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## Creator

Ao Guo, Qi Zhu, Shimeng Zhang, Xudong Sun, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361)

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[A building-block strategy for dynamic anti-counterfeiting by using (Ba,Sr)Ga2O4:Sm3+ new red persistent luminescent phosphor as an important component](https://mdr.nims.go.jp/datasets/21d04f59-8b02-4e65-a82d-9e6ea7df38ae)

## Fulltext

1  A building-block strategy for dynamic anti-counterfeiting by using (Ba,Sr)Ga2O4:Sm3+ new red persistent luminescent phosphor as an important component    Ao Guoa, Qi Zhua,*, Shimeng Zhanga, Xudong Sunb and Ji-Guang Lic,*  aKey Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China bFoshan Graduate School of Northeastern University, Foshan, Guangdong 528311, PR China cResearch Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan  *Corresponding author Dr. Qi Zhu Tel: +86-24-8367-2700 E-mail: zhuq@smm.neu.edu.cn  Dr. Ji-Guang Li Tel: +81-29-860-4394 E-mail: LI.Jiguang@nims.go.jp   Revised Manuscript (Clean Version) Click here to view linked References 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 mailto:zhuq@smm.neu.edu.cnmailto:LI.Jiguang@nims.go.jphttps://www.editorialmanager.com/ceri/viewRCResults.aspx?pdf=1&docID=156842&rev=2&fileID=2756278&msid=4506043c-6aa2-4300-ab59-ef97f16f7d9ehttps://www.editorialmanager.com/ceri/viewRCResults.aspx?pdf=1&docID=156842&rev=2&fileID=2756278&msid=4506043c-6aa2-4300-ab59-ef97f16f7d9e2  Abstract  Long persistent luminescence materials developed to commercial standards are primarily concentrated in the blue and green regions, with only a few in the red region. Red, as one of the three basic colors, can be mixed in various proportions with blue and green to yield various colors. The development of red persistent phosphors has a broader application potential but remains a challenge. A solid-state reaction method was used to synthesize new red persistent luminescent materials of Ba1-xSrxGa2O4:Sm3+ (x = 0–0.09). In BaGa2O4, both Sr2+ and Sm3+ preferentially occupy the Ba2+ site rather than the Ga3+ site. When exposed to UV light at 254 nm, the phosphors emit the characteristic red emission of Sm3+ at wavelengths ranging from 500 nm to 750 nm. After removing the UV light source, an intense red afterglow that lasted more than 1400 seconds was observed. The red afterglow signal reappears after a heating process. Doping Sr2+ reduces the trap depth and improves the red persistent luminescence significantly. Because the escaped electrons from traps compensate for the emission loss of Sm3+ during the heating process, the red phosphors have unimaginably luminescent thermal stability. Thus, the emission intensity at 200 °C is 1.6 times that at room temperature. The prepared red persistent phosphors show multimode luminescence, with the output signal being time and temperature sensitive, indicating that they are potential luminescent materials for anti-counterfeiting applications. Finally, a building-block strategy for advanced anti-counterfeiting applications of dynamic display information is proposed, with red persistent phosphors serving as an important component combined with upconversion phosphors of NaYF4:Yb3+, Tm3+, and green persistent phosphors of SrAl2O4:Eu2+, Dy3+.  Keywords:  Long persistent luminescence; Temperature-dependent luminescence; Anti-counterfeiting; BaGa2O4; Sm3+   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 3  1. Introduction There are some luminescent materials, that can continue to emit light for a certain period of time even after the external light source excitation is eliminated [1,2]. These materials are known as long persistent luminescence materials, and they are commonly used in places that require low lightings, such as night lights, traffic signs, and indoor safety escape signs [3,4]. They have recently been employed in radiation detection and optical storage media after adequate research and development [5,6]. Long persistent luminescent materials have been used in anti-counterfeiting, in vivo imaging, and medical diagnosis [7-10]. At the moment, materials with green and blue afterglow, such as SrAl2O4:Eu2+, Dy3+ (green), and CaAl2O4:Eu2+, Nd3+ (blue), have reached commercial standards, whereas red long persistent luminescence materials are difficult to attain these standards in terms of afterglow time and brightness [11,12]. Because red is one of the three basic colors, it can be mixed in various proportions with blue and green to produce various colors, giving it a broader application prospect [13-16]. Therefore, it is critical to developing red long persistent luminescence materials with superior properties. Typically, emitting centers are critical for materials with persistent luminescence. The emitting color is always determined by the doped ions and the host materials. The host is fixed, but the doped ions can be changed. In other words, the afterglow color of the synthesized afterglow materials will be affected by the choice of different doped ions. When excited under certain conditions, Eu2+, Eu3+, Mn2+, Sm3+, or Pr3+ can produce red light [17-21]. Furthermore, through energy transfer processing [22], the emitting color of persistent luminescence materials can be changed because energy transfer from one ion to another doped ion during afterglow decay contributes to another afterglow, including red afterglow. This type of red long persistent luminescence was discovered in Sr3MgSi2O8-1.5xNx:Eu2+,Dy3+,Mn2+, and BaMg2Si2O7:Eu2+,Dy3+,Mn2+ [23,24]. However, the duration and brightness of the afterglow are short and dim. Red long persistent luminescence materials such as Ca2SnO4:Sm3+, Y2O2S:Sm3+, and Sr2SnO4:Sm3+ have been studied recently [25-27].  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4  The typical transition between Sm3+ electronic configurations has been reported to contribute to a strong red emission [28]. As a result, Sm3+ is always used in red phosphors and red long persistent luminescence materials. In addition to being influenced by doped ions, the intensity and duration of the persistent afterglow are determined by phosphor traps [29]. When the excitation is stopped, the trapped electrons or holes will be released to the emitting centers via the conduction or valence bands. It is believed that the traps are related to defects. Li et al. discovered a novel red-orange-emitting phosphor, BaGa2O4:Bi3+, with BaGa2O4 as the host [30]. Because of the efficient traps formed by defects during the formation of BaGa2O4, BaGa2O4:Bi3+ exhibited a long afterglow at extremely low temperatures [31]. Furthermore, this phosphor has excellent thermal stability, as its integrated intensity at 200 °C retains more than 88.1% of that at room temperature. As a result, this persistent luminescence material has promising applications in both extremely low and high-temperature conditions. As a result, BaGa2O4 could be used as a suitable host for long persistent luminescence phosphors. Except for the host, the defects can be formed by doped ions occupying the site of host matrix ions with a different valence or ionic radius. A solid-state reaction method was used to synthesize Ba1-xSrxGa2O4:0.01Sm3+ (x = 0–0.09), a group of red persistent phosphors, with smaller Sr2+ substituting the larger Ba2+ to improve the persistent luminescence of BaGa2O4:Sm3+. The samples were then characterized using XRD, UV-Vis, HRTEM, SAED, TL, PLE/PL spectroscopy, and persistent luminescence decay analysis. Finally, the possibility of their use in anti-counterfeiting applications was investigated.  2. Experimental section  2.1. Sample preparation Ba1-xSrxGa2O4:0.01Sm3+ (x = 0, 0.01, 0.03, 0.05, 0.07, 0.09) samples, which are described as BGSO:Sm3+, were synthesized by the high-temperature solid-state reaction method. BaCO3, Ga2O3, Sm2O3, and SrCO3 were chosen as the raw materials,  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5  all purchased from Sinopharm (Shanghai, China) with a purity of 99.9%. Based on the stoichiometric ratio after calculation, all the raw materials were weighted by the electronic balance. The powders were ground for 30 min and then the mixture was calcined at 1400 oC for 8 hours. When cooling to room temperature, the powders were tested after ground again. 