| As a new concept of new solid-state light source, white light emitting diodes(LEDs) are opening a new revolution in human lighting field, because of their long life, high energy saving, environmental protection and other significant characteristics. At present, the mainstream white LED in market is phosphor convert LED(abbreviated pc-LED), wherein the luminescent material as an important part, and its performance directly affects the performance of white LED devices, which has received extensive attention. Generally, two ways can be employed in obtaining white light: one is using yellow phosphor YAG:Ce3+ to convert blue light to obtain white light; the other is using three phosphors named as blue, green and red, to convert ultraviolet light into white light. However, there are many disadvantages of the first way. The disadvantages of the combination are low color-rendering index and high color temperature(about 5000K) due to the deficiency of red emission in the visible spectrum and the lack of thermal stability at elevated temperatures during white LED operation when pumped by a blue LED chip. And,there are many disadvantages of the second way. The green phosphors and the red phosphors have lower colour purity and emission intensity. Hence, there is an urgent need to develop and improve(oxy)nitride phosphors to solve the problem. Aiming at the existed problems of phosphors for LEDs(1、the current green phosphors for(near) UV LEDs have lower colour purity 2、the current green phosphors for(near) UV LEDs do not have enough emission intensity 3、the current red phosphors for(near) UV LEDs do not have enough emission intensity), in this thesis, through designing and exploring the synthesis of materials, crystal structure refinement and spectral characterization, we systematically investigated the synthesis and luminescence properties of several new green and red light emission phosphors for(near) UV LEDs. The main research contents are as follows:1. In view of the darwbacks of the current green phosphors for(near) UV LEDs, we successfully developed green emission phosphors Ba2.9-xCaxEu0.1Si6O12N2 and Ba2.9-xMgxEu0.1Si6O12N2 by high temperature solid slate reaction. Through the crystal structure refinement and spectroscopic characterization, we studied the crystal structure, luminescent properties and the effect of crystal field on the luminescence properties. The PLE spectra of the Ba2.9-xCaxEu0.1Si6O12N2 span the UV to blue region, and the PL spectra show a green-yellow emission under 400 nm excitation. With the various crystal environments in the Ba2.9-xCaxEu0.1Si6O12N2 structure, the emission spectra can be tuned from 525 nm to 536 nm. The decrease of thermal stability with the Ca2+ incorporation can be explained from the configurational coordinate diagram. For Ba2.9-xMgxEu0.1Si6O12N2 phosphor, within an appropriate content, doping Mg2+ remarkably enhances the emission and narrows FWHM of the phosphors. The enhancement is due to partial Mg2+ enter into the interstitial sites of crystal lattice, increasing the distance between Eu2+ ions and subsequently minimizing the Eu2+–Eu2+ interaction. The narrowing FWHM is due to that after doping Mg2+ into Ba2.9Eu0.1Si6O12N2, the Eu2+ ions prefer the loose Ba2 sites with the increasing Mg2+ concentration, leading to the decrease of FWHM for Ba2.9-xMgxEu0.1Si6O12N2. This novel phosphor also has high thermal stability.2. In view of the darwbacks of the current red phosphors for(near) UV LEDs, we successfully developed two efficient red emission phosphors Ca2Si5N8:Eu2+ and Sr2Si5N8:Eu2+ by Gas-Pressed Sintering method. The color of the emission can be tuned with adding BaF2 into the Ca2Si5N8:Eu2+ host lattice. Adding BaF2 can make better single phase, emission peak shift to longer wavelength(608 nm to 622 nm) and significantly enhance the emission of Ca2Si5N8:Eu2+ phosphors. This is mainly due to the fact that the lattice structure of Ca2Si5N8:Eu2+ phosphors can be modified and the solubility of the Eu2+ ions can be increased by adding BaF2. More Eu2+ ions would incorporate the lattice because of the larger lattice expended by Ba2+. For Sr2Si5N8:Eu2+ phosphor, the luminescence of Sr1.95-xEu0.05Si5N8 phosphor with La3+-Al3+ replacing Sr2+-Si4+ is discussed and analyzed. The incorporation of La3+-Al3+ ion pairs into the host lattice leads to an additional emission band with maximum at about 760 nm. And With increasing the content of La3+-Al3+, the chromaticity coordinates(x, y) vary systematically from(0.6107, 0.3716) to(0.6452, 0.3441), corresponding to color points of the samples change gradually from orange-red to red. A series of Sr1.95-xMgxEu0.05Si5N8(0≤x≤0.2) samples have also been successfully synthesized. By doping Mg2+, the photoluminescence properties of Sr1.95Eu0.05Si5N8 have been improved. Within an appropriate content, doping Mg2+ enhances the emission of the phosphors. This novel phosphor also has high thermal stability.3. In view of the darwbacks of the current red phosphors for(near) UV LEDs, we successfully developed an efficient red emission phosphor CaSiN2:Eu2+ by Gas-Pressed Sintering method. Adding Sr2+ can enhance the emission of CaSiN2:Eu2+ phosphors. But this novel phosphor has poor thermal stability. |
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