Insight into Site Environment and Photoluminescence Properties of Cr3+-doped Gallogermanate Toward NIR pc-LEDs Applications

Insight into Site Environment and Photoluminescence Properties of Cr3+-doped Gallogermanate Toward NIR pc-LEDs Applications | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Insight into Site Environment and Photoluminescence Properties of Cr3+-doped Gallogermanate Toward NIR pc-LEDs Applications Decai Huang, Wenjie Deng, Wangzu Zou, Zhi Zhang, Shuping Huang, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8234812/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Near-infrared phosphor-converted light-emitting diodes (NIR pc-LEDs) are promising for applications in spectroscopy and bioimaging. Typical NIR phosphor designs feature isolated Cr 3+ ion centers in octahedral crystallographic sites. However, it is challenging to design broadband NIR phosphors based on tetrahedral crystallographic sites or Cr 3+ − Cr 3+ pairs. Herein, we explore a series of Cr 3+ -doped gallogermanate Ga 6 Ge 2 O 13 (GGO) phosphors that feature both octahedral, tetrahedral crystal sites, as well as Cr 3+ − Cr 3+ pairs, and evaluate their structure-property relationships. Structural and spectroscopic analysis, including low-temperature (10 K) spectroscopy, X-ray absorption near-edge structure, and electron paramagnetic resonance, reveal multiple Cr 3+ emission centers in GGO — namely, octahedral, tetrahedral, and Cr 3+ − Cr 3+ pairs, resulting in NIR luminescence at approximately 650 − 1400 nm. Through Cr 3+ concentration engineering, the emission spectra of phosphors exhibit a remarkable redshift and an increase in their full width at half maximum. The optimized GGO:6% Cr 3+ exhibits satisfactory thermal stability (60% retention at 423 K) alongside outstanding absorption efficiency (69%) and external quantum efficiency (36%). The fabricated NIR-LED device delivers a high NIR output power of 55.59 mW at a driving current of 180 mA. This work provides deeper insight into the site environment of Cr 3+ ions in phosphors and, more specifically, demonstrates the potential of gallogermanates as promising host systems for NIR phosphors. Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Inorganic LEDs Physical sciences/Physics/Electronics, photonics and device physics/Photonic devices Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Near-infrared (NIR) light serves as a versatile optical platform and has been extensively utilized in biological imaging, disease diagnosis and treatment, security surveillance, and agriculture, while leveraging the inherent advantages of NIR light, such as invisibility, deep tissue penetration, and low scattering loss 1 – 5 . Among various NIR light sources, NIR phosphor-converted light-emitting diodes (pc-LEDs) offer further practical benefits including environmental friendliness, cost-effectiveness, and stable illumination 6 – 8 . Moreover, the emission spectrum of NIR pc-LEDs can be tailored by selecting appropriate NIR phosphors, enabling adaptation to diverse application scenarios. Among various NIR activators (Cr 3+ , Fe 3+ , Ni 2+ , Mn 2+ , Eu 2+ , Bi 3+ ) 9–13 , Cr 3+ -activated phosphors have emerged as promising candidates for the light conversion layer in NIR pc-LEDs, owing to their tunable emission band and broad absorption in the blue region, which matches well with commercial GaN LED chips. Considerable progress has been made in developing Cr 3+ -activated NIR phosphors with high internal quantum efficiency and good thermal stability, as exemplified by representative host systems such as garnets 14 , borates 15 , spinels 16 , double perovskites 17 , and fluorides 18 . By tailoring the crystal field strength, the photoluminescence (PL) spectrum can be modulated within the 650 − 1100 nm range. Nevertheless, most Cr 3+ -doped phosphors still face two major challenges: limited absorption efficiency due to the parity-forbidden nature of d-d transitions, and the difficulty in achieving long-wavelength emission beyond 900 nm 19 . Although increasing the Cr 3+ doping concentration can enhance absorption, it frequently induces concentration quenching. Meanwhile, the coordination environment and site-specific luminescence behavior of Cr 3+ in many host systems remain elusive. In particular, the potential of tetrahedrally coordinated Cr 3+ to produce extended NIR emission beyond 850 nm has been largely overlooked, owing to its relatively low formation probability and poor chemical stability in common oxide hosts 20 – 23 . Additionally, while Cr 3+ − Cr 3+ pair emission has been sporadically reported in selected hosts as a route to broadband NIR emission 24 – 27 , a systematic understanding of how to controllably introduce such pairs and distinguish their emission from that of isolated ions is still lacking. Therefore, achieving coordinated control over the luminescence from isolated octahedral Cr 3+ , tetrahedral Cr 3+ , and Cr 3+ ion pairs represents a promising yet challenging pathway toward developing ultra-broadband NIR phosphors covering the 700–1400 nm region. In this work, we developed a gallium germanate solid solution, Ga 6 Ge 2 O 13 (GGO), as a host for Cr 3+ doping, motivated by its rich variety of cationic sites, including both octahedral and tetrahedral coordination, and the presence of Ga 3+ , which favors a weaker crystal field and facilitates longer-wavelength NIR emission 28 . Related materials such as Al 6 Ge 2 O 13 : Cr 3+ , Al 2.26 Ge 0.74 O 4.87 : Cr 3+ , Al 4 + 2 z Si 2−2 z O 10− z : Cr 3+ and Ga 3 Al 3 Ge 2 O 13 : Cr 3+ have been reported, 29–33 but the anomalous luminescence phenomena and precise lattice occupancy of Cr 3+ ions in the host have not been thoroughly investigated. Under 430 nm excitation, GGO:6% Cr 3+ exhibits a long-wavelength emission peak at 880 nm with a broad full width at half maximum (FWHM) of 215 nm. A notable shoulder peak at 538 nm in the excitation spectrum, which is unusual for Cr 3+ in a single site, suggests the presence of multiple luminescent centers. Through a combination of low-temperature spectroscopy, X-ray absorption fine structure (XAFS), and electron paramagnetic resonance (EPR) analysis, we identify the involvement of tetrahedrally coordinated Cr 3+ and exchange-coupled Cr 3+ − Cr 3+ ion pairs, in addition to conventional octahedral Cr 3+ . This work not only clarifies the PL mechanism and site occupancy of Cr 3+ in GGO but also provides a structural design strategy for developing long-wavelength, ultra-broadband NIR phosphors via multi-site engineering. Results Crystal structure and micromorphology In order to study the crystal structure of Cr 3+ -doped GGO materials, the crystallographic data of Ga 2.31 Ge 0.69 O 4.84 (ICSD-9005504) is used as the initial structural model to perform Rietveld refinement for GGO host and GGO:6% Cr 3+ . The refinement parameters are shown in Fig. 1 a and Table S1 -S2. The GGO crystal belongs to orthorhombic system, Pbam space group. Figure 1 b presents the crystal structure of refined GGO, which is composed of octahedral ([GaO 6 ]), tetrahedral ([Ga T /GeO 4 ] and [Ga T* O 4 ]) crystal sites. Based on the local characteristics of the material structure, the framework can be divided into two distinct units: Unit 1 (comprising two [GaO 6 ] octahedra and one [Ga T /GeO 4 ] tetrahedron) and Unit 2 (comprising two [GaO 6 ] octahedra and one [Ga T* O 4 ] tetrahedron). Notably, the distance between the two adjacent octahedra ([GaO 6 ]) is only 3.0148 Å in both units. Therefore, it can be anticipated that if Cr 3+ ions are doped into the material, the distance between Cr ions will be very short (< 5 Å). Such proximity would lead to overlapping electron clouds between neighboring Cr ions, resulting in an exchange coupling interaction among Cr 3+ ions 34 . In addition, the [GaO 6 ] octahedra are connected via edge-sharing to form octahedral chain columns along the c -axis, which are cross-linked by the irregularly occupied [Ga T /GeO 4 ] and [Ga T* O 4 ] tetrahedral sites. Meanwhile, the disordered distribution of Ga/Ge sites in the tetrahedral units of GGO provides structural flexibility for the doping of Cr 3+ ions. Figure 1 c presents the XRD patterns of GGO: y Cr 3+ ( y = 0–16%) samples. All diffraction peaks match well with the standard PDF card (Ga 6 Ge 2 O 13 , PDF#50–0354), indicating that Cr 3+ ions have been successfully incorporated into GGO host. Furthermore, the magnified view of the diffraction peak between 24.1° and 26.5° reveals a slight shift toward higher angles with increasing Cr 3+ content. Fig. S1 presents the Rietveld refinement profiles of samples with different Cr 3+ concentrations, and the unit cell parameters ( a , b , c , and V ) are summarized in Fig. 1 d. These results confirm a slight shrink of the unit cell as the Cr 3+ concentration increases. Considering the ionic radius of Ga 3+ (CN = 6, r = 0.62 Å; CN = 4, r = 0.47 Å), Ge 4+ (CN = 4, r = 0.39 Å) and Cr 3+ (CN = 6, r = 0.615 Å; CN = 4, r = 0.41 Å), the observed lattice slightly shrinkage is likely due to the substitution of Cr 3+ for Ga 3+ in the host structure. However, whether Cr 3+ occupies both octahedral and tetrahedral sites or is limited to a single site requires further investigation. Figure 1 e shows the SEM and elemental mapping images of GGO:6% Cr 3+ , and the corresponding EDS spectrum is provided in Fig. S2. The sample consists of irregularly shaped particles with sizes ranging from 10 to 40 µm. Elemental mapping demonstrates a homogeneous distribution of Ga, Ge, O, and Cr throughout the particles. Combined with EDS analysis, the elemental composition is found to be close to the theoretical stoichiometric ratio of Ga 6 Ge 2 O 13 . Photoluminescence (PL) properties The bandgap of the host material significantly influences the luminescence properties of Cr 3+ -doped near-infrared phosphors, especially in terms of thermal stability. Figure 2 a shows the diffuse reflectance (DR) spectra of the host and GGO: y Cr 3+ samples. Based on the Tauc plot and Kubelka-Munk transformation, the optical bandgap ( E g ) of GGO was determined to be approximately 4.32 eV (inset of Fig. 2 a), which is larger than that of many oxide materials 35 – 37 , and provide a suitable PL environment for Cr 3+ ions. Compared with the host, the Cr 3+ -doped samples exhibit three distinct absorption bands, located in the ranges of 300–350 nm, 400–550 nm, and 550–750 nm, respectively. Notably, the broadband 550–750 nm comprises at least two distinct electronic transitions, and further analysis will be conducted in combination with the PL spectrum. Furthermore, no additional absorption band in the 800–1200 nm range indicates that Cr 4+ ions are virtually absent in GGO host. Moreover, XPS results of GGO: 6%Cr 3+ further confirm that Cr ions exist predominantly in the trivalent state (Cr 3+ ). As shown in the survey spectrum (Fig. S3a), all expected elements were detected. The high-resolution Cr 2p spectrum (Fig. S3b) exhibits two peaks at binding energies of 576.18 and 588.38 eV, corresponding to the 2p 3/2 and 2p 1/2 orbitals of Cr 3+ , respectively 38 , 39 . Figure 2 b shows the PL excitation and emission spectra of GGO:6% Cr 3+ . The excitation spectrum agrees well with its DR spectrum. The three excitation bands are attributed to the 4 A 2 → 4 T 1 (4P) (270 nm), 4 A 2 → 4 T 1 (4F) (430 nm) and 4 A 2 → 4 T 2 (4F) (640 nm) transitions of Cr 3+ ions, which also account for the pale green color of the sample under ambient light. Notably, a shoulder peak observed around 560 nm in the excitation spectrum is consistent with the corresponding feature in the DR spectrum, which cannot be found in the 3d 3 energy level diagram of Cr 3+ ions, and will be discussed in later. Upon 430 nm excitation, the sample exhibits a broadband NIR emission, peaking at 880 nm, with a full width at half maximum (FWHM) of about 210 nm. The FWHM is significantly broader than that of many Cr 3+ -activated materials in which emission originates from a single crystallographic site 40 . Combined with the distinct features observed in the excitation spectrum, these results suggest the coexistence of emission from multiple Cr 3+ sites within the GGO host. Figure 2 c shows the concentration-dependent PL spectra of GGO: y Cr 3+ ( y = 2% – 16%). As the Cr 3+ doping concentration increases, the emission peak systematically red-shifts from 850 nm to 918 nm, accompanied by a spectral broadening of the FWHM from 209 nm to 239 nm. The optimal integrated PL intensity is achieved at a Cr 3+ doping concentration of 6 mol% (Fig. 2 d). In order to accurately assess the PL efficiency of the NIR phosphors, the PL quantum yield of GGO:6% Cr 3+ was evaluated (Fig. S4). Th internal quantum yield (IQY) reaches 53%. Notably, the absorption efficiency reaches 69%, which, to the best of our knowledge, represents the highest value reported among Cr 3+ -activated NIR phosphors in powder form (Table S3). The high absorption is likely attributed to the local structural distortion in the host lattice, which partially relaxes the parity-forbidden rule of the d-d transitions in Cr 3+ ions, thereby significantly enhancing blue light absorption. Consequently, an external quantum yield (EQY) of 36% was achieved, ranking among the highest values for Cr 3+ -activated materials with emission peaks beyond 880 nm. Building upon the discussion of the unique structure features and unusual PL properties of GGO: Cr 3+ , an in-depth investigation into the lattice occupancy and luminescence characteristics of Cr 3+ in GGO was conducted by selecting two samples with Cr 3+ doping concentrations of 0.1% and 6%. As we know, low-temperature measurement can suppress thermal broadening of spectral bands, enabling more reliable identification of energy levels. Figure 3 a presents the PLE and PL spectra of GGO:0.1% Cr 3+ measured at 10 K. Surprisingly, under blue light excitation, the emission spectrum consists of a sharp line at 697 nm superimposed on a broad band spanning from 720 nm to 1000 nm. Notably, the excitation spectra monitored at 697 nm and 850 nm are distinctly different. The excitation spectrum from 697 nm emission displays three characteristic bands corresponding to the 4 A 2 level transitions to 4 T 1 (4P), 4 T 1 (4F), and 4 T 2 (4F), respectively. In contrast, the excitation spectrum from 850 nm emission exhibits additional peaks at 565 nm and 650 nm alongside the three fundamental Cr 3+ transitions. This disparity strongly indicates that Cr 3+ ions occupy multiple crystallographic sites in the GGO host, even at ultra-low doping concentrations. Gaussian fitting of the low-temperature emission spectrum (Fig. 3 b) deconvolutes it into a sharp peak at 697 nm and a broad Gaussian band spanning 720–850 nm. The sharp emission 697 nm and broadband ~ 726 nm are unambiguously assigned to the 2 E/ 4 T 2 → 4 A 2 transition of Cr 3+ ions in [GaO 6 ] octahedral environment. Considering that the concentration of Cr 3+ ions is extremely low, it is unlikely that Cr 3+ – Cr 3+ ion pairs will be formed. Thus, the broad emission centered near 850 nm is tentatively attributed to Cr 3+ ions occupying tetrahedral sites in GGO. In addition, comparison of the PL spectra at 300 K and 10 K reveals significant thermal quenching of the sharp-line emission (Fig. 3 a and Fig. S5a), accompanied by a relative enhancement of the broadband emission due to thermal population effects between