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## Creator

Zhuowei Li, Ge Zhu, Fan Li, Qi Zhu, Yan Cong, Xue Bai, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361), Bin Dong

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[Strategy to optimize the broadband near-infrared emission based on Eu2+-doped sulfureted garnet phosphors toward emerging spectroscopy applications](https://mdr.nims.go.jp/datasets/30c11bd8-0b3b-4580-8006-a0b78e00b125)

## Fulltext

1  Strategy to optimize the broadband near-infrared emission based on Eu2+-1 doped sulfureted garnet phosphors toward emerging spectroscopy 2 applications 3 Zhuowei Li,a,b Ge Zhu,b∗ Fan Li,a Qi Zhu,a Yan Cong,b Xue Bai,c Ji-Guang Li,d∗ Bin Dongb∗ 4 a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of 5 Materials Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, P. 6 R. China 7 b Key Laboratory of New Energy and Rare Earth Resource Utilization of State Ethnic Affairs 8 Commission, Key Laboratory of Photosensitive Materials & Devices of Liaoning Province, 9 College of Physics and Materials Engineering, Dalian Minzu University, Dalian, Liaoning 10 116600, P. R. China 11 c Key Laboratory of Integrated Optoelectronics and College of Electronic Science and 12 Engineering, Jilin University, Changchun, Jilin 130012, P. R. China 13 d Research Center for Electronic and Optical Materials, National Institute for Materials Science, 14 Tsukuba, Ibaraki 305-0044, Japan 15 *Corresponding author 16 Prof. Bin Dong 17 Dalian Minzu University 18 E-mail: dong@dlnu.edu.cn 19 Dr. Ji-Guang Li 20 National Institute for Materials Science 21 E-mail: li.jiguang@nims.go.jp 22 Prof. Ge Zhu 23 Dalian Minzu University 24 mailto:dong@dlnu.edu.cnmailto:li.jiguang@nims.go.jp2  E-mail: zhuge@dlnu.edu.cn 1 Abstract 2 Eu2+-doped near-infrared (NIR) emitting phosphors, known for their high efficiency, 3 broadband emission and spectral tunability, have gained much attention. However, achieving 4 efficient NIR emission based on Eu2+ remains a challenge due to the co-existence of Eu3+, 5 especially in materials (i.e. garnets and apatites) containing trivalent lanthanide cations. In this 6 study, a Eu2+ doped sulfureted NIR-emitting garnet phosphor Ca3(Sc, Eu)2Si3(O, S)12: Eu2+ is 7 successfully designed and synthesized. Notably, a strategy for regulating the initial valence state 8 of dopants is proposed by using prepared EuS instead of the conventional Eu2O3 as raw material, 9 enhancing the NIR emission by 135%. Moreover, a sulfuration strategy is further introduced to 10 enhance the NIR-emitting intensity and internal quantum efficiency by 192% and 167.8%, and 11 to improve thermal stability by 154% at 120°C. The luminescence origin of the unusual 12 broadband NIR emission is re-examined through chemical unit co-substitution strategy by 13 introducing [Al3+-Hf4+] to replace [Sc3+-Si4+] ion pairs. Meanwhile, the spectral regulation and 14 the performance optimization mechanism are systematically discussed. Finally, a green light 15 pumped NIR LED device with a photoelectric efficiency of 9.43%@100 mA and output power 16 of 22.74 mW@100 mA is fabricated, showing remarkable potential in nondestructive testing 17 and biomedical imaging applications. 18 Keywords: Garnet structure; Eu2+; sulfuration strategy; Broadband NIR emission; pc-NIR LED. 19 1. Introduction 20 NIR spectroscopy technique is widely used in nondestructive testing, biomedical imaging, 21 medical diagnosis and food quality assessment due to its advantages of being nondestructive, 22 mailto:zhuge@dlnu.edu.cn3  rapid and cost-effective, etc.[1-3] Among the components of NIR spectroscopy system, NIR 1 light sources are crucial. They need to be compact and have high energy efficiency to meet the 2 requirements of NIR spectroscopy applications.[4] However, the ability of traditional NIR light 3 sources (such as tungsten filament and halogen lamps) to be easily combined with smartphones 4 or portable devices for various fast and convenient applications has been limited by their low 5 efficiency and large size.[5] Recently, researchers have proposed a portable phosphor-converted 6 NIR LED (pc-NIR LED) inspired by the commercially white LED technology. This device is 7 based on a blue LED chip (450 ~ 470 nm) coating on a broadband NIR-emitting phosphor, 8 effectively solving the above disadvantages.