Design, Synthesis And Afterglow Properties Of Visible Light- Long Persistent Luminescence Materials

Long persistent phosphors have attracted considerable attention due to their widespread application in emergency signs. Along with the development of long persistent phosphor, there is a strong desire for the development of multiple color-emitting long persistent phosphors in recent years. Meanwhile, the mechanism behind this phenomenon is still unclear and many details are still shrouded in mystery. In this paper, a series of long persistent phosphors were designed and synthesized and their luminescence properties were systematically investigated.We synthesized a long persistent composite phosphor Ca Al2O4:Eu2+, Nd3+ /Y3Al5O12:Ce3+(CA/YAG). The main idea is to use the store ability of traps in CA phosphor and high yellow photoluminescence intensity and quantum efficiency of YAG phosphor.The persistent luminescence color of composite can be tuned from blue to yellow through the radiation energy transfer from CA to YAG. All the composites yield brighter LPL and longer persistent time. The LPL intensity can be enhanced ~2.7-fold and the persistent time can be prolonged ~2.8-fold in comparison with the widely used CA. The mechanism research indicated that the enhancement of the LPL of CA/YAG composites is associated with the photopic luminous efficiency( V(l)) and photo releasing the energy that is stored in deep traps. Although the radiance(W/Sr/m2) of LPL decreases with increasing YAG, the brightness and persistent time increases just by mechanical mixing.A color tunable from blue to white long persistent phosphor(Ca0.65-x Sr0.35-x)7(Si O3)6Cl2:Eu3+ x,Tm3+ x(0.01≤x≤0.08) was prepared by high solid-state reaction. The blue long persistent luminescence can persist for 27 h and its initial intensity can reach 1275 mcd/m2. More importantly, the white long persistent luminescence can persist for 22 h, which is longer than the famous Cd Si O3:Dy3+(5h). The results of thermoluminescence indicate that three trap centers, [ Tm·sr], [Tm·Ca- Tm·sr] and [ Tm·Ca], are formed by the conglomeration of defects that are generated from codoping Tm3+. Meanwhile, the predominating thermoluminescence band corresponding to [ Tm·sr] probably be responsible for their long persistent luminescence, whereas the weaker thermoluminescence band corresponding to [Tm·Ca- Tm·sr] and [ Tm·Ca] may not contribute to LPL because the carrier in these deep traps can not be released and then recombine with the emitting centers. The persistent luminescence spectra prove that the distribution of the traps in(Ca0.65-x Sr0.35-x)7(Si O3)6Cl2:Eu3+ x,Tm3+ xare nonuniform. [ Tm·sr] is closer to Eu1, which results in [ Tm·sr] is easier to trap the electrons that come from Eu1, whereas [Tm·Ca- Tm·sr] is closer to Eu2 and far away from Eu1. Hence, [Tm·Ca- Tm·sr] are easy to trap the electrons from Eu2. The probability of [ Tm·Ca] of traping electrons from Eu1 and Eu2 has little difference because of the little difference of the distances between [ Tm·Ca] and the emitting centers(Eu1 and Eu2). Comparing the thermoluminescence curves under different excitation wavelengths, we can find that under 254 nm excitation, the electrons are excited to the condution band, and then captured by the traps, while under 365 nm excitation, the electrons are excited to the excited state of Eu2+, and then tunnel to the trap levels.Several novel long persistent phosphors were developed, such as blue Ca3Al2O6:Ce3+, green Sr3Al2O5Cl2:Tb3+, yellow Ba3P4O13: Eu2+,Ga3+, orange Ca3 Si O4Cl2:Eu2+,R3+(R=Dy, Ce, Nd) and red Ca4(PO4)2O:Eu2+,R3+(R=Y, Tm, Ce, Gd, La, Lu). The options for the host lattices and activators are thus broadened.Series Ce3+ doped Ca3Al2O6 phosphors were synthesized, which emit asymmetric broad bands from 400 nm to 600 nm peaking at 485 nm. This is due to the 5d-4f transitions of Ce3+ ions occupying two different Ca2+ sites. The thermoluninescence curves indicated that the traps associated with the thermoluminescence peaks at 33℃ are responsible for the long-lasting phosphorescence. A green afterglow phosphor Sr3Al2O5Cl2:Tb3+ was synthesized and its LPL can last about 1 h. By changing the excitation time, we concluded that it is not sequencable for filling of traps with different depths. It was found that codoping Ga3+ could significantly enhance the afterglow of Ba3P4O13:Eu2+, and the LPL can last about 8.5 h. Moreover, we found that codoping of rare earth ions can not improve the afterglow properties. Instead, the persistant time was decreased. This report enlarges the option of codopant about Eu2+ activated LPL materials. Novel orange long persistant phosphors Ca3 Si O4Cl2:Eu2+, R3+(R = Dy, Ce, Nd) were synthesized, which emit a broad band peaking at 615 nm under 380 nm excitation. The persistent time for Eu2+ singly doped Ca3 Si O4Cl2 is only 2 min. And codoping rare earth ions can enhance the afterglow properties of the samples. Especially when codoped with Dy3+, the persistent time of the sample can reach 40 min. Novel near-infrared persistent phosphors Ca4(PO4)2O:Eu2+,R3+(R=Y, Tm, Ce, Gd, La, Lu) were synthesized. All the codoped samples exhibited a broad band peaking at 690 nm under 467 nm excitation. With codoping Eu2+ and R3+(R = Y, Tm, Ce, Gd, La, Lu), all the samples exhibit persistent luminescence. Moreover, the luminescence of Ca3.97(PO4)2O:Eu0.012+,Y0.023+ shows the longest persistent time, which can be detedcted for 2 h after the removal of excitation. The investigation of the new near-infrared long persistent phosphor enriches the options for the activator and hosts.