The Nobel Prizes are some of the most prestigious awards in their field, and each year one is awarded in the areas of chemistry, physics, literature, economics, peace, and physiology or medicine. It is great to see chemistry recognized among the most impactful fields of research, and it is likely because Alfred Nobel, who established the prizes, was a chemist himself!
The 2023 Nobel Prize in Chemistry was awarded to three individuals, Alexei Ekimov, Louis Brus, and Moungi Bawendi, for their discovery and creation of a new class of materials called quantum dots. Quantum dots’ uniqueness stems from the fact that their physical properties can be tuned by modifying just one parameter – their size. Their redox potential, melting and boiling temperatures, and phase transition are some of the many properties that are directly affected by their size. This speaks to the core ideology of nanoscience which posits that at the scale of nanometres, materials gain new size-dependent properties.
The first Nobel Laureate, Alexei Ekimov, was recognized for his work in dealing with CuCl glass matrices. His team observed that by tightly regulating temperature and duration they could control the sizes of crystals forming as their glass materials changed phase. Using X-ray scattering, they found crystal sizes ranging from a few nanometres to tens of nanometres, and what they noticed was that the CuCl absorption lines differed depending on the crystal size. The result of this was different colours of glass depending on the size of the CuCl nanoparticle. Ekimov was quick to relate this to quantum size effects, and his work was the first ever demonstration of independent nanoparticles displaying this ability. The applicability of his work however was limited as his quantum dots were solids encased inside other solid materials.
A couple years later, and independently of Ekimov, Louis Brus also discovered these quantum size effects; however, his work involved CdS crystals in solution, which introduced new uses for these materials. He found that the smaller the particles the shorter the wavelength they emit (Figure 1). Brus used this knowledge to present a new model which described many novel aspects of quantum size effects, including the relation redox potentials, excitation energies, and Coulomb interactions.
Figure 1: Varying solution colour due to differing emission wavelengths determined by quantum dot nanoparticle size. The smaller the quantum dot, the bluer the wavelength it emits. Figure adapted from https://www.compoundchem.com/2023/10/04/2023nobelchemistry/
Up to this point, further research of quantum dots was limited by inefficient syntheses as lack of control over size, shape, and other physical properties made it difficult to determine explicitly size-dependent behaviour. This was until in 1993 when Moungi Bawendi, the third Nobel Laureate, developed a synthesis method with much greater control. Their synthesis allowed a wide range of material systems to be used, including metal nanoparticles for the first time, and therefore introduced the ability to produce quantum dots on a much larger scale.
Nanoparticles and nanotechnology have been unknowingly used for several millennia. Works dating back to the Greco-Roman period have listed techniques to alter the colour of materials, however, it wasn't until these three scientists that the relation to size was discovered and verified as well as effective synthesis methods. Presently, quantum dots are used in many applications such as QLED televisions and LED lamps, and current work is also showing future promise in the use of flexible electronics and thinner solar cells.
Material in this article is adapted from the scientific background document written by Heiner Luke as part of the presentation of the Nobel Prize announcement. Read more at https://www.nobelprize.org/uploads/2023/10/advanced-chemistryprize2023.pdf
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