the 2 E and 4 T 2 energy levels. Gaussian fitting of the 300 K PL spectrum shows that the position of the ~ 850 nm band remains largely unchanged. In contrast, the 726 nm emission peak observed at low temperature red-shifts to 742 nm at 300 K. This shift can be attributed to intensified electron-lattice interactions and a reduction in crystal field strength due to thermal expansion. Alternatively, the involvement of coupled Cr 3+ - Cr 3+ pairs, whose emission may overlap and merge with the 4 T 2 level, could also contribute to the observed spectral shift. Furthermore, the PL lifetimes at different emission wavelengths of GGO:0.1% Cr 3+ were also evaluated (Fig. 3 c). At 10 K, the decay curve at 697 nm exhibits a long lifetime ( τ = 3.11 ms) and a single exponential decay behavior, which is consistent with the previous speculation that it belongs to the 2 E → 4 A 2 forbidden transition of isolated octahedral Cr 3+ . However, the decay curve at 850 nm shows a short lifetime ( τ = 74.95 µs) and double-exponential decay behavior. As the temperature increases from 10 K to 300 K, the average lifetime ( τ av ) decreased from 74.95 µs ( τ 1 = 128.42 µs, τ 2 = 39.96 µs) to 60.78 µs ( τ 1 = 119.19 µs, τ 2 = 37.58 µs), mainly caused by the enhanced nonradiative transition probability at elevated temperature. It is noted that the decay fraction τ 1 and τ 2 are considered to be derived from two overlapping broadband emission. For comparison, temperature-dependent PL spectra of GGO:6% Cr 3+ were collected from 10 K to 300 K (Fig. 3 d and Fig. S5b). The result shows that the emission spectrum of GGO:6% Cr 3+ remains a broadband even at 10 K, with a spectral profile similar to that observed at 300 K, differing mainly in intensity. The excitation spectrum also shows little variation with temperature. Gaussian fitting of the PL spectra at 10 K and 300 K resolves two components centered near 880 nm and 960 nm (Fig. 3 e), indicating contributions from two distinct luminescent centers. Comparing the PL spectra of GGO:6% Cr 3+ and GGO:0.1% Cr 3+ reveals a significant red-shift in the highly doped sample. Given the similar ionic radii of Cr 3+ and Ga 3+ in octahedral coordination, this red-shift is unlikely to result from a substantial reduction in crystal field strength. Instead, it can be attributed to the formation of exchange-coupled Cr 3+ -Cr 3+ pairs, facilitated by the short distance (~ 3.2 Å) between adjacent [GaO 6 ] octahedra in the GGO structure. Then the emission from isolated octahedral Cr 3+ and that from Cr 3+ -Cr 3+ pairs will overlap, leading to the observed red-shift. Thus, in the GGO:6% Cr 3+ sample, the Gaussian component near 880 nm originates from both isolated octahedral Cr 3+ and Cr 3+ - Cr 3+ pairs, while the 960 nm band is attributed to Cr 3+ ions in tetrahedral sites, the same center responsible for the ~ 850 nm emission in GGO:0.1% Cr 3+ sample. Concurrently, the PL lifetime of GGO:6% Cr 3+ at 875 nm and 959 nm were also monitored (Fig. 3 f). Both decay curves exhibit double exponential decay behavior, and the fitted PL average lifetimes ( τ av ) are 23.45 µs ( τ 1 = 35.38 µs, τ 2 = 15.78 µs) and 13.47 µs ( τ 1 = 18.98 µs, τ 2 = 6.47 µs), respectively. Based on the above analysis, we speculate that the τ 1 may be derived from the energy level transition of 4 T 2 → 4 A 2 of octahedral Cr 3+ , while the τ 2 is derived from the energy level transition of 4 T 2 → 4 T 1 of tetrahedral Cr 3+ . Site occupation and local structure of Cr In order to further clarify the above inference, EPR and XAFS measurements were employed to probe the chemical state and local coordination of Cr 3+ ions in the GGO host. As shown in Fig. 4 a, the EPR spectra provide insights into the coordination environment and electronic state of Cr 3+ ions substituted into the host lattice. The material was analyzed to evaluate their local environment. Different from the GGO:0.1% Cr 3+ sample, GGO:6% Cr 3+ exhibits broader spectral features. A resonance signal is observed in both samples at g = 1.907/1.891, which is assigned to Cr 3+ ions occupying tetrahedral sites or involved in Cr 3+ - Cr 3+ exchange coupling effect, while the signal at g = 4.167/4.042 corresponds to isolated Cr 3+ ions located in octahedral sites of GGO. The signal near g = 1.891 is significantly enhanced in the GGO: 6%Cr 3+ sample due to strong Cr 3+ - Cr 3+ pair coupling. In addition, the broadening of the g = 1.891 band significantly increases with Cr 3+ concentration, indicating that this EPR signal does not originate from oxygen vacancy. Compared with GGO:0.1% Cr 3+ , GGO: 6% Cr 3+ has a signal observed in the range of g = 2.5–3.5, which may be derived from the distorted [CrO 6 ] octahedron. No EPR signal Corresponding to Cr 4+ in a tetrahedral environment was detected. The chemical state and local coordination environment of GGO:6% Cr 3+ were further investigated using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. As shown in Fig. 4 b, the XANES spectrum of GGO:6% Cr 3+ exhibits an obvious pre-edge absorption peak at 5992 eV, indicating the presence of Cr 3+ ions in tetrahedral site, while the peak at 6009 eV (white line) corresponds to the Cr 3+ ions in octahedral site, consistent with the reference spectrum of Cr 2 O 3 foil (Fig. 4 b). These features clearly demonstrate that the emission previously attributed to Cr 6+ actually originates from Cr 3+ ions in a tetrahedral crystal field environment. EXAFS analysis of the first coordination shell for GGO:6% Cr is performed using a combined model based on Cr–O (from Cr 2 O 3 ) and Cr = O (from CrO 3 ) reference paths. The fitting results show that the Cr–O path has a coordination number (CN) of 6 and a phase-corrected bond length (R) of 1.96 Å, whereas the Cr = O path has a CN of 4 and a bond length of 1.58 Å. The k 3 -weighted wavelet transforms (WT) EXAFS spectrum of GGO:6% Cr shows a profile similar to those of CrO 3 and Cr 2 O 3 in k space, and has a noticeable broadening (Fig. 4 c). These findings confirm that Cr 3+ exists in octahedron and tetrahedron sites, and the relevant supplementary information is shown in Figs. S6a-b and Table S4. Notably, the fitted of Cr-M (M = Cr, Ga or Ge) bond length is 2.94 ± 0.02 Å, indicating short interionic distances between Cr 3+ ions. This proximity facilitates the formation of Cr 3+ – Cr 3+ pairs, in agreement with the EPR results discussed above. Based on the combined results of EPR, XAFS, and PL characterization, the PL mechanism of GGO: Cr 3+ at different doping concentrations is illustrated in Fig. 4 d. In the low concentration sample ( y = 0.1%), the sharp R-line comes from the parity-forbidden 2 E→ 4 A 2 transition in the isolated octahedral ions, while the broadband emissions (726 nm and 850 nm) comes from the 4 T 2 level of the isolated octahedron and tetrahedral Cr 3+ , respectively. With the increase of Cr 3+ concentration, the formation of Cr 3+ – Cr 3+ ion pair induced the formation of ( 4 T 2 , 4 A 2 ) state, resulting in the gradual broadening of the emission band, the decrease of energy and the significant red shift of the emission peak. Energy level transition and luminescence mechanism of Cr According the previous analysis, a clear depiction of the electronic and vibronic energy diagram of Cr 3+ in GGO host is essential for understanding the luminescence behaviors. The optical transitions of Cr 3+ are sensitive to their local coordination, and the energy level splitting is determined by the local symmetry of the crystallographic site occupied by Cr 3+ ions. When Cr 3+ ions are doped into the crystal lattice of GGO, they substitute for [GaO 6 ] octahedral and [Ga T* O 4 ] tetrahedral sites with local site symmetry of C 2h and C S . Based on group theory, both spin-orbit interaction and symmetry descending will affect the energy level splitting. Detailed energy levels of Cr 3+ at the site symmetry of C 2h and C S were deduced and plotted in Fig. 5 a. All identified energy levels closely match the peak positions in the excitation spectrum of the sample at 10 K. The calculation of the crystal field strength of GGO:0.1% Cr 3+ at 10 K was presented in the supplementary information. The Dq/B values of Cr 3+ in the octahedral site and tetrahedral site were calculated to be 2.74 and 0.68, respectively (Fig. 3 b, c). The Dq/B value for Cr 3+ at the tetrahedral site demonstrates that Cr 3+ ions occupied a weak crystal field environment and gives rise to broadband 4 T 2 → 4 T 1 emission. Furthermore, a strong excitation band characteristic of octahedral Cr 3+ is observed in the excitation spectrum monitored at the tetrahedral Cr 3+ emission (850 nm, Fig. 3 a), indicating the occurrence of energy transfer (ET) from octahedral to tetrahedral Cr 3+ centers. PL thermal stability It is widely recognized that PL thermal stability is a critical parameter for the practical application of phosphors, typically evaluated by the ratio of the PL intensity at 423 K to that at room temperature (RT). In order to evaluate the thermal stability of GGO: Cr 3+ phosphor, temperature-dependent PL spectra of GGO:6% Cr 3+ were recorded from 298 K to 473 K. The relationship between the normalized PL integral intensity and temperature is shown in Fig. 6 a. A thermal quenching phenomenon is observed with increasing temperature. At 373 K and 423 K, the integrated intensity of GGO:6% Cr 3+ maintains 75% and 60% of its initial value at RT, respectively. The activation energy ( E a ) for the thermal quenching process can be determined using the single-barrier model according to the following equation: $$ \text{I}\text{T}\text{/I}\text{0}\text{ = }\left[\text{1+Aexp(-}{E}_{a}\text{/kT)}\right]\text{}\text{}\text{ (}\text{1}\text{)}$$ I 0 is the initial PL integral strength, I T is the integral strength at a given temperature T , A is a constant, k is a Boltzmann's constant (8.617 × 10 − 5 eV/K), and E a is the activation energy. The E a fitting value of GGO:6% Cr 3+ is 0.22 eV (Fig. 6 b). The lower activation energy ( E a ) means larger energy loss and stronger thermal quenching. The Electron-phonon coupling (EPC) effect is an important factor in describing thermal stability. The Huang-Rhys factor ( S ) is a crucial parameter for assessing the EPC effect and can be obtained through the following Eq. 4 1 : $$ \text{FWHM }\text{= }\text{2.36}\sqrt{S}hv\sqrt{\text{c}\text{o}\text{t}\text{h}\left(\frac{h\omega }{2kT}\right)}\text{}\text{}\text{ (}\text{2}\text{)}$$ where S refers to the Huang-Rhys factor reflecting the EPC strength, hv represents the phonon energy, and k is a constant. The equation is simplified to the following equation: $$ {FWHM}^{2}\text{}\text{= }\text{5.57 }\text{×}\text{}\text{S}\text{}\text{×}\text{}({h\omega )}^{2}\text{}\text{×}\text{ (1 + }\frac{2kT}{h\omega }\text{)}\text{}\text{ (}\text{3}\text{)}$$ The linear relationship between FWHM 2 and 2 kT was obtained by fitting the data from the variation of FWHM with temperature as displayed in Fig. 6 c. The calculated hν and S are 0.0209 eV and 7.1236, respectively. These outcomes reveal that GGO:6% Cr 3+ suffers from a strong EPC effect, and also implies that the material would exhibit more pronounced thermal quenching compared to phosphors with shorter-wavelength emission. NIR pc-LED device and multiple applications To evaluate the practical application of GGO: Cr 3+ phosphors, a prototype NIR pc-LED device was fabricated by GGO:6% Cr 3+ and commercial 430 nm LED chip (3 W) (insets in Fig. 7 a). The electroluminescence (EL) spectrum of the device is shown in Fig. 7 a, exhibiting a broad emission band from 680 nm to 1400 nm. The insets show photographs of the device in the off state, and under illumination as captured by a normal camera and an NIR camera, respectively, revealing intense NIR emission. The EL intensity increases progressively with the driving current (Fig. 7 b). The NIR radiant flux and the photoelectric conversion efficiency of the device under different driving currents (20–180 mA) are summarized in Fig. 7 c. Owing to the high EQE and good thermal stability of the GGO:6%Cr 3+ phosphor, the NIR output power reaches a maximum of 55.59 mW at 180 mA, and the photoelectric efficiency peaks at 15.0% at 20 mA. The observed decrease in photoelectric efficiency with increasing current is attributed to the efficiency droop effect inherent to the LED chip. The Gannan area is an important rare earth resource base in China, where mining activities can lead to the release of ions such as Nd 3+ and Yb 3+ into surrounding water bodies. While these ions exhibit low ecological toxicity at trace concentrations, long-term accumulation may disrupt aquatic ecosystems and pose potential health risks to humans via the food chain. Nevertheless, given their recyclable value, the efficient monitoring and recovery of these rare earth (RE) ions can serve the dual purposes of environmental remediation and resource reclamation. This calls for the development of a straightforward detection technique for Nd 3+ , Yb 3+ , and other rare earth ions. Here, we demonstrate that their concentrations can be quantitatively determined using a non-contact detection approach based on a NIR pc-LED. This method relies on the characteristic absorption peaks of these ions in the near-infrared region—for instance, Nd 3+ at ~ 880 nm and Yb 3+ at ~ 980 nm. As shown in Figs. 7 d-e, the absorption spectra of Nd 3+ and Yb 3+ aqueous solutions were measured at different concentration gradients. According to the Lambert-Beer law, the relationship between the transmitted light intensity and the concentration of Nd 3+ /Yb 3+ ions in aqueous solution can be expressed as: $$ {\text{Log}}_{\text{10}}\left({\text{I}}_{\text{0}}\text{/}\text{I}\right)\text{}\text{=}\text{}\text{ε}\text{bc}$$ 4 where I 0 is the intensity of the incident light, I is the intensity of the transmitted light, \( \text{ε}\) is the molar absorption coefficient, b is the thickness of the absorption layer, and c is the concentration of the absorbing material. As shown in Fig. 7 f, the data points exhibit a good linear fit, from which the calibration curves for Nd 3+ and Yb 3+ were established. The concentration of these ions in real water samples can thus be determined using the external standard method, demonstrating the great potential of the GGO:6% Cr 3+ phosphor for the quantitative detection of RE ions. Additionally, owing to the material-dependent absorption of near-infrared light, the fabricated NIR pc-LED also can be utilized for transmission-based imaging, where variations in transmitted NIR brightness reveal internal structures. As shown in Fig. 7 g, the vascular network in a human palm is clearly visible under illumination by the NIR pc-LED when captured with an NIR camera. Furthermore, the device can be used for non-destructive inspection; for example, the precise location of an embedded chip inside a card can be accurately identified without causing any physical damage, as shown in Figs. 7 h-i. These demonstrations indicate that the fabricated NIR pc-LED devices hold promise for multiple applications in non-destructive testing and bioimaging. Discussions In this work, a series NIR phosphor GGO: Cr 3+ were synthesized via solid-state reaction technology. The excitation spectrum of the sample is distinct from that of Cr 3+ in a single octahedral environment, and its emission spectrum exhibits a significant red-shift with increasing Cr 3+ concentration. Through structural analysis combined with low-temperature spectroscopy and XANES, we clarified that the broad emission (650–1400 nm) originates from the combined luminescence of Cr 3+ ions occupying both octahedral and tetrahedral sites, as well as from exchange-coupled Cr 3+ pairs. The optimized GGO:6% Cr 3+ phosphor achieves an extremely high blue light absorption about 69% with QY/EQY of 53%/36%. Moreover, at 423 K, the PL intensity of this phosphor still maintained 60% of that at room temperature. A fabricated NIR pc-LED delivers a NIR output power about 55.59 mW at 180 mA and a photoelectric efficiency of 15%, demonstrating potential for nondestructive testing and bioimaging. We believe that this work provides an insight into the occupation and PL behavior of Cr 3+ ions and will inspire more researchers to develop high-performance broadband NIR phosphors. Materials and methods Synthesis GGO: y Cr 3+ ( y = 0–16%) samples were prepared by high temperature solid state reaction method. Raw materials Ga 2 O 3 (99.9%, Macklin), GeO 2 (99.99%, Macklin) and Cr 2 O 3 (99.9%, Macklin) were weighed according to stoichiometry and ground in agate mortar for 20 min. The dried mixture was then placed in an alumina crucible and subjected to a multi-step solid-phase reaction in a tube furnace (at 600 ℃ for 2 h, 1350 ℃ for 6 h, respectively for burn-in and crystallization, at a heating rate of 5 ℃/min). After the final sintering, the powder was cooled to room temperature at 5 ℃/min and ground into fine powder for subsequent characterization. Characterizations The XRD patterns of samples were collected on a BRUKER D8 X-ray diffractometer with Cu Kα radiation with a count time of 15 s/step and a step size of 0.01° at room temperature. Rietveld refinements were performed using the GSAS program. The crystal structures were determined by a three-dimensional visualization system for electronic and structural analysis. The morphologies and elemental compositions of the powders were observed and detected via SEM (MLA650F, FEI) equipped with EDS. The valence states of chromium ions were confirmed through XPS using an AXIS ULTRA spectrometer from Kratos Analytical Ltd. The PL excitation and emission spectra were recorded using a spectrometer (FLS 980, Edinburgh Instruments) equipped with 450 W Xe lamps. EPR measurements on a Bruker EMXplus-6/1 EPR spectrometer. Diffuse reflection spectra were obtained using a UV-vis-NIR spectrophotometer (PerkinElmer, Lambda 950) with a reflectivity standard whiteboard as the calibration. Decay curves and temperature-dependent PL spectra were collected using the same with a flash lamp and temperature controller (THMS 600, Linkam Scientific Instruments). The PL IQE of phosphors at room temperature were performed on a Hamamatsu quantum yield measurement system (C9920-02G). XAFS was performed at the BL14B2* of SPring-8 (8 GeV, 100 mA), Japan, in which, the X-ray beam was mono-chromatized with water-cooled Si (111) double-crystal monochromator and focused with two Rh coated focusing mirrors with the beam size of 2.0 mm in the horizontal direction and 0.5 mm in the vertical direction around sample position, to obtain XAFS spectra both in near and extended edge, Cr 2 O 3 foil sample was used as references. NIR pc-LED was fabricated by blending NIR phosphors with 430 nm blue LED chip. The appropriate amount of GGO: Cr 3+ with different weights were mixed with transparent epoxy resin in a ratio of 1:1, and uniformly coated on the blue LED chip. After drying in air at 80 ℃ for 1 h, NIR pc-LED device was obtained. The performance of device was measured by photoelectric test system (HAAS-2000, 350–1100 nm and 780–1650 nm, Everfine, Hangzhou). Declarations Author details 1 College of Rare Earths, Jiangxi University of Science and Technology, Ganzhou 341000, P.R. China. 2 National Rare Earth Functional Materials Manufacturing Innovation Centre, Guorui Kechuang Rare Earth Functional Materials (Ganzhou) Co., Ltd, Ganzhou 341000, P. R. China. 3 Key Laboratory of Testing and Tracing of Rare Earth Products for State Market Regulation, Jiangxi University of Science and Technology, Ganzhou 341000, P.R. China. 4 Key Laboratory of Low Dimensional Quantum Materials and Sensor Devices of Jiangxi Education Institutes, Ganzhou 341000, China. 5 Yunnan Key Laboratory of Electromagnetic Materials and Devices, School of Materials and Energy, Yunnan University, Kunming 650091, P. R. China Competing interests The authors declare no competing interests. Author contributions W.J.D. and D.C.H. conceived the idea. W.J.D. conducted the materials synthesis. W.J.D. conducted the XRD, SEM, XPS, fluorescence steady-state and transient spectroscopy measurements, etc. W.J.D. and D.C.H. co-wrote and revised the manuscript. D.C.H. supervised the work. All authors contributed to the discussion of the results and provided comments on the manuscript. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 12274409), the Natural Science Foundation of Jiangxi Province of China (No. 20232ACB211006), the Science and Technology Program of Ganzhou City (No. 2023CYZ17837), Key Laboratory of Ionic Rare Earth Resources and Environment, Ministry of Natural Resources of the People’s Republic of China (No. 2024IRERE204), the Jiangxi Provincial Training Program for Academic and Technical Leaders of Major Disciplines (No. 20232BCJ23088), the project of Yunnan Key Laboratory of Electromagnetic Materials and Devices, Yunnan University (No. ZZ2024002) and the Jiangxi Provincial Project for Graduate Innovation Special Fund (No. YC2024-S538). Data availability The data that support the findings of this study are available from the corresponding author upon reasonable request. References Gu, S. M. et al. Laser-driven luminescent ceramic-converted near-infrared II light source for advanced imaging and detection techniques. Light Sci. Appl. 14, 317 (2025). Shen, H. C. et al. Rational design of NIR-II AIEgens with ultrahigh quantum yields for photo-and chemiluminescence imaging. J. Am. Chem. Soc. 144, 15391–15402 (2022). Hu, Z. H. et al. First-in-human liver-tumour surgery guided by multispectral fluorescence imaging in the visible and near-infrared-I/II windows. Nat. Biomed. Eng. 4, 259–271 (2019). Jia, Z. W. et al. Strategies to approach high performance in Cr 3+ -doped phosphors for high-power NIR-LED light sources. Light Sci. Appl. 9, 86 (2020). Zhong, Y. T. et al. In vivo molecular imaging for immunotherapy using ultra-bright near-infrared-IIb rare-earth nanoparticles. Nat. Biotechnol. 37, 1322–1331. (2019). Li, R. Y. et al. Boosting Applications with High-performance near-infrared phosphor-converted light-emitting diodes. Laser Photonics Rev. 18, 2300608 (2024). Wang, K. N. et al. Enabling highly efficient neodymium luminescence for near-infrared phosphor-converted light-emitting diode applications. Small Struct. 5, 2400092 (2024). Zheng, G. J. et al. Glass-crystallized luminescence translucent ceramics toward high‐performance broadband NIR LEDs. Adv. Sci. 9, 2105713 (2022). Li, H. M. et al. Ultrawideband emission of Bi 3+ ions spanning visible to near-infrared spectral regions (400 nm-1700 nm). Angew. Chem. Int. Ed. e202520587 (2025). Xiao, Y. et al. Tracing the origin of near-infrared emissions emanating from manganese. Light Sci. Appl. 14, 194 (2025). Yan, L. Q. et al. Emerging Fe 3+ doped broad NIR-emitting phosphor Ca 2.5 Hf 2.5 (Ga, Al) 3 O 12 : Fe 3+ for LWUV pumped NIR LED. Laser Photonics Rev. 18, 1 (2024). Zhang, Y. et al. Enhanced near-infrared II and III emission in Ga 2 O 3 : Ni 2+ phosphor via charge compensation for NIR spectroscopy application. Ceram. Int. 50, 31589–31597 (2024). Zhao, Q. Q. et al. Unlocking chlorine oxide-based ultra-wideband near-Infrared Phosphor and Advancing Spectral Performance via Lattice Engineering. Adv. Funct. Mater. 34, 1 (2024). Z. R. Liao. et al. Developing the Na 2 CaZr 2 Ge 3 O 12 : Cr 3+ Garnet Phosphor for Advanced NIR pc-LEDs in Night Vision and Bioimaging. Chem. Mater. 36, 4654–4663 (2024). Yue, Y. X. et al. Preparation and luminescent properties of chromium ion doped aluminum borate near-infrared phosphor (Al 18 B 4 O 33 : Cr 3+ ) and its application of broadband near-infrared LED. Ceram. Int. 51, 27243–27252 (2025). Li, C. J. et al. Laser-driven spinel-type ceramics enabling NIR-II light sources for penetration optical imaging assisted by a guided filter network algorithm. Mater. Today https://doi.org/10.1016/j.mattod.2025.10.019 (2025). Mou, T. L. et al. Efficient and thermally stable double perovskite phosphor Cs 2 NaScCl 6 : Mo 4+ with broadband long-wavelength NIR emission. Chem. Eng. J. 520, 165718 (2025) Wang, Y. J. et al. High-Temperature Negative-Thermal-Quenching in Broadband NIR Light-Emitting ScF 3 : Cr 3+ Phosphors. Adv. Opt. Mater. 13, e01552 (2025). Chen, C. H. et al. Strategies for broadening the emission spectra of Cr 3+ -doped near-infrared emitting phosphors. Mater. Chem. Front. 9, 1821–1838 (2025). Nie, W. D. et al. A highly efficient NIR-II-emitting MgGa 2 O 4 : Cr 3+ , Ni 2+ phosphor via the Cr 3+ ions in tetrahedral sites as the Cr 3+ -Ni 2+ energy transfer bridge. Sci. China Mater. 68, 1788–1801 (2025). Tian, Y. et al. Broadband emission from NIR-I to NIR-II in BaZn 3 Ga 2 O 7 :Cr 3+ phosphors achieved through double-site occupation. Ceram. Int. 51, 37861–37867 (2025). Zhu, F. M. et al. Cr 3+ -Doped LiAlO 2 NIR-I emitting phosphors with superior resistance to thermal quenching for night vision monitoring and bioimaging. ACS Appl. Mater. Interfaces. 16, 60599–60607 (2024). Zou, X. K. et al. Ultra-wide Vis–NIR Mg 2 Al 4 Si 5 O 18 : Eu 2+ , Cr 3+ phosphor containing unusual NIR luminescence induced by Cr 3+ occupying tetrahedral coordination for hyperspectral imaging. Adv. Opt. Mater. 10, 1 (2022). Chang, C. Y. et al. Ultrahigh quantum efficiency near-infrared-II emission achieved by Cr 3+ clusters to Ni 2+ energy transfer. Chem. Mater. 36 3941–3948 (2024). Rajendran, V. et al. Chromium ion pair luminescence: a strategy in broadband near-infrared light-emitting diode design. J. Am. Chem. Soc. 143, 19058–19066 (2021). Zhu, H. et al. Abnormal lattice shrinkage, site occupation, and luminescent properties of Cr 3+ -activated β-Al 2 O 3 structure phosphors. Laser Photonics Rev. 19, 2401089 (2025). Zhu, H. et al. Structural design of Cr 3+ -activated hexaaluminate phosphors with high quantum efficiency and Cr 3+ -Cr 3+ exchange coupling pairs. Chem. Mater. 37, 7227–7239 (2025). Yao, L. Q. et al. Efficient ultra-broadband Ga 4 GeO 8 : Cr 3+ phosphors with tunable peak wavelengths from 835 to 980 nm for NIR pc‐LED application. Adv. Opt. Mater. 10, 1–10 (2022). Chang, J. H. et al. Investigation of Cr 3+ -doped mullite structure near-infrared phosphors featuring with octahedral and tetrahedral dual-site emission. Chin. J. Lumin. 46, 1430–1441 (2025). Chi, F. F. et al. Luminescence properties of Cr 3+ -doped Al 6 Ge 2 O 13 broadband near-infrared phosphor. Opt. Mater. Express. 126, 112218 (2022). Han, L. et al. Ionic substitution combined with flux doping triggered near-unity efficiency and thermally stable NIR luminescence in Al 6 Ge 2 O 13 :Cr 3+ , Si 4+ phosphor for NIR spectroscopy applications. J. Alloys Compd. 1036 181870 (2025). Wang, Z. N. et al. Simultaneous luminescence color tuning and spectral broadening of Cr 3+ near-infrared emission for night vision application. ACS Appl. Opt. Mater. 1, 1088–1096 (2023). Zhang, Q. et al. Highly efficient and thermally stable broadband NIR phosphors with superlong afterglow performance and their multifunctional applications. Laser Photonics Rev. 18, 1 (2024). Yan, Y. Q. et al. Unique spectral broadening induced by exchange coupling between Cr 3+ ions in LiAl 5 O 8 :Cr 3+ phosphors for versatile optical applications. Laser Photonics Rev. 19, 1 (2024). Zhong, L. Valence and site engineering enable efficient broadband near-infrared emission at 960 nm in Cr 3+ -activated forsterite. Adv. Mater. e2508768 (2025). Lin, Q. M. et al. Near-unity internal quantum efficiency and high thermal stability of Sr 3 MgGe 5 O 14 :Cr 3+ phosphor for plant growth. Laser Photonics Rev. 19, 2402047 (2025). Wang, S. W. et al. Achieving high quantum efficiency broadband NIR Mg 4 Ta 2 O 9 :Cr 3+ phosphor through lithium-ion compensation. Adv. Mater. 35, 2300124 (2023). Z.W. et al. A dual-excited and dual near-infrared emission phosphor Mg 14 Ge 5 O 24 :Cr 3+ , Cr 4+ with a super broad band for biological detection. Dalton Trans. 50, 311–322 (2021). Zheng, Z.-H. et al. A simple and generic post-treatment strategy for highly efficient Cr 3+ -activated broadband NIR emitting phosphors for high-power NIR light sources. J. Mater. Chem. C. 10, 8797–8805 (2022). Huang, D. C. et al. A highly efficient and thermally stable broadband Cr 3+ -activated double borate phosphor for near-infrared light-emitting diodes. J. Mater. Chem. C. 9, 164–172 (2021). Huang, D. C. et al. Efficient and thermally stable broadband near-infrared emission from near zero thermal expansion AlP 3 O 9 : Cr 3+ phosphors. Inorg. Chem. Front. 9, 692–1700 (2022). Song, L. P. et al. Ultra-broad near-infrared emitting phosphor LiInF 4 : Cr 3+ with extremely weak crystal field. Inorg. Chem. 62, 11112–11120 (2023). Dalal M: electronic spectra of transition metal complexes, a textbook of inorganic chemistry, India: Dalal Institute. 1, 280–299 (2017). Nazarov, M. et al. Vibronic coupling parameters and stokes shift in thiogallate phosphors. J. Phys. Chem. Solids. 69, 2605–2612 (2008). Additional Declarations There is no conflict of interest Supplementary Files SupplementaryMaterial.docx Supplementary Material Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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13:16:14","extension":"html","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":134332,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8234812/v1/404ba918c52e84db6dd8930e.html"},{"id":97251175,"identity":"ec4a3e07-35c0-4441-8bf5-ea81e433d9c4","added_by":"auto","created_at":"2025-12-02 13:16:18","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":10162546,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePhase analysis of GGO: \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003ey\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003eCr\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e3+\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e XRD Rietveld refinement of GGO and GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e. \u003cstrong\u003eb\u003c/strong\u003e Crystal structure of GGO. \u003cstrong\u003ec\u003c/strong\u003e XRD patterns of GGO: \u003cem\u003ey\u003c/em\u003eCr\u003csup\u003e3+\u003c/sup\u003e. \u003cstrong\u003ed\u003c/strong\u003e Cell parameters of GGO: \u003cem\u003ey\u003c/em\u003eCr\u003csup\u003e3+\u003c/sup\u003e with Cr\u003csup\u003e3+\u003c/sup\u003e concentration. \u003cstrong\u003ee\u003c/strong\u003e SEM images and elemental mapping images of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-8234812/v1/d2ea18d1fc8c89567fd6ead2.png"},{"id":97244298,"identity":"e4af43e1-213b-42cc-9029-157c783db557","added_by":"auto","created_at":"2025-12-02 12:02:36","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":4404896,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eInvestigation of the DR spectra, band gap, and steady-state PL.