Synthesis Of M₄Nb₂O₉:Eu³⁺?M=Mg,Ca? Red-emitting Nanocrystal Phosphor And Its Luminescent Property

The novel phosphor for solid-state lighting is one of the most popular research topics in the field of luminescent materials as well as energy-saving technology,which plays as a critical role in white light emitting diode?LED?devices.Currently,the combination of yellow-emitting YAG:Ce³⁺phosphor coating on a blue-emitting InGaN chip is the most successful approach for commercial white LEDs.Although it could generate white light with an extreme high luminous efficacy,a low color-rendering index?CRI?could not be avoided.Therefore,for those white LEDs with high color-rendering index?CRI?and low color temperature,it is necessary to develop novel red phosphors that can be used as a complement for red light generation in the above mentioned approach.Recently,Eu³⁺-activated niobate phosphors have been extensively studied for their efficient photoluminescence under near-UV and blue light excitation,which have the good chemical stability and optical properties.In this study,novel Eu³⁺-activated M₄Nb₂O₉?M=Mg,Ca?nanophosphor were synthesized by a precursor method,and the dependence of the host structure nvoled with different spacies of alkaline earth metal ions on the coordination environment of Eu³⁺ion and their luminescence properties.The energy transfer mechanism between the europium ion and host was also explained.The results obtained in this study could be summarized as follows,?1?Red-emitting M₄Nb₂O₉?M=Mg,Ca?nanophosphor with a size range of50-100 nm were synthesized at 700^oC by a precursor method based on sol-gel technique.Verified by XRD analysis,no impurity phase rather than M₄Nb₂O₉ was found with the additive of Eu³⁺ions.?2?According to the results of XRD analysis,Eu³⁺ions were supposed to be introduced into the lattice of M₄Nb₂O₉,which results the shift of diffraction peak position in the XRD patterns.Through the high resolution transmission electron diffraction fringes from HRTEM analysis,the crystal structures should not change after Eu³⁺doping.That is,Mg₄Nb₂O₉:Eu³⁺_xshow a hexagonal structure as Mg₄Nb₂O₉,while Ca_4-xNb₂O₉:Eu³⁺_xshow a monoclinic strcuture as Ca₄Nb₂O₉.?3?For both of Mg₄Nb₂O₉:Eu³⁺_xand Ca_4-xNb₂O₉:Eu³⁺_xphosphors,their strongest excitation peak was found around 395 nm,and the second strongest peak was found around 465 nm.Under excitation at 395nm,their highest emission peaked at near 613nm and 611 nm,respectively.They should be ascribed to the characteristic line emission from ⁵D₀?⁷F₂ transition of Eu³⁺ions.Mg₄Nb₂O₉:Eu³⁺_0.02 and Ca_3.82Nb₂O₉:Eu³⁺_0.18 were found to show the highest emission intensity in their own systems,respectively.?4?With the increase of Eu³⁺additive,the emission intensity from the host decreased gradually,while the emission intensity from Eu³⁺ions increased at first and then decreased.The energy transfer from host to Eu³⁺ion was proposed to be responsible for that phenomenon,and the mechanism of that energy transfer process should be assigned to dipole-dipole interaction between the niobate host and Eu³⁺ions.?5?The thermal quenching temperature of Mg₄Nb₂O₉:Eu³⁺_xand Ca_4-xNb₂O₉:Eu³⁺_xwas found to be relatively low.When the temperature reached 100^oC,their remaiend emission intensity was lower than a half of that at room temperature.