CaWO 4 :Yb 3+ ,Tm 3+ Crystals and SrAl 2 O 4 :Eu 2+ ,Dy 3+ Phosphors in Glass-Based Composites for Green Afterglow after NIR Excitation

. Here, we present the preparation of composites with green persistent luminescence using melting process. The composites are phosphate glass (75NaPO 3 -25CaF 2 and 90NaPO 3 -10NaF (in mol%)) with embedded phosphors: CaWO 4 : Yb 3+ , Tm 3+ crystals with blue upconversion and SrAl 2 O 4 :Eu 2+ ,Dy 3+ with green persistent luminescence. Green persistent luminescence above 0.3 mcd/m 2 can be seen for ~ 30 minutes after charging with 980 nm due to energy transfer between the blue upconverter crystals and the persistent luminescent phosphors. Challenges related to the fabrication of such composites are discussed.


Introduction
Persistent luminescent (PeL) materials have been of great importance as they can emit light for hours once the excitation source is removed [1].Such materials have found uses in emergency signalization, thermal sensors and bioimaging [2][3][4].PeL occurs due to energy storage during excitation, with subsequent release of trapped charges and delayed photon emission [5].When the SrAl2O4:Eu 2+ ,Dy 3+ PeL phosphors are added in passive glass matrix, green afterglow can only be obtained when the material is exposed to UV, blue or even white light, limiting their applications.Herein, we present an alternative approach to obtain green PeL using near-infrared rechargeable glass-based materials by adding Yb 3+ ,Tm 3+ codoped CaWO4 crystals and SrAl2O4:Eu 2+ ,Dy 3+ phosphors in phosphate glasses with the composition 90NaPO3-10NaF and 75NaPO3-25CaF2 (in mol%).

Results and discussion
The composites were prepared with CaWO4 crystals doped with 0.5 mol% of Tm2O3 and 15 mol% of Yb2O3 for intense blue emission under 980 nm pumping and 1 wt% of SrAl2O4:Eu 2+ , Dy 3+ .The crystals were added in the glass melt prior to quenching the glass.After annealing, the composites prepared with large amount of crystals are opaque as expected while the composites prepared with low amount of crystals are translucent confirming the presence of the crystals in the glasses (Figure 2a and b).As shown in Figure 2c, the addition of the crystals in the glass matrix induce the precipitation of additional crystals in the glass matrices.Unfortunately, the additional crystalline phase cannot be identified due to the low number and intensity.From the XRD pattern of the composites, the survival of the CaWO4 crystals in the glass matrices during the glass preparation is confirmed.The survival of the SrAl2O4:Eu 2+ , Dy 3+ in the glass matrices is confirmed by the green afterglow seen in Figure 2a and b.The PeL phosphors appear to be homogeneously distributed in the glass matrices.However, as shown in Fig 3, the intensity of the PeL after UV charging from the composites depends not only on the amount of CaWO4 crystals but also on the glass matrix.We think it is due to their low transmittance properties that the PeL from the Ca composites is low.The various crystals in the glass matrix lead to absorption and scattering of the UV and so to efficient charging of the PeL particles.Due to the overlap between the blue UC emission between 450 and 500 nm and the thermoluminescence excitation spectrum of the PeL particles [7], green PeL was seen after charging the composites with 980 nm.The PeL properties of the composites after charging for 5 min at 980 nm are presented in Figure 4.

Conclusion
Phosphate glass-based composites with green PeL after NIR charging can be prepared by adding CaWO4:Tm 3+ , Yb 3+ crystals and SrAl2O4:Eu 2+ ,Dy 3+ in the glass melt before the quenching step.However, the amount of the crystals should be optimized to prepare a translucent composite, crucial conditions for efficient charging of the PeL particles.We successfully prepared a composite emitting green light for ~30 minutes above the threshold level of 0.3 mcd/m² after 5 min charging at 980 nm.

Fig. 1 .
Fig. 1.Process of fabrication of the glass composites.

Fig. 3 .
Fig. 3. PeL intensity of the Ca and Na composites prepared with 1 wt% of PeL and different amount of CaWO4 crystals (measured after 5 min charging at exc= 266 nm)