Achieving high performance ultra-broadband near-infrared emission through a multi-site occupancy and energy transfer strategy for NIR LED applications

Abstract

Broadband near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are considered to be at the forefront of the development of next-generation NIR light sources. However, the performance of NIR pc-LEDs is severely limited due to the narrow band emission, low quantum efficiency, and thermal quenching of NIR-emitting materials. Herein, an efficient and thermally stable broadband NIR LaMgGa₁₁O₁₉:Cr³⁺,Yb³⁺ (LMG:Cr³⁺,Yb³⁺) phosphor has been successfully designed by [Cr³⁺–Yb³⁺] co-doping. The broadband emission phenomenon of LaMgGa₁₁O₁₉:Cr³⁺ was confirmed to be due to selective lattice occupancy of Cr³⁺ ions based on the analysis of crystal structures, crystal field calculations, and fluorescence lifetimes. The NIR emission spectra in the range of 1000–1200 nm were enriched by using the highly efficient energy transfer of Cr³⁺ → Yb³⁺ ions and the energy transfer mechanism is discussed in detail. The prepared LMG:Cr³⁺,Yb³⁺ phosphors exhibit highly efficient ultra-broadband NIR emission from 650 to 1200 nm under 440 nm excitation with high internal and external quantum efficiencies of 94.2%/40.5% and excellent luminescence thermal stability of 89.3%@373 K. A NIR pc-LED prototype was fabricated by combining the optimized phosphor with a commercial 440 nm blue LED chip, providing 84.5 mW NIR output power at a 350 mA driving current. Finally, the potential applications of the phosphor in night vision lighting and non-destructive testing were demonstrated. The results show that this work is expected to provide a new strategy for efficient ultra-broadband NIR phosphor design.

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Introduction

Near-infrared (NIR) pc-LEDs have the advantages of small size, low cost, and spectral tunability, and show great potential for applications in night vision, anti-counterfeiting, non-destructive testing and medical imaging.^1–8 As the core component of NIR pc-LED devices, the NIR phosphor directly determines the peak emission, bandwidth, luminous efficacy, and long-term stability of the device.^9,10 Generally speaking, the wider the emission band of the NIR phosphor, the more the spectral information that can be obtained from the NIR pc-LED.^11–13 However, the absence of suitable NIR phosphors has resulted in the spectral characteristics and light output power of NIR pc-LEDs remaining inadequate for practical applications. Therefore, it is of great practical significance to develop broadband NIR phosphors with high efficiency and thermostability so that the NIR LEDs can meet the requirements of convenient equipment applications.

Cr³⁺ ions are considered as a favorable luminescence center for NIR-emitting phosphors due to their tunable emission bands from deep red to NIR, achieved by adjusting the strength of the crystal field in which they are located.^14,15 A variety of Cr³⁺-doped solid solutions have been successfully developed, and these materials have been widely used in pc-LEDs because they exhibit highly efficient quantum yields and excellent thermal stability in the short wavelength region from 700 to 900 nm.^16–19 Nevertheless, there is an increasing demand for phosphor materials capable of achieving intense emission in the long wavelength range in cutting-edge fields such as medicine, imaging and remote sensing detection.^20,21 To broaden the emission bandwidth of Cr³⁺-activated NIR phosphors, researchers have proposed a multi-grid occupation strategy. For example, Jiang et al. reported a broadband emission AlTaO₄:Cr³⁺ NIR phosphor induced by the simultaneous occupation of AlO₆ and TaO₆ lattices by Cr³⁺ ions.²² Sun et al. developed a new broadband NIR K₂SrGe₈O₁₈:Cr³⁺ phosphor using two Cr³⁺ lattices, resulting in a full width at half-maximum (FWHM) up to 214 nm.²³ Shi et al. prepared La₃SnGa₅O₁₄:Cr³⁺ phosphors by the crystalline site-occupation method, whose emission spectrum spans a broad 650–1300 nm range and which show strong emission in the NIR II region (>1000 nm).²⁴ These works provide a reference for further research on tunable NIR luminescent phosphor materials.

Furthermore, researchers have put forth a proposal to expand the luminescence spectrum of Cr³⁺ ion-activated NIR phosphors through the design of energy transfer mechanisms. A great deal of research has been conducted in this field, resulting in significant advancements. Miao et al. co-doped Cr³⁺/Ni²⁺ ions in the LiMgPO₄ matrix, which resulted in the generation of broadband NIR emission in the 1100–1600 nm range.²⁵ He et al. co-doped Cr³⁺ and Nd³⁺ in Ca₂LuZr₂Al₃O₁₂, resulting in the creation of a highly efficient broadband NIR phosphor with a FWHM of 350 nm; the phosphor demonstrated excellent thermal stability.²⁶ Shi et al. undertook a co-doping process with Yb³⁺ ions on the InBO₃:Cr³⁺ phosphor, resulting in broader emission spectra and higher internal quantum efficiency (IQE).²⁷ Unfortunately, it is regrettable that these studies continue to exhibit deficiencies in terms of external quantum efficiency (EQE) and thermal stability. Therefore, it is a challenging work to develop new and efficient broadband NIR phosphors to meet the demand for NIR pc-LEDs.

In this study, the successful preparation of novel LaMgGa₁₁O₁₉(LMG):Cr³⁺,Yb³⁺ ultra-broadband NIR phosphors via [Cr³⁺–Yb³⁺] cation co-doping was achieved. The luminescence properties and energy transfer mechanisms of this material have been studied in depth, which has revealed that the significant red-shift and broadening of the Cr³⁺ spectrum is attributable to the lattice occupation tendency. Furthermore, the energy transfer process from Cr³⁺ to Yb³⁺ has been determined to be a dipole–quadrupole interaction, exhibiting a maximum transfer efficiency of 71.6%. Finally, NIR pc-LED devices were prepared by combining the above phosphors with commercial 440 nm blue LED chips, and their potential applications in night vision imaging and NDT were demonstrated. The results show that this new LMG:Cr³⁺,Yb³⁺ phosphor will be promising for future NIR pc-LED applications.

