Researches On Synthesis And Photoluminescence Properties Of Multifunctional Rare Earth Ions Doped Molybdate Materials

Recently, rare earth ions doped inorganic micro/nanocrystals materials have wide application in the light source, display, imaging, optical communications, sensing,medical and other fields due to their unique electronic, optical and chemical properties, but also have low energy consumption, low pollution, high efficiency,small size and many other advantages. In numerous rare-earth doped matrix materials,molybdates with scheelite structure have been more high-profile due to their good physical and chemical stability and optical properties. In addition, metal molybdates have lower phonon energy, which can enhance radiative transition probability, and improve the quantum yield.Several chemical synthesis techniques to prepare rare earth doped phosphors are available such as high temperature solid-state, microwave heating, sol–gel, solvent,co-coprecipitation, combustion, hydrothermal and so forth. However, the hydrothermal method is the most simple and environmental synthetic route, which is not only low cost, low energy consumption, lower reaction temperature, to produce uniform and good dispersion samples, but can be very flexible in adjusting the reaction conditions（reaction time, temperature, p H value） or add different chelating agents in the preparation process to obtain the desired structure and morphology of the samples. In this paper, we have successfully synthesized different systems of rare earth molybdate by a low-temperature hydrothermal synthesis method by using ethylene diamine tetraacetic acid（EDTA） as the “shape modifiers”. The structural characteristics of the final products were systematically characterized using powder X-ray diffraction（XRD）, field emission-scanning electron microscopy（FE-SEM）,photoluminescence（PL）, photoluminescent excitation spectra（PLE）, upconversion luminescence（UCL）, diffuse reflectance spectroscopy（DRS） and the electronic structure, respectively. The concrete contents of the research as follows:（1） Uniform and well-crystallized NaY（MoO₄）₂: Eu³⁺ 3D hierarchical flower-like architectures novel self-assembled from different building blocks have been successfully synthesized by a facile EDTA-mediated hydrothermal route. It was found that the p H value in the initial solution was responsible for the crystal phase determination of final products. The experimental results showed that the amount of EDTA was a key parameter which not only determined their spacial arrangement, but also affected the down-conversion luminescence intensity of the final products. In PL spectrum, a prominent red emission was observed due to the hypersensitive 5D0 �' 7F2 transition, and the optimum doping level of Eu³⁺ was 20 %. Two strongest lines at 396 and 467 nm in excitation spectra of these phosphors matched well with the two popular emissions from near ultraviolet and blue Ga N-based LEDs, so they could be explored for an efficient red region for white light-emitting diodes.（2） A class of Yb³⁺/Er³⁺ co-doped NaY（MoO₄）₂ UC phosphors have been successfully synthesized by a facile hydrothermal route with further calcination. The UC luminescence properties of Yb³⁺/Er³⁺ co-doped NaY（MoO₄）₂ were investigated in detail. Concentration-dependent studies revealed that the optimal composition was realized for a 2% Er³⁺ and 10% Yb³⁺-doping concentration. Two-photon excitation UC mechanism further illustrated that the green enhancement arised from a novel energy-transfer（ET） pathway which entailed a strong ground-state absorption of Yb³⁺ions and the excited state absorption of Yb³⁺–Mo O42– dimers, followed by an effective energy transfer to the high-energy state of Er³⁺ ions. We have also studied the thermal properties of UC emissions between 303 and 523 K for the optical thermometry behavior under a 980 nm laser diode excitation for the first time. The higher sensitivity for temperature measurement could be obtained compared to the previous reported rare-earth ions fluorescence based optical temperature sensors. These results indicated that the present sample was a promising candidate for optical temperature sensors with high sensitivity.（3） Uniform and well-crystallized NaCe（MoO₄）₂: Er³⁺/Ho³⁺, Yb³⁺ UC phosphors have been successfully synthesized by a facile hydrothermal route. The experimental results showed that the EDTA was a key parameter which not only determined their spacial arrangement, but also affected the size distributions of the final products.Moreover, we found that EDTA could tune the band edge absorption of molybdate system by changing the lattice parameters, so as to realize their bicolour tunable luminescence of Er³⁺/Ho³⁺-Yb³⁺ co-doped NaCe（MoO₄）₂ for the first time. Studies of the behavior as a function of dopant concentration were all described, and its luminescence mechanism were analyzed in detail. The different UC mechanism of the samples with and without EDTA were systematically depicted as well. It was found that an innovative route to increase the green UC emission and simultaneously suppress red UC emission in Er³⁺/Ho³⁺-Yb³⁺ co-doped NaCe（MoO₄）₂ prepared with the assistance of 0.1 g EDTA, which enhanced the efficiency of the single green color.These results indicate that this phosphor material shows great prospect in practical application in 3D display technology, green light laser and biological field.