Excitation Wavelength-Dependent Dual-Mode Luminescence

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Excitation Wavelength-Dependent Dual-Mode Luminescence Emission for Dynamic Multicolor Anticounterfeiting Chen Shi, Xiuyu Shen, Yanan Zhu, Xiaoqiang Li, Zengyuan Pang, and Mingqiao Ge* College of Textile and Clothing, Jiangnan University, Wuxi 214122, China

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by STATE UNIV NEW YORK PLATTSBURGH on 05/13/19. For personal use only.

S Supporting Information *

ABSTRACT: Luminescent materials have become prevalent in data communication and information security because of their special optical characteristics. Conventional luminescent materials generally exhibit unicolor emission and fixed excitation mode, resulting in decreased efficiency of anticounterfeiting applications. The development of an iridescent chameleon-like material that can change its emission color under different stimulations is a significant challenge. Here, we propose that Pb2+, Mn2+, and lanthanide cations (such as Y3+, Tb3+, Yb3+) co-doped in Na2CaGe2O6 particles can be an effective tool for designing dual-mode anticounterfeiting materials based on their tunable fluorescence/ persistent luminescence transformation and excitation wavelength-dependent emission. Ultimately, a proof-of-concept anticounterfeiting fabric is obtained by using the as-prepared phosphors and exhibits a dynamic multiple color response. This work exploits the possibility of developing a new class of multimode anticounterfeit materials, which would be almost impossible to mimic or counterfeit, providing a very high level of security. KEYWORDS: excitation wavelength-dependent, lanthanide, luminescence, anticounterfeiting, dual mode

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INTRODUCTION Photo-stimuli-responsive materials, whose color and/or luminescence properties can be altered by light irradiation [e.g., ultraviolet (UV) light,1 visible light,2 near-infrared light3,4], have been extensively investigated because of their potential application in optoelectronic devices,5 cell labels,6 and information encryption/anticounterfeiting.7 A series of materials with photo-stimuli-emitting properties, such as lanthanide (Ln3+) ion-doped inorganic phosphors,8,9 organic dyes,10 metal−organic frameworks,11,12 and perovskites13,14 have been investigated. These smart materials can respond to external photo-stimuli with reversible changes. The emission wavelength of these materials can be tuned by appropriate doping of Ln3+ ions in host materials,15 controlling their microstructures16,17 or adjusting their chemical structures.18 However, the luminescent color of these ready-made materials is unchanged. Thus, they intrinsically exhibit monotonous luminescence. Unimodal luminescence has increasingly become familiar to counterfeiters and can be easily replicated, and the simple blending of different luminescent materials may lead to an uneven dispersion and a performance mismatch.19 Therefore, preparing novel multiple photo-stimuli-responsive materials in a single host material is necessary. For example, some species such as phosphorescence carbon dots,20,21 silicon nanoparticles,22 show excitation wavelength-dependent photoluminescence (PL) properties, which can achieve multicolor under different excitation wavelengths. Moreover, some inorganic nanocrystals emit dual-mode luminescence (downconversion and up-conversion luminescence) when they are excited with different laser pulses.23 Different from the above © XXXX American Chemical Society

two kind of materials, long-persistent afterglow materials show delayed luminescence even when the excitation is removed.24 Thus, the afterglow color and lifetime of this species can be used as additional authentication information. These multiple photo-stimuli materials possess extra security features compared with traditional static luminescent materials.25,26 Nevertheless, the development of wide-spectrum even full color tunable PL solid-state materials for security designs remains challenging. Here, we successfully develop a series of novel multiple photo-stimuli luminescent materials in the Na2CaGe2O6 system. By doping appropriate elements (e.g., Ln3+, Pb2+, Mn2+) and changing the synthesis temperature and environment, the trichromatic luminescent materials Na 2 CaGe 2 O 6 :Pb 2+ ,Y 3+ (blue), Na 2 CaGe 2 O 6 :Pb 2+ ,Tb 3+ (green), and Na2CaGe2O6:Pb2+,Mn2+,Yb3+ (red) are obtained. In this approach, we take advantage of simultaneously employing two different excitation wavelengths to generate synchronously tunable fluorescence. The fluorescent emission of these materials can be rapidly and reversibly fine-tuned over a wide range of UV wavelengths as a result of the different excitation efficiencies of dopant and co-dopant ions in the host material. In addition, the materials can produce blue, green, and red long-persistent luminescence, which is evidently distinct from the fluorescence color. Received: March 13, 2019 Accepted: May 3, 2019

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DOI: 10.1021/acsami.9b04213 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Rational design of multicolor luminescence through co-doping in Na2CaGe2O6 lattice. Proposed color combination of the multicolor PL mode and afterglow. (a) Between blue-emitting Pb2+ and white-emitting of the host materials. (b) Between blue-emitting Pb2+ and green-emitting Tb3+. (c) Between blue-emitting Pb2+ and red-emitting Mn2+.

