The first people to make and use quantum dots were glassmakers. Working thousands of years ago, they realized that the same chemical mixture could turn glass into different colors, depending on how they heated it.
This year’s Nobel Prize in Chemistry honors three scientists who, along with their colleagues, students, and staff, figured out why the ancient glassmakers’ methods worked — and how to control them much more precisely. During the waning days of the Cold War, Alexei Ekimov and Louis Brus, working in separate labs on opposite sides of the Iron Curtain, both discovered the same thing: that tiny crystals (just millionths of a millimeter wide) act very differently than larger pieces of the exact same material. These tiny, weird crystals are called quantum dots, and just a few years after the Berlin Wall fell, Moungi Bawendi figured out how to mass-produce them.
That changed everything. Quantum dots are crystals so small that they follow different rules of physics than the materials we’re used to. Today, these tiny materials help surgeons map different types of cells in the body, paint vivid color images on QLED screens, and give LED lights a warmer glow.
Clear As Glass
By the early 1980s, chemists understood that the same pigment could turn glass completely different colors depending on the size of the pigment crystals – but they weren’t sure why, and that bothered Ekimov. Working with nanometer-sized crystals of copper chloride in glass, he discovered that the tinier the crystals got, the more blue light they absorbed. Rhis was confusing because copper chloride normally absorbs yellow light and reflects blue. In fact, for that reason, it’s often used in fireworks to make a blue/green color.
Two years later, Brus — unaware of what Ekimov had found, because the Cold War blocked most communication between scientists in the USSR and the U.S. — noticed something very similar about tiny crystals of cadmium sulfide, which he was hoping would capture solar energy and use it to power chemical reactions.
Both Ekimov and Brus realized they’d discovered a quantum effect: something that happens when materials are so small they’re governed by quantum mechanics, not mundane physics. The particular quantum effect they’d discovered happened to be the same one that ancient glassmakers had used to get bright reds and yellows out of the same pigment — a mix of cadmium selenide and cadmium sulfide — when they heated it to different temperatures for varying lengths of time. Changing the heating process caused the pigment to form nanocrystals of different sizes, which absorbed different amounts of blue light and emitted light in different colors.
Those ancient glassmakers knew their methods worked, but they didn’t know the underlying mechanics. That’s what Ekimov and Brus discovered, and they gave the tiny crystals a name: quantum dots.
A Different Set of Rules
Ekimov and Brus discovered quantum dots because of the way these tiny crystals absorb and emit light, which changes their color – but that’s also a clue that nearly all of the material’s properties are different when it’s shrunk down to nanometers – millionths of a millimeter. At that scale, electrons act differently than they otherwise would, because the rules of regular physics have taken a backseat to the rules of quantum physics.
If you want to understand why something conducts electricity, reacts with particular chemicals, or appears a certain color, the answer probably boils down to electrons. (And if you want to know more about electrons, this year’s Nobel Prize in Physics is a great place to start.)
When Ekimov and Brus saw their tiny nanocrystals changing colors — and understood why — “they understood that, in principle, they were looking at an entirely new material,” explains the Royal Swedish Academy of Sciences, the organization that awards the Nobel Prizes. “An element’s properties are not only affected by the number of electron shells and how many electrons there are in the outer shell, but at the nano level, size also matters. A chemist who wanted to develop a new material thus had another factor to play with.”
However, it was still difficult to make quantum dots in a consistent size or with a consistent quality, which meant it would be hard to put them to any practical use. Bawendi figured out that piece of the puzzle about a decade later, in 1993. He injected material into a liquid solvent until the liquid was saturated — meaning that it couldn’t dissolve any more of the material (you can see this for yourself by stirring sugar into a glass of warm water). Then Bawendi carefully varied the temperature of a solution so it would grow crystals of exactly the right size with a smooth, even surface.
Bawendi’s work is what finally opened the door for scientists and engineers to develop new ways to use quantum dots in medicine, electronics, and chemistry. Today, you’ve probably got quantum dots in your TV screen and computer monitor, where they absorb blue light and convert it to red or green. Surgeons use quantum dots to help map different tissues in the body, which helps them find cancerous tumors. And chemists use quantum dots to kickstart chemical reactions to produce other useful materials.