[6, 7] However, as a non-visual light-source, pc-9 NIR LED may not require a blue LED chip as a necessary component, due to drawbacks such 10 as the large Stokes shift and the leakage of blue light.[8] To acquire a pc-NIR LED with high 11 efficiency and human eye security, green light pumped pc-NIR LED could be an viable 12 alternative. Unfortunately, there are few reports have reported on the work of NIR-emitting 13 phosphors based on green LED chips.[9] 14 Currently, the activator ions achieving efficient NIR emission are mainly Cr3+ and Eu2+. 15 However, the broadband NIR emission of Cr3+ requires a weak crystal field environment (Dq/B 16 < 2.3).[10] In addition, the Cr3+ shows strong absorption in ultraviolet (UV) and blue regions 17 but weak absorption in green region based on its 4A2→4T1 transition.[11, 12] In contrast, the 5d 18 orbital of Eu2+ is exposed in the outer layer, resulting in its absorption and luminescence are 19 easily affected by the surrounding crystal field environment, that is, the absorption can cover 20 the ultraviolet-visible region and produce efficient visible-NIR emission.[13-17] Recently, 21 4  several Eu2+ doped NIR-emitting phosphors with excellent optical properties have been 1 reported, such as Ba3ScB3O9: Eu2+ (λex ≈ 376 nm, λem ≈ 735 nm), SrBaSc0.5Ga1.5O5: Eu2+ (λex ≈ 2 440 nm, λem ≈ 728 nm), Ba3Lu(BO3)3: Eu2+ (λex ≈ 450 nm, λem ≈ 720 nm) and CaO: Eu2+ (λex ≈ 3 470 nm, λem ≈ 740 nm).[18-21] It is evident that the NIR emission of Eu2+ typically lies in the 4 deep red region, and the Eu2+ doped luminescent materials with long-wavelength NIR emission 5 are rarely reported. However, long-wavelength NIR luminescence is crucial for emerging NIR 6 spectroscopy applications, such as vein imaging (~ 780 nm), corneal recognition (~ 810 nm), 7 night vision (~ 850 nm), food detection (~ 970 nm), and long-wavelength fluorescent probes 8 (1000-1700 nm).[22] Therefore, there is an urgent need to develop Eu2+ doped long-wavelength 9 NIR-emitting phosphors to satisfy the multi-functional application demands of the emerging 10 NIR spectroscopy applications. 11 Berezovskaya et al. were the first to report an unusual long-wavelength NIR emission 12 phenomenon of a silicate garnet phosphor Ca3Sc2Si3O12: Eu2+ (CSSO: Eu2+, only the excitation 13 and emission spectra were reported).[23] Furthermore, Berezovskaya mentioned that the 14 emission of incompletely reduced Eu3+ was observed under 520 nm excitation. Later, Zhou et 15 al. investigated the luminescence and energy transfer process from Ce to Eu to improve the 16 energy conversion efficiency in solar cells.[24] However, the detail luminescence origin of the 17 unusual broadband NIR emission, the strategy to stabilize the valence state of Eu2+ and the 18 emerging NIR spectroscopy application have not been verified or reported. In fact, the co-19 existence of Eu3+ has become a troublesome problem in many Eu2+ doped phosphors, especially 20 in those containing trivalent lanthanide cations, such as in garnet and apatite-type 21 5  phosphors.[25-28] Therefore, exploring effective strategies to stabilize the valence state of Eu2+ 1 and enhance its NIR luminescence properties in Eu2+ doped phosphors is urgent. 2 In this work, we successfully designed and synthesized a series of Eu2+ doped NIR-3 emitting sulfureted garnet phosphors Ca3(Sc, Eu)2Si3(O, S)12: Eu2+ (CSSO: Eu2+, S2-), and 4 conducted an in-depth study on their phase component and characteristic properties of NIR 5 spectroscopy. In addition, we used a chemical unit co-substitution strategy to modify the local 6 structure of Sc3+ ions in CSSO: Eu2+, and determined the luminescence origin of the unusual 7 broadband NIR emission. Above all, we proposed a strategy for regulating the initial valence 8 state of dopants by using the prepared EuS (Eu valence state is divalent) instead of the 9 conventional Eu2O3 (Eu valence state is trivalent) as raw material, effectively reducing the Eu3+ 10 content and achieving a significant enhancement in the NIR emission intensity. Furthermore, 11 we proposed a sulfurization strategy to further enhance the NIR emission intensity and improve 12 the thermal stability. Finally, an efficient pc-NIR LED is prepared by combining a 520 nm green 13 chip with the optimum sample, and the potential application of the above pc-NIR LED on 14 nondestructive testing and biomedical imaging was demonstrated. 