Study On The Synthesis And Optical Properties Of New Inorganic Luminescent Materials Co-doped With Rare Earth Ions

White light-emitting diodes (LEDs) are considered as a mainstream of the new source in 21st century because of their excellent properties such as minute extension, lower voltage, longer operation lifetime, higher efficiency and environmental friendship. The discovery of the white LED of combing a blue LED with a yellow phosphor (YAG: Ce³⁺) in 1990s opened the exciting and challenging research on phosphor-converted LED (PC-LED), which has been a practical and potential method to fabricate the white LED chip in recent years. The phosphor-converted LED light strongly depends on the phosphor whether it can be efficient excited by blue (450～470 nm) or near ultraviolet (NUV, 350～420 nm) LED chip and show high downconverting efficiency. Base on the great attention to the luminescent materials doped by rare earth ions for their application in the white LED, the high-pressure mercury lamp and anti-courterfeiting fields, we synthesized a new series of Eu²⁺/Eu3+-doped inorganic phosphors, (CaO-CaCl₂-SiO₂): Eu²⁺, (BaO-BaCl2-SiO2): Eu²⁺, Ba2Mg(BO3)2: Eu²⁺, Sr2CaMoO6: Eu3+, Na+ and BaCaBO3F: Eu3+, Li+ with different synthesis methods, such as solid-state reaction, sol-gel and microwave-assisted sol-gel. The crystallization process, sturcuture, morphology, luminescence properties and energy transfers of the as-synthesized phosphors were characterized by thermogravimetry–differential thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) analysis, diffuse reflection spectra (UV-vis), photoluminescence (PL) spectra and PL decay curve, respectively. Further, we studied the dependence of the size, morphology and luminescence properties of the phosphors on the synthesis methods and preparation conditions. The results of the investigation of all the phosphors were shown as follows:(1) Eu²⁺-doped Alkali earth chlorosilicate phosphor of the types (CaO-CaCl₂-SiO₂): Eu²⁺was synthesized by the solid-state reaction. Under the excitation of the NUV light, the (CaO-CaCl₂-SiO₂): Eu²⁺phosphor synthesized at 900 oC emits an intense green light peaking at 510 nm, whereas the same component phosphor synthesized at 1100 oC emits an yellow light peaking at 582 nm. In addition, luminescence properties and crystal-lattice environment of Eu²⁺in (CaO-CaCl₂-SiO₂) crystal synthesized at 1100 oC were discussed according to the Van Uitert experimental equation and the possibilities of two Eu²⁺emission centers, viz, green and yellow centers were confirmed. The mechanisms of concentration quenching in (CaO-CaCl₂-SiO₂): Eu²⁺phosphor were also discussed, which were caused by the energy transfer among the nearest-neighbor ions in this phosphor system.(2) A new intense green light-emitting phosphor, Eu²⁺-doped (BaO-BaCl₂-SiO₂) phosphor system was synthesized at 800 oC by the conventional high-temperature solid-state reaction. PL spectra reveal that this phosphor can be efficiently excited by the NUV light and blue light in the region covering 280 nm and 480 nm, and emits an intense green light peaking at 502 nm,which perfectly matches with the emissions wavelength of near-UV light-emitting diodes (LEDs). Further, luminescence properties and crystal-lattice environment of Eu²⁺in (BaO-BaCl₂-SiO₂) crystal synthesized at 1100oC were discussed according to the Van Uitert experimental equation, and we suggested that it existed only one Ba site in the crystal of the host material, which Eu²⁺occupied in the (BaO-BaCl₂-SiO₂) system and emits an intense green light. The mechanisms of concentration quenching in (BaO-BaCl₂-SiO₂): Eu²⁺phosphor were also discussed, which were caused by the energy transfer among the nearest-neighbor ions in this phosphor system.(3) Fine-sized red-emitting Ba₂Mg(BO₃)₂: xEu²⁺phosphors, which emits an intense red light peaking at 608 nm in the region covering 320 nm and 450 nm,were synthesized via a microwave-assisted sol-gel route. It was found that the energy transfer among the nearest-neighbor ions resulted in the concentration quenching of Ba₂Mg(BO₃)₂: Eu²⁺phosphor, and the critical distance was determined to be about 17.32 . Base on these, we studied the dependence of the size, morphology and luminescence properties of the phosphors on the synthesis methods and preparation tempurature. Results indicated that Ba2-xMg(BO3)2: xEu²⁺phosphors prepared by the microwave-assisted sol-gel route had low synthesis temperature (900 oC), favorable size (2μm), homogeneous shape and high luminescent intensity, which was better than those of the phosphors prepared by the conventional solid-state reaction method.(4) Double-perovskite B-site substituted Sr₂CaMoO₆: Eu³⁺, Na⁺ red-emitting phosphors were prepared by the sol-gel method and their luminescence properties were investigated as a function of the sintering temperature and Eu³⁺-doping content. Sr₂CaMoO₆: Eu³⁺, Na⁺ phosphor was selected to study the thermal behavior, phase structure, microstructure and photoluminescence property under different sintering temperature. The photoluminescence studies on Sr2Ca1-2xMoO6: xEu³⁺, xNa⁺ (x = 0.02, 0.05, 0.10, 0.15 and 0.20) show that a dominant red emission line at around 594 nm, which is due to the Eu³⁺ magnetic dipole transition of ⁵D0-⁷F₁ , is observed under different Eu³⁺excitations (396 and 412 nm). Another emission line at around 615 nm can be due to the Eu³⁺electric dipole transition of ⁵D0-⁷F₂. Finally, the PL decay progress of Sr2Ca0.8MoO6: 0.10Eu³⁺, 0.10Na+ was studied in details, and the mechanisms of concentration quenching in Sr2Ca1-2xMoO₆:xEu³⁺, xNa⁺ phosphor were also discussed.(5) BaCaBO3F: Eu³+), Li⁺ phosphors were synthesized by conventional solid-state reaction. According to XRD of different sintering temperatures, 1000 oC could be chosen as the favorable synthesis temperature. The photoluminescence studies on BaCa_1-2xBO₃F: xEu³⁺,xLi⁺ (x=0.01, 0.03, 0.05, 0.10, 0.15, 0.20 and 0.25) show that at two red emission lines at around 590 and 615 nm, which are due to the Eu³⁺magnetic and electric dipole transition of⁵D0-⁷F₁ and ⁵D0-⁷F₂ is observed under different Eu³⁺excitations (394, 463 and 532 nm). Compare with Eu³⁺magnetic dipole transition of⁵D0-⁷F₁ in BaCaBO₃F host material, the intensity of ⁵D0-⁷F₂ electric dipole transition is relatively strong.