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## Creator

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

## Rights

This is the peer reviewed version of the following article: Z. Li, G. Zhu, H. Li, Q. Zhu, Y. Cong, X. Bai, J.-G. Li, B. Dong, Visible Light Excited Yb3+-Doped Phosphor Via Eu2+ Bridged Energy Transfer Toward NIR-II Spectroscopy Application. Laser Photonics Rev 2025, 19, 2402162, which has been published in final form at https://doi.org/10.1002/lpor.202402162. 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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[Visible Light Excited Yb3+-Doped Phosphor Via Eu2+ Bridged Energy Transfer Toward NIR-II Spectroscopy Application](https://mdr.nims.go.jp/datasets/fffc3992-aef6-47b4-b312-2d2a7768862b)

## Fulltext

1  Visible light excited Yb3+-doped phosphor via Eu2+ bridged 1 energy transfer towards NIR-II spectroscopy application 2 Zhuowei Li,a,b Ge Zhu,b,∗ Heyang Li,c Qi Zhu,a Yan Cong,b Xue Bai,d Ji-Guang Li,e,∗ 3 Bin Dongb∗ 4 a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), 5 School of Materials Science and Engineering, Northeastern University, Shenyang, 6 Liaoning 110819, P. R. China 7 b Key Laboratory of New Energy and Rare Earth Resource Utilization of State Ethnic 8 Affairs Commission, Key Laboratory of Photosensitive Materials & Devices of 9 Liaoning Province, College of Physics and Materials Engineering, Dalian Minzu 10 University, Dalian, Liaoning 116600, P. R. China 11 c Department of Physics & Astronomy, University College London, United Kingdom 12 d Key Laboratory of Integrated Optoelectronics and College of Electronic Science and 13 Engineering, Jilin University, Changchun, Jilin 130012, P. R. China 14 e Research Center for Electronic and Optical Materials, National Institute for Materials 15 Science, Tsukuba, Ibaraki 305-0044, Japan 16 *Corresponding author 17 Prof. Bin Dong 18 Dalian Minzu University 19 E-mail: dong@dlnu.edu.cn 20 Dr. Ji-Guang Li 21 National Institute for Materials Science 22 E-mail: li.jiguang@nims.go.jp 23 Prof. Ge Zhu 24 mailto:dong@dlnu.edu.cnmailto:li.jiguang@nims.go.jp2  Dalian Minzu University 1 E-mail: zhuge@dlnu.edu.cn 2  3 Abstract 4 Yb3+ is regarded as an efficient near-infrared-II (NIR-II) emitting activator, which 5 has been widely used in energy conversion, up-conversion luminescence and optical 6 communication. However, the simple energy level structure of Yb3+ makes it can only 7 be excited by ultraviolet or infrared light, which limits their application in the currently 8 booming phosphor converted near-infrared-II light emitting diodes (NIR-II pc-LEDs). 9 Here, we presented “Eu2+ bridge” strategy to extend Yb3+ absorption to visible range, 10 and successfully realize efficient visible light pumped NIR-II emission via energy 11 transfer from Eu2+ to Yb3+. Meanwhile, to balance the valence state of reduced Eu2+ and 12 oxidized Yb3+, the pre-prepared EuS was used as precursor instead of Eu2O3, which 13 significantly increases the NIR-II luminescence of Yb3+ by 6 times. Detailed energy 14 transfer and luminescence enhancement mechanism were discussed. Finally, a NIR-II 15 pc-LED was fabricated with photoelectric efficiency of 12.61%@50 mA and output 16 power of 74.09 mW@300 mA. Subsequently, a miniaturized and real-time test system 17 was integrated based on the convolutional neural network technology to accurate 18 predict organic solvents with different concentrations. This study not only introduces a 19 new strategy to realize visible light excited Yb3+-doped NIR-II emitting phosphors, but 20 also promotes their innovative application based on NIR spectroscopy technology. 21 Keywords: garnet structure; Yb3+ ions; energy transfer; NIR-II pc-LEDs; convolutional 22 neural network technology. 