Synthesis And Luminescent Properties Of Rare-earth Doped Molybdate And Tungstate Phosphors

White light-emitting diodes (LED) as a more preferable replacement forconventional incandescent and fluorescent lamps light up the21st century. At present,the most common method to achieve white light is based on the combination of a blueLED chip with a yellow-emitting YAG:Ce3+phosphor. However, several drawbacksappear in practical application, including low luminous efficiency and poorcolor-rendering index due to the deficiency of red emission. Another promisingmethod to obtain white light is using the tricolor phosphors to convert the radiation ofultraviolet/near ultraviolet LED chips, which is considered to be the leading of thenew-generation light source. Therefore, it is necessary to research and design somenew efficient phosphors for white LED. In the present work, series of molybdate andtungstate phosphors were prepared via solid-state reaction or hydrothermal method.The structure, morphology and luminescence property of the samples also have beensystematically studied.Eu3+-activated MRE(MoO4)2(M=Li, Na, K; RE=Gd, Y, Lu), NaRE(WO4)2(RE=Gd, Y, Lu) and MLa(WO4)2(M=Li, Na, K) efficient red phosphors have beensuccessfully prepared via the solid-state reaction method. The emission intensity ofMRE(MoO4)2:Eu3+was found to decrease with increasing the size of alkali cations ordecreasing size of rare earth ions. In MLa(WO4)2:Eu3+system, the emission intensitywas found to decrease with increasing the size of alkali ions, and inNaRE(WO4)2:Eu3+system, the emission intensity can be ordered as follows:Lu>Y>Gd. Under near UV light excitation, all compounds exhibited strong redemission at about613nm due to5D0â†'7F2transition of Eu3+. Compared with thecommercially available red phosphor Y2O3:Eu3+, the emission intensity of thesephosphors is much stronger than that of Y2O3:Eu3+. Moreover, the CIE chromaticitycoordinates are closer to the National Television Standard Committee (NTSC)standard CIE chromaticity coordinate values for red.LiLa(MoO4)X(WO4)2-X:Tm3+,Dy3+phosphors have been successfully prepared via the conventional solid-state reaction method. The emission colors of LiLa(WO4)2:0.005Tm3+,Dy3+could be tuned from blue to white through tuning the energy transfer.And their energy transfer efficiency increases gradually with increasing the Dy3+doped concentration. The energy transfer from Tm3+to Dy3+was revealed to beresonant type via the quadrupole-quadrupole mechanism. As for the complexmolybdate-tungstates LiLa(MoO4)X(WO4)2-X:0.005Tm3+,0.03Dy3+phosphors, boththe emission intensities of Tm3+and Dy3+were found to reach a maximum when themolar ratio of Mo/W is0:2. In addition, with increasing the content of MoO42-, thelocation of the chromaticity coordinates for the LiLa(MoO4)X(WO4)2-X:0.005Tm3+,0.03Dy3+phosphors changed from the edge to the center of the white area.The chromaticity coordinate when the relative ratio of Mo/W is2:0is much closer tothe standard chromaticity coordinate for white.Eu3+/Sm3+co-doped SrMoO4phosphors have been prepared via a facilehydrothermal method. The phosphor particles have the dumbbell-like shape, and theformation of the dumbbell can be explained through fast nucleation, then dissolutionand re-crystallization, and finally further growth, based on the time-dependentexperiments. The introduction of Sm3+can generate a strong excitation line at403nm,significantly broadened the excitation region for matching the near-ultravioletlight-emitting diodes. Moreover, Sm3+ions can transfer the absorbed energy to Eu3+ions efficiently, so the intensity of the main emission peak at614nm due to5D0â†'7F2transition of Eu3+is strengthened by co-doped of Sm3+. SrMoO4:0.03Eu3+,0.02Sm3+owns the strongest red emission, and considered to be a promising red-emittingphosphor.Dy3+doped and Eu3+/Tb3+co-doped MMoO4(M=Ca, Sr, Ba) phosphors havebeen prepared via a facile hydrothermal method. The as-prepared CaMoO4, SrMoO4and BaMoO4samples present cylinder-like, dumbbell-like, and micro-rods shape,respectively. The optimum doped concentration of Dy3+is4mol%for MMoO4. Mostof the chromaticity coordinates of MMoO4:Dy3+are located in the white-light region.All the characteristic emissions of Eu3+and Tb3+can be observed in Eu3+/Tb3+co-doped MMoO4phosphors, and the emission color can be turned from red, throughyellow or green-yellow, to green by simply adjusting the relative dopingconcentrations of the Eu3+and Tb3+ions.NaLa(MoO4)2:Dy3+phosphors have been prepared via a hydrothermal method.The phosphor particles have the pompon-like shape with an average diameter of about 1.0μm. The formation of such pompon-like microstructure may be achieved vianucleation, crystal growth, and self-assembly growth process. Upon390and456nmexcitations, the yellow emission at574nm (4F9/2â†'6H13/2) and blue emission at486nm (4F9/2â†'6H15/2) have been observed. The emission intensity achieved highest whenthe Dy3+concentration is2mol%. In addition, the yellow-to-blue emission intensityratio (Y/B) can be changed with the doped concentration of Dy3+ions. Allchromaticity coordinates of the obtained NaLa(MoO4)2:Dy3+phosphors are located inthe white-light region.CaXSrYBa1-X-YWO4:RE3+(RE=Pr, Ho and Er) phosphors have been successfullyprepared via a facile hydrothermal method. The morphologies of the products varygradually and regularly with the change of the host composition, in which theanisotropic growth has played a key role. The down-conversion emissions of Pr3+,Ho3+and Er3+in CaXSrYBa1-X-YWO4host have been successfully realized. Theoptimum doped concentrations of Pr3+, Ho3+and Er3+in SrWO4host are1,1and0.5mol%, respectively. Moreover, the dominant concentration quenching mechanisms forPr3+, Ho3+and Er3+in SrWO4host are electric quadrupole-quadrupole, dipole-dipoleand dipole-quadrupole interaction, respectively. The emission intensities ofCaXSrYBa1-X-YWO4:RE3+phosphors reveal a non-monotonic decreasing (or increasing)variation trend due to the multiple factors. Additionally, among these phosphors,Ca0.4Sr0.6WO4:0.01Pr3+, Ca0.8Sr0.2WO4:0.01Ho3+and Ca0.6Sr0.4WO4:0.005Er3+havethe optimal luminescent property, and these phosphors will emit orange, yellowishgreen and green light, respectively.