New materials could bring more natural lighting from LEDs

Published July 04, 2013 at
New materials could bring more natural lighting from LEDs

This story is republished with permission from Txchnologist, a digital magazine that follows innovation in science and technology.

We’re at the beginning of a new lighting revolution — the world is slowly switching over from the energy hogs of the last century to efficient light-emitting diodes (LEDs).

But the goal of making a LED bulb that produces warm, white light is still the subject of intense research. It turns out that creating materials that can emit a broad spectrum of wavelengths mimicking natural light is a tricky business.

LEDs produce light when electrons are passed through a semiconductor to meet atoms missing an electron. When they meet, the electron drops to a lower energy level and releases photons. Additives called phosphors, which glow when radiation from the LED hits them, often augment this light.

Originally capable of weak output that limited them to indicator lights and displays, LEDs have moved to the front of the pack in the race for next generation lighting because of their meager energy use, long life, low heat output and brightness.

Significantly more energy efficient than incandescent and fluorescent bulbs, LEDs can be made to produce a number of colors. And as the technology advances, engineers are starting to make composite bulbs that can produce different colors, temperatures and wattage.

Two recent advances in the technology underpinning LEDs are joining the growing field of competitors working to make light more like the sun.

Developing materials that glow in broader spectrum

Scientists from Oak Ridge and Argonne national labs and the University of Georgia are working together to develop a new group of phosphors that glow in a broader part of the electromagnetic spectrum.LumiSands cofounders Chang-Ching Tu, left, and Ji Hoo with a demo showing the warmer, softer hue of the LED bulb (left side) after a film embedded with their red-emitting silicon nanoparticles is placed underneath. The box on the right is an identical, standard LED bulb. Photo: University of Washington.

“It’s hard to get one phosphor that makes the broad range of colors needed to replicate the sun,” said John Budai, an Oak Ridge materials scientist. “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.”

They are growing and testing nanocrystals composed of europium oxide and aluminum oxide powders. Europium is a rare-earth element that has exceptional phosphorescent qualities, they say. “What’s amazing about these compounds is that they glow in lots of different colors — some are orange, purple, green or yellow,” Budai said in a release on their work. “The next question became: why are they different colors? It turns out that the atomic structures are very different.”

They are using X-rays to understand how atoms are arranged in the phosphorescent materials’ different crystal structures. Once they figure out how altering crystal growing conditions change the colors emitted, they hope to create lighting close to natural sunlight.

Their work on europium and aluminum phosphors appeared in a recent edition of the journal Advanced Functional Materials.

Looking to sand to cut costs

A company spun off from research at the University of Washington is also pursuing LEDs that produce more natural light. LumiSands also is working on another problem with the new lighting: the higher cost of LED bulbs.

A significant reason why LEDs cost more than earlier bulbs is their need for rare-earth elements to create phosphors, such as the Oak Ridge research above that uses europium. Instead, LumiSands says it is using silicon derived from sand as phosphors to convert some of the blue light emitted by standard LEDs into greens, reds and yellows.

The company’s process slices silicon wafers into nano-sized particles, which glow red when they are small enough. When these particles are used to dope the LED’s semiconductor, the lamp’s color profile changes to a warmer, softer hue. They are still developing the different phosphors and hope soon to create a mix that produces white light.

“Manufacturers could substitute traditional rare-earth elements with our material with minimal additional steps,” said Ji Hoo, LumiSands cofounder and a UW electrical engineering doctoral student, said in a university release. “It will be cheaper, better-quality lighting for users.”

Top Image: Oak Ridge National Laboratory scientists are using X-ray diffraction analysis to better understand tiny crystals that could be used in warm-white LEDs. Image courtesy ORNL.