Synthesis And Upconversion Luminescence Properties Of Rare-earth Fluoride Nanocrystals

In recent years, fluorescence imaging based on upconversion nanoparticles(UCNPs) is becoming a popular tool for biomedical assays and diagnosis due to its high spatial resolution and convenience. The UCNPs have many advantages over conventional bio-markers(Organic and inorganic quantum dots and organic dyes), such as deep penetration of NIR excitation light into biological tissue, low toxicity, and high signal-to-noise ratio as well as high resistance to photobleaching, blinking, and reduced photo-damage. Among them, the rare earth fluoride nanoparticles are excellent host materials due to its low phonon energy, which results in decreased non-radiative relaxations and increased UC emission efficiencies. The main contents in this thesis are as follows:1. Monodisperse water-soluble La F3:Yb3+/Er3+ nanocrystals(NCs) have been successfully fabricated via a fast, facile, and environmentally-friendly polyol process with PVP as an amphiphilic surfactant. The obtained NCs with a small size of 18 nm can be well dispersed in hydrophilic solutions. Meanwhile, their upconversion(UC) luminescence intensity was increased through Ca2+ doping due to the decrease of symmetry around rare earth ions. Prepared a new optical waveguide material.2. We synthesized monodispersity hexagonal phase Na Gd F4:Yb/Ho nanoparticle with less than 20 nm size via the high temperature thermal decomposition method. The fluorescence pictures show that nanoparticles exhibit high luminescence intensity. In addition, upconversion emission tuning from green to red in Na Gd F4:Yb3+/Ho3+ nanoparticles was successfully achieved by tri-doped with Ce3+ ions..3. Based on small Li Lu F4 cores to grow up the tetragonal-phase Li Gd F4 shells, we obtained Li Lu F4@Li Gd F4:Yb/Er(Tm) core-shell NPs via the high temperature thermal decomposition method. Li Lu F4@Li Gd F4:Yb/Er(Tm) nanoparticles show intense UCL than β-Na Gd F4:Yb/Er(Tm) UCNPs under continuous-wave excitation at 980 nm. We constructed an inert Li Gd F4 shell layer around Li Lu F4@Li Gd F4:Yb/Er(Tm) nanoparticles to enhance the UCL intensity. Furthermore, the feasibility of Li Lu F4@ Li Gd F4:Yb/Er(Tm)@Li Gd F4 core-shell-shell nanoparticles applied to UCL/CT/T1-weighted MR tri-modal imaging bioprobes.