Material and methods

Materials and preparation

A series of LMG phosphor samples were synthesized using a conventional high-temperature solid-phase method. The raw materials were La₂O₃ (99.99%, Aladdin), MgO (99.99%, Aladdin), Ga₂O₃ (99.99%, Aladdin), Cr₂O₃ (99.95%, Aladdin), Yb₂O₃ (99.99%, Aladdin), and H₃BO₃ (99.99%, Aladdin). The above ingredients were weighed accurately according to the stoichiometric ratios of the samples, and an amount of boric acid equivalent to 3% of the total mass was incorporated as a flux. All the raw materials were put into an agate mortar and mixed with ethanol and fully ground for 40 min. Then they were put into a corundum crucible and placed in a tube furnace and calcined at 1500 °C for 6 h and then cooled to room temperature. The cooled sample was taken out and ground again for 20 min to obtain the final sample.

Characterization

The crystal structure of the samples was confirmed using a powder X-ray diffractometer (XRD) (Ultima IV). Rietveld refinement used TOPAS 4.2. The micro-morphology and elemental distribution of the samples were tested and analyzed using a field emission scanning electron microscope (Verios 460) equipped with an energy dispersive spectroscopy (EDS) analyzer. Diffuse reflectance spectra were recorded using a UV-visible spectrophotometer (PE Lambda950s) and Ba₂SO₄ was used as a reference sample. X-ray photoelectron spectroscopy (XPS) measurements were executed using a Thermo Scientific K-Alpha system. Excitation spectra, emission spectra, variable temperature spectra and fluorescence lifetimes of the samples were obtained using an FLS1000 spectrometer (Edinburgh). The IQE and absorption efficiency (AE) of the samples were obtained using a UV-NIR absolute quantum yield meter (Quantaurus-QY Plus C13534-12, Hamamatsu) and Ba₂SO₄ was used as a reference sample.

NIR pc-LED fabrication and performance measurement

The NIR pc-LED device was made by mixing LMG:0.2Cr³⁺,0.05Yb³⁺ phosphors with AB glue at a 1 : 1 powder–glue ratio, and then coating on a 440 nm blue LED chip followed by encapsulation. The optical properties of the device, such as electroluminescence (EL) spectra, NIR output power, and photoelectric conversion efficiency, were then characterized using an LED test rig (HP8000) equipped with an integrating sphere and an HAAS2000 photoelectric measurement system. The operating temperatures of the LED were recorded using a thermal imaging camera (FLIR E40).

Results and discussion

Crystal structure

The crystal structure of LMG is shown in Fig. 1a, which belongs to the structure of magnetoplumbite, a P6₃/mmc space group with five unique M cation sites [M = Mg²⁺, Ga³⁺]. Among them, Ga^I, Ga^IV and Ga^V are 6-coordinated, and Ga^II and Ga^III are 5- and 4-coordinated, respectively. Here [Ga^IO₆] is a regular octahedron and [Ga^IVO₆] and [Ga^VO₆] are relatively irregular octahedra. Mg²⁺ selectively occupies the Ga^I and Ga^III sites.²⁸ Considering the valence and ionic radius matching rules, the effective ionic radii of Ga³⁺ and Cr³⁺ are 0.0620 Å and 0.0615 Å under six-coordination, respectively, with the same valence and similar ionic radii, so substitution is more likely to occur.²⁹ As for the trivalent rare earth ion Yb³⁺, it is more likely to occupy the La³⁺ lattice.³⁰

Fig. 1 (a) Crystal structure of LMG. (b) Powder XRD patterns of LMG, LMG:0.2Cr³⁺, LMG:0.05Yb³⁺, and LMG:0.2Cr³⁺,0.05Yb³⁺. (c) Rietveld refinement of LMG:0.2Cr³⁺,0.05Yb³⁺. (d) SEM image and EDS elemental distribution of LMG:0.2Cr³⁺,0.05Yb³⁺.

The synthesized LMG series phosphors were subjected to XRD tests to determine the phase purity of the samples. Fig. 1b shows the XRD patterns of LMG, LMG:0.2Cr³⁺, LMG:0.05Yb³⁺, and LMG:0.2Cr³⁺,0.05Yb³⁺ phosphors. The XRD diffraction peaks of all these samples are in agreement with the standard card (PDF#84-0889); no appearance of new peaks was observed, suggesting that the introduction of dopant ions did not alter the crystal structure or induce impurity. Fig. S1(a) and (b)† show the XRD patterns of LMG:xCr³⁺ (x = 0.01–0.5) and LMG:0.2Cr³⁺,yYb³⁺ (y = 0–0.1) phosphors, respectively (insets are partial enlargements of the highest diffraction peaks). The angle of the highest diffraction peak gradually becomes larger with the increase of Cr³⁺ doping, which may be due to the substitution of the larger Ga³⁺ ions by the smaller Cr³⁺ ions. Meanwhile, for the LMG:0.2Cr³⁺,yYb³⁺ series phosphors, the angle of the highest diffraction peak is shifted to a large angle with the increase of Yb³⁺ doping, which leads to cell shrinkage, thus decreasing the crystal plane spacing and shifting the diffraction peak to a higher angle.³¹ This is consistent with the designed material substitution model. To obtain more detailed crystal data, we performed a Rietveld structure refinement of the samples using LMG as the initial model, and the Rietveld structure refinement of the representative samples is shown in Fig. 1c. No impurity phase was observed, further supporting the conclusion that the samples were pure phases. The residual factor is relatively small, indicating that the results are acceptable. The detailed refinement results are presented in Tables S1 and S2.† The Ga–O bond lengths and average bond lengths of [Ga^IO₆], [Ga^IVO₆] and [Ga^VO₆] are given in Table S3.† The Cr³⁺ ions occupying the Ga^I, Ga^IV and Ga^V lattice sites are labeled as Cr^I, Cr^IV and Cr^V. The trends of the bond lengths of Ga^I, Ga^IV and Ga^V are shown in Fig. S2d,† where the Ga–O bond lengths decrease with the increase of Cr³⁺ ion doping, and the changes of Ga^IV and Ga^V are more pronounced with respect to those of Ga^I, which indicates that Cr³⁺ tends to occupy the Ga^IV and Ga^V lattice sites more. Fig. 1d shows the SEM and EDS elemental distribution of the sample LMG:0.2Cr³⁺,0.05Yb³⁺ phosphor. It can be observed that the phosphor samples exhibit a uniform dispersion of irregular shapes, with particle sizes ranging from approximately 5 to 20 μm. Further EDS analysis showed that La, Mg, Ga, Cr and Yb elements were uniformly distributed in the sample particles, and the elemental ratio of La, Mg and Ga was about 1 : 1.37 : 11, which was basically consistent with the stoichiometric ratio of the matrix.

