Scientists are discovering a process that stands in the way of making quantum dots brighter

Researchers at SLAC and Stanford made the first observations on the atomic scale of how nanocrystals known as quantum dots lose their light-creating efficiency when excited by intense light. The dots were excited by a green light (above) or more energy by a purple light (below), and the scientists observed them corresponding to an “electronic camera,” MeV-UED. When hit by the green light, the dots relaxed and the excited pairs of electrons and holes converted almost all the incoming energy into light. But when struck by the purple light, part of the energy is trapped on the surface of the point; this disrupted the arrangement of the surrounding atoms and wasted energy as heat. The results have broad implications for the development of future quantum and photon technologies where light replaces electrons in computers and liquids in refrigerators. Credit: B. Guzelturk et al., Nature Communications, March 25, 2021

Bright semiconductor nanocrystals known as quantum dots give QLED TV screens their vivid colors. But attempts to increase the intensity of that light generate heat, reducing the efficiency of light production at points.

A new study explains why, and the results have broad implications for the development of future quantum and photon technologies where light replaces electrons in computers and liquids in refrigerators, for example.

On the QLED TV screen, dots absorb blue light and turn it green or red. At the low energies at which TV screens operate, this conversion of light from one color to another is practically 100% efficient. But with the higher excitation energies needed for brighter screens and other technologies, efficiency drops sharply. Researchers had theories as to why this was happening, but no one had ever noticed it on the atomic scale.

To find out more, scientists from the National SLAC Acceleration Laboratory at the Department of Energy used a high-speed “electronic camera” to observe the points that convert incoming high-energy laser light into their incandescent light emissions.

Experiments have revealed that incoming high-energy laser light expels electrons from the point’s atoms, and their corresponding holes – empty places with free-moving positive charges – become trapped on the surface of the point, creating unwanted waste heat.

In addition, electrons and holes recombine in a way that provides additional thermal energy. This increases the mixing of the dot atoms, deforms its crystal structure and consumes even more energy that could be invested in making the dots brighter.

“This is a key way energy is sucked out of the system without generating light,” said Aaron Lindenberg, an associate professor and researcher at Stanford University at the Stanford Institute of Materials and Energy Sciences at SLAC who led the study with postdoctoral researcher Burak Guzelturk.

“Trying to figure out what underlies this process has been the subject of study for decades,” he said. “This is the first time we’ve been able to see what atoms actually do as the energy of the excited state is lost as heat.”

The research team, which included scientists from SLAC, Stanford, the University of California, Berkeley and the Lawrence Berkeley National Laboratory from DOE, described the results in Nature Communications today.

Emitting a clean, brilliant glow

Despite their small size – they are about the same diameter as four strands of DNA – quantum dot nanocrystals are surprisingly complex and highly designed. They emit extremely pure light whose color can be adjusted by adjusting their size, shape, composition and surface chemistry. The quantum dots used in this study were invented more than two decades ago and are widely used today in bright, energy-efficient displays and in imaging tools for biology and medicine.

Understanding and solving problems that stand in the way of points becoming more efficient at higher energies is currently a very hot area of ​​research, said Guzelturk, who conducted experiments with postdoctoral researcher Ben Cotts at SLAC.

Previous studies have focused on how point electrons behaved. But in this study, the team was able to see the motion of entire atoms, using an electronic camera known as MeV-UED. It hits samples with short pulses of electrons with very high energies, measured in millions of electron volts (MeV). In a process called ultrafast electron diffraction (UED), electrons scatter from the sample into detectors, creating patterns that reveal what both electrons and atoms are doing.

While the SLAC / Stanford team measured the behavior of quantum dots affected by different wavelengths and intensities of laser light, UC Berkeley graduate students, Dipti Jasrasaria and John Philbin, worked with Berkeley’s theoretical chemist Eran Rabani to calculate and understand the resulting results. electronic and atomic motions from a theoretical point of view.

“We met with the experimenters quite often,” Rabani said. “They came up with a problem and we started working together to understand it. Thoughts went back and forth, but it all came from experiments, which were a big breakthrough in being able to measure what happens to atomic quantum lattice points when very excited. “

The future of light-based technology

The study was conducted by researchers at the DOE Energy Frontier Research Center, Photonics at Thermodynamic Limits, led by Jennifer Dionne, associate professor of materials science and engineering at Stanford and senior associate vice conductor of research platforms / common facilities. Her research team collaborated with Lindenberg’s group to help develop an experimental technique for probing nanocrystals.

The ultimate goal of the center, Dionne said, is to demonstrate photonic processes, such as light absorption and emission, at the limits of what thermodynamics allows. This could lead to technologies such as cooling, heating, cooling and energy storage – as well as quantum computers and new space exploration engines – that are fully powered.

“To create photon thermodynamic cycles, you need to precisely control how light, heat, atoms and electrons interact in materials,” Dionne said. “This work is exciting because it provides an unprecedented lens on electronic and thermal processes that limit the efficiency of light emission. The particles being studied already have record quantum yields, but there is now a way to design near-perfect optical materials.” Such high light emission efficiency could open up a multitude of large futuristic applications, all powered by tiny dots probed by ultrafast electrons.

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More information:
Burak Guzelturk et al., Dynamic lattice distortions caused by surface capture in semiconductor nanocrystals, Nature Communications (2021). DOI: 10.1038 / s41467-021-22116-0

Provided by SLAC National Acceleration Laboratory

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