Design And Synthesis Of La-Si(Al)-N(O) Phosphors Activated By Rare Earth Ions

Phosphor-converted white light emitting diodes(wLEDs),recognized as the 4th generation green lighting source,have many advantages over conventional lighting technologies,such as energy saving,high efficiency,environmental friendliness,compactness and longevity.They are strategic emerging industries with key development worldwide.In the packaging option of combining red,green and blue phosphors with ultraviolet LED chips,phosphors as the core material of wLEDs play a key role in achieving the high optical quality lighting with a continuous and uniform spectrum.In this thesis,the host La3Si8N11O4 with high stability was selected as the research object in order to develop blue-green phosphors that can match well with ultraviolet chips.By means of screening luminescent centers,co-doping activator ions and doping host cation,new La-containing and silicon-based oxynitride phosphors were successfully designed and synthesized by the high-temperature solid-state method.In addition,the synthesis and luminescence properties of the phosphor were carefully studied,and some ideas for improving the thermal stability of phosphors or developing new phosphors were suggested.The La3Si8N11O4:Eu2+ phosphor was prepared and the photoluminescence properties were investigated.La3Si8N11O4:Eu2+ showed a broad and asymmetric band peaking at 481?513 nm,and the internal and external quantum efficiencies were 44%and 33%under 320 nm excitation,respectively.The above results suggested that the La3Si8N11O4:Eu2+phosphor could be a potential blue-green down-conversion luminescent material for UV-LED pumped wLEDs.In addition,the energy transfer between Eu2+ ions in La3Si8N11O4:Eu2+ phosphor was dominated by the dipole-dipole interaction through the analysis of the local crystal field,emission spectrum shape,intensity and fluorescence lifetime,which was attributed to the occupancy of two distinct Lal and La2 sites by Eur2+ions.Compared to the Ce3+-doped La3SigN11O4,the Eu2+-doped one showed poor thermal stability,predominantly owing to the photoionization and non-equivalent substitution of divalent Eu2+ for trivalent La3+.A blue-green phosphor La3Si8N11O4:Ce3+,Tb3+ was successfully designed byco-doping of Tb3+ and Ce3+,and the internal and external quantum efficiencies were 76.3%and 46.7%under 360 nm excitation,respectively.More importantly,the thermal stability of the co-doped phosphor could be significantly optimized while maintaining the original luminescence efficiency.Compared to La3Si8N11O4:Tb3+,the emission intensity of the Tb3+in La3Si8N11O4:Ce3+,Tb3+ was greatly reduced from 74%to 30%at 250?,owing to the energy transfer from Ce3+ to Tbi+.The above phenomena were mainly attributed to the fact that the contribution of the electrons transferred from Ce3+ to Tb3+ was much greater than that of thermal ionization of Tb3+ ions,understood by establishing the energy transfer model and the tracking analysis of electrons.Finally,an UV-LED driven white LED using the phosphor blend of La3Si8N11O4:0.01Ce3+,0.05Tb3+,SrSi2N202:Eu2 and Sr2Si5N8:Eu2 phosphors was fabricated,and it exhibited a correlated color temperature of 3570 K and a high color rendering index of 90.2.The oxynitride phosphors La3-xCaxSi8N11-xO4+x:Ce3+(x ?0.8)and La33i8-yalyN11-yO4+y:Eu2+(y?2.5)were synthesized by the cation substitution.The peak maximum of excitation spectra of Ca2+ doped(La,Ca)3Si8N11-xO4+x:Ce3+ phosphors gradually shifted from 325 to 360 nm,which was more suitable for ultraviolet chips to achieve the full-spectrum lighting.The Ca2+-substituted phosphors had a smaller proportion of thermally-induced electrons,the higher absorption efficiency,better thermal stability and the higher quantum efficiency due to the increase of bandgap(Eg).The blue-shift of emission spectra of Al3+-substituted La3Si8N11O4:Eu2+ was mainly ascribed to the change of local environments Eu2+associated with the replacement of Si4+ by Al3+.In addition,the quenching concentration of Eu2+ ion after the Al3+ substitution increased from 0.01 to 0.03,and the luminescence intensity was also increased by 1.4 times when compared to the unsubstituted one.