Preparation And Luminescence Properties Of Phosphors For UV Excited White Light-emitting Diodes

White light emitting diodes (WLEDs) are considered to be the next generation lighting systems because of advantages such as low power consumption, high efficiency, long lifetime, and lack of toxic mercury compared to conventional light bulbs. At present, the most significant scheme is phosphor-converted white LED (pc-WLED) by combing LED chip with the down-converting phosphors that play a key role in controlling the quality of the white light, including photo-conversion efficiency, luminous efficiency, luminous flux, correlated color temperature, chromaticity coordinates and color rendering index and so on. Recently, the developing of ultraviolet (UV) LED provides new space for phosphors. Because of the invisible UV light, UV radiation chip based white LEDs has the advantages such as high color stability and high luminous efficiency. This thesis focuses on developing UV-pumped single-phased white-emitting phosphors, the preparation of monochromatic phosphor and their luminescence properties investigation. In chapter 1, we briefly interview on the developments of LEDs, materials and their properties, chapter 2-6, describe the preparation methods, experimental observations regarding materials synthesis and characterizations, applications related to LEDs. Emphasis is given to:(1) A series of Eu²⁺ and Mn²⁺ codoped single-phased white-emitting phosphors were investigated. We first examine Ba₂Ca(B₃O₆)₂:Eu²⁺,Mn²⁺ phosphors by high temperature solid state reaction. Due to the energy transfer process from Eu²⁺ to Mn²⁺, the emission light can be changed by adjusting the doping content of Eu²⁺ and Mn²⁺ under the UV excitation. Furthermore, the white light with CIE coordinates of (x = 0.37, y = 0.25) and correlated temperature of 2654 K was obtained. Next, we investigate Ca₇Mg₂P₆O₂₄:Eu²⁺,Mn²⁺ phosphor including their crystal structure, luminescence properties and energy transfer mechanism from Eu²⁺ to Mn²⁺ were investigated. Upon the excitation of 355 nm, the emission spectra show not only a blue band ascribed to Eu²⁺ but also a yellow band originated from Mn²⁺. The energy transfer from Eu²⁺ to Mn²⁺ was proved by observing the significant overlap of the emission band of Eu²⁺ and the excitation band of Mn²⁺ as well as the series emission band of Ca₇Mg₂P₆O₂₄:Eu²⁺,Mn²⁺ phosphor. We have also found that the energy transfer mechanism was dominated by dipole-quadrupole interaction and a white light has been obtained from Ca_6.965Mg_1.5P₆O₂₄:0.035Eu²⁺,0.5Mn²⁺ phosphor with CIE chromaticity coordinates of (x = 0.32, y = 0.29) and color temperature of 6175 K.(2) Ca₂BO₃Cl:Ce³⁺,Eu²⁺ phosphors were synthesized by a solid-state reaction. The phosphor could display tunable color emission from blue to yellow under an ultraviolet (UV) source by adjusting the ratio of Ce³⁺ and Eu²⁺ appropriately. The mechanism of resonance-type energy transfer from Ce³⁺ to Eu²⁺ was established to be electric dipole–dipole interaction, and the critical distance was estimated to be 31 (?) based on the spectral overlap and concentration quenching model. Utilizing the energy transfer from Ce³⁺ to Eu²⁺, a white light was obtained from Ca1.93BO3Cl:0.06Ce³⁺,0.01Eu²⁺ phosphor with chromaticity coordinates (x = 0.31, y = 0.29) and relative color temperature of 7330 K upon excitation with 360 nm.(3) Y₄MgSi₃O₁₃:RE(RE = Bi³⁺,Eu³⁺; Ce³⁺,Tb³⁺,Eu³⁺) nanophosphors have been prepared by a facile sol–gel method. The products have been characterized by X-ray diffraction, field-emission scanning electron microscopy. Photoluminescence properties of Y₄MgSi₃O₁₃:Bi³⁺, Y₄MgSi₃O₁₃:Eu³⁺ and Y₄MgSi₃O₁₃:Bi³⁺,Eu³⁺ were investigated. It was observed that Y₄MgSi₃O₁₃:Bi³⁺ phosphor gives broad bluish-green light extending from 380 to 650 nm due to Bi³⁺ ions occupy two different Y sites in Y₄MgSi₃O₁₃ host. White-light emission has been obtained from Y₄MgSi₃O₁₃:Bi³⁺,Eu³⁺ nanophosphors upon excitation of 350 nm ultraviolet light. Warm white light has been obtained by adjusting the contents of Bi³⁺ and Eu³⁺ appropriately. Y_3.8MgSi₃O₁₃:0.08Bi³⁺,0.12Eu³⁺ phosphor with CIE coordinates and color temperature of (x = 0.38, y = 0.31, Tc = 3180K) can be considered as potential candidates for warm white light-emitting diodes (LEDs) applications. Y_4-xMgSi₃O₍13_:xRE³⁺ (RE = Ce, Tb and Eu) phosphors give blue, green and red emission light with peak wavelength located at 430, 542 and 614 nm, respectively. The CIE coordinates were calculated as (x = 0.15, y = 0.11), (x = 0.31, y = 0.53) and (x = 0.64, y = 0.36) corresponding to the optimal doping content as x = 1.5 mol%(Ce³⁺), 10 mol%(Tb³⁺) and 20 mol%(Eu³⁺).(4) Green-emitting Ba₂Si₃O₈:Eu²⁺ nanophosphors were prepared and the luminescence properties were investigated. The dependence of emission intensity on the dopant concentration of Eu²⁺ was discussed in detail. Upon 380 nm UV excitation, Ba₍1.93_Eu_0.07Si₃O₈ phosphor shows a bluish-green emission centered at 500 nm with FWHM of 126 nm and color coordination of (x = 0.25, y = 0.40). In addition, Ca₃(VO₄)₂:Eu³⁺,Li⁺ phosphors show excellent red emission centered at 612 nm with CIE coordinates (x = 0.66, y = 0.34), which is very close to the ideal red light. The addition of Li⁺ ions as charge compenstor has enhanced the red emission greatly. Utilizing the energy transfer from Sm³⁺ to Eu³⁺, the PLE spectra of Ca₃(VO₄)₂:Sm³⁺,Eu³⁺,Li⁺ has broaden and emission intensity has enhanced simultaneously.Finally, in conclusion, we provide a discussion and summary of the main points covered. The thesis concludes with recommandtion for improving our understanding on phosohor converted white LEDs.