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Minghui Jin, Tao Zhang, [Ji‐Guang Li](https://orcid.org/0000-0002-5625-7361), Qi Zhu

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This is the peer reviewed version of the following article: Incorporation of Eu3+ in ZnGa2O4:Ni2+ for improved NIR persistent luminescence located in second transparency window, which has been published in final form at https://doi.org/10.1111/jace.19442. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Incorporation of Eu3+ in ZnGa2O4:Ni2+ for improved NIR  persistent luminescence located in second transparency  window](https://mdr.nims.go.jp/datasets/53dfd6b7-b8d7-4e51-84fb-e8e5bfac98c8)

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For Peer Review1Incorporation of Eu3+ in ZnGa2O4:Ni2+ for improved NIR persistent luminescence located in second transparency windowMinghui Jin1, Tao Zhang2, 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 China2Shenyang National Laboratory for Materials Science, Northeastern University, 3-11 Wenhua Road, Shenyang 110819, China3Research Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan*Corresponding authorDr. Qi ZhuTel: +86-24-8367-2700E-mail: zhuq@smm.neu.edu.cnPage 1 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960mailto:zhuq@smm.neu.edu.cnFor Peer Review2AbstractIt is well known that near-infrared (NIR) persistent phosphors have rather low absorption coefficients of biological tissues for NIR light. However, recent research shows that the phosphors emitting NIR lights in second (NIR-II, 1000-1350 nm) and third (NIR-III, 1500-1800 nm) biological window have advantages over that in NIR-I (650-900 nm). Although ZnGa2O4:Ni2+ outputs near-infrared (NIR) emission and afterglow located in NIR-II, the weak signal significant limits its application. Here, persistent luminescent phosphors of ZnGa2O4:xNi2+, yEu3+ (x = 0-0.013, y = 0.01-0.06) (termed as ZEGN) were synthesized via a traditional high-temperature solid-state reaction, which feature a broad emission band in the second near-infrared (NIR-II) window. Upon ultraviolet (UV) or orange-red lights excitation, the phosphors exhibit a broad NIR emission at about 1300 nm, arising from the 3T2(3F)→3A2(3F) transition of Ni2+. However, incorporation of Eu3+ ions, the NIR emission intensity significantly increases with the increase of Ni2+ ion concentration, reaching the maximum by 18 times at x = 0.005. Removing the light source yields intense NIR afterglow and red afterglow, with the duration longer than 500 s. It is worth noting that the red afterglow of Eu3+ shows a dramatically decrease but the NIR afterglow increases with increasing the Ni2+ ion concentration, because of energy transfer. Under the excitation of 282-nm UV light, the ZGEN sample exhibits a good thermal stability. The phosphor offers a promising application in biological imaging due to broadband NIR-II light and afterglow.Keywords: ZnGa2O4; Ni2+; near-infrared (NIR); second transparency window; persistent phosphorsPage 2 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review31. IntroductionFluorescence imaging is widely used in fields such as life sciences and clinical medicine due to its advantages of non-invasive, high sensitivity, and high spatiotemporal resolution.1-3 Indeed, near-infrared (NIR) light can penetrate biological tissues, such as skin and blood, more efficiently than visible light, while the light in the second NIR window (1000-1400 nm) has lower absorption, lower scattering and more efficient tissue penetration than the light in the first NIR window (650-950 nm).4 Therefore, the optical signal in the second window range of near-infrared can greatly improve the penetration depth, resolution, and signal-to-noise ratio of live imaging.1, 5 Traditional fluorescence imaging requires the use of external excitation light sources to excite real-time fluorescent probes inside the organism, inevitably resulting in the generation of biological tissue background fluorescence, which affects the resolution and signal-to-noise ratio of imaging.6 Long afterglow is a unique optical phenomenon that refers to the slow release of stored light energy by a material after stopping excitation (usually ultraviolet, X-ray, etc.), lasting for several minutes, hours, or even days. 7-9 Generally speaking, unlike traditional fluorescence imaging, long afterglow imaging does not require real-time excitation from external light sources, thus effectively avoiding interference