Ostwald-ripening and particle size focussing of sub-10 nm NaYF4 upconversion nanocrystals

We have studied the growth behaviour of sub-10 nm NaYF4 upconversion nanocrystals of the hexagonal beta-phase and the cubic alpha-phase. Ostwald-ripening of such particles in oleic acid/octadecene solvent results in broadening of the particle size distribution if the colloid contains particles of one crystal phase only. Narrow size distributions are formed only if beta-phase particles grow in the presence of an excess of alpha-phase particles. Such binary mixtures of alpha-phase and beta-phase particles form intrinsically when colloids of alpha-phase particles are heated for a sufficiently long time, because seeds of the beta-phase nucleate in the solution after some time at high temperatures. Since the number of seeds determines the final size of the beta-phase product, control of the nucleation is crucial for controlling the final particle size. We show that the number of beta-phase seeds strongly depends on the composition of the alpha-phase known to form solid solutions Na1-xYF4-x in the range from x = 0 to x = 4/9. Sodium-deficient alpha-phase particles form a negligible number of beta-phase seeds whereas alpha-phase particles with high sodium content yield a very large number of seeds. By taking advantage of this dependence and modifying the synthesis of the alpha-phase particles accordingly, small phase-pure beta-NaYF4:Yb,Er particles with a size smaller than 6 nm can be prepared in oleic acid/octadecene just as well as much larger particles.