15  16  17  18  19  20  21 6  2. Results and discussion 1 2.1 Crystal structure, phase purity and morphological analysis of CSSO: Eu2+ and CSSO: 2 Eu2+, S2- samples. 3  4 Figure 1(a) Rietveld refinement of CSSO: 1%Eu2+, 3%S2- sample; (b) The average bond length 5 distributions histograms of the CSSO: 1%Eu2+ and CSSO: 1%Eu2+, S2- samples; (c) HRTEM and SAED 6 image of CSSO: 1%Eu2+, 3%S2- sample; (d) and (e) EDS spectra and elemental mapping of a single CSSO: 7 1%Eu2+, 3%S2- sample particle. 8 The XRD diffraction patterns of prepared EuS and samples with different Eu2+ and S2-9 concentrations are shown in Fig. S1-S3, indicating that the prepared EuS is single phase. 10 Furthermore, the XRD patterns of CSSO: xEu2+, yS2- (0 ≤ x ≤ 5%, 0 ≤ y ≤ 7%) samples are 11 consistent with that of Ca3Sc2Si3O12 (PDF#72-1969), implying that all samples are pure phases. 12 In order to compare the changes in crystal structure information between the CSSO: 1%Eu2+ 13 and CSSO: 1%Eu2+, 1%S2- samples, Rietveld refinement analysis is performed on the XRD 14 diffraction patterns within the 2θ range from 10° to 80° using GSAS software, and the 15 7  refinement results are shown in Fig. 1a, S4 and Table S1-S6. The refinement results show that 1 CSSO: 1%Eu2+ and CSSO: 1%Eu2+, 3%S2- samples have cubic structure with an Ia-3d (230) 2 space group. It is worth noting that with the addition of S, the unit cell parameter a increases 3 from 12.21 Å to 12.25, and V expands from 1820.32 Å3 to 1838.26 Å3, which is due to the 4 substitution of larger S2- for smaller O2-.[29] Additionally, the [Ca-O/S] bond length remains 5 unchanged, the [Si-O/S] bond length slightly increases, and the [Sc/Eu-O/S] bond length 6 becomes shorter, as shown in Fig. 1b. Consequently, the contraction of [Sc/Eu-O/S]6 octahedron 7 will lead to the increase of the crystal field strength around Eu2+ and further cause a red shift of 8 the photoluminescence spectrum, which will be discussed later.  9 Figures S5 and S6 show the SEM images of CSSO: 1%Eu2+ and CSSO: 1%Eu2+, 3%S2- 10 samples and the tendency to form agglomerates can be observed, which results from the rapid 11 aggregation of the samples during high-temperature sintering.[30] Furthermore, with the 12 doping of S2-, the surface morphology of the particles transitions from sharp to smooth, and the 13 average grain size increases from 34.01 to 51.56 μm. These changes indicate that the doping of 14 S2- has a fluxing effect and can improve the crystallinity of the phosphor.[31, 32] In addition, 15 the high crystallinity of the sample is further confirmed by the clear lattice stripes in the 16 HRTEM image, as shown in Fig.1c. At the same time, the selected area electron diffraction 17 pattern clearly presents the d(220) = 4.32 Å and d(420) = 2.71 Å planes of the garnet 18 structure,[33] which also demonstrates the successful synthesis of the targeted phosphors. 19 Meanwhile, the distribution of Ca, Sc, Si, O, Eu and S in the whole particle can be observed 20 from the elemental mapping images of a single sample particle, which further confirms that S 21 8  has been successfully doped into CSSO: 1%Eu2+, 3%S2-, as shown in Fig. 1d and 1e. 1 2.2 The luminescence origin of the NIR emission in Eu2+ doped CSSO: Eu2+ sample. 2  3 Figure 2(a) The emission spectra of CSSO: 1%Eu2+, Ca3Sc1.98Si3O12 and CSSO samples under 518 nm 4 excitation; (b) The excitation and emission spectra of CSSO: 1%Eu2+ sample under the λex = 355 nm and 5 λem = 480 nm; (c) The decay time of different wavelengths for CSSO: 1%Eu2+ sample; (d) The XRD 6 patterns of Ca3Sc1.98-zHfzSi3-zAlzO12: 1%Eu2+ (0 ≤ z ≤ 1) samples; (e) The normalized emission spectra of 7 Ca3Sc1.98-zHfzSi3-zAlzO12: 1%Eu2+ (0 ≤ z ≤ 1) samples; (f) The expansion mechanism of the EuO6 8 polyhedron with [Al3+-Hf4+] substituted for [Sc3+-Si4+] ion pairs. 9 Berezovskaya and Zhou et al. mentioned that the unusual broadband NIR emission in 10 CSSO: Eu2+ likely originates from Eu2+ ions occupying the octa-coordinated Ca2+ sites.