23 mailto:zhuge@dlnu.edu.cn3  1. Introduction 1 In recent years, the portable near-infrared-II (NIR-II) light sources, which can be 2 easily integrated into smartphones and wearable devices, have exhibited broad 3 application prospects in various fields such as non-destructive testing, medical 4 diagnosis, and food quality assessment.1-3 Phosphor converted near-infrared-II light 5 emitting diodes (NIR-II pc-LEDs), considered ideal for next generation portable NIR-6 II light sources, are expected to replace traditional tungsten halogen lamps and lasers 7 due to their noteworthy attributes of high efficiency, compact design and good 8 durability.4, 5 Currently, one of the most widely used NIR pc-LED is the SFH 4735 9 introduced by OSRAM in 2016.6, 7 Unfortunately, the emission intensity of this NIR pc-10 LED in the NIR-II region (900–1050 nm) is significantly lower than that in the NIR-I 11 region (650–900 nm), limiting its use in food detection (~970 nm ) and long-wavelength 12 fluorescent probes (~1000 nm).8 Although OSRAM has subsequently developed the 13 SFH 4737 NIR pc-LED light source,9 and researchers have also developed a variety of 14 NIR pc-LED devices based on fluorine and oxide luminescent materials.10-16 There is 15 still a lack of effective means to broaden the spectral range of the NIR-II region and 16 improve the luminous intensity, which is a huge challenge. 17 As a key component of NIR-II pc-LED, the luminescence characteristics of NIR-18 II emitting phosphors directly affect the spectral range, conversion efficiency and 19 output power of the device. Currently, Eu2+ and Cr3+ ions are among the most widely 20 used activators for generating NIR emission.17-20 However, despite substantial efforts, 21 the NIR emission wavelength of Eu2+ ions still remains difficult to significantly extend 22 4  to longer wavelengths (λem > 900 nm).11 Cr3+ ion in a strong crystal field environment 1 will show the narrow-band emission characteristics related to the 2Eg→4A2g transition. 2 On the contrary, it will exhibit broadband emission caused by the spin-allowed 4T2g→3 4A2g transition. Therefore, Cr3+-activated phosphors usually exhibit long-wavelength 4 emission and excellent NIR luminescence properties.21, 22 Unfortunately, they are prone 5 to oxidation during high-temperature processing, transforming into harmful Cr4+ and 6 Cr6+ forms.23 The rare-earth ion Yb3+ is widely regarded as a highly efficient activator 7 for NIR-II emission with a peak wavelength approximately at 1000 nm.24 However, the 8 parity-forbidden 4f-4f transition characteristic of Yb3+ ions results in its weak light 9 absorption capacity. Additionally, the 4f13 electronic configuration restricts Yb3+ ions to 10 only two energy levels: the 2F7/2 ground state and the 2F5/2 excited states. Consequently, 11 Yb3+ ions can only be excited by ultraviolet (e.g. Yb3+-ligand charge transfer band) or 12 infrared light (2F7/2→2F5/2 transition),25-27 but cannot be excited by the commonly used 13 visible LED chips. Thus, developing Yb3+-doped NIR-II emitting phosphors for 14 portable pc-LED applications remains a significant challenge. 15 Constructing an efficient energy transfer “bridge” to sensitize Yb3+ ions emission 16 is considered as an effective strategy to address this problem. At present, Cr3+ ions are 17 the main activator ion to sensitize Yb3+ ions.28-30 However, due to the parity-forbidden 18 transition inherent to Cr3+ ions, their excitation spectra typically exhibit strong 19 absorption in the blue light region, making it challenging to achieve efficient emission 20 excited by other visible light. In contrast, Eu2+ usually possesses efficient broadband 21 5  excitation due to its parity-allowed 4f65d1→4f7 transitions.31-33 Moreover, the excitation 1 and emission energy of Eu2+ can be easily adjusted in the whole visible light region 2 depending on different crystal field strength,34-37 which is more suitable to realize the 3 visible light excitation of Yb3+ through energy transfer.  