The Optical Storage Properties And Mechanism Of Near Infrared Photostimulated Persistent Luminescence Phosphors

Long lasting phosphors are a kind of very important and usable materials. Some long lasting phosphors can be excited by sun light so that they can save solar energy in daytime and release the stored solar energy slowly at night through emitting luminescence. They have been widely used in weak light illumination and safety signage field. Some of the investigation shows that photostimulated persistent Luminescence was observed in some long lasting phosphors. Photostimulated persistent luminescence refers to persistent luminescence under the stimulation of a low energy photon after the excitation with high-energy radiation. Using this kind of phenomenon, optical information storage can be carried on, broadening the application field of long lasting phosphors from weak light illumination and safety signage to optical information and high energy beam survey. In conventional photostimulable storage phosphors, the stored optical information is usually read out as a transient luminescence which vanishes after the excitation stops. Unlike the transient luminescence, photostimulated persistent Luminescence phosphors exhibit a long lasting luminescence, making it possible to replace traditional photostimulable storage phosphors in novel storage device, high energy beam survey as well as image and optical information storage. In recent years, new photostimulable persistent phosphors gradually became the research hot spot. In this work, three kinds of long persistence materials SrAl2O4:Eu2+, Dy3+(SED); SrAl2O4:Eu2+, Bi3+(SEB) and ZnGa2O4:Cr3+(ZGC) are prepared by a solid state reaction method. The crystal structure and optical properties were analyzed systematically. The conduction electrons recombination model is used to interpret the mechanism of long persistent luminescence and photo-stimulated luminescence in the studied phosphors. The main research content is as follows:1) Long persistent luminescence phosphor with high bright green emission of SED is synthesized by a solid state reaction method. The result of X-ray diffraction shows that the sample is monoclinic crystal. The result of the fluorescence spectrum experiment shows that the peak of the excitation spectrum located at 360 nm caused by the transition from the ground state 4f7 to 4f65d of Eu2+. The phosphor emitted 515 nm bright green light when exposed under the UV irradiation. The result of the phosphorescence excitation test shows that the peak of the spectrum located at 430 nm instead of 356nm. It is different with the result of fluorescence excitation test. The persistent luminescence decay curve indicates the decay time of the sample is over an hour and the peak of the phosphorescence spectrum located at 515nm. There are two peaks in thermoluminescence spectrum. One peak is at 85℃ and the other located over 400℃. The thermoluminescence spectrum is used to explain the photostimulated persistent luminescence phenomenon. Result in photo-stimulated luminescence test shows that the peak of photo-stimulated luminescence excitation located at 760nm. The sample displays obvious photo-stimulated luminescence properties.2) Solid stated reaction method is used to prepare the green long afterglow phosphor SEB and the influence of the transition metal Bi doping on the performance of afterglow and the photo-stimulated persistent luminescence was investigated. The experiment result shows that the excitation spectrum of the sample ranges from 300-420nm, wider than that in SED. There is an obvious peaks located at 330nm caused by characteristic emission of Bi3+. The persistent luminescence decay curve shows good green afterglow and the decay time of SED is over 20min. Thermoluminescence test verifies the long persistent luminescence properties and the peak of thermoluminescence spectrum located at 92℃. The result of photo-stimulated experiment shows a weak photo-stimulated luminescence property of the sample. Compared with SED, the photostimulated persistent luminescence became weaker obviously. It is caused by the characteristic emitting of Bi3+ in the cell which hindered the conduction electrons to be captured by the trap level. Because of the ultraviolet light emission of Bi3+, the persistent luminescence and photostimulated persistent luminescence of SEB became weaker compared with SED.3) The near infrared persistent luminescent materials ZGC was prepared by a high temperature solid state method and the photoluminescence, long persistent photoluminescence, photostimulated luminescence and the thermoluminescence properties of ZGC were studied in detail. The X-ray diffraction result shows that the sample is cubic crystal. There are four peaks in the excitation spectrum located at 250nm, 300nm,407nm,545nm respectively. The 256 nm and 300 nm peaks originate the charge transfer transition from the 2p of O2- to 4s4p of Ga3+, the 407 nm band originates from the 2A2â†'4T1 transition of Cr3+, and the 545nm originating from the 2A2â†'4T2 transition of Cr3+. The persistent luminescence excitation spectra revealed that the persistent luminescence of ZGC come from the charge transfer transition from O2- to Ga3+ instead of the intrinsic transition of Cr3+. The investigation in photo-stimulated persistent luminescence indicated the sample could emit bright persistent luminescence again under infrared light excitation after the UV light excited persistent luminescence had decayed completely. The result shows that the UV light wrote-in information in ZGC can be read out by infrared light. The writing-in and reading-out wavelengths are located at 335nm and 855nm, respectively.4) The experimental results show strong near-infrared photostimulated persistent luminescence in SED and ZGC powders. Moreover, the conduction electrons recombination model is employed to reveal the mechanism of persistent luminescence and photo-stimulated luminescence of the three samples.