from spontaneous fluorescence of biological tissues caused by excitation, thereby improving the resolution of in vivo imaging.Transition metal ions, as the candidate dopants to realize a broadband NIR luminescence, have attracted considerable attention.10 Among them, Ni2+ ions give a broadband NIR emission with a center around ~ 1300 nm when occupying an octahedral site in various materials, such as Ni2+-doped ZnAl2O4 nanostructures,11, 12 Ni2+-doped β-Ga2O3 nanocrystals,13 Ni2+-doped yttrium aluminum gallium garnet phosphors14 and so on, which is attributed to both its 3d8 electronic configuration and strong influence on the splitting of the energy level caused by the electrostatic potential, owing to the contribution of neighborhood ions. Researchers have made many attempts to improve the emission intensity of this fluorescent powder, such as increasing the excess or deficiency of Zn2+ in the raw material,15 doping Ge4+, Mg2+, or Li+ with Page 3 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review4average charge,16, 17 and adjusting the crystal field by doping Al3+, Sn4+ or other elements to change the proportion of ions in the matrix.6, 14, 18 All of the above methods essentially adjust the matrix lattice, that is, the direct environment in which the activated ions are located, thereby achieving an increase in material emission intensity, but rarely involving in energy transfer. As a rule of thumb, if the emission spectrum of the donor ions overlaps with the excitation spectrum of the acceptor ions, energy transfer usually occurs, which could increase the emission intensity of the phosphor.19-21 Eu3+ ion, which shows an intense emission at about 616 nm (5D0→7F2 transition), is the activator of red fluorescent powder for color television. It is interesting to note that the NIR absorption of octahedral Ni2+ and emission of Eu3+ partially overlap in spectra.18, 19, 21 Therefore, energy transfer from Eu3+ to Ni2+ in Eu3+/Ni2+ co-doped ZnGa2O4 may occur, making the emission intensity of Ni2+ increase.Herein, a series of ZnGa2O4:Ni2+, Eu3+ NIR phosphors were successfully synthesized using a solid-state reaction. Under the excitation of ultraviolet light, the energy transfer from Eu3+ to Ni2+ results in a significant increase by 18 times in the near-infrared emission and afterglow of Ni2+ ions, indicating that the phosphors are potential fluorescent materials for biological imaging field.2. Experimental Section2.1. Materials and synthesisZnGa2O4:yEu3+ (y = 0.01, 0.02, 0.04, 0.06) and ZnGa2O4:xNi2+,0.02Eu3+, (x = 0, 0.003, 0.005, 0.008, 0.01, 0.013) (called as ZGE and ZGEN in the following content) phosphor powders were prepared by a conventional high-temperature solid-state reaction. The raw materials (ZnO, Ga2O3, Eu2O3, and NiO) were all purchased from Sinopharm (Shanghai, China) with a high purity of 99.9%. All the materials were calculated accurately and were ground for 30 minutes to be fully mixed at the beginning. Then, the powder mixture was pre-sintered at 1000 ℃ for 4 hours under air conditions and ground for 30 min again, they were finally sintered at 1350 ℃ in air atmosphere for 8 h. The samples were slowly cooled to room temperature and grinded again about 30 min for the later test.Page 4 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review52.2. Characterization. Phase identification was performed via X-ray diffractometry (XRD, Bragg measurement, SmartLab, Rigaku, Tokyo, Japan) under 40 kV/200 mA using nickel-filtered Cu-Kα radiation and a scanning speed of 10 2θ per minute from 10 to 70. Product morphology and microstructure were analyzed by transmission electron microscopy (TEM, Model JEM-2000FX, JEOL) under 200 kV. Photoluminescence of Eu3+ was analyzed with an FP-8600 fluorospectrophotometer (JASCO, Tokyo) equipped with a Φ60 mm integrating sphere (Model ISF-834, JASCO), a 150 W xenon lamp for excitation, a slit width of 5 nm and a scan speed of 100 nm/min for both excitation and emission. The spectral response of the spectrophotometer was corrected with a Rhodamine-B solution (5.5 g⋅L−1 in ethylene glycol) and a standard light source unit (ECS-333, JASCO) for the ranges of 220-600 nm and 350-850 nm, respectively. The fluorescence decay of Eu3+ was also analyzed with the FP-8600 instrument and long-last persistent luminescence decay curves of Eu3+ was recorded by using a Horiba JY Fluorolog-3 (Kyoto, Japan) spectrofluorometer equipped with 150 W xenon lamps at room temperature. Meanwhile, Horiba JY