Preparation, Growth Mechanism, Upconversion, and Near-Infrared Photoluminescence Properties of Convex-Lens-like NaYF4 Microcrystals Doped with Various Rare Earth Ions Excited at 808 nm

Preparation of rare earth ions doped photoluminescence materials with controlled morphology was desired to fulfill the requirement of different applications. In the work, convex-lens-like NaYF4 microcrystals doped with various rare earth ions were prepared by adjusting preparation parameters including the reaction time, reaction temperature, NaOH concentration, ratio of oleic acid to 1-octadecene, and types of doping ions. A possible growth mechanism of convex-lens-like NaYF4 microcrystals is proposed based on reaction time and temperature-dependent morphology evolution. The formation of micro-convex-lens includes the three processes of NaYF4 nanoparticles self-assemble, dissolution–nucleation, and regrowth. Doping ions dependent near-infrared and upconversion luminescence properties of convex-lens-like NaYF4 microcrystals were investigated excited at 808 nm. The visible upconversion luminescence was observed in the Er3+, Yb3+/Er3+, Nd3+/Er3+, Nd3+/Yb3+/Er3+ doped convex-lens-like NaYF4 microcrystals, and near-infrared luminescence was obtained in the Nd3+, Nd3+/Er3+, Yb3+/Er3+, Nd3+/Yb3+, Nd3+/Yb3+/Er3+ doped NaYF4 convex-lens-like NaYF4 microcrystals. The Er3+, Yb3+/Er3+, Nd3+/Er3+, Nd3+/Yb3+/Er3+ doped convex-lens-like NaYF4 microcrystals exhibit various upconversion luminescence mechanisms. The energy transfer of the Er3+ → Yb3+ and the Nd3+ → Er3+ was observed in the Yb3+/Er3+ and Nd3+/Er3+ doped convex-lens-like NaYF4 microcrystals, respectively. The upconversion emission of Nd3+/Yb3+/Er3+ doped convex-lens-like NaYF4 microcrystals is from the energy transfer mechanisms of Nd3+ → Yb3+ → Er3+.