# Fileset [Manuscript-Xiahou-ICF.pdf](https://mdr.nims.go.jp/filesets/ddda48fb-ac02-4311-a5db-2be35d7c4256/download) ## Creator Junqing Xiahou, Qi Zhu, Fan Li, Minghui Jin, Lin Zhu, Sai Huang, Tao Zhang, Xudong Sun, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361) ## Rights [In Copyright](http://rightsstatements.org/vocab/InC/1.0/) ## Other metadata [Regulating the trap distribution of ZnGa2O4:Cr3+ by Li+ /Ga3+ doping for upconversion-like trap energy transfer NIR persistent luminescence](https://mdr.nims.go.jp/datasets/3c3c8e39-30c8-4d05-a8df-9cb85bb88ef4) ## Fulltext Regulating the trap distribution of ZnGa2O4:Cr3+ by Li+/Ga3+ doping for upconversion-like trap energy transfer NIR persistent luminescence Junqing Xiahou1, Qi Zhu1*, Fan Li1, Minghui Jin1, Lin Zhu2, Sai Huang1, Tao Zhang3, Xudong Sun4, and Ji-Guang Li5* 1Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, PR China 2College of Sciences, Northeastern University, Shenyang Liaoning 110819, PR China 3Shenyang National Laboratory for Materials Science, Northeastern University, 3-11 Wenhua Road, Shenyang, Liaoning 110819, PR China 4Foshan Graduate School of Northeastern University, Foshan, Guangdong 528311, PR China 5Research 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 tel:+86-24-8368-1680tel:+81-29-860-4394mailto:Jiguang@nims.go.jpAbstract ZnGa2O4:Cr3+ persistent luminescent phosphors (PLPs) have been widely applied in bioimaging and photonics due to their ultra-long near-infrared (NIR) afterglow. However, UV and visible excitation currently in use have shallow penetration depth or harmful effect on organism, which limits the long-term bioimaging. Therefore, developing NIR PLPs excited by NIR light is urgent for bioimaging. Here, Zn1-x(Li/Ga)xGa2O4:Cr3+ (x=0-1) NIR PLPs were synthesized. All the newly introduced Ga3+ ions occupy the tetrahedral sites. However, with increasing Li+/Ga3+ content, Li+ ions firstly occupy the tetrahedral position, then partially enter octahedral sites, and completely occupy the octahedral sites at x=1. Incorporation of Li+/Ga3+ contributes to the weakened crystal field strength, which leads to a deeper trap depth and a wider trap energy level. A complete replacement of Zn2+ by Li+/Ga3+ ions leads to the splitting of the trap energy level into two-divided ones, which reduces the electron transfer between deep/shallow traps and makes the deep trap energy level be near to the 2E energy level of Cr3+. Therefore, an enhanced NIR afterglow excited by the low-energy NIR light is found for the Li+/Ga3+ doped sample. This work provides a new category for the NIR-absorptive-NIR-emissive PLPs and proposes a new application for long-term bioimaging. Keywords: near-infrared persistent luminescence; crystal field strength; trap depth; low-energy excitation 1. Introduction In recent decades, near-infrared (NIR) persistent luminescent phosphors (PLPs) have shown broad application prospects in the field of biological imaging mainly due to their two advantages: (1) The avoidance of self-fluorescence from biological tissue due to excitation in vitro; (2) The high spatial resolution, deep penetration depth, and low optical absorption of NIR optical signals through the organism. Trivalent chromium ion (Cr3+) is a near-infrared red doping ion because of its emission range from 650 to 1600 nm.1-3 Based on the above reasons, spinel long persistent luminescence materials of Cr3+-doped ZnGa2O4, Zn3Ga2Ge2O10, and other similar gallate hosts have become the research hotspots in the field of luminescence and biological imaging due to their near-infrared emission and long afterglow characteristics.4-6 Subsequently, a variety of functionalized zinc gallates or zinc gallogermanates nanoparticles with NIR long afterglow were applied in biological imaging, life sciences, and biomedicine.7-9 The research on the spinel-structure NIR persistent luminescence materials mainly focuses on two aspects: 1. Using monodisperse luminescent particles as the fluorescent probes for different biological applications, including live animal imaging, drug delivery, tumor imaging, etc. 2. Regulating the crystal structure for improved persistent luminescence, such as co-doping Cr3+/Ge4+/Sn4+, Cr3+/Mg2+/Ge4+, Cr3+/Zn2+/Sn4+, Cr3+/Li+, Cr3+/Pr3+, Cr3+/Al3+/Bi3+, and Cr3+/Gd3+/Sn4+ in zinc gallate. 