2.2. Characterization The powder X-ray diffraction patterns were collected by X-ray diffraction (XRD, Model SmartLab, Rigaku, Tokyo, Japan) at the scanning rate of 8 min-1 in the 2θ range from 10o to 70o，operating at 40 mA and 40 kV using monochromated Cu Kα as radiation. The photoluminescence spectra (PL)/photoluminescence excitation spectra (PLE) of samples were obtained by the FP-8600 (JASCO, Tokyo) with a 150 W Xe-lamp as the excitation source. The diffuse reflectance spectra of the samples were acquired by a UV-vis spectrophotometer (UV-3600 Plus, Shimadzu, Kyoto, Japan). Long persistent luminescence spectra were recorded via the Horiba JY Fluorolog-3 (Kyoto) spectrofluorometer. An FJ-427A TL spectrofluorometer (Beijing Nuclear Instrument Factory, Beijing, China) at a heating rate of 1 K s-1 was used to gain Thermoluminescence (TL) curves after the samples were excited for 5 min by a UV lamp. In order to study the application in anti-counterfeiting, the commercial persistent phosphor of SrAl2O4:Eu3+, Dy3+ (Luming Science and Technology Group Co., Ltd, China) was chosen here as a green light source. The afterglow luminance of sample was measured by a spectroradiometer (HS-1000, Photal Otsuka Electronics, Osaka, Japan) with a built-in software of the system.  3. Results and discussion 3.1. Synthesis and crystal structure Fig. 1a depicts the XRD patterns of BGSO:Sm3+ (x = 0–0.09). All the XRD diffraction peaks are consistent with the standard patterns of BaGa2O4 (JCPDS NO. 46-0415), indicating the formation of a single phase of BGSO:Sm3+. Because the ionic radii of Sm3+ (1.02 Å, CN = 7; 0.958 Å, CN = 6) and Sr2+ (1.21 Å, CN = 7; 1.18  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 6  Å, CN = 6) are close to that of Ba2+ (1.38 Å, CN = 7) and much larger than that of Ga3+ (0.47 Å, CN = 4), Sm3+ and Sr2+ ions are thought to substitute for Ba2+ sites. Furthermore, because the ionic radius of Sm3+ and Sr2+ is smaller than that of Ba2+, the incorporation of Sm3+ and Sr2+ causes the shrink of lattice. The enlarged drawing of the (222) main diffraction peaks is show in Fig.1b. As the x value increases from 0 to 0.09, the (222) diffraction peak shifts to the higher angle side. This is primarily due to that smaller Sr2+ ions substitute the larger Ba2+ ions, causing the lattice to shrink. To test this inference, the Rietveld refinement results of the XRD pattern for BGSO (x = 0.07 sample) are analyzed and shown in Figure S1. The calculated results agree with the experimental data. The Rwp, Rp, and Rexp values are quite low, indicating that the results reliable. Interstitial oxygen is generated when Ba2+ ions are replaced by Sm3+ ions to compensate for the charge difference. This could contribute to long persistent luminescence. The unit cell structure of BaGa2O4 is depicted in Fig. 2a. The hexagonal structure of BaGa2O4 belongs to the P63 space group. BaGa2O4 comprises six-membered ring layers of GaO4 tetrahedra that are perpendicular to the c-axis, with its layers stacked to form a three-dimensional framework. Tunnels are formed by the superposition of the layers parallel to the c-axis, and the barium cations are in these tunnels. There are two types of barium ions: one with nine oxygen atoms located in a more symmetrical Ga-O tetrahedron six-membered ring, and the other with seven and eight oxygen atoms located in a more irregular Ga-O tetrahedron six-membered ring (Fig. 2b) [32]. Figures S2 and 2c show the FE-SEM and TEM morphology of BGSO:Sm3+ (x = 0.07) sample, indicating a particle size of 1–5 μm. The HRTEM image (Fig. 2c) and SAED patterns were examined (Fig. 2d). The HRTEM image's clear lattice fringes indicate that the sample has good crystallinity. The distance between the two crystal planes is about 0.325 nm. As a result, they correspond to the (411) crystallographic plane (d(411) = 0.32612 nm, JCPDS No. 46-0415). The calculated values of the diffraction spots, as shown in Fig. 2d, are approximately 0.325 nm, 0.352 nm, and 0.432 nm, which correspond to the (411), (410), and (00-1) planes, respectively (d(410) =  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 7  0.35216 nm, d(00-1) = 0.43326 nm, JCPDS No. 46-0415). The elemental distribution results in Fig. 2e-g show that Sr2+ and Sm3+ are successfully doped and that the chemical elements are homogeneously distributed.  3.2. Photoluminescence properties Fig. 3a depicts the diffuse reflectance spectra of the BGSO (x = 0, 0.05, and 0.09) samples. The peak from 200 nm to 300 nm is attributed to the absorption of BaGa2O4 host [33]. The substitution of Sr2+ ions for Ba2+ sites did not affect the absorption. The Kubelka-Munk formula was used to calculate the band gap energy of the hosts [34]:  RRF(R)2)-(1 = 2 )-A(= )( 2gEhναhν  where R denotes the reflectivity of the sample, α denotes the absorption coefficient, which is proportional to F(R), h denotes Planck’s constant, ν denotes the light frequency, and A denotes the proportionality constant. The intercept of the diffuse reflection spectra transformed by the Kubelka-Munk function F(R) in Fig.3b can be used to calculate the bandgap energy. The bandgap of BaGa2O4 is approximately 5.01 eV. The band gap shrinks slightly as the Sr2+ doping content increase because the determined values are around 4.96 eV for x = 0.05 and x = 0.09 samples, which are smaller than 5.01 eV. The PLE and PL spectra of BaGa2O4:0.01Sm3+ are shown in Fig.4a. The O2-→Ga3+ charge transfer band is assigned to the strong band at 254 nm in the PLE spectra [33]. Because the host absorption of BaGa2O4 is close to the charge transfer band, the strong band could overlap the charge transfer band and the host absorption band. Sharp peaks ranging from 300 nm to 450 nm correspond to the Sm3+ intra-4f5 transitions, with the 6H5/2→4K11/2 transition at 404 nm dominating [28]. The PL spectra obtained at an excitation wavelength of 254 nm show four narrow emission bands attributed to the 4G5/2 to the 6H5/2 (568 nm), 6H7/2 (608 nm), 6H9/2 (651 nm), and 6H11/2 (707 nm) transitions of Sm3+, with the sharp peak at 608 nm dominating [35,36].  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 8  The persistent luminescence spectra of BaGa2O4:0.01Sm3+ at room temperature are shown in Fig. 4b, which are nearly identical to the emission spectra of Sm3+. When the sample is excited by a 254 nm UV light, four emission bands are observed at 568 nm, 608 nm, 651 nm, and 707 nm. Fig. 4c depicts the decay curve of persistent luminescence. A UV lamp was used to excite the sample for 5 min at room temperature. The red afterglow of the samples lasts more than 1400 seconds. The schematic energy level diagrams of BaGa2O4:0.01Sm3+ in Fig. 4d show the possible luminescence mechanism. The ground-state electrons of Sm3+ ions are promoted to the conduction band by UV irradiation. The excited states of Sm3+ are degenerated with free electron-hole states, with an electron at the bottom of the conduction band and a hole at the top of the valence band, because of the UV light excitation. Some excited electrons are delocalized in the CB and are captured by the GaO4 tetrahedron. The electrons released from the GaO4 tetrahedron to VB contribute to the blue emission [37], as evidenced by the host emission (Figure S3). Simultaneously, some electrons are relaxed to the excited levels of Sm3+ and relaxed to the 4G5/2 level, and the energy relaxes through the transitions from 4G5/2 to the 6H5/2, 6H7/2, 6H9/2, and 6H11/2 levels, contributing to the emission at 568, 608, 651, and 707 nm. Furthermore, the possible energy transfer from the GaO4 tetrahedron to the 4G5/2 energy level contributes to Sm3+ emission. When electrons are excited from the valence band to the conduction band by UV excitation, some electrons are captured and retained by the electron trap. During the illumination period, the electron traps are filled. When the ultraviolet irradiation excitation stops, the trapped electrons escape to the conduction band and then to the excited energy levels of Sm3+, contributing to the afterglows [37, 38]. Electronic traps are critical for all long persistent luminescence materials. Shallow traps require less energy to allow captured electrons to escape more easily, comparing to deep traps. We used thermoluminescence (TL) to determine the trap depth of the samples. The TL glow curves for the samples as shown in Fig. 5a. The TL curves are not symmetrical, and there are two peaks in the TL curves corresponding to the shallow and deep traps, respectively. These two peaks are caused  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 9  by two different defects: charge compensation defects caused by Sm3+ replacing Ba2+ and the lattice distortion defects formed by the substitution of Sm3+ and Sr2+ for Ba2+.  