Research And Application On Upconversion Luminescence Of Rare Earth Doped Micro Nanocrystals

Temperature measurement is of great significance in industrial production,aerospace,biomedical and other fields.With the development of science and technology,higher requirements have been put forward for temperature measurement.Traditional contact temperature sensors are difficult to meet the strict temperature measurement environment and measurement accuracy requirements.The upconversion micro nano materials based on rare earth doping have great development prospects as new non-contact temperature sensors,especially the temperature measurement method based on fluorescence intensity ratio technology,which can resist laser disturbance and eliminate propagation losses.However,for rare earth doped micro/nano luminescent materials,their further development is severely constrained by low luminescence efficiency.Increasing the doping concentration of rare earth ions,which is the most direct enhancement method,will instead result in concentration quenching（i.e.,a sharp decrease in luminescence intensity as the doping concentration increases）,resulting in the doping concentration of luminescent ions being usually limited to below 10 mol%.On the other hand,for the use of nano core-shell structures to enhance upconversion luminescence,the mechanism of shell effect on upconversion luminescence and its impact on temperature sensing performance still need to be further discussed.In view of the above problems,this paper takes Er³⁺doped micro and nano powders as the research object,and the specific research work is as follows:A series of up conversion micro phosphor based on NaY（WO₄）₂,Er³⁺single doped and Yb³⁺/Er³⁺codoped were synthesized by high temperature solid-state method.In the Er³⁺single doped system,concentration quenching occurs only when the doping concentration reaches 70 mol%.The reason is that the micro phosphor has high crystallinity,few defects,and energy is not easily transferred to the quenching center,leading to a high concentration quenching threshold in this environment.Through steady-state and transient spectra,it was found that the photothermal effect caused by high doping concentration is the fundamental cause of its quenching,causing energy to be dissipated in the form of thermal energy.We investigated the fluorescence intensity ratio temperature measurement performance of Na Er（WO₄）₂ phosphors based on thermal coupling energy levels.We excited them using 980 nm and 1550 nm lasers,and investigated the temperature sensing characteristics of the heavily doped system for the²H_11/2 and ⁴S_3/2 energy levels,⁴I_11/2 and ⁴I_9/2 energy levels of Er³⁺.The research results indicate that the temperature measurement sensitivity of the two green light levels is relatively high at low temperatures,with a relative sensitivity of 0.56%K^-1 at 303 K.Under 1550 nm laser excitation,the temperature measurement sensitivity of the ⁴I_9/2 and⁴I_11/2 levels can reach 0.51%K^-1 at 493 K.In addition,research has found that doping a small amount of Yb³⁺on this basis can increase the absorption of laser energy,further enhancing the upconversion luminescence through the energy transfer from Yb³⁺to Er³⁺.A series of core-shell structure nanoparticles based on NaYF₄ were prepared by solvothermal method.The bare core nanoparticles had serious concentration quenching when the single doping concentration of Er³⁺was up to 30 mol%.After a layer of NaYF₄inert shell was coated,the concentration quenching occurred when the doping concentration was up to 50 mol%.With the increase of Er³⁺doping concentration,the upconversion luminescence changed from green to red.Research has shown that energy migration to the surface quenching center is the main reason for concentration quenching.After coating the shell,surface defects are suppressed,and energy is effectively transferred between Er³⁺.At the same time,high concentration doping reduces the distance between ions,enhances cross relaxation,and significantly enhances upconversion red light.Limiting energy transfer to the surface quenching center combined with strong cross relaxation,forming a phenomenon in the heavily doped system of core-shell structure nanoparticles.The temperature sensing performance of rare earth heavily doped nanoparticles was investigated.The relative sensitivity of NaYF₄:50 mol%Er³⁺@NaYF₄sample was 1.01%K^-1 at 303 K.The relative sensitivity of NaErF₄@NaYF₄ achieved a maximum value of 0.90%K^-1 at 303 K.This indicates that the core-shell structure can reduce laser thermal effects and is beneficial for temperature sensing.In addition,the influence of shell properties on upconversion luminescence was analyzed.NaYF₄ was prepared by solvothermal method,and 20 mol%Yb³⁺/1 mol%Er³⁺doped bare core structure nanoparticles were prepared.The results show that the way of coating the shell can effectively reduce the surface defects of nanoparticles and enhance the upconversion luminescence.For NaYF₄:20 mol%Yb³⁺/1 mol%Er³⁺@NaYF₄@NaYF₄ nanoparticles,the fluorescence intensity ratio of ²H_11/2 and ⁴S_3/2 of Er³⁺,which are heat coupled energy levels,is used to measure the temperature.At 303 K,the maximum relative sensitivity can reach 1.10%K^-1.The scattering effect of nano core-shell structures on light was investigated using Lumeric FDTD software,and the accuracy of the FDTD numerical simulation method was demonstrated through comparison with Mie theory.The results show that the scattering intensity is far greater than the absorption intensity,and the scattering effect gradually increases with the increase of the radius of the nanoparticles,which proves that the core-shell structure will enhance the scattering of nanoparticles to laser,and reduce the absorption of nanoparticles to laser.To verify the results,a series of bare core,core-shell,core-shell,and core-three-layer shell structural samples were prepared by solvothermal method.The fluorescence spectrum excited by 980 nm laser was measured.It was found that with the increase of the number of NaYF₄ inert shell coating layers,the fluorescence lifetime of each energy level was extended,and the upconversion fluorescence intensity first increased and then decreased,indicating that the scattering of excited light by nanoparticles led to the reduction of the utilization ratio of pump light energy.