4 Garnet-type compounds are considered as ideal host materials for luminescent 5 materials because of their deliberate luminescent properties and abundant cation sites, 6 which have important applications in white LED, optical anti-counterfeiting, biological 7 imaging, solar cells and so on.38-40 Among them, Zhou et al. reported a garnet-based 8 solar cell light-converting material based on the energy transfer from Ce3+ to Eu2+ and 9 finally to Yb3+ ions, utilizing the carbothermal reduction method.41 However, the 10 quantum efficiency of Eu2+, Yb3+ co-doped sample is only 7.9%, insufficient for current 11 NIR spectroscopy applications. The reason may be that Eu3+ ions cannot be completely 12 reduced to Eu2+ in a weak reducing atmosphere, and some Yb3+ ions may be reduced to 13 Yb2+ ions, which ultimately hinders the energy transfer process. Thus, there is an urgent 14 need to develop effective strategies to balance the valence states of reduced Eu2+ ions 15 and oxidized Yb3+ ions, thereby enhancing the NIR-II luminescence of Yb3+ ions via 16 Eu2+ ions sensitization.  17 In this work, a series of NIR-II emitting garnet phosphors Ca3Sc2Si3O12: xEu2+, 18 yYb3+ (CSSO: xEu2+, yYb3+, x = 0 and 1%, 0 ≤ y ≤ 7%) are synthesized using the pre-19 prepared EuS instead of traditional Eu2O3 as dopant under N2 atmosphere. The result 20 indicates that the proposed strategy not only realizes the NIR-II emission of Yb3+ ions 21 6  by visible light excitation, but also effectively inhibits the formation of Eu3+ and Yb2+ 1 ions and enhances the energy transfer from Eu2+ to Yb3+ ions, resulting in the largely 2 increased NIR-II luminescence by 6 times. Furthermore, we successfully fabricated a 3 portable NIR-II pc-LED based on the optimized CSSO: 1%Eu2+, 5%Yb3+, and used 4 absorption spectroscopy techniques in conjunction with convolutional neural network 5 (CNN) technology to achieve accurate prediction of various organic solvents at 6 different concentrations. We hope that this study will inspire the development of Yb3+ 7 activated NIR-II emitting phosphors and their innovative application based on NIR 8 spectroscopy technology in the future. 9 2. Results and discussion 10 2.1 Phase purity and structure transition analysis of CSSO: Eu2+, Yb3+ samples. 11  12 Figure 1(a) The XRD patterns of CSSO: xEu2+, yYb3+ (x = 0 and 1%, 0 ≤ y ≤ 7%) samples; (b) The 13 crystallographic parameters a and V of CSSO: 1%Eu2+, yYb3+ (0 ≤ y ≤ 7%) samples; (c) The 14 Raman spectra of CSSO, CSSO: 1%Yb3+ and CSSO: 1%Eu2+, 1%Yb3+ samples; (d) The 15 thermoluminescence curve of CSSO: 1%Yb3+ and CSSO: 1%Eu2+, 1%Yb3+ samples excited for 10 16 min; (e) Rietveld refinement of the XRD pattern for CSSO: 1%Eu2+, 5%Yb3+ sample; (f) The 17 crystal structure of CSSO: 1%Eu2+, 5%Yb3+ sample. 18 7  Figure 1a presents the XRD patterns of CSSO: xEu2+, yYb3+ (x = 0 and 1%, 0 ≤ y 1 ≤ 7%) samples. The diffraction peaks observed in all synthesized samples are 2 completely consistent with the standard data for Ca3Sc2Si3O12 (PDF#72-1969), 3 confirming the successful synthesis of pure-phase phosphors. Notably, the 4 crystallographic parameters a and V of CSSO: 1%Eu2+, yYb3+ (0 ≤ y ≤ 7%) samples 5 follow Vegard's law.42 That is, the crystallographic parameters gradually increase due 6 to the larger ionic radii of Yb3+ ions (CN = 6, R = 0.868 Å) compared with those of Sc3+ 7 ions (CN = 6, R = 0.745 Å) (Fig. 1b),43 which is also confirmed by the low-angle shift 8 of the XRD pattern peak as shown in Fig. S1. According to the previous analysis, 9 broadband NIR emission will generate when Eu2+ ions occupy the six-coordinated Sc3+ 10 sites.44 To further explore the effect of doping Yb3+ and Eu2+ ions on the crystal structure 11 of the samples, the Raman spectra of CSSO, CSSO: 1%Yb3+ and CSSO: 1%Eu2+, 12 1%Yb3+ samples are measured. As shown in the Fig. 1c, the vibrational modes in the 13 range of 200–415 cm-1 originate from the rotational and translational motions of the 14 CaO8 dodecahedra (ν1), ScO6 octahedra (ν2) and SiO4 tetrahedra (ν3). The vibrational 15 modes lying from 450 to 700 cm-1 are assigned to bending motions of SiO4 tetrahedra 16 (ν4). The vibration modes within the range of 800–1200 cm-1 are attributed to the 17 symmetric (ν5) and asymmetric (ν6) internal stretching vibrations of the SiO4 18 tetrahedra.45-47 It is worth noting that the Raman peak of CSSO: Yb3+ sample does not 19 shows significantly shift or broadening compared with CSSO sample. However, when 20 Eu2+ and Yb3+ ions are co-doped, the Raman peak broadening is observed in the range 21 8  of 200–300 cm-1 and 800–900 cm-1. This broadening may arise from the unbalanced 1 substitution of Eu2+ for Sc3+ ions, which destroys the lattice order in the crystal 2 structure.48 To further verify the above inference, we perform the electron paramagnetic 3 resonance (EPR) test on CSSO: 1%Eu2+, 1%Yb3+ sample, as shown in Fig. S2. The 4 EPR signals at g = 6.1, 4.4 and 2.7 are corresponding to the unpaired electrons in Eu2+ 5 ions.49, 50 The EPR signal at g = 2 originates from electrons trapped in oxygen vacancies. 