[23, 24] 11 However, in some other Eu2+ doped garnet-type phosphor, reports indicate that if Eu2+ occupy 12 the octa-coordinated sites with weak crystal field strength, blue or cyan emission will be 13 obtained, resulting from the 5d→4f transitions of Eu2+, such as in Lu2CaMg2Si3O12: Eu2+ (λex 14 ≈ 365 nm, λem ≈ 450 nm), Lu2MgAl4SiO12: Eu2+ (λex ≈ 365 nm, λem ≈ 463 nm) and Ba3Y2Si3O12: 15 Eu2+ (λex ≈ 365 nm, λem ≈ 505 nm).[34-36] Recently, Xia et al. demonstrated that short bond 16 lengths around the six-coordinated Eu2+ could contribute to its NIR emission, although Eu2+ 17 occupy an inequivalent Y3+ sites in (Sr, Ba)Y2O4: Eu2+.[37] Additionally, oxygen vacancies are 18 9  demonstrated to be able to generate a broadband NIR emission.[38] Thus, it is necessary to re-1 examine the luminescence origin of the unusual broadband NIR emission in CSSO: Eu2+. First, 2 considering the reductive atmosphere during the sintering process and the possible inequivalent 3 substitution when Eu2+ enter Sc3+ sites (as described by defect equation S1). It is necessary to 4 determine that whether the unusual broadband NIR emission originates from the oxygen 5 vacancies. Therefore, the Sc3+ absent sample Ca3Sc1.98Si3O12 is successfully synthesized (Fig. 6 S7) and the emission spectra of Ca3Sc1.98Si3O12, CSSO and CSSO: 1%Eu2+ samples prepared 7 in a reductive atmosphere are measured under 518 nm excitation, as shown in Fig. 2a. However, 8 the Ca3Sc1.98Si3O12 and CSSO samples do not exhibit any visible or NIR emission, which 9 confirms that the unusual broadband NIR emission in CSSO: Eu2+ should not be related to the 10 oxygen vacancies luminescence but stem from the 5d→4f transition of Eu2+. 11 Moreover, it is worth noting that a blue emission band centered at 480 nm is observed 12 under 355 nm excitation (Fig. 2b) for CSSO: 1%Eu2+ sample, which is not mentioned in 13 Berezovskaya and Zhou’s report.[23, 24] Regarding this, we hypothesize that the blue emission 14 should be attributed to the substitution of Eu2+ for the octa-coordinated Ca2+ ions, as reported 15 in Eu2+ doped other garnet systems, [34-36] while the latter broadband NIR emission should 16 due to the substitution of Eu2+ for the six-coordinated Sc3+ ions. To further confirm the above 17 hypothesis and determine the relation between the site occupation and luminescence 18 characteristic, we compared the crystal field strength (εCFS) around Eu2+ using the following 19 equation,[37] 20 εCFS=βpolyQRav-2                               (1) 21 10  Rav=1n∑ (Ri-0.6(RM-RLn))CNi=1                       (2) 1 Where Q is 2+ for Eu2+; βQ  poly  is a constant; Ri is the individual bond length to the CN 2 coordinating anions in the unrelaxed lattice; RM is the cationic radius and RLn is the ionic radius 3 of lanthanide. According to the formulas (1,2), it can be found that εCFS is negatively correlated 4 with Rav, that is, Eu2+ occupy cationic sites with a small coordination number, which enhances 5 εCFS and reduces the energy of the 5d excited state.[39] In this work, the Rav value of Sc3+ (~ 6 2.44 Å) is smaller than that of Ca2+ (~ 2.52 Å). Therefore, the emission peak located in the blue 7 region (~ 480 nm) is attributed to the Eu2+ occupying the Ca2+ site, while the emission peak 8 located in the NIR region (~ 858 nm) is attributed to the Eu2+ occupying the Sc3+ site. The decay 9 time of the CSSO: 1%Eu2+ sample monitored at 480 and 858 nm is measured to be 4.20 and 10 7.13 μs, as shown in Fig. 2c and Table S7, which further supports that the blue and NIR 11 emissions in CSSO: 1%Eu2+ sample are attributed to the 5d→4f transition of Eu2+ occupying 12 two different luminescence centers, respectively.  