Luminescence properties and energy transfer

Fig. 2a shows the diffuse reflectance spectra of LMG and LMG:0.2Cr³⁺ phosphors, and the inset shows the body color of LMG and LMG:0.2Cr³⁺ under daylight. The LMG sample displays a pronounced ultraviolet absorption band, whereas no intrinsic absorption is discernible in the visible and far-red regions. In contrast, the LMG:0.2Cr³⁺ sample exhibited two additional absorption peaks at 436 nm and 600 nm, in addition to the pronounced UV absorption of the substrate, which was attributed to the ⁴A₂ → ⁴T₁(⁴F) and ⁴A₂ → ⁴T₂(⁴F) absorption bands of the Cr³⁺ ion.³² These phenomena correspond to the LMG matrix base color showing white and the LMG:0.2Cr³⁺ base color showing gray-green. The optical band gap of the LMG matrix can be calculated using the Kubelka–Munk equation:³³where R denotes the reflection coefficient, hν denotes the photon energy, A denotes the absorption constant, E_g denotes the optical band gap, and F(R) denotes the absorption. A band gap of 5.07 eV was calculated using the data obtained for the LMG sample (shown in the inset of Fig. 2a).

Fig. 2 (a) Diffuse reflectance spectra of the LMG host and LMG:0.2Cr³⁺ samples. (b) PLE and PL spectra of LMG:0.2Cr³⁺. (c) PL spectra of LMG:xCr³⁺ (x = 0.05–0.5) samples and (d) the change of PL intensity and FWHM with the doping concentration. (e) Fluorescence decay curves of LMG:xCr³⁺ (x = 0.05–0.5) samples. (f) Peak differentiation and imitation of LMG:0.2Cr³⁺. (g) Excitation spectra with three peaks. (h) Tanabe–Sugano energy level diagram for Cr³⁺ in octahedron coordination. (i) Variation of three Gaussian fitted peaks of the LMG:xCr³⁺ (x = 0.05–0.5) PL spectra with the doping concentration.

Fig. 2b illustrates the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the LMG:0.2Cr³⁺ sample, which demonstrate that the two excitation peaks are attributed to the energy levels ⁴A₂ → ⁴T₁(⁴F) and ⁴A₂ → ⁴T₂(⁴F) of Cr³⁺, and that the emission at 770 nm is due to the transitions from ²E → ⁴A₂ and ⁴T₂(⁴F) → ⁴A₂. The PL spectra of the LMG:xCr³⁺ (x = 0.05–0.5) samples are shown in Fig. 2c. The fluorescence intensity of the samples under blue light excitation at 440 nm shows an increase and then a decrease with the increase of the Cr³⁺ doping concentration, and reaches a maximum at x = 0.2. This phenomenon can be attributed to the concentration quenching that occurs as a result of the increasing concentration, which involves a non-radiative transition process.³⁴Fig. 2d demonstrates the trend of luminescence intensity and FWHM of Cr³⁺ doped samples with different concentrations. The fluorescence decay curves of different concentrations of LMG:xCr³⁺ (x = 0.05–0.5) samples are shown in Fig. 2e, and the fluorescence lifetime of each sample can be fitted by eqn (3):³⁵

A₁, A₂, and A₃ represent fitting constants, t is the time, and τ₁, τ₂ and τ₃ represent three different lifetime indices. The three different lifetimes τ₁, τ₂ and τ₃ may be related to the three Cr³⁺ sites in this host.

Fig. 2f depicts the Gaussian spectral results of the PL spectrum of the LMG:0.2Cr³⁺ sample, wherein the emission peaks are accurately represented by three Gaussian peaks. The reliability of the fitted data is indicated by an R² value of 0.997. In general, it is useful to monitor the excitation spectra at different emission wavelengths to distinguish multiple Cr³⁺ sites. The excitation spectra monitored at different emission wavelengths (730, 805 and 900 nm) are shown in Fig. 2g. The three excitation spectra are very different, showing a red-shift and broadening, and the calculated crystal field parameters D_q/B are 2.37, 2.11, and 1.93 respectively, suggesting that the three peaks are contributed by three different luminescence centers. The three Cr³⁺ ion luminescence centers correspond to Cr^I, Cr^V and Cr^IV, and their octahedron coordination Tanabe–Sugano energy levels are shown in Fig. 2h. Among them, Cr^I is in the crystal field where it is located in the strongest of the three sites, corresponding to the [Ga^IO₆] octahedron with the shortest bond length, emitting NIR light centered at 730 nm, while Cr^IV and Cr^V are in the weak crystal-field environment corresponding to the [Ga^IVO₆] and [Ga^VO₆] octahedra, emitting NIR light of long wavelengths centered at 900 nm and 805 nm. Additionally, the fluorescence decay curves of the three aforementioned peaks were monitored and are illustrated in Fig. S5.† The fluorescence lifetimes of peaks I, II and III were 214.29 μs, 143.37 μs and 132.94 μs, respectively, exhibiting a significant discrepancy, which further substantiates the presence of three distinct luminescence centers.

