At low energy levels, quantum dots are able to absorb light and convert it to a different color at nearly 100% efficiency. At higher excitation energies required for brighter screens and other technologies, the level of efficiency drops dramatically. The conclusions are a result of experimentation conducted by researchers at the Department of Energy’s SLAC National Accelerator Laboratory.
To make sense of the process, the researchers employed a high-speed electron camera to watch the dots turn incoming high-energy laser light into their own glowing light emissions. Experiments showed that the incoming high-energy laser light ejected electrons from the dots’ atoms, and that their corresponding holes — empty spots with positive charges that move without obstruction — become trapped at the surface of the dots, producing unwanted waste heat.
A quantum dot is excited by a green laser (top). A quantum dot is stimulated by a high-energy purple laser (bottom). Courtesy of B. Guzelturk et al., Nature Communications
The electrons and holes also recombined in a way that emitted additional heat energy. This increased the jiggling of the dots’ atoms, deformed their crustal structure, and wasted unused energy.
“This represents a key way that energy is sucked out of the system without giving rise to light,” said Aaron Lindenberg, a Stanford University associate professor and investigator with the Stanford Institute for Materials and Energy Sciences at SLAC.
Researchers have been trying to figure out the inefficiencies for decades, Lindenberg said. “This is the first time we could see what the atoms are actually doing while excited state energy is being lost as heat.”
Previous studies have focused on electron behavior, and by deploying the researchers’ approach, the movements of the whole atom were visible. While measuring the behavior of the quantum dots as they were hit with various wavelengths and intensities of light, University of California, Berkeley graduate students Dipti Jasrasaria and John Philbin worked with theoretical chemist Eran Rabani, also of UC Berkeley, to calculate and understand the resulting interplay of electronic and atomic motions from a theoretical standpoint.
“We met with the experimenters quite often,” Rabani said. “They came with a problem and we started to work together to understand it. Thoughts were going back and forth, but it was all seeded from the experiments, which were a big breakthrough in being able to measure what happens to the quantum dots’ atomic lattice when it’s intensely excited.”
The researchers determined that minimizing the excess energy of the holes is crucial to suppress energy losses associated with surface trapping. High-energy excitation in nanocrystal lasers and energetic hole injection in LEDs, they said, should be avoided to minimize that surface trapping.
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