Luminescence And Mechanism Studies Of Multi-colored Phosphors With Different Rare Earth Ions

With growing global energy shortage and increasing environmental pressure, energy conservation and environmental protection has become one of the important subjects nowadays. In the field of general lighting, white LED products are attracting more and more attention for their advantages: lightning with high efficiency, long usage life, small size, low power and environmental protection, which is recognized as a new lighting source and would become the future trend of development. And as the most important components in white LEDs, phosphors have significant influence on its performance index: luminous efficiency, service life, color temperature and color rendering index. Generally, phosphors can be doped with rare earth(RE) ions for its special electronic structure, which has strong absorption ability, high conversion rate, strong emitting ability particularly in the visible region and the stable chemical and physical properties. Among them, the most widely used methods to fabricate white LEDs are ultraviolet or near ultraviolet LEDs with tricolor phosphors(red, green, blue) combination, which has the best color and high fluorescent material luminous efficiency; The other is to combine blue LED with the yellow phosphor.In the chapter three, un-doped and Eu3+-doped glaserite-type orthovanadates Cs K2Gd1-xEux[VO4]2 with various Eu3+ concentrations of x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 were synthesized via the solid-state reaction. The formation of single phase compound was verified through the X-ray diffraction studies. The photoluminescence(PL) and PL excitation(PLE) spectra, PL decay curves, and absolute quantum efficiency(QE) were investigated. These Eu3+-doped samples show not only several sharp emission lines due to Eu3+ but also a broad emission band with maximum at 530 nm due to [VO4]3- host. The optimum doping concentration of Eu3+ is 0.6. The host emission is never quenched even if in the Eu3+-condensed host of Cs K2Eu(VO4), indicating inefficient energy transfer from host [VO4]3- to Eu3+. Like the 530 nm charge transfer [VO4]3- emission, two broad and intense PLE bands with maxima at 330 and 312 nm are observed for the Eu3+ emission. Maximum QE of 38.5 % is obtained from Cs K2Gd1-xEux[VO4]2(x=0.6). A white-color emission is obtained by combination of the broad 530 nm emission band and intense sharp lines due to Eu3+ at 590-620 nm.In the fourth chapter, green-emitting phosphor of Eu2+-doped sodium barium silicate Na2 Ba Si2O6 was prepared by chemical sol-gel method. The X-ray powder diffraction, SEM, photoluminescence spectra and time resolved emission spectra were measured. It can be well excited by the near UV-LED chips. The luminescence was suggested to originate from two kinds of Eu2+ centres, which occupy both Ba2+ and Na(2) sites in Na2 Ba Si2O6 lattices. Energy transfer among the Eu2+-centres was discussed by analyzing the photoluminescence spectra, concentration-dependent luminescence intensity and decay lifetimes. The activation energy(ΔE) for thermal quenching was reported. The high luminescence properties of Na2 Ba Si2O6: Eu2+ were compared with that of Ba Si O3:Eu2+ on the base of luminescence properties and microstructure of Eu2+ ion.In the chapter five, Eu2+-doped borate fluoride Ba Al BO3F2 was synthesized by the conventional solid state reaction. The crystal phase formations were confirmed by XRD measurements. The PL spectra and the decay curves were also investigated. Eu2+-doped Ba Al BO3F2 phosphor presents narrow blue luminescence excited by near-UV light. The maximum absolute quantum efficiency of Ba Al BO3F2:0.05Eu2+ phosphor was 76 % excited at 398 nm light at 300 K. The phosphor shows an excellent thermal stability with high thermal activation-energy on temperature quenching effects because of the rigid crystal lattices.In the sixth chapter, a yellow-emitting nanophosphor of Ce3+-activated Sr3 Lu Al2O7.5 was prepared by chemical sol-gel method. The XRD and SEM were applied to characterize the phosphors. The luminescence properties such as the photoluminescence(PL) excitation and emission spectra, the internal quantum efficiency(QE) and the luminescence decay curve(lifetime) were investigated. Ce3+-activated Sr3 Lu Al2O7.5 exhibits brightest yellow emission with the internal quantum efficiency of 57 %, its thermal stability was evaluated by the temperature-dependent luminescence and the activation energy for thermal quenching. The luminescence properties of Ce3+-activated Sr3 Lu Al2O7.5 was discussed in comparisons with famous yellow-emitting YAG:Ce3+. This new nanophosphor could be a potential yellow-emitting phosphor for an application in lighting and display.Finally, systematically studied multiple-colored phosphors used in white LEDs, including red, green, blue, yellow phosphor. This dissertation’s novelties are as followings: Firstly: the tunable luminescence of Cs K2Gd[VO4]2:Eu3+ was studied for the first time, and showed nonquenching of the host emission even under high Eu3+ concentration, which can be applied in the WLEDs as a red phosphor. Secondly, the phase composition and luminous performance of Na2 Ba Si2O6:Eu2+ were investigated, which has 3.3 times higher luminescent intensity than Ba Si O3:Eu2+ and has two kinds of luminescent centers. Thirdly, the structure and luminescent properties of Ba Al BO3F2:Eu2+ blue phosphors were studied, having a very narrow FWHM and with a QE of 76 %. Finally, the nano-phosphor Sr3 Lu Al2O7.5:Ce3+ has an quantum efficiency of 57 %, which has a higher activation energy than that of YAG:Ce3+ and can be applied in the WLEDs as a yellow phosphor.