The trap depth can be calculated by the following equation [39]:  500 = mTE  where Tm denotes the temperature for which the glow curve reaches a maximum, and E denotes the approximate trap depth. The trap depth for each sample is summarized in Table 1. The peaks of T1 and T2 shift to the lower temperature side as the x value increases, indicating that Sr2+ doping makes the traps shallower on the overall trend. Fig. 5b depicts the decay curves of persistent luminescence. A UV lamp was used to excite all the samples for 5 min at room temperature. The red afterglow in all samples lasts longer than 1400 seconds, because the afterglow luminance at 1400 s is estimated to be ~0.7–1.1 mcd/m2, which is greater than the value of 0.32 mcd/m2 [40]. Obviously, there are two types of laws for the decay curves at the initial and later parts. Increasing the x value from 0 to 0.07 results in a more intense afterglow for the initial duration (before 35 s), but increasing the x value from 0.07 to 0.09 results in a decline. However, for the later duration, a higher x value from 0 to 0.09 results in a more intense afterglow (after 35 s). This is because the intensity of the afterglow is affected by trap number and trap depth. The captured electrons can easily escape from the shallower traps, contributing to the stronger afterglow. Additionally, more electrons captured by more electron traps may contribute to a stronger afterglow. Because the x = 0.07 sample has the shallowest trap depth and the more trap number compared to the samples with x smaller than 0.07, it has the most intense afterglow at the initial stage. However, because of the larger electron trap numbers, the x = 0.09 sample exhibits a more intense afterglow than that for the x = 0.07 sample at the later stage. The results show that lattice distortion caused by substituting Ba2+ with Sr2+ can enhance the afterglow intensity.     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 10  Table 1 Calculated trap depths of BGSO:Sm3+ (x = 0, 0.05, 0.07, 0.09). x T1 T2 Trap I depth/eV Trap II depth/eV 0 98 139 0.742 0.824 0.05 83 121 0.712 0.788 0.07 79 115 0.704 0.776 0.09 87 118 0.720 0.782  In Fig. 6a, the change in emission peaks can be observed with increasing heating temperature. In the case of BGSO:Sm3+ (x = 0.07), increasing the temperature yields an enhanced emission intensity under 254-nm UV lamp irradiation. The emission intensity at 200 °C is more than 160% of that at room temperature and about 80% of that at room temperature at 300 °C (Fig. 6b). The increased emission intensities are caused by captured electrons released from electron traps that combine with the excited state of Sm3+ by heat processing, which compensates for the emission loss caused by thermal quenching. This phenomenon was discovered previously in our research on ZnGa2O4:Cr3+ persistent luminescent materials [41,42]. Fig. 6c shows the persistent luminescence decay curves for the samples after 30 s of heating at 150 °C. Clearly, afterglow can be seen for all samples, with more intense afterglow found at a higher x value, which is nearly the same as that without heat processing. The afterglow intensity of the x = 0.07 sample, however, exhibits the fastest decline among the samples because it has the shallowest trap depth and the electrons are emptied most easily. As a result, the afterglow of the x = 0.07 sample is highly sensitive to temperature. The BGSO:Sm3+ (x = 0.07) sample was placed in cylindrical containers. As shown in Fig. 6d, as the temperature rises, the phosphors exhibit increased red emission when excited by 254-nm UV light. It can be seen that the emission intensity of BGSO:Sm3+ (x = 0.07) is the highest at 200 °C. Increasing the temperature further contributes to a decrease of the red signal. The luminescence mechanism after heat processing is depicted schematically in Fig. 6e. At elevated temperatures, the trapped electrons escape to the CB and combine with the excited energy levels of Sm3+, which compensates for the emission loss caused by the thermal  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 11  quenching of Sm3+. As a result, the intensity of the red emission increases after heating, and the sample exhibits a temperature-dependent luminescence.  3.3. Anti-counterfeiting applications To investigate the anti-counterfeiting application, BGSO:Sm3+ (x = 0.07) and NaYF4:Yb3+, Tm3+ phosphors were mixed with a mass ratio of 1:1. The PL spectra and the appearance of the mixture phosphor under 980 nm laser excitation (2.5 W) are shown in Fig. 7a. The PL spectra show several emission bands corresponding to the 1D2→3H6 (361 nm, purple), 1D2→3F4 (450 nm, blue), 1G4→3H6 (473 nm, blue), 1G4→3F4 (646 nm, red), 3F2→3H6 (695 nm, red), and 3F3→3H6(723 nm, red) transitions of Tm3+ [43]. The images show that the mixture phosphor emits a blue upconversion light. The PL spectra and appearance of the mixture phosphor obtained after excitation at 254 nm are shown in Fig. 7b. Sm3+ also exhibits four narrow emission peaks at 568 nm, 608 nm, 651 nm, and 707 nm. The red emission of the mixture phosphors can be seen by the naked eye. The persistent luminescence spectra of the mixture phosphor at room temperature, which is assigned to the red afterglow of Sm3+, are shown in Fig. 7c. The persistent luminescence decay curve of the mixture phosphor, which was excited by a 254 nm UV lamp for 5 min, is shown in Fig. 7d. The red afterglow from the mixture phosphor lasted more than 600 seconds. The prepared samples could be successfully employed in advanced anti-counterfeiting applications due to their different emission performances at different exciting wavelengths. To capture the emission signals of the samples under different excitation conditions, the BGSO:Sm3+ (x = 0.07) sample and the mixture phosphor are placed in the “B” container fabricated by metal 3D printing (Fig. 7e). The phosphor 1 is BGSO:Sm3+ (x = 0.07), and the phosphor 2 is a mixture of the two. Under natural light, all the patterns are white “B”. The four patterns showed bright red “B” and dark red “B” emissions at 254-nm UV excitation and after removing the 254-nm UV excitation lamp, respectively (Fig. 7e). The four patterns, however, exhibited blue “B”, “p”, “b”, and “D” under 980-nm laser excitation.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 12  BGSO:Sm3+ (x = 0.07) and SrAl2O4:Eu2+, Dy3+ phosphors were mixed with weight ratios of BGSO:Sm3+ (x = 0.07) to SrAl2O4:Eu2+, Dy3+ at 200:1 and 150:1, respectively. The PL spectra of the two phosphors mixtures obtained under excitation at 254 nm are shown in Fig. 8a. The PL spectra of the two mixture phosphors show four emission peaks from Sm3+ at 568 nm, 608 nm, 651 nm, and 707 nm, but the emission intensity of Sm3+ is weaker than that before mixing. The green emission of SrAl2O4: Eu2+, Dy3+ is visible with the broadband at 515 nm [44]. The two emission intensities for the two mixed powders were compared. The I(515)/I(608) ratio for the mixture with the weight ratio of BGSO:Sm3+ (x = 0.07) to SrAl2O4:Eu2+, Dy3+ as 200:1 is approximately 0.6. However, I(515)/I(608) is about 1.2 for the mixture with the weight ratio of BGSO:Sm3+ (x = 0.07) to SrAl2O4:Eu2+, Dy3+ as 150:1. It implies that increasing the BGSO:Sm3+ content would cause the emission color shifting to the red range. The persistent luminescence spectra of the two mixture phosphors are shown in Fig. 8c. In the absence of 568 nm and 707 nm emission peaks for the phosphors mixture (the weight ratio of BGSO:Sm3+ to SrAl2O4:Eu2+, Dy3+ is 200:1), afterglow emission peaks at 515 nm, 608 nm and 651 nm are observed, indicating that the afterglow is yellow. The persistent luminescence spectra of the mixture phosphors (weight ratio of BGSO:Sm3+ to SrAl2O4:Eu2+, Dy3+ is 150:1) only has a 515-nm peak, and the Sm3+ emission peak disappears, illustrating that the afterglow is green. The relative intensities of 515 nm and 608 nm at different temperatures are shown in Fig. 8d, with the weight ratio of BGSO:Sm3+ (x = 0.07) to SrAl2O4:Eu2+, Dy3+ of 150:1. As the temperature rises, the intensity of the green emission peak at 515 nm decreases steadily. It is worth nothing that the red emission of the mixture increases with an increased temperature. The red emission intensity of the mixture is highest at 150 °C, in contrast to that of BGSO:Sm3+, which has the highest intensity at 200 °C.  