Scientists find quantum dots with extremely bright light emission
An international team of scientists from ETH Zurich, IBM Research Zurich, the Swiss material science institute Empa and four American research institutes has found the explanation why a class of nanocrystals, which has been extensively studied in recent years, shines in such significant light colours. The nanocrystals are those made of caesium-lead halide compounds arranged in a perovskite lattice structure.
Three years ago, Maksym Kovalenko, professor at ETH Zurich and Empa, succeeded in producing nanocrystals – or quantum dots, as they are also known – from this semiconductor material. These tiny crystals turned out to be extremely bright and fast-emitting light sources, brighter and faster than any other type of quantum dots studied so far. By varying the composition of the chemical elements and the nanoparticle size, he was also able to produce different nanocrystals that emit light in the colors of the entire visible spectrum. These quantum dots are therefore also considered as components of future light emitting diodes and displays.
In a study recently published in the journal Nature, the international research team investigated these nanocrystals in detail. The scientists were able to confirm that the nanocrystals emit light extremely quickly. Quantum dots investigated so far typically emit light at room temperature for about 20 nanoseconds after they are excited. That’s very fast. “Cesium lead halide cesium halide quantum dots, on the other hand, emit light at room temperature after only one nanosecond,” explains Michael Becker, the first author of the study. He is a PhD student at ETH Zurich and conducts his doctoral thesis at IBM Research.
As the researchers have now been able to show, the cesium-lead halide quantum dots differ from other quantum dots: With the caesium-lead halide quantum dots, the most likely excited energy state is not a “dark” state. Instead, excited electron-hole pairs are in a state from which they can immediately emit light. “This is why they shine so brightly,” says David Norris, Professor of Material Engineering at ETH Zurich.
The researchers came to this conclusion based on their new experimental data and theoretical considerations, led by Alexander Efros, a theoretical physicist at the Naval Research Laboratory in Washington. He is a pioneer in quantum dot research and 35 years ago, together with other scientists, discovered how traditional semiconductor nanocrystals work. Since the cesium-lead halide quantum dots examined are not only bright but also very cost-efficient to produce, they are suitable for use in screens. Several companies, both in Switzerland and worldwide, are involved in development work in this area. Because the quantum dots emit photons very quickly, they are also relevant for optical data communication within data centers and supercomputers. “Fast, small and efficient components are particularly important here,” says Rainer Mahrt, scientist at IBM Research. Another future application would be the optical simulation of quantum systems, which is important in basic research and materials science.
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