10-16 ZnGa2O4:Cr3+ belongs to cubic spinel structure, which contains 56 atoms in the F-d3_m. In ZnGa2O4, Zn2+ ions occupy the 8a sites (CN=4, CN: coordination number), Ga3+ ions occupy the 16d sites (CN=6).2, 10, 11, 17 For ZnGa2O4:Cr3+, Cr3+ would like to replace Ga3+ in octahedron site, because Cr3+ and Ga3+ ions have similar ionic radii (RCr=0.63 Å; RGa=0.62 Å) in octahedron. Cr3+ ions are doped in spinel-structured gallates as the emitters, and the crystal field as well as defect regulation are crucial for the improvement of photoluminescence and persistent luminescence. The optical absorption and luminescence of the Cr3+ ions can be remarkably influenced by host materials because the energy levels of the d electrons of Cr3+ ions are dependent on the symmetry of oxygen coordination and crystal field strength.3, 18, 19 Cr3+ ions can emit a broadband emission at ~650-1600 nm (the spin-allowed 4T2 → 4A2 transition) and a narrow-band emission at ~700 nm (the spin forbidden 2E → 4A2 transition) in a weak and strong crystal field, respectively. However, the 2E and 4T2 levels are overlapped in the intermediate crystal field, leading to coincidence of these two transitions from 4T2/2E to 4A2. Recently, the investigations on the influence of crystal field on luminescence by co-doping different ions have been widely reported. However, investigations about the effect of crystal field on the electron traps and the persistent luminescence are rather limited. The afterglow behavior has been attributed to the Cr3+ ion and nearby anti-site defect interactions in the spinel ZnGa2O4 based on the currently accepted model. Research showed that traps play an important role as active centers in persistent phosphors of ZnGa2O4:Cr3+. Changing the Zn ratio or incorporating other metal cations is the effective ways to change the trap distribution and depth of phosphors, and further improve their persistent luminescence. However, the exact effect of the trap depths and distributions on persistent luminescence have not been defined.14, 20-22 On one hand, the slowly released electrons from the deep traps influence the afterglow of persistent phosphor, and the deeper traps result in the slower release of electrons from traps, which contributes to a longer afterglow;22-24 On the other hand, higher density and larger number of traps may contribute to a stronger afterglow.22, 25 Moreover, some studies indicate that the trap distribution also affects the light storage of various energies or various wavelengths.19, 26, 27 In general, low-energy excitation can not excite the electrons to the traps. However, continuously distributed traps result in up-convertible-like trap energy storage and electron transfer between deep traps (DTs) and shallow traps (STs), which facilitates biological imaging of in situ long-wavelength excitations.26 However, deep traps and shallow traps are continuously distributed for most materials, which leads to rapid energy transfer between traps and accelerates the release of electrons from deep traps. Therefore, the reconstruction and segmentation of trap distribution may regulate the electron transfer between DTs and STs and thus improve the near-infrared persistent luminescence of phosphors. Herein, a series of Zn1-x(Li/Ga)xGa2O4:Cr3+ (x=0-1) NIR PLPs were synthesized using a solid-state reaction. As Li+/Ga3+ content increases, the crystal field strength of phosphors decreases, which makes a redshift of excitation and emission spectra. The crystal field also affects the trap depth and distribution. A complete replacement of Zn2+ by Li+/Ga3+ ions leads to the splitting of the trap energy level, which reduces the electron transfer between DTs and STs, but enhances the NIR afterglow by low-energy excitation. The outcome in this work provides a new category for the persistent luminescence nanoparticles with a NIR-absorptive-NIR-emissive feature and a new application for biological imaging. 2. Experimental section All the experimental methods are described in detail in supporting information. 