The samples were placed in cylindrical containers labeled A, B, C, and D to capture the emission signal under different excitation conditions. In each condition, sample A emits red and sample D emits green, as shown in Fig. 9. With the increased content of SrAl2O4:Eu2+, Dy3+, the phosphors change color from red to green under the excitation of 254-nm UV light. In contrast, samples B and C show a color change  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 13  from red to yellow after 30 seconds of excitation. After 5 min of continuous excitation with 254 nm UV light, the sample emits an afterglow that changes from red to yellow and then to green with increased content of SrAl2O4:Eu2+, Dy3+. Sample B's afterglow is yellow, whereas the afterglow of sample C is green. The red-light emission of samples A, B, and C increases with increasing the heating temperatures. The green emission of sample D, however, decreases gradually. Because the mixed phosphor demonstrated time- and temperature-dependent multimode luminescence, it has great potential in advanced anti-counterfeiting applications. Here, an apple tree logo was prepared using four powders as raw materials. Fig. 10 shows the images under 254 nm UV excitation and the afterglow images at room temperature. Under the excitation of 254-nm UV light, the three apples were red and ripe. After 30 seconds of excitation, apples B and C turned yellow and immature. The afterglow of apple B was also yellow, but the afterglow of apple C turned green. Under UV excitation at 254 nm, all three apples turned bright red at 100 °C and 200 °C. The red light from three apples became stronger, while the green glow from the tree trunk became weaker. At 300 °C, the light from trunk became more weaker, while the red light from the three apples weakened but remained bright.  4. Conclusion  Red long persistent luminescent materials of Ba1-xSrxGa2O4:Sm3+ (x = 0–0.09) were synthesized using a solid-state reaction method and characterized using XRD, UV-Vis, HRTEM, SAED, TL, PLE/PL spectroscopy, and persistent luminescence decay analysis. Both Sr2+ and Sm3+ preferentially occupy the Ba2+ site rather than the Ga3+ site in BaGa2O4. Doping with Sr2+ and Sm3+ does not significantly affect the crystal structure but causes lattice shrinkage and a narrower band gap. The samples exhibit characteristic Sm3+ emission with emission peaks ranging from 500 nm to 750 nm, belonging to G5/2 to the 6H5/2 (568 nm),6H7/2 (608 nm), 6H9/2 (651 nm), and 6H11/2 (707 nm) transition of Sm3+. Doping Sr2+ reduces the trap depth and improves red  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 14  persistent luminescence significantly. The Ba0.93Sr0.07Ga2O4:0.01Sm3+ sample exhibits the brightest and strongest red afterglow. Because the escaped electrons from traps compensate for the emission loss of Sm3+ during the heating process, the red phosphors have unimaginably luminescent thermal stability. The emission intensity at 200 °C is 1.6 times that at room temperature. The prepared red persistent phosphors exhibit time- and temperature-dependent luminescence, indicating that they could be used as multimode luminescent materials in anti-counterfeiting applications. Finally, a building-block strategy for advanced anti-counterfeiting applications is proposed, using the red phosphor as an important component combined with an upconversion phosphor and a green persistent phosphor.  Acknowledgments This work was supported in part by the Natural Science Foundation of Liaoning Province (Grant 2020-MS-081), and National Natural Science Foundation of China (Grant 51302032, 51972047, 52172112).  References [1] Y. Li, M. 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Yin, Thermometric and optical heating bi-functional properties of upconversion phosphor Ba5Gd8Zn4O21:Yb3+/Tm3+, J. Mater. Chem. C. 3 (28) (2015) 7379-7385. [44] Hai, M. K. Pei, Q. Ren, X. L. Wu, E. L. Yang, D. Xu, J. F. Zhu, Ag nanoparticles significantly improve the slow decay brightness of SrAl2O4:Eu2+, Dy3+ by the surface plasmon effect, Dalton. T. 51 (6) (2022) 2287-2295.                1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 18   Fig. 1. (a) X-ray diffraction patterns and (b) enlarged (222) diffraction of BGSO:Sm3+ (x = 0- 0.09).               Fig. 2. (a) Crystal unit cell of BaGa2O4 viewed along the c-axis, (b) two different types of six-membered rings consisted of Ga-O tetrahedron, (c) TEM image, (d) HR-TEM image, (e) SAED pattern, and (f-j) element distribution of BGSO:Sm3+ (x = 0.07).  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 19    Fig. 3. (a) Diffuses reflection spectra and (b) the bandgap energies of the BGSO (x = 0, 0.05, 0.09).   Fig. 4. (a) PLE and PL spectra, (b) persistent luminescence spectra, (c) persistent luminescence decay curve of BaGa2O4:0.01Sm3+, and (d) schematic illustration of the luminescence mechanism for BaGa2O4:0.01Sm3+.    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 20   Fig. 5. (a) TL glow curves for BGSO:Sm3+ (x = 0, 0.05, 0.07, 0.09) and (b) the persistent luminescence decay curves (monitored at 608 nm after 254-nm UV light illumination for 5 min).                  Fig. 6. (a) PL spectra of BGSO:Sm3+ (x = 0.07) at different temperatures, (b) the relative intensities of 608 nm at different temperatures for BGSO:Sm3+ (x = 0.07), (c) persistent luminescence decay curves of BGSO:Sm3+ (x = 0-0.09) after heating at 150 oC, (d) appearances of BGSO:Sm3+ (x = 0.07) phosphors at different temperatures  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 21  under 254 nm light excitation, and (e) schematic illustration of the luminescence mechanism for BGSO:Sm3+ after heat processing.      Fig. 7. PL spectra and the appearance of the mixture phosphor (a) under 980 nm laser excitation (2.5 W) and (b) under 254-nm UV light excitation, respectively. (c) and (d) are the persistent luminescence spectra and the afterglow and the persistent 550 600 650 700 7501000200030004000500060004G5/2-6H11/24G5/2-6H9/24G5/2-6H7/24G5/2-6H5/2Intensity/a.u.Wavelength/nmλex=254 nm(b)0 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 22  luminescence decay curves of the mixture phosphor. The mixture phosphor is composed of Ba0.93Sr0.07Ga2O4:0.01Sm3+ and NaYF4:Yb3+,Tm3+ with the weight ratio of 1:1. (e) is the designed logo for anti-counterfeiting application, with phosphor 1 as Ba0.93Sr0.07Ga2O4:0.01Sm3+ and phosphor 2 as the mixture phosphor.                            Fig. 8. (a) PL spectra, (b) the persistent luminescence spectra, and (c) persistent luminescence decay curves of two typical mixture phosphors, with the weight ratio of Ba0.93Sr0.07Ga2O4:0.01Sm3+ to SrAl2O4:Eu3+, Dy3+ as 200:1 and 150:1, respectively. (d) is the relative intensities of 515 nm and 608 nm at different temperatures for the mixtures, with the weight ratio of Ba0.93Sr0.07Ga2O4:0.01Sm3+ to SrAl2O4:Eu3+, Dy3+ as 150:1.    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 23  .  Fig. 9. Appearances of four typical samples under daylight, 254 nm UV light for 0s and 30 s at different temperatures, and the afterglows after removing the UV light source. Sample A is Ba0.93Sr0.07Ga2O4:0.01Sm3+ and sample D is the mixture of BaSO4 and SrAl2O4:Eu3+, Dy3+ (the weight ratio is 100:1). The weight ratio of Ba0.93Sr0.07Ga2O4:0.01Sm3+ to SrAl2O4:Eu3+, Dy3+ is 200:1, 150:1 for samples B and C respectively.  Fig. 10. Appearances of the signals for the pattern of "apple tree" with four typical samples placed in the designed container fabricated by metal 3D printing.       1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 24   Supporting Information  A building-block strategy for dynamic anti-counterfeiting by using (Ba,Sr)Ga2O4:Sm3+ new red persistent luminescent phosphor as an important component    Ao Guoa, Qi Zhua,*, Shimeng Zhanga, Xudong Sunb and Ji-Guang Lic,*  aKey Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China bFoshan Graduate School of Northeastern University, Foshan, Guangdong 528311, PR China cResearch Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan   *E-mail: zhuq@smm.neu.edu.cn and LI Jiguang@nims.go.jp            1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 mailto:zhuq@smm.neu.edu.cnmailto:Jiguang@nims.go.jp25      Figure S1. Rietveld refinements of BGSO (x=0.07 sample).       Figure S2. FE-SEM image of BGSO:Sm3+ (x = 0.07) powder.   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 26        250 300Intensity (a.u.)Wavelength (nm) BaGa2O4λem=463 nm(a)300 400 500 600 700Intensity (a.u.)Wavelength (nm) BaGa2O4λex=238 nm(b) Figure S3. (a) PLE spectra and (b) PL spectra of BaGa2O4.    