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H. Yang, G. Shen, S. Tang, [J.-G. Li](https://orcid.org/0000-0002-5625-7361), Q. Zhu

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[Breaking the balance of chemical ratio in ZnAl2O4 to regulate activators and traps for multimode luminescence - A new strategy](https://mdr.nims.go.jp/datasets/ec3cb829-19f3-4da9-a5d0-069335d9564c)

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1  Breaking the balance of chemical ratio in ZnAl2O4 to regulate activators and traps for multimode luminescence - A new strategy   Huan Yang1, Guichang Shen2, Shuai Tang2, Ji-Guang Li3, Qi Zhu1* 1Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, PR China 2State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China 3Research 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  mailto:zhuq@smm.neu.edu.cn2  Abstract As a common transition metal activator, the non-rare earth element Mn has good luminescent properties, so it has been widely concerned. It is reported that Mn2+ and Mn4+ can coexist in the matrix, and the luminescence of Mn with different valence states is usually controlled by co-doping of other ions. In this work, we controlled the green light emission of Mn2+, the red light emission of Mn4+, and the NIR emission of defects by breaking the balance of chemical ratio in ZnAl2O4:Mn to achieve multimode luminescence. The spinel-structured ZnAl2O4 phosphors with deficiency of zinc were synthesized by high temperature solid state reaction, which were characterized by a series of techniques, including XRD, DFT calculation, PLE/PL spectroscopy, TL, persistent luminescence decay curves, and temperature-dependent PL spectra analysis. The deficiency of zinc results in the appearance of zinc vacancy (VZn) and oxygen vacancy (VO), and the increased oxygen vacancy defects inhibit the self-reduction of Mn4+. Under the excitation of 325 nm and 426 nm ultraviolet light, the phosphor showed green emission at 510 nm, red emission at 678 nm and near-infrared emission at 767 nm. Due to the increase of vacancy defects, high concentration of zinc deficiency leads to stronger emission of green light, red light and near-infrared light. The phosphor exhibited different light signals at different excitation wavelengths, and the thermal stability of Mn2+ and Mn4+ luminescence is inconsistent. The above characteristics show that the multimode luminescent phosphor synthesized in this work has broad application prospects in the field of fluorescence anti-counterfeiting.  Keyword: ZnAl2O4, Mn2+, Mn4+, vacancy defect, multimode luminescence, anti-counterfeiting      3  1. Introduction In the field of luminescent materials, non-rare earth element Mn is usually used as activator, because Mn possesses good optical properties and it is cheap and easy to obtain [1]. Therefore, Mn is widely concerned as a good activator. Due to the specific electronic configuration of manganese ion, its luminescence properties also change with the different valence states of manganese ion. Among the valence states of manganese ions, Mn2+ and Mn4+ are the most common [2]. Mn2+ with 3d5 electron configuration shows green-light emission in tetrahedral environment (weak crystal field), and Mn4+ with 3d3 electron configuration shows red-light emission in octahedral environment (strong crystal field) [3]. For example, Zn2SiO4:Mn2+ [4], SrAl2Si2O8:Mn2+ [5], Cs2HfF6:Mn4+ [6], K2SiF6:Mn4+ [7] used as green and red phosphors show good luminescence properties. Among them, Mn4+ fluoride phosphor showing red-light emission is considered to be used for commercialization because of its high luminous efficiency, which has great application prospects [7]. However, it is challenging to control the valence change of manganese [8]. Moreover, the use of HF as a necessary raw material also greatly limits its application [7, 9, 10]. Therefore, the oxide phosphors doped Mn4+ without using hazardous raw materials have received extensive attention. Mn2+ and Mn4+ ions can co-exist in some hosts, which may contribute to multimode luminescence. For example, co-doping Mn2+ and Mn4+ in Ca3M2Ge3O12 [11], LaMgAl11O19 [3], MgB2O4 (B=Al, Ga) [12], ZnGa2O4 [13], ZnAl2O4 [14] yielded green and deep red emissions under UV light excitation. Among above hosts, ZnAl2O4 with broad band gap has a typical spinel structure. In