Figure 2. XRD patterns of the as-prepared (a)NCG:Pb2+,Tb3+ ; (b) NCG:Pb2+,Mn2+,Yb3+; (c)NCG:Pb2+,Y3+ .

Figure 3. Multiple PL and afterglow properties of NCG:Pb2+,Y3+ phosphors, NCG:Pb2+,Tb3+ phosphors, and NCG:Pb2+,Mn2+,Yb3+. (a) Excitation spectra of the trichromatic materials monitored at 400 nm. (b) Excitation spectra of the trichromatic materials monitored at 400, 550, 600 nm, respectively. (c) Afterglow emission spectra.

creating photochargeable energy traps that participated in the deep-blue delayed emission under UV stimuli. Y3+ created the suitable trapping levels. As for Na 2 CaGe 2 O 6 :Pb 2+ ,Tb 3+ (abbreviated as NCG:Pb2+,Tb3+ or G) particles, Tb3+ is a typical green-emitting dopant,27 and was co-doped to endow Na 2 CaGe 2 O 6 with the color-tunable (green to blue) fluorescent emissions under the different UV excitation wavelengths (Figure 1b). For Na2CaGe2O6:Pb2+,Mn2+,Yb3+ phosphors (abbreviated as NCG:Pb2+,Mn2+,Yb3+ or R), the blue-emitting Pb2+ and red-emitting Mn2+ displayed a collective multiple PL mode under different UV excitations, thereby showing a color-tunable emission via excitation wavelength fine-selectivity. The incorporated Yb3+ ions acted as new trapping levels within the system, resulting in the red long-persistent luminescence (Figure 1c). The morphological characterization and crystal analysis of the materials were examined (Figures S1−S3). Scanning

Besides, an anticounterfeiting fabric was printed with a concealed pattern that is invisible under indoor light and can be revealed under UV light; then, a different afterglow color can be observed in a dark environment. The printed image obtained from these phosphors is impossible to imitate or counterfeit, and thus suitable for anticounterfeiting applications.

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RESULTS AND DISCUSSION In this study, we demonstrated the excitation wavelengthdependent bitemporal colorful luminescence in Na2CaGe2O6 for the first time. In the proof-of concept experiment, multilevel luminescent phosphors were constructed in a sequence from wavelength-dependent fluorescent emission to long-persistent luminescence. In Na2CaGe2O6:Pb2+,Y3+ phosphors (abbreviated as NCG:Pb2+,Y3+ or B) (Figure 1a), Na2CaGe2O6 was used as the host lattice and Pb2+ act as B

DOI: 10.1021/acsami.9b04213 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) Decay curves of the afterglow; (b) afterglow images taken at different delay times.

Figure 5. Dependence of the PL spectra on the excitation wavelengths. The inset shows the color-tunable PL photographs. (a) NCG:Pb2+,Y3+ phosphors, (b) NCG:Pb2+,Tb3+ phosphors, (c) NCG:Pb2+,Mn2+,Yb3+ phosphors, and (d) colorful luminescence photographs with a typical wavelength response.

ishing tendencies from 230 to 320 nm (Figure 3b), which are attributed to the transition of Tb3+ and Mn2+, respectively. NCG:Pb2+,Y3+ monitored at 400 nm shows another relatively weak excitation peak from 230 to 260 nm, which possibly belongs to the excitation of Na2.54Ca1.73Ge3O9. Interestingly, the excitation region from Pb2+ partially overlaps with the excitation bands of the co-doping ions. This overlapping indicates that the emission color of phosphors can be tuned by adjusting the excitation wavelength. Figure 3c shows the afterglow luminescence spectra after 254 nm excitation for 1 min. The resulting NCG:Pb2+,Tb3+ phosphors show a sharp afterglow emission band centered at 550 nm, which is highly consistent with the emission band of Tb3+.30 Moreover, Pb2+ does not display any afterglow emission band in the phosphors, and this finding agrees well with the result of the green afterglow images (Figure 4b). The afterglow color remains constant with decay time, indicating that the green persistent luminescence originates from the Tb3+ emitting centers and exists in the whole afterglow emission process. To enhance our understanding of the function of Pb2+ and Tb3+ in the host materials, we performed a comparison experiment of NCG:Tb3+ phosphors without Pb2+ doping at the same heating conditions. The results show that the PL spectra only show the characteristic peak of Tb3+ at 550 nm under 254 nm excitation (Figure S5). This finding indicates that the Pb2+ dopant in the phosphors is not the

