Research On Upconversion Luminescence Properties And Temperature Sensing Of Rare Earth Doped Alkaline Earth Fluoride Core-shell Nanocrystals

Rare earth fluoride upconversion multicolor light-emitting nanocrystals have shown promising applications in display,optical storage,bioanalytical imaging,photodynamic therapy,temperature probes and many other areas due to their unique optical properties,excellent chemical and physiological properties,which have attracted the attention of many researchers and have been developed rapidly in recent years.Since smaller upconversion nanocrystals have a large surface-to-volume ratio,most of the atoms of such upconversion nanocrystals are surface atoms.These surface atoms are unsaturated and easily form surface defects,resulting in excited states with high non-radiative relaxation chances and greatly reduced upconversion luminescence intensity.Although strategies such as surface plasmon coupling,photonic crystal engineering,optical micro-ring resonators,and organic dye sensitization have been employed to enhance upconversion luminescence,these strategies involve the design of complex structures,which limits their practical applications.So far,the synthesis methods of fluoride nanocrystals with core-shell structures have been well developed to enhance the luminescence of upconversion nanocrystals to some extent.The common core-shell structures NaYF₄@NaYF₄,NaYF₄@NaGdF₄,NaErF₄@NaGdF₄,where the crystal structure of the shell layer is similar to that of the core,are only used to passivate the surface defects of the core.However,there are fewer studies on the improvement of the upconversion luminescence properties and on the improvement of their temperature sensing properties by using the compositional differences between the core and the shell to modulate the microstructure of the nanocrystals.Therefore,we carried out the following studies.（1）We prepared a series of Yb/Er co-doped SrYF and SrGd F upconversion nanocrystals coated with different shell layers by thermal decomposition method to optimize the effect of interfacial lattice mismatch of core-shell upconversion nanocrystals and enhance the upconversion emission intensity of core-shell nanocrystals.The experimental results show that the Ba Gd F shell layer encapsulating M²⁺/RE³⁺ metal ions with large ionic radii reduces the phonon energy and phonon density of states at the core-shell interface,decreases the nonradiative jump probability of the activator,and significantly enhances the upconversion luminescence.It is demonstrated that constructing a shell layer with a different composition from the nucleus can change the interfacial microstructure between the nucleus and the shell layer,thus changing the phonon energy as well as the phonon density of states of the upconversion nanocrystals,which has a significant effect on their upconversion luminescence.It is suggested that for smaller core-shell nanocrystals,a rational design of the shell layer and the core-shell interface may be an effective way to greatly reduce the chance of nonradiative leap in the nucleus.The core-shell structured nanocrystals studied in this chapter are also successfully applied to the field of dynamic display and have great application in the field of dynamic display and advanced information storage.（2）We prepared a series of SrYF:Yb/Nd with different Nd³⁺ion doping concentrations by thermal decomposition method,and enhanced the negative thermal quenching performance of the nanocrystals by modulating the doping concentration of Nd³⁺ions.The negative thermal quenching properties of the samples were characterized to determine the optimal doping concentration of Nd³⁺ions.In addition,we prepared core-shell nanocrystals with different core materials（BaF₂,SrF₂,CaF₂）encapsulated with the same SrYb F:Nd shell layer,and tuned the phonon energy of the nanocrystals by adjusting the core materials,which finally improved the negative thermal quenching performance of the nanocrystals.The enhancement of the temperature sensing performance of nanocrystals by the differences between the core and the shell is shown.By summarizing and using the above findings,we finally designed and successfully prepared SrYF:10Yb/Er@CaF₂@SrYb F:40Nd triple core-shell-shell nanocrystals by thermal decomposition method,and the maximum relative sensitivity obtained was calculated to be 10.01%K^-1,which is larger than that of similar experimentally reported temperature probes,facilitating their application to upconversion temperature probes.