Up-conversion monodispersed spheres of NaYF4:Yb3+/Er3+: green and red emission tailoring mediated by heating temperature, and greatly enhanced luminescence by Mn2+ doping

Abstract

Submicron sized, monodispersed spheres of Mn²⁺, Yb³⁺/Er³⁺ and Mn²⁺/Yb³⁺/Er³⁺ doped α-NaYF₄ were easily autoclaved from mixed solutions of the component nitrates and ammonium fluoride (NH₄F), in the presence of EDTA-2Na. Detailed characterizations of the resultant phosphors were obtained using XRD, Raman spectroscopy, FE-SEM, HR-TEM, STEM, PLE/PL spectroscopy, and fluorescence decay analysis. Finer structure and better crystal perfection was observed at a higher calcination temperature, and the spherical shape and excellent dispersion of the original particles was retained at temperatures up to 600 °C. Under the 980 nm infrared excitation, the Yb³⁺/Er³⁺-doped sample (calcined at 400 °C) exhibits a stronger green emission centered at ∼524 nm (²H_11/2 → ⁴I_15/2 transition of Er³⁺) and a weaker red emission centered at ∼657 nm (⁴F_9/2 → ⁴I_15/2 transition of Er³⁺). A 200 °C increase in the temperature from 400 °C to 600 °C resulted in the dominant red emission originating from the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺, instead of the previously dominant green one. Mn²⁺ doping induced a remarkable more enhanced intensity at ∼657 nm and ∼667 nm (red emission area) than that at ∼524 nm and ∼546 nm (green emission area), because of the non-radiative energy transfer between Mn²⁺ and Er³⁺. However, a poor thermal stability was induced by Mn²⁺ doping. The observed upconversion luminescence of the samples calcined at 400 °C and 600 °C followed the two photon process and the four photon process, respectively.