Notably, the PL spectra exhibited a large red-shift (110 nm) and the FWHM of the spectra unfolded dramatically with the doping concentration, which may be related to the tendency of Cr³⁺ ions to occupy different luminescence centers. To further investigate this phenomenon, Gaussian peak splitting was performed on the PL spectra of LMG:xCr³⁺ (x = 0.05–0.5) solid solutions (shown in Fig. S6†). The PL spectra of all samples can be well fitted by the three Gaussian peaks. The relationship between the luminous intensity of Cr^I, Cr^IV and Cr^V and the concentration of Cr³⁺ is shown in Fig. 2i. The three luminescence centers do not contribute equally to the PL spectrum at different concentrations (Fig. S6e†). When the doping concentration is less than 0.2 mol, Cr^I contributes more to PL than Cr^IV and Cr^V, while when the doping concentration is greater than or equal to 0.2 mol, Cr^IV and Cr^V contribute more to PL than Cr^I, which implies that Cr³⁺ occupies different lattice positions in a selective manner as the doping concentration increases. At higher doping concentrations, Cr³⁺ ions tend to occupy the [Ga^IVO₆] and [Ga^VO₆] sites of the weak crystal field, exhibiting red-shift and broadening in the spectra. This phenomenon can be attributed to the fact that the various octahedron structures present in the matrix exhibit varying degrees of stability.^36,37 The superposition of the three luminescent centers shows the best luminescence when x = 0.2 mol, which is consistent with the phenomenon observed by PL spectroscopy.

Enhancement of LMG:Cr³⁺ solid solutions in the short-wave NIR is important for their NIR spectroscopic applications. Inspired by the previous work, the absorption of Yb³⁺ and the emission of Cr³⁺ partially overlap in the NIR region, and the emission band can be broadened by constructing Cr³⁺ → Yb³⁺ energy transfer to improve the luminescence performance of the phosphor.^38–42 Therefore, Yb³⁺ ions were co-doped into LMG:0.2Cr³⁺ phosphors and their luminescence properties and mechanisms were investigated.

The diffuse reflectance spectra of the LMG:0.2Cr³⁺ and LMG:0.2Cr³⁺, 0.05Yb³⁺ samples and the emission spectrum of the LMG:0.2Cr³⁺ sample are depicted in Fig. 3a. The diffuse reflectance spectra of LMG:0.2Cr³⁺,0.05Yb³⁺ samples show absorption bands in the range of 900–1000 nm, which belong to the characteristic absorption of Yb³⁺ ions, and are attributed to the ²F_5/2 → ²F_7/2 transition; no characteristic absorption of Cr⁴⁺ ions, which seriously impairs the phosphor performance, is observed in the diffuse reflectance spectra. In addition, the absorption of Yb³⁺ has a large overlap with the emission spectrum of the LMG:0.2Cr³⁺ sample, suggesting that energy transfer may be constructed in LMG:0.2Cr³⁺.

Fig. 3 (a) Diffuse reflectance spectrum of LMG:0.2Cr³⁺ and LMG:0.2Cr³⁺,0.05Yb³⁺ and PL spectrum of LMG:0.2Cr³⁺. (b) XPS full spectrum of LMG:0.2Cr³⁺,0.05Yb³⁺ on Cr 2p and (c) XPS fine spectrum. (d) PLE and PL spectra of LMG:0.2Cr³⁺ and LMG:0.2Cr³⁺,0.05Yb³⁺. (e) PL spectra of LMG:0.2Cr³⁺,yYb³⁺. (f) Cr³⁺ and Yb³⁺ emission intensity of LMG:0.2Cr³⁺,yYb³⁺ at different Yb³⁺ concentrations. (g) IQE, AE and EQE of LMG:0.2Cr³⁺,0.05Yb³⁺. (h) Lifetime decay curves of LMG:0.2Cr³⁺,yYb³⁺. (i) Energy transfer efficiency (η and η′) versus Yb³⁺ concentration calculated from PL intensity (i) and average lifetime (τ) data, respectively.

Fig. 3b and c show the XPS fine spectra as well as the XPS full-spectrum profiles of the LMG:0.2Cr³⁺,0.05Yb³⁺ samples at the Cr 2p level, respectively. The results show that the peak in the fine spectrum appears at 576.08 eV, corresponding to the 2p^3/2 orbital of the Cr³⁺ ion. No Cr⁴⁺ characteristic peak appeared, which further confirmed the absence of Cr⁴⁺ in the LMG:0.2Cr³⁺,0.05Yb³⁺ phosphor. In addition, the characteristic peaks of La, Mg, Ga, Cr, and Yb elements appeared in the XPS full-spectrum mapping, which further proved the successful synthesis of phosphor samples with LMG as the matrix and Cr³⁺/Yb³⁺ as the luminescence center, in line with the result of EDS mapping. Fig. 3d demonstrates the excitation and emission spectra of the LMG:0.2Cr³⁺ sample and the LMG:0.2Cr³⁺,0.05Yb³⁺ sample. Following the introduction of Yb³⁺, LMG:0.2Cr³⁺,0.05Yb³⁺ exhibited additional strong emission peaks in the 1000–1200 nm range in comparison with the LMG:0.2Cr³⁺ sample, in addition to the emission characteristics of the Cr³⁺ ion. On monitoring the emission wavelengths of the LMG:0.2Cr³⁺,0.05Yb³⁺ samples at 770 nm and 1004 nm, respectively, it was found that the excitation spectra obtained are basically consistent with those of the single-doped samples with Cr³⁺ ions, which proves that an effective energy transfer from the Cr³⁺ ions to the Yb³⁺ ions has occurred.

