Bengaluru: The 2023 Nobel Prize in chemistry has been awarded to Moungi G. Bawendi (Massachusetts Institute of Technology, US), Louis E. Brus (Columbia University, US), and Alexei I. Ekimov (Nanocrystals Technology Inc., US) for their work in the discovery and development of extremely tiny particles called quantum dots.
These are nanoparticles so small that they are subject to quantum phenomena that change depending on the size of the particle.
These particles have different colours as well, depending on their size, and thus offer new opportunities for creating coloured light. They find uses in television display and computers as QLED technology, where Q stands for quantum.
They are also used in research and medicine: Quantum dots can be attached to biomolecules and injected into the body to map cells and organs. They can also be used to track cancer cells and their growth, and guide surgeons when they remove tumour tissue.
In chemistry, they are used for catalysing and speeding up chemical reactions.
When particles are shrunk to a point where they can be measured in nanoscale, or one millionth of a millimetre, strange phenomena called quantum effects begin to occur.
These are analysed under a different field of study called ‘quantum mechanics’, aimed at assessing the behaviour and interactions of microscopic particles, as opposed to the traditional macroscopic study of classical physics.
The field began to develop gradually beginning in the 1920s, and it allows for mathematical calculation of properties and behaviours of quantum particles, which are typically atomic and subatomic.
Quantum systems and particles are subject to a whole new set of rules, and are difficult to work with and understand. But they are finding increasing use everywhere, including in applications like quantum computers, as technology advances.
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Quantum mechanics and coloured glass
Even during the times of theoretical physicist Albert Einstein (1879-1955), physicists theoretically knew that nanoparticles would not behave like macro particles.
Physicist Herbert Fröhlich worked on Schrödinger’s equations, which determine that as particles become smaller, there is less space for electrons in an atom. As a result, they are squeezed together.
Electrons, just like light, are both waves and particles, and Fröhlich concluded that this dual quality would create drastic changes in the properties of the material. More theoretical and mathematical work continued on quantum mechanics, resulting in a large number of predictions about quantum particles’ properties.
However, it was not until the 1970s that researchers were able to succeed in creating a nanostructure with which to perform experiments.
Working with quantum materials (and quantum computers) requires extremely cold temperatures going down to absolute zero (zero kelvin or −273.15 degrees celsius) and ultra-high vacuum, and was thus quite expensive and inaccessible.
The first material created was a thin layer of coating on a larger material, which was made using a molecular beam or a stream of molecules that resulted in the coating being only nanometres thin. This experiment also showed that as the thickness of the coating changed, so did its optical properties.
However, it was thought that such quantum phenomena would not find real-world applications. But the study of coloured glass put this theory to rest.
Glassmaking goes back thousands of years, and coloured glass is found in various archaeological sites around the world.
When physicists began experimenting with and understanding the properties of light in the early 19th century, they started to understand the creation of coloured glass, which gets its colour by filtering different wavelengths of light.
Coloured glass is made by adding various substances like gold and silver, and, at various temperatures, these result in different colours. Physicists realised that just the addition of one substance to the raw mixture could produce a completely different coloured glass.
Additionally, varying the temperatures for the same mix with the same substances also resulted in different colours. Changing the methods by which hot newly made glass was cooled also resulted in different colours.
Scientists concluded that the different colours in glass came because of various particles that form inside during the cooking process, and that the colour was dependent on the size of these particles.
Development of quantum dots
In the early 1980s, Alexei Ekimov, then a grad student, experimented with coloured glass while working on semiconductors. He produced glass tinted with copper chloride, heating it to various temperatures for varied periods of time.
When he x-rayed the cooled glass, he found that tiny crystals of copper chloride had formed inside, and the way the glass was made affected the size of these tiny particles.
Ekimov realised that he had observed a size-dependent quantum effect, which was the first time someone had purposefully produced what would come to be known as quantum dots.
His findings were published in the former Soviet Union in 1981, but, since it was cut off from western nations on account of the Cold War, scientists in Europe and America were unaware of the paper.
In 1983, Louis Brus was at Bell Laboratories, working on chemical reactions that utilise solar energy.
He used cadmium sulfide floating in a solution for his experiments.
During the course of his work, he observed that the optical properties of these tiny particles and the colour they produce changed after he left the solution on his desk for some time.
He thought this could be because the particles had grown in size, which his observations then confirmed. Brus then realised he had succeeded in synthesising quantum dots by discovering size-dependent effects of tiny particles floating in a solution.
Thus, in the 1980s, physicists became aware that the properties of an element not only depended on the number of electrons in an atom of a particle, but also the size of the particle.
However, neither Ekimov nor Brus were able to consistently produce particles of similar quality and same size.
This changed in 1993, when Brus’s former student Moungi Bawendi — the third winner of this Nobel — finally succeeded in his efforts to standardise the size of particles synthesised in one experiment.
He and his team found that when they injected substances that form nanocrystals into certain specific solvents at specific temperatures, they could create tiny crystals of consistent size.
This method revolutionalised the production of quantum dots, with more and more chemists now working with nanotechnology and quantum mechanics.
The properties and potential of quantum dots have been understood only recently, and they now find applications in a wide variety of areas, including in electronics, where they are used in tiny sensors, and optimal solar cells.
(Edited by Sunanda Ranjan)