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e DR spectra of the host and GGO: \u003cem\u003ey\u003c/em\u003eCr\u003csup\u003e3+\u003c/sup\u003e. \u003cstrong\u003eb\u003c/strong\u003e PLE, PL and DR spectra of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e, photograph of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e under ambient light. \u003cstrong\u003ec\u003c/strong\u003e Normalized PL spectra of GGO: \u003cem\u003ey\u003c/em\u003eCr\u003csup\u003e3+\u003c/sup\u003e at room temperature. \u003cstrong\u003ed\u003c/strong\u003e Normalized PL intensity and FWHM of GGO: \u003cem\u003ey\u003c/em\u003eCr\u003csup\u003e3+\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-8234812/v1/071ec8fa90a213abe8d19946.png"},{"id":97244303,"identity":"01494abd-280f-48eb-bf51-0daa7d847d3e","added_by":"auto","created_at":"2025-12-02 12:02:36","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":7008556,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDeciphering the multi-site luminescence of Cr\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e3+\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e through concentration- / temperature-dependent spectral analyses.\u003c/strong\u003e \u003cstrong\u003ea, b\u003c/strong\u003e PL spectra of GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e at 10 K and 300 K and their Gaussian fitting peak splitting. \u003cstrong\u003ec\u003c/strong\u003e Decay curves of Cr\u003csup\u003e3+\u003c/sup\u003e in GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e. \u003cstrong\u003ed, e\u003c/strong\u003e PL spectra of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e at 10 K and 300 K and their Gaussian fitting peak splitting. \u003cstrong\u003ef\u003c/strong\u003e Decay curves of Cr\u003csup\u003e3+\u003c/sup\u003e in GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-8234812/v1/dd899c1aed0815953a0a7870.png"},{"id":97244302,"identity":"78837d62-5921-4c63-b0d5-5ce2fb7203dc","added_by":"auto","created_at":"2025-12-02 12:02:36","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":7732992,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe site occupation and luminescence mechanism of Cr\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e3+\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e EPR patterns of GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e and GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e measured at room temperature. \u003cstrong\u003eb\u003c/strong\u003e Cr-K edge XAFS spectra of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and CrO\u003csub\u003e3\u003c/sub\u003e foils. \u003cstrong\u003ec\u003c/strong\u003e k\u003csup\u003e3\u003c/sup\u003e-weighted WT-EXAFS contour plot for GGO:6% Cr, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e foil, and CrO\u003csub\u003e3\u003c/sub\u003e foil. \u003cstrong\u003ed\u003c/strong\u003e Energy-level diagram and electronic transitions of isolated Cr\u003csup\u003e3+\u003c/sup\u003e ions, Cr\u003csup\u003e3+\u003c/sup\u003e-Cr\u003csup\u003e3+\u003c/sup\u003e ion pair and tetrahedral Cr\u003csup\u003e3+\u003c/sup\u003e with different Cr concentration.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-8234812/v1/8bc96875f31cb40a60d3f5a2.png"},{"id":97244313,"identity":"730e1509-2ab5-41ea-bbda-b4baaf51cd40","added_by":"auto","created_at":"2025-12-02 12:02:37","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":11404845,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe identification of energy level transitions of Cr\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e3+\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e occupying different sites. a\u003c/strong\u003e Energy level splitting of Cr\u003csup\u003e3+\u003c/sup\u003e ions at crystal sites of different symmetry (octahedron and tetrahedron) and the PLE spectra of GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e at 10 K. \u003cstrong\u003eb, c\u003c/strong\u003e Tanabe-Sugano energy level diagrams of 3d\u003csup\u003e3\u003c/sup\u003e and 3d\u003csup\u003e7\u003c/sup\u003e in octahedral and tetrahedral coordination, respectively.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-8234812/v1/48bc189a934e73d0c655e68e.png"},{"id":97244304,"identity":"332de7de-67f9-4153-aae1-c3bc2d3c3dd2","added_by":"auto","created_at":"2025-12-02 12:02:36","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":2767034,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePL thermal stability analysis and thermal quenching mechanism of GGO:6% Cr\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e3+\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Temperature-dependent PL spectra of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e and normalized PL intensity of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e versus temperature. \u003cstrong\u003eb\u003c/strong\u003e The plot of ln(\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/I\u003c/em\u003e-1) versus 1/\u003cem\u003ekT\u003c/em\u003e for the representative sample (GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e) \u003cstrong\u003ec\u003c/strong\u003e Temperature-dependent FWHM of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e and its fitting curve.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-8234812/v1/4b00d89b25786fccdf7e36df.png"},{"id":97250476,"identity":"9c3f1bca-edd9-4729-ab13-310b56b41ad8","added_by":"auto","created_at":"2025-12-02 13:14:35","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":14314954,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eProperties and applications of NIR pc-LEDs.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e EL spectrum of the NIR pc-LED at 100 mA. \u003cstrong\u003eb\u003c/strong\u003e Driven current-dependent EL spectra of the NIR pc-LED. \u003cstrong\u003ec\u003c/strong\u003e NIR radiant power and photoelectric efficiency of the NIR-LED device depending on driven current. \u003cstrong\u003ed, e\u003c/strong\u003e Transmission spectra of NIR pc-LED light source through different concentrations of Nd\u003csup\u003e3+\u003c/sup\u003e and Yb\u003csup\u003e3+\u003c/sup\u003e. \u003cstrong\u003ef\u003c/strong\u003e Relationship between Ln\u003csup\u003e3+\u003c/sup\u003e (Nd\u003csup\u003e3+\u003c/sup\u003e and Yb\u003csup\u003e3+\u003c/sup\u003e) concentration and log\u003csub\u003e10\u003c/sub\u003e(\u003cem\u003eI\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e/\u003cem\u003eI\u003c/em\u003e). \u003cstrong\u003eg\u003c/strong\u003e Images of the palm under illumination of the as-fabricated NIR pc-LED. \u003cstrong\u003eh, i\u003c/strong\u003e A bank card was photographed under natural light and NIR light respectively.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-8234812/v1/b2e041dd269358aab2950e27.png"},{"id":97664618,"identity":"75c0e157-1a03-423e-9082-bbb1fdd250e2","added_by":"auto","created_at":"2025-12-08 09:11:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":50974992,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8234812/v1/51f7941e-7d3c-486c-951d-7b99ec12ef9b.pdf"},{"id":97244300,"identity":"dbfbe63d-96f0-475b-8747-7b9f8d2b7cee","added_by":"auto","created_at":"2025-12-02 12:02:36","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":4808613,"visible":true,"origin":"","legend":"Supplementary Material","description":"","filename":"SupplementaryMaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-8234812/v1/9e3e279e8e35f95cc32735fd.docx"}],"financialInterests":"There is no conflict of interest","formattedTitle":"Insight into Site Environment and Photoluminescence Properties of Cr3+-doped Gallogermanate Toward NIR pc-LEDs Applications","fulltext":[{"header":"Introduction","content":"\u003cp\u003eNear-infrared (NIR) light serves as a versatile optical platform and has been extensively utilized in biological imaging, disease diagnosis and treatment, security surveillance, and agriculture, while leveraging the inherent advantages of NIR light, such as invisibility, deep tissue penetration, and low scattering loss \u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Among various NIR light sources, NIR phosphor-converted light-emitting diodes (pc-LEDs) offer further practical benefits including environmental friendliness, cost-effectiveness, and stable illumination \u003csup\u003e\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. Moreover, the emission spectrum of NIR pc-LEDs can be tailored by selecting appropriate NIR phosphors, enabling adaptation to diverse application scenarios. Among various NIR activators (Cr\u003csup\u003e3+\u003c/sup\u003e, Fe\u003csup\u003e3+\u003c/sup\u003e, Ni\u003csup\u003e2+\u003c/sup\u003e, Mn\u003csup\u003e2+\u003c/sup\u003e, Eu\u003csup\u003e2+\u003c/sup\u003e, Bi\u003csup\u003e3+\u003c/sup\u003e) \u003csup\u003e9\u0026ndash;13\u003c/sup\u003e, Cr\u003csup\u003e3+\u003c/sup\u003e-activated phosphors have emerged as promising candidates for the light conversion layer in NIR pc-LEDs, owing to their tunable emission band and broad absorption in the blue region, which matches well with commercial GaN LED chips.\u003c/p\u003e\u003cp\u003eConsiderable progress has been made in developing Cr\u003csup\u003e3+\u003c/sup\u003e-activated NIR phosphors with high internal quantum efficiency and good thermal stability, as exemplified by representative host systems such as garnets \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e, borates \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, spinels \u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e, double perovskites \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, and fluorides \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. By tailoring the crystal field strength, the photoluminescence (PL) spectrum can be modulated within the 650\u0026thinsp;\u0026minus;\u0026thinsp;1100 nm range. Nevertheless, most Cr\u003csup\u003e3+\u003c/sup\u003e-doped phosphors still face two major challenges: limited absorption efficiency due to the parity-forbidden nature of d-d transitions, and the difficulty in achieving long-wavelength emission beyond 900 nm \u003csup\u003e19\u003c/sup\u003e. Although increasing the Cr\u003csup\u003e3+\u003c/sup\u003e doping concentration can enhance absorption, it frequently induces concentration quenching. Meanwhile, the coordination environment and site-specific luminescence behavior of Cr\u003csup\u003e3+\u003c/sup\u003ein many host systems remain elusive. In particular, the potential of tetrahedrally coordinated Cr\u003csup\u003e3+\u003c/sup\u003e to produce extended NIR emission beyond 850 nm has been largely overlooked, owing to its relatively low formation probability and poor chemical stability in common oxide hosts \u003csup\u003e\u003cspan additionalcitationids=\"CR21 CR22\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Additionally, while Cr\u003csup\u003e3+\u003c/sup\u003e \u0026minus; Cr\u003csup\u003e3+\u003c/sup\u003e pair emission has been sporadically reported in selected hosts as a route to broadband NIR emission \u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, a systematic understanding of how to controllably introduce such pairs and distinguish their emission from that of isolated ions is still lacking. Therefore, achieving coordinated control over the luminescence from isolated octahedral Cr\u003csup\u003e3+\u003c/sup\u003e, tetrahedral Cr\u003csup\u003e3+\u003c/sup\u003e, and Cr\u003csup\u003e3+\u003c/sup\u003e ion pairs represents a promising yet challenging pathway toward developing ultra-broadband NIR phosphors covering the 700\u0026ndash;1400 nm region.\u003c/p\u003e\u003cp\u003eIn this work, we developed a gallium germanate solid solution, Ga\u003csub\u003e6\u003c/sub\u003eGe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e (GGO), as a host for Cr\u003csup\u003e3+\u003c/sup\u003e doping, motivated by its rich variety of cationic sites, including both octahedral and tetrahedral coordination, and the presence of Ga\u003csup\u003e3+\u003c/sup\u003e, which favors a weaker crystal field and facilitates longer-wavelength NIR emission \u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. Related materials such as Al\u003csub\u003e6\u003c/sub\u003eGe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e, Al\u003csub\u003e2.26\u003c/sub\u003eGe\u003csub\u003e0.74\u003c/sub\u003eO\u003csub\u003e4.87\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e, Al\u003csub\u003e4\u0026thinsp;+\u0026thinsp;2\u003cem\u003ez\u003c/em\u003e\u003c/sub\u003eSi\u003csub\u003e2\u0026minus;2\u003cem\u003ez\u003c/em\u003e\u003c/sub\u003eO\u003csub\u003e10\u0026minus;\u003cem\u003ez\u003c/em\u003e\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e and Ga\u003csub\u003e3\u003c/sub\u003eAl\u003csub\u003e3\u003c/sub\u003eGe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e have been reported, \u003csup\u003e29\u0026ndash;33\u003c/sup\u003e but the anomalous luminescence phenomena and precise lattice occupancy of Cr\u003csup\u003e3+\u003c/sup\u003e ions in the host have not been thoroughly investigated. Under 430 nm excitation, GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e exhibits a long-wavelength emission peak at 880 nm with a broad full width at half maximum (FWHM) of 215 nm. A notable shoulder peak at 538 nm in the excitation spectrum, which is unusual for Cr\u003csup\u003e3+\u003c/sup\u003e in a single site, suggests the presence of multiple luminescent centers. Through a combination of low-temperature spectroscopy, X-ray absorption fine structure (XAFS), and electron paramagnetic resonance (EPR) analysis, we identify the involvement of tetrahedrally coordinated Cr\u003csup\u003e3+\u003c/sup\u003e and exchange-coupled Cr\u003csup\u003e3+\u003c/sup\u003e \u0026minus; Cr\u003csup\u003e3+\u003c/sup\u003e ion pairs, in addition to conventional octahedral Cr\u003csup\u003e3+\u003c/sup\u003e. This work not only clarifies the PL mechanism and site occupancy of Cr\u003csup\u003e3+\u003c/sup\u003e in GGO but also provides a structural design strategy for developing long-wavelength, ultra-broadband NIR phosphors via multi-site engineering.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003eCrystal structure and micromorphology\u003c/h2\u003e\n\u003cp\u003eIn order to study the crystal structure of Cr\u003csup\u003e3+\u003c/sup\u003e -doped GGO materials, the crystallographic data of Ga\u003csub\u003e2.31\u003c/sub\u003eGe\u003csub\u003e0.69\u003c/sub\u003eO\u003csub\u003e4.84\u003c/sub\u003e (ICSD-9005504) is used as the initial structural model to perform Rietveld refinement for GGO host and GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e. The refinement parameters are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea and Table \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e-S2. The GGO crystal belongs to orthorhombic system, \u003cem\u003ePbam\u003c/em\u003e space group. