Preparation And NUV Photoluminescence Of Several Typical Rare-earth Silicates Phosphors For White-LED

Silicates are excellent hosts for photoluminescence (PL) materials for their various crystal structures and high physical-chemical stability. The luminescent properties of rare earth doped silicates are closely related to the NUV absorption band, charge transfer band, 4f-5d transition and 4f-4f transition, which are all of Rare-earth (RE) ions. In this thesis, the NUV PL properties of RE ions in several typical silicate hosts were systematically investigated. Our investigation is focused on the crystal structure, doping and the energy transfer mechanism of phosphor. We summed up the critical conditions and influencing factors about the rare-earth silicate-based phosphor, which have high-efficiently excited at NUV region. The results derived from the above research would contribute greatly to the developing of phosphors for NUV-LED and advance their application in LEDs.In this article, one-dimensional chains structure phsophor of Sr2Ce04:RE (RE= Sm3+, Ti4+/Eu3+, Eu3+/Sm3+) and several typical silicate phosphors of Ca2Al2Si07:RE (RE= Ce3+/Tb3+, Dy3+, Eu3+, Eu2+), Sr2Al2(Si, Mo)O7:Eu3+, CaSrAl2SiO7:Eu3+, CaAl2Si2O8:RE (RE= Eu, Eu2+/Mn2+, Ce3+/Tb3+), BaMg2Si207:Eu2+, Mn2+, Mg2Si04:RE (RE= Eu3+, Eu2+),SrZrSi2O7::RE (RE= Ce3+, Tb3+, Eu3+, Dy3+) powder samples were synthesized by the solid-state reaction. Their luminescent properties under NUV excitation were investigated in detail. Based on the current demands about the development of white LED phosphors, our investigation is mainly focused on the exploitation of a new and efficient red phosphor and the exploration of a single matrix white phosphor. The results indicate that:1. A system of rare earth doped phsophor Sr2Ce04 with one-dimensional chains structure1) A tunable full color emitting phosphor Sr2Ce04:Sm3+ was synthesized. Its emission color could be changed by tuning concentration of rare-earth activator Sm3+. The doped compound emits strong white light when the concentration of the doped Sm3+ is low (<3%), When the Sm3+ concentration is increased (3%-15%), it emits red light. Among all the samples, Sr2CeO4:1%Sm3+ phosphor show not only strong emission intensity but also good color purity, its chromaticity coordinates is (0.334,0.320), which is more close to the the NTSC standard values (0.33,0.33). 2) By doping small amount of Ti4+ into Ce4+ sites, the excitation intensity of charge transfer (CT) band of 300-380 nm was significantly broaden and enhanced. As a result, under 334nm excitation, compared with the undoped phosphor Sr2CeO4, the emission intensity of phosphor Sr2Ce1-xTixO4 has improved about 85% by doping 0.01mol Ti4+. When doping Eu3+ ion, the optimum phosphor Sri.95Ceo.99Ti0.01O4:0.05Eu3+ with the integrated intensity 3 times higher than that of the red commercial phosphor Y2O2S:0.05Eu3+, and owns excellent color purity with chromaticity coordinates of (0.665,0.33).2. A single matrix white phosphor. By adjusting the concentration of luminous center Eu2+, Mn2+, and with the energy transfer of Eu2→Mn2+, we obtained a single matrix white phosphor with good color purity and high brightness in two types of silicate system BaMg2Si207:Eu2+,Mn2+ and CaAl2Si20g:Eu2+,Mn2+. We first report a novel rare earth activated a single matrix white phosphor CaaAl2SiO7:Ce3+, Tb3+, and demonstrated that energy transfer from Ce3+ to Tb3+ in Ca2Al2SiO7 results in a white emission activated with long-UV radiation. Moreover, we prepared a single ion activated white phosphor Ca2Al2SiO7:Dy3+, under 350 nm excitation, the phosphor Ca1.97Al2SiO7:0.07Dy3+ exhibits the strongest emission intensity and the phosphor Ca1.97Al2SiO7:0.01Dy3+ owns excellent color purity with chromaticity coordinates of (0.340,0.338) and relative color temperature of 5164 K. These results show that the phosphor Ca2Al2SiOy:Dy3+ can be considered as a potential warm white light phosphor for application in LEDs.3. A new and efficient red phosphor. Photoluminescence properties of Ca2Al2SiO7:Eu3+, Sr2Al2SiO7:Eu3+, CaSrAl2SiO7:Eu3+ and Mg2Si04:Eu3+ were evaluated under NUV excitation. It is found that there existed an absorption peak in 393 nm, which was assigned to the absorption of Eu3+. In Ca2Al2SiO7:Eu3+ and Sr2Al2SiO7:Eu3+, compared with the CTB absorption of O2-→Eu3+ in the 200-350 nm, the intensity of 393 nm absorption peak is weaker. By introducing Mo ion and charge compensator ion Na, but also strengthen the absorption of the phosphors at-400 nm and strongly enhanced the emission intensity of Eu3+ under 393 nm excitations. The intense red-emitting phosphor is Sr1.56Euo.22Na0.22Al2Si0.9gMo0.02O7 with the integrated intensity 1.5 times higher than that of the red commercial phosphor. Its chromaticity coordinates of (0.659,0.331) is more close to the NTSC standard values (0.67,0.33) than that of the red commercial phosphor and the quantum efficiency of Sr1.56Eu0.22Na0.22Al2Si0.98Mo0.02O7 is 50%. These results indicate that this novel red phosphor could be considered as a promising red phosphor for application in LEDs. In CaSrAl2SiO7:Eu3+ system, optical properties were studied as a function of Eu3+ concentration x, when x>14%, the intensity of absorption of the f-f transitions of Eu3+ at 393 nm is stronger than that of the broad charge transfer transition band (CTB) around 250 nm, and which is matches well with the output lights of NUV-LED. As a result, the optimum red phosphor is Ca0.78SrAl2Si07:0.22Eu3+ with the chromaticity coordinates of (0.650,0.350). The possibility of Mg2Si04:Eu3+ as novel promising NUV red phosphors were exploited. The results indicate that the optimum concentration of Eu3+ is 0.09 mol while the intensity of absorption at 393 nm is the strongest and its quantum efficiency is around 38%.4. Other Silicate phosphors. The Eu2+ doped Ca2Al2SiO7 system exhibited a redshift in its emission wavelength, and explaied the reason of red shift from the crystal field. CaAl2Si2O8:Eu was prepared in air atmosphere by the solid state reaction. It was found that a reduction of Eu3+to Eu2+ in CaAl2Si2O8 in air condition. And the reduction mechanism from Eu3+ to Eu2+ in this compound has been discussed from the charge compensation model. In CaAl2Si2O8:Ce3+, Tb3+ system, CaAl2Si2O8:Ce3+, Tb3+phosphor with the emission intensity 10 times higher than that of the green phosphor CaAl2Si2O8:Tb3+ phosphor. The result indicated that CaAl2Si2O8:Ce3+, Tb3+would be a potential NUV-LED green phosphor. Moreover, the NUV photoluminescence properties of Mg2SiO4:Eu2+ and SrZrSi2O7:RE (RE = Ce3+, Tb3+, Eu3+) were investigated in detail. The result indicated that Mg2SiO4:Eu2+ and SrZrSi2O7:RE (RE= Ce3+, Tb3+, Eu3+) are not suitable for NUV-LED phosphor because the absorption of these phosphors in NUV region are very weak.