Fluorolog-3 (Kyoto, Japan) was used to detect the photoluminescence spectrum (PL)/photoluminescence excitation spectrum (PLE) and long-last persistent luminescence decay curves of Ni2+. Before persistent luminescence tests, the samples were excited by a 365-nm UV lamp for 5 min.3. Results and discussion3.1 Phase identification and crystal structureZnGa2O4, an oxide compound with a spinel structure, belongs to the cubic crystal system with the formula AB2O4, in which the oxygen ions are densely packed in a face-centered cubic configuration with Zn2+ ions in the A site (tetrahedral site) and Ga3+ ions in the B site (octahedral site).22 According to the relevant literature,23 Eu3+ and Ni2+ should occupy the octahedral coordination of Ga3+, because Ni2+ and Eu3+ for ions have the same valence state and similar ionic radii and Ga3+ ions, respectively. Figure.1a shows a schematic diagram of the crystal structure with the ions occupying tendency Page 5 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review6FIGURE 1 (a) Crystal structure and (b) XRD patterns of ZGEN samplefor better understand. The XRD patterns of ZGE and ZGEN synthesized by the traditional high-temperature solid-state reaction were shown in Figure S1 and Figure 1b (we get the best content of Eu3+ from the following experiments, y = 0.02). It can be seen that all of the samples’ diffraction peaks are well matched to the standard diffraction of ZnGa2O4 (JCPDS No. 38-1240), indicating that the prepared samples are a single phase.Figure 2a and d show the TEM morphology of the typical sample of ZGEN (x = 0.005). It can be indicated that they are irregularly shaped particles of approximately 250 nm in size. Selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HR-TEM) were performed for the single particle, as shown in Figure 2b and c. The high crystallinity of the sample is confirmed by the clear lattice stripes in the HR-TEM image. The distances between the two crystal planes with Page 6 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review7different orientations are respectively measured to be ~ 0.296 and 0.253 nm with an angle of about 58.5°, so they are corresponding to the (220) and (311) crystallographic planes, matching well with the d (220) = 0.295 nm and the d (311) = 0.253 nm of the cubic structured ZnGa2O4 (JCPDS No. 38-1240). The SAED pattern shows symmetric lattice-grid characteristics, indicating the sample is single crystal. The elemental distribution results in Figure 2e-i show the successful doping of Ni and Eu and the homogeneous distribution of the chemical elements.FIGURE 2 (a and d) TEM image, (b) HR-TEM image, (c) SAED pattern, (e-i) elemental mapping of the x = 0.005 sample. (e-i) Element distributions of (e) Zn, (f) Ga, (g) O, (h) Ni, and (i) Eu, respectively3.2. Photoluminescence of the ZGE phosphorsPLE and PL spectra of ZGE phosphors (x = 0) were shown in Figure 3. Monitored at 616 nm, a strong band was found from 250 nm to 350 nm centered at ~ 282 nm, which can be attributed to the O2--Eu3+ charge transfer. According to the reports,24, 25 the other bands centered at 361, 392 and 464 nm corresponding to the transitions 7F0→5D4, 7F0→5L6 and 7F0→5D2 of Eu3+ ions respectively. Under excitation with 282 nm UV light, the ZGE phosphors show sharp lines from 550 nm to 750 nm, which are related to the typical transition of Eu3+ from the excited 5D0 to the 7FJ (J = 0, 1, 2, 3, 4) emission states with the 5D0→7F2 red one being the most prominent.25, 26 In addition, as shown in the inset of Figure 3, the emission intensity gradually increases with increasing the Page 7 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review8content of Eu3+. After the intensity reaching a maximum value when y = 0.02, it gradually decreases with further increasing the concentration of Eu3+ due to the concentration quenching effect.FIGURE 3 PLE and PL spectra of ZGE samples3.3. Photoluminescence and energy transfer of the ZGEN phosphorsFigure 4a shows the PL spectrum of the ZnGa2O4:0.02Eu3+ phosphor under excitation at 282 nm and the PLE spectrum of the ZnGa2O4:0.005Ni2+ phosphor monitored at 1310 nm. It can be seen that the emission spectrum of ZnGa2O4:0.02Eu3+ and the excitation spectrum of ZnGa2O4:0.005Ni2+ have a large overlap in the wavelength range from 570 nm to 630 nm. Theoretically, if the emission spectrum of the donor ion overlaps with the excitation spectrum of the recipient ion, energy transfer usually occurs between them. So maybe, there will