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb presents the crystal structure of refined GGO, which is composed of octahedral ([GaO\u003csub\u003e6\u003c/sub\u003e]), tetrahedral ([Ga\u003csub\u003eT\u003c/sub\u003e/GeO\u003csub\u003e4\u003c/sub\u003e] and [Ga\u003csub\u003eT*\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e]) crystal sites. Based on the local characteristics of the material structure, the framework can be divided into two distinct units: Unit 1 (comprising two [GaO\u003csub\u003e6\u003c/sub\u003e] octahedra and one [Ga\u003csub\u003eT\u003c/sub\u003e/GeO\u003csub\u003e4\u003c/sub\u003e] tetrahedron) and Unit 2 (comprising two [GaO\u003csub\u003e6\u003c/sub\u003e] octahedra and one [Ga\u003csub\u003eT*\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e] tetrahedron). Notably, the distance between the two adjacent octahedra ([GaO\u003csub\u003e6\u003c/sub\u003e]) is only 3.0148 \u0026Aring; in both units. Therefore, it can be anticipated that if Cr\u003csup\u003e3+\u003c/sup\u003e ions are doped into the material, the distance between Cr ions will be very short (\u0026lt;\u0026thinsp;5 \u0026Aring;). Such proximity would lead to overlapping electron clouds between neighboring Cr ions, resulting in an exchange coupling interaction among Cr\u003csup\u003e3+\u003c/sup\u003e ions \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. In addition, the [GaO\u003csub\u003e6\u003c/sub\u003e] octahedra are connected via edge-sharing to form octahedral chain columns along the \u003cem\u003ec\u003c/em\u003e-axis, which are cross-linked by the irregularly occupied [Ga\u003csub\u003eT\u003c/sub\u003e/GeO\u003csub\u003e4\u003c/sub\u003e] and [Ga\u003csub\u003eT*\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e] tetrahedral sites. Meanwhile, the disordered distribution of Ga/Ge sites in the tetrahedral units of GGO provides structural flexibility for the doping of Cr\u003csup\u003e3+\u003c/sup\u003e ions.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec presents the XRD patterns of GGO: \u003cem\u003ey\u003c/em\u003eCr\u003csup\u003e3+\u003c/sup\u003e (\u003cem\u003ey\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0\u0026ndash;16%) samples. All diffraction peaks match well with the standard PDF card (Ga\u003csub\u003e6\u003c/sub\u003eGe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e, PDF#50\u0026ndash;0354), indicating that Cr\u003csup\u003e3+\u003c/sup\u003e ions have been successfully incorporated into GGO host. Furthermore, the magnified view of the diffraction peak between 24.1\u0026deg; and 26.5\u0026deg; reveals a slight shift toward higher angles with increasing Cr\u003csup\u003e3+\u003c/sup\u003e content. Fig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e presents the Rietveld refinement profiles of samples with different Cr\u003csup\u003e3+\u003c/sup\u003e concentrations, and the unit cell parameters (\u003cem\u003ea\u003c/em\u003e, \u003cem\u003eb\u003c/em\u003e, \u003cem\u003ec\u003c/em\u003e, and \u003cem\u003eV\u003c/em\u003e) are summarized in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed. These results confirm a slight shrink of the unit cell as the Cr\u003csup\u003e3+\u003c/sup\u003e concentration increases. Considering the ionic radius of Ga\u003csup\u003e3+\u003c/sup\u003e (CN\u0026thinsp;=\u0026thinsp;6, \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.62 \u0026Aring;; CN\u0026thinsp;=\u0026thinsp;4, \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.47 \u0026Aring;), Ge\u003csup\u003e4+\u003c/sup\u003e (CN\u0026thinsp;=\u0026thinsp;4, \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.39 \u0026Aring;) and Cr\u003csup\u003e3+\u003c/sup\u003e (CN\u0026thinsp;=\u0026thinsp;6, \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.615 \u0026Aring;; CN\u0026thinsp;=\u0026thinsp;4, \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.41 \u0026Aring;), the observed lattice slightly shrinkage is likely due to the substitution of Cr\u003csup\u003e3+\u003c/sup\u003e for Ga\u003csup\u003e3+\u003c/sup\u003e in the host structure. However, whether Cr\u003csup\u003e3+\u003c/sup\u003e occupies both octahedral and tetrahedral sites or is limited to a single site requires further investigation.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ee shows the SEM and elemental mapping images of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e, and the corresponding EDS spectrum is provided in Fig. S2. The sample consists of irregularly shaped particles with sizes ranging from 10 to 40 \u0026micro;m. Elemental mapping demonstrates a homogeneous distribution of Ga, Ge, O, and Cr throughout the particles. Combined with EDS analysis, the elemental composition is found to be close to the theoretical stoichiometric ratio of Ga\u003csub\u003e6\u003c/sub\u003eGe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003ePhotoluminescence (PL) properties\u003c/h3\u003e\n\u003cp\u003eThe bandgap of the host material significantly influences the luminescence properties of Cr\u003csup\u003e3+\u003c/sup\u003e-doped near-infrared phosphors, especially in terms of thermal stability. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea shows the diffuse reflectance (DR) spectra of the host and GGO: \u003cem\u003ey\u003c/em\u003eCr\u003csup\u003e3+\u003c/sup\u003e samples. Based on the Tauc plot and Kubelka-Munk transformation, the optical bandgap (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e) of GGO was determined to be approximately 4.32 eV (inset of Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea), which is larger than that of many oxide materials \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e, and provide a suitable PL environment for Cr\u003csup\u003e3+\u003c/sup\u003e ions. Compared with the host, the Cr\u003csup\u003e3+\u003c/sup\u003e-doped samples exhibit three distinct absorption bands, located in the ranges of 300\u0026ndash;350 nm, 400\u0026ndash;550 nm, and 550\u0026ndash;750 nm, respectively. Notably, the broadband 550\u0026ndash;750 nm comprises at least two distinct electronic transitions, and further analysis will be conducted in combination with the PL spectrum. Furthermore, no additional absorption band in the 800\u0026ndash;1200 nm range indicates that Cr\u003csup\u003e4+\u003c/sup\u003e ions are virtually absent in GGO host. Moreover, XPS results of GGO: 6%Cr\u003csup\u003e3+\u003c/sup\u003e further confirm that Cr ions exist predominantly in the trivalent state (Cr\u003csup\u003e3+\u003c/sup\u003e). As shown in the survey spectrum (Fig. S3a), all expected elements were detected. The high-resolution Cr 2p spectrum (Fig. S3b) exhibits two peaks at binding energies of 576.18 and 588.38 eV, corresponding to the 2p\u003csub\u003e3/2\u003c/sub\u003e and 2p\u003csub\u003e1/2\u003c/sub\u003e orbitals of Cr\u003csup\u003e3+\u003c/sup\u003e, respectively \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb shows the PL excitation and emission spectra of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e. The excitation spectrum agrees well with its DR spectrum. The three excitation bands are attributed to the\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e1\u003c/sub\u003e (4P) (270 nm), \u003csup\u003e4\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e1\u003c/sub\u003e (4F) (430 nm) and \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e2\u003c/sub\u003e (4F) (640 nm) transitions of Cr\u003csup\u003e3+\u003c/sup\u003e ions, which also account for the pale green color of the sample under ambient light. Notably, a shoulder peak observed around 560 nm in the excitation spectrum is consistent with the corresponding feature in the DR spectrum, which cannot be found in the 3d\u003csup\u003e3\u003c/sup\u003e energy level diagram of Cr\u003csup\u003e3+\u003c/sup\u003e ions, and will be discussed in later. Upon 430 nm excitation, the sample exhibits a broadband NIR emission, peaking at 880 nm, with a full width at half maximum (FWHM) of about 210 nm. The FWHM is significantly broader than that of many Cr\u003csup\u003e3+\u003c/sup\u003e-activated materials in which emission originates from a single crystallographic site \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. Combined with the distinct features observed in the excitation spectrum, these results suggest the coexistence of emission from multiple Cr\u003csup\u003e3+\u003c/sup\u003e sites within the GGO host. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec shows the concentration-dependent PL spectra of GGO: \u003cem\u003ey\u003c/em\u003eCr\u003csup\u003e3+\u003c/sup\u003e (\u003cem\u003ey\u003c/em\u003e\u0026thinsp;=\u0026thinsp;2% \u0026ndash; 16%). As the Cr\u003csup\u003e3+\u003c/sup\u003e doping concentration increases, the emission peak systematically red-shifts from 850 nm to 918 nm, accompanied by a spectral broadening of the FWHM from 209 nm to 239 nm. The optimal integrated PL intensity is achieved at a Cr\u003csup\u003e3+\u003c/sup\u003e doping concentration of 6 mol% (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed).\u003c/p\u003e\n\u003cp\u003eIn order to accurately assess the PL efficiency of the NIR phosphors, the PL quantum yield of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e was evaluated (Fig. S4). Th internal quantum yield (IQY) reaches 53%. Notably, the absorption efficiency reaches 69%, which, to the best of our knowledge, represents the highest value reported among Cr\u003csup\u003e3+\u003c/sup\u003e-activated NIR phosphors in powder form (Table S3). The high absorption is likely attributed to the local structural distortion in the host lattice, which partially relaxes the parity-forbidden rule of the d-d transitions in Cr\u003csup\u003e3+\u003c/sup\u003e ions, thereby significantly enhancing blue light absorption. Consequently, an external quantum yield (EQY) of 36% was achieved, ranking among the highest values for Cr\u003csup\u003e3+\u003c/sup\u003e-activated materials with emission peaks beyond 880 nm.\u003c/p\u003e\n\u003cp\u003eBuilding upon the discussion of the unique structure features and unusual PL properties of GGO: Cr\u003csup\u003e3+\u003c/sup\u003e, an in-depth investigation into the lattice occupancy and luminescence characteristics of Cr\u003csup\u003e3+\u003c/sup\u003e in GGO was conducted by selecting two samples with Cr\u003csup\u003e3+\u003c/sup\u003e doping concentrations of 0.1% and 6%. As we know, low-temperature measurement can suppress thermal broadening of spectral bands, enabling more reliable identification of energy levels. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea presents the PLE and PL spectra of GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e measured at 10 K. Surprisingly, under blue light excitation, the emission spectrum consists of a sharp line at 697 nm superimposed on a broad band spanning from 720 nm to 1000 nm. Notably, the excitation spectra monitored at 697 nm and 850 nm are distinctly different. The excitation spectrum from 697 nm emission displays three characteristic bands corresponding to the \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e level transitions to \u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e1\u003c/sub\u003e (4P), \u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e1\u003c/sub\u003e (4F), and \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003eT\u003csub\u003e2\u003c/sub\u003e (4F), respectively. In contrast, the excitation spectrum from 850 nm emission exhibits additional peaks at 565 nm and 650 nm alongside the three fundamental Cr\u003csup\u003e3+\u003c/sup\u003e transitions. This disparity strongly indicates that Cr\u003csup\u003e3+\u003c/sup\u003e ions occupy multiple crystallographic sites in the GGO host, even at ultra-low doping concentrations. Gaussian fitting of the low-temperature emission spectrum (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb) deconvolutes it into a sharp peak at 697 nm and a broad Gaussian band spanning 720\u0026ndash;850 nm. The sharp emission 697 nm and broadband\u0026thinsp;~\u0026thinsp;726 nm are unambiguously assigned to the \u003csup\u003e2\u003c/sup\u003eE/\u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e2\u003c/sub\u003e \u0026rarr; \u003csup\u003e4\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e transition of Cr\u003csup\u003e3+\u003c/sup\u003e ions in [GaO\u003csub\u003e6\u003c/sub\u003e] octahedral environment. Considering that the concentration of Cr\u003csup\u003e3+\u003c/sup\u003e ions is extremely low, it is unlikely that Cr\u003csup\u003e3+\u003c/sup\u003e \u0026ndash; Cr\u003csup\u003e3+\u003c/sup\u003e ion pairs will be formed. Thus, the broad emission centered near 850 nm is tentatively attributed to Cr\u003csup\u003e3+\u003c/sup\u003e ions occupying tetrahedral sites in GGO.\u003c/p\u003e\n\u003cp\u003eIn addition, comparison of the PL spectra at 300 K and 10 K reveals significant thermal quenching of the sharp-line emission (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea and Fig. S5a), accompanied by a relative enhancement of the broadband emission due to thermal population effects between the \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003eE and \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003eT\u003csub\u003e2\u003c/sub\u003e energy levels. Gaussian fitting of the 300 K PL spectrum shows that the position of the ~\u0026thinsp;850 nm band remains largely unchanged. In contrast, the 726 nm emission peak observed at low temperature red-shifts to 742 nm at 300 K. This shift can be attributed to intensified electron-lattice interactions and a reduction in crystal field strength due to thermal expansion. Alternatively, the involvement of coupled Cr\u003csup\u003e3+\u003c/sup\u003e - Cr\u003csup\u003e3+\u003c/sup\u003e pairs, whose emission may overlap and merge with the \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003eT\u003csub\u003e2\u003c/sub\u003e level, could also contribute to the observed spectral shift.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFurthermore, the PL lifetimes at different emission wavelengths of GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e were also evaluated (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec). At 10 K, the decay curve at 697 nm exhibits a long lifetime (\u003cem\u003e\u0026tau;\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3.11 ms) and a single exponential decay behavior, which is consistent with the previous speculation that it belongs to the \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003eE \u0026rarr; \u003csup\u003e4\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e forbidden transition of isolated octahedral Cr\u003csup\u003e3+\u003c/sup\u003e. However, the decay curve at 850 nm shows a short lifetime (\u003cem\u003e\u0026tau;\u003c/em\u003e\u0026thinsp;=\u0026thinsp;74.95 \u0026micro;s) and double-exponential decay behavior. As the temperature increases from 10 K to 300 K, the average lifetime (\u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003eav\u003c/sub\u003e) decreased from 74.95 \u0026micro;s (\u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;128.42 \u0026micro;s, \u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;39.96 \u0026micro;s) to 60.78 \u0026micro;s (\u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;119.19 \u0026micro;s, \u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;37.58 \u0026micro;s), mainly caused by the enhanced nonradiative transition probability at elevated temperature. It is noted that the decay fraction \u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e and \u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e are considered to be derived from two overlapping broadband emission.