The Synthesis And Optical Properties Of Multicolor Long Persistent Luminescence Materials By Eu²⁺/Sm³⁺

Long persistent luminescence materials have attracted much attention due to their advantages of solar energy utilization and environment friendly.In order to meet the requirements in the practical application,the afterglow performance need to be improved and their color should be enriched.At present,only green long persistent luminescence materials are commercially available.Others,especially the red long persistent luminescence materials,are still in great scarcity and none of them is good enough to be applied to practical applications.In addition,the exact mechanisms governing phosphorescence in materials have not yet to be clarified and then researchers have no scientific and instructional approach to develop new good long persistent luminescence materials.In view of these problems,based on the activators Eu²⁺/Sm³⁺and a variety of matrix materials,several long persistent luminescence materials with different colors are prepared in this doctoral thesis.Their optical properties are studied in detail,and then possible mechanism is proposed.Specific research contents can be divided into the following:Cyan long persistent luminescence material:novel long persistent luminescence materials Ba₂Gd₂Si₄O₁₃:Eu²⁺,RE³⁺(RE³⁺=Dy³⁺,Ho³⁺,Tm³⁺,Nd³⁺and Tb³⁺)are successfully synthesized by a solid state reaction.The green emitting phosphors show a cyan long persistent luminescence phenomenon induced by the emission of Eu²⁺ion only occupying the Ba²⁺site.Co-doping the RE³⁺ions can improve the long persistent luminescence performance effectively,and with the moderate introduction of Nd³⁺ion the most persistent cyan emission can last for 3 h approximately.It is revealed that the introduction of the RE^2+/3+ions results in larger generation of the intrinsic defects with a trap depth about 0.713 eV,which exactly contributes to the long persistent luminescence performance.This work provides a promising cyan long persistent luminescence material for practical application,and also proposes a new approach for the design of long persistent luminescence materials to control the long persistent luminescence color by the distinct emission of Eu²⁺ion at the different crystallographic sites in one host.Green long persistent luminescence materials:Eu²⁺in BaSc₂Si₃O₁₀ and NaBaScSi2O7 are synthesized by a solid state reaction.First,for the BaSc₂Si₃O₁₀:Eu²⁺material,Eu²⁺shows an asymmetric blue emission band located at 443 nm,because it occupies both the Ba and Sc sites in BaSc₂Si₃O₁₀.The blue emitting phosphors show a green long persistent luminescence phenomenon induced by the emission of Eu²⁺ion only occupying the Sc site.The result provides a direct experimental evidence that conduction band plays an important role in the long persistent luminescence process.Co-doping the RE³⁺ions(RE³⁺=Nd³⁺,Tm³⁺,Dy³⁺and Tb³⁺),especially Nd³⁺or Dy³⁺ions,can improve the long persistent luminescence performance effectively.The green long persistent luminescence can last 1h at least.For another one,the NaBaScSi2O7:Eu²⁺material,by co-doping Nd³⁺ion,both long persistent luminescence and optically stimulated luminescence properties have a great improvement.In NaBaScSi2O7:Eu²⁺material,it is exactly the relatively shallow T₁ traps with a continuous trap depth distribution that lead to the long persistent luminescence phenomenon of five more hours and the other deep traps of T₂,T₃ and T₄,corresponding to a separated trap depth distribution,should contribute to the optically stimulated luminescence performance.Orange long persistent luminescence materials:a novel orange long persistent luminescence material based on LiSr₄?BO₃?₃ host matrix is synthesized by solid-state reaction.The Eu²⁺single-doped samples show a faint afterglow,whereas the orange afterglow time is prolonged drastically by co-doping with Dy³⁺and could reach about 20 min at the optimum doping concentration.It is revealed that the introduction of Dy³⁺results in large generation of the suitable energy trap T₂ with a trap depth 0.76 eV,which exactly contributes to the orange afterglow time.Red long persistent luminescence materials:based on the activator Sm³⁺and three matrix materials,Sm³⁺in Sr₂ScGaO₅,Gd_9.33?SiO₄?₆O₂ and GdSr₂AlO₅ are synthesized by a solid state reaction.First,for Sr₂ScGaO₅:Sm³⁺material,its photoluminescence and phosphorescence are both attributable to the well-known intra-4f transitions of Sm³⁺ion,resulting in the reddish emission.However,they show different emissions of Sm³⁺ion which originate from different splitting excited energy levels of ⁴G_5/2.It can be explained by the fact that the conduction band plays an important role in long persistent luminescence process,but doesn't participate in the photoluminescence process.Also,the material can be stimulated effectively within a short time and would have great potentials for the applications in AC-LED.This work enriches the content of long persistent luminescence mechanism and novel long persistent luminescence material well.Second,for Gd_9.33?SiO₄?₆O₂:Sm³⁺material,the red afterglow time reaches more than 20 min when the Sm³⁺ion doping concentration is 1.5%.It is revealed that the introduction of Sm³⁺ions occupying the Gd³⁺sites in Gd_9.33?SiO₄?₆O₂ host matrix results in large generation of oxygen vacancies acting as the predominant electron traps,which have a great contribution to the emission intensity and afterglow time of the Gd_9.33?SiO₄?₆O₂:Sm³⁺material.Finally,for GdSr₂AlO₅:RE³⁺(RE³⁺=Eu³⁺,Sm³⁺,Pr³⁺and Dy³⁺)material,all obtained phosphors show a long-wavelength emission at approximately 600nm,and exhibit the afterglow for several minutes.In addition,they show excellent paramagnetism characteristics simultaneously.So the red-emitting,paramagnetic and long persistent luminescence materials GdSr₂AlO₅:RE³⁺should be promising candidates for multifunctional applications.