Preparation And Luminescence Properties Of Tungstate Red Phosphors For White LED

White light emitting diodes?WLEDs?have been considered as the most promising green lighting optical source for the fourth generation due to the merit of a long operation lifetime,energy conservation and non-pollution.Presently,the general approach to fabricate WLED is to combine a near ultra violet or bluc LED chip with a yellow-emitting phosphor or red phosphors which have been commercial.Therefore,it is crucial to synthesize red phosphors which excitation wavelength is matched with the emission of commercial LED chips to improve the efficiency of white light LED devices.In this paper,perovskite structure tungstate and scheelite structure tungstate red phosphors were synthesized by conventional solid-state method.The details are as follows:1.Lummescem properties of NaLaMgWO₆ red phosphor for white LEDs?1?Series of NaLa_1-xMgWO₆:x Eu³⁺?0?x?1?novel red phosphors were successfully synthesized via the conventional solid state method.The crystal structure,morphology,and luminescence properties of NaLa_1-xMgWO₆:x Eu³⁺phosphors were characterized by X-ray powder diffraction,scanning electron microscopy and fluorescence spectra,respectively.The results show that NaLaMgWO₆ phosphors have monoclinic double perovskite structure with space group C2/m.Eu³⁺doped NaLaMgWO₆ phosphors can be effectively excited by near-UV and blue light,and exhibit the strongest red emission peak located at617 nm,which is ascribed to ⁵D₀?⁷F₂ transition of Eu³⁺ion.The optimal doping concentration of Eu³⁺is determined to be x=0.5.It can be confirmed that the dipole-dipole interaction type plays an important role in the energy transfer in NaLaMgWO₆:Eu³⁺phosphors through the concentration quenching curve.In addition,Judd-Ofelt theory was employed to evaluate various radiative parameters such as the refractive index,the J-O parameters??_??and the branching ration???.?2?Series of NaLaMgWO₆:Sm³⁺phosphors were successfully prepared by high temperature solid phase method.The studies showed that the crystal structure of the prepared NaLaMgWO₆:Sm³⁺sample was a single perovskite structure.NaLaMgWO₆:Sm³⁺phosphor can be effectively excited by 406 nm and emits orange-red light of 600 nm.In addition,the concentration quenching mechanism and thermal quenching effect of different concentrations of Sm³⁺doped NaLaMgWO₆:Sm³⁺phosphors were investigated.The results show that the optimal doping concentration of Sm³⁺is 5 mol%,and it has excellent thermal stability.The activation energy is 0.326eV.?3?A series of red-emitting NaLaMgWO₆:Sm³⁺,Eu³⁺phosphors were successfully synthesized by the solid-state reaction method.The energy transfer mechanism between Sm³⁺ions and Eu³⁺ions was mainly studied.The studies showed that the mechanism of energy transfer and energy transfer efficiency were confirmed by the decay time of Sm³⁺ion in NaLaMgWO₆:Sm³⁺,Eu³⁺phosphors.The mechanism of energy transfer between Sm³⁺and Eu³⁺was proved to be dipole-dipole interaction,and the energy transfer efficiency of NaLa_0.65MgWO₆:0.05Sm³⁺,0.3Eu³⁺phosphor was calculated to be 38.89%.The temperature dependent emission spectra showed that the NaLaMgWO₆:Sm³⁺,Eu³⁺phosphors had a good thermal stability with an thermal activation energy?E of 0.241 eV.The CIE coordinate was calculated to be?x=0.661,y=0.339?.2.Lummescem propernes of CaGd₂?WO₄?₄ red phosphor for white LEDs?1?The pure phase CaGd₂?WO₄?₄:Sm³⁺phosphor was successfully prepared by high temperature solid phase method.The results show that the crystal structure of the prepared CaGd₂?WO₄?₄:Sm³⁺sample is a monoclinic scheelite structure.CaGd₂?WO₄?₄:Sm³⁺phosphor can be effectively excited by 405nm,and emits orange-red light of 600nm,which belongs to ⁴G_5/2?⁶H_7/2 electric dipole transition of Sm³⁺ion.At the same time,the concentration quenching mechanism of different concentrations of Sm³⁺doped CaGd₂?WO₄?₄:Sm³⁺phosphors was studied,and the optimal doping concentration was determined to be 3 mol%.?2?A series of Eu³⁺and Sm³⁺co-doped CaGd₂?WO₄?₄:Eu³⁺,Sm³⁺red phosphors were successfully prepared by high temperature solid phase method.The effects of different sensitizer Sm³⁺concentrations on the luminescence properties of CaGd₂?WO₄?₄:0.7 Eu³⁺,y Sm³⁺phosphors were investigated.And the results show that the optimal doping concentrations of Eu³⁺and Sm³⁺ions are x=0.7 and y=0.03,respectively.In addition,the calculation of the critical distance and the measurement of the fluorescence lifetime prove that the Sm³⁺ions can effectively transfer the absorbed energy to the Eu³⁺ions,thereby increasing the luminous intensity of the phosphor.?3?A series of red-emitting CaGd₂?WO₄?_4?1-x??MoO₄?_4x:Eu³⁺phosphors were successfully synthesized through the conventional solid-state reaction method.The influence of molybdenum ions doping on the crystal structure and photoluminescence properties of CaGd₂?WO₄?₄:Eu³⁺were investigated in detail.Results show that Mo⁶⁺ions are introduced into CaGd₂?WO₄?₄ to induce the structure transformation from?3+1?D incommensurately modulation of the monoclinic system with superspace group I2/b to?3+2?D of the tetragonal system with superspace group I4₁/a.Under UV light excitation,the emission spectra of CaGd₂?WO₄?_4?1-x??MoO₄?_4x:Eu³⁺phosphors exhibit the strongest red emission peak at 617nm corresponding to the ⁵D₀?⁷F₂ transition of Eu³⁺ions.The luminescent properties of Eu³⁺are improved significantly when Mo⁶⁺ions are incorporated into the host lattice.In the case of x=0.1,the emission intensity of CaGd₂?WO₄?_3.6?MoO₄?_0.4:Eu³⁺phosphor is enhanced by 50%compared to undoped one,and the colour purity and quantum yield are calculated to be 97.4%and 53.5%,respectively.Meanwhile,the as-prepared phosphors exhibit unusual temperature sensitive emission,and the thermal quenching mechanism is analyzed resultantly by using the configurational coordinate diagram.Furthermore,the Judd-Oflet theory is employed to study the local structure and bonding in the vicinity of Eu³⁺ ions.