6 51 However, no EPR signal is detected in the CSSO: 1%Yb3+ sample. The 7 thermoluminescence spectra of CSSO: 1%Yb3+ and CSSO: 1%Eu2+, 1%Yb3+ samples 8 also provide strong evidence for the above inference, as shown in Fig. 1d. The intensity 9 of the main thermoluminescence peak (80oC) of the CSSO: 1%Eu2+, 1%Yb3+ sample is 10 600% higher than that of CSSO: 1%Yb3+ sample, and the trap depth is calculated to be 11 0.706 eV. This phenomenon indicates that the unbalanced substitution of Eu2+ for Sc3+ 12 significantly increases the oxygen vacancies concentration,52 as represented by the 13 reaction: 2Eu2+ + 2Sc3+ → 2EuʹSc + Vӧ. This further supports the above view that Eu2+ 14 ions can replace Sc3+ ions, thereby reducing the order of CSSO host. In addition, to 15 obtain structural information of the CSSO: 1%Eu2+, 5%Yb3+ sample, the Rietveld 16 refinement of XRD pattern is performed using garnet Ca3Sc2Si3O12 as the initial model, 17 as shown in Fig. 1e. The refined parameters Rwp = 12.17%, Rp = 9.73% and χ2 = 2.11, 18 indicating the reliability of the refined structural model.53 The refined cell parameters, 19 atomic sites and bond lengths are listed in the Table S1-S3, respectively. CSSO: 1%Eu2+, 20 5%Yb3+ sample exhibits a cubic crystal structure in the Ia-3d(230) space group, with a 21 9  cell parameter a = 12.23 Å and cell volume V = 1829.28 Å3. Based on the refinement 1 results, the crystal structure of the CSSO: 1%Eu2+, 5%Yb3+ sample is illustrated in Fig. 2 1f. This structure consists primarily of CaO8 dodecahedra, Sc/Eu/YbO6 octahedra and 3 SiO4 tetrahedra, with average bond lengths of 2.48, 2.09 and 1.64 Å (Table S3), 4 respectively. Additionally, the Sc/Eu/YbO6 octahedra share edges with CaO8 5 dodecahedra, and vertices with the SiO4 tetrahedra, thus forming a stable unit mode. 6 2.2 Morphological, XPS and EDS analysis of CSSO: Eu2+, Yb3+ sample. 7  8 Figure 2(a) and (b) The SEM, HR-TEM and SAED images of CSSO: 1%Eu2+, 5%Yb3+ sample; 9 (c) The XPS spectrum of CSSO: 1%Eu2+, 5%Yb3+ sample, the inset shows an enlarged view of the 10 yellow region; (d) and (e) The EDS spectrum and elemental mapping of CSSO: 1%Eu2+, 5%Yb3+ 11 sample particles. 12 The scanning electron microscope (SEM) images and particle size distribution 13 histograms of CSSO: 1%Eu2+, 5%Yb3+ sample are given in Fig. 2a and S3, with an 14 average particle size of 3.3 μm. In addition, a single particle has undergone high-15 resolution transmission electron microscopy (HR-TEM) and selected area electron 16 diffraction (SAED) analysis, as illustrated in Fig. 2b. The HR-TEM image indicates 17 10  that the sample possesses good crystallinity, and the lattice stripe spacings of 2.70 and 1 4.28 Å, calculated from the SAED pattern clearly reveal the existence of (420) and (220) 2 planes of the garnet structure Ca3Sc2Si3O12. In order to study the electronic states and 3 charges of the elements in CSSO: 1%Eu2+, 5%Yb3+ sample, we have used X-ray 4 photoelectron spectroscopy (XPS) technology to analyze them. The XPS spectrum and 5 the core level spectra of all elements are shown in Fig. 2c and S4, respectively. The 6 characteristic peaks at 347, 402, 102, 532, 1154 and 185 eV are observed, corresponding 7 to the energy level characteristics of Ca 2p, Sc 2p, Si 2p, O 1s, Eu 3d and Yb 4d, 8 respectively. These results further confirm the successful preparation of CSSO: 1%Eu2+, 9 5%Yb3+ sample. Furthermore, the elemental composition and distribution of CSSO: 10 1%Eu2+, 5%Yb3+ sample are analyzed using the energy dispersive spectroscopy (EDS), 11 as shown in Fig. 2d and 2e. The EDS spectrum and elemental mapping further confirm 12 that Eu2+ and Yb3+ ions are successfully doped into the host as well as the uniform 13 distribution of Ca, Sc, Si, O, Eu and Yb in the phosphor particles. 