Scientists at Oak Ridge National Laboratory are reportedly using x-ray diffraction analysis to help understand tiny crystals that could be used in warm white LEDs. The team's most recent study is published as the inside front cover article in the April 25 issue of Advanced Functional Materials.

The researchers note that developing an LED that emits a broad spectrum of warm white light on par with sunlight has proven difficult. Conventional White LEDs produce light by passing electrons through a semiconductor material, coupled with phosphors that glow when excited by radiation from the LED.

"It's hard to get one phosphor that makes the broad range of colors needed to replicate the sun," commented John Budai, a scientist in ORNL's Materials Science and Technology division. "One approach to generating warm-white light is to hit a mixture of phosphors with ultraviolet radiation from an LED to stimulate many colors needed for white light."

Budai is 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 produce 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.

"What's amazing about these compounds is that they glow in lots of different colors—some are orange, purple, green or yellow," Budai said. "The next question became: why are they different colors? It turns out that the atomic structures are very different."

Budai and the other scientists have used x-rays from Argonne's Advanced Photon Source to studying the atomic structure of the materials. Budai says 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.

"Only the green ones were a known crystal structure," Budai said. "The other two, the yellow and blue, don't grow in big crystals; they only grow with these atomic arrangements in these tiny nanocrystals. That's why they have different photoluminescent properties."

X-ray diffraction analysis is helping the scientists figure out the arrangement of the atoms in each of the different crystal types. The different-colored phosphors exhibit distinct diffraction patterns when they are hit with x-rays, depending on the crystal structure. So the diffraction patterns can be used to analyze the crystal structures.

"What that means in terms of how the electrons around the atoms interact to make light is much harder," Budai said. "We haven't completely solved that yet. That's the continuing research. We have a lot of clues, but we don't know everything."

The atomic-scale analysis is helping the research team improve the phosphorescent crystals. Different factors in the growth process such as 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. The scientists note that improving the material's luminescence efficiency is key to making it useful for commercial LED products and other applications.

Budai concluded, "You can keep growing the crystals and measuring them, or you can understand why it's doing what it's doing, and figure out how to make it better. That's what we're doing—basic research. We have to figure out nature first."