3. Results and discussion 3.1 Synthesis and local structure of Zn1-x(Li/Ga)xGa2O4:0.005Cr3+ 10 20 30 40 50 60 70Intensity (a.u.)x=0x=0.8x=1x=0.4ZnGa2O4 (JCPDS no. 38-1240)(111)(220)(222)(311)(400)(422)(511)(440)4500 cps2q (degree)LiGa5O8 (JCPDS no. 38-1371)(111)(220)(311)(222)(400)(422)(511)(440)(331)(531)x=0.2x=0.6(110)(211)(210)(510)(520)(441)(521)(530)35 36 372q (degree)(311)(311) (531)(421)(221)(310)(320)(321)(410)(411)(332)(430)(610) Figure 1. XRD patterns for Zn1-x(Li/Ga)xGa2O4:0.005Cr3+ (x=0-1) calcined at 1300 oC. A series of Zn1-x(Li/Ga)xGa2O4:0.005Cr3+ (x=0-1) powders were synthesized and the XRD were performed to analyze the crystal structure and phase transition in Figure 1. The measured XRD patterns of x=0-0.8 indicate a crystalline structure corresponding to the reference pattern for ZnGa2O4 (ZGO) of spinel structure (JCPDS File No. 38-1240). However, LiGa5O8 (LGO) cubic phase (JCPDS File No. 38-1371) with new diffraction peaks appears only at the larger x value of 1. The reference patterns of LiGa5O8 (JCPDS File No. 38-1371) correspond to a spinel structure with the space group of P4332, a derivative of the Fd3_m space group of spinel structure. Furthermore, the (311) main diffraction of spinel gradually shifts to the bigger angle side with increasing the x value, and the lattice constant of the powders decreases from a=8.338 to 8.203 Å with the value of x increasing from 0 to 1 (Table S1), indicating that the unit cell is shrink by substitution of Zn2+ with the Li+/Ga3+ pair. For convenient description, Zn1-x(Li/Ga)xGa2O4:0.005Cr3+ and Zn1-x(Li/Ga)xGa2O4 are denoted as ZLGGC and ZLGGO hereafter. -315.5-316.0-316.5-317.0-317.5 Li4-Ga4 Li8-Ga4Formation energy (eV)Different confrigurations(a)lowest -319.0-319.5-320.0-320.5-321.0-321.5-322.0 Formation energy (eV)Different confrigurations(b) Li4-Ga4 Li8-Ga4lowest -323-324-325-326 Formation energy (eV)Different confrigurations(c) Li4-Ga4 Li8-Ga4lowest -327-328-329-330-331 Formation energy (eV)Different confrigurations(d) Li4-Ga4 Li8-Ga4lowest Figure 2. The DFT total energy of the unit cell with different doping contents of Li/Ga: (a) 25%, (b) 50%, (c) 75%, (d) 100%. Table 1. The calculated energy difference (ΔEd) with various Li/Ga doping content. Li/Ga content E(Li4-Ga4) (eV) E(Li8-Ga4) (eV) ΔEf (eV) 25% -317.043 -316.643 0.4 50% -321.396 -321.209 0.187 75% -325.750 -325.676 0.074 100% -330.109 -330.721 -0.612 It is known that, in the structure of ZGO spinel, Ga3+ ions occupy the 16d sites, Zn2+ ions occupy the 8a sites, with O occupying the 32e sites. In this work, co-substitution of Li+/Ga3+ for Zn2+ results in a smaller lattice constant. A complete replacement of Zn2+ by Li+/Ga3+ ions leads to the LGO spinel structure: The Ga3+ ions occupy both the 8c (CN = 4) and 12d sites (CN = 6), and the Li+ ions occupy only the 4b sites (CN = 6). Four oxygens are surrounding the tetrahedral site, and six oxygens surround the octahedral site.17, 28, 29 It is noticed that Li+, Zn2+, and Ga3+ ions have different radii in different coordination environments: Li+ (rIV = 0.59 Å, rVI = 0.67 Å), Zn2+ (rIV = 0.60 Å, rVI = 0.74 Å), and Ga3+ (rIV = 0.47 Å, rVI = 0.62 Å). Referring to the ionic radius, Li+/Ga3+ ions prefer occupying Zn2+ sites, but the specific occupations need to be determined by a variety of structural detection.11, 22, 30 Different configurations were built for formation energy calculation to explore the site preference of Li+/Ga3+ doping in ZnGa2O4. There are two conditions for Li/Ga doping in spinel ZnGa2O4: (1) Li and Ga occupy the tetrahedral sites; (2) Li occupies the octahedral sites and Ga occupies the tetrahedral sites, which are defined as Li4-Ga4 and Li8-Ga4, respectively. Figure 2 demonstrates the unit cell’s density functional theory (DFT) total energy with different Li/Ga content doping. As shown in Figure 2, two, seven, seven, and four crystal structure models are created for the tetrahedral positions occupied by Li and Ga (Li4-Ga4) with 25%, 50%, 75%, and 100% Li/Ga content, respectively. There are 5, 66, 106, and 22 crystal structure models created for calculating the DFT total energy of Li in the octahedron and Ga in the tetrahedron (Li4-Ga8) with 25%, 50%, 75%, and 100% Li/Ga content, respectively. The energy difference (ΔEd) of conditions for Li/Ga doping was calculated as follows:22, 31, 32 ΔEd = E(Li4-Ga4)- E(Li8-Ga4) (1) where E(Li4-Ga4) is the unit cell’s lowest DFT total energy with Li and Ga occupying the tetrahedral sites, E(Li8-Ga4) is the unit cell’s lowest DFT total energy with Li in the octahedron and Ga in the tetrahedron. All the calculated results are demonstrated in Table 