Synthesis of upconversion NaYF₄:Yb³⁺,Er³⁺ particles with enhanced luminescent intensity through control of morphology and phase

Preparation of well-defined NaYF₄ crystals with bright upconversion emission remains a major challenge. The complicated chemical reactions as well as the effect of structure, phase and morphology on the emission efficiency require fine tuning of multiple parameters during the growth of NaYF₄ crystals. In this study, we successfully synthesized NaYF₄:Yb³⁺,Er³⁺ microcrystals with well-controlled morphologies (e.g., sphere and tube) and enhanced luminescent intensity through tuning pH values and ion concentrations in the initial reaction solution. With increasing reaction time, the phase of NaYF ₄:Yb³⁺,Er³⁺ changes from cubic to hexagonal, while the morphology follows the change from spheres to microtubes and then to microrods. Upon excitation by 980 nm infrared light, hexagonal NaYF ₄:Yb³⁺,Er³⁺ microtubes show a significant enhancement in green upconversion emission, which is much stronger than that observed in particles with other morphologies. This phase and morphology dependent strong upconversion emission holds great potential for applications in photonic devices and bioanalyses.