Gd 3+ -Doping Effect on Upconversion Emission of NaYF 4 : Yb 3+ , Er 3+ /Tm 3+ Microparticles.

β-NaYF ₄ microcrystals co-doped with Yb ³⁺ , Er ³⁺ /Tm ³⁺ , and Gd ³⁺ ions were synthesized via a hydrothermal method using rare-earth chlorides as the precursors. The SEM and XRD data show that the doped β-NaYF ₄ form uniform hexagonal prisms with an approximate size of 600-800 nm. The partial substitution of Y by Gd results in size reduction of microcrystals. Upconversion luminescence spectra of microcrystals upon 980 nm excitation contain characteristic intra-configurational ff bands of Er ³⁺ /Tm ³⁺ ions. An addition of Gd ³⁺ ions leads to a significant enhancement of upconversion luminescence intensity with maxima at 5 mol % of dopant.