Synthesis And Upconversion Luminescence Of Multilayered Lanthanide-doped Nanoparticles

Lanthanide-doped upconversion nanoparticles(UCNPs)have a variety of applications in display,anti-counterfeiting,biomedical fields due to their fascinating luminescent properties,such as non-blinking,large anti-Stokes shift,long lifetime,sharp multiline emissions.However,it is still challenging for their applications because of limited upconversion efficiency and brightness.Multilayered UCNPs have attracted extensively attention in recent years thanks to their enhanced upconversion efficiency and controlled luminescence.Considering that stable and tunable luminescence is of great importance for applications of multilayered UCNPs,we synthesized NaYF₄:Er/Tm@NaYbF₄@NaYF₄UCNPs in this thesis to investigate the effect of host lattices on stable luminescence of these multilayered UCNPs.Based on the host lattice assuring stable luminescence,we explored the effects of Tm³⁺and Li⁺concentrations on upconversion luminescence of multilayered UCNPs.The main research contents are as follows:(1)Crystalline phase-dependent ions migration of multilayered UCNPs.We synthesized cubic and hexagonal NaYF₄:Er@NaYbF₄@NaYF₄ and Na Er F₄@NaYF₄ UCNPs via epitaxial growth.The morphology,crystal phase,surface chemical compositions and upconversion luminescence of these UCNPs were examined before and after annealing at certain temperature in Ar atmosphere.We demonstrated that cation migration among different layers easily occurrs in cubic multilayerd UCNPs by monitoring the variation in surface chemical composition of Na Er F₄@NaYF₄ UCNPs,resulting in composition change in core or shells and thus luminescence quenching after annealing at 200°C.In contrast,the elemental migration of hexagonal multilayered UCNPs occurrs at relatively higher temperature,ensuring high stable upconversion luminescence at 200°C.These results demonstrate the effect of crystalline-phase-dependent ions migration on upconversion luminescence should be considered for pratical application of multilayered UCNPs.(2)Regulation of upconversion luminescence of?-NaYF₄:Tm@NaYbF₄@NaYF₄multilayered UCNPs.We synthesized?-NaYF₄:Tm@NaYbF₄@NaYF₄ multilayered UCNPs and explored the effect of Tm³⁺concentrations on their upconversion luminescence.It was found that the optimum Tm³⁺concentration is 5%for these UCNPs and the 4 and 5photons emission intensity of Tm³⁺at this concentration enhances by 3 orders compared with canonical core-shell UCNPs.The temperature sensitivity of the 4 and 5 photons emission enables highly sensitive temperature sensor with maximum absolute and relative sensitivity of 24%K^-1and 5.1%K^-1,respectively.In addition,the upconversion luminescence of multilayered UCNPs was further enhanced via doping Li⁺into core.This work provides new strategy to modulate upconversion luminescence and exhibits potential applications of multilayered UCNPs.