this crystal structure, Zn occupies the tetrahedral site and Al occupies the octahedral site. The structure is classic and there is no reverse defect. ZnAl2O4 has high chemical and thermal stability, so it is a perfect matrix for metal ions doping [15-17]. Due to the existence of intrinsic defects or defects caused by ion doping in ZnAl2O4, excessive electrons will be released [18, 19]. This will lead to Mn4+ ions transforming to Mn2+ ions, which is called as a self-reduction phenomenon. Therefore, the proportion of 4  Mn2+/Mn4+ in the host is uncontrollable [20-23]. Therefore, it is possible to regulate the luminescent properties of phosphors by controlling the self-reduction process of Mn4+. Co-doping ions are frequently used to change the electronic environment in the matrix lattice and thus affecting the proportion of Mn2+/Mn4+. Recently, the molar ratio of Mn2+ and Mn4+ in ZnAl2O4 and Mg2SnO4 has been adjusted by doping Li+ to achieve dual-mode luminescence [1, 14, 24]. Usually, co-doping ions with various valence is selected to regulate the electronic environment in the lattice [13, 25]. However, the lattice domain environment becomes complex in presence of new ions. Therefore, it is necessary to explore a simple way for electronic environment regulation. In addition to cations and anions with fixed lattice positions, there are also some types of defects in the lattice of phosphors, such as vacancies [26]. These defects are often caused by cationic substitution or atomic loss [27]. It is reported that vacancy defects could affect the luminescent properties of phosphors [28, 29]. Recently, it was found that concentration of oxygen vacancy in zinc aluminate could be regulated by the content of zinc deficiency, with the oxygen vacancy acting as the luminescent center, which contributes to near-infrared emission in the range of 650-800 nm [16]. In addition, vacancy defects are not only conducive to the formation of effective hole/electron traps for persistent luminescence, but also can interact with their neighboring ions to adjust their valences [30, 31].  In this work, we prepared ZnAl2O4 doped Mn ions by a high-temperature solid-state method. Through changing the chemical ratio of ZnAl2O4, the activators and traps were successfully regulated. The series of phosphors were characterized by XRD, DFT calculation, PLE/PL spectra, persistent luminescence spectra and temperature-dependent PL spectra analysis. The phosphors exhibit multimode luminescence characteristics, indicating they have a prospect in the field of optical anti-counterfeiting. 2. Experiment Section 2.1. Sample preparation 5  The samples of ZnAl2O4:yMn (y = 0-0.012) and Zn1-xAl2O4:yMn (x = 0-0.13, y = 0-0.012) were synthesized by a high temperature solid-state reaction in air atmosphere. The starting materials were Al2O3 (99.99%), ZnO (99.99%) and MnCO3 (99.99%). The sources were purchased from China pharmaceutical group chemical reagent Co. LTD (Shanghai, China). Based on the composition, the above materials were weighed stoichio-metrically and mixed homogeneously in an agate mortar. The mixtures were grounded for 30 min homogeneously. After that, the mixtures were pre-fired at 1000 oC for 4 h in air. After cooling to room temperature and regrinding for 30 minutes, the samples were sintered at 1500 oC for 8 h in air. The heating rate for the two stages of heating is 5 oC min-1. Finally, after cooling down to room temperature naturally, ZnAl2O4:yMn and Zn1-xAl2O4:yMn phosphors were obtained for further measurement.  2.2. Characterization techniques X-ray diffractometry (XRD, Model SmartLab, Rigaku, Tokyo, Japan) was employed for phase identification, which was operated at 40 kV/40 mA using nickel-filtered Cu Kα radiation and the data was measured at a speed of 10.0o 2θ per minute, and the scanning range was 10o-70o. The XRD data for Rietveld refinement was gained through the step-scan mode, using a step interval of 0.02 and a counting time of 0.85 s per step. Rietveld refinement was performed using the TOPAS software. Inductively coupled plasma (ICP) spectroscopy (Model Optima 8300 and Optima 4300, Perkin Elmer, Shelton, CT) was employed to analyze the composition of the product with a detection limit of 0.01 wt%. The photoluminescence and persistent luminescence of samples were detected using a JY FL3-21 spectrophotometer (HORIBA) in the spectral range of 200-800 nm at room temperature, and using a slit width of 10 nm in the kinetics mode. The FJ-427A thermoluminescence (TL) instrument (Beijing Nuclear Instrument Factory) was used to measure the TL glow curves. The samples were first exposed to 302 nm UV light for 5 min at room temperature, and the TL glow curves were subsequently recorded from 300 K (27 °C) to 410 K (137 °C) at a heating rate of 1 K·s-1.  