\u003c/p\u003e\n\u003cp\u003eFor comparison, temperature-dependent PL spectra of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e were collected from 10 K to 300 K (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed and Fig. S5b). The result shows that the emission spectrum of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e remains a broadband even at 10 K, with a spectral profile similar to that observed at 300 K, differing mainly in intensity. The excitation spectrum also shows little variation with temperature. Gaussian fitting of the PL spectra at 10 K and 300 K resolves two components centered near 880 nm and 960 nm (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ee), indicating contributions from two distinct luminescent centers. Comparing the PL spectra of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e and GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e reveals a significant red-shift in the highly doped sample. Given the similar ionic radii of Cr\u003csup\u003e3+\u003c/sup\u003e and Ga\u003csup\u003e3+\u003c/sup\u003e in octahedral coordination, this red-shift is unlikely to result from a substantial reduction in crystal field strength. Instead, it can be attributed to the formation of exchange-coupled Cr\u003csup\u003e3+\u003c/sup\u003e-Cr\u003csup\u003e3+\u003c/sup\u003e pairs, facilitated by the short distance (~\u0026thinsp;3.2 \u0026Aring;) between adjacent [GaO\u003csub\u003e6\u003c/sub\u003e] octahedra in the GGO structure. Then the emission from isolated octahedral Cr\u003csup\u003e3+\u003c/sup\u003e and that from Cr\u003csup\u003e3+\u003c/sup\u003e-Cr\u003csup\u003e3+\u003c/sup\u003e pairs will overlap, leading to the observed red-shift. Thus, in the GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e sample, the Gaussian component near 880 nm originates from both isolated octahedral Cr\u003csup\u003e3+\u003c/sup\u003e and Cr\u003csup\u003e3+\u003c/sup\u003e - Cr\u003csup\u003e3+\u003c/sup\u003e pairs, while the 960 nm band is attributed to Cr\u003csup\u003e3+\u003c/sup\u003e ions in tetrahedral sites, the same center responsible for the ~\u0026thinsp;850 nm emission in GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e sample.\u003c/p\u003e\n\u003cp\u003eConcurrently, the PL lifetime of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e at 875 nm and 959 nm were also monitored (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ef). Both decay curves exhibit double exponential decay behavior, and the fitted PL average lifetimes (\u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003eav\u003c/sub\u003e) are 23.45 \u0026micro;s (\u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;35.38 \u0026micro;s, \u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;15.78 \u0026micro;s) and 13.47 \u0026micro;s (\u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;18.98 \u0026micro;s, \u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;6.47 \u0026micro;s), respectively. Based on the above analysis, we speculate that the \u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e may be derived from the energy level transition of \u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e2\u003c/sub\u003e\u0026rarr;\u003csup\u003e4\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e of octahedral Cr\u003csup\u003e3+\u003c/sup\u003e, while the \u003cem\u003e\u0026tau;\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e is derived from the energy level transition of \u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e2\u003c/sub\u003e\u0026rarr;\u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e1\u003c/sub\u003e of tetrahedral Cr\u003csup\u003e3+\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eSite occupation and local structure of Cr\u003c/h3\u003e\n\u003cp\u003eIn order to further clarify the above inference, EPR and XAFS measurements were employed to probe the chemical state and local coordination of Cr\u003csup\u003e3+\u003c/sup\u003e ions in the GGO host. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea, the EPR spectra provide insights into the coordination environment and electronic state of Cr\u003csup\u003e3+\u003c/sup\u003e ions substituted into the host lattice. The material was analyzed to evaluate their local environment. Different from the GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e sample, GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e exhibits broader spectral features. A resonance signal is observed in both samples at \u003cem\u003eg\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.907/1.891, which is assigned to Cr\u003csup\u003e3+\u003c/sup\u003e ions occupying tetrahedral sites or involved in Cr\u003csup\u003e3+\u003c/sup\u003e - Cr\u003csup\u003e3+\u003c/sup\u003e exchange coupling effect, while the signal at \u003cem\u003eg\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4.167/4.042 corresponds to isolated Cr\u003csup\u003e3+\u003c/sup\u003e ions located in octahedral sites of GGO. The signal near \u003cem\u003eg\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.891 is significantly enhanced in the GGO: 6%Cr\u003csup\u003e3+\u003c/sup\u003e sample due to strong Cr\u003csup\u003e3+\u003c/sup\u003e - Cr\u003csup\u003e3+\u003c/sup\u003e pair coupling. In addition, the broadening of the \u003cem\u003eg\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.891 band significantly increases with Cr\u003csup\u003e3+\u003c/sup\u003e concentration, indicating that this EPR signal does not originate from oxygen vacancy. Compared with GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e, GGO: 6% Cr\u003csup\u003e3+\u003c/sup\u003e has a signal observed in the range of \u003cem\u003eg\u003c/em\u003e\u0026thinsp;=\u0026thinsp;2.5\u0026ndash;3.5, which may be derived from the distorted [CrO\u003csub\u003e6\u003c/sub\u003e] octahedron. No EPR signal Corresponding to Cr\u003csup\u003e4+\u003c/sup\u003e in a tetrahedral environment was detected.\u003c/p\u003e\n\u003cp\u003eThe chemical state and local coordination environment of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e were further investigated using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb, the XANES spectrum of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e exhibits an obvious pre-edge absorption peak at 5992 eV, indicating the presence of Cr\u003csup\u003e3+\u003c/sup\u003e ions in tetrahedral site, while the peak at 6009 eV (white line) corresponds to the Cr\u003csup\u003e3+\u003c/sup\u003e ions in octahedral site, consistent with the reference spectrum of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e foil (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb). These features clearly demonstrate that the emission previously attributed to Cr\u003csup\u003e6+\u003c/sup\u003e actually originates from Cr\u003csup\u003e3+\u003c/sup\u003e ions in a tetrahedral crystal field environment. EXAFS analysis of the first coordination shell for GGO:6% Cr is performed using a combined model based on Cr\u0026ndash;O (from Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) and Cr\u0026thinsp;=\u0026thinsp;O (from CrO\u003csub\u003e3\u003c/sub\u003e) reference paths. The fitting results show that the Cr\u0026ndash;O path has a coordination number (CN) of 6 and a phase-corrected bond length (R) of 1.96 \u0026Aring;, whereas the Cr\u0026thinsp;=\u0026thinsp;O path has a CN of 4 and a bond length of 1.58 \u0026Aring;. The \u003cem\u003ek\u003c/em\u003e\u003csup\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sup\u003e-weighted wavelet transforms (WT) EXAFS spectrum of GGO:6% Cr shows a profile similar to those of CrO\u003csub\u003e3\u003c/sub\u003e and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in \u003cem\u003ek\u003c/em\u003e space, and has a noticeable broadening (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec). These findings confirm that Cr\u003csup\u003e3+\u003c/sup\u003e exists in octahedron and tetrahedron sites, and the relevant supplementary information is shown in Figs. S6a-b and Table S4. Notably, the fitted of Cr-M (M\u0026thinsp;=\u0026thinsp;Cr, Ga or Ge) bond length is 2.94\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 \u0026Aring;, indicating short interionic distances between Cr\u003csup\u003e3+\u003c/sup\u003e ions. This proximity facilitates the formation of Cr\u003csup\u003e3+\u003c/sup\u003e \u0026ndash; Cr\u003csup\u003e3+\u003c/sup\u003e pairs, in agreement with the EPR results discussed above.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBased on the combined results of EPR, XAFS, and PL characterization, the PL mechanism of GGO: Cr\u003csup\u003e3+\u003c/sup\u003e at different doping concentrations is illustrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed. In the low concentration sample (\u003cem\u003ey\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.1%), the sharp R-line comes from the parity-forbidden \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003eE\u0026rarr;\u003csup\u003e4\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e transition in the isolated octahedral ions, while the broadband emissions (726 nm and 850 nm) comes from the \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003eT\u003csub\u003e2\u003c/sub\u003e level of the isolated octahedron and tetrahedral Cr\u003csup\u003e3+\u003c/sup\u003e, respectively. With the increase of Cr\u003csup\u003e3+\u003c/sup\u003e concentration, the formation of Cr\u003csup\u003e3+\u003c/sup\u003e \u0026ndash; Cr\u003csup\u003e3+\u003c/sup\u003e ion pair induced the formation of (\u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e2\u003c/sub\u003e, \u003csup\u003e4\u003c/sup\u003eA\u003csub\u003e2\u003c/sub\u003e) state, resulting in the gradual broadening of the emission band, the decrease of energy and the significant red shift of the emission peak.\u003c/p\u003e\n\u003ch3\u003eEnergy level transition and luminescence mechanism of Cr\u003c/h3\u003e\n\u003cp\u003eAccording the previous analysis, a clear depiction of the electronic and vibronic energy diagram of Cr\u003csup\u003e3+\u003c/sup\u003e in GGO host is essential for understanding the luminescence behaviors. The optical transitions of Cr\u003csup\u003e3+\u003c/sup\u003e are sensitive to their local coordination, and the energy level splitting is determined by the local symmetry of the crystallographic site occupied by Cr\u003csup\u003e3+\u003c/sup\u003e ions. When Cr\u003csup\u003e3+\u003c/sup\u003e ions are doped into the crystal lattice of GGO, they substitute for [GaO\u003csub\u003e6\u003c/sub\u003e] octahedral and [Ga\u003csub\u003eT*\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e] tetrahedral sites with local site symmetry of \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003e2h\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003eS\u003c/em\u003e\u003c/sub\u003e. Based on group theory, both spin-orbit interaction and symmetry descending will affect the energy level splitting. Detailed energy levels of Cr\u003csup\u003e3+\u003c/sup\u003e at the site symmetry of \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003e2h\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003eS\u003c/em\u003e\u003c/sub\u003e were deduced and plotted in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea. All identified energy levels closely match the peak positions in the excitation spectrum of the sample at 10 K. The calculation of the crystal field strength of GGO:0.1% Cr\u003csup\u003e3+\u003c/sup\u003e at 10 K was presented in the supplementary information. The \u003cem\u003eDq/B\u003c/em\u003e values of Cr\u003csup\u003e3+\u003c/sup\u003e in the octahedral site and tetrahedral site were calculated to be 2.74 and 0.68, respectively (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb, c). The \u003cem\u003eDq/B\u003c/em\u003e value for Cr\u003csup\u003e3+\u003c/sup\u003eat the tetrahedral site demonstrates that Cr\u003csup\u003e3+\u003c/sup\u003e ions occupied a weak crystal field environment and gives rise to broadband \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003eT\u003csub\u003e2\u003c/sub\u003e\u0026rarr;\u003csup\u003e4\u003c/sup\u003eT\u003csub\u003e1\u003c/sub\u003e emission. Furthermore, a strong excitation band characteristic of octahedral Cr\u003csup\u003e3+\u003c/sup\u003e is observed in the excitation spectrum monitored at the tetrahedral Cr\u003csup\u003e3+\u003c/sup\u003e emission (850 nm, Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea), indicating the occurrence of energy transfer (ET) from octahedral to tetrahedral Cr\u003csup\u003e3+\u003c/sup\u003e centers.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003ch3\u003ePL thermal stability\u003c/h3\u003e\n\u003cp\u003eIt is widely recognized that PL thermal stability is a critical parameter for the practical application of phosphors, typically evaluated by the ratio of the PL intensity at 423 K to that at room temperature (RT). In order to evaluate the thermal stability of GGO: Cr\u003csup\u003e3+\u003c/sup\u003e phosphor, temperature-dependent PL spectra of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e were recorded from 298 K to 473 K. The relationship between the normalized PL integral intensity and temperature is shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ea. A thermal quenching phenomenon is observed with increasing temperature. At 373 K and 423 K, the integrated intensity of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e maintains 75% and 60% of its initial value at RT, respectively. The activation energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e) for the thermal quenching process can be determined using the single-barrier model according to the following equation:\u003c/p\u003e\n\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equa\" class=\"mathdisplay\"\u003e$$ \\text{I}\\text{T}\\text{/I}\\text{0}\\text{ = }\\left[\\text{1+Aexp(-}{E}_{a}\\text{/kT)}\\right]\\text{}\\text{}\\text{ (}\\text{1}\\text{)}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e is the initial PL integral strength, \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eT\u003c/em\u003e\u003c/sub\u003e is the integral strength at a given temperature \u003cem\u003eT\u003c/em\u003e, \u003cem\u003eA\u003c/em\u003e is a constant, \u003cem\u003ek\u003c/em\u003e is a Boltzmann's constant (8.617 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e eV/K), and \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e is the activation energy. The \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e fitting value of GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e is 0.22 eV (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eb). The lower activation energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e) means larger energy loss and stronger thermal quenching. The Electron-phonon coupling (EPC) effect is an important factor in describing thermal stability. The Huang-Rhys factor (\u003cem\u003eS\u003c/em\u003e) is a crucial parameter for assessing the EPC effect and can be obtained through the following Eq.