Copyright John Jarnestad/The Royal Swedish Academy of Sciences
Three scientists have been awarded the 2023 Nobel Prize in Chemistry for the discovery and synthesis of quantum dots. Moungi G. Bawendi (MIT), Louis E. Brus (Columbia University), Alexei I. Ekimov (Nanocrystals Technology Inc., NY) have split the prize for their contributions toward the discovery and development of quantum dots—nanoparticles so tiny that their size determines their properties.
These smallest components of nanotechnology are now most well-known for their role in providing light for televisions and LED lamps. They have also been used clinically to map biological tissue.
Two researchers, one discovery
At the end of the 1970s, physicists had just started to investigate the optical properties of light. They discovered they could use colored glass to filter out selected wavelengths of life. They also learned that a single substance could result in completely differently colored glass. Eventually, scientists were able to show that the colors came from particles forming inside the glass and that the color depended on the particles’ size.
That was the state of the industry when Ekimov, a recent doctoral graduate, started his work at the S. I. Vavilov State Optical Institute in what was then the Soviet Union. Having previously studied optical methods for assessing the quality of semiconducting materials, Ekimov applied this knowledge to glass.
Ekimov experimented by producing glass that was tinted with copper chloride, altering multiple variables such as the temperature and heating time. Subsequent X-rays showed that tiny crystals of copper chloride had formed inside the glass and the altered manufacturing processes affected the size of these particles. For examples, in some of the glass samples the particles were only about two nanometers, while they were up to 30 nanometers in others.
Interestingly, Ekimov noticed the glass’ light absorption was also affected by the size of the particles. The biggest particles absorbed the light in the same way that copper chloride normally does, but the smaller the particles, the bluer the light that they absorbed. This was the first time someone had succeeded in deliberately producing quantum dots.
Ekimov published his discovery in a Soviet scientific journal in 1981, but access out of the country was limited. That’s why Brus was unaware of Ekimov’s discovery when, two years later, he became the first to discover size-dependent quantum effects in particles floating freely in a solution.
Brus was initially working on solar energy at Bell Laboratories in the U.S. In his research, he was using particles of cadmium sulphide, which can capture light and utilize its energy to drive reactions. To extend their surface area, Brus put them in a solution and made them very small.
One time, after leaving the tiny particles on the lab bench for a while, Brus noticed that their optical properties changed. To test this, he produced cadmium sulphide particles that were just about 4.5 nanometers in diameter. Brus then compared the optical properties of these newly made particles with larger ones that had a diameter of about 12.5 nanometers. The larger particles absorbed light at the same wavelengths as cadmium sulphide, but the smaller particles had an absorption that shifted toward blue. Investigating particles made from a range of other substances, Brus kept seeing the same pattern—the smaller the particles, the bluer the light they absorbed.
Continuing research with nanocrystals
Unfortunately, the methods Brus had used to fabricate the particles generally resulted in unpredictable quality. Bawendi started his postdoctoral training at Louis Brus’ laboratory in 1988, where intensive work was underway to improve the methods used to produce quantum dots. When Bawendi later moved to MIT, he continued his efforts.
The major breakthrough came in 1993, when Bawendi’s research group injected the substances that would form nanocrystals into a heated and carefully chosen solvent. They injected as much of the substances as was necessary to precisely saturate the solution, which led to tiny crystal embryos beginning to form simultaneously.
Then, by dynamically varying the temperature of the solution, Bawendi succeeded in growing nanocrystals of a specific size. During this phase, the solvent helped give the crystals a smooth and even surface.
The nanocrystals that Bawendi produced were almost perfect, giving rise to distinct quantum effects. Because the production method was easy to use, it was revolutionary—more and more chemists started working with nanotechnology and began to investigate the unique properties of quantum dots.
“Quantum dots are bringing the greatest benefit to humankind, and we have just begun to explore their potential. Researchers believe that in the future quantum dots can contribute to flexible electronics, miniscule sensors, slimmer solar cells and perhaps encrypted quantum communication. One thing is certain—there is a lot left to learn about amazing quantum phenomena,” said The Royal Swedish Academy of Sciences.