Simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF₄:Yb³⁺/Er³⁺ microcrystals by simply tuning the KF dosage

A strategy has been adopted for simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF₄:Yb³⁺/Er³⁺ microcrystals by simply tuning the KF dosage. X-ray power diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectra (PL) were used to characterize the samples. The influence of molar ratio of KF to Y3+ on the crystal phase and morphology has been systematically investigated and discussed. It is found that the molar ratio of KF to Y3+ can strongly control the morphology of the as-synthesized β-NaYF₄ samples because of the different capping effect of F- ions on the different crystal faces. The possible formation mechanism has been proposed on the basis of a series of time-dependent experiments. More importantly, the upconversion luminescence of β-NaYF₄:Yb³⁺/Er³⁺ was greatly enhanced by increasing the molar ratio of KF to RE³⁺ (RE = Y, Yb, Er), which is attributed to the distortion of local crystal field symmetry around lanthanide ions through K+ ions doping. This synthetic methodology is expected to provide a new strategy for simultaneous morphology control and remarkable upconversion luminescence enhancement of yttrium fluorides, which may be applicable for other rare earth fluorides.