Imagine a nanocrystal so minuscule that it behaves like an atom. Moungi G. Bawendi, Louis E. Brus and Alexei I. Ekimov have been awarded the 2023 Nobel Prize in Chemistry for discovering a category of such minute marvels, now known as quantum dots, and for developing a precise method of synthesizing them. Quantum dots are already playing important roles in electronics and in biomedicine, such as in drug delivery, imaging and medical diagnoses, and have more promising applications in the future, the Nobel Committee for Chemistry said in its announcement of the prize.
Quantum dots, sometimes called artificial atoms, are precise nanocrystals made of silicon and other semiconductor materials that are just a few nanometers wide — small enough to exhibit quantum properties just as individual atoms do, although they are a hundred to a few thousand atoms in size. Because electrons can be trapped at certain energy levels within them, the nanocrystals can emit only certain wavelengths of light. By controlling the size of the particles, researchers can program precisely what color the quantum dots will flash when stimulated.
Onstage at the Nobel Prize announcement this morning, Johan Åqvist, chair of the Nobel Committee for Chemistry, displayed a series of five flasks, each containing liquid glowing a different color. The fluids held liquid solutions of quantum dots only a few millionths of a millimeter in size. At this tiny size, “quantum mechanics starts to play all kinds of tricks,” Åqvist said.
Quantum mechanics predicts that if you take an electron and squeeze it into a small space, the electron’s wave function gets compressed, explained Heiner Linke, a member of the Nobel Committee for Chemistry and a professor of nanophysics. The smaller you make the space, the larger the energy of the electron, which means it can give more energy to a photon. In essence, a quantum dot’s size determines what color it shines. The smallest particles shine blue, while the larger ones shine yellow and red.
By the 1970s, physicists knew that quantum phenomena should in theory be associated with particles of extremely small size, just as they were with ultrathin films, but that prediction seemed impossible to test: There seemed to be no good way of making and handling particles except inside other materials that would mask their properties. In 1981 at the S.I. Vavilov State Optical Institute in the Soviet Union, however, Ekimov changed that. While adding compounds of copper and chlorine to a glass, he discovered that the color of the glass depended entirely on the size of those added particles. He quickly recognized that quantum effects were the likely explanation.
In 1983 at Bell Labs, Brus was running experiments on the use of light to drive chemical reactions. Brus (now at Columbia University) noticed that the size of nanoparticles also affected their optical properties even when they were floating freely in a liquid solution. “This triggered a lot of interest,” Linke said.
The potential optoelectronic utility of such particles was not lost on technologists, who followed the lead of Mark Reed of Yale University in referring to them as quantum dots. But for the next decade, researchers struggled to precisely control the size and quality of these particles.
In 1993, however, Bawendi invented an “ingenious chemical method” for making perfect nanoparticles, Åqvist said. He was able to control the exact moment in time when the crystals formed, and he was then able to stop and restart further growth in a controlled manner. His discovery made quantum dots widely useful in a variety of applications.
The applications for these nanoparticles range from LED displays and solar cells to imaging in biochemistry and medicine. “These achievements represent an important milestone in nanotechnology,” Åqvist said.
What are quantum dots?
They are human-made nanoparticles so small that their properties are governed by quantum mechanics. Those properties include the emission of light: The wavelength of light they emit depends solely on the size of the particles. Electrons in larger particles have less energy and emit red light, whereas electrons in smaller particles have more energy and emit blue light.
Researchers can precisely determine what color of light will emerge from the quantum dots simply by regulating their size. That offers a huge advantage over the use of other kinds of fluorescent molecules, for which a new type of molecule is needed for every distinct color.
This advantage in controllability isn’t limited to the color of quantum dots. By adjusting the size of the nanoparticles, researchers can also adjust their electrical, optical and magnetic effects, as well as physical properties like their melting point or how they influence chemical reactions.
How did Bawendi’s work make quantum dots practical?
In 1993, Bawendi and his team at the Massachusetts Institute of Technology developed a method to produce quantum dots more precisely and with higher quality than previously possible. They found a way to grow the nanocrystals in an instant by injecting their chemical precursors into an extremely hot solvent. The researchers then immediately stopped the growth of the crystals by lowering the temperature of the solvent, creating infinitesimal crystalline “seeds.” By slowly reheating the solution, they could regulate further growth of the nanocrystals. Their method produced crystals of a desired size reproducibly, and it was adaptable to different systems.
Where are quantum dots being used?
If you’ve ever watched programs on a QLED TV, you’ve seen these nanoparticles at play. But they’re also being put to use in biomedical imaging and lighting. Researchers are still exploring additional applications for these nanoparticles in quantum computing and communications, flexible electronics, sensors, efficient solar cells, and catalysis for solar fuels.