Bright semi­conductor nanocrystals – quantum dots – give QLED TV screens their vibrant colors. But attempts to increase the intensity of that light generate heat instead, reducing the dots' light-producing effi­ciency. A new study explains why, and the results have broad impli­cations for developing future quantum and photonics techno­logies where light replaces electrons in computers and fluids in refri­gerators, for example.

In a QLED TV screen, dots absorb blue light and turn it into green or red. At the low energies where TV screens operate, this conversion of light from one color to another is virtually 100 % efficient. But at the higher exci­tation energies required for brighter screens and other techno­logies, the effi­ciency drops off sharply. Researchers had theories about why this happens, but no one had ever observed it at the atomic scale until now. To find out more, scientists at the Department of Energy's SLAC National Acce­lerator Labora­tory used a high-speed “electron camera” to watch dots turn incoming high-energy laser light into their own glowing light emissions.

The experiments revealed that the incoming high-energy laser light ejects electrons from the dot's atoms, and their corresponding holes become trapped at the surface of the dot, producing unwanted waste heat. In addition, electrons and holes recombine in a way that gives off addi­tional heat energy. This increases the jiggling of the dot's atoms, deforms its crystal structure and wastes even more energy that could have gone into making the dots brighter.

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

Despite their tiny size quantum dot nano­crystals are sur­prisingly complex and highly engineered. They emit extremely pure light whose color can be tuned by adjusting their size, shape, compo­sition and surface chemistry. The quantum dots used in this study were invented more than two decades ago, and today they're widely used in bright, energy-effi­cient displays and in imaging tools for biology and medicine.

Understanding and fixing problems that stand in the way of making dots more efficient at higher energies is a very hot field of research right now, post­doctoral researcher Burak Guzelturk, who carried out experiments at SLAC with post­doctoral researcher Ben Cotts. Previous studies had focused on how the dots' electrons behaved. But in this study, the team was able to see the movements of whole atoms, too, with an electron camera known as MeV-UED. It hits samples with short pulses of electrons with very high energies, measured in millions of electron­volts (MeV). By ultrafast electron diffrac­tion (UED), the electrons scatter off the sample and into detectors, creating patterns that reveal what both electrons and atoms are doing.

As the team measured the behavior of quantum dots that had been hit with various wavelengths and intensities of laser light, UC Berkeley graduate students Dipti Jasrasaria and John Philbin worked with Berkeley theoretical chemist Eran Rabani 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.”

“This work is exciting because it provides an unpre­cedented lens on the electronic and thermal processes that limit the light emission efficiency. The particles studied already have record quantum yields, but now there is a path toward designing almost-perfect optical materials”, said Jennifer Dionne, a Stanford associate professor of materials science and engi­neering. Such high light emission efficiencies could open a host of big futuristic appli­cations, all driven by tiny dots probed with ultrafast electrons. (Source SLAC)

Reference: B. Guzelturk et al.: Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals, Nat. Commun. 12, 1860 (2021); DOI: 10.1038/s41467-021-22116-0

Link: Dept. of Materials Science and Engineering, Stanford University, Stanford, USA