Moungi Bawendi, Louis Brus, and Alexei Ekimov share award for developing nanoparticles with beguiling properties determined by their size.
Three researchers have been awarded this year’s Nobel Prize in Chemistry for work on quantum dots—tiny crystals a few dozen atomic diameters wide that have highly tunable optical and electronic properties.
Moungi Bawendi of the Massachusetts Institute of Technology, Louis Brus of Columbia University, and Alexei Ekimov of Nanocrystals Technology Inc. will share a prize of roughly $1 million for discovering the crystals and showing how to produce them reliably. In doing so, they “planted an important seed for nanotechnology,” the Royal Swedish Academy of Sciences said in a press release this morning. The dots, which fluoresce in brilliant colors, have found applications in television and computer displays, LED lighting, and medical imaging. Scientists now envision using them to create tiny lasers, improved solar cells, and quantum computers.
The award “is a great example of fundamental science connected to things where you already see applications from the work,” says Cherie Kagan, a materials engineer at the University of Pennsylvania who was a former graduate student of Bawendi’s. “There’s a lot more in front of us.”
Theorists speculated about the powers of such tiny structures as long ago as the 1930s, in the early days of quantum mechanics. The theory, which describes the behavior of the atomic world, implied that crystals a millionth the size of a pinhead would act like a box, confining electrons in a way that alters their properties. A smaller box would compress the electron’s wavelike properties to shorter wavelengths. When stimulated by an external source of light, the electrons of a smaller quantum dot should emit bluer, shorter wavelength light. A larger dot should emit longer wavelength yellow or red light.
In the late 1970s, Ekimov, then at the Vavilov State Optical Institute in Russia, first managed to make nanometer-size crystals of copper chloride, embedded in glass. He confirmed that dots of different size fluoresced in different colors.
A few years later, Brus, then at Bell Labs and working independently of Ekimov, was looking for catalysts to capture the energy of sunlight in a chemical reaction. When Brus’s team crystallized particles of cadmium sulfide out of a solution, they noticed that the larger ones reacted to light differently than the smaller ones and realized it was the same quantum phenomenon. This research also removed a major limitation of Ekimov’s dots, which were “frozen” in glass. Brus’s dots were suspended in solution, which gave them the ability to flow and made them more attractive for applications like displays.
Still, defects in these early quantum dots—especially their variable sizes—kept them from wide commercialization. In 1993, Bawendi and his team, also at Bell Labs, devised a way to make high-quality crystals of a well-defined size that would produce sharp, vivid light of one specific color.
Bawendi’s breakthrough involved injecting the chemical ingredients for quantum dots into a hot solvent to immediately form crystal seeds, and then quenching their growth by rapidly cooling and diluting the solvent. To make dots of a required size—usually 2 to 10 nanometers nowadays—the solvent is slowly warmed up again to continue crystal growth in a more controlled way. “It was just an exciting time,” says David Norris, a materials engineer at ETH Zürich and a former graduate student of Bawendi’s. “It was a really great environment that Moungi had created in those early days.”
Many companies now use variations of the process as they compete to produce quantum dots cheaply for different technologies, using semiconductor materials such as zinc selenide, cadmium selenide, or indium phosphide. The market for quantum dot applications in the United States alone reached $4 billion in 2021, and some 8% of the global TV market now relies on quantum dots to add brilliant colors.
But researchers are testing other applications. In medicine, doctors want to use quantum dots as tissue-specific beacons to hunt for tumors or other problems. For example, quantum dots covered in organic materials to make them more biocompatible inside cells and blood could map blood vessels and lymph nodes or monitor changes in tumors. The dots could also help track the movements of drugs throughout the body.
In solar cells, quantum dots could be tuned to absorb a wider spectrum of light and convert it more efficiently into electrical energy. Because the dots produce such specific wavelengths of light, they could also act as microscopic lasers to optically shuttle information around computer chips, reducing heat loss and making chips more energy efficient.
Perhaps the most exciting potential use is in quantum computers, which have the potential to outstrip even supercomputers in some applications. Some researchers are trying to manipulate the spins of quantum dot electrons as the gates and switches of a quantum computer, while others hope to exploit individual photons produced by quantum dots for a light-driven quantum computer. “That’s not in the commercial domain yet, but that could be the future,” says Mark Fox, an optical physicist at the University of Sheffield.
However, quantum computing would require quantum dots that differ from those in television displays, says Sofia Patomäki, a recent Ph.D. graduate at University College London. Among other reasons, quantum dots for displays often rely on materials that are optimized for producing light in the visible spectrum, whereas materials that best retain quantum dot spin properties may be better for quantum computing.
The names of the three chemistry laureates—normally a closely guarded secret—were leaked to Swedish newspapers 4 hours before the Nobel committee’s official announcement. It’s not clear what happened, and flustered officials said they are investigating the breach. But the news did not seem to have reached at least one of the laureates. Speaking on the phone during the press conference, Bawendi pronounced himself “surprised, sleepy, shocked … and very honored” by the award.
He added, “There’s still a lot of exciting work to be done in this field, that’s for sure.”