The QLED television that is in many living rooms would not have existed without Alexei Ekimov, Louis Brus and Moungi Bawendi. This year they will receive the Nobel Prize in Chemistry for discovering and developing quantum dots. These are particles that are so small that their size determines the properties. They are currently mainly used to manipulate light: the smaller the nanoparticle, the bluer the light. In QLED screens they provide very intense color reproduction. They are also used in LED lighting, in solar panels and as biomarkers.
It’s a good thing that all three researchers work at American institutions, and that it was still practically night there when the official call came from the Nobel Committee that they would receive the prize. On this side of the ocean, their names had been circulating for several hours, after the press release about the announcement had accidentally reached Swedish media at half past seven in the morning.
“Very unfortunate,” Secretary General Hans Ellegren of the Royal Swedish Academy of Sciences said about the leaking of the names. Strict secrecy until the press conference that starts at a quarter to twelve in Stockholm is an important part of the Nobel circus. Leaking has never happened before. “We don’t know why that email was sent, but it had no influence on the awarding of the prize. That process takes a very long time.”
Quantum dots are one of the most important discoveries in the field of nanotechnology. They are made of semiconductor materials, such as silicon or cadmium sulfide. Semiconductors contain energy bands between which electrons can move. When a photon, a light particle, hits the semiconductor material, an electron jumps to a higher energy band. When an electron falls back to the lower band, a photon is released that cancels out the energy difference. The distance between the two energy bands, the so-called band gapdetermines the color of the emitted light.
The crux of the quantum dots is their size band gap can be influenced by making the nanoparticles larger or smaller. Smaller quantum dots have a larger one band gap, and therefore emit blue light (light with short wavelengths, which contains a lot of energy). Larger quantum dots have smaller ones band gaps and thus emit longer wavelengths, or red light. This concerns ‘large’ and ‘small’ at the nano level, crystals with a diameter between 2 and 10 nanometers (that is between a hundred and tens of thousands of atoms, a nanometer is one billionth of a meter).
Theorists already said in the 1930s that this quantum effect existed, but it was still difficult to demonstrate it. Ekimov, initially working in St. Petersburg, was one of the first to demonstrate the effect in colored glass. The glass was colored with copper chloride, and the color varied depending on how long and how hot it was heated. This turned out to be because heating influenced the crystal formation of the copper chloride. Ekimov published about it in 1981, but in Russian, so the research did not immediately spread around the world.
Cited ten million times
Brus meanwhile worked at Bell Laboratories in the US, where he conducted research into the use of solar energy to drive chemical reactions of small cadmium sulfide particles in a solution. He noticed that the optical properties of the particles changed after leaving them for a day and suspected that the older particles had formed new, larger crystals. He also realized that the color change was due to a quantum effect related to size. This was in 1983.
It remained difficult to make the particles for ten years, and it was especially difficult to control the start of the growth of the crystals. Bawendi, who worked as a postdoc under Brus at Bell Laboratories, managed to make them in a controlled manner in 1993, when he was already at MIT. Suddenly it was easy to control exactly how large the particles became. He thus made large-scale production of quantum dots possible.
“We had expected for a long time that the prize would be awarded to this,” said Andries Meijerink, professor of solid state chemistry at Utrecht University, in response to the announcement. “Nanosciences are very popular, everything is nano these days. But a lot of nanoscience is about making large surfaces. That is of course clever, but also quite trivial. This really is the pinnacle of nanoscience. Here they control the physical properties of the particles by varying dimensions on the nanoscale.”
“I think Bawendi in particular has been quoted ten million times. And then I exaggerate just a little bit,” says Willem Vos, professor of complex photonic systems at the University of Twente. “Thanks to him, the synthesis of the particles has really become much easier. As a result, research has exploded since the late 1990s. Not long after, it also stormed into my lab.”
It is groundbreaking fundamental science that has really led to something in society, according to both Vos and Meijerink. In addition to screens and lighting, Vos also mentions applications in biology as one of the important areas of attention. He is referring to quantum dots that are connected to proteins and can visualize pathogens or processes in the body. “Organic materials are now often used for this, but they are not very stable. Quantum dots are more robust. You have to pack them well, the materials are not that healthy. You would prefer non-toxic quantum dots, a lot of work is now being done on that.”
A version of this article also appeared in the October 5, 2023 newspaper.