1. When the content of Li+/Ga3+ is not more than 75%, the energy difference is calculated as 0.4 (25%), 0.187 (50%), and 0.074 (75%) eV, which indicates that the most likely placeholder in ZLGGO is Li and Ga both in tetrahedral sites. However, the energy difference decreases with increasing Li/Ga content. When the Li/Ga content reaches 100%, the energy difference was calculated to be -0.612 eV, which indicates that the most likely placeholder in the x=1 sample is Li in octahedral and Ga both in octahedral and tetrahedral sites. According to the calculation results, the lowest energy configurations with different contents of Li/Ga doped in spinel ZnGa2O4 are shown in Figure 3. Figure 3. 56-atom unit cells crystal structure with different contents of Li/Ga doped in spinel ZnGa2O4 at condition of the lowest energy: (a) 25%, (b) 50%, (c) 75%, (d) 100%. To explore the occupations of Li+ and Ga3+ ions in tetrahedral and octahedral sites, the 7Li and 71Ga solid-state MAS NMR spectra of ZLGGO powder were obtained (Figure S1 and Figure 4a). With increasing Li+/Ga3+ concentration, the peaks in 7Li NMR spectra move to the high chemical shift, which means the Li+ ions move from tetrahedron to octahedron. There are interactions between electric field gradients and nucleus quadrupolar moment for 71Ga, a half-integer quadrupolar nucleus (I=3/2), which leads to the NMR spectra to be correspondingly broadened.10, 33 Fast sample spinning frequencies and high magnetic fields are required for the resolution of the corresponding resonances in MAS spectra. 10 The coordination numbers and local symmetries significantly influence the shifts of 71Ga MAS NMR line of the central transition.33, 34 The range of Ga in an octahedral or tetrahedral environment will be 80 to 42 ppm, and 107 to 222 ppm, respectively.10, 34-36 Solid nuclear magnetic field-tested at high speed will produce rotating sidebands, which is the peak caused by the rotor rotating at high speed in the probe. The sideband peaks are easy to be identified because they are symmetric relative to the main peak, and the distance from sideband peak to main peak is the frequency of rotation. As shown in Figure 4a, the peak of GaIV overlaps with the sideband, which makes it difficult to directly quantify the content of Ga in octahedron. The peak integral area ratio in NMR spectra is used to quantitatively analyze the content of Ga3+ in tetrahedron and octahedron. The areas of ~-110-~15 ppm, ~15-~90 ppm, and ~90-~230 ppm are defined as Regin1 (R1), Regin2 (R2), and Regin3 (R3), respectively. The quantification of GaIV/GaVI proportion can be determined from the three areas of signal integration using the following formula: (R3-R1)/R2 (Figure S2). As shown in Table 2, the quantification of GaIV/GaVI in ZGO determined from 71Ga NMR spectra were of ~1.7%, ~5%, ~21%, and ~66% for the x = 0, 0.2, 0.6, and 1 samples, respectively. The GaIV/GaVI ratios of x=0 and x=1 samples are 1.7% and 66%, which agree with the spinel inversion in ZGO and the sites allocation in LGO reported in previous results.10, 11, 28, 36 The GaIV/GaVI ratio of ZLGGO measured from the MAS NMR spectrum increases from 1.7% to 66% with the Li+/Ga3+ content increases from 0 to 100%. This indicates that more Ga3+ ions and Li+ ions occupy the tetrahedron and octahedron respectively at a higher x value. Raman spectroscopy was performed to further reveal the changes for structure in Figure 4b. Cubic spinel contains 42 vibrational modes in its group theory. The following modes of spinels are predicted by group theory analysis at the Γ point:18, 37-40 Γ=A1g(R)+Eg(R)+T1g+3T2g(R)+2A2u+2Eu+4T1u(IR)+ T1u+2T2u (2) where most modes are silent or acoustic modes except the indicated infrared-active and Raman-active modes. There are only four infrared active modes and five Raman active modes in normal non-defective spinels. In general, the mode frequencies follow the sequence: T2g 0.8), it drops. Figure S8b shows the persistent luminescence spectra with x = 0.8 phosphor as an example. It is shown that the spectra own a band with the peak at ~708 nm, indicating that the phosphors outputs excellent NIR afterglow, which can last more than 2 h. The TL curve of ZLGGC samples was measured at the temperature of 300-450 K to explore the trap depth and distribution (Figure 8b, Figure S9). The trap density