US chemists shrink GaN into nanocrystals
A group of chemists with the University of Chicago and Argonne National Laboratory has announced a way to make nanocrystals from metal nitrides, including GaN.
Their study 'Ammonia pressure controls colloidal metal nitride synthesis in molten salts', published July 15 in Nature, opens new doors for electronics and other technologies, such as flexible lighting or medical implants.
“We were able to show how to make a series of nearly a dozen materials that could not be synthesised by traditional methods,” said Ruiming Lin, graduate student at UChicago and first author on the new paper.
The ability to make nanocrystals out of such materials fundamentally increases what can be done with them. Beyond just rigid films, they could someday be blended into polymers, inkjet-printed, and integrated into fabrics or other flexible devices, according to rhe team.
“This expands the boundaries of the field beyond what were previously fundamental constraints, and lays the foundation for the use of nitrides as nanomaterials,” said Dmitri Talapin, the Ernest DeWitt Burton Distinguished Service Professor of Chemistry and Molecular Engineering at UChicago, a scientist at Argonne and the senior author on the paper.
In theory, nanocrystals can be made from almost anything. But in practice, that versatility has been elusive. Lin and other scientists with Talapin’s laboratory wanted to see if they could change that. When crystals form, their ions must swap around a bit before settling down into their final configurations, like dance partners switching during a square dance. Metal nitrides, though, form extremely strong bonds—like tango dancers—so they are reluctant to change partners.
“If bonds cannot break during this process, that’s a death sentence for nanocrystals,” said Talapin. “Once you make an incorrect bond, everything goes south.”
The solution to the problem came in two parts, the team said.
First, the Talapin lab built on a previous discovery that using molten salts as the liquid in the recipe could stabilise the formation of nanocrystals. Then, experimenting further, they found a “sweet spot” of temperature and ammonia pressure conditions that allowed the metal-nitrogen bonds to detach and reattach more readily.
“This process is very unusual — it goes against every bit of common sense in the field,” said Talapin. “We had to entirely rethink the approach.”
The team showed how to make nanocrystals not only out of GaN, but a range of other related nitrides. These included titanium nitride, which is used in medical implants; niobium nitride, an industrially important superconductor; and molybdenum nitride, a common catalyst.
