Uniform NaYF₄:Yb, Tm hexagonal submicroplates: Controlled synthesis and enhanced UV and blue upconversion luminescence

We reported the preparation of cubic (α-) and hexagonal (β-) NaYF₄ particles in high boiling organic solvents 1-octadecene (ODE) and oleic acid (OA), through a thermal decomposition synthesis route. The as-prepared products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), photoluminescence (PL) spectra. By tuning the OA/ODE volume ratio and reactant concentration, we could manipulate the morphology, size, and crystal structure of the products. Highly uniform β-NaYF₄ submicroplates were obtained from α-NaYF₄ nanoparticles by increasing the OA/ODE volume ratio, while the phase kept unchanged with the increasing of reactant concentration. Upconversion emissions from UV to NIR emissions were observed in β-NaYF₄ hexagonal submicroplates under 980 nm laser diode excitation. In addition, the enhanced UV and blue upconversion emissions were obtained by varying Tm³⁺ and Yb³⁺ ion concentration.