A pot of molten salt and a stream of ammonia gas have accomplished what the field of colloidal nanocrystals considered impossible for decades: synthesizing colloidal metal nitride nanocrystals. In late August, the Talapin group at the University of Chicago, working with Argonne National Laboratory, published in Nature a synthesis covering roughly a dozen nitrides. The paper's title keywords are refreshingly honest: ammonia pressure, molten salt.
Background: why nitrides resisted colloidal synthesis
Metal nitrides are everywhere in modern industry—gallium nitride in phone-screen backlights, titanium nitride coatings on orthopedic implants, niobium nitride in superconducting circuits, molybdenum nitride in refining catalysts. They are hard, heat-resistant, and corrosion-proof.
But they could not be made as colloidal nanocrystals. Crystal growth can be pictured as a square dance: ions hold hands and circle, the music changes, partners swap positions, and the lattice locks in a new layer. The problem is that metal–nitrogen bonds are too strong—when the music changes, the partners refuse to let go. Once a wrong bond forms, the crystal's order is destroyed on the spot.
Talapin put it bluntly: "If bonds cannot break during bonding, it's a death sentence for nanocrystals." He added that the approach "goes against all conventional wisdom in this field."
The nanoscale amplifies the problem: the smaller the crystal, the larger the surface-area-to-volume ratio, and the higher the "box office share" of a single wrong bond. Bulk crystals can be brute-forced at high temperature (the physical vapor deposition route), but that yields hard films—not colloidal particles that disperse in liquid and can be printed or coated.
The recipe: heat the dance floor, let the partners release
The solution has two halves, each handling one job:
- Molten salt as the reaction medium — it "supports" freshly nucleated crystal facets, shielding them from solvent attack.
- The temperature × ammonia-pressure combination — tuned to a sweet spot, metal–nitrogen bonds become breakable and re-formable. Bonds that can reconnect allow the square-dance partner swap to work, giving wrong bonds a second chance to correct.
- Flexible lighting (gallium nitride as printable luminescent ink on plastic instead of rigid LED chips)
- Inkjet-printed electronics
- Polymer composites
- Textile electronics
- Implantable devices (titanium nitride coatings conforming flexibly to biological tissue)
First author Ruiming Lin, a UChicago PhD student, summarized: "We demonstrated a route to nearly a dozen materials that traditional methods simply could not synthesize."
From hard films to inks: the real news
The synthesis itself is a means to an end. The valuable part is the change of form: for the first time these nitrides exist as colloids—dispersible in solution and formable into inks.
Potential applications include:
Some family history: the previous highlight of colloidal nanocrystal research was quantum dots, honored with the 2023 Nobel Prize in Chemistry—laureate Bawendi is himself a UChicago alumnus. This work extends the quantum-dot colloidal toolbox to a family of heavy-industry materials that previously could not enter.
The funding roster also reveals industrial intent: the U.S. Department of Energy, NSF, the Air Force Office of Scientific Research, plus a Samsung quantum-dot cluster collaboration. Samsung's interest is clearly next-generation display and semiconductor materials, not soup spoons.
The next milestone is on the device side: whoever first prints a light-emitting device from gallium nitride nanocrystal ink turns this pot of molten salt from a Nature paper into a production line. On the chemistry side, the door is open.