Metal Oxides

Luminescence that persists after the removal of the excitation is called afterglow or persistent phosphorescence. Long-lasting phosphorescence (LLP) is a phenomenon due to the thermal stimulated recombination of holes and electrons at traps which leave holes or electrons in a long-lived excited state at room temperature. As novel functional materials, the long afterglow phosphors are drawing more and more attention in recent years because of their applications in traffic signs, emergency signage, watches and clocks, textile printing, etc. As a new generation of long afterglow phosphors, lanthanide ion-doped alkaline earth silicates or aluminates yield much better characteristics, such as longer duration time of the phosphorescence, brighter luminosity, and improved chemical stability, than the conventional sulfide materials used earlier. At present, more than 250 species of persistent phosphors have been discovered and identified, mostly through a trial-and-error approach. Apart from Sr₂MgSi₂O₇:Eu²⁺, Dy³⁺ and SrAl₂O₄:Eu²⁺, Dy³⁺, several metal oxides exhibit interesting long persistent luminescence. Some examples are Ca₂Sn₂Al₂O₉:Mn²⁺, Sn⁴⁺, KGaGeO₄:Bi³⁺, Ba₂Ga₂GeO₇:Pr³⁺, Lu₃Al₂Ga₃O₁₂:Ce³⁺, Cr³⁺, BaSi₂O₅:Gd³⁺, Mn²⁺, etc. Such phosphors are reviewed in this chapter. The mechanisms of persistent emissions are discussed. In most of the proposed models, absorption of UV is related to Eu²⁺. LLP has been often explained on basis of redox reactions. Though this model was proposed for alkaline earth aluminates, it had been used for several other compounds exhibiting LLP. In Ce³⁺-doped, garnet-related phosphors, which exhibited LLP, the UV absorption is not related to the activator but to antisite defects. Various applications of LLP are also discussed at length which makes this topic more interesting.