2.3. Computational Details 6  Density functional theory (DFT) calculations are carried out using the Vienna ab initio simulation package (VASP) in the study [32]. The electron interaction with ions was replaced by the projector augmented wave (PAW) method, and exchange-correlation potentials between electrons were parameterized by the generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) function [33, 34]. The 3s23p1, 3d104s2 and 2s22p4 electrons are the valence electrons of Al, Zn and O, respectively. The tetrahedral and octahedral sites of O were selected as substitution sites. All calculations are performed in 54 atoms without defects. The conjugate-gradient algorithm was used to relax all atomic positions and cell size to achieve the minimum total energy of the systems until the average force on each atom is less than 0.01 eV/atom. The energy cut-off for the plane-wave basis set was kept at a fixed value of 450 eV. The 4 × 4 × 4 k-points mesh obtained by the Monkhorst-Pack method was employed to sample the Brillouin zone [35]. 2.4 Preparation of anti-counterfeiting logo This study used phosphor and aluminum grooves to design multimode fluorescent anti-counterfeit digital patterns. The groove was filled with Zn0.87Al2O4 and Zn0.87Al2O4:0.012Mn phosphors. The anti-counterfeit pattern was presented in the form of fluorescent numbers. 3. Result and Discussion According to the first-principle calculation, the energy of perfect zinc aluminate is determined as -374.157 eV. The energies for Mn atoms to enter tetrahedral and octahedral sites to replace Zn and Al atoms in the zinc aluminate model are shown in Table 1. According to the formula [25, 36, 37]: 𝐸(𝑒𝑉) = 𝐸𝑑𝑜𝑝𝑒𝑑 − 𝐸𝑝𝑟𝑒𝑓𝑒𝑐𝑡                                                                                    (1) the formation energy of Mn replacing Zn into tetrahedral site is -4.168 eV, and that of Mn replacing Al into octahedral site is -0.353 eV. It can be seen that Mn is more likely to occupy the Zn position when it enters the zinc aluminate lattice.    7  Table 1. Formation energy of Mn substituting Zn and Al in ZnAl2O4 lattice. ZnAl2O4:Mn Mn-Zn Mn-Al ED (eV) -374.51 -378.325 EF (eV) -4.168 -0.353  XRD diffraction patterns of ZnAl2O4:yMn (y = 0-0.012) samples are shown in Figure 1(a). The diffraction positions in the XRD pattern are completely consistent with the PDF standard card of ZnAl2O4 (JCPDS NO.05-0669). There are no other diffraction peaks, and no obvious diffraction shift is found, indicating that Mn ions were well doped into ZnAl2O4 lattice to form a homogeneous solid solution. Moreover, with the increase of Mn content, the main lattice of ZnAl2O4 matrix did not change significantly. Figure 1(b) simulates the atomic site occupation of ZnAl2O4 lattice doped Mn ions. ZnAl2O4 is a standard spinel structure, in which Zn occupies tetrahedral position and Al occupies octahedral position [38, 39]. When Mn ions enter the main lattice, both Mn2+ and Mn4+ ions exist in the crystal cell structure. However, since the ionic radius of Mn2+ is 0.66 Å, which is close to that of Zn2+ (0.6 Å), Mn2+ will preferentially replace Zn2+ after entering the lattice and occupy the tetrahedral position. Similarly, the ionic radius of Mn4+ is 0.53 Å, which is close to the ionic radius of Al3+ ion (0.535 Å), so Mn4+ ion preferentially replaces Al3+ ion and occupies octahedral position [40, 41].  10 20 30 40 50 60 70 2-Theta (deg.) JCPDS NO.05-0669Intensity (a.u.)(111)(220) (311)(222)(400) (331)(422)(511) (440)  y=0(a)  y=0.001  y=0.007   y=0.012 8   Figure 1. (a) XRD patterns of ZnAl2O4:yMn (y = 0-0.012) calcined at 1500 oC, and (b) the unit cell crystal structure of ZnAl2O4 and ZnAl2O4: Mn.  