Controlled Synthesis And Upconversion Luminescence Of Lanthanide Doped α-NaYbF₄@CaF₂ Core-Shell Nanocrystals

Recently, lanthanide-doped inorganic upconversion nanoparticles（UCNPs） have attracted a great deal of attention owing to their eminent advantages, such as large anti-Stokes shift, high contrast bioimaging, low toxicity and deep penetration depth of excitation light in biological tissues. The ability to control synthesis of UCNPs is of fundamental importance to tailor their luminescent properties. Moreover, the utilization of an epitaxial core/shell structure allows to isolate numerous surface lanthanide dopants on the core nanoparticles from the surrounding quenching sites, thus significantly reducing surface-related quenching effect. In this thesis, we intend to prepare mondodisperse core and core/shell fluoride nanocrystals doped with a range of lanthanide activators through a thermolysis method, and investigate their upconverting luminescence properties and mechanisms behind.The α-NaYbF₄ was selected as host material to serve our purpose. It is found that variations of reaction temperature, reaction time, amount of precursors, and oleylamine（OM） volume allows us to precisely tune the nucleation and growth process of nanocrystals in solution, thus allowing us to prepare monodisperse spherical α-NaYbF₄ nanocrystals with an average size of 20-25 nm. Moreover, the size of resulting nanocrystals generally is found had a close relationship with the aforementioned reaction parameters. The optimized synthetic parameters for the core nanocrystals were experimentally determined to be temperature of 280 oC, rare earth trafluoroacetate precursor of 1 mmol, CF₃ COONa of 2 mmol, reaction time of 30 min, and OM of 16 ml.Then selected the calcium fluoride（CaF₂） material to coat the optimized α-NaYbF₄:Er core nanocrystals prepared in the first step. The CaF₂ is utilized here, since it is characteristic of, good optical transparency and high chemical stability, and more importantly, has a low lattice mismatch with the α-NaYbF₄ material. The size and morphology, crystallographic phase, and upconversion luminescence of the resulting core/shell nanocrystals were characterized by transmission electron microscope（TEM）、X-ray diffraction（XRD） and fluorescence spectroscopy, respectively. Upconversion photoluminescence（UCPL）spectra indicated that the intensity of the core/shell α-NaYbF₄:Er @CaF₂ nanocrystals is about 74 times higher than that of the α-NaYbF₄:Er core nanocrystals, and about 5 times higher than the widely investigated α-NaYbF₄:Er@NaYF₄ core/shell nanocrystals. In addition, the dependence of the UCPL bands on the laser pump power in a double logarithmic plot has been measured to obtain the effective photon numbers involved to generate specific UCPL emission bands. It has been shown that the α-NaYbF₄: Er@CaF₂ core/shell nanocrystals manifest a smaller slope value for the green UCPL than both the α-NaYbF₄: Er core and the well-established α-NaYbF₄: Er@NaYF₄ core/shell nanocrystals, revealing that the ratio of upconverting rate to relaxation rate at the intermediate energy level of Er3+ is the highest for the α-Na YbF₄: Er@CaF₂ core/shell nanocrystals developed here. Meanwhile, we showed that the coating of CaF₂ shell layer are also able to significantly enhance UCPL of the α-NaYbF₄ core nanocrystals doped with other lanthanide activators such as Ho3+ and Tm3+, suggesting the generality of the CaF₂ shell coating on UCPL enhancement.