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1  A building-block strategy for dynamic anti-counterfeiting by using (Ba,Sr)Ga2O4:Sm3+ new red persistent luminescent phosphor as an important component    Ao Guoa, Qi Zhua,*, Shimeng Zhanga, Xudong Sunb and Ji-Guang Lic,*  aKey Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China bFoshan Graduate School of Northeastern University, Foshan, Guangdong 528311, PR China cResearch Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan  *Corresponding author Dr. Qi Zhu Tel: +86-24-8367-2700 E-mail: zhuq@smm.neu.edu.cn  Dr. Ji-Guang Li Tel: +81-29-860-4394 E-mail: LI.Jiguang@nims.go.jp   Revised Manuscript with Changes Marked 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 mailto:zhuq@smm.neu.edu.cnmailto:LI.Jiguang@nims.go.jp2  Abstract  Long persistent luminescence materials developed to commercial standards are primarily concentrated in the blue and green regions, with only a few in the red region. Red, as one of the three basic colors, can be mixed in various proportions with blue and green to yield various colors. The development of red persistent phosphors has a broader application potential but remains a challenge. A solid-state reaction method was used to synthesize new red persistent luminescent materials of Ba1-xSrxGa2O4:Sm3+ (x = 0–0.09). In BaGa2O4, both Sr2+ and Sm3+ preferentially occupy the Ba2+ site rather than the Ga3+ site. When exposed to UV light at 254 nm, the phosphors emit the characteristic red emission of Sm3+ at wavelengths ranging from 500 nm to 750 nm. After removing the UV light source, an intense red afterglow that lasted more than 1400 seconds was observed. The red afterglow signal reappears after a heating process. Doping Sr2+ reduces the trap depth and improves the red persistent luminescence significantly. Because the escaped electrons from traps compensate for the emission loss of Sm3+ during the heating process, the red phosphors have unimaginably luminescent thermal stability. Thus, the emission intensity at 200 °C is 1.6 times that at room temperature. The prepared red persistent phosphors show multimode luminescence, with the output signal being time and temperature sensitive, indicating that they are potential luminescent materials for anti-counterfeiting applications. Finally, a building-block strategy for advanced anti-counterfeiting applications of dynamic display information is proposed, with red persistent phosphors serving as an important component combined with upconversion phosphors of NaYF4:Yb3+, Tm3+, and green persistent phosphors of SrAl2O4:Eu2+, Dy3+.  Keywords:  Long persistent luminescence; Temperature-dependent luminescence; Anti-counterfeiting; BaGa2O4; Sm3+   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 3  1. Introduction There are some luminescent materials, that can continue to emit light for a certain period of time even after the external light source excitation is eliminated [1,2]. These materials are known as long persistent luminescence materials, and they are commonly used in places that require low lightings, such as night lights, traffic signs, and indoor safety escape signs [3,4]. They have recently been employed in radiation detection and optical storage media after adequate research and development [5,6]. Long persistent luminescent materials have been used in anti-counterfeiting, in vivo imaging, and medical diagnosis [7-10]. At the moment, materials with green and blue afterglow, such as SrAl2O4:Eu2+, Dy3+ (green), and CaAl2O4:Eu2+, Nd3+ (blue), have reached commercial standards, whereas red long persistent luminescence materials are difficult to attain these standards in terms of afterglow time and brightness [11,12]. Because red is one of the three basic colors, it can be mixed in various proportions with blue and green to produce various colors, giving it a broader application prospect [13-16]. Therefore, it is critical to developing red long persistent luminescence materials with superior properties. Typically, emitting centers are critical for materials with persistent luminescence. The emitting color is always determined by the doped ions and the host materials. The host is fixed, but the doped ions can be changed. In other words, the afterglow color of the synthesized afterglow materials will be affected by the choice of different doped ions. When excited under certain conditions, Eu2+, Eu3+, Mn2+, Sm3+, or Pr3+ can produce red light [17-21]. Furthermore, through energy transfer processing [22], the emitting color of persistent luminescence materials can be changed because energy transfer from one ion to another doped ion during afterglow decay contributes to another afterglow, including red afterglow. This type of red long persistent luminescence was discovered in Sr3MgSi2O8-1.5xNx:Eu2+,Dy3+,Mn2+, and BaMg2Si2O7:Eu2+,Dy3+,Mn2+ [23,24]. However, the duration and brightness of the afterglow are short and dim. Red long persistent luminescence materials such as Ca2SnO4:Sm3+, Y2O2S:Sm3+, and Sr2SnO4:Sm3+ have been studied recently [25-27].  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4  The typical transition between Sm3+ electronic configurations has been reported to contribute to a strong red emission [28]. As a result, Sm3+ is always used in red phosphors and red long persistent luminescence materials. In addition to being influenced by doped ions, the intensity and duration of the persistent afterglow are determined by phosphor traps [29]. When the excitation is stopped, the trapped electrons or holes will be released to the emitting centers via the conduction or valence bands. It is believed that the traps are related to defects. Li et al. discovered a novel red-orange-emitting phosphor, BaGa2O4:Bi3+, with BaGa2O4 as the host [30]. Because of the efficient traps formed by defects during the formation of BaGa2O4, BaGa2O4:Bi3+ exhibited a long afterglow at extremely low temperatures [31]. Furthermore, this phosphor has excellent thermal stability, as its integrated intensity at 200 °C retains more than 88.1% of that at room temperature. As a result, this persistent luminescence material has promising applications in both extremely low and high-temperature conditions. As a result, BaGa2O4 could be used as a suitable host for long persistent luminescence phosphors. Except for the host, the defects can be formed by doped ions occupying the site of host matrix ions with a different valence or ionic radius. A solid-state reaction method was used to synthesize Ba1-xSrxGa2O4:0.01Sm3+ (x = 0–0.09), a group of red persistent phosphors, with smaller Sr2+ substituting the larger Ba2+ to improve the persistent luminescence of BaGa2O4:Sm3+. The samples were then characterized using XRD, UV-Vis, HRTEM, SAED, TL, PLE/PL spectroscopy, and persistent luminescence decay analysis. Finally, the possibility of their use in anti-counterfeiting applications was investigated.  2. Experimental section  2.1. Sample preparation Ba1-xSrxGa2O4:0.01Sm3+ (x = 0, 0.01, 0.03, 0.05, 0.07, 0.09) samples, which are described as BGSO:Sm3+, were synthesized by the high-temperature solid-state reaction method. BaCO3, Ga2O3, Sm2O3, and SrCO3 were chosen as the raw materials,  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5  all purchased from Sinopharm (Shanghai, China) with a purity of 99.9%. Based on the stoichiometric ratio after calculation, all the raw materials were weighted by the electronic balance. The powders were ground for 30 min and then the mixture was calcined at 1400 oC for 8 hours. When cooling to room temperature, the powders were tested after ground again. 2.2. Characterization The powder X-ray diffraction patterns were collected by X-ray diffraction (XRD, Model SmartLab, Rigaku, Tokyo, Japan) at the scanning rate of 8 min-1 in the 2θ range from 10o to 70o，operating at 40 mA and 40 kV using monochromated Cu Kα as radiation. The photoluminescence spectra (PL)/photoluminescence excitation spectra (PLE) of samples were obtained by the FP-8600 (JASCO, Tokyo) with a 150 W Xe-lamp as the excitation source. The diffuse reflectance spectra of the samples were acquired by a UV-vis spectrophotometer (UV-3600 Plus, Shimadzu, Kyoto, Japan). Long persistent luminescence spectra were recorded via the Horiba JY Fluorolog-3 (Kyoto) spectrofluorometer. An FJ-427A TL spectrofluorometer (Beijing Nuclear Instrument Factory, Beijing, China) at a heating rate of 1 K s-1 was used to gain Thermoluminescence (TL) curves after the samples were excited for 5 min by a UV lamp. In order to study the application in anti-counterfeiting, the commercial persistent phosphor of SrAl2O4:Eu3+, Dy3+ (Luming Science and Technology Group Co., Ltd, China) was chosen here as a green light source. The afterglow luminance of sample was measured by a spectroradiometer (HS-1000, Photal Otsuka Electronics, Osaka, Japan) with a built-in software of the system.  