In order to explore the luminescent properties of ZnAl2O4:yMn phosphors, the series of samples were tested. The samples were monitored by the typical emission wavelength of Mn2+ at 510 nm, and the corresponding excitation peaks appeared at 360 nm, 385 nm, 426 nm and 456 nm in PLE spectra (Figure 2(a)). These peaks are corresponding to the 6A1-4E(4D), 6A1-4T2, 6A1-[4A1(4G), 4E(4G)], 6A1-4T1(4G) transitions of Mn2+, respectively [14, 42]. Under the excitation at 426 nm, the sample emits green light at 510 nm (Figure 2(b)), which is assigned to the 4T1-6A1 transition of Mn2+. With the increase of Mn doping concentration, the green light emission intensity gradually increases, and the optimal doping concentration of Mn2+ is y = 0.012. However, the PLE spectrum in Figure 2(c) exhibits two obviously wide excitation bands at 325 nm (Mn-O2- CTB) and 456 nm (6A1-4T1 transition of Mn2+) by monitoring at 650 nm, indicating there are two different lattice positions of Mn2+ in ZnAl2O4 lattice. It can be recognized from Figures 2(b) and 2(d) that Mn2+ ions in two lattice positions can be selectively excited by ultraviolet light of different wavelengths. However, based on the formation energy calculated previously, the formation energy of Mn occupying tetrahedron is lower. Moreover, comparing with the spectral intensity in Figures 2(b) and 2(d), the emission intensity of green light is much higher than that of red light, which further verifies that Mn ion tends to replace Zn2+ ion. This is consistent with the first-principles calculation results. It is discovered in Figure 2(d) that under the excitation of 325 nm wavelength, the samples not only display the red emission of Mn2+ at 650 nm, but also display the near-infrared emission in the range of 700-800 nm. Through the comparison of PL spectra of 9  undoped ion sample (y = 0), it is uncovered that the near-infrared emission band has no relationship with Mn doping. It is speculated that it is caused by oxygen vacancy defect in ZnAl2O4 main lattice, which has been explained in detail in previous work [16]. With the increase of Mn ion concentration, the luminous intensity of the samples increases first and then decreases in Figure 2(d). The quenching concentration of Mn2+ ion red light emission is found at y = 0.007. The main reason for concentration quenching is the non-radiative energy transfer between Mn2+ ions in the host lattice, which may occur through exchange interaction, radiation reabsorption and multipolar-multipolar interaction [43-45]. When the doping concentration is y = 0.001, the near-infrared light intensity of the sample is the strongest. Therefore, the optimized doping concentration of Mn ion for green light, red light and near-infrared light of the samples is y = 0.012, y = 0.007, and y = 0.001, respectively. 320 340 360 380 400 420 440 46001000000200000030000004000000Intensity (a.u.)Wavelength (nm) y=0 y=0.001 y=0.007 y=0.012λem=510 nm(a)6A1-4E(4D)6A1-4T2(4D)6A1-[4A1(4G), 4E(4G)]6A1-4T1(4G)475 500 525 550 575 600 625 6500500000100000015000002000000Intensity (a.u.)Wavelength (nm) y=0 y=0.001 y=0.007 y=0.0124T1     6A1λex=426 nm(b)  Figure 2. (a, c) PLE and (b, d) PL spectra of ZnAl2O4:yMn (y = 0-0.012) samples. In order to investigate the interaction between defects and Mn ions, the samples with different zinc deficiency concentrations were prepared and the XRD diffraction patterns are shown in Figure 3. The XRD diffraction peaks of all samples correspond to the standard card one by one, and the peak positions do not shift significantly, indicating that zinc deficiency did not significantly change the crystal structure of 250 275 300 325 350 375 400 425 450 4751200010000400060008000Intensity (a.u.)Wavelength (nm) y=0 y=0.001 y=0.007 y=0.012λem=650 nm2000Mn2+-O2- CTB6A1-4T1(4G)(c)1600004000080000120000200000240000625 650 675 700 725 750 775 800020000400006000080000100000120000140000160000180000200000λex=325 nmIntensity (a.u.)Wavelength (nm) y=0 y=0.001 y=0.007 y=0.012(d)4T1-6A110  ZnAl2O4:Mn. XRD Rietveld refinements for Zn0.87Al2O4:0.012Mn sample calcined at 1500 oC is performed using the TOPAS software. The refinement results are shown in Figure S1. The calculation results are consistent with the experimental data. The lattice parameters and the values of Rwp, Rp, Rexp and χ2 are provided in the corresponding figure. The values of Rwp, Rp and χ2 are quite low, suggesting that the results are credible. Inductively coupled plasma (ICP) spectroscopy was employed to analyze the composition of the product, and elemental analysis identified the molar ratio of Zn: Al: Mn as 0.85: 1.98: 0.01, which is close to the composition of Zn0.87Al2O4:0.012Mn. Zinc vacancies will appear in ZnAl2O4 lattice due to zinc deficiency. However, positively charged oxygen vacancies will also appear for charge balance [16].  