They have developed a technology that is found in electronic devices and biomedical markers: quantum dots. For this, Alexei Ekimov, Louis Brus and Moungi Bawendi will receive the 2023 Nobel Prize in Chemistry. The three researchers discovered the optical and electronic properties of these tiny nanoparticles in solid or liquid materials. With this knowledge, they also developed methods to create customized nanodots.
So-called quantum dots are found in QLEDs, in many televisions and are used as marker substances in medicine. Crucial to most applications of quantum dots is their ability to emit photons of a specific wavelength when excited by current or radiation. The basic principle behind this is a band gap in the energy levels of their electrons, similar to a semiconductor. The width of this band gap depends on the size of the quantum dots. The quantum dots emit light when, after charge separation due to excitation, a recombination of electrons and positive “holes” occurs. The previously absorbed energy is released in the form of photons - as light of a specific wavelength.
Because the wavelength of this light is specific depending on the quantum dot, the colors of such quantum dots are more intense and sharply defined than, for example, classic LEDs. Other characteristics such as the redox potential in chemical reactions or the melting temperature of these nanoparticles also depend directly on their size.
The secret of colored glasses decoded
The first prize winner, Alexei Ekimov from Nanocrystals Technology in New York, took the first step towards the discovery of quantum dots in 1979 when he was researching colored glasses - at that time still in the former Soviet Union. He wanted to understand how targeted contamination of such glasses with foreign particles produced their colors and how the growth of such particles in the glass melt could be influenced more specifically. Using the example of copper chloride nanocrystals in glass, Ekimov discovered that the spectrum of the absorbed light changed depending on the size of the crystal particles: the smaller the nanocrystals were, the more blue the light color was. Ekimov attributed this effect to the interaction of electrons and holes in the material and created an equation that described the relationship between photon energy and particle size. This marked the discovery of the first crystalline semiconductor quantum dots.
The next step was taken in 1983 by a team led by Louis Brus from Columbia University in New York. Without knowing about Ekimov's discovery, they were researching colloidal nanoparticles. During experiments with cadmium sulfide particles in solution, they also discovered the effect of particle size on the spectrum of the excited particles and set up a corresponding model. Such size-dependent quantum effects should occur below a particle size of five nanometers and also influence the photochemical redox potentials.
Quantum dots made to measure
However, the targeted production of such quantum dots was still complex and only limited to certain particles. However, that changed in 1993 with the work of the third prize winner, Moungi Bawendi from the Massachusetts Institute of Technology (MIT). He was the first to develop a method through which quantum dots with a defined size and high optical quality could be produced. For this synthesis, organometallic compounds are injected into a hot solvent with a high boiling temperature. This leads to an abrupt supersaturation of the solution and tiny particles begin to crystallize. By quickly reducing the temperature and diluting, this crystallization process can be stopped or continued. “The hot injection method developed by Bawendi and his team paved the way for the use of quantum dots on a large scale,” said the Nobel Prize Committee in praise of the development.
Together, the three Nobel Prize winners in chemistry have made a technology possible that is already an important component of modern electronics. “The discovery of quantum dots and the ability to produce such materials with high precision but relatively simple chemical methods was a crucial step in the development of nanoscience and nanotechnology,” explains the Nobel Prize Committee. However, with the emergence of quantum physics applications such as quantum computers and quantum communication, it could become even more important in the future.
Source: Nobelprize.org