Quantum dots are a kind of artificial atom: just a few nanometres in size and made of semiconductor materials, they can emit light of a specific colour or even single photons, which is important for quantum technologies. The discoverers and pioneers of the 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 of such materials 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 brighter than many other quantum dots. They can also be produced less expensively, which makes them interesting for applications in displays, for instance.
A team of 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 the prestigious scientific journal Nature.
Unhappy atoms reduce brightness
Brightness is an important measure for quantum dots and is related to the number of photons the quantum dot emits per second. Quantum dots radiate photons of a specific colour (and hence 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 energetic band structure of the material. The excited electron can fall back to 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.
This doesn’t always work, however. “At the surface of the perovskite nanocrystals are ‘unhappy’ atoms that are missing a neighbour in the crystal lattice,” senior researcher Gabriele Raino explains. These 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 being emitted as light. As a result, the quantum dot “blinks”, meaning that it doesn’t shine continuously.
Protective coating made of phospholipids
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,” Kovalenko explains. An important difference: the researchers optimised their molecules so that the polar, or electrically sensitive, part of the molecule latches on to the surface of the perovskite quantum dots and makes sure that the ‘unhappy’ atoms are provided with a charge partner.