The enhanced upconversion fluorescence and almost unchanged particle size of β-NaYF4:Yb3+, Er3+ nanoparticles by codoping with K+ ions

In this work, the improved upconversion (UC) intensity, the same morphology and almost unchanged particle size of β-NaYF4:Yb3+, Er3+ via codoping with low content K+ ions is reported. Hexagonal NaYF4 nanoparticles with a spherical morphology are synthesized by introducing 0-15 mol% K+ ions. The morphology of the nanoparticles changed to a hexagonal plate-like shape when the K+ ion concentration continuously increased to 20 mol% or more. The UC fluorescence intensities of the green and red emissions of Er3+ are gradually improved and reached a maximum when the concentration of K+ ions was 15 mol%. The decay lifetimes of 4S3/2 and 4F9/2 were determined. The enhancement in UC intensity was mainly ascribed to tailoring the local crystal field around Er3+ ions. Furthermore, we tested the optimized concentration by monitoring the decay lifetime of the intermediate 4I11/2 state followed by theoretical steady-state rate equations.