10 20 30 40 50 60 70 2-Theta (deg.) JCPDS NO.05-0669(111)(220)(311)(400) (331)(422)(511)(440) Intensity (a.u.) x=0  x=0.07   x=0.13 Zn1-xAl2O4:0.001Mn(a) 10 20 30 40 50 60 70Zn1-xAl2O4:0.007Mn(111) 2-Theta (deg.) JCPDS NO.05-0669(220) (311)(400) (331)(422)(511)(440)(b)  x=0 Intensity (a.u.) x=0.07   x=0.13 11  10 20 30 40 50 60 70 2-Theta (deg.) JCPDS NO.05-0669(111)(220)(311)(400) (331)(422)(511) (440)  x=0 Intensity (a.u.) x=0.07Zn1-xAl2O4:0.012Mn   x=0.13(c) Figure 3. XRD patterns of Zn1-xAl2O4:Mn (x = 0-0.13) calcined at 1500 oC. In the PL spectra (Figure 4), it is found that the emission intensity of red light and green light increased gradually with the increase of zinc deficiency concentration, which is consistent with the real-time emissions of samples excited by UV light (insets in Figure 4). The PLE spectra are shown in Figure S2. Because of the zinc vacancies in ZnAl2O4 lattice due to zinc deficiency, it provides a better opportunity for Mn2+ to enter tetrahedral sites easily, which outputs green emission under the activation of ultraviolet light. According to the PL spectrum in Figure 4(b), the samples exhibit two emissions at 650 nm and 767 nm with the x value not larger than 0.07, and the emission intensity increases with x value increasing from 0 to 0.07. However, increasing the x value from 0.07 to 0.13, a new emission peak at 678 nm appears in the PL spectrum, and the emission intensity sharply increases compared to that for x = 0.07 sample. The red light emission at 650 nm is mainly due to the 4T1-6A1 transition of Mn2+, while the deep red light emission at 678 nm is due to the 2E-4A2 energy level transition of Mn4+ [21]. When the x value is larger than 0.07, a large number of zinc vacancies appear, accompanied by the generation of many oxygen vacancies, which contributes to the transformation from Mn2+ to Mn4+ in octahedral environment, thus resulting in deep red emission at 678 nm. In addition, the enhancement of luminescence intensity is due to the oxygen vacancy defects, because they can serve as sensitizers for effective energy transfer to Mn2+ and Mn4+ [46-48]. When the concentration of zinc deficiency in Zn1-xAl2O4:Mn phosphor is x = 0, the 12  oxygen vacancy in the lattice is the intrinsic defect in ZnAl2O4 lattice. When the concentration of zinc deficiency increased, the oxygen vacancy increased significantly. This also enhances the emission intensity of near-infrared luminescence with defects as the emission center. 480 500 520 540 560 580 600 620 64005000001000000150000020000002500000300000035000004000000UV-Light  λ=365 nmx=0.13x=0.07Intensity (a.u.)Wavelength (nm) x=0 x=0.07 x=0.13λex=426 nmx=0(a)600 650 700 750 8000100000200000300000400000500000600000700000Intensity (a.u.)Wavelength (nm) x=0 x=0.07 x=0.13λex=325 nm x=0.13x=0.07x=0UV-Light  λ=302 nm(b) Figure 4. PL spectra of Zn1-xAl2O4:0.012Mn (x = 0-0.13) samples Figure 5 shows the PL spectra of samples with various Mn concentrations at the same zinc deficiency concentration (x = 0.13). The emission intensities at 510 nm (Mn2+ in tetrahedron site) and 678 nm (Mn4+ in octahedron site) all gradually increase at a higher Mn concentration (y value), because of the increased contents of Mn2+ in tetrahedron site and Mn4+ in octahedron site. However, the strongest red emission at 650 nm and NIR emission at 767 nm are found at y = 0.007 rather than at the largest y value of 0.012. Because Mn2+ ions in octahedron site contribute to the emission at 650 nm and oxygen vacancy defects contribute to the emission at 767 nm, there are some Mn2+ ions in octahedron site [21]. At a higher Mn content (y value), more Mn2+ ions appear in octahedron site, which contributes to more oxygen vacancy defects because of charge balance. Therefore, enhanced emissions at 650 nm and 767 nm are found by increasing the y value from 0.001 to 0.007. But enhanced emission at 678 nm and decreased emissions at 650 nm and 767 nm are found by increasing the y value from 0.007 to 0.012, indicating that further increasing more Mn4+ in octahedron site would reduce the number of oxygen vacancy defects and result