Upconversion Spectral Control And Luminescence Mechanism Of Rare Earth Doped Fluoride Core-Shell Nanocrystals

Photon upconversion of lanthanide rare earth ions has become a research hotspot in many fields such as super-resolution nanoimaging,information security and encryption,temperature sensing,biological labeling and therapy owing to their characteristics of sharp emission spectrum,large anti-Stokes shift and photochemical stability.Among varieties of upconversion materials,upconversion nanoparticles with multilayer core-shell structure have recently gained substantial attention due to the advantages of adjustable rare earth element distribution on nanoscale,controllable energy transfer processes,tunable upconversion properties and modifiable physical and chemical properties.Although many regulation methods have been reported on rare earth upconversion luminescence using core-shell structure design,there are still some scientific problems which hinder the in-depth understanding and analysis of rare earth upconversion luminescence kinetics,such as the unclear luminescence mechanism,the lack of scheme to regulate the energy transfer of rare earth ions on nanoscale,the difficulty to fabricate multilayer core-shell structure,the luminescence quenching under high doping concentration.Therefore,it is of great significance to make the research on realizing the fine-tuning of lanthanide's spatial distributions on nanoscale,depressing the concentration quenching effect,improving the color purity as well as the multi-wavelength excitations for both basic research and practical application of upconversion materials.In this thesis,the interface energy transfer?IET?model was proposed and optimized to separate the lanthanide activator and sensitizer through core-shell structure design,which enabled the upconversion luminescence from a series of rare earth ions as well as the control of the ionic interactions between rare earth ions on the nanometer length scale.In addition,upconversion luminescence with 100%Er³⁺dopant was achieved in the Na Er F₄ nanoparticles by inert shell protection and the mechanism of upconversion luminescence was studied.Finally,the thermal enhancement of red upconversion emission was successfully achieved by adjusting the shell thickness of the lithium core-shell structure,showing good performance in temperature sensing applications.The main achievements of this thesis are as follows:?1?The upconversion luminescence process through the IET model was studied.Firstly,a double-layer core-shell structure was constructed based on the IET model,where the activator and sensitizer were separated in different shell.The results showed that IET in core-shell structure design is applicable to the upconversion of Yb-Er/Tm/Ho coupled systems.It can effectively suppress the back energy transfer from Er³⁺to Yb³⁺,which further enhanced the upconversion by increasing the doping concentration of the sensitizer.The upconversion kinetics of the Gd-Eu/Tb system was also analyzed in energy migration upconversion?EMU?and IET designs.By comparing the spectra and lifetime,it was found that Gd³⁺preferentially transfers energy to the activator rather than to nearby Gd³⁺ions via energy migration at the core/shell interface.A tri-layer core-shell structure was designed and fabricated to control the ionic interaction between the donor and acceptor by tuning their separation through altering the inert layer thickness.The physical mechanism of upconversion luminescence of rare earth ions on the nanoscale was deeply studied.It was confirmed that the effective energy transfer in the pairs of Yb-Er/Tm/Ho,Gd-Eu/Tb and Nd-Yb is within a range of 1.6-2.1 nm.The upconversion luminescence of Eu³⁺and Tb³⁺under the 808/980 nm dual-wavelength excitation also realized in the tri-layer core-shell structure samples by taking advantage of IET mechanism.The time-gating technology was further used to decode the information with different lifetimes,suggesting the application of multi-wavelength excited upconversion nano-luminescent materials in the field of information security and storage.?2?The self-sensitized upconversion of highly doped Er³⁺under 1530/980/808 nm excitation was systematically investigated by core-shell structure design with inert shell protection.It was found that increasing the thickness of the inert shell layer could significantly enhance the upconversion luminescence intensity and lifetime of the sample.This indicated that surface quenching caused by the energy migration is one of major factors for the upconversion luminescence quenching in nanoparticles doping with high concentration of Er³⁺.It was confirmed by the emission-excitation power dependence result,and the was found to be the key ways to realize the red upconversion under 1530 nm excitation.The effect of codoping other rare earth ions on the upconversion luminescence properties of Na Er F₄ was also investigated,and it was found that introducing a small amount of Yb³⁺,Tm³⁺,and Ho³⁺can further purify and enhance the red upconversion emission.Finally,the energy migration among the erbium lattice was studied by designing the low-concentration/high-concentration/inert-layer core-shell-shell structure upon which the energy migration was depressed with resultant enhancement of upconversion luminescence.?3?The upconversion luminescence properties of Er³⁺in lithium lattice host was systematically studied under temperature field.By adjusting the sample size and inert layer thickness,it was found that the upconversion luminescence intensity of Li Er F₄@Li YF₄nanoparticles was closely related to the temperature under 1530 nm excitation.The red upconversion light?668 nm?increased with increasing temperature,which was 7.1 times enhanced at 573 K than that at room temperature.This phenomenon of thermal enhancement of upconversion is sensitive to the surface protection that it disappears with increasing the thickness of the inert Li YF₄ shell layer.Further experiment results suggested that the lattice expansion occurs with increasing the temperature which reduces the surface quenching caused by energy migration.Using this unique optical property,the temperature probe at 303-573 K was successfully prepared based on the relationship between fluorescence intensity ratio and temperature.The probe has high absolute sensitivity with a maximum absolute sensitivity value of 0.187 K^-1 at 573 K,providing new possiblities for the development of new-generation high-sensitivity fluorescent temperature probes.