high enough could lend them to applications in fiber-optic technologies.
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