Nitridosilicate-based phosphors have received extensive attention owing to the high covalency, large crystal field effect, high chemical stability, and reliable thermal stability 1- 3. These phosphors usually exhibit relatively long excitation and emission wavelengths because of the strong crystal field effect. Y2Si4N6C: Ce3+ exhibits a green emission band at around 540-550 nm under blue excitation 4. MSiN2: Eu2+ (Sr, Ba) shows a red emission band in 600-670 nm upon blue excitation 5, and CaAlSiN3: Eu2+ displays a red emission at around 650-660 nm under blue excitation 6. Among the nitridosilicate-based phosphors, M2Si5N8: Eu2+ (M = Ca, Sr, Ba) materials have been reported as promising orange-red emitting phosphors for white light-emitting diodes due to the high quantum efficiency under blue excitation and high thermal stability 7- 8. In addition, M2Si5N8 hosts can be synthesized under ambient pressure, leading to the relatively low cost in production than other nitridosilicate-based phosphors such as MAlSiN3 (M=Ca, Sr), LaSi3N5, and ?-sialon 9- 11.
White light-emitting diodes (WLEDs), exhibiting high brightness, low energy consumption, high reliability, long lifetime and eco-friendliness, have attracted significant attention as illumination sources 12- 15. The conventional technique for generating white light is the combination of blue InGaN LED chips and yellow-emitting Y3Al5O12: Ce3+ (YAG) phosphors 16. However, the present WLEDs usually exhibit several disadvantages including high color temperatures and low thermal stability. Furthermore, the WLEDs usually show low color-rendering index (CRI) values owing to the lack of red emission in the LED spectra 17. Therefore, red-emitting nitride phosphors with reliable thermal stability and quantum efficiency are necessary to be designed in order to solve the mentioned problems.
Long-persistent phosphors are important in various applications including instrumental displays, night illumination, traffic labels and emergency signs 18- 20. Blue and green-emitting afterglow materials such as CaAl2O4: Eu2+, Nd3+, Sr4Al14O25: Eu2+, Dy3+, and SrAl2O4: Eu2+, Dy3+ usually exhibit high intensity and long afterglow time, and have been investigated and used widely 21- 22. However, red-emitting afterglow phosphors such as CaS: Eu2+, Tm3+, and Y2O2S: Eu3+, Mg2+, Ti4+ encounter several disadvantages such as insufficient chemical stability, high toxicity, low persistent intensity and short afterglow time 13, 23. To solve the drawbacks of WLEDs and long-persistent phosphors, Sr2Si5N8 with high chemical stability and strong crystal field is selected as the host for developing new red-emitting phosphors.
Terbium ions-doped luminescent materials have been widely investigated for application in displays and fluorescent lamps. However, these phosphors are seldom used in WLEDs owing to the insufficient excitation efficiency of Tb3+ ions in the range of 300-500 nm 24- 26. In the present work, terbium ions were deliberately doped into the Sr2Si5N8 host matrix. The structure of Sr2-xTbxSi5N8 was refined and the corresponding structural evaluations were presented. The variation of luminescence properties for the prepared phosphors with the synthesized temperatures was investigated. The red-emitting Sr2-xTbxSi5N8 and commercial yellow-emitting Y3Al5O12: Ce3+ phosphors were combined with a blue LED chip for the fabrication of WLEDs with high CRI values. The decay time and afterglow properties of the prepared phosphors after various excitation wavelengths were discussed in detail.