Fig. 3e shows the PL spectra of the LMG:0.2Cr³⁺,yYb³⁺ phosphor with different Yb³⁺ concentrations under 440 nm excitation, and Fig. 3f demonstrates the trend of the luminescence intensity of the LMG:Cr³⁺,yYb³⁺ phosphor at 770 nm and 1004 nm. When Yb³⁺ ions were introduced, additional emission bands in the range of 1000–1200 nm were observed from the emission spectra in addition to the characteristic emission of Cr³⁺. With the increase of the Yb³⁺ doping concentration, the positions of the two characteristic emission peaks did not change significantly, and the emission intensity of the Yb³⁺ characteristic peaks gradually increased, accompanied by a decrease in the emission intensity of Cr³⁺, proving that there might be an energy transfer between Cr³⁺ → Yb³⁺. Fig. S7a† demonstrates the emission spectra of LMG:0.05Yb³⁺,xCr³⁺ phosphors with different Cr³⁺ concentrations under 440 nm excitation. No characteristic emission was observed from the emission spectra when the concentration of Cr³⁺ ions was 0, which indicated that LMG:0.05Yb³⁺ was not able to produce characteristic emission under 440 nm blue light excitation. With the introduction of Cr³⁺ ions, the characteristic emission of Yb³⁺ ions appeared in addition to the characteristic peaks of Cr³⁺ ions, and the emission intensity of the characteristic peaks of Yb³⁺ gradually increased with the increase of the doping concentration of Cr³⁺, which also indicated that an effective energy transfer from Cr³⁺ ions to Yb³⁺ ions occurred. The quantum efficiency of the solid solution was tested using a BaSO₄ reference sample due to the high stability and reflectivity of BaSO₄ and the absence of fluorescence interference. Fig. 3g illustrates the PL quantum yield of the LMG:0.2Cr³⁺,0.05Yb³⁺ phosphor, which exhibits excellent performance, with an IQE of 94.2% and an EQE of 40.5%. This phenomenon can be attributed to the high EQE observed in the singly doped samples (IQE/EQE = 93.8%/43.0%, shown in Fig. S8†), coupled with the high energy transfer efficiency of Cr³⁺ → Yb³⁺. This efficiency is much better than those of many recently reported related phosphors such as SrLaGa₃O₇:Cr³⁺,Yb³⁺ (IQE = 60.5%),⁴³ SrGe₄O₉:Cr³⁺ (IQE = 66.9%),⁴⁴ RbAl₃P₆O₂₀:Cr³⁺ (IQE = 66.%),⁴⁵etc. In addition, the emission bandwidth of the co-doped Yb³⁺ sample was further broadened compared with that of the single-doped LMG:0.2Cr³⁺ phosphor. This proves that the phosphor is expected to have potential applications in fields such as nondestructive testing.

To further investigate the energy transfer relationship between Cr³⁺ and Yb³⁺ ions, the fluorescence decay curves of all co-doped Yb³⁺ phosphors were tested. The fluorescence decay curves of LMG:0.2Cr³⁺,yYb³⁺ phosphors at 770 nm were monitored under 440 nm excitation, as shown in Fig. 3h, and the fluorescence lifetime of each sample can be fitted by eqn (3). As the concentration of Yb³⁺ ions increased, the fluorescence lifetime values gradually shortened, indicating a significant energy transfer from Cr³⁺ → Yb³⁺, which is the main reason for the decrease in Cr³⁺ luminescence.

In general, the energy transfer efficiency can be calculated based on the changes in Cr³⁺ emission intensity or fluorescence lifetime with increasing Yb³⁺ doping concentration.⁴⁶ The specific calculation formula is as follows:

where I_Cr, I_Cr0, τ_Cr, and τ_Cr0 are the emission intensity and fluorescence lifetime of Cr³⁺ when doped and undoped with Yb³⁺, respectively. The energy transfer efficiencies calculated according to the variation of emission intensity and fluorescence lifetime of Cr³⁺, respectively, are shown in Fig. 2i. Note that the efficiency calculated from the spectral intensity is higher than that calculated from the lifetime. This interesting phenomenon is also observed in many energy transfer systems.^39,46–48 Considering the spectral distribution of Cr³⁺ emission and the choice of measurement wavelength, the lifetime decay curves of LMG:0.2Cr³⁺,yYb³⁺ at 730, 805, and 900 nm wavelengths were monitored, as shown in Fig. S9(a), (b), and (c),† respectively. Calculations show that the energy transfer efficiency is not the same for different luminescence centers (Fig. S9(d)†). Therefore, the energy transfer efficiency calculated by monitoring the lifetime of Cr³⁺ ions on the main peak (770 nm) does not directly reflect the energy transfer efficiency between Cr³⁺ and Yb³⁺, which results in a lower observed energy transfer efficiency than the one calculated from the spectral intensity.^49,50 It is reasonable to calculate the energy transfer efficiency based on the change in spectral intensity, and the energy transfer efficiency from Cr³⁺ to Yb³⁺ ions is estimated to be 71.6% when the doping concentration of Yb³⁺ ions is 0.1 mol.

To further investigate the energy transfer mechanism, based on the Dexter energy transfer formula for multipolar interactions, eqn (6) is obtained:⁵¹

where η₀ and η are the luminescence quantum efficiencies of Cr³⁺ in the absence and presence of Yb³⁺, respectively; C is the total concentration of Cr³⁺ and Yb³⁺, and n = 6, 8, and 10 corresponds to the dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. In general, the values of η and η₀ can be calculated in a simplified way by the ratio of the relevant luminous intensities:⁵²where I_S0 and I_S are the luminescence intensities of Cr³⁺ in the absence and presence of Yb³⁺, respectively. The relationship of I_S0/I_S as a function of C^n/3 (n = 6, 8, 10) is shown in Fig. 4a. The linear behavior of the fit is best when n = 8 (R² = 0.9893), which suggests that the energy transfer from Cr³⁺ to Yb³⁺ in this study is predominantly a dipole–quadrupole mechanism, which is consistent with the previously reported studies.^53,54

Fig. 4 (a) Dependence of I₀/I of Cr³⁺ on C^6/3, C^8/3, and C^10/3. (b) Simplified diagram of the Cr³⁺ → Yb³⁺ energy transfer process.

