Synthesis And Luminescence Properties Of Rere-Earth Doped Fluoride Nanoparticles

With recent tremendous advances in nanotechnology, increasing attention have been paid to lanthanide (Ln³⁺) ions doped luminescence nanocrystals (NCs) owing to their superior properties. Thesed NCs have narrow-band width (<10 nm) emission, long luminescence lifetime (μs–ms range) and low toxicity, etc. Besides the applications in the fields of solid-state lasers, multicolor three dimensional displays and solar cells, they are consided to be the most suitable candicate for luminescent bio-label in contrast to conventional luminescent materials such as organic fluorescent dyes and semiconductor QDs. It is well-known that in the luminescence of Ln³⁺ ions, the highest phonon frequencies of the host lattice are responsible for nonradiative relaxations. To overcome the suppression of luminescent effeciency due to nonradiative relaxations, it is necessary to search an appropriate lattice with lower phonon energy (the highest vibration frequency vmax of host lattice). Rare earth fluorides including binary REF3 and complex AREF₄ (RE = rare earth; A = alkali) have been considered as an excellent luminescent host matrix for various optically active Ln³⁺ ions, because they normally possess a high refractive index and low phonon energy. On the basis of our group's previous studies, in this thesis, we achieved some important results on the bifunctional NCs showing paramagnetism and upconversion emissions were achieved through adjusing host matrix and the combination of lanthanide dopant. Main results obtained are described as follows:1. Rare earth (RE) ions doped KGdF₄ (RE=Yb/Er, Eu, Tb) NCs are synthesized by a hydrothermal method. The results of X-ray diffraction and transmission electron microscope reveal that the prepared NCs crystallized in the cubic phase asα-NaGdF₄ with a mean diameter of 12 nm. When excited by the 274 nm line of xenon lamp, orange and red emissions are observed in Eu³⁺ doped KGdF₄ NCs, indicate the existence of energy transfer between Gd³⁺ and Eu³⁺. Under 980 nm infrared excitation, red and green up-conversion emissions attributed to energy transfer process betweent Yb³⁺ and Er³⁺ are obserbed in Yb³⁺/Er³⁺ codoped KGdF₄ NCs. The measured field dependence of magnetization of Eu³⁺ doped KGdF₄ NCs exhibits paramagnetic properties at both 293K and 77K.2. Yb³⁺/Tm³⁺ co-doped hexagonal NaGdF₄ NCs with the size of approximately 22 nm are synthesized via a hydrothermal method. Under the excitation of a 980 nm diode laser, these NCs emit intensive near infrared UC emission at 800nm and blue one at 475nm . The ratio of the emission intensities at 800 nm to that at 475 nm can be enhanced to be 56 by doping a small amount of Sm³⁺ ions into NaGdF₄ host. Our results indicate that the kind of reported magnetic NaGdF₄ NCs may have potential applications in high contrast in vitro and in vivo imaging and magnetic resonance imaging (MRI).3. Tm³⁺ doped orthorhombic KYb₂F₇ NCs with the average size of of 42 nm are prepared by a hydrothermal method. Under the excitation of a 980 nm diode laser, near infrared UC emissions at 800 nm, blue ones at 475 and 450 nm, and ultraviolet one at 360nm are observed from these NCs. The intensity of high energy UV and blue upconverted emissions of Tm³⁺ ions in Tm³⁺ doped KYb₂F₇ NCs can be selectively suppressed compared to the NIR emission at 800 nm by doping a small amount of Sm³⁺ ions into the host matrix. Our results indicate that the obtained NCs showing excellent near infrared UC emissions have potential applications in high contrast imaging for deeper tissues and organs.