Quantum Dots Emit Extremely Bright Light

A caesium lead bromide nanocrystal under the electron microscope. Individual atoms are visible as points. (Source: M. Kovalenko, ETHZ / Empa)

An inter­national team of researchers from ETH Zurich, IBM Research Zurich, Empa and four American research insti­tutions have found the expla­nation for why a class of nano­crystals that has been intensively studied in recent years shines in such incre­dibly bright colours. The nano­crystals contain caesium lead halide compounds that are arranged in a perov­skite lattice structure.

Three years ago, Maksym Kovalenko succeeded in creating nano­crystals from this semi­conductor material. “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,” says Kovalenko. By varying the compo­sition of the chemical elements and the size of the nano­particles, he also succeeded in producing a variety of nano­crystals that light up in the colours of the whole visible spectrum. These quantum dots are thus also being treated as components for future light-emitting diodes and displays.

Now the team examined these nano­crystals indi­vidually and in great detail. The scientists were able to confirm that the nano­crystals emit light extremely quickly. Previously-studied quantum dots typically emit light around 20 nano­seconds after being excited when at room tempera­ture, which is already very quick. “However, caesium lead halide quantum dots emit light at room tempera­ture after just one nano­second,” explains Michael Becker. He is a doctoral student at ETH Zurich and is carrying out his doctoral project at IBM Research.

Under­standing why caesium lead halide quantum dots are not only fast but also very bright entails diving into the world of individual atoms, photons and electrons. “You can use a photon to excite semi­conductor nano­crystals so that an electron leaves its original place in the crystal lattice, leaving behind a hole,” explains David Norris, Professor of Materials Engi­neering at ETH Zurich. The result is an electron-hole pair in an excited energy state. If the electron-hole pair reverts to its energy ground state, light is emitted.

Under certain condi­tions, different excited energy states are possible; in many materials, the most likely of these states is called a dark one. “In such a dark state, the electron hole pair cannot revert to its energy ground state imme­diately and therefore the light emission is suppres­sed and occurs delayed. This limits the brightness”, says Rainer Mahrt, a scientist at IBM Research. The researchers were able to show that the caesium lead halide quantum dots differ from other quantum dots: their most likely excited energy state is not a dark state. Excited electron-hole pairs are much more likely to find themselves in a state in which they can emit light imme­diately. “This is the reason that they shine so brightly,” says Norris.

The researchers came to this conclusion using their new experi­mental data and with the help of theoretical work 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, was among the first scientists to explain how traditional semi­conductor quantum dots function.

As the examined caesium lead halide quantum dots are not only bright but also inex­pensive to produce they could be applied in tele­vision displays, with efforts being under­taken by several companies, in Switzerland and world-wide. “Also, as these quantum dots can rapidly emit photons, they are of parti­cular interest for use in optical communi­cation within data centres and super­computers, where fast, small and efficient components are central,” says Mahrt. Another future appli­cation could be the optical simu­lation of quantum systems which is of great impor­tance to funda­mental research and materials science. Norris is also interested in using the new knowledge for the develop­ment of new materials. “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,” he says. (Source: ETHZ)

Reference: M. A. Becker et al.: Bright triplet excitons in caesium lead halide perovskites, Nature 553, 189 (2018); DOI: 10.1038/nature25147

Link: Optical Materials Engineering Lab, Dept. of Mechanical and Process Engineering, ETH Zurich, Zurich, Switzerland

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