14  15  16  17  18  19  20  21 11  2.3 The luminescence properties and energy transfer analysis of the CSSO: Eu2+, 1 Yb3+ samples. 2  3 Figure 3(a) The excitation and emission spectra of CSSO: 1%Yb3+ sample; (b) The excitation 4 spectra of CSSO: 1%Eu2+, yYb3+ (0 ≤ y ≤ 7%) samples; (c) The emission spectra mapping at 5 different excitation wavelengths (λex = 350–700 nm); (d) The emission spectra of CSSO: 1%Eu2+, 6 yYb3+ (0 ≤ y ≤ 7%) samples; (e) The schematic diagram of the energy transfer process from Eu2+ 7 to Yb3+ ions in the CSSO: Eu2+, Yb3+ sample; (f) The decay curves of CSSO: 1%Eu2+, yYb3+ (0 ≤ 8 y ≤ 7%) samples. 9 Figure 3a shows the excitation and emission spectra of CSSO: 1%Yb3+ sample. 10 Specifically, the NIR-II emission centered at 976 nm can be clearly observed under the 11 UV light excitation, attributed to the 2F5/2→2F7/2 transition of Yb3+ ions. Simultaneously, 12 a broad excitation band ranging from 270 to 400 nm is recorded when monitoring the 13 emission at 976 nm, attributed to the Yb3+-O2- charge transfer bands.25 As we know, the 14 presence of Eu3+ and Yb2+ ions will significantly hinder the NIR luminescence of Eu2+ 15 and Yb3+ ions in CSSO host. To address this issue, we utilized pre-prepared EuS as a 16 raw material to substitute for the commonly used Eu2O3 and synthesized a series of 17 garnet structure CSSO: 1% Eu2+, yYb3+ (0 ≤ y ≤ 7%) samples under N2 atmosphere, the 18 12  excitation spectra of these samples are presented in the Fig. 3b. At a monitored of 976 1 nm, the excitation spectra of the Eu2+ and Yb3+ co-doped samples, compared to the 2 spectrum of the Yb3+-doped sample, not only exhibit the characteristic excitation of 3 Yb3+-O2- charge transfer bands, but also reveals strong absorption in visible light region, 4 corresponding to the 4f→5d transitions of Eu2+ ions, which indicates that there exists 5 effective energy transfer between Eu2+ and Yb3+ ions. In addition, CSSO: Eu2+, Yb3+ 6 samples can be effectively excited by almost the entire visible light (350–700 nm) and 7 emit broad NIR-II emission, as shown in Fig. 3c and S5. Under the optimal excitation 8 of 520 nm, the emission spectra simultaneously display the broadband NIR-I emission 9 of Eu2+ ions at 860 nm, along with the characteristic NIR-II emission peak of Yb3+ ions 10 at 976 nm, attributed to 5d→4f and 2F5/2→2F7/2 transition, respectively. It is noteworthy 11 that the emission intensity of Eu2+ ions significantly decreases with the increase of Yb3+ 12 concentration, which confirms the existence of energy transfer between Eu2+ and Yb3+ 13 ions.54 Meanwhile, the NIR-II emission intensity of Yb3+ ions reaches its maximum at 14 a concentration of 5%, as shown in Fig. 3d. To better understand the energy transfer 15 process from Eu2+ to Yb3+ ions, a schematic of the energy transfer is constructed, as 16 shown in Fig. 3e. Upon 520 nm excitation, the electrons are excited to the 5d excited 17 state of Eu2+ ions, and then transfer to the 2F5/2 excited state of Yb3+ based on the electric 18 multipole resonant energy transfer, and finally resulting in the NIR-II luminescence 19 enhancement of Yb3+ ions through 2F5/2 to 2F7/2 transitions. Moreover, the decay time of 20 CSSO: 1%Eu2+, yYb3+ (0 ≤ y ≤ 7%) samples monitored at 860 nm decrease from 4.70 21 13  to 4.35 μs (Fig. 3f), further strongly supporting the energy transfer process from Eu2+ 1 to Yb3+ ions.55 As Yb3+ ions doping concentration increases, the energy transfer 2 efficiency is evaluated, gradually rising to a maximum of 78.44% at y = 7%, as shown 3 in Fig. S6. This increase of the energy transfer efficiency can be attributed to the 4 shortened distance between sensitizer and activator, effectively enhancing the energy 5 transfer from Eu2+ to Yb3+ ions.56 6 2.4 Stabilization of Eu2+ and Yb3+ valence state and luminescence enhancement 7 mechanism investigation. 