Fig. 4b demonstrates the schematic energy transfer between Cr³⁺ → Yb³⁺ ions. Under the excitation of 440 nm blue light, the electrons in the ground state of the Cr³⁺ ion are excited to jump to the excited state, and the electrons in the high excited state will first release energy to jump back to the lowest excited state ⁴T₂. Finally, these electrons may release energy through two pathways. One is that the electrons directly return to the ground state ⁴A₂ and emit NIR light, and the other is that the electrons are transferred from the excited state ⁴T₂ of Cr³⁺ to the ²F_5/2 energy level of Yb³⁺, which then emits NIR light of a longer wavelength through the ²F_5/2 → ²F_7/2 energy level leap, resulting in an energy transfer between Cr³⁺ → Yb³⁺.

Temperature-dependent properties

Due to the accumulation of heat within the device, the pc-LED operates at temperatures exceeding 100 °C, which results in a loss of luminescence due to thermal quenching. It is therefore essential to evaluate the PL thermal stability of the target NIR phosphor. The temperature-dependent emission spectra of the LMG:0.2Cr³⁺,0.05Yb³⁺ phosphor in the range of 293–450 K were tested as an example. The temperature-dependent PL spectra as well as the contour plots of its temperature-dependent PL spectra in the temperature range of 293–450 K are given in Fig. 5a and b. The variation curves of the integral intensity of the emission spectra of LMG:0.2Cr³⁺,0.05Yb³⁺ phosphors with temperature are shown in Fig. 5c. The luminescence intensity of the phosphor decreases with increasing temperature due to thermal quenching. The fluorescence emitted by Cr³⁺ and Yb³⁺ was integrated separately, and it was found that the luminescence of Yb³⁺ was consistently better than that of Cr³⁺, indicating that Yb³⁺ has better thermal stability. This is important for long wavelength applications of pc-LED devices.⁵⁵ The thermal stability of LMG:0.2Cr³⁺,0.05Yb³⁺ was further evaluated by calculating the activation energy E_a through eqn (8):⁵⁶where I₀ represents the initial emission intensity, I_T represents the emission intensity at temperature T, K is the Boltzmann constant, C is a constant, and E_a is the activation energy. The activation energy of LMG:0.2Cr³⁺,0.05Yb³⁺ was calculated to be 0.259 eV (Fig. 5d). The large activation energy also reveals excellent thermal quenching resistance.

Fig. 5 (a) Temperature dependent PL spectra of LMG:0.2Cr³⁺,0.05Yb³⁺ in the temperature range of 293–450 K. (b) Temperature dependent PL spectrum contour plot of LMG:0.2Cr³⁺,0.05Yb³⁺. (c) Normalized integral intensity of the LMG:0.2Cr³⁺,0.05Yb³⁺ sample. (d) Plot of ln[(I₀/I_T) − 1] vs. 1/(KT) of the optimal LMG:0.2Cr³⁺,0.05Yb³⁺ sample.

Upon increasing the temperature to 373 K, the luminous intensity of LMG:0.2Cr³⁺,0.05Yb³⁺ remains at 89.3% of its initial intensity. Upon increasing the temperature to 423 K, the luminous intensity of the LMG:0.2Cr³⁺,0.05Yb³⁺ phosphor remains at 72.7% of the initial intensity, demonstrating excellent thermal stability. Table S4† presents the IQE, EQE, thermal stability and photovoltaic efficiency data for a selection of Cr³⁺–Yb³⁺ co-doped NIR-emitting phosphors that have been previously reported in the literature. The results demonstrate that the prepared LMG:0.2Cr³⁺,0.05Yb³⁺ solid solution exhibits significant advantages in terms of quantum efficiency and thermal stability properties.

Applications of the NIR pc-LED

The optimized LMG:0.2Cr³⁺,0.05Yb³⁺ NIR phosphor was packaged with a commercial 440 nm blue chip for device packaging. Fig. 6a shows the EL curves of the NIR pc-LED device at different driving currents. The images of the NIR pc-LED device before and after energization are shown in the insets of Fig. 6a, respectively. The results demonstrate that the LMG:0.2Cr³⁺,0.05Yb³⁺ phosphor is capable of producing efficacious long-wavelength emitted light within the wavelength range of 1000–1200 nm, which is of considerable importance for practical spectral non-destructive testing. With the increase of the driving current from 20 mA to 350 mA, the luminous intensity of the NIR pc-LED device increased continuously and there was no light saturation phenomenon, with 9.9% photoelectric conversion efficiency at 100 mA, and the NIR output power was as high as 84.5 mW at 350 mA (Fig. 6b), which indicated that the LMG:0.2Cr³⁺,0.05Yb³⁺ phosphors in the high-throughput NIR pc-LED devices have good potential for application. Table S5† lists the detailed data of the pc-LED output power and efficiency under different current drives.

Fig. 6 (a) EL spectra of the fabricated NIR pc-LED prototype device. (b) The output power and photoelectric conversion efficiency. (c) Photographs obtained under white light and NIR light, respectively. (d) Relationship between the temperature of leaves and the duration of NIR pc-LED exposure.

