Unraveling Epitaxial Habits in the NaLnF4 System for Color Multiplexing at the Single-Particle Level.

We report an epitaxial growth technique for scalable production of hybrid sodium rare-earth fluoride (NaLnF4 ) microcrystals, including NaYF4 , NaYbF4 , and NaLuF4 material systems. The single crystalline nature of the as-synthesized products makes them strong upconversion emission. The freedom of combining a lanthanide activator (Er(3+) or Tm(3+) ) with a sensitizer (Yb(3+) ) at various doping concentrations readily gives access to color multiplexing at the single-particle level. Our kinetic and thermodynamic investigations on the epitaxial growth of core-shell microcrystals using NaLnF4 particle seeds suggest that within a certain size regime it is plausible to exert precise control over shell thickness and growth orientation under hydrothermal conditions.