Preparation And Luminescent Properties Of Rare Earth Doped CaWO₄ Phosphors For LEDS

The fabrication and properties of phosphor used for white LED were paid more attention with the extension of applied field and the development of preparation technology in white LED.The phosphor-conversion white LED becomes the mainstream.Especially,the properties of phosphor excited by blue and near-ultraviolet light can effect directly the color rendering of white LED.The main purpose of this work is to obtained the phosphors for the near-ultraviolet based white LEDs.CaWO4:Eu3+,CaWO4:Sm3+ and CaWO4:Eu3+,Sm3+ were synthesized and their crystal structures,luminescent properties and energy-transfer process have been studied.The main contents and results are listed below.1.The CaWO4:Eu3+ red phosphors have been prepared by co_precipitation method.Here,Ca(NO3)2、NaWO4、2H2O、Eu2O3、HNO3、[C4H10O3]n were used as source materials.The crystal structure,morphology and luminescent properties of the as-synthesized CaWO4:Eu3+ were investigated.The x-ray powder diffraction showed that the tungstate was body-centred tetragonal structure.The as-synthesized CaWO4:Eu3+ phosphors can exhibit sharply red characteristic elissions of Eu3+ at room temperature.The strong emission at 613 nm was observed,originating from the SD0→7F2 transition of Eu3+ ions.While monitoring 613 nm emission from Eu3+,the energy transfer was observed from the host to the Euhe.The excitation spectrum of the CaWO4:Eu3+ red phosphors consisted of a broad band peak and a series of sharp peaks,which located at 395 nm(the broad band center),and 465 nm,respectively.This result shows that the as-synthesized CaWO4:Eu3+ red phosphors can be effectively excited by the ultraviolet light emitting diode and blue light emitting diode.The absorption intensities of near-ultravio let(395nxn)and blue light(465nm)were controlled by adjusting Eu3+ doping concentration,and then the crystal constapnts,the crystal symmetries and the luminescent properties can be adjusted,respectively.This red emission had the characteristics of the high intensity and brightness,pure chroma.2.The CaWO4:Sm3+ red phosphors have been prepared by co_precipitation method.Here,Ca(NO3)2、NaWO4·2H2O、Sm2O3、HNO3、[C4H10O3]n were used as source materials.The x-ray powder diffraction showed that the tungstate was body-centred tetragonal structure.The as-synthesized CaWO4:Sm3+ phosphors can exhibit sharply red characteristic emissions of Sm3+ at room temperature.The emission spectrum excited by 642 nm showed a strong emission peak at 594 nm,originating from the 4G5/2→6H7/2 transition of Sm3+ ions.While monitoring 642 nm emission from Sm3+,the energy transfer was observed from the host to the Sm3+.Thus,the as-synthesized CaWO4:Sm3+ red phosphors can be effectively excited by the ultraviolet(405 nm)light emitting diode and blue(481 nm)light emitting diode.The doped Sm3+ concentration has a great influence on the crystal orientation,crystal constants and the luminescent properties.The absorption intensities of near-ultraviolet and blue light were controlled by adjusting Sm3+ doping concentration,and then the red emission intensity under 405 nm and the 481 nm excitation can be adjusted,respectively.3.A series sample of CaWO4:Eu3+-Sm3+ phosphors have been prepared by co-precipitation method.Here,Ca(NO3)2、NaWO4·2H2O、Eu2O3、Sm2O3、HNO3、[C4H10O3]n were used as source materials.The red light emission of CaWO4:Eu3+-Sm3+ was observed,which was excited by 346,362,377,395 and 405nm.The excitation spectrum consisted of two emission bands,which were attributed to Eu3+and Sm3+,respectively.The energy transfer between Eu3+ and Sm3+ was observed.The light color can be tunable by varying the rare earth ions concentrations.The excitation wavelengths can affect the intensity ratio,and the doping concentration has a great influence on the crystal constants and the luminescent properties.Thus,Luminous intensity of the CaWO44:Eu3+-Sm3+ can be controlled by the excitation wavelengths and the rare earth ions concentrations of Eu3+ and Sm3+.