Preparation And Propetries Of The Phosphors For LED

The white light emitting diodes (LEDs) have attracted increasing attention inrecent years due to their excellent properties such as low power consumption, longoperating time, and environmental benefit. There are several ways to gain white light.Among these, the most common and simple method to realize white-light LEDs is tocombine an InGaN-based blue LED with a yellow phosphor material, such asYAG:Ce. However, the efficiency index of white LED is influenced by phosphors andblue LED. In higher electrical flow, the blue light intensity of photoelectric strengthincreases quickly than yellow. Therefore, the emission from an InGaN-based blueLED coated with YAG:Ce is deficient in the red spectral region and its colorrendering property is poor.White LEDs can also be made by coating a near ultraviolet (UV) or nearultraviolet emitting LED with a mixture of high efficient red, green and blue emittingphosphors. This method yields light with better spectral characteristics, which rendercolor better. Therefore, it is necessary to provide phosphor compositions that areexcitable in the UV range and emit in the visible range.In the present work, some new phosphors excited by UV have been synthesized.The synthesis method, structure and spectral characteristics have been systematicresearches.Sr₃Al₂O₆:Ce³⁺, MAl₂O₄:Eu³⁺,Li+(M=Ca, Ba), Sr₂Al₂SiO₇:Ce³⁺,Eu²⁺andNaLaMoO₄:Ce³⁺,Tb³⁺phosphors were prepared by the solid-state reaction technique.X-ray diffraction diagrams confirmed that all samples exhibit a well-defined phases.The effects of concentration of Ce³⁺and H₃BO₃on structure and photoluminescenceof Sr₃Al₂O₆:Ce³⁺phosphor have been presented and analyzed. The emission spectrumSr₃Al₂O₆:Ce³⁺phosphor showed blue band and peaking at460nm excited at395nm.The MAl₂O₄:Eu³⁺,Li+(M=Ca, Ba) showed red light excited at254nm. In addition, theluminescence intensities are remarkably enhanced when the alkali metal ions areco-doped in Sr₃Al₂O₆:Ce³⁺samples. Under the excitation of345nm, the emissionspectra of Sr₂Al₂SiO₇:Ce³⁺,Eu²⁺appear not only blue emission of Ce³⁺ions but also green emission of Eu²⁺ions. With increasing Eu²⁺content, the color tone of thesamples changes gradually from blue to green. Similarly, under the excitation of339nm, the emission spectra of NaLa(MoO₄)₂:Ce³⁺,Tb³⁺appear not only blue emission ofCe³⁺ions but also green emission of Eu²⁺ions. The color tone of the samples changesgradually from indigo to green by simply adjusting the relative doping concentrationsof the Ce³⁺and Tb³⁺ions. The results affirm that there is energy transfer from theCe³⁺to Eu²⁺ions in Sr₂Al₂SiO₇:Ce³⁺,Eu²⁺and Ce³⁺to Tb³⁺ions inNaLa(MoO₄)₂:Ce³⁺,Tb³⁺. Through calculation, The energy transfer from Ce³⁺to Eu²⁺ions occurred via a dipole-dipole mechanism. The critical distance has been estimatedto be about1.83nm and the energy transfer from the Ce³⁺to the Tb³⁺ions is thedipole-quadrupole mechanism.Eu³⁺and Tb³⁺co-doped NaLa(MoO₄)₂phosphors with various shapes,LaPO₄:Eu³⁺,Sm³⁺nanorods and uniform YF3nanocrystals have been successfullyprepared by hydrothermal method. The morphology of the materials was found to bemanipulated by changing the pH value of the precursor solution. UniformNaLa(MoO₄)₂3D microspheres could be trapped at pH5and the spindle-likeNaLa(MoO₄)₂microstructures were obtained at pH7. When the pH value wasadjusted to9, the flower-like NaLa(MoO₄)₂microstructures were obtained.NaLa(MoO₄)₂:Eu³⁺samples show red light and NaLa(MoO₄)₂:Tb³⁺samples showgreen light. The results show that the emission intensity of the product obtained at pH5is the strongest. Besides, the luminescence colors of NaLa(MoO₄)₂samplesco-doped with Eu³⁺and Tb³⁺under ultraviolet excitation can be tuned from red,through yellow and green-yellow, to green by simply adjusting the relative dopingconcentrations of the activator ions. LaPO₄: Eu³⁺,Sm³⁺nanorods were prepared by adesigned two-step hydrothermal method. Firstly, the lanthanum precursor has beenprepared and then LaPO₄:Eu³⁺,Sm³⁺nanorods were prepared. Their structure,morphology, formation process and photoluminescence properties have beeninvestigated. Research shows that the precursor is comprised of microprisms of whichdiameters is about500nm and length is about600nm and small nanorods of whichdiameters and lengths are about50nm and200nm respectively which consists of the hexagonal phase of La(OH)3and the hexagonal phase of LaCO₃OH. The LaPO4:Eu³⁺,Sm³⁺is composed of nanorods which are around10nm in diameter and lengthranging from600to800nm. The doped Sm³⁺is found to be efficient to sensitize theemission of Eu³⁺and be effective to extend the absorption of the near-UV light withwavelength of400-405nm. OmimPF6was introduced to prepare uniform YF3:Ln³⁺(Ln=Eu, Tb, Ce, Dy) nanocrystals in large scale for the first time. In the systems, theILs acted as templates and a fluorine source. Uniform nanocrystal resulting fromself-assembly of little nanoparticles were successfully synthesized in these systemswith a large scale. The photoluminescence (PL) properties of YF₃:Ln³⁺(Ln=Eu, Tb,Ce, Dy) nanocrystals were also studied in detail.The highly uniform gamma-alumina hollow spheres can be synthesizedsuccessfully on a facile and green method using colloidal carbon spheres as templatesand OSA as alumina source. The optimal synthesis conditions are confirmed,calcination temperature700oC, leaching temperature100oC, leaching time5h,sulfuric acid concentration30wt%, solid-to-liquid ratio1:3. under the excitation of246nm, the emission spectra of Al2O₃:Eu³⁺hollow spheres appears a strong emissionpeak at612nm, which is attributed to the transition from⁵D₀to⁷F₂.