8  9 Figure 4(a) The emission spectra of CSSO: 1%Eu2+, 5%Yb3+sample with Eu2O3 as raw 10 material in a carbothermal atmosphere and EuS as raw material in a N2 atmosphere; (b) and (d) 11 The XPS spectra of Eu 3d and Yb 4d energy levels of CSSO: 1%Eu2+, 5%Yb3+sample with Eu2O3 12 as raw material in a carbothermal atmosphere; (c) and (e) The XPS spectra of Eu 3d and Yb 4d 13 energy levels with EuS as raw material in a N2 atmosphere; (f) The QE of CSSO: 1%Eu2+, 14 5%Yb3+ sample prepared by EuS in N2 atmosphere. 15 To substantiate that samples prepared using EuS under N2 atmosphere enhance the 16 NIR-II emission performance of Yb3+ ions, we compared the emission intensity of 17 samples prepared by Eu2O3 in a carbothermal atmosphere with those synthesized using 18 14  the aforementioned method. The NIR-II emission intensity of the sample prepared by 1 our proposed method is 6 times higher than that of the sample prepared by Eu2O3 in a 2 carbothermal atmosphere, as shown in Fig. 4a. To gain a deeper understanding of the 3 luminescence enhancement mechanism, the valence state of Eu and Yb is analyzed in 4 detail by XPS spectra. As shown in Fig. 4b and 4c, the Eu 3d XPS spectrum has been 5 deconvoluted into two signal peaks through Gaussian fitting, attributed to Eu2+ and Eu3+ 6 ions, respectively.57 In order to more clearly quantify the XPS signal intensity 7 relationship between Eu2+ and Eu3+ ions, the XPS signal intensity ratio R1 has been 8 proposed, specifically defined as the ratio of XPS signal intensity between Eu2+ and 9 Eu3+ ions. Excitingly, the R1 value of EuS-doped sample is 1.21, significantly higher 10 than that of Eu2O3-doped sample (R1 = 0.91), indicating a higher content of Eu2+ ions 11 in the EuS-doped sample. Additionally, Gaussian fitting is applied to deconvolute the 12 XPS spectra of the Yb3+ 4d peak within the range of 180 to 200 eV. The results indicate 13 that the peak at lower energy (~192 eV) corresponds to Yb2+ ions, while the peak at 14 higher energy (~186 eV) corresponds to Yb3+ ions.58-60 The XPS signal intensity ratio 15 R2 of Yb3+ and Yb2+ ions in the powder synthesized in N2 atmosphere (R2 = 11.92) is 16 much higher than that in the carbothermal atmosphere (R2 = 1.71) as shown in Fig. 4d 17 and 4e. In addition, the internal and external quantum efficiency (IQE and EQE) of 18 CSSO: 1%Eu2+, 5%Yb3+ sample prepared with EuS as the raw material under N2 19 atmosphere reach 44.68% and 18.62%, respectively, which is 6 times higher than that 20 of sample prepared with Eu2O3 as the raw material under a carbothermal atmosphere 21 15  (IQE = 7.51%，EQE = 3.87%), as shown in Fig. 4f and S7. The XPS and QE results 1 further suggest that employing EuS as a raw material in a N2 atmosphere leads to 2 improved stabilization of the valence states of Eu2+ and Yb3+ ions, thereby facilitating 3 an NIR-II emission enhancement of Yb3+ ions. 4 2.5 The temperature-dependent NIR luminescence properties of CSSO: 1%Eu2+, 5 5%Yb3+ sample. 6  7 Figure 5(a) Temperature-dependent emission spectra mapping of CSSO: 1%Eu2+, 5%Yb3+ 8 sample under 520 nm excitation; (b) Twenty cycles of intensity ratio variations measured at 9 120oC; (c) The variable temperature Raman spectra; (d) Normalized temperature-dependent 10 emission spectra; (e) Decay curves in the temperature ranges of 30–200oC; (f) Emission spectra at 11 77 K and 300 K. 12 The temperature-dependent emission spectra of CSSO: 5%Yb3+ and CSSO: 13 1%Eu2+, 5%Yb3+ samples were thoroughly investigated, as depicted in Fig. 5a, S8-S10. 14 Apparently, the NIR luminescence intensity of the CSSO: 1%Eu2+, 5%Yb3+ sample 15 exhibits a downward trend as the temperature elevates from 30 to 200°C. At 120°C, the 16 relative intensity is 55% of the initial value after twenty cycles test (Fig. 5b), which is 17 lower than that of CSSO: 5%Yb3+ sample (95%@120°C). This is attributed to the 18 16  introduction of Eu2+ ions in the CSSO: 5%Yb3+ sample, which destroys the lattice order 1 in the crystal structure, significantly reducing the structural rigidity and resulting in 2 degenerate thermal stability.61 In addition, the temperature dependent Raman spectra 3 can directly reflect the variation in chemical bond vibrations of a sample.62 As shown 4 in Fig. 5c, at high temperature of 200oC, the SiO4 tetrahedra exhibit significant bending 5 motions (ν4), symmetric (ν5) and asymmetric (ν6) internal stretching vibrations. The 6 Raman shift ΔR is obvious, especially for the Raman peak at 790 cm-1, which shift 7 reaches 10 cm-1. This indicates the relatively weak rigidity of CSSO: 1%Eu2+, 5%Yb3+ 8 sample, which further supports the above viewpoint. 