Phosphor-in-glass with time-evolving multicolor luminescence for dynamic anticounterfeiting

The conventional luminescent materials used in the anticounterfeiting field generally exhibit unicolor output and a single-wavelength excitation mode, resulting in a poor anticounterfeiting effect. Herein, we successfully achieved a new transparent SrAl2O4:Eu2+,Dy3+/CaAlSiN3:Eu2+ phosphor-in-glass (PiG) with dynamic luminescence color variation from red to yellow in hundreds of seconds during the ultraviolet lamp irradiation and green persistent luminescence (PersL) after ceasing excitation. Additionally, the prepared PiG owns excitation wavelength- or doping weight ratio-dependent color-tunable luminescence. As a proof-of-concept experiment, the specially designed SrAl2O4:Eu2+,Dy3+/CaAlSiN3:Eu2+ PiG based flower patterns exhibit dynamic luminescence color variation under a simple ultraviolet lamp, and can be observed for quick discrimination. This work exploits a new perspective for the design of dynamic multicolor anticounterfeiting materials.


Introduction
Counterfeiting has long been a major hazard to society, causing huge economic losses to the state, businesses and individuals [1,2]. To address the rising threat of forgery, many anticounterfeiting technologies, such as radio frequency identification, laser holographic printing and optical anticounterfeiting, have been adopted [3,4]. Among them, luminescent anticounterfeiting technology has attracted great attention due to its visual identification, facile design, and low cost [5]. However, traditional anticounterfeiting luminescent materials generally exhibit a single security feature, such as mono-mode emission (downshifting or upconversion luminescence (UCL)), which can be easily replicated by substitutes [6,7]. It is possible to integrate multiple security features on a product by the blending of different luminescent materials; however, the simple mixing of materials may result in an uneven dispersion and a performance degradation [8].
To solve such problems, a multimode luminescent anticounterfeting strategy has been developed. For example, a variety of anticounterfeiting luminescent materials with multi-mode (such as possessing both photoluminescence (PL)/UCL, PL/PersL capabilities) or excitation wavelength-dependent emission have been reported [9,10]. They mainly contain carbon quantum dots [11], perovskite [12], dye [13], and so on. More recently, a few literatures reported that materials with dynamic multicolor tunable luminescence can provide extra security feature in the time dimension, which makes them applicable for advanced anticounterfeiting [14,15]. However, realizing dynamic multicolor tunable luminescence in a single host material is still a significant challenge.
Glass is an excellent optical host matrix owing to its high optical transparency, good chemical and physical stability, easy manipulation and low cost [16]. Integrating optically active particles within a glass matrix can lead to a new functional composite with novel optical properties, potential promoting the development of Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Experimental section
Precursor glass with the composition of 25Na 2 O-45P 2 O 5 -10Li 2 O-10NaF-10ZnO (mol%) was prepared by a conventional melting process. The Na 2 O, P 2 O 5 and Li 2 O were introduced in the form of Na 2 CO 3 , (NH 4 ) 2 HPO 3 and Li 2 CO 3 , respectively. The reagent grade chemicals were mixed uniformly, and melted in a corundum crucible at 700°C for 30 min. Subsequently, the glass melt temperature was reduced to 450°C to add SAO (purchased from Jinan G L New Materials Co. Ltd) and CAS (purchased from XinLi Illuminant Co. Ltd) MPs with defined amount. The SEM images of the SAO and CAS commercial MPs with sizes in the range of 7-25 μm are shown in figure 1. Then, the glass melt was swirled for 30 s to homogeneously disperse the doped MPs. 3 min after adding the MPs, the mixture melt was poured into a preheated copper mould to form PiGs. Finally, the PiGs samples were annealed at 200°C for 2 h to remove residual stress from the quench.
