The Study On Rare Earth Phosphate Phosphors

Rare earth elements are used widely as fluorescent(luminescent) materials because of their characteristic electron structures.The rare earth fluorescent materials have many advantages such as high absorption ability,high transformation ratio,strong emission ability(especially at visible light regions),and stable physicochemical properties.The rare earth phosphates are one of the most important parts in the rare earth phosphors as they have been applied widely in illumination industry.In this dissertation,we review the properties and the synthetic methods of the rare earth phosphates in the first chapter.The main types,characteristics,preparation,and improvement of nano phosphors are also discussed.Our experimental work consists of four parts:improvement of nano phosphors by self-assembly,design and synthesis of a novel color-tunable particle by core-shell structure,reduction manufacture cost of phosphor by core-shell complex,and development of biological probe of rare earthdoped nanoparticles.The above-mentioned studies can be summarized as followings.(1) LnPO₄(LnPO₄=LaPO₄:Ce,Tb) system is used as a model system to explore a new strategy to improve the luminescence of nano phosphors.The results indicate that the luminescent efficiency of the nano phosphors can be enhanced significantly by 1D ordered aggregation.Compared with the individual nanoparticles,the quantum efficiency of the 1D linear assemblies can increase from 21%to 80%.This value is even close to that of the corresponding bulk ones.It is also found that the fluorescent life is also increased by the oriented assembly.Generally,nano phosphors are featured by the relatively low luminescence,which limits their industrial application.We suggest that the assembly of nano phosphors can be used as an easy but effective pathway to improve the luminescent intensities of these materials.(2) A novel color tunable phosphor with core-shell structure is designed and synthesized using two materials,LnPO₄(LnPO₄=LaPO₄:Ce,Tb )(green phosphor) and Y₂O₃:Eu(red phosphor),which are widely used in industry.By adjusting the proportion of core and shell,the fluorescence colors of the phosphor complex can be tuned conveniently.It is important that the individual core-shell structured particle can emit the similar lights even at micron scale.This interesting result is well confirmed by photoluminescence examination,fluorescence spectrum,and confocal laser scanning microscopy.It is suggested that the core-shell structure can provide a new method to design and fabricate color-tunable phosphors. (3) The core-shell structure can also be used to reduce the manufacture cost of the rare earth phosphors.The relative high price of LnPO₄ is attributed to the expensive Tb₃O₄.We use the cheap LaPO₄ micron particle as the core to induce heterogeneous precipitation of LnPO₄ to form the LaPO₄-LnPO₄ core-shell complex. After calcination,the surface of resulted particles become smooth and the crystallinity is also improved.The luminescence of the core-shell particles is examined and the brightness is almost as same as the commercial LnPO₄ precursor.The optimum synthesis condition for this novel structure is also summarized.This method is easy but can greatly reduce the industrial cost in the production of LnPO₄ phosphors.(4) The incorporation of rare earth ions into biominerals is developed as a new approach to the biological probes.As the main inorganic component of biological bone and tooth enamel,hydroxyapatite(HAP) is featured by its excellent biocompatibility and bioactivity.But the particle itself is a non-luminescent material.The in situ studies of the biological function of this important mineral in living organisms is one of the unsolved issues in biomineralization.By partially replacing the calcium ions on the surfaces of 20 nm HAP particles with Tb³⁺,the Tb-HAP nanoparticles can be obtained readily.After such a treatment,the HAP nanoparticles become luminescent and their maximum emission intensity is observed at 544 nm under an excitation of 488 nm visible light.The experiment also confirms that the surface doping do not change the original morphology and biological activity of HAP particles.The observations of biological transmission electron microscopy and the confocal laser scanning microscopy confirm that these Tb-HAP nanoparticles can be internalized by the rabbit mesenchymal stem cells readily and their luminescence in the cells can be detected clearly under a fluorescent microscope.Therefore,the surface modification of rare earth doped nanoparticles can be developed as a functional treatment to design the new biological probes.