Researchers unlock a ‘new synthetic frontier’ for quantum dots

The type of semiconducting nanocrystals known as quantum dots are both expanding the cutting edge of pure science and hard at work in practical applications, including lasers, quantum QLED televisions and displays, solar cells, medical devices and other electronics.

A new technique to grow these microscopic crystals, published this week in Sciencehas not only found a new, more efficient way to build a useful type of quantum dot, but has also opened up a whole group of new chemical materials for future exploration by researchers.

“I’m excited to see how researchers around the world can use this technique to prepare previously unimaginable nanocrystals,” said first author Justin Ondry, a former postdoctoral researcher in UChicago’s Talapin Lab.

The team – including researchers from the University of Chicago, University of California Berkeley, Northwestern University, the University of Colorado Boulder and Argonne National Laboratory – achieved these remarkable results by organic solvents usually used to make nanocrystals molten salt-literally overheated sodium chloride the type sprinkled on baked potatoes.

“Sodium chloride isn’t a liquid in your mind, but suppose you heat it to such a crazy temperature that it becomes a liquid. It looks like a liquid. It has the same viscosity as water. It’s colorless. The only problem was that no one has ever thought of these liquids as media for colloidal synthesis,” says Prof. Dmitri Talapin of the UChicago Pritzker School of Molecular Engineering (UChicago PME) and the Department of Chemistry.

Why salt?

Quantum dots are among the better known nanocrystals, not only because of their wide commercial applications, but also because of the recent Nobel Prize in Chemistry 2023 given to the team that discovered them.

“If there is a material from the world of nano that has had an impact on society in terms of applications, it is the quantum dot,” says UC Berkeley Prof. Eran Rabani, co-author of the paper.

However, much of the previous research is about quantum dotsincluding the Nobel work, consisted of dots grown using combinations of elements from the second and sixth groups of the periodic table, Rabani said. These are called “II-VI” (two-six) materials.

More promising materials for quantum dots can be found elsewhere in the periodic table.

Materials found in the third and fifth groups of the periodic table (III-V materials) are used most efficiently solar cellsbrightest LEDs, most powerful semiconductor lasers and fastest electronic devices. They could potentially make large quantum dots, but with a few exceptions it was impossible to use them to grow nanocrystals in solution. The temperatures required to make these materials were too high for any known organic solvent.

Molten salt can handle the heat, making these previously inaccessible materials accessible.

“These marked advances in molten salt synthesis have allowed Prof. Talapin’s group to develop for the first time many materials for which previously colloidal synthesis was simply not available,” says co-author Richard D. Schaller, who has a joint appointment with Argonne National Laboratory and Northwestern University. “Both fundamental and applied advances can now be made with many of these newly available materials and at the same time, a whole new synthetic frontier is now available to the community.”

The Quantum Age

One of the reasons researchers overlooked molten salt when synthesizing nanocrystals was because of its strong polarity, said UChicago student Zirui Zhou, second author of the new paper.

The positively charged ions and the negatively charged ions of salt strongly attract each other. Small things like nanocrystals have small surface charges, so researchers assumed that the charge would be too weak to push back as the salt ions pull in. Any growing crystals would be crushed before they could form a stable material.

At least that’s what previous researchers thought.

“It’s a surprising observation,” said Zhou. “This is very much at odds with what scientists traditionally think about these systems.”

The new technique could mean new building blocks for better, faster quantum and classical computers, but for many on the research team, the really exciting part is opening up new materials for research.

“Many eras in human history are defined by the materials humanity had at their disposal – think of the ‘Bronze Age’ or ‘Iron Age,'” Ondry said. “In this work, we have unlocked the possibility of synthesizing nearly a dozen new nanocrystal compositions that will enable future technologies.”