The story so far: Alexei I. Ekimov, Louis E. Brus, and Moungi G. Bawendi have been awarded the 2023 Nobel Prize for chemistry “for the discovery and synthesis of quantum dots”.
What is a quantum dot?
A quantum dot is a really small assembly of atoms (just a few thousand) around a few nanometres wide. The ‘quantum’ in its name comes from the fact that the electrons in these atoms have very little space to move around, so the crystal as a whole displays the quirky effects of quantum mechanics — effects that otherwise would be hard to ‘see’ without more sophisticated instruments. Quantum dots have also been called ‘artificial atoms’ because the dot as a whole behaves like an atom in some circumstances.
Why are they of interest?
There are two broad types of materials: atomic and bulk. Atomic of course refers to individual atoms and their specific properties. Bulk refers to large assemblies of atoms and molecules. Quantum dots lie somewhere in between and behave in ways that neither atoms nor bulk materials do. One particular behaviour distinguishes them: the properties of a quantum dot change based on how big it is. Just by tweaking its size, scientists can change, say, the quantum dot’s melting point or how readily it participates in a chemical reaction.
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When light is shined on a quantum dot, it absorbs and then re-emits it at a different frequency. Smaller dots emit bluer light and larger dots, redder light. This happens because light shone on the dot energises some electrons to jump from one energy level to a higher one, before jumping back down and releasing the energy at a different frequency. So, quantum dots can be easily adapted for a variety of applications including surgical oncology, advanced electronics, and quantum computing.
What did the Nobel laureates do?
For centuries, people have been creating coloured glass by tinting it with a small amount of some compound. How much of the compound, or dopant, is added and how the glass is prepared changed which colour the glass finally had. By the late 1970s, scientists had developed techniques to deposit very thin films on other surfaces and observe quantum effects in the films. But they didn’t have a material per se — an object wholly reigned by quantum effects. In the early 1980s, Alexei Ekimov, from the erstwhile Soviet Union, and his colleagues changed this. They added different amounts of copper chloride to a glass before heating it to different temperatures for different durations, tracking the dopants’ structure and properties. They found that the glass’s colour changed depending on the size of the copper chloride nanocrystals (which depended on the preparation process) — a telltale sign that the crystals were quantum dots. In 1983, a group led by Louis Brus in the U.S. succeeded in making quantum dots in a liquid — rather than trapped within glass, as in Dr. Ekimov’s work. Both Dr. Brus and Dr. Ekimov further studied quantum dots, working out a mathematical description of their behaviour and how it related to their structure. But both of them lacked one thing: a simple way to make quantum dots with just the right properties.
A team led by Moungi Bawendi at the Massachusetts Institute of Technology achieved this in 1993, with the hot-injection method. A reagent is injected into a carefully chosen solvent (with a high boiling point) until it is saturated, and heated until the growth temperature, that is, when the reagent’s atoms clump together to form nanocrystals in the solution. Larger crystals form if the solution is heated for longer. Their birth within a liquid makes their surfaces smooth. Finally, crystals of the desired size can simply be filtered out. This method accelerated the adoption of quantum dots in a variety of technologies.
What are quantum dots’ applications?
An array of quantum dots can be a TV screen by receiving electric signals and emitting light of different colours. Scientists can control the path of a chemical reaction by placing some quantum dots in the mix and making them release electrons by shining light on them. If one of the energy levels an electron jumps between in a quantum-dot atom is the conduction band, the dot can operate like a semiconductor. Also, solar cells made with quantum dots are expected to have a thermodynamic efficiency as high as 66%. A quantum dot can also highlight a tumour that a surgeon needs to remove, hasten chemical reactions that extract hydrogen from water, and as a multiplexer in telecommunications.