The potential applications of the NIR pc-LED in night vision lighting and non-destructive testing were demonstrated. A pc-LED encapsulated with LMG:0.02Cr³⁺,0.05Yb³⁺ phosphors was used as a NIR light source, and a NIR camera was used to take images of plants and boxes with text under daylight and in dark environments with or without NIR light. Under NIR light, succulents can be clearly photographed and imaged, and text and graphics obscured by filters are clearly visible (Fig. 6c). The freshness of the leaves was evaluated through the utilization of the NIR pc-LED and thermal imaging instrumentation. Leaves #1, #2 and #3 were the same leaves that had been dried at room temperature for 1, 2 and 3 days, respectively, following harvesting. It was observed that the temperature of the leaves increased gradually with increasing irradiation time. The temperature at the apex of #2 and #3 was markedly lower than that of the mid-region of the leaves, due to the effects of drying and dehydration, in comparison with #1 (Fig. 6d). This discrepancy may be attributed to the variability in water and sugar contents among leaves at varying degrees of freshness. This can be attributed to the differing rates of absorption of NIR light by different functional groups.⁵⁷ It is noteworthy that NIR pc-LEDs do not significantly elevate the temperature of the leaves, and the resulting temperature remains harmless. This phenomenon was also observed in grape berries (Fig. S10a†). These findings have significant implications for the integration of NIR spectroscopy into mobile phones or smart wearable devices. Fig. S10b† shows the operating temperature of the fabricated pc-LED versus the driving current and the thermograms after 5 min of operation driven by a 50 mA–300 mA operating current, respectively. When the operating current was increased from 50 mA to 300 mA, the pc-LED device emitted brighter light, while the operating temperature rose from 33.5 °C to 76.1 °C, but the temperature increase is within acceptable limits and the device still operated stably. These results demonstrate that the synthesized LMG:0.02Cr³⁺,0.05Yb³⁺ phosphors have a wide range of applications in the field of nondestructive testing as well as imaging techniques.

Conclusion

In summary, the synthesis of the LMG:Cr³⁺,Yb³⁺ broadband NIR phosphor with excellent comprehensive performance was achieved by constructing an efficient energy transfer between Cr³⁺ and Yb³⁺. Under 440 nm excitation, the LMG:Cr³⁺,Yb³⁺ phosphor exhibits a broad emission spectrum from 650 to 1200 nm, shows strong emission near 1000 nm, and exhibits high internal/external quantum efficiencies of 94.8%/40.5%. In addition, it has excellent thermal stability (89.3%@373 K and 72.7%@423 K). Finally, a NIR pc-LED device was fabricated by combining a 440 nm blue LED chip with the LMG:Cr³⁺,Yb³⁺ phosphor, with an output power of 84.5 mW at 350 mA and a photovoltaic conversion efficiency of 9.9% at 100 mA, which was successfully applied to non-destructive testing and bioimaging. These results indicate that this novel compound has great potential for future NIR pc-LED applications. This work provides a reference for the development of new efficient broadband NIR phosphors.

Data availability

Some of the data that support the findings of this study are available in the ESI† of this article. The remaining data are available from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (No. 12274144, 22361132525, and 52472161), the Guangdong Provincial Science and Technology Project (No. 2022A1515010229), the Russian Science Foundation (grant 24-43-00006), and the Undergraduate Innovation and Entrepreneurship Training Program grant for Zixi Chen.

