How to make quantum dots even brighter

Quantum dots are a kind of artificial atom: measuring just a few nanometers and made of semiconductor materials, they can emit light of a specific color or even single photons, which is important for quantum technologies. The discoverers and pioneers of commercial production of quantum dots were awarded the Nobel Prize in Chemistry in 2023.

In recent years, quantum dots made of perovskites have attracted particular attention. Perovskites belong to a class of materials that have a similar structure to the mineral perovskite (calcium titanate). Quantum dots made from such materials were first produced by ETH Zurich in 2015.

These quantum dots made of perovskite nanocrystals can be mixed with liquids to form a dispersion, making them easier to process further. Additionally, their special optical properties make them shine brighter than many other quantum dots. They can also be produced at lower cost, which makes them interesting for applications in screens, for example.

A team of researchers led by Maksym Kovalenko from ETH Zurich and Empa, working in collaboration with Ukrainian and American counterparts, 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 been observed before in perovskite quantum dots. The researchers recently published their results in two articles Nature.

Unhappy atoms reduce brightness

Brightness is an important measurement for quantum dots and is related to the number of photons emitted by the quantum dot per second. Quantum dots emit photons of a specific color (and therefore frequency) after being excited, for example, by ultraviolet light of a higher frequency.

This leads to the formation of an exciton consisting of an electron, which can now move more freely, and a hole – in other words, a missing electron – in the energy band structure of the material. The excited electron can fall back into a lower energy state and thus recombine with the hole. If the energy released during this process is converted into a photon, the quantum dot emits light.

However, this doesn’t always work. “On the surface of perovskite nanocrystals there are ‘unfortunate’ atoms that are missing a neighbor in the crystal lattice,” explains lead researcher Gabriele Raino. These edge atoms disrupt the balance between positive and negative charge carriers inside the nanocrystal and can cause the energy released during recombination to be converted into lattice vibrations instead of being emitted as light. As a result, the quantum dot “flashes,” meaning it does not glow continuously.

Phospholipid-based protective coating

To prevent this from happening, Kovalenko and his team developed tailor-made molecules called phospholipids. “These phospholipids are very similar to the liposomes in which, for example, the coronavirus mRNA vaccine is integrated in a way that makes it stable in the blood until it reaches the cells,” explains Kovalenko.

An important difference: the researchers optimized their molecules so that the polar (electrically sensitive) part of the molecule attaches to the surface of the perovskite quantum dots and ensures that the “unlucky” atoms have a partner dump.

The non-polar part of the phospholipid which protrudes outside also makes it possible to transform the quantum dots into dispersion within non-aqueous solutions such as organic solvents. The lipid coating on the surface of perovskite nanocrystals is also important for their structural stability, as Kovalenko points out: “This surface treatment is absolutely essential for anything we might want to do with quantum dots.”

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

Even brighter thanks to Superradiance

Thanks to the lipid surface, it was possible to reduce the blinking of the quantum dots to such an extent that they emit a photon in 95% of electron-hole recombination events. However, to make the quantum dot even brighter, the researchers had to increase the speed of the recombination itself, which requires quantum mechanics.

An excited state, such as an exciton, decays when a dipole (positive and negative charges move relative to each other) interacts with the electromagnetic field of the vacuum. The larger the dipole, the faster the decay. One possibility to create a larger dipole is to coherently couple several smaller dipoles to each other. This can be compared to pendulum clocks which are mechanically linked and run in time with each other after a certain time.

The researchers were able to show experimentally that coherent coupling also works in perovskite quantum dots, with a single exciton dipole which, thanks to the effects of quantum mechanics, extends across the entire volume of the quantum dot, creating multiple copies of itself, as it was. The larger the quantum dot, the more copies can be created. These copies can cause an effect called superradiance, whereby the exciton recombines much more quickly.

The quantum dot is therefore also ready to absorb a new exciton more quickly and can thus emit more photons per second, making it even brighter. An important detail to note is that the faster quantum dot continues to emit single photons (not multiple photons at once), making it suitable for quantum technologies.

Enhanced perovskite quantum dots are of interest not only for light production and displays, Kovalenko says, but also in other less obvious areas. For example, they could be used as light-activated catalysts in organic chemistry. Kovalenko is conducting research on these and several other applications, notably as part of the NCCR Catalysis.