3. Results and discussion 3.1. Synthesis and crystal structure Fig. 1a depicts the XRD patterns of BGSO:Sm3+ (x = 0–0.09). All the XRD diffraction peaks are consistent with the standard patterns of BaGa2O4 (JCPDS NO. 46-0415), indicating the formation of a single phase of BGSO:Sm3+. Because the ionic radii of Sm3+ (1.02 Å, CN = 7; 0.958 Å, CN = 6) and Sr2+ (1.21 Å, CN = 7; 1.18  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 6  Å, CN = 6) are close to that of Ba2+ (1.38 Å, CN = 7) and much larger than that of Ga3+ (0.47 Å, CN = 4), Sm3+ and Sr2+ ions are thought to substitute for Ba2+ sites. Furthermore, because the ionic radius of Sm3+ and Sr2+ is smaller than that of Ba2+, the incorporation of Sm3+ and Sr2+ causes the shrink of lattice. The enlarged drawing of the (222) main diffraction peaks is show in Fig.1b. As the x value increases from 0 to 0.09, the (222) diffraction peak shifts to the higher angle side. This is primarily due to that smaller Sr2+ ions substitute the larger Ba2+ ions, causing the lattice to shrink. To test this inference, the Rietveld refinement results of the XRD pattern for BGSO (x = 0.07 sample) are analyzed and shown in Figure S1. The calculated results agree with the experimental data. The Rwp, Rp, and Rexp values are quite low, indicating that the results reliable. Interstitial oxygen is generated when Ba2+ ions are replaced by Sm3+ ions to compensate for the charge difference. This could contribute to long persistent luminescence. The unit cell structure of BaGa2O4 is depicted in Fig. 2a. The hexagonal structure of BaGa2O4 belongs to the P63 space group. BaGa2O4 comprises six-membered ring layers of GaO4 tetrahedra that are perpendicular to the c-axis, with its layers stacked to form a three-dimensional framework. Tunnels are formed by the superposition of the layers parallel to the c-axis, and the barium cations are in these tunnels. There are two types of barium ions: one with nine oxygen atoms located in a more symmetrical Ga-O tetrahedron six-membered ring, and the other with seven and eight oxygen atoms located in a more irregular Ga-O tetrahedron six-membered ring (Fig. 2b) [32]. Figures S2 and 2c show the FE-SEM and TEM morphology of BGSO:Sm3+ (x = 0.07) sample, indicating a particle size of 1–5 μm. The HRTEM image (Fig. 2c) and SAED patterns were examined (Fig. 2d). The HRTEM image's clear lattice fringes indicate that the sample has good crystallinity. The distance between the two crystal planes is about 0.325 nm. As a result, they correspond to the (411) crystallographic plane (d(411) = 0.32612 nm, JCPDS No. 46-0415). The calculated values of the diffraction spots, as shown in Fig. 2d, are approximately 0.325 nm, 0.352 nm, and 0.432 nm, which correspond to the (411), (410), and (00-1) planes, respectively (d(410) =  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 7  0.35216 nm, d(00-1) = 0.43326 nm, JCPDS No. 46-0415). The elemental distribution results in Fig. 2e-g show that Sr2+ and Sm3+ are successfully doped and that the chemical elements are homogeneously distributed.  3.2. Photoluminescence properties Fig. 3a depicts the diffuse reflectance spectra of the BGSO (x = 0, 0.05, and 0.09) samples. The peak from 200 nm to 300 nm is attributed to the absorption of BaGa2O4 host [33]. The substitution of Sr2+ ions for Ba2+ sites did not affect the absorption. The Kubelka-Munk formula was used to calculate the band gap energy of the hosts [34]:  RRF(R)2)-(1 = 2 )-A(= )( 2gEhναhν  where R denotes the reflectivity of the sample, α denotes the absorption coefficient, which is proportional to F(R), h denotes Planck’s constant, ν denotes the light frequency, and A denotes the proportionality constant. The intercept of the diffuse reflection spectra transformed by the Kubelka-Munk function F(R) in Fig.3b can be used to calculate the bandgap energy. The bandgap of BaGa2O4 is approximately 5.01 eV. The band gap shrinks slightly as the Sr2+ doping content increase because the determined values are around 4.96 eV for x = 0.05 and x = 0.09 samples, which are smaller than 5.01 eV. The PLE and PL spectra of BaGa2O4:0.01Sm3+ are shown in Fig.4a. The O2-→Ga3+ charge transfer band is assigned to the strong band at 254 nm in the PLE spectra [33]. Because the host absorption of BaGa2O4 is close to the charge transfer band, the strong band could overlap the charge transfer band and the host absorption band. Sharp peaks ranging from 300 nm to 450 nm correspond to the Sm3+ intra-4f5 transitions, with the 6H5/2→4K11/2 transition at 404 nm dominating [28]. The PL spectra obtained at an excitation wavelength of 254 nm show four narrow emission bands attributed to the 4G5/2 to the 6H5/2 (568 nm), 6H7/2 (608 nm), 6H9/2 (651 nm), and 6H11/2 (707 nm) transitions of Sm3+, with the sharp peak at 608 nm dominating [35,36].  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 8  The persistent luminescence spectra of BaGa2O4:0.01Sm3+ at room temperature are shown in Fig. 4b, which are nearly identical to the emission spectra of Sm3+. When the sample is excited by a 254 nm UV light, four emission bands are observed at 568 nm, 608 nm, 651 nm, and 707 nm. Fig. 4c depicts the decay curve of persistent luminescence. A UV lamp was used to excite the sample for 5 min at room temperature. The red afterglow of the samples lasts more than 1400 seconds. The schematic energy level diagrams of BaGa2O4:0.01Sm3+ in Fig. 4d show the possible luminescence mechanism. The ground-state electrons of Sm3+ ions are promoted to the conduction band by UV irradiation. The excited states of Sm3+ are degenerated with free electron-hole states, with an electron at the bottom of the conduction band and a hole at the top of the valence band, because of the UV light excitation. Some excited electrons are delocalized in the CB and are captured by the GaO4 tetrahedron. The electrons released from the GaO4 tetrahedron to VB contribute to the blue emission [37], as evidenced by the host emission (Figure S3). Simultaneously, some electrons are relaxed to the excited levels of Sm3+ and relaxed to the 4G5/2 level, and the energy relaxes through the transitions from 4G5/2 to the 6H5/2, 6H7/2, 6H9/2, and 6H11/2 levels, contributing to the emission at 568, 608, 651, and 707 nm. Furthermore, the possible energy transfer from the GaO4 tetrahedron to the 4G5/2 energy level contributes to Sm3+ emission. When electrons are excited from the valence band to the conduction band by UV excitation, some electrons are captured and retained by the electron trap. During the illumination period, the electron traps are filled. When the ultraviolet irradiation excitation stops, the trapped electrons escape to the conduction band and then to the excited energy levels of Sm3+, contributing to the afterglows [37, 38]. Electronic traps are critical for all long persistent luminescence materials. Shallow traps require less energy to allow captured electrons to escape more easily, comparing to deep traps. We used thermoluminescence (TL) to determine the trap depth of the samples. The TL glow curves for the samples as shown in Fig. 5a. The TL curves are not symmetrical, and there are two peaks in the TL curves corresponding to the shallow and deep traps, respectively. These two peaks are caused  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 9  by two different defects: charge compensation defects caused by Sm3+ replacing Ba2+ and the lattice distortion defects formed by the substitution of Sm3+ and Sr2+ for Ba2+.  The trap depth can be calculated by the following equation [39]:  500 = mTE  where Tm denotes the temperature for which the glow curve reaches a maximum, and E denotes the approximate trap depth. The trap depth for each sample is summarized in Table 1. The peaks of T1 and T2 shift to the lower temperature side as the x value increases, indicating that Sr2+ doping makes the traps shallower on the overall trend. Fig. 5b depicts the decay curves of persistent luminescence. A UV lamp was used to excite all the samples for 5 min at room temperature. The red afterglow in all samples lasts longer than 1400 seconds, because the afterglow luminance at 1400 s is estimated to be ~0.7–1.1 mcd/m2, which is greater than the value of 0.32 mcd/m2 [40]. Obviously, there are two types of laws for the decay curves at the initial and later parts. Increasing the x value from 0 to 0.07 results in a more intense afterglow for the initial duration (before 35 s), but increasing the x value from 0.07 to 0.09 results in a decline. However, for the later duration, a higher x value from 0 to 0.09 results in a more intense afterglow (after 35 s). This is because the intensity of the afterglow is affected by trap number and trap depth. The captured electrons can easily escape from the shallower traps, contributing to the stronger afterglow. Additionally, more electrons captured by more electron traps may contribute to a stronger afterglow. Because the x = 0.07 sample has the shallowest trap depth and the more trap number compared to the samples with x smaller than 0.07, it has the most intense afterglow at the initial stage. However, because of the larger electron trap numbers, the x = 0.09 sample exhibits a more intense afterglow than that for the x = 0.07 sample at the later stage. The results show that lattice distortion caused by substituting Ba2+ with Sr2+ can enhance the afterglow intensity.     