Study On The Fluorescent Properties Of NaYF₄ Nanocrystals Doped By Ho³⁺ Ions

Rare-earth doped luminescent nanomaterials have unique optical properties such as colorful fluorescence emission,because of rich energy level structure of rare-earth ions,and have been widely applied in the optoelectronics,solar energy,biomedical,fluorescent display,and anti-counterfeiting technologies.the selective regulation of the fluorescence characteristics of rare earth luminescent materials is one of the hot topics that researchers are generally concerned about.It is well known that luminescence process of the rare earth ion mainly includes down-conversion luminescence and up-conversion luminescence,and most studies have focused on the process and mechanism of up-conversion luminescence.Rare-earth upconversion luminescence nanomaterials have been applied to biomarkers and bioimaging because of their strong detection sensitivity.The spectrum detection range is mainly located in the"optical window"of biological tissues,the wavelength range is 600-1100 nm.Therefore,obtaining a strong red light emission or achieving single-band red light emission is one of the preconditions for achieving biological target detection.Therefore,the thesis mainly uses NaYF₄ nanocrystals as the host material,achieving the regulation of the luminescent properties of the luminescent light Ho³⁺ion by co-doping ion,increasing the excitation power,constructing core-shell structure and so on.Under the excitation of near-infrared 980nm laser,the fluorescence emission of Ho³⁺ions in NaYF₄:Yb³⁺/Ho³⁺upconverted nanocrystals was regulated by the introduction of Ce³⁺ions and achieving the transition from green to red.The red light emission intensity and color purity of Ho³⁺ions are further improved by increasing the excitation power and constructing the core-shell structure.The main work and conclusions are as follows:1)NaYF₄:Yb³⁺/Ho³⁺nanocrystals with a hexagonal phase structure and a size of about 20 nm were synthesized by a high temperature coprecipitation method.Under 980nm laser excitation,the effect of Ce³⁺ion concentration on the up-conversion luminescence of the Ho³⁺ion in NaYF₄ nanocrystals and the energy transfer efficiency between ions were systematically studied.The study found that with the increase of Ce³⁺ion concentration,Ho³⁺fluorescence emission was changed from green emission to red emission,and the red-green ratio increased by nearly 11 times.The increase of red emission intensity is mainly due to the two effective cross-relaxation processes between the Ho³⁺and Ce³⁺ions.By studying the luminescence kinetics,it was found that⁵S₂/⁵F₄(Ho³⁺)+²F_5/2(Ce³⁺)?⁵F₅(Ho³⁺)+²F_7/2(Ce³⁺)cross-relaxation 2?CR2?played a leading role,and ⁵I₆(Ho³⁺)+²F_5/2(Ce³⁺)?⁵I₇(Ho³⁺)+²F_7/2(Ce³⁺)cross-relaxation 1?CR1?plays an auxiliary role.At the same time,by increasing the excitation power,the red upconversion emission intensity of the NaYF₄:Yb³⁺/Ho³⁺/Ce³⁺nanocrystals was increased.2)To further increase the red emission upconversion emission intensity,NaYF₄:20%Yb³⁺/2%Ho³⁺/12%Ce³⁺@NaYF₄:Yb³⁺core-shell structure nanocrystal was constructed by epitaxial growth.By adjusting the concentration of the sensitizer ion Yb³⁺in the shell,the enhancement of the red upconversion luminescence intensity of Ho³⁺was achieved.The experimental results showed that obvious fluorescence enhancement was observed in the NaYF₄:20%Yb³⁺/2%Ho³⁺/12%Ce³⁺@NaYF₄ core-shell nanocrystals.The reason was that the protective effect of the shell inhibited the nonradiative transition processes of the activator Ho³⁺in the nucleus.When Yb³⁺ions in the shell are changed,the red upconversion luminescence intensity of the core-shell nanocrystals first increased and then decreased,and it reached the maximum when the Yb³⁺ion concentration are 10%mol.