13 Furthermore, considering that the emission of Eu2+ is quite sensitive to the crystal field 14 strength, we utilized a chemical unit co-substitution strategy to regulate the lattice environment 15 around the Sc3+ site by replacing [Sc3+-Si4+] with [Al3+-Hf4+] ion pairs, and then design and 16 successfully synthesized a series of new solid solution phosphors Ca3Sc1.98-zHfzSi3-zAlzO12: 17 1%Eu2+ (0 ≤ z ≤ 1) samples, as confirmed by XRD analysis in Fig. 2d. It is worth mentioning 18 that a significant preferred orientation occurs with increasing [Al3+-Hf4+] doping concentration, 19 and the XRD pattern of the Ca3Sc0.98HfSi2AlO12: 1%Eu2+ (z = 1) sample is consistent with the 20 standard card of the garnet-structured compound Ca3In2Si3O12 (PDF#74-1579). Notably, due to 21 11  the large ion radius of Al3+ and Hf4+, the mean peak at 36 degree (2θ) is found to slightly shift 1 toward the small angle direction, as shown in Fig. S8. This shift indicates the successful 2 replacement of [Al3+-Hf4+] ion paris by [Sc3+-Si4+]. The normalized excitation and emission 3 spectra of Ca3Sc1.98-zHfzSi3-zAlzO12: 1%Eu2+ (0 ≤ z ≤ 1) samples are recorded in Fig. S9 and 2e. 4 As expected, the emission spectra are largely blue shifted from 858 to 785 nm, and the 5 corresponding excitation spectra shifts from 518 to 470 nm. This shift can be assigned to the 6 inhibited the splitting of the 5d energy level of Eu2+ due to the decrease of the crystal field 7 strength after the replacement of [Sc3+-Si4+] with larger [Al3+-Hf4+] ion pairs as shown in Fig. 8 2f.[29, 40] The above results further support that the unusual broadband NIR emission in CSSO: 9 1%Eu2+ sample should stem from the 5d→4f transitions of Eu2+ occupying the six-coordinated 10 Sc3+ sites. 11 2.3 Stabilization of Eu2+ ions and luminescence enhancement investigation through EuS 12 doping. 13  14 Figure 3(a) The excitation spectra of CSSO: 1%Eu2+ sample with EuS and Eu2O3 as raw material; (b) The 15 emission spectra of CSSO: xEu2+ (0.25% ≤ x ≤ 5%) and CSSO: 1%Eu2+ samples with Eu2O3 as raw 16 12  material; (c)-(e) XPS spectra of the Eu 3d3/2 level from samples prepared with varying Eu raw materials 1 and EuS concentrations; (f) The emission spectra of CSSO: 1%Eu2+ sample under 393 nm excitation with 2 different Eu raw materials. 3 Due to the inequivalent substitution in CSSO: Eu2+, stabilizing the valence state of Eu2+ is 4 crucial. Although some methods have been used in the previous works, such as carbothermal 5 reaction method, additional annealing in a reducing atmosphere, adding additional ZnO as a 6 compensating agent or controlling the reaction temperature,[23, 41-43] but the results are not 7 satisfactory and universal. In this work, we proposed a strategy to control the initial valence 8 state of the dopant by using the prepared EuS (Eu valence state is divalent) instead of 9 conventional Eu2O3 (Eu valence state is trivalent) as the raw material, and the comparison of 10 their excitation and emission spectra are illustrated in Fig. 3a and 3b. It is obvious that the 11 spectra are almost the same except for the intensity, that is, the spectra intensity of EuS doped 12 sample can be increased by about 135% than that of using Eu2O3 as a raw material. The optimal 13 doping content of EuS in CSSO is then determined to be 1%.  14 To clarify the luminescence optimization mechanism, the XPS spectra on the Eu 3d3/2 15 levels of 1% Eu2O3 and 1% EuS samples are measured, as shown in Fig. 3c and 3d. Excitingly, 16 the XPS signal intensity ratio R of Eu2+ and Eu3+ in the EuS-doped sample is determined to be 17 1.15, which is much higher than that of the Eu2O3-doped sample (R = 0.74). Furthermore, in 18 order to avoid the error caused by the low concentration doping of EuS, we also specially 19 performed XPS tests on the sample doped with high concentrations of EuS, as shown in Fig. 20 3e. The results indicate that high-concentration doping of EuS can also significantly enhance 21 the R value of the sample (R = 1.31). This demonstrates that Eu2+ can be effectively stabilized 22 through the proposed strategy by the regulating initial valence state of the dopant.[14, 42, 44, 23 13  45] The above inference can also be further demonstrated by utilizing the selective excitation 1 characteristics of Eu3+ ions. To address this, the emission spectral of the above two samples 2 under 393 nm excitation (the characteristic excitation wavelength of Eu3+ ions) are shown in 3 Fig. 3f. For the sample doped with the prepared EuS, the emission spectra show weaker line-4 sharp emissions peaked at 594, 615, 653 and 711 nm, corresponding to the 5D0→7Fi (i = 1, 2, 5 3, 4) transitions of Eu3+,[41, 46] further indicating that more Eu2+ can be stabilized at high 6 temperatures by using EuS as raw material. This stabilization is beneficial for enhancing the 7 NIR-emitting intensity of Eu2+. 