Growth phase diagram and upconversion luminescence properties of NaLuF4:Yb3+/Tm3+/Gd3+ nanocrystals

Sub-25 nm beta-NaLuF4 nanocrystals (NCs) with high homogeneity, monodispersity, and good crystallinity were synthesized through thermal decomposition at different temperatures (285, 295, 305, and 315 degrees C), using an automatic nanomaterial synthesizer. The phase transition of NaLuF4 NCs from alpha-phase to beta-phase suggests that the entire growth process consists of four stages: a cubic phase, the coexistence of cubic and hexagonal phases, a homogeneous hexagonal phase, and an inhomogeneous hexagonal phase, caused by Ostwald ripening. Based on this observation, we obtained the growth phase diagram of NaLuF4: Yb3+/Tm3+/Gd3+ NCs, relating to the reaction time, temperature, crystal size, and crystal phase. From spectral analysis, the products prepared in the third stage were shown to exhibit the characteristics of having small size (sub-25 nm), uniformity, and high upconversion luminescence (UCL) intensity. On the other hand, the luminescence properties of NaLuF4: Yb3+, Tm3+ were studied via incorporating different concentrations of Gd3+ ions. These results indicated that 10% Gd3+ was the best doping concentration for obtaining NaLuF4 NCs with relatively small size and high brightness. Further increasing the Gd3+ concentration induced a drastic decrease in UCL intensity.