n and trap depth E are estimated according to the equations: 22, 55, 56 E = Tm/500 (10) n = ωIm/{β×[2.52+10.2×(μg-0.42)]} (11) 300 350 400 450 500 550 60005000100001500020000 x=0 x=0.2 x=0.4 x=0.6 x=0.8 x=1(b)Intensity (a.u.)Temperature (K) 0.0 0.2 0.4 0.6 0.8395400405410415135140145150155160165170175 FWHMTm (K)The x value TmFWHM (K)(c) Figure 8. (a) Time-dependent NIR afterglows of ZLGGC after 254 nm UV light illumination for 10 min. (b) TL curves of ZLGGC after 254 nm UV light illumination for 10 min. (c) Tm and FWHM of the TL bands for different x values. (d) Schematics illustrating the multi-trap energy level diagram with the x increasing. All statistics are demonstrated in Table S7. The x=0.8 sample outputs the best persistent luminescence because it has the deepest trap depth and the highest electron trap density. It is known that most of the TL bands have asymmetric shapes. The trap energy is assumed to have a "quasi-continuous" distribution, in which case a well-defined TL peak is not observed, but a broad peak composed of several closely spaced TL peaks superimposed. In this study, with the increase of x value (higher Li+/Ga3+ content), the TL peaks gradually moved to high temperature, the full-width at half maximum (FWHM) gradually increased and split into two peaks when x=1 (Figure 8b). It’s fantastic that two broad bands appear in the TL curve of the x=1 sample, and the centers are at 405 K and 544 K, respectively, which indicates that the x=1 sample owns two kinds of traps, including deep traps (DTs) and shallow traps (STs). Figure 8c additionally shows the FWHM of the TL band and the peak temperatures of the TL bands (Tm), as a function of the x value. It is seen that the FWHM and peak temperatures of the TL bands are 139/396, 141/398, 144/401, 153/402 and 173/415 K for x=0, 0.2, 0.4, 0.6 and 0.8, respectively. This phenomenon is closely related to the crystal structure and crystal field strength: it may be that the increase of Li+/Ga3+ content leads to the decrease of crystal field, which further deepens the trap depth and widens the energy level, similar to the centroid shift of Ce3+ ion luminescence, which has also been observed in previous studies.22, 57 The multi-trap energy level diagram (Figure 8d) was drawn according to Figure 8b and 8c: with the increase of the x value, there are two effects :(1) the trap energy level center moved down; (2) the trap energy level coverage area gradually increased. When x ≤ 0.8, there are many lattice defects in the sample, which are in disordered state. The crystal field strength is greater than 2.10, thus Cr3+ ions are in strong crystal field with 2E→4A2 transition dominating, which causes continuously distributed trap energy level gradually moves down and becomes wider. When x reaches 1, the crystal field strength is less than 2.10. Therefore, Cr3+ ions are in weak crystal field, which causes the trap level splitting into two divided traps including deep traps (DTs) and shallow traps (STs). This further makes the deep trap energy level and 2E energy level are prone to electron transfer by tunneling. The splitting of two kinds of traps in x=1 sample means that the rate of electron transfer between DTs and STs may decrease, which favors the DTs to preserve the charged electrons and sets the stage for the subsequent prolonged the low-energy-excitation afterglow.26, 55, 56, 58 To further explore the spatial distributions of trap energy levels following optical excitation, TL spectra of the x=1 sample were detected at 0 min, 30 min, and 24 h after ceasing the UV excitation (Figure 9a, Figure S10). The low-temperature TL band shifts from 405 to 476 K when the interval time following UV irradiation increases from 0 s to 30 min and 24 h, while little change was observed in the high-temperature band intensity peaked at 544 K, indicative of carrier exhaustion in STs and little release of the carriers in DTs. These results suggest that the trapping process consists of two steps, the shallow trapping step and DTs capture step (Figure 9b): first, following pumping of high-energy photon, the charge carriers are introduced into shallow traps; second, the carriers are further filled into deeper traps via nonradiative relaxation. In addition to the conventional capture of carriers from STs to DTs, the mechanism also involves an