X-ray powder diffraction (XRD) pattern was carried out on a powder diffractometer (UItima IV, Japan Kuraray Co) using Cu K α1 radiation (λ = 0.1546 nm). The surface morphology of the samples etched by hydrofluoric acid was observed by a scanning electron microscopy (SEM, JSM-6700F). PL, PL excitation (PLE) and PersL spectra as well as afterglow decay curves were measured by a spectrofluorometer (FS5, Edinburgh Instruments) with 150 W xenon lamp. Quantum yield (PLQY) was evaluated by a spectrofluorimeter (FS5) with an integrating sphere. All photographs of the PL and PersL phenomena were recorded by using a mobile phone (Mate 30, Huawei). To form luminescent inks, SAO/CAS PiG powders and polyvinyl chloride with a weight ratio of 1:15 were mixed uniformly. Using the prepared inks, the designed anticounterfeiting patterns were directly printed onto a black paper.

Results and discussion
The survival of the doped MPs in a glass matrix plays an important role in ensuring the excellent optical performances of the PiGs [19]. Hence, XRD patterns of 5 wt% SAO and 2 wt% CAS single-doped PiGs (denoted as SAO-PiG and CAS-PiG, respectively) were measured and displayed in figure 2(a). For comparison, XRD patterns of the SAO and CAS MPs are also shown in the figure. Obviously, all diffraction peaks of the PiGs are almost consistent with those of the corresponding MPs, implying that the doped MPs can survive in the glass matrix without producing any impurity and their structures keep unchanged during the sintered process. However, compared to the standard JCPDS data, some diffraction peaks of CAS-PiG are shift and split, possible due to the form of some impurity AlN and Si 2 N 2 O during the preparation of the commercial CAS MPs [20]. As shown in figure 2(b), a typical SEM image of the SAO and CAS co-doped PiG (denoted as SC-PiG) demonstrates the distribution of some MPs among the glass matrix, which may be SAO and CAS MPs. The problem will be further studied in the future. To evaluate PL performances of the prepared PiGs, normalized PLE and PL spectra of the CAS and SAO single-and co-doped PiGs are shown in figure 3. For comparison, the spectra of the corresponding CAS and SAO MPs are also exhibited in the figure. Obviously, the line-shapes of PL and PLE spectra for the single-doped PiGs are similar to those of the corresponding MPs, implying that the doped CAS and SAO MPs are not eroded by the glass melting during the sintering process and consequently keep their undamaged microstructure and excellent PL properties. However, compared to the single-doped PiGs, the excitation spectra of the 1.2 wt% SAO and 0.4 wt% CAS co-doped one monitored at the same wavelengths change significantly, possible due to the reabsorption of the co-doped MPs. As shown in figure 3(c), the PL spectrum of the SC-PiG sample excited at 365 nm consists of a green emission band at 520 nm and a red emission one at 620 nm, assigned to the 4f 6 5d 1 → 4f 7 transition of Eu 2+ ions in SrAl 2 O 4 [21,22] and CaAlSiN 3 [22][23][24], respectively.
Interestingly, the excitation spectra of CAS partially overlaps with that of SAO, indicating that the PL color can be tuned by the excitation wavelength (Inset in figure 4(a)). To evaluate the influence of the excitation wavelength on the PL color, the PL spectra of the SC-PiGs excited at different wavelengths are shown in figure 4(a). The relative intensity of the green emission band gradually decreases with the increase of the excitation wavelength from 340 nm to 430 nm. To visualize the color tuning, the CIE coordinates obtained from the PL spectra and the PL photographs of PiGs are shown in figure 4(b). Obviously, the PL color changes from yellow to red with the change of the excitation wavelength.
