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Nobel Prize in Chemistry 2023: In the 1939 film The Wizard of Oz, the protagonist Dorothy was transported from a black-and-white and sepia-toned world to a magical, technicolour realm because her house was swept away by a powerful tornado. While this is just a fictional movie, such scenarios occur in real life. In the world of nano dimensions, where everything is extremely tiny, the properties of particles change, and so do the colours emitted by them. This is because of the quantum effects that occur in the nanoworld, where the size of matter is measured in millionths of a millimetre, or in the scale of 10^(-9).
This year, the Nobel Prize in Chemistry was awarded to three scientists for conducting research on quantum effects in the nanoworld. Moungi G Bawendi from the Massachusetts Institute of Technology (MIT) in Cambridge, Louis E Brus from Columbia University in New York, and Alexei I Ekimov from Nanocrystals Technology in New York were awarded the 2023 Chemistry Nobel for the discovery and synthesis of quantum dots.
According to the Nobel Prize organisation, Dorothy landing in a technicolour world in The Wizard of Oz is similar to quantum dots exhibiting effects such as the emission of colours different from what larger sizes of the particles would have emitted.
Before understanding the work of each laureate in detail, let us know what quantum dots are.
The colourful world of quantum dots
Quantum dots are extremely tiny particles in the order of a nanometre in size, and are made up of a hundred to a thousand atoms, according to the University of Wisconsin-Madison. Quantum dots are nanoparticles so tiny that they exhibit quantum effects.
Quantum dots are semiconductor materials which can be made from elements such as silicon and germanium, or compounds such as cadmium sulphide or cadmium selenide, and differ in colour depending on their size. The duration for which quantum dots are allowed to form determines the final size they acquire. The particle size of quantum dots determines the colour emitted by them.
Ekimov and Brus created quantum dots in the 1980s, independently of each other.
Bawendi made extremely high quality quantum dots in 1993.
How size determines the properties of matter
Scientists had theorised that extremely tiny nanoparticles have unusual characteristics, years before Ekimov and Brus discovered nanoparticles. German-British physicist Herbert Fröhlich predicted in 1937 that nanoparticles would behave differently than other particles.
The Schrödinger equation demonstrates that extremely small particles have less space for the material’s electrons, as a result of which electrons, which are both waves and particles, are squeezed together. Fröhlich realised that it is the reduced space available for electrons that results in drastic changes in the material’s properties.
These are called size-dependent quantum effects.
Researchers achieved a breakthrough in this field in the 1970s, when they successfully created a nano-thin layer of coating material on coating material on the surface of a bulk material, and demonstrated that the optical properties of the coating were dependent on how thin it was. This was an important observation because it proved size-dependent quantum effects.
However, advanced technology was required to create this nano-thin layer-coated bulk material, and hence, scientists decided to study in detail an ancient invention: coloured glass.
How coloured glass shows size-dependent quantum effects
Coloured glass was made several thousand years ago using substances such as silver, gold and cadmium, and then regulating the temperatures to produce different colours. When researchers started making coloured glass in the 19th and 20th centuries to study the optical properties of light, they observed that a single substance can be used to form different colours of glass.
For instance, one can make yellow or red glass from a mixture of cadmium selenide and cadmium sulphide, depending on how much the molten glass is heated and how it is cooled.
The researchers observed that particles forming inside the glass emitted different colours, depending on their size. One can determine the materials which a substance is made up of, and what the crystal structure is, by shining light on the substance, and measuring the absorbance.
The phenomenon of the same substance producing different colours depending on the size of the particles inside caught the interest of Ekimov, who started studying coloured glass.
How Ekimov produced quantum dots
Ekimov produced glass that was tinted with copper chloride, heated the molten glass to high temperatures, cooled it, and then X-rayed it to observe the scattered rays.
From the scattered rays, Ekimov noticed that tiny crystals of copper chloride had formed inside the glass, and that the size of the particles was determined by the manufacturing process.
Some glass samples had copper chloride particles of size two nanometres, while others had sizes of up to 30 nanometres.
The size of the particles affected the light absorbed by them. Ekimov observed that the biggest particles absorbed light in the same way as normal particles of copper chloride, but as the particle size decreased, the light absorbed became bluer.
This is how coloured glass exhibited size-dependent quantum effects.
The larger the nanoparticle, the more the space available for electrons, and the smaller the nanoparticle, the lesser the space available for nanoparticles. The amount of space available for electrons determines the optical properties of the particle.
The wavelength of the light absorbed by quantum dots is different from the wavelength of light emitted by them. The colour is determined by the size of the particles.
Ekimov’s observations were a major milestone because these marked the first time someone had deliberately produced quantum dots.
How Brus showed size-dependent quantum effects
In 1983, Brus, who was unaware of Ekimov’s discovery, became the first researcher in the world to discover size-dependent quantum effects in freely-floating particles in a solution. He conducted experiments using particles of cadmium sulphide, which have the ability to capture light, and use the energy to drive reactions. Brus ensured that the cadmium sulphide particles in the solution were very small because this would give him a greater surface area to perform his experiments.
On one occasion, Brus had left the particles on his lab bench when he noticed that the optical properties of cadmium sulphide particles had changed. He suspected that it could be because the size of the particles had increased, and to confirm this, he made cadmium sulphide particles which were about 4.5 nanometres in diameter.
The size of the particles that had grown was 12.5 nanometres. Brus compared the optical properties particles, and observed that the wavelengths of light absorbed by the larger particles were the same as the wavelengths of light generally absorbed by cadmium sulphide. Meanwhile, the smaller particles absorbed bluer wavelengths of light.
This was another size-dependent quantum effect. Brus concluded that the smaller the particles, the bluer the light they observed.
All these observations are interesting because they prove that as particles shrink in size, the space available for electrons decreases, and since electrons determine the optical properties of a substance, and also other properties such as the ability to conduct electricity or catalyse chemical reactions, the quantum dots proved to be an entirely new material.
However, one drawback was that the quantum dots produced by Brus were of varying sizes, and contained defects.
This was where Bawendi came into the picture.
How Bawendi created extremely high-quality quantum dots
Initially, Bawendi used a range of solvents, temperatures and techniques to produce quantum dots, and while well-organised nanocrystals were formed, their quality was not good enough.
In 1993, Bawendi, while working at MIT, injected substances that can form cadmium selenide crystals into a hot solvent, used a stabilising gas, and ensured that the volume was enough to saturate the solvent around the needle.
Bawendi and his research group were able to form small crystals of cadmium selenide, but the injection cooled the solvent, and stopped the formation of the crystals.
Therefore, the team increased the temperature of the solvent, and to their surprise, nanocrystals started forming. The longer the process continued, the larger the crystals became. The crystals had a smooth and even surface because of the solvent.
Since Bawendi’s quantum dots were almost perfect, they exhibited distinct size-dependent quantum effects.
Practical applications of quantum dots
Quantum dots are used in innumerable fields. Since quantum dots absorb blue light on being illuminated with it, and emit a different colour, scientists leverage these luminous properties in making computer and television screens based on QLED technology. Here, the Q stands for quantum dot.
The screens generate blue light with the help of energy-efficient diodes, and quantum dots change the colour of some of the blue light, emitted red or green light. In this way, a computer or television screen produces the three primary colours of light.
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