electron microscopy (SEM) exhibits irregularly shaped particles (average size of ≈20 μm). The energy-dispersive spectrum (EDS) shows the presence of Na, Ca, Ge, and O elements but does not reveal the presence of the low-content Mn and the corresponding lanthanide ions. The EDS mapping images indicate that all of the constituent elements are uniformly distributed over the as-prepared single particles. The X-ray diffraction (XRD) patterns (Figure 2) of the representative NCG:Pb 2+ ,Y 3 + , NCG:Pb 2+ ,Tb 3 + , and NCG:Pb2+,Mn2+,Yb3+ were indexed to the trigonal phase. NCG:Pb2+,Tb3+ and NCG:Pb2+,Mn2+,Yb3+ are in agreement with the Joint Committee on Powder Diffraction Standards (JCPDS) card no. 79-2074,28 indicating the formation of purephase and highly crystalline Na2CaGe2O6. However, the phase structure of the NCG:Pb 2 + ,Y 3 + was attributed to Na2.54Ca1.73Ge3O9 (Inorganic Crystal Structure Database, ICSD no. #280853). The main reason is that the main phase converted from Na2CaGe2O6 to Na2.54Ca1.73Ge3O9 with the increased heating temperature. These results are similar to the published research.29 In Figure 3a, the PL excitation spectra monitored at an emission of 400 nm reveal that all of the phosphors can be efficiently excited by UV light from 250 to 320 nm which is assigned to the 1S0−3P1 transition of Pb2+. Moreover, the excitation spectra monitored at 550 and 600 nm for NCG:Pb2+,Tb3+, NCG:Pb2+,Mn2+,Yb3+ show gradually diminC

DOI: 10.1021/acsami.9b04213 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces luminescence centers. The afterglow images of NCG:Tb3+ exhibit a rapid decrease, and the green afterglow is no longer visible after 3 s of irradiation. The results demonstrate that Pb2+ doping into NCG:Tb3+ can significantly increase the decay time of afterglow, possibly because Pb2+ is indispensable to the creation of deep traps in the host material. The decay curves of the phosphors with and without Pb2+ dopant can be well fitted by treble exponential decay function and the average decay lifetimes were calculated. The result indicated that with the Pb2+ doping, the average decay lifetimes increased from 2.17 to 6.23 s (Figure S7, Table S1). The afterglow luminescence spectra of NCG:Pb2+,Mn2+,Yb3+ show orange-red emission assigned to the 4T1−6A1 transitions of Mn2+.31 The characteristic transition 3P1−1S0 of Pb2+ at approximately 400 nm is not detected, indicating that the luminescence centers in the phosphors are Mn2+ rather than Pb2+. For comparison, the NCG samples co-doped only with Mn2+, Yb3+ were prepared and show a red afterglow which is similar to that of NCG:Pb2+,Mn2+,Yb3+ (Figure S6). The average decay lifetimes were calculated according to the fitting result of the decay curves. The result indicated that the addition of Pb2+ ions in NCG:Mn2+,Yb3+ increased the average decay lifetimes from 3.39 to 105.7 s (Figure S8, Table S1). The afterglow luminescence spectra of NCG:Pb2+,Y3+ show the characteristic transition 3P1−1S0 of Pb2+ at approximately 400 nm.32 The luminescent properties of Pb2+ in host materials appear to be quite diverse. If Pb2+ is co-doped with luminescent ions such as Eu3+, then it unlikely shows the characteristic emission. By contrast, a broadband blue emission peak is detected, if dopant ion, such as Y3+ is a nonluminescent ion. This finding is consistent with results reported by Dai.33 Although Y3+ does not show luminescence, Y3+ co-doping plays a role in creating more electron traps. Figure 4 illustrates the afterglow decay curves of phosphors and the corresponding digital images of afterglow in all the samples. After the samples are exposed to an artificial light source irradiation (1000 lx) for 15 min at room temperature, blue, green, and red long-persistent luminescence can be observed by the naked eye for more than 30 min. The decay curves show that the decay rates of the three materials are similar, because the different luminescent centers are doped in the similar crystal field. The afterglow intensity of NCG:Pb2+,Mn2+,Yb3+ is slightly higher than the NCG:Pb2+,Y3+ and NCG:Pb2+,Tb3+. The increased afterglow intensity in NCG:Pb2+,Mn2+,Yb3+ is expected because of the considerably efficient thermal release of the trapped charges. Figure 5 shows the properties of the excitation wavelengthdependent PL emission from the samples. As show in Figure 5a, under the excitation of 250 nm, NCG:Pb2+,Y3+ exhibits a bright white-blue emission peak probably because of the superposition of the characteristic blue emission of Pb2+ and the white emission from the self-radiation of host materials. The blue emission is likely attributed to electron−phonon coupling because the phonon energy of the Pb2+ dopant matches well with that of the germanate-containing host. Thus, the blue emission may be attributed to the 3P1−1S0 transition from Pb2+ in the germanate system. The white emission of host materials may be a result of self-trapped exciton recombination which can be achieved by controlling the reaction temperature.32 Along with the increase of sintering temperature from 960 to 1100 °C, the PL emission at 254 nm irradiation gradually changes from deep-blue to white-blue, and then to white (Figure S4a). Besides, a control experiment without Pb2+

