A groundbreaking method for developing quantum dots has not only led to a more efficient approach for creating a beneficial type of quantum dot, but has also unveiled a range of innovative chemical materials for researchers to further investigate. By using molten salt instead of organic solvents, scientists can fabricate ‘previously unimaginable nanocrystals.’
Quantum dots, a class of semiconducting nanocrystals, are advancing both scientific research and practical applications, including lasers, quantum QLED TVs and displays, solar panels, medical instruments, and various electronic devices.
This newly introduced method for growing these tiny crystals, featured this week in Science, not only provides a more effective way to produce a valuable kind of quantum dot, but also sets the stage for a host of new chemical materials ripe for future exploration by researchers.
“I am thrilled to see how scientists around the world can utilize this technique to create nanocrystals that were previously thought to be impossible,” remarked first author Justin Ondry, formerly a postdoctoral researcher at UChicago’s Talapin Lab.
The research team, which comprised scholars from the University of Chicago, UC Berkeley, Northwestern University, the University of Colorado Boulder, and Argonne National Laboratory, made these significant breakthroughs by swapping out traditional organic solvents for molten salt—essentially superheated sodium chloride similar to that used on baked potatoes.
“Sodium chloride isn’t typically viewed as a liquid, but if you heat it to an extreme temperature, it becomes liquid. It appears like water, has a similar viscosity, and is colorless. The challenge was that no one had considered these liquids as a medium for colloidal synthesis,” explained Prof. Dmitri Talapin from the UChicago Pritzker School of Molecular Engineering (UChicago PME) and the Chemistry Department.
Why Use Salt?
Quantum dots are notably recognized not only for their wide-ranging commercial applications but also due to the recent 2023 Nobel Prize in Chemistry awarded to the team that first identified them.
“If there’s a nanomaterial that has significantly impacted society through its applications, it’s the quantum dot,” stated UC Berkeley Prof. Eran Rabani, a co-author of the study.
However, much of the prior research on quantum dots, including the Nobel-winning work, focused on dots made with elements from the second and sixth groups of the periodic table, known as “II-VI” materials, Rabani noted.
More promising resources for quantum dots exist in other areas of the periodic table.
Materials from the third and fifth groups (III-V materials) are commonly employed in the most efficient solar cells, the brightest LEDs, the most powerful semiconductor lasers, and the fastest electronic devices. These materials would potentially make excellent quantum dots, but up until now, it was nearly impossible to grow nanocrystals from them in liquid solution, as the heat needed was too much for any known organic solvent.
Molten salt can withstand these high temperatures, making these previously unattainable materials available.
“This significant advancement in molten salt synthesis that Prof. Talapin’s group has developed is allowing for many materials that were previously impossible to synthesize colloidally,” explained co-author Richard D. Schaller, who holds joint positions with Argonne National Laboratory and Northwestern University. “Both fundamental and applied advances can now be achieved with many of these newly accessible materials, opening a new synthetic domain for the research community.”
The Quantum Era
A reason why researchers have previously overlooked molten salt as a synthesis medium is due to its strong polarity, said UChicago graduate student Zirui Zhou, the second author of the study.
The strong attraction between salt’s positively and negatively charged ions led scientists to believe that small charged entities like nanocrystals would never be able to overcome this pull, causing any growing crystals to collapse before forming stable structures.
However, researchers may have underestimated this process.
“It’s a surprising finding,” Zhou said, “and runs contrary to what scientists have traditionally held about these systems.”
This new method could serve as a foundation for developing enhanced quantum and classical computers, but for many team members, the most exhilarating aspect is the new array of materials it makes available for investigation.
“Throughout human history, we often define eras by the materials that were used—consider ‘Bronze Age’ or ‘Iron Age,'” Ondry remarked. “This work enables us to synthesize nearly a dozen new nanocrystal compositions, paving the way for future technological advancements.”
