The Research In Novel Structure And Physical Properties Of NaErF4 Upconversion Luminescence Nanoparticles Doped With Alkali Ions

Lanthanide-based upconverting luminescence nanoparticles?UCNPs?can convert long-wavelength near-infrared light to short-wavelength UV/visible light.They have many unique optical properties,such as high optical stability,long luminescence lifetime,low optical toxicity and narrow emission bands.Therefore the lanthanide-based upconvertion luminescence nanomaterials have broad application prospects in biomedical fields such as photodynamic therapy and biomedical detection,as well as in the field of solar cells.However,the low luminescence efficiency hindered the further development of UCLPs due to various energy loss processes,such as energy transfer to surface defects,radiative relaxation,and cross relaxation between ions,etc.Therefore,improving the luminescent efficiency is significant for the practical application of UCL nanomaterials.In the past,the common lanthanide-based UCL nanosystem was the sensitizer-activator codoping system.However,the doping concentration of the activator is lower than 3mol%because of the concentration quenching effect.It is difficult to increase the luminescence efficiency since the activator concentration is too low.Recently,our research group and another two groups have reported a new type of UCL system,namely the NaErF₄@NaYF₄ and NaErF₄@NaLuF₄ core-shell nanostructure with Er³⁺doping concentration as high as 100mol%.This innovative result has led us to a new understanding of the role of concentration quenching effects in nanoparticles,and it has enabled the efficient luminescence,and it also has achieved quasi-monochromatic red emission.However,the luminescence efficiency of this new system still cannot meet the requirement of application.Therefore,in this paper,we further study the new method of improving the upconversion luminescence efficiency of this new system.The main research contents are as follows:?1?We design NaErF₄@Na LuF₄ core-shell UCL nanoparticles,synthesize the nanoparticles with good luminescence properties by solvent-thermal method,and study their luminescence mechanism and optimization of synthesis conditions.The experimental results show that the luminescence of Er³⁺at 540nm and 650nm is a two-photon process.The red light intensity at 650nm is increasing gradually as the thicknesses increase.The inflection point is formed at the shell thickness of 5nm.Taking into account factors of the experimental process and fluorescence intensity,we confirm that the optimal shell thickness is 5nm.?2?We study the luminescence properties of Na_1-xK_xErF₄@NaLu F₄ core-shell nanoparticles with different K⁺concentrations,and the concentration is 0⁸mol%.A new method is proposed to increase the luminescence intensity of NaEr F₄@NaLuF₄system.The results of XRD analysis show that these nanoparticles with different doping concentrations of K⁺are all?-phase nanostructure.The emission intensity of the red band of Er³⁺at 650nm increase first,and then decrease as the doping ions increasing when K⁺ions have been doped into NaErF₄@NaLuF₄ system.The luminescence intensity is the strongest when the K⁺doping concentration is 4mol%.At this time,the K⁺ions exist in the system in two ways:substitution and interstitial.?3?We study the effects of different concentrations of Li⁺doping on the upconversion luminescence of NaErF₄@NaLuF₄ core-shell nanostructure.We propose a new method to improve the NaErF₄@Na LuF₄ UCL system by doping Li⁺.The results show that under the premise of keeping the crystal size constant and?-phase nanostructure,the red light intensity of Er³⁺at 650nm increase first,and then decrease with the doping concentration of Li⁺increasing.When the doping concentration of Li⁺is 8mol%,namely Na_0.92Li_0.08ErF₄@NaLuF₄,the intensity is the strongest.At this time,Li⁺ions exist in the system in the modes of substitution and interstitial.