Stockholm: Moungi Bawendi, Louis Brus and Alexei Ekimov were awarded the 2023 Nobel Prize in Chemistry on Wednesday for the discovery and development of quantum dots. These tiny particles have unique properties and now spread their light from television screens and LED lamps. They catalyse chemical reactions and their clear light can illuminate tumour tissue for a surgeon.
Size matters on the nanoscale:
The 2023 Nobel Laureates in Chemistry have all been pioneers in the exploration of the nanoworld. In the early 1980s, Louis Brus and Alexei Ekimov succeeded in creating – independently of each other – quantum dots, which are nanoparticles so tiny that quantum effects determine their characteristics. In 1993, Moungi Bawendi revolutionised the methods for manufacturing quantum dots, making their quality extremely high – a vital prerequisite for their use in today’s nanotechnology.
Thanks to the work of the laureates, humanity is now able to utilise some of the peculiar properties of the nanoworld. Quantum dots are now found in commercial products and used across many scientific disciplines, from physics and chemistry to medicine – but we are getting ahead of ourselves.
For decades, quantum phenomena in the nanoworld were just a prediction:
When Alexei Ekimov and Louis Brus produced the first quantum dots, scientists already knew that they could – in theory – have unusual characteristics. In 1937, the physicist Herbert Fröhlich had already predicted that nanoparticles would not behave like other particles. He explored the theoretical consequences of the famous Schrödinger equation, which shows that when particles become extremely small there is less space for the material’s electrons. In turn, the electrons – which are both waves and particles – are squeezed together. Fröhlich realised that this would result in drastic changes to the material’s properties.
Researchers were fascinated by this insight and, using mathematical tools, they succeeded in predicting numerous size-dependent quantum effects. They also worked to try to demonstrate them in reality, but this was easier said than done because they needed to sculpt a structure that was about a million times smaller than a pinhead.
A single substance can give glass different colours:
Glass, an ancient material dating back thousands of years, exhibits a variety of colors due to the intentional addition of substances like silver, gold, or cadmium, combined with precise temperature adjustments during production.
In the nineteenth and twentieth centuries, physicists leveraged this understanding to utilise coloured glass for filtering specific light wavelengths, enhancing their experiments. They realized that a singular substance could yield diverse glass colors, depending on heating and cooling conditions. Notably, the colors resulted from particles forming within the glass, with the particle size dictating the hue.
This was more or less the state of the knowledge at the end of the 1970s, when one of this year’s laureates, Alexei Ekimov, a recent doctoral graduate, started working at the S.I. Vavilov State Optical Institute in what was then the Soviet Union.
Alexei Ekimov maps the mysteries of coloured glass:
Alexei Ekimov, a Nobel Prize Laureate, delved into the enigma of colour variation in glass, combining his knowledge of semiconductors and optical methods. Through meticulous experiments with copper chloride in molten glass, he discovered quantum dots—nanoparticles exhibiting size-dependent quantum effects for the first time.
During his doctoral degree, Ekimov studied semiconductors – important components in microelectronics. In this field, optical methods are used as diagnostic tools for assessing the quality of semiconducting material. Researchers shine light on the material and measure the absorbance. This reveals what substances the material is made from and how well-ordered the crystal structure is.
Ekimov was familiar with these methods, so he began using them to examine coloured glass. After some initial experiments, he decided to systematically produce glass that was tinted with copper chloride. He heated the molten glass to a range of temperatures between 500°C and 700°C, varying the heating time from 1 hour to 96 hours. Once the glass had cooled and hardened, he X-rayed it. The scattered rays showed that tiny crystals of copper chloride had formed inside the glass and the manufacturing process affected the size of these particles. In some of the glass samples they were only about two nanometres, in others they were up to 30 nanometres.
Interestingly, it turned out that the glass’ light absorption was affected by the size of the particles. The biggest particles absorbed the light in the same way that copper chloride normally does, but the smaller the particles, the bluer the light that they absorbed. As a physicist, Ekimov was well acquainted with the laws of quantum mechanics and quickly realised that he had observed a size-dependent quantum effect.
This was the first time someone had succeeded in deliberately producing quantum dots – nanoparticles that cause size-dependent quantum effects. In 1981, Ekimov published his discovery in a Soviet scientific journal, but this was difficult for researchers on the other side of the Iron Curtain to access. Therefore, this year’s next Nobel Prize Laureate in Chemistry – Louis Brus – was unaware of Alexei Ekimov’s discovery when, in 1983, he was the first researcher in the world to discover size-dependent quantum effects in particles floating freely in a solution.
Brus shows that the strange properties of particles are quantum effects:
Louis Brus, independently and unaware of Ekimov's discovery, observed similar size-dependent quantum effects in particles like cadmium sulphide. This groundbreaking revelation indicated that altering a substance's size could revolutionize its properties, akin to a third dimension added to the periodic table.
However, challenges in controlling particle size hindered progress until Moungi Bawendi's innovative approach revolutionized quantum dot production. By injecting precise substances into a carefully chosen solvent and dynamically adjusting temperature, Bawendi achieved consistent nanocrystal growth, paving the way for broader applications.
Now, quantum dots, with their tunable luminescence based on size, have become integral to nanotechnology, finding applications in QLED screens, LED lamps, biochemistry, and more. They offer promising avenues for future technologies, including flexible electronics, sensors, and efficient solar cells, revealing a world of potential waiting to be explored in the realm of quantum phenomena.