8 2.4 Enhanced NIR emission analysis through sulfuration strategy. 9  10 Figure 4(a) DRS spectra of CSSO, CSSO: 1%Eu2+ and CSSO: 1%Eu2+, 3%S2- samples, the inset shows 11 digital photo of the CSSO: 1%Eu2+, 3%S2- sample; (b) and (c) The excitation and emission spectra of 12 CSSO: 1%Eu2+, yS2- (0 ≤ y ≤ 7%) samples; (d) The decay curves of CSSO: 1%Eu2+, yS2- (0 ≤ y ≤ 7%) 13 samples; (e) The internal quantum efficiency of CSSO: 1%Eu2+, 3%S2- sample; (f) XPS spectra of the Eu 14 3d3/2 level of CSSO: 1%Eu2+, 3%S2- sample. 15 To further optimize the NIR luminescence behavior, we proposed a sulfuration strategy by 16 co-doping S2- into CSSO: 1%Eu2+ sample. The diffuse reflection spectra (DRS) of the CSSO, 17 CSSO: 1%Eu2+ and CSSO: 1%Eu2+, 3%S2- samples are shown in Fig. 4a. The spectra contain 18 14  two strong absorbance bands in 250-800 nm region with maxima at around 400 and 520 nm, 1 attributed to the 5d→4f transition of the Eu2+. Notably, the S2- doping significantly enhances 2 the phosphor absorption in the corresponding region. The inset of Fig 4a and S10 are digital 3 photos of the CSSO: 1% Eu2+, 3% S2- and CSSO samples, respectively. The Eu2+ doped sample 4 show an attractive purple color, consistent with its strong absorption in the violet and green 5 light region. The excitation and emission intensity are gradually enhanced with increasing S2- 6 doping content, as shown in Fig. 4b and 4c. When the doping content of S2- reaches 3%, the 7 emission intensity of CSSO: 1%Eu2+, 3%S2- sample is enhanced to 192% compared to that of 8 CSSO: 1%Eu2+ sample using Eu2O3 as raw material. Furthermore, both the excitation and 9 emission spectra exhibit slight red shifts, attributed to the contraction of the [Sc/Eu-O/S]6 10 octahedron that enhances the crystal field strength around Eu2+, resulting in an increase in 11 crystal field splitting.[37] Meanwhile, the larger electron cloud expansion effect caused by the 12 smaller electronegativity of S2- (2.45) compared to O2- (3.44),[47, 48] leads to a decrease in the 13 5d orbital, which also contributes to the red shifts in both spectra. This further confirms the 14 successful doping of S2- into the sample. Fig. 4d and Table S8 illustrate the decay curves of 15 CSSO: 1%Eu2+, yS2- (0 ≤ y ≤ 7%) samples excited at 521 nm and monitored at 860 nm. The 16 microsecond order of the lifetime is consistent with that of Eu2+ doped NIR-emitting 17 phosphors.[20, 49] In addition, the lifetime shows a slight decrease along with increasing S2- 18 content, implying that the doping of S2- has a small effect on the probability of radiative energy 19 transition involving Eu2+ ions. 20 Excitingly, the internal quantum efficiency (IQE) of the CSSO: 1%Eu2+, 3%S2- sample 21 15  reaches 30.58%, marking a staggering 167.8% enhancement in quantum efficiency compared 1 to the CSSO: 1%Eu2+ sample prepared using Eu2O3 as the raw material (IQE = 18.22%), as 2 shown in Fig. 4e and S11. In order to further verify and reveal the luminescence enhancement 3 mechanism, the XPS spectra on the Eu 3d3/2 levels of CSSO: 1%Eu2+, 3%S2- sample is 4 measured as shown in Fig. 4f. The R value of Eu2+ and Eu3+ in the EuS and S2- co-doped sample 5 is determined to be 2.64, which is much higher than that of the Eu2O3-doped sample (R = 0.74). 6 This is because the addition of S2- can promote the spontaneous transformation of Eu3+ to 7 Eu2+,[50] thereby increasing the R value and enhancing the luminescence intensity. Additionally, 8 the improvement in crystallinity and absorption due to sulfuration also enhances the 9 luminescence intensity and quantum efficiency, as supported by SEM and DRS analysis (Fig. 10 S5, S6 and 4a).  11 2.5 The thermal stability property investigation of CSSO: Eu2+ and CSSO: Eu2+, S2- samples. 