Preparation And Fluorescence Properties Of NaYbF₄@NaYF₄:Yb/Er Core-shell Nanocrystals

The upconversion luminescence emission,converting two or more near-infrared photons into high-energy photons,is a typical optical nonlinear process that.Rare earth doped upconversion nanomaterials possess the following advantages: strong light penetration,long luminescence lifetime,high sensitivity,large anti-Stokes displacement,low fluorescence background,etc.The relatively low luminescence emitting efficiency restricts its wide applications.The researchers thus focus on the upconversion efficiency enhancement.Increasing doping content of sensitizers in single upconversion nanoparticle is an effective strategy for improving the upconversion luminescence emission.However,the doping concentration is limited by its intrinsic characteristics.The construction of core-shell architecture has been established to be a potential approach for high concentration of sensitizers and enhanced upconversion luminescence.We designed a core-shell structured NaYbF₄@NaYF₄:Yb/Er nanocrystal,which enables the double non-radiative energy resonance transfer from the Yb³⁺sensitizer in the core and the Yb³⁺ doped in the shell layer to Er³⁺activatorslocated in shell.Thisas-prepared NaYbF₄@NaYF₄:Yb/Er nanocrystals achieved highly-efficiency upconversion emission.This thesis mainly focus the preparation and optimization of the core-shell NaYbF₄@ NaYF₄:Yb/Er nanocrystals:The thermal cracking method was utilized for preparing the NaYbF₄ core nanocrystals.Based on the optimizing to reaction parameters,the high-quality core nanoparticles with small size was obtained.This benefits the successful and further synthesis of core-shell nanoparticles.The TEM image and XRD results demonstrated that the core-shell structure nanocrystals are hexagonal phases with uniform size,meeting the expected requirements for preparing core-shell architecture.The NaYbF₄@NaYF₄:Yb/Er core-shell structure nanocrystals are optimized.The amount of sodium oleate was researched.The doping amount of Yb³⁺ in the core was significantly studied for tuning the optical emissions of core-shell nanocrystal.The doping content of Er³⁺ ions was further optimized for producing intense upconversion lights.Subsequently,the optimal shell thickness of NaYF₄:Yb/Er was also determined.Through comparing the designed core-shell NaYbF₄@NaYF₄:Yb/Er nanocrystals with the NaYF₄:Yb/Er nanocrystal,the well-established enhancement in upconversion emitting intensity is confirmed.In order to obtain the supporting evidences,another two control experiments were carried out by comparing the NaYbF₄@NaYF₄:Yb/Tm,NaYbF₄@NaYF₄:Yb/Ho with NaYF₄:Yb/Tm and NaYF₄:Yb/Ho samples.The comparison further supports the conclusion that this kind of core-shell structure nanocrystal can improve the upconversion luminescence emissions efficiently.