Synthesis of Colloidal A2BF6 (B = Zr4+, Ti4+, Si4+) Nanocrystals: Controlling Composition and Morphology

We report the synthesis of a library of colloidal Zr4+-, Ti4+-, and Si4+-based A2BF6 (A = Na+, Cs+, NH4+, N(CH3)4+) nanocrystals. These lattices are synthesized without the use of aqueous HF but instead are prepared using safer fluoride salt precursors dissolved in non-polar solutions. We show that nanocrystal morphologies, including shape anisotropy, can be controlled by adjusting various synthetic parameters including reaction temperature, time, and solvent polarity. Nucleation and growth pathways of A2ZrF6 compositions are compared. An in-situ A-site cation exchange requiring internal anion sublattice reorganization is described that allows conversion of anisotropic (NH4)2ZrF6 nanorods into Cs2ZrF6 nanorods that could not be prepared directly. The development of colloidal nanocrystals as a new form factor for A2BF6 lattices addresses fundamental synthesis challenges in inorganic nanoscience and lays the groundwork for future work into doping such nanocrystals for solution-phase photonics applications, processing, or printing.