Controllable synthesis and luminescence properties of NaYF4@YOF composite microcrystals with multiform morphologies

A variety of α-NaYF4@YOF composites with diverse morphologies and particles sizes have been fabricated via an easy-to-operate approach. First of all, the YB(OH)4CO3 precursor was obtained by a urea-based homogeneous precipitation route. Subsequently, the uniform and well-dispersed β-NaYF4 microcrystals with a variety of crystal shapes were synthesized by a simple hydrothermal process. The morphology and particle size of the β-NaYF4 samples can be adjusted in a controlled manner by simply tuning the reaction temperature and time, feeding molar ratio of precursor/NaF, or total amount of the reactants. Finally, the β-NaYF4 microcrystals converted to α-NaYF4@YOF composites which inherited the morphology and dispersity of the hydrothermal products after an annealing process in air. Upon UV or NIR excitation, the as-synthesized lanthanide ions Ln3+ (Eu3+, Tb3+, Yb3+/Er3+, Yb3+/Ho3+) doped α-NaYF4@YOF samples show the intense characteristic down-conversion (DC) or up-conversion (UC) multicolor emissions. Moreover, the calcination process not only broadens the excitation range but also improves the emission efficiency of the phosphors due to the enhanced crystallintiy and formation of YOF shell. The as-synthesized Ln3+-doped luminescent materials may find potential applications in fields of color displays, optoelectronic devices, and light-emitting diodes (LEDs).