Scientists have discovered why a certain class of nanocrystals — caesium lead halide quantum dots — shines in such bright colors. They have also confirmed that these nanocrystals emit light extremely quickly. Previously studied quantum dots typically emit light about 20 ns after excitation at room temperature. Experiments showed that caesium lead halide quantum dots emit light at room temperature after just 1 ns. “These tiny crystals have proved to be extremely bright and fast emitting light sources, brighter and faster than any other type of quantum dot studied so far,” said professor Maksym Kovalenko of ETH Zurich and Empa. A cesium lead bromide nanocrystal under the electron microscope (crystal width: 14 nm). Individual atoms are visible as points. Courtesy of ETH Zürich/Empa/Maksym Kovalenko. Kovalenko, who created quantum dots containing caesium lead halide, found that by varying the composition of the chemical elements and the size of the nanoparticles, he could produce a variety of nanocrystals that light up in the colors of the entire visible spectrum. Now, a research team from ETH Zurich, Empa, IBM Research Zurich and four U.S. research institutions, in a study into how caesium lead halide quantum dots differ from other QDs, has shown that the lowest exciton in caesium lead halide perovskites involves a highly emissive triplet state. First the team used an effective-mass model and group theory to demonstrate the possibility of such a state existing. Then, it applied this model to caesium lead halide perovskites and measured size- and composition-dependent fluorescence at the single-nanocrystal level. The team found that the bright triplet character of the lowest exciton caused the anomalous photon-emission rates of caesium lead halide perovskites, and that these materials emitted about 20 and 1000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. Researchers analyzed the fine structure in low-temperature fluorescence spectra to confirm the existence of this bright triplet exciton. A sample with several green glowing perovskite quantum dots excited by a blue laser. Courtesy of IBM Research/Thilo Stoeferle. In many materials, the most likely excited energy state is a dark state, where light emission is delayed and suppressed. The team showed that unlike many materials, the most likely excited energy state for caesium lead halide quantum dots is not a dark state. “This is the reason that they shine so brightly,” said David Norris, who is a professor of materials at ETH Zurich. The caesium lead halide quantum dots are not only bright but also inexpensive to produce and could be used in television displays. Another future application could be the optical simulation of quantum systems. “Also, as these quantum dots can rapidly emit photons, they are of particular interest for use in optical communication within data centers and supercomputers, where fast, small and efficient components are central,” said IBM researcher Rainer Mahrt. Researchers are also interested in applying their results to the development of new materials. The research provides criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices. “As we now understand why these quantum dots are so bright, we can also think about engineering other materials with similar or even better properties,” said Norris. The research was published in Nature (doi:10.1038/nature25147).