doping was conducted, and the result indicated that white fluorescence of the sample comes from the host lattices (Figure S4b). In Figure 5a, the emission intensity and emission color vary obviously as a function of the excitation wavelength. As the excitation wavelength increases from 250 to 320 nm, the intensity of the blue emission increases and reaches the maximum at 290 nm excitation. Afterward, it decreases rapidly. The result is highly consistent with the corresponding excitation spectrum. The white emission band only appears at a short excitation wavelength (250−260 nm). Therefore, we can simultaneously pump different ratios of blue/white emissions based on the excitation wavelength fine-selectivity, thereby obtaining a continuously tunable color from whiteblue to deep-blue. In addition, it was noted that the blue emission peak positions tended to shift to longer wavelengths as the excitation wavelength was increased. These results are in agreement with the previously reported band gap energies.34 Moreover, the PL quantum yields (QYs) at 254 and 295 nm are approximately 7.3 and 25.8%, respectively (Figure S10). Figure 5b shows multiple PL emission properties from NCG:Pb2+,Tb3+. The result presents the colorful fluorescence mode obtained by combining the blue emission of Pb2+ and the green emission of Tb3+, and this observation is attributed to the 3P1−1S0 transition of Pb2+ and 5D4−7F6 transitions of Tb3+. When the excitation wavelength changes from 250 to 320 nm, the trend of the blue emission is similar to that of NCG:Pb2+,Y3+ under the same excitation condition, whereas the green emission band gradually diminishes as the excitation wavelength increases. This result is due to the different excitation efficiencies of the two co-dopants. Thus, the luminescence color of NCG:Pb2+,Tb3+ gradually changes from green-blue to blue, because of the shift and relative change in the strength of Pb2+ and Tb3+ emission bands. The measured QYs are approximately 2.8% at the excitation of 254 nm and approximately 15.9% at 295 nm (Figure S11). The PL spectra of the NCG:Pb2+,Mn2+,Yb3+ powder also shows excitation-dependent features (Figure 5c). At 250 nm excitation, the phosphors have luminescent bands centered at 400 and 600 nm, which can be assigned to the 3P1−1S0 and 4 T1(G)−6A1(S) transitions of Pb2+, Mn2+ respectively. A similar condition is observed in NCG:Pb2+,Tb3+ exhibiting multiple trends. The fluorescence color changes from redviolet to blue-violet by adjusting the excitation wavelength based on the different change tendencies of Pb2+ and Mn2+ of emission band intensities. The phosphors exhibit a high QY of 29.3% at the excitation of 295 nm for the blue emission color and a low QY (6.8%) under 254 nm (red) (Figure S12). Typically, fluorescence and luminescence images show different emission colors, when these phosphors are excited at indoor light, 254 nm of UV light, 295 nm of UV light, and then ceasing the excitation (Figure 5d). Under indoor conditions, all of the phosphors are white. Under excitation of 254 nm, white-blue emission of NCG:Pb2+,Y3+, green-blue emission of NCG:Pb2+,Tb3+, and red-violet emission of NCG:Pb2+,Mn2+,Yb3+ were revealed. However, the emission colors of three phosphors change to deep-blue when the samples are stimulated at 295 nm. In addition, the phosphors emit deep-blue, green, and red long-persistent luminescence after excitation is removed, which is distinguished to be different with the corresponding fluorescence colors. Subsequently, the repeatability of the phosphors was tested. The PL measurement was executed every 1 h. The intensity did not D