9 The different thermal quenching behaviors of Eu2+ and Yb3+ ions emission centers 10 have also attracted our attention. As shown in Fig. 5d, by normalizing the emission peak 11 of Yb3+ (976 nm), it is observed that the emission intensity of Eu2+ (860 nm) gradually 12 increased, which is attributed to the phonon-assisted tunneling from the excited state of 13 Yb3+ to Eu2+ ions.63 In order to analyze this phenomenon, we have tested the decay 14 curve within the temperature range of 30 to 200oC as illustrated in Fig. 5e. Generally, 15 with the increase of temperature and the energy transfer between Eu2+ and Yb3+ ions, 16 the decay time of Eu2+ should gradually decreases.55, 64 However, the calculated decay 17 time does not decrease with increase of temperature. Instead, it shows an irregular up 18 and down change around the average value 7.23 μs. This phenomenon indicates a 19 competitive relationship between the thermal quenching of Eu2+ ions and the energy 20 transfer of Eu2+ and Yb3+ ions in CSSO: 1%Eu2+, 5%Yb3+ sample.29 Furthermore, we 21 17  have also measured the emission spectra of CSSO: 1%Eu2+, 5%Yb3+ samples at 77 K, 1 as shown in Fig. 5f. The results show that there is no obvious change in the peak shape 2 of the emission spectrum at low temperature, but the emission intensity is 220% higher 3 than that at 300 K. 4 2.6 Performance and application of the fabricated NIR-II pc-LED. 5  6 Figure 6 (a) and (b) PL spectra, output power and photoelectric efficiency of the fabricated 7 NIR-II pc-LED under drive current of 50–300 mA; (c) Schematic diagram of the designed 8 absorption spectroscopy testing system; The linear relationship between the absorbance and 9 concentration of (d) methanol, (e) ethanol, (f) glycol and (g) cyclohexane; (h) The training and 10 testing models of convolutional neural networks for the absorption spectra, types and 11 concentrations of four organic solvents. 12 The NIR-II pc-LED is fabricated by combing the optimized CSSO: 1%Eu2+, 13 5%Yb3+ phosphor with a 520 nm LED chip. Fig. 6a and 6b display the PL spectra, 14 output power, and photoelectric efficiency of the NIR-II pc-LED measured under drive 15 currents ranging from 50 to 300 mA. The results show that the PL emission intensity 16 increases significantly with the increase of current. The output power simultaneously 17 18  rises from 9.11 to 74.09 mW. Nevertheless, the photoelectric efficiency has decreased 1 from 12.61% to 8.79%. Additionally, we compared the electroluminescent properties 2 of our NIR-II pc-LED with similar green and blue light-excited NIR-II pc-LEDs, as 3 well as the commercial pc-NIR LEDs like Osram SHF 4735 and 4737, as detailed in 4 Table S4. The results reveal that the our NIR-II pc-LED demonstrates superior NIR 5 output power and photoelectric efficiency, which is not only significantly better than 6 other NIR-II pc-LEDs, but also comparable to the commercial Osram SHF 4737. The 7 operating temperature of the NIR-II pc-LED rises from 35.6 to 65.4°C as current rises 8 from 50 to 300 mA. When the NIR-II pc-LED is driven continuously at 100 mA for 30 9 to 120 min, its operating temperature increases from 37.7 to 60.1°C as shown in Fig. 10 S11. The minimal temperature variation of the NIR-II pc-LED during operation 11 highlights its potential as a high-power NIR light source suitable for scientific and 12 technical applications.29 13 In order to highlight the potential application value of NIR-II pc-LED in the field 14 of NIR spectroscopy technology, we designed and constructed a miniaturized, real-time 15 absorption spectroscopy testing system using this NIR-II pc-LED as the light source as 16 shown in Fig. 6c. Since C-H, N-H, O-H and other functional groups with different 17 energy