HF-Free Synthesis of Colloidal A₂BF₆ Nanocrystals

Fluoride-based lattices are widely studied due to their low toxicity, high photostability, and optical transparency. These properties, along with their generally low phonon energies compared to oxides, make fluoride lattices attractive hosts for luminescent impurities. In particular, A₂BF₆ (A = Na, K, Cs, NH₄; B = Si, Ti, Zr, Ge) compounds doped with Mn⁴⁺ in the B⁴⁺ site are leading candidates for narrow-line red emitters in display and lighting technologies.¹ Solution-processability of these materials would enable additive manufacturing of next-generation classical and quantum photonic structures or devices, but fluoride lattices containing tetravalent cations are essentially entirely unexplored as colloidal nanomaterials, in large part because of the current reliance upon concentrated hydrofluoric acid (HF) in nearly all doped A₂BF₆ syntheses. This talk will describe results from a versatile new synthesis² developed to eliminate the handling of HF entirely. Colloidal nanocrystals of several A₂BF₆ lattices ((NH₄)₂ZrF₆, Cs₂ZrF₆, (N[CH₃]₄)₂ZrF₆, K₂SiF₆) have now been prepared for the first time. In-situ A-site cation exchange has also been demonstrated in these materials, allowing the post-synthetic conversion of, e.g., (NH₄)₂ZrF₆ nanocrystals into Cs₂ZrF₆ nanocrystals. Under some conditions, a nanoscale Kirkendall effect occurs that yields hollow A₂BF₆ nanostructures. Recent results showing nanocrystal size and shape control will also be discussed. These results highlight the versatility of this bottom-up synthesis for preparing a wide variety of previously inaccessible colloidal fluoride nanostructures.

References
1. “Narrow-Band Phosphors for Next Generation MiniLED and MicroLED Displays.” Murphy; Camardello; Dohert; Liu; Smigelski; Setlur SID, 2021, 52(S1), 902-905.
2. "HF-Free Synthesis of Colloidal Cs₂ZrF₆ and (NH₄)₂ZrF₆ Nanocrystals.” Tzanetopoulos, E.; Schwartz, J.; Gamelin, D. R. Chem. Commun., 2023, 59, 5451-5454.