Sol-gel Synthesis And Properties Of ZnGa_2-x(Mg²⁺/Si⁴⁺)_xO₄:0.005Cr³⁺,yLi⁺ Spinel Phosphors With Long-Persistent Luminescence

In this dissertation,near-infrared(NIR)and long-persistent luminescent nanoparticles of spinel-structured ZnGa2-x(Mg2+/Si4+)xO4:0.005Cr3+,yLi+(ZGMSO:Cr3+,Li+)(0≤x≤0.15,0≤y≤0.07)have been synthesized via sol-gel processing at the calcination temperature of 1000℃.Regulation of the local microstructure was achieved by varying the composition,in order to optimizing the NIR afterglow performance.Finally,a white-emitting phosphor without any rare earth elements has been originally developed.Phase structure,local micro structure,defect depth,luminescence properties and persistent luminescence behaviour of the samples were analyzed by XRD,XPS,FE-SEM,TEM,TSL,UV-vis,FT-IR,PL-PLE,Raman and long-persistent decay curve.The main conclusions are listed as follows.Incorporation of Mg2+/Si4+ induces smaller particle size,broader band gap,more Zn/Ga anti-defects,and increased lattice distortion.However,it affects the crystal structure insignificanly.Li+doping induces increased particle size,better crystalline,broader band gap,and decreased lattice distortion of[ZnO4]tetrahedron that caused by Si4+ doping.But the lattice constant is independent on the Li+incorporation.Under UV excitation at 265 nm,ZnGa2-x(Mg2+/Si4+)xO4:0.005Cr3+(ZGMSO:Cr3+)exhibits typical a NIR emission at 694 nm,which is originated from the 2E→4A2 transition of Cr3+.Mg2+/Si4+ doping induces the red shift of 4T2 level.The samples show intense afterglow after the stoppage of ultraviolet light irradiation,and the x=0.01 sample exhibits the best persistent luminescence,which can last for 48 hours.After re-excitation by a red light,an intense afterglow appears again and can last for more than 3 hours.The x=0.15 sample exhibits strong afterglow emission several times under cyclic heat processing at 150℃,mainly due to its deepest trap depth.Incorporation of Li+contributes to better afterglow performance,and the best persistent luminescence was observed at y=0.05.Li+ occupying Zn2+ site gives rise to the formation of oxygen vacancy defects,leading to the appearance of oxygen vacancy defect level in the band gap,which is near the conduction band bottom at 280 nm.Under the excitation at 280 nm,the samples emit a blue emission at 490 nm and a NIR emission at 694 nm.The visible emission is significantly affected by the contents of Li+ and Mg2+/Si4+,but the NIR emission is not.Stronger blue emission intensity is found at a higher content of Li+,while more incorperation of Mg2+/Si4+ contrubutes to the weakened emission intensity.Therefore,the emission color of the materials can be easily regulated by changing the composition.The ZnGa1.97(Mg2+/Si4+)0.03O4:0.005Cr3+,0.07Li+ exhibits a white color emission with the CIE coordinate of(0.31,0.32)indicating a new potential rare earth-free phosphor of white light emission.