DOI: 10.1021/acsami.9b04213 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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invisible (Figure 7a) but can be clearly felt by touch. At 254 nm UV excitation, the white-blue butterfly, the red-violet

show significant recession with irradiation cycles which shows a good reversibility of over 30 times (Figure S9). Inspired by the unique emission features of as-prepared particles, the dynamic bitemporal emission mode was designed by combining colorful fluorescence and afterglow for anticounterfeiting. As displayed in Figure 6, all of the

Figure 7. Dynamic multicolor anticounterfeiting design using the trichromatic materials: (a) anticounterfeiting image under indoor light; (b) at 254 nm excitation; (c) after removal of the excitation; (d) at 295 nm excitation.

flower, and the green-blue leaf can be clearly observed (Figure 7b). After switching off the UV light, the blue fluorescence disappears immediately, and the deep-blue luminescent butterfly, the red flower, and the green leaf appear, thereby achieving the second color change (Figure 7c). At 295 nm UV excitation, all of the parts of the image change to the deep-blue region, resulting in the third color change. This variability of luminescent images can clearly inspire many creative anticounterfeiting designs. Unlike conventional persistent luminescence phosphors, fluorescent quantum dots, and dyes, the as-prepared bitemporal luminescence materials exhibit dynamic multiple color change and thus show a higher security level in anticounterfeiting and security printing. Meanwhile, the toxicity of Pb in the phosphors should be considered in future applications. However, it would not be a fatal problem because many researchers have paid attention to lead-free luminescence materials and made great progress. For example, Leng et al.35 replaced Pb with nontoxic Bi ions in perovskite luminescent materials, indicating that bismuth may be a promising alternative. Zhang et al.24 successfully synthesized lead-free piezoelectric particles which show multiple-responsive colorful luminescence. In addition, Tao et al.36 reported an efficient organic ultralong room-temperature phosphorescence, suggesting that organic materials are another potential choice for toxic metal-free luminescent materials. We believe that our research will serve as guidelines for multiple-responsive luminescent materials in anticounterfeiting applications.

Figure 6. Illustration of excitation wavelength-dependent bitemporal colorful luminescence of the trichromatic materials (a) NCG:Pb2+,Y3+ phosphors; (b) NCG:Pb2+,Tb3+ phosphors; and (c) NCG:Pb2+,Mn2+,Yb3+ phosphors.

phosphors show distinguishable fluorescence colors that progressively shift to the deep-blue region from white blue, green-blue, and red-violet regions as the excitation wavelength increases. For NCG:Pb2+,Y3+ (Figure 6a), the deep-blue afterglow appears, which corresponds to the transition of Pb2+ after ceasing the special UV wavelength irradiation. The initial afterglow intensities under different wavelengths highly correspond to the PL excitation intensities of Pb2+ with a maximum at 290 nm. For NCG:Pb2+,Tb3+ and NCG:Pb2+,Mn2+,Yb3+ phosphors (Figure 6b,c), when UV irradiation at different wavelengths is stopped, the blue fluorescence from Pb2+ disappears, and the phosphors show only green and red afterglow from Tb3+, Mn2+, respectively. Moreover, the initial afterglow intensity gradually diminishes as the excitation wavelength increases because the excitation efficiencies of Tb3+ and Mn2+ decrease in a longer excitation wavelength. Considering the significantly different colors and similar decay time between the fluorescence and luminescence emission of the doped Na2CaGe2O6 materials, the newconcept time-resolved and color-coded multiple informational image can be designed using these promising persistent luminescence materials and cotton fabric. The multiple anticounterfeiting image contains three parts: the butterfly pattern made of NCG:Pb2+,Y3+ phosphors, the flower pattern composed of NCG:Pb2+,Tb3+ phosphors, and the leaf consisting of NCG:Pb2+,Mn3+,Yb3+ phosphors. Under natural light, the printed image on common cotton fabric is almost

