Moungi Bawendi, Louis Brus and Alexei Ekimov. [Image: Ill. Niklas Elmehed © Nobel Prize Outreach]
The 2023 Nobel Prize in Chemistry has gone to three pioneers in the development of the semiconductor nanocrystals known as quantum dots: Alexei I. Ekimov of Nanocrystals Technology Inc., USA, Louis E. Brus of Columbia University, USA, and Moungi G. Bawendi of the Massachusetts Institute of Technology (MIT), USA.
It’s all about the size
Quantum dots (QDs) are nanoscale crystals of a semiconducting material whose physical properties—particularly their emission wavelength, or color—depend solely on their size. As the name implies, the property/size relationship is a quantum effect. At the nanometer scale, smaller particle size implies tighter electron confinement, higher energy and, thus, a shorter emission wavelength. In essence, a smaller nanocrystal will emit light tending toward the blue end of the visible spectrum; a larger crystal will emit at the red end.
This simple relationship has spawned a significant market (currently estimated at more than US$4 billion, according to the Nobel Commission)—and a vast application space. QDs have been used as tiny emitters in certain displays, TVs and illumination; for biomedical sensing; as single-photon sources in certain quantum-computing and communications setups; in lasers; in photodetectors; in experiments in photocatalysis and solar-energy conversion; and much more.
Looking at glass suspensions
So much for QDs today. But how, practically, do you make them? Solving that problem required the substantial intellectual and experimental contributions of the three 2023 Chemistry Laureates.
The notion that, at sufficiently small length scales, a crystal’s properties might vary with its size goes back to experiments and modeling in the late 1930s, and was followed up by several decades of work fleshing out quantum size effects. This was particularly true in semiconducting thin films, where researchers during the 1960s documented quantum size effects in resistivity and other properties. Such studies also made it clear, however, that taking advantage of such size effects at the required nanometer length scales would require materials of very high quality.
[Image: © Nobel Prize Outreach. Photo: Clément Morin]
In 1979, one of the three 2023 Chemistry Laureates, Alexei Ekimov—then at the S.I. Vavilov State Optical Institute in the Soviet Union—was studying silica glasses that had been doped with a few weight percent of copper and chlorine. Ekimov and his team discovered that the emission spectra of the glasses varied depending on the temperature and duration details of the heat treatment used to create the glasses.
What’s more, the observed wavelengths correlated nicely with calculations of the expected average size of calcium chloride nanocrystals that formed in the glass melt and were frozen in when the material solidified. In particular, there was an increasing blue shift as the crytsals became smaller (down to a few nanometers in diameter). In classic papers in two Soviet journals, Ekimov made the crucial connection, tying these observations to quantum size effects.
QDs in liquids
Frozen in glass, quantum dots aren’t of much use. At around the same time as Ekimov’s pioneering work, the second of the three 2023 Chemistry Laureates, Louis Brus—then working at Bell Labs in Murray Hill, NJ, USA—was pushing some of the same ideas in a more potentially practical dimension.
Brus and his colleagues were interested in exploiting cadmium sulfide (CdS) nanocrystals for specific applications in photochemistry and photocatalysis. To do the work, the team used previously developed synthesis methods to create suspensions of particles with an average diameter narrowly distributed around 4.5 nm. However, in one solution that had been allowed to age for a day, the particles in the suspension recrystallized to a size distribution clustered around a larger diameter—approximately 12.5 nm.
Brus and the team discovered that the emission spectrum of the suspension with the larger-sized crystals had been red-shifted relative to that of the smaller-sized crystals. Like Ekimov, Brus attributed this to quantum size effects, and he and his team reported the finding in the Journal of Chemical Physics in 1983. (Ekimov had actually made the same discovery in glasses two years earlier—but the Brus team was unaware of those findings, as they had been reported only in Soviet journals that were not readily available in the United States.) Brus and colleagues went on to flesh things out in a detailed model tying particle size to chemical potential and energy.
For their pioneering work in quantum dots and quantum size effects, Ekimov and Brus shared the 2006 R.W. Wood Prize of The Optical Society (now Optica).
Toward better synthesis—and beyond
The third of the 2023 Chemistry Laureates, Moungi Bawendi, and his MIT colleagues took these early observations and approaches to synthesis, and brilliantly extended them (in work published in 1993 in the Journal of the American Chemical Society) into a practical method for synthesizing quantum dots of well-defined dimension and optical quality in colloidal suspensions. The specific method, known as hot-injection synthesis, enabled the scaling up of production of quantum dots to industrial levels—and, thus, their use in practical applications from TV screens to biomedicine to quantum computing.
The Swedish Academy, in initially announcing the prize, paid tribute to QDs’ place in laying the foundations of modern nanotechnology.
Since these early days, significant work has continued in QD design, development and synthesis. Of particular interest has been the emergence of so-called core–shell nanoparticles, such as cadmium-selenium (CdSe) quantum dots, for applications such as frequency up-conversion and energy conversion. Water-soluble QDs are making inroads into in vivo biomedical labeling, imaging and treatment. And the tiny crystals are finding use in photovoltaics, photodetection and other core optical areas.
In a larger sense, the Swedish Academy, in initially announcing the prize, paid tribute to QDs’ place in laying the foundations of modern nanotechnology. This year’s Laureates, the Academy concluded, “provided seeds for the rich field of nanoscience to grow.”
A little Nobel drama
While the Nobel Prize announcements have become rather stylized media events, this year’s unveiling of the Chemistry prize featured a bit of unintentional drama. A few hours before the formal prize announcement, a press release revealing the Laureates was apparently mistakenly leaked to Swedish media—so that the identities of the Laureates, usually a closely guarded secret until the moment of the announcement, was already being circulated beforehand.
While the Nobel Prize announcements have become rather stylized media events, this year’s unveiling of the Chemistry prize featured a bit of unintentional drama.
Questioned about the glitch, Hans Ellegren, the secretary general of the Royal Swedish Academy, said that the Nobel authorities “deeply regret what happened, for sure” and “have been very active this morning trying to find out what actually happened.” He went on to stress, however, that “the important thing is that it did not affect the … prize recipients in any way.”
The nomination and selection “is a very long process that goes on for a very long time,” Ellegren said. “The decision about the prize is not taken until the Academy has met, and the Academy met this morning.”