be energy transfer between Ni2+ and Eu3+ in ZnGa2O4 system. Energy level diagram between Ni2+ and Eu3+ was shown in Figure 4b, it can be seen that some of the energy levels of the two ions are adjacent, therefore, there is a high possibility of energy transfer from Eu3+ to Ni2+ during the emission process. Based on the previous investigations, the electron transitions and energy transfer process of the samples may be as following. Under ultraviolet excitation, the electrons in the Ni2+ ion are excited from the ground state to the excited state energy level, then relax to the lowest excited state 3T2 and then returned to the ground state Page 8 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review93A2, producing NIR emission at 1310 nm.27 In addition, the electrons in Eu3+ ions are excited from the ground state 7F0 to the excited states 5DJ (J=1,2,3.4) and then part of the electrons relax to the lowest excited state 5D0, and then return to the ground state in the form of radiative transitions, resulting in a red emission of 616 nm.25, 26 Meanwhile, part of the electron energy at the excited state level 5D0 is transferred to the excited state level 3T1(3F) of adjacent Ni2+ ions, making the NIR emission of Ni2+ improve. FIGURE 4 (a) PLE spectra of ZnGa2O4:0.005Ni2+ and PL spectra of ZnGa2O4:0.02Eu3+ and (b) Schematic illustration for the energy transfer process between Ni2+ and Eu3+ in ZGEN phosphors, with the energy level location referring to the literaturesPage 9 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review10To determine whether this energy transfer exists in the system, we synthesized a series of ZGEN (x = 0-0.013) phosphors doping both Ni2+ and Eu3+ ions (the optimal doping amount of Eu3+ is y = 0.02). Figure 5 shows the PLE spectrum of ZGEN phosphors monitored at 1310 nm, which are consistent with a typical fluorescence pattern of ZnGa2O4 doped with Ni2+. The first excitation band centered at 250 ~ 350 should be generated by a combination of overlapping bands: the Ga3+ - O2- charge transfer band (CTB) at ~ 244 nm,28 Eu3+-O2- CTB of at ~ 282 nm 26 and the Ni2+-O2- CTB at ~ 310 nm.27 In addition, the bands centered at ~ 378 and ~ 610 nm respectively correspond to the 3d-3d transition of 3A2(F)→3T1(P) and 3A2(F)→3T1(F) of the Ni2+ ion 29. The PL spectra of ZGEN phosphors at 282 nm excitation spectrum are shown in Figure 5b. The PL spectrum shows a same broadband emission caused by the 3T2(F)→3A2(F) transition of Ni2+. It can also be seen that, the emission intensity increases gradually with increasing Ni2+ content, and the intensity reaches a maximum value at a Ni2+ concentration of 0.5 mol%, then, the intensity gradually decreases with furthering increasing the Ni2+ content, which is due to the concentration quenching effect. With the increase of nickel ion content, however, the intensity of Eu3+ dramatically decreases. The variations of Ni2+ and Eu3+ emissions are similar to the typical variation pattern presented in other sensitizer-activator systems,10, 20 based on this feature, it can be inferred that there is a phenomenon of energy transfer between Ni2+ and Eu3+.FIGURE 5 (a) PLE and (b) PL spectra of ZGEN samplesPage 10 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review11The energy transfer efficiency can be calculated by the following equation:20, 30                              (1)ητ = 1 ― 𝐼𝑠/𝐼𝑠0where,  represents the integral intensity of the emission spectrum of Eu3+ in the 𝐼𝑠presence of Ni2+,  represents the integral intensity of the emission spectrum of Eu3+ 𝐼𝑠0in the absence of Ni2+, is the energy transfer efficiency. Figure 6a-c shows ητ fluorescence decay curves of ZGEN samples (x = 0, 0.003 and 0.005) and based on the calculation above, energy transfer efficiencies of 21.77% and 45.51% were determined for x = 0.003 and 0.005, respectively. Rc (the critical distance) is the key parameter for assessing the average distance between activator and activator or sensitizer and activator when a concentration quenching effect plays a role and it can be calculated by the following equation:31                             (2)𝑅𝑐 = 2[3𝑉4𝜋𝑥𝑐𝑁]13where, xc is the critical concentration of Ni2+ ions, V is the volume of the unit cell and N represents the number of positions per unit cell available for the dopant (Ni2+). In this study, the values for V, xc and N are 578.01 Å3, 0.005 and 16, respectively. Based on these values, Rc of 30.21 Å was calculated. The exchange interaction and the electric