| White light-emitting diodes (LED), known as a pivotal component of solid-state lighting (SSL), have considered being the new-generation lighting source because of high luminous efficiency, long lifetime, and low pollution. Typically, the common approach to making white LED is combining blue LED chip and yellow-emitting phosphor (Y3Al5O12:Ce3+), which leading to the low colour rendering index (CRI) and high correlated colour temperature (CCT) due to lack of red and green light in the light spectrum. The red and green phosphors should be added to fulfill the novel white LED symbolized by high CRI and low CCT. The fact that presently used red and green phosphors is insufficient for novel white LED, which boosted investigations of new multinary phosphors. Nitride/oxynitride phosphors have demonstrated outstanding photoluminescent properties and high stability, which resulting in the extensive applications in white LED.(Ca,Sr)AlSiN3:Eu2+and Sr2Si5N8:Eu2+for red phosphor have been commercialized; green phosphors confront some problems:layered MSi2O2N2:Eu2+has strong thermal quenching, and the synthesis of β-sialon:Eu2+is not facile. It is thus necessary to develop highly efficient green oxynitride/nitride phosphors that are suitable for white LED. The oxynitride Ba3Si6O12N2:Eu2+phosphor with high stability and excellent luminescent property has been developed in2008. However, the research of this oxynitride phosphor is on its primary state, and the preparation techniques, luminescent properties and spectral tuning properties need to be further researched.In this paper, Eu2+-activated (Ba,Sr)3Si6O12N2phosphors were synthesized via the solid-state reaction. The spectral tuning properties have been investigated on the basis of optimized preparation techniques for the destination of improving luminescent property. The activator-doping, alkaline metal cation substitution, anion substitution of N/O and co-activator-doping in oxynitride host are discussed according to fundamental theories of spectrum tuning such as crystal field effect, centroid shift and Stokes shift.It is believed that the preparation technique plays an important role on the crystal structure and luminescence properties. The phosphor with pure Ba3Si6O12N2phase and higher luminous intensity could be obtained while choosing the BaCO3as Ba source, and calcining raw material mixture at1300℃for8h under N2/H2atmosphere. The twice firing can enhance the emission intensity of phosphors, and the introduction of fluxes will promote the luminescence property, especially BaF2and BaCl2flux. It is discovered that the luminescent intensity will reach the peak while adding2wt%BaF2flux.It is found that excesses in Si3N4raw material will be conducive to the formation of pure Ba3Si6O12N2phase. Phase analysis results for those phosphors which synthesized through variational Si3N4content in raw mixture and different calcination temperature, disclose the excess Si3N4dependence of pure phase and the reaction mechanism. The results show the SiO2and BaCO3raw material would generate the metasilicate phase at relatively low temperature, and then this synthetic metasilicate phase would react with Si3N4to form the pure Ba3Si6O12N2phase. However, this intermediate metasilicate phase is proved to be Ba5Si8O21rather than the preconceived Ba2Si3O8phase, which leads to the changes in the following steps and more Si3N4consumptions.The crystal structure and luminescent properties of this Eu2+-doped Ba3Si6O12N2phosphor have been systematic investigated. It is revealed a broad excitation spectrum extending from250to500nm along with a green-emitting band of470-560nm with peak wavelength of525nm and full-width at half-maximum (FWHM) of70nm from photoluminesccnce(PL) and photoluminescence excitation (PLE) spectra. The activator Hit2+ion would locate at the site of Ba2in the lattice. Ba3Si6O12N2:Eu2+phosphor shows excellent thermal stability, and still64%of the emission intensity remains when heated up to200°C, which is clearly better than commercial silicate green phosphors. It is found that the emission band shifts toward high energy due to reduction of crystal field strength caused by lattice expansion. It is revealed that the activator is a key component in luminescent material. The crystal field strength will be boost as the increasing of Eu2+concentration, and then lower the4f65d1-4f7transfer energy, which leads to the red shift of emission wavelength from520nm to530nm. Meanwhile, the relationship between emission intensity and Eu2+concentration reveals a parabola curve, and the quenching concentration is x=0.3in Ba3-xSi6O12N2:xEu2+phosphor. The electric dipole-dipole interaction is assumed to be the mechanism of concentration related quenching.The alkaline earth cation substitution will improve photoluminescent properties of phosphors by adjusting crystal structure and changing crystal field strength. High efficiency green-emitting solid solution (Ba3-xSrx)Si6O12N2:Eu2+phosphors were produced, and the Limited solid solubility of Sr in Ba3Si6O12N2has been discovered. All emission spectra show a tendency of redshift while increasing the Sr/Ba ratio because of large crystal field splitting and Stokes shift. The x value dependence of emission intensity is discovered, which is explained by the enhanced probability of excited electron via nonradiative transition. In addition, white LED with chromaticity coordinates of (0.2262,0.4181) to (0.2650,0.3092) and CRI of49.7to59.5due to the broaden FWHM, are fabricated by packing blue chips and as-synthesized phosphors.The adjustments of N and O contents in oxynitride luminescent material have significant impacts on photoluminescent property. The XRD patterns and N/O automatic analyzer results show that the N/O substitution becomes available in Ba3Si6O12N2lattice for this Eu2+doped Ba3Si6O12-δN2+2δ/3phosphors in the range of δ=-0.6-1.8. The lattice expansion has been proved due to the N/O substitution according to the XRD patterns. The quneching concentration displays a parabola with a valley value of0.3as the δ value increasing from-0.6to1.8due to the combined impacts of N induced lattice expansion and charge enhancement. All the quenching mechanisms can ascribe to the electric dipole-dipole interaction on the basis of PL and PLE spectra. The8value dependence of luminescent intensity reveals a parabola curve with the peak of δ=0, and the blue shift of emission wavelength has been testified because the lattice expansion by substituting more O with N. The chromaticity coordinates shift from green (0.2756,0.6167) to yellow-green (0.3507,0.6067), and the colour temperature decreased from6241K to6098K in Ba3Si6O12-δN2+2δ/3:Eu2+phosphors.The activator co-doping in phosphors is affirmed to be a common approach to improve luminescent properties due to the energy transfer process of different activators. The Ba3+x-ySi6O12N2:xEu2+,yCe3+phosphors have been prepared, and the energy transfer from Ce3+ions to Eu2+ions have been demonstrated. The emission intensity of Eu2+will be enhanced due to the energy transfer from Ce3+to Eu2+at lower Ce3+ion doped phosphors, and then the emission intensity begin to diminish while doping more Ce3+ions for the domination of energy transfer between Ce3+ions. The critical concentration of Ce3+is confirmed to be y=0.6in Ba2.7-ySi6O12N2:0.3Eu2+,yCe3+phosphors. The Ce3+ion dependence of emission spectrum redshift is discovered, which is explained by the crystal field enhancement caused by the lattice shrink due to the Ce3’ion substitution. The Ba3Si6O12N2:Eu2+,Mn2+phosphors have been synthesized, and the energy transfer from Eu2+to Mn2+is found because of the decreased luminescent intensity by adding more Mn2+ion in Ba3Si6O12N2:Eu2+. The nonlinear relationship between energy transfer efficiency and Mn2+concentration has been verified, and the energy transfer mechanism from Eu2+to Mn2+is assumed to be the dipole-dipole transition. |
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