Synthesis And Luminescence Of ABPO₄Phosphate Phosphors For Near Ultraviolet Light Emitting Diodes

White light-emitting diodes (WLEDs) have attracted substantial attention owing totheir extraordinary luminous efficiency, low power consumption, reliability, andenvironmental friendliness. Thus it is generally accepted that they will replaceconventional lamps for general lighting in the near future. Near ultraviolet (NUV) LEDand multiple phosphors can cover a broad range of emission wavelengths, giving goodcolor rendering and stable emission color. This strategy of a NUV LED combined withRGB phosphors becomes advantageous if the efficiency of NUV LEDs exceeds that ofblue LEDs. However, there still exist some problems that cannot be overcome so far forWLEDs fabricated with ultraviolet-LED chips and tri-color phosphors. Generally, theluminescent efficiency is relatively low in this system owing to the strong reabsorptionof the blue light by the red and green phosphors. Phosphate based phosphor are ofinterest due to their unusual stability and useful luminescent properties. They are usedfor different applications such as phosphors for lamps, color TV screens, long-lastingdevices, laser hosts, scintillators, and pigments.In this thesis, phosphate phosphor having the general formula ABPO4, where A is amonovalent cation (Li, Na, K, Rb, Cs) and B is a divalent cation (Mg, Ca, Sr, Ba, Zn),was synthesized by a solid state reaction or sol gel process, and factors on morphologyand luminescence were also investigated. Finaly, single host white light phosphors usedfor NUV LEDs were obtained by co-doping Eu2+, Tb3+and Mn2+activator. Energytransfer processes is discussed by analyzing the photoluminescence (PL) andphotoluminescence excitation (PLE) spectra. The main points of the thesis are asfollowing:1) The luminescence mechanism, realization methods and phosphor of white LEDare reviewed. And then phosphate phosphor having the general formula ABPO4weresummaried and analysised. Finally, characteristics of various phosphors synthetictechnologies for white LED are discussed.2) According to Dexter’s energy transfer theory of multipolar interaction, theinteraction type between sensitizers or sensitizer and activator can be determined by theslope of the relation curve ln I1C~ln Cof the emission intensity and thecomposition in the double logarithm coordinates when the concentration is high enough. Among the concentration quenching caused by the electric multipole interaction, theslopes of dipole–dipole (d–d), dipole–quadripole (d–q) and quadripole–quadripole (q–q)are located in [2,1],[8/3,5/3] and [10/3,7/3], respectively. The results areagreement with Huang’s conclusion.3) The optimal reaction conditions for LiCaPO4:Eu2+phosphor prepared by sol-gelroutine are respectively,3mol%for Eu2+contents,110%for Li element dosage and1:1:2for molar ratio of metal: citric acid: polyethylene glycol. Then the obtainedprecursor is calcined at1100℃for2h. For higher doping above0.03, the dipole–dipoleinteraction is the major mechanism for concentration quenching of luminescence.4) The decay time of LiCaPO4:Eu2+phosphor prepared by sol-gel routine is990.93ns. The particle size has a narrow size distribution and its Dav, D50is2.531μm,2.235μm, respectively. The activation energy for thermal quenching was found to be0.356eV, which indicates LiCaPO4:Eu2+phosphors have an excellent thermal stabilityon the temperature quenching effect. The CIE chromaticity coordinates ofLiCaPO4:Eu2+is (0.1238,0.1473).5) The emission peak of Eu2+in ABPO4system can be varied from around410nmto505nm. This provides a very wide, and useful, range of blue phosphors for use innear-UV pumped LED. Compared to ACaPO4:Eu2+, emitting peaks in ASrPO4:Eu2+shifts towards blue. The emission wavelength and decay time of Eu2+in LiBaPO4were467nm and990.93ns.6) The Stokes shift of emission of the5d levels of Eu2+and Ce3+appear to belinearly related in ABPO4host, ΔS(Eu)=(2.0536±0.0569)ΔS(Ce)-(3878.81±231.89)cm-1.The relationships found help in predicting spectroscopic properties for Eu2+fromproperties known for Ce3+and vice versa.7) In LiCaPO4host, the Eu2+photoluminescence band overlaps well withExcitation spectra of Tb3+, indicating the possibility of energy transfer of Eu2+â†'Tb3+. Awhite light could be generated by co-doping Eu2+, Mn2+and Tb3+ions. Theoptimal-composition for white light is LiCaPO4:0.02Eu,0.09Tb,0.07Mn, which givesthe CIE coordinates of (0.3256,0.3008) and color temperature of5907K. The energytransfer from Eu2+to Mn2+and Eu2+to Tb3+in the LiCaPO4host have been observed.By the energy transfers of Eu2+to Tb3+and Eu2+to Mn2+, the emitting intensity ofphosphor samples at541nm and650nm could be increased, respectively.8) Under the excitation of UV light, Na(Ba0.45,Sr0.55)PO4:Eu2+,Na(Ba0.45,Sr0.55)PO4:Tb3+, and Na(Ba0.45,Sr0.55)PO4:Mn2+exhibit the emissions of Eu2+ (439nm, blue), Tb3+(541nm, green), and Mn2+(571nm, red), respectively. Thephotoluminescence spectrum can be decomposed into two well-separated Gaussiancomponents with peak centers at531nm and561nm, which was because Eu2+ionsinhabited two different Sr2+sites: Eu(I) coordinate with six nearest oxygen atoms andEu(II) with four atoms in the host lattice, respectively. By adjusting the dopingconcentration of Eu2+and Mn2+in Na(Ba0.45,Sr0.55)PO4host, a white emission in asingle composition was obtained under the excitation of360nm. Theoptimal-composition Na(Ba0.45,Sr0.55)PO4:0.02Eu,0.07Mn phosphor has a CIEcoordinates (0.3013,0.3124).The better performance could be obtained by codopingwith Tb3+, and optimal-composition Na(Ba0.45,Sr0.55)PO4:0.02Eu,0.07Tb,0.07Mnphosphor has a CIE coordinates (0.3153,0.3382).9) In the Na2.5Y0.5Mg7(PO4)6host, the emission bands centered at423,543, and610nm ascribed to the contribution from Eu2+, Tb3+and Mn2+, respectively. Theoptimum Eu2+activator for Na2.5Y0.5Mg7(PO4)6is0.02, and the dipole–dipoleinteraction is the major mechanism for concentration quenching of luminescence.Theenergy transfer process between Eu2+and Mn2+had been demonstrated to be anexchange mechanism. The optimal-composition Na2.5Y0.5Mg7(PO4)6:0.02Eu,0.07Tb,0.03Mn phosphor emits white light at a360nm NUVexcitation. This whiteemission has a color temperature (6735K), and its CIE coordinates (0.31,0.33) arecloser to ideal white light (0.33,0.33). These results indicate that the phosphor could beused as a single-composition white-emitting phosphor for NUV LED applications.