multipolar interaction are the two models which can be used to understood the concentration quenching effect.20 In this work, the concentration quenching effect should be excluded from the exchange interaction model, because the value of Rc (30.21 Å) is much larger than 5 Å.32 According to Dexter’s theory, the type of work for electric multipolar interaction can be estimated by the following equation:20, 33                               (3)𝜂𝑜𝜂 ∝ 𝐶𝑛3Here, η0/η represents the ratio of the initial luminescence quantum efficiency of Eu3+ to the luminescence quantum efficiency of Eu3+ in the Ni2+ doped phosphor, which can be approximated by the ratio of the initial luminescence intensity of Eu3+ to the luminescence intensity of Eu3+ in the Ni2+ doped phosphor instead. Values of 6, 8 and 10 for n represent dipole-dipole (d-d), dipole-quadrupole (d-p) and quadrupole-quadrupole (q-q) interactions respectively. Figure 6e-g represent IS0/IS as a function of Page 11 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review12   FIGURE 6 (a-c) Fluorescence decay curve and fitting results for the 616 nm emission of ZGEN (x = 0, 0.003, 0.005) and (d) Relative emission intensities of Ni2+ and Eu3+ at 282 nm and energy transfer efficiencies as functions of Ni2+-doping concentrations (x). Relationships between IS0/IS and Cn/3 with (e) n = 6, (f) n = 8 and (g) n = 10Page 12 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review13Cn/3 (n = 6, 8, 10). Based on a linear fit for each case, the correlation coefficient (R2) is found to be a maximum of 0.92582 for n equals to 8. Therefore, the concentration burst effect in ZGEN phosphors is dominated by the d-p interaction mechanism.The decay time reflects the luminescence dynamics of the energy transfer process.20, 26 Monitored at 616 nm and 1310 nm, Eu3+ and Ni2+ ions respectively exhibit red afterglow and long near-infrared afterglow (Figure 7). After cessation of the UV excitation, the two persistent luminescence signals of the phosphors last at least 500 s. When x = 0, the intensity of the red afterglow is the maximum one. However, as the concentration of Ni2+ increases, the red afterglow sharply decreases, and the signal almost disappears when x = 0.013. This further illustrates the existence of energy transfer from Eu3+ ions to Ni2+ ions in this system. The near-infrared afterglow becomes more intense at higher Ni2+ content, reaching the maximum at x = 0.005, which also corresponds to the above spectral test results. The persistent luminescence mechanism in this work can be summarized in Figure 8b.  FIGURE 7 Persistent luminescence decay curves of ZGEN samples after 302 nm UV irradiation for 5 minPage 13 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review14FIIGURE 8 A schematic representation of the possible persistent red and NIR luminescence mechanism in this workUnder the excitation of UV lamp (302 nm), the ground-state electrons of Eu3+ and Ni2+ ions are promoted to conduction band. The excited electrons are subsequently captured by the electron traps near the conduction band, which are filled during a sufficient illumination time. After the stopping the UV irradiation, recombination between the conduction electrons released from the electron traps and the excited energy levels of Eu3+ and Ni2+ finally contributes to both the NIR and red afterglows. However, electrons falling back to the excited energy level of Eu3+ (5D0) were partly transferred to excited energy level of Ni2+ (3T1), resulting in a significant decrease of red afterglow and an enhancement of near-infrared afterglow. The near-infrared afterglow intensity of x = 0.005 sample is 1.25 times that of x = 0 at the beginning of the decay time.Page 14 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review15FIGURE 9 (a) Temperature-dependent luminescence spectra excited at 282 nm, (b) relative integral intensity of 616 nm emission bands, and (c) activation energy of thermal quenching for 616-nm emission bands at the temperature from 298 K to 573 K of ZEGN samplesThermal quenching is one of the most important factors affecting the effectiveness of fluorescent powder. As shown in Figure 9a, the temperature-dependent luminescence spectra of the sample located in the red-light region (x = 0.005) were measured at the range of 298 K ~ 573 K under 282-nm UV excitation. Due to the thermal quenching behavior, however, it can be seen that the luminescence intensity of the fluorescent powder decreases with the temperature