Study Upon Optical Upconversion Properties Of Homo/Hetero-phase Core-shell Structures Based On NaLuF₄:Yb/Er Nanocrystal

In the upconversion?UC?research field of rare-earth fluorides,NaLuF₄ with hexagonal phase??-NaLuF₄?demonstrates great potential applications in vivo imaging and cell diagnosis due to its bright upconversion luminescence,low cytotoxicity and excellent chemical stability.However,the hexagonal structure is too sensitive to the reaction conditions,and the size of prepared particles is usually too large to be absorbed by the tissue cells.Conversely,NaLuF₄ with cubic phase??-NaLuF₄?shows low upconversion efficiency,but it is easy to obtain small-sized nanoparticles.In order to resolve the contradiction between particle size and upconversion efficiency,?-NaLuF₄:Yb/Er core with small mean size was prepared and homo/hetero-phase shells were coated on it,both of which realized significant enhancement of UC fluorescence.The UC energy transfer process is discussed by investigating the spectra of different structures.The main research contents of the dissertation are as follows:Firstly,we synthesized NaLuF₄:Yb/Er nanomaterials via solvothermal method.The crystal phase and size of the samples were controlled by changing the nucleation temperature,reaction time and the amount of NH₄F.The UC fluorescence spectra of different particles were studied and the intensity ratio of UC red to green was found to be severely dependent on crystal phase and crystal size.Considering the spatial distribution of rare earth ions in different phase structures and the role of surface groups in different size materials,it is believed that the change of spectral structure is derived from the competition between the cross-relaxation process and the non-radiative relaxation process.Secondly,homo-phase core-shell structure,which keep the same or similar crystal phase in the core and the shell,such as?-NaLuF₄:Yb/Er@?-NaYF₄ and?-NaLuF₄:Yb/Er@?-NaYF₄,were successfully prepared by coating NaYF₄ shell onto the?-NaLuF₄:Yb/Er nanoparticles.It is found that the increase of the reaction temperature and the decrease of the amount of core can promote the phase transformation from the common-cubic to common-hexagonal for core-shell structure.The homo-phase core-shell structures all possess the ability to enhance UC fluorescence,especially the?-NaLuF₄:Yb/Er@?-NaYF₄ sample,for which the red and green UC emissions were remarkably enhanced by 56.2 and 358 times,respectively.Temperature sensing experiment based on the common-cubic phase samples?-NaLuF₄:Yb/Er@?-NaYF₄ indicated that the homo-phase shell can only improve the accuracy of temperature measurement but not the sensitivity.Finally,we coated the?-NaLuF₄:Yb/Er nano-core with different shells NaReF₄?Re=Lu,Yb,Y?and found that the core induced the formation of hexagonal phase for the heterogeneous shell.Experiments show that the larger the equivalent radius of rare earth ionic in the shell layer,the stronger the induction effect of the core.The hetero-phase?-NaLuF₄:Yb/Er@?-NaYF₄:Yb was prepared by the core induction method,and its ability to enhance fluorescence was much higher than that of the homo-phase NaLuF₄:Yb/Er@-NaYF₄ core-shell structure prepared under the same condition.Adjusting the concentration of sensitizer Yb³⁺in the shell of hetero-phase core-shell structure?-NaLuF₄:20%Yb/2%Er@?-NaYF₄:Yb,the UC luminescence is brightest when the Yb³⁺concentration is the same for core and shell,due to its efficient absorption of near-infrared photons and suppression of the energy-back-transfer process.Furthermore,it is found that as Yb³⁺increased the number of photons required for the green UC emission is basically unchanged while that for the red is increased from 2 to 3,indicating the variation of Yb³⁺concentration in shell has no influence upon the UC population path of green emission,but affects the the red emission through the cross-relaxation process.