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.