12  13 Figure 5(a) Contour plot of the temperature-dependent emission spectra; (b) The configuration coordinate 14 diagram of thermal quenching; (c) and (d) The relationship of ln[(I0/IT)-1] with 1/kT, Huang-Rhys factor 15 and phonon energy of CSSO: 1%Eu2+ and CSSO: 1%Eu2+, 3%S2- samples; (e) Emission spectra of CSSO: 16 1%Eu2+, 3%S2- sample at 77K and 300K; (f) The relative luminous intensity of CSSO: 1%Eu2+, 3%S2- 17 sample in humid environment for ten days. 18 16  The temperature-dependent emission spectra of the CSSO: 1%Eu2+ and CSSO: 1%Eu2+, 1 3%S2- samples are measured as shown in Fig. 5a, S12 and S13. The thermal stability of CSSO: 2 1%Eu2+, 3%S2- (51%@120°C) is significantly improved compared with that of CSSO: 1%Eu2+ 3 (33%@120°C) sample. Generally, the thermal quenching behavior can be expressed by the 4 configuration coordinate diagram in Fig. 5b. As the temperature increases, the electrons at the 5 lowest 5d level gain more energy, overcoming the energy barrier and resulting in thermal 6 quenching behavior (i.e. A→B→C→F→D→A). Among these transitions, the energy 7 difference from C to F indicates the difficulty of this thermally activated process, this is, the 8 activation energy (ΔE), which can be determined by the Arrhenius formula,[51] 9 I(T) ≈I01+cexp(−∆EkT)                        (3) 10 Where I(T) is the emission intensity at temperature T(K), I0 is the initial emission intensity, c is 11 a constant related to the host material and k is Boltzmann’s constant. By rearranging equation 12 (3) and plotting ln[(I0/IT)-1] against 1/kT, as shown in Fig. 5c, the thermal activation energy 13 ΔE2 is calculated to be 0.2933 eV, which is higher than the ΔE1 (0.2865 eV). This is consistent 14 with the better thermal stability of CSSO: 1%Eu2+, 3%S2- sample. The bond energy of [Sc/Eu-15 O/S] is higher than that of the [Sc/Eu-O] when the S2- enters the CSSO: 1%Eu2+ sample, the 16 valence electrons of Eu2+ in the outer layer strongly interact with the lattice environment, 17 resulting in the rigid lattice structure and achieving a large activation energy ΔE.[52] Huang's 18 theory indicates that the value of ΔE has dependence on the strength of electron-phonon 19 coupling, reflected by the Huang-Rhys factor and obtained via the following formula,[53]  20 FWHM (T)=√8ln2S(hv)√coth (hv2kT)                   (4) 21 17  Where S is the Huang-Rhys factor, hv is the mean phonon energy, and k is the Boltzmann’s 1 constant. S is determined to be 4.94 and 4.87 for CSSO: 1%Eu2+ and CSSO: 1%Eu2+, 3%S2- 2 samples (Fig. 5d), respectively. The smaller S for CSSO: 1%Eu2+, 3%S2- sample indicates that 3 the electron-phonon coupling is weaker than CSSO: 1%Eu2+ sample, and the spectral 4 broadening is less as the temperature increases (Table S9), which also contributes the excellent 5 thermal quenching properties.[54]  6 Moreover, we measured the decay curves of the CSSO: 1%Eu2+, 3%S2- sample in the 7 temperature range of 30 to 240°C as shown in Fig. S14 and Table S10. The lifetime decreases 8 from 5.05 to 4.52 μs as the temperature increases. This indicates that the interaction between 9 Eu2+ ions diminish at high temperature.[55, 56] In order to further prove that CSSO: 1%Eu2+, 10 3%S2- sample possesses excellent thermal stability and more spectral details, the emission 11 spectrum of CSSO: 1%Eu2+, 3%S2- sample is measured at 77K, as shown in Fig. 5e. The 12 emission intensity of the sample at 77K remains 8.33 times that at 300K, indicating that the 13 optimal sample still exhibits high emission efficiency at low temperatures. In addition, we also 14 studied the luminescence stability of CSSO: 1%Eu2+, 3%S2- in humid environments at 85°C 15 and 85% humidity for ten days as shown in Fig. 5f. The results indicate that the sample can still 16 maintain good stability in humid environments. 17  18  19  20  21 18  2.6 The application of CSSO: 1%Eu2+, 3%S2- sample for pc-NIR LED. 1  2 Figure 6(a) Electroluminescent spectra of the fabricated pc-NIR LED under different drive current, inset 3 shows the fabricated pc-NIR LED with the light off and on; (b) The NIR output power and photoelectric 4 efficiency under different drive current; (c) Vein imaging; (d) Photographs of the pc-NIR LED, NFC chip, 5 campus card and capsules taken by the NIR camera.  6 Finally, a pc-NIR LED is fabricated by coating the CSSO: 1%Eu2+, 3%S2- sample on a 7 commercial 520 nm LED chip, and the photoelectric efficiency of the pc-NIR LED is measured 8 as shown in Fig. 6a and b. It can be clearly observed that the NIR luminous intensity is enhanced 9 with increase of driving current, resulting in the output power increases from 16.09 mW@50 10 mA to 68.22 mW@350 mA, while the conversion efficiency decreases from 11.37%@50 mA 11 to 5.93%@350 mA. Furthermore, the electroluminescent performance of the pc-NIR LED is 12 compared with that of similar green light-excited pc-NIR LEDs, the commercial pc-NIR LEDs 13 (Osram SHF 4735 and 4776) and other blue light-excited pc-NIR LEDs, as shown in Table S11. 