increasing. The decrease of luminescence intensity with the increase of temperature is due to the enhancement of molecular thermal motion at high temperature, which intensifies the nonradiative transition. In Page 15 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review16addition, a small blue shift of the emission peak from 616 nm to 610 nm was found by increasing the temperature, mainly due to the stokes displacement. When the temperature increases gradually, the molecular thermal motion increases gradually, and the crystal field intensity decreases, resulting in a lattice shrinkage distortion and the decrease of Stokes displacement.34, 35 Taking the integral area of the emission peak as the object of intensity comparison and the emission peak intensity of 298 K as the benchmark, we calculated the intensity ratio of the integral intensity compared with the benchmark at different temperatures. Figure 9b shows its relative integrated emission intensity curves and the luminous intensity of the sample still reaches 70.50% even if the temperature rises to 100 ℃. Figure 9c exhibits the ln (A0/A − 1) versus 1/kT plot for the emission of Eu3+, from which the value of thermal activation energy ( ) was 𝐸𝑎determined to be ~ 0.180 eV by the following formula:                             (4)𝐴 =𝐴01 + 𝑐 𝑒𝑥𝑝 ( ―𝐸𝑎𝑘𝑇)where A0 and A are the integral area of the emission peaks at room temperature (298 K) and experimental temperature T, respectively, k is the Boltzmann constant (8.62 × 10-5 eV), and c is a constant. The Ea value falls in the value range of 0.06 ~ 0.57 eV that reported in the literatures.36-38 The results above show that the ZGEN sample has a good thermal stability under the excitation of 282-nm UV light.4. ConclusionIn this work, we have successfully synthesized ZnGa2O4:Ni2+,Eu3+ persistent luminescent phosphors via a traditional high temperature solid-state reaction, which feature a broad emission band located in the second near-infrared (NIR-II) window. The samples were characterized by XRD, TEM, PLE/PL spectroscopy, lifetime, persistent luminescence decay and temperature-dependent PL spectra analysis. Ni2+ and Eu3+ are likely to occupy Ga3+ site in ZnGa2O4, forming a six-coordinated octahedron. The samples can not only be excited by ultraviolet (UV) light but also by orange-red light, in which, the emission spectrum of ZnGa2O4:0.02Eu3+ and the excitation spectrum of ZnGa2O4:0.005Ni2+ have a large overlap. Besides, under 282 nm UV excitation, an energy transfer from Eu3+ to Ni2+ ions could be observed which improves Page 16 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review17the NIR emission intensity. After removing the light source, the sample outputs both red and NIR afterglow, which can last more than 500 s. With the increase of Ni2+ ion concentration, however, the red afterglow monitored at 616 nm decreases significantly but the NIR afterglow increases, mainly due to the energy transfer. The ZGEN sample has a good thermal stability. The prepared persistent phosphors are the potential materials for bio-imaging application in NIR-II window, due to their charming emissions and afterglows.AcknowledgementsThis 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, U21A2045, 52172112).References[1] Pei P, Chen Y, Sun C, Fan Y, Yang Y, et al. X-ray-activated persistent luminescence nanomaterials for NIR-II imaging. Nat Nanotechnol. 2021;16:1011-1018.[2] Mao M, An M, Zhang X, Chu P. In vitro and in vivo antibacterial activity of graphene oxidemodified porous TiO2 coatings under 808-nm light irradiation. 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Ceram Int. 2016;42:14086-14093.Page 20 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer ReviewIncorporation of Eu3+ in ZnGa2O4:Ni2+ for improved NIR persistent luminescence located in second transparency windowMinghui Jin1, Tao Zhang2, 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 China2Shenyang National Laboratory for Materials Science, Northeastern University, 3-11 Wenhua Road, Shenyang 110819, China3Research Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan*Corresponding authorDr. Qi ZhuTel: +86-24-8367-2700E-mail: zhuq@smm.neu.edu.cnPage 21 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960mailto:zhuq@smm.neu.edu.cnFor Peer ReviewFig. S1 XRD spectra of ZnGa2O4:yEu3+ samplesPage 22 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review 314x222mm (300 x 300 DPI) Page 23 of 23Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960