Synthesis And Up-conversion Luminescent Properties Of Rare Earth Doped NaYF₄Nanocrystals

Lanthanide-doped near-infrared （NIR）-to-visible up-conversion nanocrystals have become aninteresting research topic over the past several decades for their wide applications in solid-state lasers,flat-panel displays, solar cells, and biotechnology controllable synthesis of high-quality nanoparticleswith uniform dimension and shape is very important, because the electronic structure, bonding, surfaceenergy, and chemical reactivity are directly related to their surface morphology.Yb³⁺/Er³⁺co-doped NaYF₄nanoparticles have been obtained by a facile hydrothermal method at200oC for2hours. Nanoparticles transformed from cubic phase to hexagonal phase just by doping Gd³⁺.All samples were characterized by XRD, SEM, SAED, PL and EDX. The effects of the different Gd³⁺concentration on the structure and emission properties of the samples have been investigated. Purehexagonal phase of NaYF₄was obtained as the Gd³⁺ions concentration reached30mol%. Thetransitions²H_11/2â†'⁴I_15/2，⁴S_3/2â†'⁴I_15/2，⁴I_9/2â†'⁴I_15/2came from dual-photon, respectively.High quality hexagonal phase Yb³⁺/Tm³⁺/Gd³⁺triply doped NaYF₄nanorods have beensynthesized at200oC for2hours by a facile hydrothermal method. The effects of different Tm³⁺concentrations on emission have been investigated. These results demonstrate that the aspect ratio, sizeand emission intensity have been tuned by the concentration of Tm³⁺dopant. When doped with0.5%Tm³⁺, the hexagonal nanorods have the biggest aspect ratio, size and intensity of up-conversion （UC）luminescence. The cross-relaxation mechanism between Tm³⁺ions and the change of crystal size can beexplained this phenomenon. It was shown that the power dependence of the UC emission intensities: n=4.13,3.61, and2.54for360,450and474nm emissions, respectively. This means that the transitions¹D₂â†'³H₆,¹D₂â†'³H₄,¹G₄â†'³H₆,¹G₄â†'³F₄,³H₄â†'³H₆came from four-photon, four-photon, andthree-photon UC processes, respectively.