The prize will be awarded to three innovators in electronic security system
The Nobel Prize in Chemistry this year will be awarded to three pioneers of tiny devices called Quantum Dots. The laureates are Moungi Bawendi from the Massachusetts Institute of Technology in the United States, Louis Brus from Columbia University in New York, and Alexey Ekimov, a Russian chemist who currently works in a pharmaceutical company in the United States. Quantum Dots are nano-particles, that is, tiny crystals of material that function as semiconductors, and can be tailored for a vast range of applications due to the ability to control their electric and optical properties. Quantum Dots are used in electronic components, solar cells, medical imaging devices, microscopes, lasers, lighting devices, and more.
A Breakthrough with many potential applications. From right: Ekimov, Brus, and Bawendi | Illustration from the Nobel Prize website, Nobelprize.org
In a typical crystal, such as a salt crystal, the atoms or molecules are arranged in a repeating pattern that theoretically extends infinitely. In contrast, nano-crystals are constituted by the same atoms or molecules, yet they are finite in size and very diminutive. These nano-scale particles consist of several thousands or even tens of thousands of atoms, with the dimensions of each particle measured in nanometers (i.e. the millionths of a millimeter). The realm of nano-particles encompasses a diverse array of variations and a wide variety of nano-particles with different geometric shapes and chemical compositions. These intrinsic characteristics dictate their physical properties, such as magnetism, electricity, and optics, as well as their chemical properties, i.e., their tendency to react with certain substances and not with others. Researchers have the capacity to precisely control the structure and composition of nano-particles by manipulating several parameters of the chemical reaction that creates them, such as the choice of solvent, temperature, and external fields
A quantum dot refers to a nano-particle in which a phenomenon known as quantum confinement takes place, causing it to behave as a semiconductor, meaning it can change its electrical conductivity under certain conditions. Quantum confinement occurs when an electron becomes essentially "trapped" within a molecule or a crystal. The smaller the crystal, the more restricted is the mobility of the electron. In physical terms, it has a narrower energy band: fewer quantum states available for the electron, or in other words, different energy quantizations. As long as the quantum dot doesn't receive energy from an external source, the electrons are confined to this energy band, called the "valence band".
When a quantum dot absorbs energy from an external source, such as a beam of light hitting it, the electrons have the potential to transition to higher energy levels even beyond the highest level in the valence band; the lowest of these levels is called the "conduction band". Essentially, the electron is captured within the crystal, but if the external energy supply ceases, it falls back to the valence band, releasing a single photon – a discrete unit of light – with energy corresponding to the energy difference between the levels the electron jumped from and to. In a typical solid material, many electrons can collapse back to the valence band at once, emitting many photons. Nevertheless, specific applications in optics and biology demand the emission of a single photon at precise moments, underscoring the unique and critical role of quantum dots in these scenarios.
The ability to engineer quantum dots - in terms of the electron's "lifetime", stability, etc., to fit research or application needs, makes them a subject of intensive research and many applications. Innovative televisions today employ quantum dots instead of LED lights to provide higher resolution. Another vital application is in medical imaging - the stability of these particles for high-resolution images of biological tissues. Some use quantum dots for real-time tracking of molecules and cells in their movement, and even for drug delivery to specific target sites in the human body..
A diverse range of materials can form quantum dots, including carbon, silicon, germanium, cadmium selenide (CdSe), and indium gallium arsenide (InGaAs). Each has its properties; over the years, core-shell type quantum dots have been engineered, meaning they contain a core of one material and a shell of another. This structure enables the trapping of electrons on the surface of the dot, offering advantages in creating specific of optical devices
The size of the quantum dot affects its optical properties and the wavelength, that is, the color of the light emitted by it | Illustration: © Johan Jarnestad/The Royal Swedish Academy of Sciences
The possibility to engineer quantum dots in terms of the "lifetime" of the electronic excitation, the stability, and more, so as to make them suitable for research or application purposes, makes them a subject of intensive research with many potential applications. Some innovative TVs, for instance, have replaced traditional LED lights with quantum dots to achieve higher resolutions. An even more significant application of quantum dots is in medical imaging, where the inherent stability of these particles allows for high-resolution imaging of biological tissues. Some also employ quantum dots for real-time monitoring of molecules and cells in motion, and even for targeted drug delivery within the body.
Among the materials that can be organized in the form of quantum dots are carbon, silicon, germanium, cadmium selenide (CdSe), and indium gallium arsenide (InGaAs). Each material has different properties. Over the years, core-shell quantum dots have been engineered so as to, as their name suggests, contain a core of one material and a shell of another. This design allows trapping of electrons on the surface of the dot, which offers an advantage in the fabrication of certain optical devices.
A quantum dot is typically composed of only a few thousand atoms, and thus the size ratio between a quantum dot and a soccer ball is akin to the size ratio between a soccer ball and planet Earth | Illustration: ©Johan Jarnestad/The Royal Swedish Academy of Sciences.
A 40-Year Journey
Alexei Ekimov was one of the first pioneers of research into quantum dots. Born in Russia in 1945, he studied physics at Saint Petersburg State University, and earned his doctorate in physics from the Ioffe Institute. He then delved into research of semi-conductive nano-crystals at the Vavilov State Optical Institute. In 1981, he published the first paper on the unique quantum properties of microscopic semiconductor crystals within a glass matrix, before these were named "quantum dots". In an attempt to "engineer" these dots, he adjusted the temperature and heating duration of the glass, thus creating dots in a relatively wide range of sizes; later, he delved into their quantum, optical, and electrical properties. His research suggested that the dots represented "artificial atoms", in the sense that the energies of the dots, such as those of atoms, are discretely defined, and both contain a relatively small number of electrons
Louis Brus, born in 1943, worked at Bell Laboratories from 1973 - a major private research laboratory in the United States. There, in the early 1980s, independently of Ekimov's work, he developed quantum dots that operated in solution. Similar to Ekimov, he was intrigued by the relationship between the size of quantum dots and their properties. In 1996, he left Bell Laboratories to take up a research position at Columbia University in New York, where he continues to work to this day. His research there, among other things, focused on Rayleigh scattering, better known as the phenomenon that makes the sky appear blue - carbon nanotubes.
Moungi Bawendi, born in 1961, is a French-American chemist of Tunisian descent and a professor at the Massachusetts Institute of Technology (MIT). Bawendi also conducts research on quantum dots in solutions, with a particular emphasis on organic fluorophores - quantum dots based on organic materials that emit colorful light through fluorescence mechanisms. He is particularly known for his invention of the "rapid injection" method, which is widely used for creating quantum dots. Additionally, his research of quantum dots has applications in biology and spectroscopy
Last year, the chemistry Nobel Prize was awarded to Carolyn Bertozzi, Morten Meldal, and Karl Barry Sharpless, for developing new methods of building molecules and attaching them to each other. Sharpless became the fourth person in history to win the Nobel Prize in a scientific field twice. In 2021, the Nobel Prize in Chemistry prize was awarded to Benjamin List and David MacMillan for developing the use of small molecules as catalysts for organic reactions, especially for distinguishing between two forms that are "mirror images" of the same molecule, each having a different biological activity. In 2020, the prize was awarded to two developers of the CRISPR genome editing method, Emmanuelle Charpentier and Jennifer Doudna.