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CONCLUSIONS In summary, we have introduced a new approach to luminescent materials for anticounterfeiting, more specifically doping of Pb2+, Mn2+, and Ln3+ elements in a single germanatecontaining host material that combine colorful fluorescence and long-persistent luminescence. As the excitation wavelength increases from 250 to 320 nm, the color of the formed phosphors changes from white-blue to deep-blue, green-blue to deep-blue, and red-violet to blue-violet, respectively. In addition, these phosphors can emit deep-blue, green, and red long-persistent afterglow after cutting off the excitation. This unique optical characteristic is more distinguishable than the monochrome emission and thus shows potential application in anticounterfeiting, information encryption, and photonic displays. The present study serves as a guide for the creation E

DOI: 10.1021/acsami.9b04213 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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of advanced anticounterfeiting materials and devices because of plenty of available candidate host materials and luminescent ions.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Natural Science Foundation of Jiangsu Province of China (BK20171140), the China Postdoctoral Science Foundation (2018M630521), the Key Laboratory of Textile Fiber & Product (Wuhan Textile University) (FZXW2017001), the Ministry of Education and Innovation Program for Graduate Education in Jiangsu Province (KYCX18_1822), the Jiangnan University Student Entrepreneurship Training Program (2018CYJ003), the National Natural Science Foundation of China (no. 51803076), the Natural Science Foundation of Jiangsu Province of China (no. BK20180629), China Postdoctoral Science Foundation (no. 2018M632231), China Postdoctoral Science Foundation (2017M611696), and Postdoctoral Science Foundation of Jiangsu Province (1701012B).

EXPERIMENT SECTION Synthesis of Phosphors. All of the samples were prepared by the high-temperature solid-state method. Na2CaGe2O6:Pb2+,Y3+ materials were prepared by heating amounts of Na2CO3, CaCO3, Ge2O3, PbO and Y2O3 at 1030 °C in air for 6 h. Na2CaGe2O6:Pb2+,Tb3+ samples were synthesized at 950 °C for 6 h in air among Na2CO3, CaCO3, Ge 2 O 3 , PbO, and Tb 4 O 7 . Na 2 CaGe 2 O 6 :Pb 2+ ,Mn 2+ ,Yb 3+ phosphors were sintered at 950 °C for 6 h under a reducing atmosphere. The starting materials were Na2CO3, CaCO3, Ge2O3, PbO, MnCO3, and Yb2O3. Then, all the sintered powders were ground and screened (30 μm sieve) to microparticles. Preparation of Anticounterfeiting Fabrics. Anticounterfeiting fabrics were prepared by screen-printing technique. The printing stock paste was obtained by directly incorporating and homogeneously dispersing the as-prepared phosphors and commercial printing ink. Then, a cotton matrix was directly printed on a flat printing screen with the slurry. The anticounterfeiting fabric was formed in an oven at 80 °C for 30 min (Figure S13). Structural Characterization. The as-prepared phosphors were characterized by using a standard X-ray powder diffractometer (D8, Bruker AXS, Germany) with Cu Kα radiation at room temperature. Surface morphology and elemental distribution maps were characterized by a fieldemission SEM equipped with an EDS (S-1510, Hitachi, Japan). The PL spectra of the materials were recorded by using a fluorescence spectrometer (FS-5, Edinburgh, UK) equipped with a 450 W xenon arc lamp at room temperature. The persistent luminescence decay curves of the materials were measured by a long afterglow instrument (PR-305, Zhejiang University Sensing Instrument Co., China). The samples were first exposed to an artificial light source (1000 lx) for 15 min, and data were recorded after irradiation is terminated. The photographs of the fluorescent or luminescent phenomena were obtained by using a digital camera (G15, Canon, Japan).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04213. SEM images, EDS data, and EDS mapping of the phosphors; effect of sintered temperature on PL spectra; contrast of PL spectra and photographs between Pb2+ doping samples and without Pb2+ doping samples; afterglow decay curves and the corresponding exponential fitting; reversible PL intensity of 30 consecutive cycles; PL QYs of the phosphors; and schematic diagram of the screen printing process (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mingqiao Ge: 0000-0003-0517-3499 F

DOI: 10.1021/acsami.9b04213 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.9b04213 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX