Kim Kisslinger, a researcher at Brookhaven Lab's Center for Functional Nanomaterials seen here with a focused-ion beam instrument, reduced the indium gallium nitride (InGaN) samples to a thickness of just 20 nanometers to prepare them for electron microscopy. | Photo courtesy of Brookhaven National Laboratory.
Scientists build their careers on unquenchable curiosity and a constant quest for bright ideas. That’s true for researchers at the Energy Department's National Labs, especially as a couple of their recent studies on light-emitting diodes (LEDs) show.
LEDs are the blink-blinks in traffic lights and children’s toys. They also form the images on flat television screens, the numbers on digital clocks and have a wide range of other uses. However, they also have real limitations that Energy Department researchers are trying to overcome: They could be even more efficient, and they only shine in single colors.
In principle, LEDs could be nearly 100 percent efficient at turning their energy into visible light. In practice, they’re somewhere near 20 percent, which is still far greater than the five percent of conventional incandescent bulbs. To better understand the source of those efficiencies -- and hopefully improve them further -- a team of researchers at Brookhaven National Lab and the Massachusetts Institute of Technology (MIT) took a close look at indium gallium nitride (InGaN), a material used in many LEDs.
InGaN is an alloy, a mix of metals, and scientists had seen clusters of indium within it, which they suspected might contribute to its useful properties. However, other scientists disagreed, suggesting that the indium-rich clusters were instead artifacts, artificial structures created by the electron microscope used in previous studies.
To settle the question, scientists turned to the outstanding capabilities of Brookhaven Lab’s Center for Functional Nanomaterials. Using the Center’s aberration-corrected scanning transmission electron microscope coupled with advanced imaging techniques, they found that the indium-rich clusters were experimental artifacts, rather than inherent structures of the material. The scientist’s insights may show the way to making even more efficient LEDs, especially since their methods could be applied to studying many other materials.
At the same time, a team of researchers at Oak Ridge and Argonne National Laboratories and the University of Georgia have been studying ways to, in a sense, invert a rainbow; to turn the single hues of LEDs into a warm white light similar to sunlight. White light appears colorless, but it is actually a combination of colors. This may seem a bit abstract, but the proof could be hanging in a nearby window: Prisms separate white light into its component rainbow, which makes them popular household ornaments.
However, making LEDs that glow in many hues is difficult. Materials called phosphors are often embedded near LEDs to achieve specific-colored glows, and so the research team took it a step further. They grew a new family of phosphorescent crystals containing atoms of the elements europium (Eu), aluminum (Al) and oxygen (O) that shine in a wide range of specific colors -- blue and orange and green and others. They then used the intense x-rays at the Advanced Photon Source at Argonne Lab to take a close look at how and why the crystals glow.
Scientists found that tiny, atomic level, differences caused the crystals to glow in all different hues. They also discovered that growing the crystals with different ingredients (changing the recipe beyond EuAlO) altered the colors that they threw.
These insights could lead to LED phosphors that not only throw something closer to sunlight, but are also far more efficient at both creating and carrying light inside long, nanowire crystals. And that, in turn, could eventually lead to a variety of other applications, such as in fiber-optic technologies.
That’s the Energy Department at its best: A constant quest for bright ideas … especially when it comes to building better LEDs.