14 The results shows that our prepared pc-NIR LED has excellent NIR output power and 15 conversion efficiency at drive current of 100 mA, which are significantly superior to those of 16 other green or blue light excited pc-NIR LEDs, and even more superior compared with the 17 19  commercial Osram pc-NIR LEDs at driven current of 350 mA. The operating temperature of 1 the pc-NIR LED for currents ranging from 100 to 350 mA is shown in Fig. S15, and the 2 temperature increases from 26.1 to 51.4°C. Additionally, we recorded a temperature increase 3 from 31.9 to 57.2°C when driving the pc-NIR LEDs for 30 to 90 min at 100 mA. Moreover, 4 Figure 6c shows the application for human vein imaging, indicating great potential in the field 5 of biomedical imaging. Meanwhile, benefiting from the high penetrating power of NIR light 6 from the designed pc-NIR LED, the inner structure and some hidden details of the opaque 7 objects (such as NFC chip, campus card and capsules) can be clearly detected, as shown in Fig. 8 6d and S16. Overall, the fabricated pc-NIR LED shows great potential applications in non-9 destructive inspection and biomedical imaging. 10 3. Conclusion 11 In this work, we have successfully synthesized a series of Eu2+ doped NIR-emitting 12 sulfureted garnet phosphors CSSO: Eu2+, S2-. The broad NIR emission centered at 860 nm 13 corresponds to the 5d→4f transition of Eu2+ occupying 6-coordinated Sc3+ sites rather than 8-14 coordinated Ca2+ sites through chemical unit co-substitution strategy. Importantly, we proposed 15 two methods to optimize the NIR emission of garnet phosphor CSSO: Eu2+. Firstly, the prepared 16 EuS is used instead of the traditional Eu2O3 as the raw material to regulate the initial valence 17 state of the dopant, enhancing the NIR emission by 135%. Secondly, the sulfuration strategy 18 can further enhance the NIR-emitting intensity and internal quantum efficiency by 192% and 19 167.8%, and the thermal stability is improved by 154%@120°C. It can be seen that both the 20 utilization of pre-prepared EuS and the sulfuration strategy are equally crucial in enhancing the 21 20  luminous intensity of CSSO: Eu2+ phosphors. Finally, a pc-NIR LED is fabricated with 1 excellent photoelectric efficiency of 9.43%@100 mA and NIR output powers of 22.74 2 mW@100 mA. Its performance is superior to that of some other green or blue light-excited pc-3 NIR LEDs, and offers potential applications in nondestructive internal defect detection and vein 4 imaging. In summary, this work not only confirms the luminescence origin of Eu2+-doped 5 garnet NIR-emitting phosphor, but also provides two new strategies to achieve high-6 performance NIR luminescence of Eu2+, encouraging further researchers into efficient Eu2+-7 doped garnet NIR-emitting phosphor for photonic applications. 8  9 Acknowledgements 10 This work was supported by the National Natural Science Foundation of China (Grant Nos. 11 62375038, 12174046, 12274057, U21A2074 and U21A2068). Liaoning BaiQianWan Talents 12 Program (Grant No. 2021921012), Science and Technique Foundation of Dalian (Grant Nos. 13 2022JJ11CG003 and 2022JJ12GX041), Science and Technique Foundation of Liaoning 14 Province (Grant Nos. 2023JH2/101800033, 2023JH2/101700058, 2023JH2/101700239, 15 2023JH1/10400057 and 2023JH1/10400080). LiaoNing Revitalization Talents Program (Grant 16 No. XLYC2007048).  17  18 References 19 [1] E. H. Song, H. Ming, Y. Y. Zhou, F. Q. He, J. C. Wu, Z. G. Xia, Q. Y. Zhang, Cr3+-Doped 20 Sc-Based Fluoride Enabling Highly Efficient Near Infrared Luminescence: A Case Study of 21 21  K2NaScF6: Cr3+, Laser Photonics Rev. 15 (2021) 2000410. 1 [2] K. T. Ly, R. W. Cheng, H. W. Lin, Y. J. Shiau, S. H. Liu, P. T. Chou, C. S. Tsao, Y. C. Huang, 2 Y. Chi, Near-infrared organic light-emitting diodes with very high external quantum efficiency 3 and radiance, Nat. Photonics. 11 (2017) 63. 4 [3] C. X. Yuan, R. Y. Li, Y. F. Liu, L. L. Zhang, J. H. Zhang, G. Leniec, P. Sun, Z. H. 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