\u0026nbsp;4\u003csup\u003e1\u003c/sup\u003e:\u003c/p\u003e\n\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equb\" class=\"mathdisplay\"\u003e$$ \\text{FWHM }\\text{= }\\text{2.36}\\sqrt{S}hv\\sqrt{\\text{c}\\text{o}\\text{t}\\text{h}\\left(\\frac{h\\omega }{2kT}\\right)}\\text{}\\text{}\\text{ (}\\text{2}\\text{)}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003ewhere \u003cem\u003eS\u003c/em\u003e refers to the Huang-Rhys factor reflecting the EPC strength, \u003cem\u003ehv\u003c/em\u003e represents the phonon energy, and \u003cem\u003ek\u003c/em\u003e is a constant. The equation is simplified to the following equation:\u003c/p\u003e\n\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equc\" class=\"mathdisplay\"\u003e$$ {FWHM}^{2}\\text{}\\text{= }\\text{5.57 }\\text{\u0026times;}\\text{}\\text{S}\\text{}\\text{\u0026times;}\\text{}({h\\omega )}^{2}\\text{}\\text{\u0026times;}\\text{ (1 + }\\frac{2kT}{h\\omega }\\text{)}\\text{}\\text{ (}\\text{3}\\text{)}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eThe linear relationship between FWHM\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and 2\u003cem\u003ekT\u003c/em\u003e was obtained by fitting the data from the variation of FWHM with temperature as displayed in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ec. The calculated \u003cem\u003eh\u0026nu;\u003c/em\u003e and \u003cem\u003eS\u003c/em\u003e are 0.0209 eV and 7.1236, respectively. These outcomes reveal that GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e suffers from a strong EPC effect, and also implies that the material would exhibit more pronounced thermal quenching compared to phosphors with shorter-wavelength emission.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n\u003ch2\u003eNIR pc-LED device and multiple applications\u003c/h2\u003e\n\u003cp\u003eTo evaluate the practical application of GGO: Cr\u003csup\u003e3+\u003c/sup\u003e phosphors, a prototype NIR pc-LED device was fabricated by GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e and commercial 430 nm LED chip (3 W) (insets in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003ea). The electroluminescence (EL) spectrum of the device is shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003ea, exhibiting a broad emission band from 680 nm to 1400 nm. The insets show photographs of the device in the off state, and under illumination as captured by a normal camera and an NIR camera, respectively, revealing intense NIR emission. The EL intensity increases progressively with the driving current (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003eb). The NIR radiant flux and the photoelectric conversion efficiency of the device under different driving currents (20\u0026ndash;180 mA) are summarized in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003ec. Owing to the high EQE and good thermal stability of the GGO:6%Cr\u003csup\u003e3+\u003c/sup\u003e phosphor, the NIR output power reaches a maximum of 55.59 mW at 180 mA, and the photoelectric efficiency peaks at 15.0% at 20 mA. The observed decrease in photoelectric efficiency with increasing current is attributed to the efficiency droop effect inherent to the LED chip.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe Gannan area is an important rare earth resource base in China, where mining activities can lead to the release of ions such as Nd\u003csup\u003e3+\u003c/sup\u003e and Yb\u003csup\u003e3+\u003c/sup\u003e into surrounding water bodies. While these ions exhibit low ecological toxicity at trace concentrations, long-term accumulation may disrupt aquatic ecosystems and pose potential health risks to humans via the food chain. Nevertheless, given their recyclable value, the efficient monitoring and recovery of these rare earth (RE) ions can serve the dual purposes of environmental remediation and resource reclamation. This calls for the development of a straightforward detection technique for Nd\u003csup\u003e3+\u003c/sup\u003e, Yb\u003csup\u003e3+\u003c/sup\u003e, and other rare earth ions. Here, we demonstrate that their concentrations can be quantitatively determined using a non-contact detection approach based on a NIR pc-LED. This method relies on the characteristic absorption peaks of these ions in the near-infrared region\u0026mdash;for instance, Nd\u003csup\u003e3+\u003c/sup\u003e at ~\u0026thinsp;880 nm and Yb\u003csup\u003e3+\u003c/sup\u003e at ~\u0026thinsp;980 nm. As shown in Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003ed-e, the absorption spectra of Nd\u003csup\u003e3+\u003c/sup\u003e and Yb\u003csup\u003e3+\u003c/sup\u003e aqueous solutions were measured at different concentration gradients. According to the Lambert-Beer law, the relationship between the transmitted light intensity and the concentration of Nd\u003csup\u003e3+\u003c/sup\u003e/Yb\u003csup\u003e3+\u003c/sup\u003e ions in aqueous solution can be expressed as:\u003c/p\u003e\n\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equ1\" class=\"mathdisplay\"\u003e$$ {\\text{Log}}_{\\text{10}}\\left({\\text{I}}_{\\text{0}}\\text{/}\\text{I}\\right)\\text{}\\text{=}\\text{}\\text{\u0026epsilon;}\\text{bc}$$\u003c/div\u003e\n\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003ewhere \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e is the intensity of the incident light, \u003cem\u003eI\u003c/em\u003e is the intensity of the transmitted light, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\( \\text{\u0026epsilon;}\\)\u003c/span\u003e\u003c/span\u003e is the molar absorption coefficient, \u003cem\u003eb\u003c/em\u003e is the thickness of the absorption layer, and \u003cem\u003ec\u003c/em\u003e is the concentration of the absorbing material. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003ef, the data points exhibit a good linear fit, from which the calibration curves for Nd\u003csup\u003e3+\u003c/sup\u003e and Yb\u003csup\u003e3+\u003c/sup\u003e were established. The concentration of these ions in real water samples can thus be determined using the external standard method, demonstrating the great potential of the GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e phosphor for the quantitative detection of RE ions.\u003c/p\u003e\n\u003cp\u003eAdditionally, owing to the material-dependent absorption of near-infrared light, the fabricated NIR pc-LED also can be utilized for transmission-based imaging, where variations in transmitted NIR brightness reveal internal structures. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003eg, the vascular network in a human palm is clearly visible under illumination by the NIR pc-LED when captured with an NIR camera. Furthermore, the device can be used for non-destructive inspection; for example, the precise location of an embedded chip inside a card can be accurately identified without causing any physical damage, as shown in Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003eh-i. These demonstrations indicate that the fabricated NIR pc-LED devices hold promise for multiple applications in non-destructive testing and bioimaging.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Discussions","content":"\u003cp\u003eIn this work, a series NIR phosphor GGO: Cr\u003csup\u003e3+\u003c/sup\u003e were synthesized via solid-state reaction technology. The excitation spectrum of the sample is distinct from that of Cr\u003csup\u003e3+\u003c/sup\u003e in a single octahedral environment, and its emission spectrum exhibits a significant red-shift with increasing Cr\u003csup\u003e3+\u003c/sup\u003e concentration. Through structural analysis combined with low-temperature spectroscopy and XANES, we clarified that the broad emission (650\u0026ndash;1400 nm) originates from the combined luminescence of Cr\u003csup\u003e3+\u003c/sup\u003e ions occupying both octahedral and tetrahedral sites, as well as from exchange-coupled Cr\u003csup\u003e3+\u003c/sup\u003e pairs. The optimized GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e phosphor achieves an extremely high blue light absorption about 69% with QY/EQY of 53%/36%. Moreover, at 423 K, the PL intensity of this phosphor still maintained 60% of that at room temperature. A fabricated NIR pc-LED delivers a NIR output power about 55.59 mW at 180 mA and a photoelectric efficiency of 15%, demonstrating potential for nondestructive testing and bioimaging. We believe that this work provides an insight into the occupation and PL behavior of Cr\u003csup\u003e3+\u003c/sup\u003e ions and will inspire more researchers to develop high-performance broadband NIR phosphors.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003eSynthesis\u003c/h2\u003e\u003cp\u003eGGO: \u003cem\u003ey\u003c/em\u003eCr\u003csup\u003e3+\u003c/sup\u003e (\u003cem\u003ey\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0\u0026ndash;16%) samples were prepared by high temperature solid state reaction method. Raw materials Ga\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (99.9%, Macklin), GeO\u003csub\u003e2\u003c/sub\u003e (99.99%, Macklin) and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (99.9%, Macklin) were weighed according to stoichiometry and ground in agate mortar for 20 min. The dried mixture was then placed in an alumina crucible and subjected to a multi-step solid-phase reaction in a tube furnace (at 600 ℃ for 2 h, 1350 ℃ for 6 h, respectively for burn-in and crystallization, at a heating rate of 5 ℃/min). After the final sintering, the powder was cooled to room temperature at 5 ℃/min and ground into fine powder for subsequent characterization.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003eCharacterizations\u003c/h2\u003e\u003cp\u003eThe XRD patterns of samples were collected on a BRUKER D8 X-ray diffractometer with Cu Kα radiation with a count time of 15 s/step and a step size of 0.01\u0026deg; at room temperature. Rietveld refinements were performed using the GSAS program. The crystal structures were determined by a three-dimensional visualization system for electronic and structural analysis. The morphologies and elemental compositions of the powders were observed and detected via SEM (MLA650F, FEI) equipped with EDS. The valence states of chromium ions were confirmed through XPS using an AXIS ULTRA spectrometer from Kratos Analytical Ltd. The PL excitation and emission spectra were recorded using a spectrometer (FLS 980, Edinburgh Instruments) equipped with 450 W Xe lamps. EPR measurements on a Bruker EMXplus-6/1 EPR spectrometer. Diffuse reflection spectra were obtained using a UV-vis-NIR spectrophotometer (PerkinElmer, Lambda 950) with a reflectivity standard whiteboard as the calibration. Decay curves and temperature-dependent PL spectra were collected using the same with a flash lamp and temperature controller (THMS 600, Linkam Scientific Instruments). The PL IQE of phosphors at room temperature were performed on a Hamamatsu quantum yield measurement system (C9920-02G). XAFS was performed at the BL14B2* of SPring-8 (8 GeV, 100 mA), Japan, in which, the X-ray beam was mono-chromatized with water-cooled Si (111) double-crystal monochromator and focused with two Rh coated focusing mirrors with the beam size of 2.0 mm in the horizontal direction and 0.5 mm in the vertical direction around sample position, to obtain XAFS spectra both in near and extended edge, Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e foil sample was used as references. NIR pc-LED was fabricated by blending NIR phosphors with 430 nm blue LED chip. The appropriate amount of GGO: Cr\u003csup\u003e3+\u003c/sup\u003e with different weights were mixed with transparent epoxy resin in a ratio of 1:1, and uniformly coated on the blue LED chip. After drying in air at 80 ℃ for 1 h, NIR pc-LED device was obtained. The performance of device was measured by photoelectric test system (HAAS-2000, 350\u0026ndash;1100 nm and 780\u0026ndash;1650 nm, Everfine, Hangzhou).\u003c/p\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eAuthor details\u003c/h2\u003e\u003cp\u003e\u003csup\u003e1\u003c/sup\u003eCollege of Rare Earths, Jiangxi University of Science and Technology, Ganzhou 341000, P.R. China. \u003csup\u003e2\u003c/sup\u003eNational Rare Earth Functional Materials Manufacturing Innovation Centre, Guorui Kechuang Rare Earth Functional Materials (Ganzhou) Co., Ltd, Ganzhou 341000, P. R. China. \u003csup\u003e3\u003c/sup\u003eKey Laboratory of Testing and Tracing of Rare Earth Products for State Market Regulation, Jiangxi University of Science and Technology, Ganzhou 341000, P.R. China. \u003csup\u003e4\u003c/sup\u003eKey Laboratory of Low Dimensional Quantum Materials and Sensor Devices of Jiangxi Education Institutes, Ganzhou 341000, China. \u003csup\u003e5\u003c/sup\u003eYunnan Key Laboratory of Electromagnetic Materials and Devices, School of Materials and Energy, Yunnan University, Kunming 650091, P. R. China\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003ch2\u003eCompeting interests\u003c/h2\u003e\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e\u003cp\u003eW.J.D. and D.C.H. conceived the idea. W.J.D. conducted the materials synthesis. W.J.D. conducted the XRD, SEM, XPS, fluorescence steady-state and transient spectroscopy measurements, etc. W.J.D. and D.C.H. co-wrote and revised the manuscript. D.C.H. supervised the work. All authors contributed to the discussion of the results and provided comments on the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eThis work was supported by the National Natural Science Foundation of China (No. 12274409), the Natural Science Foundation of Jiangxi Province of China (No. 20232ACB211006), the Science and Technology Program of Ganzhou City (No. 2023CYZ17837), Key Laboratory of Ionic Rare Earth Resources and Environment, Ministry of Natural Resources of the People\u0026rsquo;s Republic of China (No. 2024IRERE204), the Jiangxi Provincial Training Program for Academic and Technical Leaders of Major Disciplines (No. 20232BCJ23088), the project of Yunnan Key Laboratory of Electromagnetic Materials and Devices, Yunnan University (No. ZZ2024002) and the Jiangxi Provincial Project for Graduate Innovation Special Fund (No. YC2024-S538).\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGu, S. M. et al. Laser-driven luminescent ceramic-converted near-infrared II light source for advanced imaging and detection techniques. \u003cem\u003eLight Sci. Appl.\u003c/em\u003e 14, 317 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eShen, H. C. et al. Rational design of NIR-II AIEgens with ultrahigh quantum yields for photo-and chemiluminescence imaging. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e 144, 15391\u0026ndash;15402 (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHu, Z. H. et al. First-in-human liver-tumour surgery guided by multispectral fluorescence imaging in the visible and near-infrared-I/II windows. \u003cem\u003eNat. Biomed. Eng.\u003c/em\u003e 4, 259\u0026ndash;271 (2019).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eJia, Z. W. et al. Strategies to approach high performance in Cr\u003csup\u003e3+\u003c/sup\u003e-doped phosphors for high-power NIR-LED light sources. \u003cem\u003eLight Sci. Appl.\u003c/em\u003e 9, 86 (2020).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhong, Y. T. et al. In vivo molecular imaging for immunotherapy using ultra-bright near-infrared-IIb rare-earth nanoparticles. \u003cem\u003eNat. Biotechnol.\u003c/em\u003e 37, 1322\u0026ndash;1331. (2019).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi, R. Y. et al. Boosting Applications with High-performance near-infrared phosphor-converted light-emitting diodes. \u003cem\u003eLaser Photonics Rev.\u003c/em\u003e 18, 2300608 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang, K. N. et al. Enabling highly efficient neodymium luminescence for near-infrared phosphor-converted light-emitting diode applications. \u003cem\u003eSmall Struct.