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 10  Table 1 Calculated trap depths of BGSO:Sm3+ (x = 0, 0.05, 0.07, 0.09). x T1 T2 Trap I depth/eV Trap II depth/eV 0 98 139 0.742 0.824 0.05 83 121 0.712 0.788 0.07 79 115 0.704 0.776 0.09 87 118 0.720 0.782  In Fig. 6a, the change in emission peaks can be observed with increasing heating temperature. In the case of BGSO:Sm3+ (x = 0.07), increasing the temperature yields an enhanced emission intensity under 254-nm UV lamp irradiation. The emission intensity at 200 °C is more than 160% of that at room temperature and about 80% of that at room temperature at 300 °C (Fig. 6b). The increased emission intensities are caused by captured electrons released from electron traps that combine with the excited state of Sm3+ by heat processing, which compensates for the emission loss caused by thermal quenching. This phenomenon was discovered previously in our research on ZnGa2O4:Cr3+ persistent luminescent materials [41,42]. Fig. 6c shows the persistent luminescence decay curves for the samples after 30 s of heating at 150 °C. Clearly, afterglow can be seen for all samples, with more intense afterglow found at a higher x value, which is nearly the same as that without heat processing. The afterglow intensity of the x = 0.07 sample, however, exhibits the fastest decline among the samples because it has the shallowest trap depth and the electrons are emptied most easily. As a result, the afterglow of the x = 0.07 sample is highly sensitive to temperature. The BGSO:Sm3+ (x = 0.07) sample was placed in cylindrical containers. As shown in Fig. 6d, as the temperature rises, the phosphors exhibit increased red emission when excited by 254-nm UV light. It can be seen that the emission intensity of BGSO:Sm3+ (x = 0.07) is the highest at 200 °C. Increasing the temperature further contributes to a decrease of the red signal. The luminescence mechanism after heat processing is depicted schematically in Fig. 6e. At elevated temperatures, the trapped electrons escape to the CB and combine with the excited energy levels of Sm3+, which compensates for the emission loss caused by the thermal  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 11  quenching of Sm3+. As a result, the intensity of the red emission increases after heating, and the sample exhibits a temperature-dependent luminescence.  3.3. Anti-counterfeiting applications To investigate the anti-counterfeiting application, BGSO:Sm3+ (x = 0.07) and NaYF4:Yb3+, Tm3+ phosphors were mixed with a mass ratio of 1:1. The PL spectra and the appearance of the mixture phosphor under 980 nm laser excitation (2.5 W) are shown in Fig. 7a. The PL spectra show several emission bands corresponding to the 1D2→3H6 (361 nm, purple), 1D2→3F4 (450 nm, blue), 1G4→3H6 (473 nm, blue), 1G4→3F4 (646 nm, red), 3F2→3H6 (695 nm, red), and 3F3→3H6(723 nm, red) transitions of Tm3+ [43]. The images show that the mixture phosphor emits a blue upconversion light. The PL spectra and appearance of the mixture phosphor obtained after excitation at 254 nm are shown in Fig. 7b. Sm3+ also exhibits four narrow emission peaks at 568 nm, 608 nm, 651 nm, and 707 nm. The red emission of the mixture phosphors can be seen by the naked eye. The persistent luminescence spectra of the mixture phosphor at room temperature, which is assigned to the red afterglow of Sm3+, are shown in Fig. 7c. The persistent luminescence decay curve of the mixture phosphor, which was excited by a 254 nm UV lamp for 5 min, is shown in Fig. 7d. The red afterglow from the mixture phosphor lasted more than 600 seconds. The prepared samples could be successfully employed in advanced anti-counterfeiting applications due to their different emission performances at different exciting wavelengths. To capture the emission signals of the samples under different excitation conditions, the BGSO:Sm3+ (x = 0.07) sample and the mixture phosphor are placed in the “B” container fabricated by metal 3D printing (Fig. 7e). The phosphor 1 is BGSO:Sm3+ (x = 0.07), and the phosphor 2 is a mixture of the two. Under natural light, all the patterns are white “B”. The four patterns showed bright red “B” and dark red “B” emissions at 254-nm UV excitation and after removing the 254-nm UV excitation lamp, respectively (Fig. 7e). The four patterns, however, exhibited blue “B”, “p”, “b”, and “D” under 980-nm laser excitation.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 12  BGSO:Sm3+ (x = 0.07) and SrAl2O4:Eu2+, Dy3+ phosphors were mixed with weight ratios of BGSO:Sm3+ (x = 0.07) to SrAl2O4:Eu2+, Dy3+ at 200:1 and 150:1, respectively. The PL spectra of the two phosphors mixtures obtained under excitation at 254 nm are shown in Fig. 8a. The PL spectra of the two mixture phosphors show four emission peaks from Sm3+ at 568 nm, 608 nm, 651 nm, and 707 nm, but the emission intensity of Sm3+ is weaker than that before mixing. The green emission of SrAl2O4: Eu2+, Dy3+ is visible with the broadband at 515 nm [44]. The two emission intensities for the two mixed powders were compared. The I(515)/I(608) ratio for the mixture with the weight ratio of BGSO:Sm3+ (x = 0.07) to SrAl2O4:Eu2+, Dy3+ as 200:1 is approximately 0.6. However, I(515)/I(608) is about 1.2 for the mixture with the weight ratio of BGSO:Sm3+ (x = 0.07) to SrAl2O4:Eu2+, Dy3+ as 150:1. It implies that increasing the BGSO:Sm3+ content would cause the emission color shifting to the red range. The persistent luminescence spectra of the two mixture phosphors are shown in Fig. 8c. In the absence of 568 nm and 707 nm emission peaks for the phosphors mixture (the weight ratio of BGSO:Sm3+ to SrAl2O4:Eu2+, Dy3+ is 200:1), afterglow emission peaks at 515 nm, 608 nm and 651 nm are observed, indicating that the afterglow is yellow. The persistent luminescence spectra of the mixture phosphors (weight ratio of BGSO:Sm3+ to SrAl2O4:Eu2+, Dy3+ is 150:1) only has a 515-nm peak, and the Sm3+ emission peak disappears, illustrating that the afterglow is green. The relative intensities of 515 nm and 608 nm at different temperatures are shown in Fig. 8d, with the weight ratio of BGSO:Sm3+ (x = 0.07) to SrAl2O4:Eu2+, Dy3+ of 150:1. As the temperature rises, the intensity of the green emission peak at 515 nm decreases steadily. It is worth nothing that the red emission of the mixture increases with an increased temperature. The red emission intensity of the mixture is highest at 150 °C, in contrast to that of BGSO:Sm3+, which has the highest intensity at 200 °C.  The samples were placed in cylindrical containers labeled A, B, C, and D to capture the emission signal under different excitation conditions. In each condition, sample A emits red and sample D emits green, as shown in Fig. 9. With the increased content of SrAl2O4:Eu2+, Dy3+, the phosphors change color from red to green under the excitation of 254-nm UV light. In contrast, samples B and C show a color change  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 13  from red to yellow after 30 seconds of excitation. After 5 min of continuous excitation with 254 nm UV light, the sample emits an afterglow that changes from red to yellow and then to green with increased content of SrAl2O4:Eu2+, Dy3+. Sample B's afterglow is yellow, whereas the afterglow of sample C is green. The red-light emission of samples A, B, and C increases with increasing the heating temperatures. The green emission of sample D, however, decreases gradually. Because the mixed phosphor demonstrated time- and temperature-dependent multimode luminescence, it has great potential in advanced anti-counterfeiting applications. Here, an apple tree logo was prepared using four powders as raw materials. Fig. 10 shows the images under 254 nm UV excitation and the afterglow images at room temperature. Under the excitation of 254-nm UV light, the three apples were red and ripe. After 30 seconds of excitation, apples B and C turned yellow and immature. The afterglow of apple B was also yellow, but the afterglow of apple C turned green. Under UV excitation at 254 nm, all three apples turned bright red at 100 °C and 200 °C. The red light from three apples became stronger, while the green glow from the tree trunk became weaker. At 300 °C, the light from trunk became more weaker, while the red light from the three apples weakened but remained bright.  