The Nobel Prize in Chemistry: Quantum Dots
D. Indumathi, The Institute of Mathematical Sciences, Chennai

The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Chemistry 2023 to
Moungi G. Bawendi,
Massachusetts Institute of Technology (MIT), Cambridge, MA, USA,
Louis E. Brus,
Columbia University, New York, NY, USA,
Alexei I. Ekimov,
Nanocrystals Technology Inc., New York, NY, USA,
“for the discovery and synthesis of quantum dots.”
According to the Nobel Prize website, the Nobel Prize in Chemistry 2023 rewards the discovery and development of quantum dots, nanoparticles so tiny that their size determines their properties. These smallest components of nanotechnology now spread their light from televisions and LED lamps, and can also guide surgeons when they remove tumour tissue, among many other things.
If you study chemistry in 11th and 12th standards, you will learn that an element’s properties are governed by how many electrons it has. For example, sodium likes to lose an electron and chlorine likes to take one, and so they combine to form sodium chloride or common salt. However, when matter shrinks to nano-dimensions quantum phenomena arise; these are governed by the size of the matter. The Nobel Laureates in Chemistry 2023 have succeeded in producing particles so small that their properties are determined by quantum phenomena. The particles, which are called quantum dots, are now of great importance in nanotechnology. Let us try to understand these quantum phenomena.

Size matters
An atom like a hydrogen atom is typically one tenth of a nanometer in size. One nanometer is one billionth of a meter, that is 1,00,00,00,000 times smaller than one meter. So an object that is nanometers in size is really small, just a collection of a few hundred or thousand atoms. (You may have heard of Avogadro number: 1 gram of water for instance, has 6x10^{23} molecules of water in it, so a few thousand atoms is a really really small collection.) In this atomic world, the laws of science work in strange and unusual ways, and this is called quantum mechanics. In this quantum world, particles can show behaviour that is not seen in the normal world of meter or even millimeter size around us.
There are so many quantum phenomena in our world and they have been explored in different ways. Here, it is explored for the relation of size to colour. In fact, quantum dots are special because they have different colours depending on their size.

BOX on The Nano World
If an enchanted tornado were to sweep into our lives and shrink everything to nano dimensions, we would almost certainly be astonished. Our surroundings would be dazzlingly colourful and everything would change. Our gold earrings would suddenly glimmer in blue, while the gold ring on our finger would shine a ruby red. If we tried to fry something on the gas hob, the frying pan might melt. And our white walls – whose paint contains titanium dioxide – would start generating lots of reactive oxygen species. The picture shows a solution of gold nanoparticles. Depending on the size, its colour is different. The colour changes from red to blue, through all the colours of the rainbow, with the largest nano-particles having the blue colour, and the smallest the red.
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Almost 100 years ago, in 1937, the physicist Herbert Fröhlich had already predicted that nanoparticles would not behave like other particles. He realised also that when particles become extremely small (nano-sized) there is less spacde for their electrons, which then get squeezed together. This will drastically change the properties of the materials. But all this was theory. In order to demonstrate these effects, you needed to make small objects – nano-particles – that were about a million times smaller than a pin-head!

The First Break-through
In the 1970s, researchers succeeded in making such a nanostructure using a type of molecular beam. Since a nano-particle is so fragile and invisible to the human eye (which can only see object larger than about 0.1 mm, or about 100 microns), the clever solution they found was to make a nano-thin layer of coating material on top of a bulk material. Then they could probe the properties of the ultra-thin coating and show that they indeed had the strange nano-behaviour. In fact, their observations agreed with the theoretical predictions.
This was a major breakthrough, but the experiment required very advanced technology. Researchers needed both an ultra-high vacuum and temperatures close to absolute zero, so few people expected that quantum mechanical phenomena would be put to practical use. However, now and again science offers up the unexpected and, this time, the turning point was due to studies of an ancient invention: coloured glass.

Glass nano-technology
The oldest archaeological finds of coloured glass are from several thousand years ago. Glassmakers did not know the chemical theories but by trial and error they learned how glass can be produced in all the colours of the rainbow. They added substances such as silver, gold and cadmium and then played with different temperatures to produce beautiful shades of glass.
In the nineteenth and twentieth centuries, when physicists started to investigate the optical properties of light, the glassmakers’ knowledge was put to use. Physicists could use coloured glass to filter out selected wavelengths of light. To optimise their experiments, they started to produce glass themselves, which led to important insights. One thing they learned was that a single substance could result in completely differently coloured glass. For example, a mixture of cadmium selenide and cadmium sulphide could make glass turn either yellow or red – which one it became depended on how much the molten glass was heated and how it was cooled. Eventually, they were also able to show that the colours came from particles that were formed inside the glass and that the colour depended on the particles’ size.