\u003c/em\u003e 5, 2400092 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZheng, G. J. et al. Glass-crystallized luminescence translucent ceramics toward high‐performance broadband NIR LEDs. \u003cem\u003eAdv. Sci.\u003c/em\u003e 9, 2105713 (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi, H. M. et al. Ultrawideband emission of Bi\u003csup\u003e3+\u003c/sup\u003e ions spanning visible to near-infrared spectral regions (400 nm-1700 nm). \u003cem\u003eAngew. Chem. Int. Ed.\u003c/em\u003e e202520587 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXiao, Y. et al. Tracing the origin of near-infrared emissions emanating from manganese. \u003cem\u003eLight Sci. Appl.\u003c/em\u003e 14, 194 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYan, L. Q. et al. Emerging Fe\u003csup\u003e3+\u003c/sup\u003e doped broad NIR-emitting phosphor Ca\u003csub\u003e2.5\u003c/sub\u003eHf\u003csub\u003e2.5\u003c/sub\u003e(Ga, Al)\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e: Fe\u003csup\u003e3+\u003c/sup\u003e for LWUV pumped NIR LED. \u003cem\u003eLaser Photonics Rev.\u003c/em\u003e 18, 1 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang, Y. et al. Enhanced near-infrared II and III emission in Ga\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e: Ni\u003csup\u003e2+\u003c/sup\u003e phosphor via charge compensation for NIR spectroscopy application. \u003cem\u003eCeram. Int.\u003c/em\u003e 50, 31589\u0026ndash;31597 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhao, Q. Q. et al. Unlocking chlorine oxide-based ultra-wideband near-Infrared Phosphor and Advancing Spectral Performance via Lattice Engineering. \u003cem\u003eAdv. Funct. Mater.\u003c/em\u003e 34, 1 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZ. R. Liao. et al. Developing the Na\u003csub\u003e2\u003c/sub\u003eCaZr\u003csub\u003e2\u003c/sub\u003eGe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e Garnet Phosphor for Advanced NIR pc-LEDs in Night Vision and Bioimaging. \u003cem\u003eChem. Mater.\u003c/em\u003e 36, 4654\u0026ndash;4663 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYue, Y. X. et al. Preparation and luminescent properties of chromium ion doped aluminum borate near-infrared phosphor (Al\u003csub\u003e18\u003c/sub\u003eB\u003csub\u003e4\u003c/sub\u003eO\u003csub\u003e33\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e) and its application of broadband near-infrared LED. \u003cem\u003eCeram. Int.\u003c/em\u003e 51, 27243\u0026ndash;27252 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLi, C. J. et al. Laser-driven spinel-type ceramics enabling NIR-II light sources for penetration optical imaging assisted by a guided filter network algorithm. \u003cem\u003eMater. Today\u003c/em\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.mattod.2025.10.019\u003c/span\u003e\u003cspan address=\"10.1016/j.mattod.2025.10.019\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMou, T. L. et al. Efficient and thermally stable double perovskite phosphor Cs\u003csub\u003e2\u003c/sub\u003eNaScCl\u003csub\u003e6\u003c/sub\u003e: Mo\u003csup\u003e4+\u003c/sup\u003e with broadband long-wavelength NIR emission. \u003cem\u003eChem. Eng. J.\u003c/em\u003e 520, 165718 (2025)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang, Y. J. et al. High-Temperature Negative-Thermal-Quenching in Broadband NIR Light-Emitting ScF\u003csub\u003e3\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e Phosphors. \u003cem\u003eAdv. Opt. Mater.\u003c/em\u003e 13, e01552 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChen, C. H. et al. Strategies for broadening the emission spectra of Cr\u003csup\u003e3+\u003c/sup\u003e-doped near-infrared emitting phosphors. \u003cem\u003eMater. Chem. Front.\u003c/em\u003e 9, 1821\u0026ndash;1838 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eNie, W. D. et al. A highly efficient NIR-II-emitting MgGa\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e, Ni\u003csup\u003e2+\u003c/sup\u003e phosphor via the Cr\u003csup\u003e3+\u003c/sup\u003e ions in tetrahedral sites as the Cr\u003csup\u003e3+\u003c/sup\u003e-Ni\u003csup\u003e2+\u003c/sup\u003e energy transfer bridge. \u003cem\u003eSci. China Mater.\u003c/em\u003e 68, 1788\u0026ndash;1801 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTian, Y. et al. Broadband emission from NIR-I to NIR-II in BaZn\u003csub\u003e3\u003c/sub\u003eGa\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e:Cr\u003csup\u003e3+\u003c/sup\u003e phosphors achieved through double-site occupation. \u003cem\u003eCeram. Int.\u003c/em\u003e 51, 37861\u0026ndash;37867 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhu, F. M. et al. Cr\u003csup\u003e3+\u003c/sup\u003e-Doped LiAlO\u003csub\u003e2\u003c/sub\u003e NIR-I emitting phosphors with superior resistance to thermal quenching for night vision monitoring and bioimaging. \u003cem\u003eACS Appl. Mater. Interfaces.\u003c/em\u003e 16, 60599\u0026ndash;60607 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZou, X. K. et al. Ultra-wide Vis\u0026ndash;NIR Mg\u003csub\u003e2\u003c/sub\u003eAl\u003csub\u003e4\u003c/sub\u003eSi\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e18\u003c/sub\u003e: Eu\u003csup\u003e2+\u003c/sup\u003e, Cr\u003csup\u003e3+\u003c/sup\u003e phosphor containing unusual NIR luminescence induced by Cr\u003csup\u003e3+\u003c/sup\u003e occupying tetrahedral coordination for hyperspectral imaging. \u003cem\u003eAdv. Opt. Mater.\u003c/em\u003e 10, 1 (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChang, C. Y. et al. Ultrahigh quantum efficiency near-infrared-II emission achieved by Cr\u003csup\u003e3+\u003c/sup\u003e clusters to Ni\u003csup\u003e2+\u003c/sup\u003e energy transfer. \u003cem\u003eChem. Mater.\u003c/em\u003e 36 3941\u0026ndash;3948 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eRajendran, V. et al. Chromium ion pair luminescence: a strategy in broadband near-infrared light-emitting diode design. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e 143, 19058\u0026ndash;19066 (2021).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhu, H. et al. Abnormal lattice shrinkage, site occupation, and luminescent properties of Cr\u003csup\u003e3+\u003c/sup\u003e-activated β-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e structure phosphors. \u003cem\u003eLaser Photonics Rev.\u003c/em\u003e 19, 2401089 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhu, H. et al. Structural design of Cr\u003csup\u003e3+\u003c/sup\u003e-activated hexaaluminate phosphors with high quantum efficiency and Cr\u003csup\u003e3+\u003c/sup\u003e-Cr\u003csup\u003e3+\u003c/sup\u003e exchange coupling pairs. \u003cem\u003eChem. Mater.\u003c/em\u003e 37, 7227\u0026ndash;7239 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYao, L. Q. et al. Efficient ultra-broadband Ga\u003csub\u003e4\u003c/sub\u003eGeO\u003csub\u003e8\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e phosphors with tunable peak wavelengths from 835 to 980 nm for NIR pc‐LED application. \u003cem\u003eAdv. Opt. Mater.\u003c/em\u003e 10, 1\u0026ndash;10 (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChang, J. H. et al. Investigation of Cr\u003csup\u003e3+\u003c/sup\u003e-doped mullite structure near-infrared phosphors featuring with octahedral and tetrahedral dual-site emission. \u003cem\u003eChin. J. Lumin.\u003c/em\u003e 46, 1430\u0026ndash;1441 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChi, F. F. et al. Luminescence properties of Cr\u003csup\u003e3+\u003c/sup\u003e-doped Al\u003csub\u003e6\u003c/sub\u003eGe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e broadband near-infrared phosphor. \u003cem\u003eOpt. Mater. Express.\u003c/em\u003e 126, 112218 (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHan, L. et al. Ionic substitution combined with flux doping triggered near-unity efficiency and thermally stable NIR luminescence in Al\u003csub\u003e6\u003c/sub\u003eGe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e:Cr\u003csup\u003e3+\u003c/sup\u003e, Si\u003csup\u003e4+\u003c/sup\u003e phosphor for NIR spectroscopy applications. \u003cem\u003eJ. Alloys Compd.\u003c/em\u003e 1036 181870 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang, Z. N. et al. Simultaneous luminescence color tuning and spectral broadening of Cr\u003csup\u003e3+\u003c/sup\u003e near-infrared emission for night vision application. \u003cem\u003eACS Appl. Opt. Mater.\u003c/em\u003e 1, 1088\u0026ndash;1096 (2023).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang, Q. et al. Highly efficient and thermally stable broadband NIR phosphors with superlong afterglow performance and their multifunctional applications. \u003cem\u003eLaser Photonics Rev.\u003c/em\u003e 18, 1 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYan, Y. Q. et al. Unique spectral broadening induced by exchange coupling between Cr\u003csup\u003e3+\u003c/sup\u003e ions in LiAl\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e:Cr\u003csup\u003e3+\u003c/sup\u003e phosphors for versatile optical applications. \u003cem\u003eLaser Photonics Rev.\u003c/em\u003e 19, 1 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhong, L. Valence and site engineering enable efficient broadband near-infrared emission at 960 nm in Cr\u003csup\u003e3+\u003c/sup\u003e-activated forsterite. \u003cem\u003eAdv. Mater.\u003c/em\u003e e2508768 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLin, Q. M. et al. Near-unity internal quantum efficiency and high thermal stability of Sr\u003csub\u003e3\u003c/sub\u003eMgGe\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e14\u003c/sub\u003e:Cr\u003csup\u003e3+\u003c/sup\u003e phosphor for plant growth. \u003cem\u003eLaser Photonics Rev.\u003c/em\u003e 19, 2402047 (2025).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang, S. W. et al. Achieving high quantum efficiency broadband NIR Mg\u003csub\u003e4\u003c/sub\u003eTa\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e9\u003c/sub\u003e:Cr\u003csup\u003e3+\u003c/sup\u003e phosphor through lithium-ion compensation. \u003cem\u003eAdv. Mater.\u003c/em\u003e 35, 2300124 (2023).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZ.W. et al. A dual-excited and dual near-infrared emission phosphor Mg\u003csub\u003e14\u003c/sub\u003eGe\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e24\u003c/sub\u003e:Cr\u003csup\u003e3+\u003c/sup\u003e, Cr\u003csup\u003e4+\u003c/sup\u003e with a super broad band for biological detection. \u003cem\u003eDalton Trans.\u003c/em\u003e 50, 311\u0026ndash;322 (2021).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZheng, Z.-H. et al. A simple and generic post-treatment strategy for highly efficient Cr\u003csup\u003e3+\u003c/sup\u003e-activated broadband NIR emitting phosphors for high-power NIR light sources. \u003cem\u003eJ. Mater. Chem. C.\u003c/em\u003e 10, 8797\u0026ndash;8805 (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHuang, D. C. et al. A highly efficient and thermally stable broadband Cr\u003csup\u003e3+\u003c/sup\u003e-activated double borate phosphor for near-infrared light-emitting diodes. \u003cem\u003eJ. Mater. Chem. C.\u003c/em\u003e 9, 164\u0026ndash;172 (2021).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHuang, D. C. et al. Efficient and thermally stable broadband near-infrared emission from near zero thermal expansion AlP\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e9\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e phosphors. \u003cem\u003eInorg. Chem. Front.\u003c/em\u003e 9, 692\u0026ndash;1700 (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSong, L. P. et al. Ultra-broad near-infrared emitting phosphor LiInF\u003csub\u003e4\u003c/sub\u003e: Cr\u003csup\u003e3+\u003c/sup\u003e with extremely weak crystal field. \u003cem\u003eInorg. Chem.\u003c/em\u003e 62, 11112\u0026ndash;11120 (2023).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDalal M: electronic spectra of transition metal complexes, a textbook of inorganic chemistry, India: Dalal Institute. 1, 280\u0026ndash;299 (2017).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eNazarov, M. et al. Vibronic coupling parameters and stokes shift in thiogallate phosphors. \u003cem\u003eJ. Phys. Chem. Solids.\u003c/em\u003e 69, 2605\u0026ndash;2612 (2008).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8234812/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8234812/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNear-infrared phosphor-converted light-emitting diodes (NIR pc-LEDs) are promising for applications in spectroscopy and bioimaging. Typical NIR phosphor designs feature isolated Cr\u003csup\u003e3+\u003c/sup\u003e ion centers in octahedral crystallographic sites. However, it is challenging to design broadband NIR phosphors based on tetrahedral crystallographic sites or Cr\u003csup\u003e3+\u003c/sup\u003e \u0026minus; Cr\u003csup\u003e3+\u003c/sup\u003e pairs. Herein, we explore a series of Cr\u003csup\u003e3+\u003c/sup\u003e-doped gallogermanate Ga\u003csub\u003e6\u003c/sub\u003eGe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e13\u003c/sub\u003e (GGO) phosphors that feature both octahedral, tetrahedral crystal sites, as well as Cr\u003csup\u003e3+\u003c/sup\u003e \u0026minus; Cr\u003csup\u003e3+\u003c/sup\u003e pairs, and evaluate their structure-property relationships. Structural and spectroscopic analysis, including low-temperature (10 K) spectroscopy, X-ray absorption near-edge structure, and electron paramagnetic resonance, reveal multiple Cr\u003csup\u003e3+\u003c/sup\u003e emission centers in GGO \u0026mdash; namely, octahedral, tetrahedral, and Cr\u003csup\u003e3+\u003c/sup\u003e \u0026minus; Cr\u003csup\u003e3+\u003c/sup\u003e pairs, resulting in NIR luminescence at approximately 650\u0026thinsp;\u0026minus;\u0026thinsp;1400 nm. Through Cr\u003csup\u003e3+\u003c/sup\u003e concentration engineering, the emission spectra of phosphors exhibit a remarkable redshift and an increase in their full width at half maximum. The optimized GGO:6% Cr\u003csup\u003e3+\u003c/sup\u003e exhibits satisfactory thermal stability (60% retention at 423 K) alongside outstanding absorption efficiency (69%) and external quantum efficiency (36%). The fabricated NIR-LED device delivers a high NIR output power of 55.59 mW at a driving current of 180 mA. This work provides deeper insight into the site environment of Cr\u003csup\u003e3+\u003c/sup\u003e ions in phosphors and, more specifically, demonstrates the potential of gallogermanates as promising host systems for NIR phosphors.\u003c/p\u003e","manuscriptTitle":"Insight into Site Environment and Photoluminescence Properties of Cr3+-doped Gallogermanate Toward NIR pc-LEDs Applications","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-02 12:02:29","doi":"10.21203/rs.3.rs-8234812/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a9fef751-4636-43db-bba8-230af2012bfa","owner":[],"postedDate":"December 2nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":58868294,"name":"Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Inorganic LEDs"},{"id":58868295,"name":"Physical sciences/Physics/Electronics, photonics and device physics/Photonic devices"}],"tags":[],"updatedAt":"2025-12-02T12:02:32+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-02 12:02:29","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8234812","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8234812","identity":"rs-8234812","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}