The Nobel Prize in Chemistry 2023 is awarded to three researchers for their discovery and development of nanostructures that have already transformed the technology of television and may in the future have a huge impact on medical imaging.
It’s safe to say that most of us had not heard of quantum dots before this year’s Nobel Prize in Chemistry, awarded to Moungi G. Bawendi, Louis E. Brus and Aleksey Yekimov for the discovery and development of these particles. So, what are quantum dots and why do they warrant a Nobel Prize? Quantum dots are miniscule particles, between two and ten nanometers in size. To help in understanding exactly how small this is, Professor Heiner Linke, member of the Nobel Committee for Chemistry and of the Council for the Lindau Nobel Laureate Meetings, makes the following analogy: If we can imagine how much smaller a soccer ball is than the entire planet earth, then a quantum dot is that much smaller than that soccer ball. In fact, quantum dots consist only of a few hundred atoms. However, it’s not their size that is so amazing. Rather, it is their unique properties. Quantum dots can conduct electricity and then emit light in response. Amazingly, just adjusting the size of the particle changes its light-emitting activity, that is the wavelength of the light, and therefore the colour, that is emitted. Why is that the case? The answer lies in quantum mechanics. When an electron is part of such a small nanoparticle, then its wave is compressed, changing the properties of the whole material. When a collection of differently sized quantum dots is then exposed to a light source, each dot emits a colour of a specific wavelength: the larger the dot, the more the emitted light will skew toward red, while for smaller dots, light is skewed more toward the green end of the spectrum.
Tiny but Mighty: The Story of the Quantum Dots
The story of quantum dots began 40 years ago in Soviet Russia and the USA. Behind the Iron Curtain, Yekimov and Alexander Efros, who were based at the S. I. Vavilov State Optical Institute and A. F. Ioffe Institute, Russia, respectively, began studying semiconductor-doped glasses. At the same time, on the other side of the world, Louis E. Brus at Bell Laboratories in New Jersey was also investigating semiconductor particles. Independently, and unaware of each other’s progress, the Russians and the Americans managed to develop semiconductor nanocrystals and arrive at a theoretical explanation for their size-dependent optical properties. It took several years before the two groups learned of each other’s results and progress, however, with the first link established when Brus obtained English translations of the Ekimov papers, prompting him to contact the author. The last piece of this Nobel puzzle was Moungi G. Bawendi’s breakthroughs in the early 1990s in reliably synthesising quantum dots in a controlled manner, which until then had not been possible.
As with any great breakthrough in science, many unsung heroes also made important contributions. To name but two who were highlighted in a recent article in Nature: Christopher Murray, who was one of the first PhD students hired by Bawendi, contributed to developing a method to synthesise quantum dots and Manoj Nirmal, also an early graduate student of Bawendi, explored the properties of quantum dots in the early 1990s working together with the group of Louis E. Brus.
Applications for Quantum Dots
Although their unique properties mean they hold huge intrinsic interest for chemists and physicists, quantum dots have attracted broader attention due to their highly diverse and important commercial applications: they are used in television displays and lighting, improving the brightness and colour of light-emitting diodes (LEDs). They also hold great promise for medical diagnostics as fluorescent markers that can bind to cancer cells, for instance.
In televisions, quantum dot technology essentially allows producers to make sets with screens that enable an enormous number of intense colours, therefore promoting accuracy and providing for a visual experience that closely resembles the world that we see around us. In LED lights, the use and mixing of quantum dots allows for minimising blue light – perceived by many to be cold and unpleasant – and instead to provide a light that mimics sunlight. In cancer diagnosis and treatment meanwhile, quantum dots could be used to track tumour growth and response to therapy. Specifically, as quantum dots can emit fluorescence in a much more stable manner than other molecules that re-emit light upon light excitation, this makes them potentially highly promising candidates for imaging during fluorescence-guided surgery. Excitingly, Linke is convinced that these are only some of the very many applications that quantum dots will find in the future.