up-conversion-like electron transition from DTs to STs. Based on previous reports, ZGC has a NIR persistent luminescence repeatedly activated by red light (~650 nm). NIR excitation (Longer-wavelength) maybe more beneficial for bioimaging because it can reduce tissue scattering, thereby increasing effective in vivo penetration depth.26 To investigate the existence of electron transition from DTs to STs in the system, ~650 nm red (25 W) and ~740 nm NIR LED (25 W) were used to estimate the ability to re-excitation. Time-dependent afterglows and afterglow decay curves of ZLGGC phosphors were demonstrated in Figure S11 and S12. The results indicate that samples own a better afterglow after the ~650 nm red LED excitation (There’s not much difference in intensity.) and the x=1 sample exhibits the best afterglow after low energy excitation, which is mainly due to that trap splitting reduces the rate of electron transfer from the deep trap to the shallow trap. However, under the same thickness of pork (5 mm), the x=1 sample particles demonstrate a better afterglow after the ~740 nm NIR LED excitation than the ~650 nm red LED excitation since longer-wavelength NIR excitation can decrease tissue scattering (Figure S13). Therefore, a large irradiation area NIR LED (~740 nm) was utilized as light source in the subsequent tests (Figure S14). With a pork thickness of 2 cm, the x=1 sample particles covered by pork still maintained ~30% emission intensity compared to the x=1 sample particles not covered by pork under the 254 nm-UV light (Figure S15). After ceasing the ~740 nm NIR LED excitation, the afterglow decay curve in Figure S16 clearly shows that NIR irradiation leads to the afterglow at 718 nm lasting more than 1500 s. 300 350 400 450 500 550 60002000400060008000100001200014000 0 min 30 min 24 h(a)Intensity (a.u.)Temperature (K) 300 350 400 450 500 550 60002004006008001000 200 s 600 s 1200 s (c)Intensity (a.u.)Temperature (K) Figure 9. (a) TL spectra of x=1 sample after ceasing the UV excitation (excitation wavelength: 254 nm; exposure time: 10 min; interval time: 30 s, 30 min, and 24 h). (b) Schematics illustrating the normal trapping mode of deep traps (DTs). (c) TL curves of x=1 sample were measured using the ~740 nm NIR LED (25 W) with different excitation times (exposure time: 200, 600, and 1200 s). (d) Schematics illustrating the up-conversion trapping process from DTs to STs under the low energy excitation. (e) In vivo NIR afterglow images of a nude mouse (BALB/cA-nu adult mouse) after intramuscular injection of LGC-NH2 solution (1 mL, 2 mg·mL-1, 10 min exposure to a 254 nm UV light before injection) and recharging NIR afterglow decay images after 20 min secondary excitation with a ~740 NIR LED lamp. To investigate the existence of electron transition from DTs to STs in this system, the TL curves of the x=1 sample were measured using the ~740 nm NIR LED as excitation source with different excitation times. After a 200-s irradiation with NIR LED (~740 nm), a distinct TL peak was observed at 530 K in the TL spectra (Figure 9c, Figure S17). With time increasing from 200 to 1200 s, the TL peak shifted from 540 to 507 K, and a new low-temperature TL peak was observed at 410 K, suggesting that the exposure time can affect the distribution of carriers in DTs and STs. Meanwhile, the afterglow has been extended with the exposure time changing from 200 to 1200 s, which further indicates the transfer of electrons from DTs to STs javascript:;(Figure S18). All the results imply that the trapping process and NIR excited persistent luminescence are as follows (Figure 9d): Under the NIR excitation, the electrons are pumped from 2E to 4A2, and the excited electrons are captured by the DTs through tunneling; under the NIR irradiation, some electrons are stored in the DTs and some are pumped into the STs like up-conversion energy transfer, which leads to the NIR emission; removing the NIR light source, the stored electrons release from DTs to STs and then to the excited energy level of Cr3+ ions, which finally contributes to the NIR afterglow. The energy storage based on the up-conversion-like trapping process also requires reproducibility and photostability under irradiation of NIR, which are essential for reliable biological imaging. 