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Empa and ETH Zurich make perovskite quantum dots into brighter emitters

14 Feb 2024

Tailor-made phospholipids form protective layer around the dots to improve performance.

Researchers at Empa and ETH Zurich, both in Switzerland, have developed methods for making perovskite quantum dots faster and more efficient emitters, thereby significantly improving their brightness.

Quantum dots made of perovskites were produced for the first time by ETH Zurich in 2014. These quantum dots made of perovskite nanocrystals can be mixed with liquids to form a dispersion, making them easy to further process. Moreover, their special optical properties make them shine more brightly than many other quantum dots. They can also be produced less expensively, which makes them interesting for applications in displays, for instance.

Researchers led by Maksym Kovalenko at ETH Zurich and Empa, working in collaboration with their counterparts in Ukraine and the USA, have now demonstrated how these promising properties of perovskite quantum dots can be further improved. They used chemical methods for surface treatment and quantum mechanical effects that had never before been observed in perovskite quantum dots.

The researchers recently published their results in two papers in Nature: Designer phospholipid capping ligands for soft metal halide nanocrystals; and Single-photon superradiance in individual caesium lead halide quantum dots.

‘Unhappy atoms’ reduce brightness

Quantum dots radiate photons of a specific frequency after being excited, for example, by UV of a higher frequency. This leads to the formation of an exciton consisting of an electron in the energetic band structure of the material. The excited electron can fall back to a lower energy state and thus recombine with a hole. If the energy released during this process is converted into a photon, the quantum dot emits light.

This does not always work, however. “At the surface of the perovskite nanocrystals are ‘unhappy’ atoms that are missing a neighbour in the crystal lattice,” explained senior researcher Gabriele Raino. The edge atoms disturb the balance between positive and negative charge carriers inside the nanocrystal and can cause the energy released during a recombination to be converted into lattice vibrations instead of light. As a result, the quantum dot “blinks”.

To prevent this from happening, Kovalenko and his team have developed tailor-made molecules known as phospholipids. “These phospholipids are very similar to the liposomes in which, for instance, the mRNA vaccine against the coronavirus is embedded in such a way as to make it stable in the bloodstream until it reaches the cells,” said Kovalenko.

The nonpolar part of the phospholipid that protrudes on the outside also makes it possible to turn quantum dots into a dispersion inside non-aqueous solutions such as organic solvents. The lipid coating on the surface of the perovskite nanocrystals is also important for their structural stability, as Kovalenko commented, “This surface treatment is absolutely essentially for anything we might want to do with the quantum dots.”

So far, Kovalenko and his team have demonstrated the treatment for quantum dots made of lead halide perovskites, but it can also be easily adapted to other metal halide quantum dots, they say.

Even brighter thanks to superradiance

With the lipid surface it was possible to reduce the blinking of the quantum dots to enable emission of a photon in 95 percent of electron-hole recombination events. To make the quantum dot even brighter, however, the researchers had to increase the speed of the recombination itself – and that requires quantum mechanics.

One possibility of creating a larger dipole involves coherently coupling several smaller dipoles to each other. This can be compared to pendulum clocks that are mechanically connected and tick in step with each other after a certain length of time.

The researchers were able to show experimentally that the coherent coupling also works in perovskite quantum dots – with only a single exciton dipole that spreads out all over the volume of the quantum dot, thereby creating several copies of itself.

These copies can bring about an effect known as superradiance, by which the exciton recombines much faster. The quantum dot is consequently also ready more quickly to take up a new exciton and can thus emit more photons per second, making it even brighter.

The improved perovskite quantum dots are not only of interest for light production and displays, said Kovalenko, but also in other, less obvious fields. For instance, they could be used as light-activated catalysts in organic chemistry. Kovalenko is conducting research into such applications and several others, including within the framework of NCCR Catalysis.

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