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 beta-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 S-4(3/2) and F-4(9/2) 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 I-4(11/2) state followed by theoretical steady-state rate equations. (C) 2014 Elsevier B.V. All rights reserved.