Formation of nanocrystalline matrix of sodium rare earth fluoride up-conversion phosphors

Until now, ytterbium and erbium co-doped sodium yttrium fluoride (NaYF4:Yb, Er) is among the most efficient up-conversion phosphors. Its potential for applications in biological and medical fields has interested many scientists. However, the formation mechanism of the co-doped sodium yttrium fluoride nanoparticles is still unknown. Based on the similarity of rare earth elements, this work started with studying the formation mechanism of NaYF4 nanoparticles without doping in aqueous phase. Effects of reactant concentrations, pH, and reaction time on the particle formation were investigated. As the reaction time increased, NaYF4 nanoparticles grew sporadically. Effect of seeding was also studied. The reaction was not accelerated and the particle size was not altered by seeding. The above investigations suggested that in aqueous solution NaYF4 nanoparticles were formed by the aggregation model. The aggregation of the primary particles may play a key role for the formation and size control of the nanoparticles. Smaller particles were achieved by the presence of chelators of Na-2-ethylenediaminetetraacetic acid and sodium nitrilotriacetate. This probably resulted from the adsorption of chelators on the NaYF4 nanoparticles and increased the repulsive force between the primary particles, preventing primary particles to form large particles by aggregation.