(Ill)Logic of colour
This was more or less the state of the knowledge at the end of the 1970s, when one of this year’s laureates, Alexei Ekimov, started working at the S. I. Vavilov State Optical Institute in what was then the Soviet Union. He had just finished his PhD and was puzzled by the following question: if you use cadmium red to paint a picture, it remains red always. How is it that the same cadmium could make the glass red or yellow depending on the way it is heated and cooled? Sounds illogical, doesn't it?

Crystal ordering
During his doctoral degree, Ekimov studied semiconductors – important components in microelectronics. In this field, optical methods are used as tools to assess the quality of semiconducting material. Researchers shine light on the material and measure how much of it is absorbed: the absorbance. From this they can not only tell what substances the material is made of, but also how well-ordered the crystal structure is (See box for explanation).

BOX on Crystals
Crystals have an ordered and repeating pattern of atoms or molecules. This repeating structure gives rise to many of the unique properties of crystalline solids. Some crystalline solids examples include diamonds, quartz, and table salt. The figure shows the periodic arrangement of sodium (purple) and chlorine (green) in the crystal structure of salt (sodium chloride).
The regular structure of a crystal can cause light to diffract, or bend as it passes through the material. This can give rise to some spectacular visual effects, such as the sparkling of diamonds or the colorful patterns seen in some types of quartz.
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Ekimov was familiar with these methods, so he began using them to examine coloured glass. After some initial experiments, he decided to systematically produce glass that was tinted with copper chloride. He heated the molten glass to a range of temperatures between 500°C and 700°C, varying the heating time from 1 hour to 96 hours. Once the glass had cooled and hardened, he X-rayed it.
The scattered rays showed that tiny crystals of copper chloride had formed inside the glass and the manufacturing process affected the size of these particles. In some of the glass samples they were only about two nanometres, in others they were up to 30 nanometres. Interestingly, it turned out that the glass’ 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, 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 – nanoparticles that cause size-dependent quantum effects.

Freely Floating Quantum Dots
Louis Brus was working at Bell Laboratories in the US. He was trying to make chemical reactions happen using solar energy. To achieve this, he was using particles of cadmium sulphide, which can capture light and then utilise its energy to drive different reactions. The particles were in a solution and Brus made them very small, because this gave him a larger area on which the chemical reactions could take place; the more a material is chopped up, the greater the surface area it will expose to its surroundings.
Brus noticed something strange – the optical properties changed after he had left the particles on the lab bench for a while. He guessed that this could be because the particles had grown. To confirm this, he compared newly-made cadmium sulphide particles that were about 4.5 nanometres in diameter with the older particles, which had a diameter of about 12.5 nano- metres. The larger particles absorbed light at the same wavelengths as cadmium sulphide generally does, but the smaller particles had an absorption that shifted towards blue colour; see figure. although Brus did not know about Ekimov's work, he also understood that he had observed a size-dependent quantum effect.

How does size matter?
A substance’s optical properties are governed by its electrons. The same electrons also govern the substance’s other properties, such as its ability to catalyse chemical reactions or conduct electricity. So when researchers detected the changed absorption they understood that, in principle, they were looking at an entirely new material. It was almost as if suddenly gained a third dimension. An element’s properties are not only affected by the number of electron shells and how many electrons there are it the outer shell but, at the nano level, size also matters.
There was just one problem. It was difficult to control the size of these particles. Hence they were of unpredictable quality.

Moungi Bawendi revolutionises the production of quantum dots
Moungi Bawendi started his postdoctoral training at Louis Brus’ laboratory. In 1988, the lab was producing well-organised nano-crystals. And the crystals were getting better, but were still not good enough. Bawendi tried to produce higher quality nanoparticles. The major breakthrough came in 1993, when the 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, Moungi Bawendi and his research group 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.

Commercial Uses
Thirty years later, quantum dots are now an important part of nanotechnology’s toolbox and are found in commercial products, mainly to create coloured light. In Q-LED computer and TV screens, blue light is generated using energy-efficient diodes. Quantum dots are used to change the colour of some of the blue light, transforming it into red or green. This makes it possible to produce the three primary colours of light (red, green, blue) needed in a television screen. Similarly, quantum dots are used in some LED lamps to adjust the cold light of the diodes. The light can then become as energising as daylight or as calming as the warm glow from a dimmed bulb.
The light from quantum dots can also be used in biochemistry and medicine. Biochemists attach quantum dots to biomolecules to map cells and organs, and perhaps even to track tumour tissue in the body. Quantum dots thus bring the greatest benefit to humankind.
Source: nobelprize.org