4. Conclusion  Red long persistent luminescent materials of Ba1-xSrxGa2O4:Sm3+ (x = 0–0.09) were synthesized using a solid-state reaction method and characterized using XRD, UV-Vis, HRTEM, SAED, TL, PLE/PL spectroscopy, and persistent luminescence decay analysis. Both Sr2+ and Sm3+ preferentially occupy the Ba2+ site rather than the Ga3+ site in BaGa2O4. Doping with Sr2+ and Sm3+ does not significantly affect the crystal structure but causes lattice shrinkage and a narrower band gap. The samples exhibit characteristic Sm3+ emission with emission peaks ranging from 500 nm to 750 nm, belonging to G5/2 to the 6H5/2 (568 nm),6H7/2 (608 nm), 6H9/2 (651 nm), and 6H11/2 (707 nm) transition of Sm3+. Doping Sr2+ reduces the trap depth and improves red  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 14  persistent luminescence significantly. The Ba0.93Sr0.07Ga2O4:0.01Sm3+ sample exhibits the brightest and strongest red afterglow. Because the escaped electrons from traps compensate for the emission loss of Sm3+ during the heating process, the red phosphors have unimaginably luminescent thermal stability. The emission intensity at 200 °C is 1.6 times that at room temperature. The prepared red persistent phosphors exhibit time- and temperature-dependent luminescence, indicating that they could be used as multimode luminescent materials in anti-counterfeiting applications. Finally, a building-block strategy for advanced anti-counterfeiting applications is proposed, using the red phosphor as an important component combined with an upconversion phosphor and a green persistent phosphor.  Acknowledgments This work was supported in part by the Natural Science Foundation of Liaoning Province (Grant 2020-MS-081), and National Natural Science Foundation of China (Grant 51302032, 51972047, 52172112).  References [1] Y. Li, M. 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Wu, S. Y. Liu, Y. C. Sun, W. Chen, L. F. Huang, G. L. Chen, Z. S. Zheng, Energy transfer of wide band long persistent phosphors of Sm3+-Doped ZrSiO4, Mater. Chem. Phys. 251 (2020) 123086. [39] K. Van den Eeckhout, P. F. Smet, D. Poelman, Persistent luminescence in Eu2+-doped compounds: A Review. Materials. 3 (2010) 2536. [40] J. Xu, S. Tanabe, Persistent luminescence instead of phosphorescence: History, mechanism, and perspective, J. Luimn. 205 (2019) 581-620.  [41] J. Q. Xiahou, Q. Zhu, L. Zhu, S. Huang, T. Zhang, X. D, Sun, J. G. Li, Lattice-site engineering in ZnGa2O4:Cr3+ through Li+ doping for dynamic luminescence and advanced optical anti-counterfeiting, J. Mater. Chem. C. 10 (2022) 7935-7948.  [42] T. Si, Q. Zhu, T. Zhang, X. D. Sun, J. G. Li, Co-doping Mn2+/Cr3+ in ZnGa2O4 to fabricate chameleon-like phosphors for multi-mode dynamic anti-counterfeiting, Chem. Eng. J. 426 (2021) 131744. [43] H. Suo, C. Guo, Z. Yang, S. Zhou, C. Duan, M. Yin, Thermometric and optical heating bi-functional properties of upconversion phosphor Ba5Gd8Zn4O21:Yb3+/Tm3+, J. Mater. Chem. C. 3 (28) (2015) 7379-7385. [44] Hai, M. K. Pei, Q. Ren, X. L. Wu, E. L. Yang, D. Xu, J. F. Zhu, Ag nanoparticles significantly improve the slow decay brightness of SrAl2O4:Eu2+, Dy3+ by the surface plasmon effect, Dalton. T. 51 (6) (2022) 2287-2295.                1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 18   Fig. 1. (a) X-ray diffraction patterns and (b) enlarged (222) diffraction of BGSO:Sm3+ (x = 0- 0.09).               Fig. 2. (a) Crystal unit cell of BaGa2O4 viewed along the c-axis, (b) two different types of six-membered rings consisted of Ga-O tetrahedron, (c) TEM image, (d) HR-TEM image, (e) SAED pattern, and (f-j) element distribution of BGSO:Sm3+ (x = 0.07).  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 19    Fig. 3. (a) Diffuses reflection spectra and (b) the bandgap energies of the BGSO (x = 0, 0.05, 0.09).   Fig. 4. (a) PLE and PL spectra, (b) persistent luminescence spectra, (c) persistent luminescence decay curve of BaGa2O4:0.01Sm3+, and (d) schematic illustration of the luminescence mechanism for BaGa2O4:0.01Sm3+.    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 20   Fig. 5. (a) TL glow curves for BGSO:Sm3+ (x = 0, 0.05, 0.07, 0.09) and (b) the persistent luminescence decay curves (monitored at 608 nm after 254-nm UV light illumination for 5 min).                  Fig. 6. (a) PL spectra of BGSO:Sm3+ (x = 0.07) at different temperatures, (b) the relative intensities of 608 nm at different temperatures for BGSO:Sm3+ (x = 0.07), (c) persistent luminescence decay curves of BGSO:Sm3+ (x = 0-0.09) after heating at 150 oC, (d) appearances of BGSO:Sm3+ (x = 0.07) phosphors at different temperatures  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 21  under 254 nm light excitation, and (e) schematic illustration of the luminescence mechanism for BGSO:Sm3+ after heat processing.      Fig. 7. PL spectra and the appearance of the mixture phosphor (a) under 980 nm laser excitation (2.5 W) and (b) under 254-nm UV light excitation, respectively. (c) and (d) are the persistent luminescence spectra and the afterglow and the persistent 550 600 650 700 7501000200030004000500060004G5/2-6H11/24G5/2-6H9/24G5/2-6H7/24G5/2-6H5/2Intensity/a.u.Wavelength/nmλex=254 nm(b)0 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 22  luminescence decay curves of the mixture phosphor. The mixture phosphor is composed of Ba0.93Sr0.07Ga2O4:0.01Sm3+ and NaYF4:Yb3+,Tm3+ with the weight ratio of 1:1. (e) is the designed logo for anti-counterfeiting application, with phosphor 1 as Ba0.93Sr0.07Ga2O4:0.01Sm3+ and phosphor 2 as the mixture phosphor.                            Fig. 8. (a) PL spectra, (b) the persistent luminescence spectra, and (c) persistent luminescence decay curves of two typical mixture phosphors, with the weight ratio of Ba0.93Sr0.07Ga2O4:0.01Sm3+ to SrAl2O4:Eu3+, Dy3+ as 200:1 and 150:1, respectively. (d) is the relative intensities of 515 nm and 608 nm at different temperatures for the mixtures, with the weight ratio of Ba0.93Sr0.07Ga2O4:0.01Sm3+ to SrAl2O4:Eu3+, Dy3+ as 150:1.    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 23  .  Fig. 9. Appearances of four typical samples under daylight, 254 nm UV light for 0s and 30 s at different temperatures, and the afterglows after removing the UV light source. Sample A is Ba0.93Sr0.07Ga2O4:0.01Sm3+ and sample D is the mixture of BaSO4 and SrAl2O4:Eu3+, Dy3+ (the weight ratio is 100:1). The weight ratio of Ba0.93Sr0.07Ga2O4:0.01Sm3+ to SrAl2O4:Eu3+, Dy3+ is 200:1, 150:1 for samples B and C respectively.  Fig. 10. Appearances of the signals for the pattern of "apple tree" with four typical samples placed in the designed container fabricated by metal 3D printing.       1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 24   Supporting Information  A building-block strategy for dynamic anti-counterfeiting by using (Ba,Sr)Ga2O4:Sm3+ new red persistent luminescent phosphor as an important component    Ao Guoa, Qi Zhua,*, Shimeng Zhanga, Xudong Sunb and Ji-Guang Lic,*  aKey Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China bFoshan Graduate School of Northeastern University, Foshan, Guangdong 528311, PR China cResearch Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan   *E-mail: zhuq@smm.neu.edu.cn and LI Jiguang@nims.go.jp            1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 mailto:zhuq@smm.neu.edu.cnmailto:Jiguang@nims.go.jp25      Figure S1. Rietveld refinements of BGSO (x=0.07 sample).       Figure S2. FE-SEM image of BGSO:Sm3+ (x = 0.07) powder.   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 26        250 300Intensity (a.u.)Wavelength (nm) BaGa2O4λem=463 nm(a)300 400 500 600 700Intensity (a.u.)Wavelength (nm) BaGa2O4λex=238 nm(b) Figure S3. (a) PLE spectra and (b) PL spectra of BaGa2O4.    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Declaration of Interest Statement There are no conflicts of interest to declare. Declaration of Interest Statement