Blue-glowing Quantum Dots for Use in Next-Gen LEDs

Article By : Gary Hilson

Researchers develop quantum dots that could lead to brighter, sharper TV screens...

If one day you buy a new TV with even brighter, more pristine colors, then you can thank the “bright” people at the Canadian Light Source (CLS) in Saskatoon, Saskatchewan.

Researchers at the facility based at the University of Saskatoon have heard all the puns before. It’s one of the largest science projects in Canada’s history. The facility is Canada’s first national synchrotron, a football field-sized building that produces “really bright lights” for all types of research. The national research facility produces millions of times brighter light sources than even the sun.

The X-rays produced can be used for imaging spectroscopy, diffraction and scattering, and a wide variety of materials challenges. More than 1,000 scientists from around the world use the CLS to conduct health, agricultural, environmental, and advanced materials research.

The work of University of Toronto researcher Yitong Dong and his collaborators falls under the latter category and explores the behavior of quantum dots, which are nanocrystals that glow. The goal has been to develop blue-glowing quantum dots, as they are crucial for creating a full range of color, as they could aid in the development of next-generation LEDs. Today’s conventional LEDs in TV screens produce white light that is filtered to achieve desired colors, but the process leads to less bright and muddier colors.

You might say blue-glowing quantum dots are the Holy Grail in that if you have a blue LED, you have everything, said Dong in a telephone interview with EE Times. “We can always downconvert the light from blue to green and red,” says Dong. “Let’s say you have green, then you cannot use this lower-energy light to make blue.” Getting the blue pixels is more challenging, he said, because of the high energy of their photons, he said. “You have to put more energy into the material to get more photons, and these materials are not well established. That’s why our researchers and scientists put more efforts on blue LEDs.”

University of Toronto researcher Yitong Dong and his collaborators leveraged the facilities of the CLS to explore behavior of quantum dots — which are nanocrystals that glow — to build better LEDs. (Image source: Canadian Light Source)

Long term, the potential practical outcome of this research is going to be better TV screens, although there are limits to what the human eye can appreciate, said Dong. “There are boundaries. Human eyes are able to differentiate about 16 million different colors. You can tell difference between deep red and light red, and deep blue and light blue.” Today’s screens and monitors are designed to reproduce the colors seen in the natural world, he said, but they aren’t doing such a good job. “It’s a tricky thing.”

Dong and his collaborators have been able develop quantum dots that produce green light at an external quantum efficiency (EQE) of 22% and blue at 12.3%. The theoretical maximum efficiency is not far off at 25%, and this is the first blue perovskite LED reported as achieving an EQE higher than 10%. Researchers were able to verify the structures achieved in the team’s quantum dot films by using techniques on the HXMA beamline at the CLS so they could validate their results and helped clarify what the structural changes achieve in terms of LED performance.

HXMA stans for Hard X-ray Micro-Analysis; it’s a multipurpose hard X-ray beamline, based on a 63-pole superconducting wiggler. It has been designed to provide the research community with X-ray absorption fine structure (XAFS), which is a specific structure observed in X-ray absorption spectroscopy (XAS). By analyzing the XAFS, information can be acquired on the local structure and on the unoccupied local electronic states.

The researchers use a special surface structure to stabilize the quantum dot and compared to the films made with long chain molecules capped quantum dots, their film has 100 times higher conductivity, sometimes even 1000 times higher, said Dong.

This is just one example of the work being done at CLS using its various beamline stations, said Adam Leontowich, an associate scientist in Material and Chemical Sciences at the CLS. His role at CLS is to help researchers make use of the eight beamlines dedicated to material sciences.

Blue-glowing quantum dots, are crucial for creating a full range of color and could aid in the development of next-generation LEDs. (Image source: Canadian Light Source)

“Advanced materials” is broad, he said, but common ones are battery cathode and anode materials and superconductor candidates. Some of the work is being done by researchers at universities — including Dong — but the facility also supports corporate clients including large technology companies such as IBM, as they can justify the budget to build their own synchrotron, said Leontowich. “Producing X-rays is very energy intensive and to produce really bright X-rays requires a synchrotron. They’re multimillion dollar facilities.”

In some cases, researchers have a specific problem and tightly focused experiment that has them using the CLS facilities for short periods, he said, while others have longer-term, ongoing projects, even as plans are in the works for a new synchrotron. “Everything electronic goes through changes. Our facility has been in operation for over 15 years now. Some people have been with us from the very beginning and they continue to be involved in the next phase of the Canadian Light Source.”

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