On the other hand, the PL color of the co-doped PiGs can also be tuned by changing of the weight ratio of SAO/CAS. Figure 5(a) displays the PL spectra of PiGs with various weight ratio of SAO/CAS (SAO: 1.2 wt%, CAS: 0.1 ∼ 0.6 wt%). All spectra consist of two discrete emission bands originating from the CAS and SAO MPs, respectively. When the intensity of the 520 nm green emission band is normalized to 1, that of the 620 nm one increases monotonously with the CAS content. The CIE coordinates and PL photographs ( figure 5(b)) demonstrate that the PL color shifts gradually from green to red with increasing of CAS fraction. Impressively, we found that the emission color of the SC-PiG sample changes with the 365 nm UV lamp irradiation time. As shown in figure 6(a), the PL color changes from red to yellow with prolonging the irradiation time from 0 to 200 s, and then remains almost unchanged with further extending the irradiation time. After the cease of irradiation, the SC-PiG sample appears green PersL originating from the SAO MPs, which weakens gradually with the passage of time. To investigate this phenomenon, the PL spectra of SC-PiG irradiated by the 365 nm UV lamp for different dwell time were measured by using a fiber spectrometer (HR2000, Ocean). With  Inset in (a) shows the excitation spectra of SC-PiG monitored at 520 nm and 620 nm, respectively. prolonging the excitation dwell time from 30 s to 200 s, the green emission band intensifies gradually, whereas the red emission band remains almost unchanged, as exhibited in figure 6(b). Further extending the excitation dwell time, the PL spectrum keeps almost unchanged. After the cease of irradiation, only a green PersL band at 520 nm is observed.
To further explore the irradiation time-dependent PL [24], the afterglow decay curves of the CAS-PiG and SAO-PiG samples were measured and shown in figure 6(c). The excitation wavelength was set to 365 nm, and the emission wavelengths were set to respective optimized ones. Remarkably, the red emission quickly reaches the maximum intensity, while the green emission intensifies slowly with the irradiation time. Moreover, no PersL is observed in the CAS-PiG sample, while PersL in SAO-PiG lasts for about 12 min. Hence, the PL color of SC-PiG   [20,21] and PersL of Eu 2+ in CaAlSiN 3 [22,23], respectively. When CAS-PiG excited at 365 nm, electrons are promoted from the ground state (4f 7 ) to the excited state (4f 6 5d 1 ) (process 1), and then most of the excited electrons return to the ground state and generate red PL (process 2). The interval time between the process 1 and 2 is very short (about 10 −8 s [21]). However, for the SAO-PiG sample, most of the excited electrons would be captured and stored by the metastable state traps ( ⋅ Dy Sr ) (process 3), and only a small number of them can return to the ground state and give green emission (process 2). With extending the irradiation time, the traps gradually saturate, and green emission gradually intensifies. When reaching an equilibrium between detrapping (process 3) and retrapping (process 4) of electrons in traps, the green emission would keep unchanged. After ceasing excitation, the captured electrons gradually leap out of the traps by thermal activation, transfers to Eu 2+ and then yields PersL. Typical, the emission of PersL phosphors comprises a very slow initial rising step followed by a stabilizing state [22,23].
To evaluate the potential application in the anticounterfeiting, PLQYs of CAS-PiG, SAO-PiG, 1.2 wt% SAO and 0.4 wt% co-doped PiG, as well as the SAO and CAS MPs were measured and listed in table 1. The values of CAS-PiG and SAO-PiG are slight smaller than that of the corresponding MPs, indicating that the embedded MPs maintain their excellent PL properties [25]. And the PLQY of the CS-PiG was about 68.63%, between those of the CAS-PiG and SAO-PiG, suggesting that the prepared PiG is suitable for anticounterfeiting application.
Compared with static luminescent materials, the SC-PiG sample with the irradiation time-dependent PL color would be well applied for the luminescent anticounterfeiting. To evaluate the remarkable dynamic multicolor luminescence, a series of luminescent patterns were designed and shown in figure 7. All patterns display distinct color changes with the variation of irradiation time and the cease of irradiation. Based on the above results, we propose that the SC-PiG sample are suitable for dynamic multicolor anticounterfeiting applications.

Conclusions
In this work, a facile and effective strategy was proposed to prepared multicolor dynamic luminescent materials via introducing SAO and CAS MPs into the specifically selected inorganic glass. The luminescence color of the prepared PiG changes from red to yellow in hundreds of seconds during the irradiation of UV lamp, as well as  intense green PersL after ceasing excitation. Additionally, the luminescence color can be tuned by controlling the blending weight ratio of SAO/CAS, and the excitation wavelength. Based on the unique optical characteristic, an anticounterfeiting with the irradiation time-dependent dynamic color-changing luminescence and green PersL is realized. We believe that our research results will provide some guidance for the development of dynamic luminescent materials for anticounterfeiting applications.