Preparation And Research On Luminescent Properties Of Molybdate And Vanadate Phosphors

Light emitting diode (LED) is a solid state semiconductor devices converting into electricity to light, showing merits of little volume, low power consumption, long lifetime, high color rendering index, lower thermal resistance and environment-friendly and so on. Currently, phosphor conversion LEDs (pc-LEDs) are attracting significant attention, which have two alternative ways to archive white light. The first way using a blue InGaN chip (450-470 nm) in combination with a yellow phosphor is commercially available. However, such a combination exhibits a poor color rendering index (<80) because of the lack of a red light component (above 600 nm). On the other hand, the combination of a near-UV InGaN chip with blue, green and red phosphors is another way to generate white light. But the red phosphor that can be efficiently excited is still lacking at present. Therefore, in order to overcome the above-mentioned drawbacks, it is necessary to search for novel red-emitting phosphors which can be effectively excited by UV or blue LED chips.Tungstates and molybdates are two important optical materials that have been intensely investigated as host materials because of their excellent thermal and chemical stability and intense charge transfer (CT) absorption bands. Undoped and rare-earth ion-doped vanadates are known as luminescent materials with rich colors, high luminescence efficiency and excellent chemical stability. The CT energy generally can be efficiently transferred between VO43- and rare earth ions which will enhance the emission intensity of rare earth ions. In this work, a series of new phosphors Sr3MoO6:Eu3+, Ca9Y(VO4)7:Dy3+and K3Y(VO4)2:Eu3+ were synthesized by convenient solid-state method. The luminescence properties of the obtained phosphors are characterized by the XRD, SEM, luminescent spectra and the results are as follows:1) Double perovskite Sr3MoO6:Eu3+ phosphors were first synthesized by solid-state reaction method. Excitation and emission spectra demonstrate that the phosphor shows a broad charge-transfer band at around 365 nm and an intense red emission (616 nm,5D0â†'7F2 transition) under 466 nm excitation. Through regulating doped concentration of Eu3+ and the substitution of Mo6+with W6+ results in an enhancement of luminescence intensity and improved red color purity with the CIE chromaticity coordinates (0.619,0.376). The optimal concentration is 30 mol% and the concentration quenching mechanism are determined to be the dipole-quadrupole interaction.2) Ca9Y1-x(VO4)7:xDy3+ phosphors were successfully synthesized by the solid-state reaction. Ca9Y(VO4)7 has the same structure with Ca3(VO4)2 belonging to the rhombohedral system. In this structure, Dy3+ is located at a low symmetry site (without inversion symmetry), which indicates that the emission spectra of the Dy3+ doped compound Ca9Y(VO4)7 will be dominated by yellow emission from 4F9/2â†'6H13/2 transition. Upon the excitation of charge transfer band in near UV, the phosphors present warm white light emissions and with increasing the Dy3+ concentration, the emission color of the phosphors changes from greenish blue to yellowish green. The concentration quenching occurs when the concentration of Dy3+ is beyond 30 mol% due to the cross relaxation processes and the nonradiative energy transfer between Dy3+ ions resulting from the exchange interaction.3) A series of K3Y(VO4)2:Eu3+ phosphors were first synthesized by a high temperature solid state reaction method. The samples with lower Eu3+ doping concentration (x=0,0.03 and 0.1), the experimental XRD patterns of K3Y(VO4)2:Eu3+ phosphors are well in agreement with the JCPDS card No.49-1227 of K3Y(VO4)2. When the samples with heavy Eu doping concentration (x=0.4 and 0.6), the samples well match with the PDF2 standard cards JCPDS No.50-0218 of K3Eu(VO4)2. We can find that Eu3+ ion occupies the centrosymmetric site for the samples with lower Eu contents (x<0.2). While for the samples with higher Eu contents (x≥0.2) Eu3+ ion occupies the site with non-inversion symmetry. The phosphors showed red light emission with high color purity and strong luminous intensity under the charge transfer band excitation. The CIE chromaticity coordinates of K3Y0.5(V04)2:0.5Eu3+ were x=0.63 and y=0.37, which were close to the standard chromaticity of x=0.67 and y=0.33 for the NTSC system. By proper tuning of the Eu3+ content and excitation wavelength, CIE chromaticity coordinates change regularly, and lighting color moves from green, warm white to red. K3Y(VO4)2:Eu3+ can potentially serve as a red-light emitting phosphor and a single-phase white-light emitting phosphor.