The 2023 Nobel Prize in Chemistry was announced by the Sweden’s Royal Academy of Sciences on October 4, 2023. It was awarded to Moungi G. Bawendi (Massachusetts Institute of Technology, USA), Louis E. Brus (Columbia University, USA), and Aleksey Yekimov (Nanocrystals Technology Inc., USA) for their discovery and development of quantum dots. Based on Quantum dot Light Emitting Diode (QLED) technology, these quantum dots currently illuminate television, LED lamps, computer monitors, etc. Quantum dots infuse chemical reactions and their clear lights could help surgeons by guiding them while removing tumour tissues.
The prize amount, 11 million Swedish kronor, is to be shared equally among the three scientists. In addition to the money, winners would also receive an 18-carat gold medal and a diploma.
Quantum Dots
Due to the emergence of quantum effects, unique characteristics of small nanoparticles arise. Minute particles like electrons have radically different and strange properties. Their motion and behaviour are different from that of familiar objects in normal human experience. The Quantum Theory, developed by physicists a century ago, describes the strange behaviour at the sub-atomic level. When compared with atoms, nanoparticles are much larger in size. A nanoparticle could pack in numerous atoms, depending on the size of the atom. However, it was theorised in the 1930s that when the size of particles was reduced to nanoscale, it could give rise to quantum effects. It was due to the fact that electrons were constrained in a limited space. Generally, electrons move outside the nucleus of the atom. However, electrons are tightly packed if the size of the particles is reduced considerably. This could lead to strange quantum effects. Aleksey Yekimov and Louis Brus noticed these strange quantum effects and created nano-sized particles in their laboratories. These nano-sized particles having different characteristics than that of the larger particles of the same element are called the quantum dots.
Significance of the Discovery
Three decades after their discovery, quantum dots have become an important part of nanotechnology toolbox and are found in many commercial products. In computer and television screens, QLED technology is used. In these screens, blue light is generated using the energy-efficient diodes. Quantum dots are used to change the colour of some of the blue light, transforming it into green or red. The three primary colours in the television screen are produced due to this.
For adjusting the cold light of diodes, quantum dots are used in some LED lamps for energising the light as daylight or calming it as a dimmed bulb.
Quantum dots lights could be used in biochemistry and medicine. Researchers opine that quantum dots could contribute to flexible electronics, miniscule sensors, slimmer solar cells, encrypted quantum communication, surgical oncology, and quantum computing, etc.
Aleksey Yekimov’s Contribution
While doing his Ph.D. research, Aleksy Yekimov, studied about the semiconductors which are important components in microelectronics. For evaluating the quality of the semiconducting material, optical methods are used as diagnostic tools. To measure the absorbance of the material, scientists used to shine light on it. With this, they assessed the substances with which the material was made from and also, the orderliness of its crystal structure.
As Aleksy Yekimov was familiar with these methods, he started to use those methods for exploring coloured glass. He heated molten glass to a range of 500 °C and 700 °C by varying the heating time from 1 hour to 96 hours. When the molten glass was X-rayed, there were tiny copper chloride (CaCl) crystals that had formed inside the glass and the size of the particles were affected by the manufacturing process. The size of the particles varied in the glass samples, some were about two nanometres, and some were up to 30 nanometres. Further, the light absorption was affected by the size of the particles—the biggest particles absorbed the light in the same way that copper chloride normally does, whereas the smaller particles absorbed the blue light. Yekimov understood the size-dependent quantum effect. Thus, Yekimov was the first person to succeed in producing quantum dots, nanoparticles that caused size-dependent quantum effects. He published his discovery in a Soviet scientific journal in 1981.
Louis Brus’s Contribution
Unaware of Aleksy Yekimov’s discovery, Louis E. Brus discovered, in 1983, size-dependent quantum effects in particles floating freely in a solution. He was working on making chemical reactions happen using solar energy. Particles of cadmium sulphide (CdS) were used as it could capture light and then utilise its energy to drive reactions. Brus made the particles in the solution tiny so that it would give a larger area on which the chemical reaction could take place. When it was left for a while, their optical properties changed and had a diameter of about 12.5 nanometres compared to the CdS particles which were just about 4.5 nanometres in diameter. The larger particles absorbed light at the same wavelengths as CdS generally does, but the smaller particles absorbed blue light. Brus published his discovery of size-dependent quantum effect in 1983. He also investigated particles made from a range of other substances only to observe that the smaller the particles, the bluer the light they absorbed.
The experiments by Yekimov and Brus proved that electrons govern the optical and other properties of a substance such as its ability to catalyse chemical reactions or conduct electricity. This proved that the properties of an element are not only affected by the number of electron shells and the number of electrons in the outer shell, but the size of the element too matters at the nano level.
Thus, the quantum dots at that time were of varying sizes. Hence, if researchers wanted all particles in a solution to be of the same size, they would have to sort them after they were made. Since it was a difficult process, it hindered development.
Moungi Bawendi’s Contribution
Moungi G. Bawendi worked with Brus in the initial part of the career. Later, he developed easier methods to efficiently produce nanoparticles that showed some desired deviant behaviour. He achieved the breakthrough in 1993. He injected substances that could form cadmium selenide (CdSe) into hot solvent. The volume was enough to saturate the solvent around the needle. This led to the immediate formation of small crystals of CdSe. However, when the injection was cooled, the solvent crystals stopped forming. Bawendi then increased the temperature of the solvent, and the crystals once again started growing. The longer this continued, the larger the crystals became. Thus, Moungi Bawendi and his research group succeeded in growing nanocrystals of a specific size. These nanocrystals gave rise to distinct quantum effects.
As this production method of nanocrystals was easy to use, more and more chemists stated to work with nanotechnology and began to investigate the unique properties of quantum dots.
About the Nobel Prize
The Nobel Prizes are awarded to those who confer the greatest benefit to mankind. Instituted after the name of Alfred Nobel, the prizes are given in the field of Physics, Chemistry, Physiology (Medicine), Literature, Peace, and Economic Sciences. These prizes are widely regarded as the most prestigious awards available in their respective fields. The first Nobel Prizes were awarded in 1901.
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