levels in organic molecules absorb NIR light at their specific vibrational 18 frequencies.11, 65 Therefore, we tested four organic solvents using this homemade device 19 and determined their absorption spectra. As shown in Fig. S12, the absorption of 20 methanol and ethanol is mainly concentrated between 900 and 1100 nm,66 the 21 19  absorption of glycol is mainly concentrated in the range of 800 to 1050 nm,67 and 1 cyclohexane only has an absorption peak at 930 nm.68 Furthermore, the absorption 2 spectra show a typical linear relationship with the change of concentration, conforming 3 to the Lambert-Beer law, which proves its accuracy in the detection of the concentration 4 of organic substances, and the goodness of fit and fitting formula as shown in Fig. 6d-5 6g and Eq. S1-S4. To further improve the accuracy of the organic solvent concentration 6 tests, we have introduced a CNN model to learn from the measured absorption spectra, 7 as shown in Fig. 6h. During the training phase, the CNN is trained to identify the 8 absorption spectra of different concentrations of methanol, ethanol, glycol and 9 cyclohexane (0, 25%, 50%, 75%, and 100%) to enhance its generalization ability. 10 During the testing phase, we collected 100 different absorption spectra at various 11 concentrations for each organic solvent and performed 100 predictions, achieving a 12 maximum accuracy of up to 98%. The successful prediction of organic solvent 13 concentrations further highlights the significant advantages of the NIR-II pc-LED 14 prepared by CSSO: 1%Eu2+, 5%Yb3+ sample, enabling its suitability for various 15 portable NIR detection instruments. 16 3. Conclusion 17 In this work, the energy transfer from Eu2+ to Yb3+ ions are achieved through 18 rational design to enable the excitation of Yb3+ ions by visible light. Importantly, we 19 proposed a strategy for synthesizing a series of Ca3Sc2Si3O12: xEu2+, yYb3+ (x = 0 and 20 1%, 0 ≤ y ≤ 7%) samples under N2 atmosphere by replacing Eu2O3 with EuS. This 21 20  strategy not only effectively enhances the valence stability of Eu2+ and Yb3+ ions, but 1 also enhances the NIR-II emission quantum efficiency of Yb3+ ions by approximately 2 6 times. Finally, we combined the optimized sample with a 520 nm LED chip to prepare 3 NIR-II pc-LED. The photoelectric efficiency is 12.61%@50 mA and the output power 4 is 74.09 mW@300 mA, which is better than the current commercial NIR-II pc-LEDs. 5 Additionally, thanks to the efficient NIR-II emission of CSSO: 1%Eu2+, 5%Yb3+ sample, 6 and combined with CNN technology to accurately distinguish different concentrations 7 of organic solvents, the accuracy rate reaches 98%. In summary, this work successfully 8 achieved visible light excitation of Yb3+ ions through the establishment of an “Eu2+ 9 bridge”, which opened up a new way for the application of Yb3+ ions in the field of 10 portable NIR-II light source. 11 Acknowledgements 12 This work was supported by the National Key Research and Development 13 Program of China (Grant No. 2024YFA1409900), National Natural Science Foundation 14 of China (Grant Nos. U21A2074, U24A2018, U21A2068, 62375038, 12174046 and 15 12274057), Science and Technique Foundation of Dalian (Grant Nos. 2022JJ11CG003, 16 2023JJ12GX011 and 2022JJ12GX041), Science and Technique Foundation of Liaoning 17 Province (Grant Nos. 2023JH1/10400057, 2023JH1/10400080, 2023JH2/101800033, 18 2023JH2/101700058, 2023JH2/101700239 and 2024JH2/102400021) and 19 Fundamental Research Funds for the Central Universities (Grant No. 04442024068).  20  21 21  References 1 1. F. F. Wang, Y. T. Zhong, O. Bruns, Y. Y. Liang, H. J. Dai, Nat. Photonics 2024, 18, 2 535. 3 2. Y. Xie, W. S. Liu, W. Y. Deng, H. M. Wu, W. P. Wang, Y. C. Si, X. W. Zhan, G. 4 Gao, X. K. Chen, H . B. Wu, J. B. Peng, Y. Cao, Nat. Photonics 2022, 16, 752. 5 3. S. K. Gupta, K. Sudarshan, R. M. Kadam, Mater. Today. Commun. 2021, 27, 6 102277. 7 4. S. Q. Liu, J. X. Du, Z. Song, C. G. Ma, Q. L. Liu, Light: Sci. Appl. 2023, 12, 181. 8 5. T. Z. Wang, Y. Z. Wang, W. B. Chen, Z. G. Xia, Laser. Photonic. 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