in the decrease of Mn2+ in octahedron site. 13  480 500 520 540 560 580 600 620 64005000001000000150000020000002500000300000035000004000000Zn0.87Al2O4:yMnλex=426 nmIntensity (a.u.)Wavelength (nm) y=0.001 y=0.007 y=0.012(a)600 650 700 750 8000100000200000300000400000500000600000700000Zn0.87Al2O4:yMnλex=325 nmIntensity (a.u.)Wavelength (nm) y=0.001 y=0.007 y=0.012(b) Figure 5. PL spectra of Zn0.87Al2O4:yMn (y = 0.001-0.012) samples Figure 6 and Figure S3 show the persistent luminescence decay curves of Zn1-xAl2O4:0.012Mn (x = 0-0.13) samples after ultraviolet excitation for 5 min. Obviously, the samples exhibit green and deep red afterglows by removing the UV light source (Figure 6), but no obvious NIR afterglow is found (Figure S3). In addition, the zinc deficiency concentration does not significantly affect green afterglow (Figure 6a). But improved deep red afterglow is found at a higher x value, and the x = 0.13 sample shows the best afterglow performance (Figure 6b). The afterglow can last at least for 800 seconds. The electron traps play an important role on afterglow. Under the excitation of ultraviolet light, the traps constantly capture electrons. After stoppage of excitation, due to the influence of thermal disturbance on the electrons in the trap, the electrons will slowly escape from the trap and combine with the excited energy level, thus producing afterglow [49, 50]. The higher the trap concentration, the more electrons trapped and released by the trap, which helps to improve the afterglow [51, 52].  Figure 6. Persistent luminescence decay curves of the Zn1-xAl2O4:0.012Mn (x = 0-0.13) samples monitored at 510 nm (a) after 365 nm UV light illumination for 5 min, 678 nm (b) after 302 nm 0 200 400 600 800500100015002000250030003500Intensity (a.u.)Time (s) x=0 x=0.07 x=0.13 backgroundZn1-xAl2O4:0.012Mnmonitored at 510 nm(a)0 200 400 600 8000100020003000400050006000700080009000Intensity (a.u.)Time (s) x=0 x=0.07 x=0.13 backgroundZn1-xAl2O4:0.012Mnmonitored at 678 nm(b)14  UV light illumination for 5 min. Shallow traps below 100 oC are conducive to the occurrence of the afterglow because the electrons they captured can be excited at room temperature [53]. Figure 7 shows the Gaussian fitting TL luminescence curve of the samples, and the approximate trap depth can be estimated by the following formula [51, 54-56]: 𝐸 = 𝑇𝑚 500⁄                                                                                                            (2) Tm is the Kelvin temperature corresponding to the maximum peak in the TL curve. Figure 7(b-d) show that the fitted spectra of two peaks. The low temperature peak (T1) and the high temperature peak (T2) correspond to two shallow traps respectively. These two traps are formed by different defects, and the depth of the electron trap of the sample is shown in Table 2. With the increase of zinc deficiency concentration, it is clear that the concentration of oxygen vacancies traps increases significantly, and the concentration of MnAl+ traps arising from Mn4+ replacing Al3+ increases slowly. In Table 2, by comparing the peak area ratio of the Gaussian fitting spectra in Figure 7, it is found that the ratio gradually decreases with the increase of zinc deficiency concentration. Therefore, it speculates that T1 corresponds to MnAl+ trap and T2 corresponds to Vo trap. As shown in Figure 6, the green afterglow intensity monitored at 510 nm did not significantly increase with the change of defect concentration, while the red afterglow intensity monitored at 678 nm gradually increased with the increase of defect concentration. With the increase of zinc vacancy, the depth of oxygen vacancy trap decreases from 0.756 eV to 0.73 eV, and electrons are easier to be released. However, the 4T1 energy level of Mn4+ is located below the conduction band, and the 4T1 energy level is closer to the conduction band comparing with the 4E energy level of Mn2+, so electrons relaxed to the 4T1 excited state energy level can be preferentially accepted. Therefore, with more defects, the red afterglow of Mn4+ gradually increases, while the green afterglow intensity of Mn2+ almost remains unchanged. 