Syntheses, Luminescent Properties And Mechanisms Of Multi-color Phosphors For Led

According to the principle of complementary color light, the commercial W-LED is achieved by a combination of InGaN blue chip and YAG yellow phosphors. However, this kind of white light shows a low color-rendering index（Ra < 80） for lack of sufficient red emission. In order to overcome this problem, based on the red light compensation strategy or trichromatic（red, green and blue） composite technology, several multi-color（red, green and yellow-emitting） phosphors with strong absorption of blue light are studied, which are meant to improve the color-temperature and Ra values of W-LED. In addition, different preparation methodologies as well as the possible luminescent mechanisms are investigated in detail.1. Effectively enhancing blue excitation of red phosphor Mg₂TiO₄: Mn4+, Bi3+ by high temperature solid-state method and the corresponding luminescent ceramic. It is the first time to report about asynchronous increases of different excitation bands with Bi3+ as a sensitizer, primarily resulting from the perturbation of symmetry and lattice vibration along with crystal defect in the host lattice with larger Bi3+ occupied at a small octahedral site of Mg2+. The sensitization of Bi3+ works not only in the case of powder phosphors but also even better in ceramic phosphors. For powder phosphor, the intensity of the blue excitation band of Mg₂TiO₄: Mn4+, Bi3+, Li+ grows to at least 1.5 times than that of Mg₂TiO₄: Mn4+, following with a uplift at about 410 nm. Meanwhile, the corresponding luminescent ceramic manifests with a more than 2.5 times increase. The red phosphors with a broad emission band in the pure red light region show a promising application in blue LEDs as a red component. In addition, the Mg₂TiO₄-based luminescent ceramic by one-setp high-temperature process can be made into versatile ceramic artwork.2. Alkaline earth sulfide red phosphors by an intermediate phase transition metathesis strategy and the corresponding solar conversion composite. A novel intermediate phase transition metathesis strategy is adopted to prepare（Ca,Sr）S: Eu²⁺ materials on basis of green chemistry. ZnS acts as vulcanizing agent and contributes to the formation of CaS from CaO through thermal decomposition of an intermediate state of Ca ZnOS. No other S-contained toxic vulcanizing agent is adopted. The effect of Sr/Ca ratios on the compositions and luminescent properties of（Ca,Sr）S: Eu²⁺ phosphors is also investigated. A solar energy conversion resin and laminated glass are promoted to overcome the drawback of decomposition of phosphors to moisture and recycling of plastic films, which provides with a promising remote red component for LEDs as well as a permanent functional green-to-red sunlight spectrum conversion device as greenhouse for green agriculture.3. One-step structure-directing approach to Ca S-based green phosphors by solvothermal method. A one-step ligand-surfactant co-assisted solvothermal technique is proposed for the preparation of CaS-based luminescent materials at 200 oC with ethylenediamine as solvent, and acetylacetone, 1-dodecanethiol and poluvinyl pyrrodidone as the capping agents. The morphologies of the obtained particles can be adjusted from an octahedron to a hexagonal prism or a cube by changing the solvent volume ratios of ethylenediamine and different capping agent（s）, and the mean sizes of the crystals can be changed in a relatively wide range from tens of nanometers to two micrometers. The growth process and dynamic mechanism are discussed. A series of CaS: xCe3+ nanocrystals are prepared and a small red shift of emission positions is observed from green to yellowish-green with increasing Ce3+ concentrations.4. Color tunable yellow-green phosphors（Sr,Ca）₃B₂O₆: Eu²⁺ by sol-gel method. Adjusting the ratio of Sr/Ca, a series of color-tunable solid solution phosphors Sr_3-mCamB₂O₆: Eu²⁺（m = 0-3） are obtained with emissions from orange-yellow to green. All the phosphors hold a broad full-width at half maximum（> 70 nm）, which benefits for the improvement of the Ra value of LED. Meanwhile, the linear structural evolution of the iso-structural Sr3-mCamB2O6 solid solutions has been investigated by Rietveld refinement in order to study the mechanism of the abnormal spectral blue shift when larger ions Sr2+ are substituted by smaller ions Ca2+ in Sr3B2O6 lattices.