长余辉材料Sr_2MgSi_2O_7∶Eu^(2+),Dy^(3+)中稀土离子的发光特性

Luminescent Properties of the Rare Earth Ions in Long Afterglow Phosphor Sr_2MgSi_2O_7∶Eu^(2+),Dy^(3+)

利用同步辐射光源(德国HASYLAB实验室的SUPERLUMI实验站)和真空紫外激光(157.6nm)对新型蓝光发射长余辉材料Sr2MgSi2O7∶Eu2+(0·2%),Dy3+(8%)进行了光谱研究。在170nm同步辐射光源激发下,观察到对应Eu2+:5d-4f跃迁的477nm发射带和对应Dy3+:4f-4f跃迁的两组线谱发射,其中只有来自Eu2+的5d-4f发射对长余辉光谱有贡献。在157.6nm激光激发下,除了上述发射外,还明显观察到对应Eu3+的红色线谱(590,614,626nm)。结合这些光谱特性,对Sr2MgSi2O7∶Eu2+,Dy3+中稀土离子的发光特性以及长余辉发光机理进行了讨论,并提出了Eu2+充当空穴陷阱的可能性。

As novel functional materials, long afterglow phosphors are drawing more and more attention in recent years because of a constantly growing market for their applications in traffic signs, emergency signage, watches and clocks, textile printing, among others. Amid the newly developed long afterglow materials which have already found commercial use, blue-emitting Sr2MgSi2O7： Eu^2＋ , Dy^3＋ is of special interest because of its excellent persistent luminescence combined with an easy processability. In this paper, the luminescent properties and long afterglow of Sr2MgSi2O7： Eu^2＋ , Dy^3＋ were investigated using synchrotron radiation and VUV laser （157.6 nm） as excitation sources. The emission spectra of Sr2MgSi2O7 ：Eu^2＋, Dy^3＋ under 170 nm synchrotron radiation excitation consists of a 477 nm broad band corresponding to 5d-4f transitions of the Eu^2＋ ions and two group of line spectra corresponding to ^4F9/2→^6H15/2 transitions （478, 483 and 493 nm） and ^4F9/2→^6H13/2 transitions （574, 576, 579, 581 and 585 nm） of Dy^3＋, respectively. The excitation spectra of 477 nm emission is dominated by an excitation band at wavelengths longer than 220 nm, which shows a complicated structure, comprising at least three sub-bands （around 250, 280 and 310nm）, corresponding to the transition from sST/2 to the higher crystal field components of 4f^65d^1 states of Eu^2＋. The relatively weaker 170 nm excitation band was ascribed to spin allowed f-d transition of Dy^3＋. At different temperature, the long afterglow emission after irradiation of 170 nm synchrotron radiation was measured -30 seconds after closing the beam-shutter. As in the case of Eu^2＋ and Dy^3＋ codoped MAl2O4 （M = Ca and Sr）, the long afterglow emission of Sr2MgSi2O7： Eu^2＋ , Dy^3＋ is due to the 5d-4f transitions of the Eu^2＋ ions. The laser excited emission spectra exhibit some new features comparing to the synchrotron radiation excited emission spectra. The most obvious new features are three groups of sharp emission lines centred around590, 614 and 626 nm, which are known to be the red emissions of trivalent europium corresponding to intraconfigurational 4f-4f transitions. The strong emission from Eu^3＋ indicates an efficient photon-induced process, promoting Eu^2＋ to Eu^3＋ , caused by the much higher excitation density of laser source. The high efficiency of Eu^2＋ -Eu^3＋ conversion by laser perhaps implies a high possibility of Eu^2＋ ions acting as hole traps （＂Eu^2＋ ＋ h＂ ）, which is neglected by now in the study of Eu^2 ＋ doped long afterglow phosphors, but may playan important role in the process of the persistent luminescence. At room temperature,the holes are thermally released continuously from these hole traps at Eu site, preparing the Eu^2＋ ions to accept the energy released by the electron-hole recombination （ perhaps via cross relaxation）, and finally yielding the long afterglow emission from Eu^2＋ ions.