15  300 320 340 360 380 4000102030405060Intensity (a.u.)Temperature (K) x=0 x=0.07 x=0.13Zn1-xAl2O4:0.012Mnλex=302 nm(a)300 320 340 360 380 4001520253035404550Intensity (a.u.)Temperature (K)ZnAl2O4:0.012Mnλex=302 nm(b) 320 340 360 380 400283032343638Intensity (a.u.)Temperature (K)Zn0.93Al2O4:0.012Mnλex=302 nm(c)  Figure 7. (a) Thermo-luminescence (TL) curves of Zn1-xAl2O4:0.012Mn (x = 0-0.13), gaussian fitting spectra of (b) ZnAl2O4:0.012Mn, (c) Zn0.93Al2O4:0.012Mn and (d) Zn0.87Al2O4:0.012Mn samples excited at 302 nm. The energy level splitting of Mn2+ in tetrahedral and octahedral coordination environment and Mn4+ in octahedral coordination environment can be represented by Tanabe-Sugano energy level diagram, as shown in Figures 8(a) and 8(b). In order to explore the luminescence mechanism of Mn2+ and Mn4+ in Zn1-xAl2O4:yMn, the mechanism diagram is speculated combining with the Tanabe-Sugano energy diagram, as shown in Figure 8(c). Table 2 Zn1-xAl2O4: 0.012Mn (x = 0-0.13) trap depth and TL fitting peak area ratio Zn1-xAl2O4:0.012Mn T1 T2 E1 E2 Peak area ratio (A1/A2) x = 0 332 K 378 K 0.664 eV 0.756 eV 3.025 x = 0.07 331 K 367 K 0.662 eV 0.734 eV 0.2863 x = 0.13 331 K 365 K 0.662 eV 0.730 eV 0.2574  Under the excitation of ultraviolet light, the electrons of Mn2+ in tetrahedral site undergo 6A1-4E, 6A1-4T2, 6A1-4A1, 6A1-4T1 energy level transitions, then are transferred to the 4T1 level via non-radiative relaxation, and finally returned to the 6A1 state with green light output. Mn2+ in octahedral coordination environment emit red 320 340 360 380 40012141618202224Intensity (a.u.)Temperature (K)Zn0.87Al2O4:0.012Mnλex=302 nm(d)16  light after undergoing this process. The electrons of Mn4+ in ZnAl2O4:yMn are excited from the ground state level 4A2 to the excited state levels 4T2, 2T2, 4T1 and Mn4+-O2- charge transfer energy band, then are transferred to the 2E level via non-radiative relaxation, and finally returned to the 4A2 state with an excellent red light output. In addition, due to the existence of VZn and VO traps in the sample, 325 nm ultraviolet light can excite the electrons from the VZn level to the VO level and conduction band and leave holes in the VZn level. Among them, the electrons excited to the VO level return to the VZn level through non-radiative transition and recombine with the holes to form the electron-hole pairs, thus realizing the emission of near-infrared light [16]. However, the electrons excited to the conduction band will be trapped by VO traps and MnAl+ traps. After stopping the excitation, the electrons will slowly release from the trap and relax to the excited energy levels of Mn2+ and Mn4+ and finally contribute to the red and green afterglow of Mn2+ and Mn4+.   0.0 0.5 1.0 1.5 2.0 2.50102030405060E/BDq/B4G4P4D6S 6A14T14T24E1 4A14T24E4T1g(a)0 1 2 3 4 501020304050  E/BDq/B2G4P4F 4A22Eg2T1g2T2g4T2g4T1g2A1g4T1g(b)17     Figure 8. Tanabe-Sugano spectra of Mn2+ (a) and Mn4+ (b) ions, (c) luminescence mechanism diagram of Zn1-xAl2O4:yMn. Figure 9 shows the varied emission intensity of the samples at different temperatures under different excitation wavelengths. With the increase of temperature, the decrease of luminous intensity is due to the enhancement of molecular thermal motion at high temperature, which intensifies the non-radiative transition [57]. When the sample is excited by ultraviolet light, the electrons of Mn2+ and Mn4+ are pumped from the ground state level to the excited state level, and then fall back to the ground state level to realize the emission of red and green light. In addition, some electrons are trapped by electron traps (Vo and MnAl+ traps). In the process of thermal excitation, although the electrons in the shallow electron trap are greatly consumed, the electrons in the deep electron trap are easily released under thermal disturbance [58-59]. Because the deep electron trap is closer to the Mn2+ energy level, the tunneling effect between the oxygen vacancy level and the Mn2+ energy level results in more electrons accumulating at the excited state level of Mn2+ [60]. With the increase of heating temperature, although the luminous intensity of the sample decreases gradually, the increase of temperature promotes the release of electrons from the trap to compensate for the emission loss [61]. Therefore, when the temperature rises to 125 oC, the luminous intensity of Mn2+ can still reach 79.17% of that at room temperature. However, the luminous intensity of Mn4+ is relatively low (58.36%) at 125 oC.  18    Figure 9. Temperature-dependent