4, 5 Therefore, the attenuating spectra after four ~740 nm NIR LED irradiation are shown in Figure S19. The initial intensity and decay duration remained essentially unchanged even following more than eight on/off cycles (Figure S20). In order to realize the near infrared light renewable charging in biological imaging, the nanoparticles of x=1 sample with good dispersion and size within 200 nm were collected by sieving (Figure S21). Before the in vivo imaging experiment, the Cell Counting Kit-8 assay was performed on RAW264, A549-1, and HEK293T cells to assess the cytotoxicity of LGC-NH2 (Figure S22), which confirms the low toxicity of LGC-NH2 to the cells. The nanoparticles-NH2 glucose solution (1 mg·mL-1) was exposure to the 254-nm UV light for 10 minutes for in vivo imaging. After irradiation process, 500 μL of the irradiated aqueous solution was injected subcutaneously into a nude adult mouse on the belly and the NIR afterglow signal was tested for bioimaging analysis. 16, 59 The intense signal of NIR afterglow could be detected during the test time throughout the mouse body with no further excitation treatment, although signal loss took place over time. The afterglow luminescence signal lasted for at least 2 h (Figure 9e) under such NIR excitation. To assess the repeatability under such NIR excitation, the nude adult mice were subjected to a 20-min in situ excitation using NIR LED lamp (~740 nm). Following red-light excitation, repeatable NIR afterglow signal appeared, which could last for more than 1 h (Figure 9e). The above results suggest that the prepared Cr3+-doped gallate probes are the effective fluorescent dyes for NIR light recovery afterglow to enhance bioimaging by connecting the electron transport channel from DTs to STs. 4. Conclusion In this study, a series of Zn1-x(Li/Ga)xGa2O4:Cr3+ (x=0-1) NIR persistent luminescent phosphors were synthesized using a solid-state reaction. The characterization of the samples was achieved by Raman, DFT calculations, XRD, UV-Vis-NIR, NMR, Rietveld refinement, TL, PLE/PL spectroscopy, and persistent luminescence decay analysis. With the increase of Li+/Ga3+ content in ZnGa2O4, all the newly introduced Ga3+ ions occupy the tetrahedral sites, but Li+ ions firstly occupy the tetrahedral sites, then partially enter the octahedral sites, and completely occupy the octahedral sites when x reaches 1. These contribute to the contraction of tetrahedron and the expansion of octahedron, as well as changing the structure from order to disorder and back to order. Incorporation of Li+/Ga3+ results in the decrease of lattice constant and the increase of band gap. The crystal field strength of phosphors decreases as Li+/Ga3+ content increases, which makes a redshift of the emission and excitation spectra. The crystal field also affects the trap depth and distribution. When the crystal field becomes weaker, the trap depth becomes deeper and the trap energy level becomes wider and tends to splitting into two-divided energy levels. This makes deep trap energy level be near to the 2E energy level of Cr3+. A complete replacement of Zn2+ by Li+/Ga3+ ions leads to the splitting of the trap energy level, which reduces the electron transfer between DTs and STs, but enhances the NIR afterglow of the samples, which are excited by the low-energy NIR light. This work provides a new category for the NIR-absorptive-NIR-emissive phosphors and a new application for long-term biological imaging. Author Contributions Qi Zhu and Ji-Guang Li conceived the project; Junqing Xiahou, Minghui Jin, and Fan Li carried out the experiments; Junqing Xiahou carried out the data analysis; Lin Zhu carried out the DFT calculations; Qi Zhu and Junqing Xiahou drafted the manuscript and performed the analysis. All the authors were involved in the results discussion, and have read and approved the final manuscript. Conflicts of interest The authors declare that they have no conflict of interest. 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 (Grants 51302032, U21A2045, 52172112). References 1. Z. Pan, Y. Y. Lu and F. Liu, Sunlight-Activated Long-Persistent Luminescence in the Near-Infrared from Cr3+-Doped Zinc Gallogermanates, Nat. Mater., 2011, 11, 58-63. 2. A. Bessière, S. K. Sharma, N. Basavaraju, K. R. Priolkar, L. Binet, B. Viana, A. J. J. Bos, T. Maldiney, C. 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