| Lanthanide ion-doped upconversion nanocrystals are of great research importance as a typical luminescent material with excellent optoelectronic and magnetic properties for a wide range of applications in bioimaging,medical diagnostics and optoelectronic devices.The sensitiser and activator in upconversion nanocrystals are the core components of the upconversion luminescence process,most of which employ trivalent rare earth ions for f-f electron leap.At the same time,the matrix crystal,as a platform for energy migration and transfer,has an important impact on the upconversion luminescence process,luminescence performance and temperature response.In this thesis,we have explored the upconversion luminescence properties in three oxide crystal matrices,Y6MoO12,La2MoO6 and Bi3TiNbO9,and have successfully prepared the above three upconversion nanocrystals by sol-gel method with the starting point of luminescence colour modulation and luminescence intensity enhancement,and by means of modulating the doping concentration and type of sensitizer and activator and their synergistic combination,and analysed the The energy transfer mechanism and luminescence properties of the upconversion process were analysed.Based on this,the above three upconversion luminescent nanomaterials were initially explored for temperature sensing applications,which are expected to provide high-performance substrates for noncontact optical temperature sensor applications.The main research content and the results obtained in this thesis are as follows:1.Y6MoO12 was used as a substrate for the preparation of Yb3+,Tm3+ and Ho3+ ion doped upconversion luminescent materials using the sol-gel method.By varying the concentration and type of doping ions,the luminescence intensity and the colour of the luminescence can be adjusted.From XRD and SEM analyses,it was found that the increase in the doping concentration of the sensitiser ion Yb3+did not cause any significant lattice distortion or nanocrystal morphology changes in the matrix crystals.It was further found that the optimal luminescence intensity of Y6MoO12 was achieved at a doping level of 20 mol%of Yb3+.The energy transfer processes of Yb3+→Tm3+,Yb3+→Ho3+and Yb3+→Tm3+→Ho3+were discussed and analysed by luminescence mapping,pumping power dependence and changes in fluorescence lifetime.In addition,temperature sensing detection was carried out using Y6MoO12:20Yb,0.3Tm,0.5Ho materials in the temperature interval 298 K-498 K.The best absolute and relative sensitivities were 0.0032 K-1 and 0.85%K-1,respectively.2.La2MoO6 and its lanthanide-doped(Yb3+,Gd3+,Ho3+)upconversion luminescent materials were successfully prepared by the sol-gel method,achieving adjustable luminescence intensity under 980 nm excitation.The optimal doping concentration of sensitizer ion Yb3+ was found to be 15 mol%,and the introduction of Gd3+ions further enhanced the upconversion luminescence intensity of La2MoO6.The optimal doping concentration of Gd3+ion was found to be 1 mol%,resulting in a 1.5-fold increase in luminescence intensity.The crystalline quality and morphology were characterized by XRD and SEM.By analyzing the reason for the increase in lifetime,it was found that Gd3+ion doping improved the sensitizer’s ability to store the absorbed pump energy and transport energy to a longer distance.The relationship between luminescence intensity and incident light power was used to determine that the transitions 5F4,5S2→5I8;5F5→5I8 and(5S2→5I7)of Ho3+ions all involve two-photon absorption.After Gd3+ion doping,the number of photons absorbed by the upconversion luminescent material increased,but the process remained a two-photon absorption process.The thermal coupling energy levels of Ho3+ ions(5F4,5S2;5F5)were analyzed using the fluorescence intensity ratio method,and the absolute sensitivity and relative sensitivity maxima in the temperature range of 398 K-648 K were found to be 0.00424 K-1 and 0.409%K-1,respectively.3.The upconversion luminescent material Bi3TiNbO9 co-doped with Yb3+,Gd3+and Ho3+was prepared by the sol-gel method with an optimum doping concentration of 10 mol%of the sensitizer Yb3+ ions.to 2.2 times higher.The crystalline quality and material morphology were characterised using XRD and SEM,respectively,and it was found that the Gd3+ions entered the lattice as substituent impurities.As the ionic radius of Gd3+is smaller than the radius of Bi3+ions,the substitution leads to a contraction of the lattice and more efficient energy transfer,resulting in a significant increase in luminescence intensity.The mechanism underlying the increase in fluorescence lifetime was analysed with the help of the change in fluorescence lifetime and the relationship between luminescence intensity and pumping energy.In addition,we have used the Generalized Gradient Approximation(GGA)-Perdew-Burke-Ernzerhof(PBE)algorithm in VASP software to carry out theoretical simulations of pure phase Bi3TiNbO9 and Yb3+,Gd3+-doped Bi3TiNbO9,briefly analysing.The variations of their energy band structure,density of states(DOS),local charge density and electron localization function were briefly analysed,and the reasons for the enhanced luminescence were tentatively discussed.In addition,the material was subjected to temperature sensing analysis in the 298 K-573 K temperature interval,and the best absolute and relative sensitivities were 0.00687 K-1 and 0.486%K-1,respectively. |
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