emission intensities of 510 nm and 678 nm for Zn0.87Al2O4:0.012Mn sample excited at (a) 426 nm and (b) 325 nm. Figure 10(a) shows the appearance of Zn1-xAl2O4:0.012Mn (x = 0-0.13) samples at different heating temperatures with different excitation wavelengths. Obviously, when the temperature rises to 498 K, the sample still maintains intense green and near-infrared light, but there is almost no red light. Based on the luminescence characteristics and thermal stability of the samples, multimode anti-counterfeiting application using Zn0.87Al2O4 and Zn0.87Al2O4:0.012Mn as component is shown in Figure 10(b). Zn0.87Al2O4 phosphor only emits near-infrared light, while Zn0.87Al2O4:0.012Mn phosphor emits green light, red light and near-infrared light. The phosphors were filled into the designed aluminum groove template with “370” 300 350 400 450 500 550 6000.40.60.81.01.2Intensity (a.u.)Temperature (K) Zn0.87Al2O4:0.012Mnλex=426 nmλem=510 nm(a)300 350 400 450 500 550 6000.00.20.40.60.81.01.2Intensity (a.u.)Temperature (K) Zn0.87Al2O4:0.012Mnλex=325 nmλem=678 nm(b)19  number as the correct password pattern. First, under the irradiation of different wavelengths of ultraviolet light (365 nm and 302 nm), the phosphors showed green visible light and red visible light of “370” at room temperature, respectively. However, the pattern under 302 nm ultraviolet light showed near-infrared “898” number, using night vision instrument. Another anti-counterfeiting method is to use the thermal stability of the sample to verify the correct digital pattern. When the sample was heated to 225 oC, the green visible light “370” excited by 365 nm ultraviolet light still kept an intense signal, but the red visible light “370” excited by 302 nm is almost invisible. It can be seen that the multimode luminescence phosphors have a potential application in the field of fluorescence anti-counterfeiting.   Figure 10. (a) Appearance of Zn1-xAl2O4:0.012Mn (x = 0-0.13) samples at different temperatures, (b) digital demonstration of multimode anti-counterfeiting logo using Zn0.87Al2O4 and Zn0.87Al2O4:0.012Mn phosphors.  4. Conclusion 20  Zn1-xAl2O4:yMn (x = 0-0.13, y = 0-0.012) phosphors were synthesized by a high temperature solid state reaction, which were characterized by a series of techniques, including XRD, DFT calculation, PLE/PL spectroscopy, TL, persistent luminescence decay curves, and temperature-dependent PL spectra analysis. In this spinel solid solution, Mn2+ and Mn4+ ions replace the tetrahedral position (Zn position) and octahedral position (Al position), respectively. They emit 510 nm green light with afterglow (4T1 (G)-6A1 (S) transition of Mn2+) and 678 nm red light with afterglow (2E-4A2 transition of Mn4+). The introduction of vacancy defects does not significantly affect the crystal structure. Increasing the value of x leads to more zinc vacancies and oxygen vacancies, and vacancy defects inhibit the self-reduction of Mn4+ ions at the Al site, providing more opportunities for Mn2+ ions to occupy the Zn position. The introduction of vacancy defects effectively regulates the emission centers of Mn2+ and Mn4+ in ZnAl2O4. The increase of vacancy defects can not only induce enhanced near-infrared emission at 767 nm, but also improve green emission at 510 nm and red emission at 678 nm. The prepared samples show multimode luminescence behavior by changing the excitation wavelength, indicating that they are the potential materials in application of advanced optical anti-counterfeiting.   Conflicts of interest The authors declare that they have no conflict of interest.  Acknowledgments This work was supported in part by the Fundamental Research Funds for the Central Universities (Grant N2302004) and National Natural Science Foundation of China (Grants 51302032, U21A2045, 52172112).   21  References and Notes [1] J.P. Xue, T. Hu, F.Q. Li, F.W. Liu, H. M. Noh, B.R. Lee, B.C. Choi, S. Park, J.H. Jeong, P. Du, Suppressed self-reduction of manganese in Mg2SnO4 via Li+ incorporation with polychromatic luminescence for versatile applications, Laser Photonics Rev. 17 (2023) 2200832. [2] A. Banerjee, Y. Shilina, B. Ziv, J. M. Ziegelbauer, S. Luski, D. Aurbach, I. C.Halalay, On the oxidation state of manganese ions in Li-ion battery electrolytesolutions, J. Am. Chem. 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