References

1. K. Elzbieciak-Piecka, M. Suta and L. Marciniak, Structurally induced tuning of the relative sensitivity of LaScO₃:Cr³⁺ luminescent thermometers by co-doping lanthanide ions, Chem. Eng. J., 2021, 421, 129757.
2. S. Liu, H. Cai, S. Zhang, Z. Song, Z. Xia and Q. Liu, Site engineering strategy toward enhanced luminescence thermostability of a Cr-doped broadband NIR phosphor and its application, Mater. Chem. Front., 2021, 5, 3841–3849.
3. X. Wu, Q. Lin, Y. Li, J. Peng, W. You, D. Huang and X. Ye, Phase transition and Nd³⁺/Pr³⁺ co-doping induced enhancement of NIR emission in ScP₃O₉: Cr³⁺ phosphor, Mater. Res. Bull., 2024, 173, 112676.
4. K. Gu and H. Zhong, A general methodology to measure the light-to-heat conversion efficiency of solid materials, Light: Sci. Appl., 2023, 12, 120.
5. C. Li and J. Zhong, Highly Efficient Broadband Near-Infrared Luminescence with Zero-Thermal-Quenching in Garnet Y₃In₂Ga₃O₁₂:Cr³⁺ phosphors, Chem. Mater., 2022, 34, 8418–8426.
6. A. Balhara, S. K. Gupta, B. Modak, A. K. Yadav, B. S. Naidu and K. Sudarshan, Broadband Shortwave Infrared-Emitting Cr³⁺- and Ni²⁺-Codoped Y₃Al₂Ga₃O₁₂Phosphor with Excellent Thermal Stability for Multifunctional Applications, ACS Appl. Mater. Interfaces, 2025, 17, 1509–1521.
7. B. B. Srivastava, S. K. Gupta, S. Mohan and Y. Mao, Molten-Salt-Assisted Annealing for Making Colloidal ZnGa₂O₄:Cr Nanocrystals with High Persistent Luminescence, Chem. – Eur. J., 2021, 27, 11398–11405.
8. B. B. Srivastava, S. K. Gupta and Y. Mao, Remarkable enhancement of photoluminescence and persistent luminescence of NIR emitting ZnGa₂O₄:Cr³⁺ nanoparticles, CrystEngComm, 2020, 22, 2491–2501.
9. D. Liu, G. Li, P. Dang, Q. Zhang, Y. Wei, L. Qiu, H. Lian, M. Shang and J. Lin, Valence conversion and site reconstruction in near-infrared-emitting chromium-activated garnet for simultaneous enhancement of quantum efficiency and thermal stability, Light: Sci. Appl., 2023, 12, 248.
10. F. Zhu, Y. Gao, C. Zhao, J. Pi and J. Qiu, Achieving Broadband NIR-I to NIR-II Emission in an All-Inorganic Halide Double-Perovskite Cs₂NaYCl₆:Cr³⁺Phosphor for Night Vision Imaging, ACS Appl. Mater. Interfaces, 2023, 15, 39550–39558.
11. T. Lang, Q. Zhao, X. Jing, G. Guan, S. Fang, Q. Qiang, L. Peng, T. Han, A. N. Yakovlev and B. Liu, Highly efficient near-infrared solid solution phosphors with excellent thermal stability and tunable spectra for pc-LED light sources toward NIR spectroscopy applications, Phys. Chem. Chem. Phys., 2023, 25, 25985–25992.
12. Z. Jia, C. Yuan, Y. Liu, X.-J. Wang, P. Sun, L. Wang, H. Jiang and J. Jiang, Strategies to approach high performance in Cr³⁺-doped phosphors for high-power NIR-LED light sources, Light: Sci. Appl., 2020, 9, 86.
13. L. Fang, L. Zhang, H. Wu, H. Wu, G. Pan, Z. Hao, F. Liu and J. Zhang, Efficient Broadband Near-Infrared CaMgGe₂O₆:Cr³⁺Phosphor for pc-LED, Inorg. Chem., 2022, 61, 8815–8822.
14. Z. Wu, J. Xiang, C. Chen, Z. Li, X. Zhou, Y. Jin and C. Guo, Multi-site occupancy high-efficient Mg₂LaTaO₆:Cr³⁺ phosphor for application in broadband NIR pc-LEDs, Ceram. Int., 2024, 50, 5242–5249.
15. S. K. Gupta, K. Sudarshan, A. Balhara, S. K. Shaw, J. Bahadur and N. K. Prasad, Highly uniform microspheres of broadband near-infrared-emitting Ba(Hf_1(-xCr_x)O₃ perovskite phosphor, Solid State Commun., 2024, 380, 115443.
16. S. Liu, Y. Zheng, D. Peng, J. Zhao, Z. Song and Q. Liu, Near-Infrared Mechanoluminescence of Cr³⁺Doped Gallate Spinel and Magnetoplumbite Smart Materials, Adv. Funct. Mater., 2023, 33, 2209275.
17. S. Wu, B. Xiao, Y. Xiao, P. Shao, Y. Wang and P. Xiong, Cr³⁺-activated broadband near-infrared mechanoluminescence in garnet compound, Nano Energy, 2023, 116, 108811.
18. C. Yang, D. Zheng, X. Zou, X. Dai, B. Tang, M. S. Molokeev, X. Zhang, H. Zhang, Y. Liu and B. Lei, Highly-Efficiency Far-Red Emission in Cr³⁺Activated Ca_1.8Mg_1.2Al₂Ge₃O₁₂ toward Plant Precise Lighting, Adv. Opt. Mater., 2024, 12, 2303235.
19. H. Zhu, Y. Li, Y. Xi, C. Xin, C. Zhou, Z. Yang, L. Ruan, Y. Li, Y. Peng, M. S. Molokeev, A. Zolotov, J. Wang, Z. Zhou and M. Xia, Abnormal Lattice Shrinkage, Site Occupation, and Luminescent Properties of Cr³⁺-Activated β-Al₂O₃ Structure Phosphors, Laser Photonics Rev., 2025, 19, 2401089.
20. D. Wang, L. Yan, G. Zhu, S. Ma, N. Zhou, S. Li, Z. Li, Y. Cong, X. Bai, X. Han and B. Dong, Achieving Flat Ultra-Broadband NIR Emission in Cr³⁺Doped Garnet-Type Phosphors Enabled by Structural Regulation Toward Multi-Functional Spectroscopy Applications, Laser Photonics Rev., 2025, 19, 2401256.
21. C. Zhong, Y. Xu, X. Wu, S. Yin, X. Zhang, L. Zhou and H. You, High Output Power and High Quantum Efficiency in Novel NIR Phosphor MgAlGa_0.7B_0.3O₄:Cr³⁺with Profound FWHM Variation, Adv. Mater., 2024, 36, 2309500.
22. L. Jiang, L. Zhang, X. Jiang, G. Lv and Y. Su, Multiple lattice sites occupied AlTaO₄:Cr³⁺phosphor for luminescence ratiometric thermometry and NIR light source, J. Alloys Compd., 2024, 970, 172544.
23. Y. Sun, M. Shang, Y. Wang, Y. Zhu, X. Xing, P. Dang and J. Lin, The ultra-wideband near-infrared luminescence properties and applications of K₂SrGe₈O₁₈:Cr³⁺phosphor, Ceram. Int., 2023, 49, 32619–32627.
24. Y. Shi, Z. Wang, J. Peng, Y. Wang, S. He, J. Li, R. Li, G. Wei, Y. Yang and P. Li, Achieving the ultra-broadband near-infrared La₃SnGa₅O₁₄:Cr³⁺phosphor via multiple lattice sites occupation for biological nondestructive detection and night-vision technology, Mater. Today Adv., 2022, 16, 100305.
25. S. Miao, Y. Liang, Y. Zhang, D. Chen and X. Wang, Blue LED-Pumped Broadband Short-Wave Infrared Emitter Based on LiMgPO₄:Cr³⁺,Ni²⁺Phosphor, Adv. Mater. Technol., 2022, 7, 2200320.
26. S. He, L. Zhang, J. Zhang, Z. Hao, H. Wu, H. Wu, Y. Luo, G. Pan, F. Liu and J. Zhang, Cr³⁺and Nd³⁺co-activated garnet phosphor for NIR super broadband pc-LED application, Mater. Res. Bull., 2022, 151, 111797.
27. M. Shi, Q. Shao, L. Yao, S. Yu, Y. Dong and J. Jiang, Molten Salt Synthesis of Broad-Band Near-Infrared InBO₃:Cr³⁺Submicron Phosphor and Its Luminescent Enhancement by Lanthanide Ion Codoping, Inorg. Chem., 2022, 61, 12275–12283.
28. Q. Zhang, D. Liu, Z. Wang, P. Dang, H. Lian, G. Li and J. Lin, LaMgGa₁₁O₁₉:Cr³⁺,Ni²⁺as Blue-Light Excitable Near-Infrared Luminescent Materials with Ultra-Wide Emission and High External Quantum Efficiency, Adv. Opt. Mater., 2023, 11, 2202478.
29. J. Wang, C. Gong, S. Yang, Q. Zhu, X. Wang and J.-G. Li, Cr³⁺activated Na3RESi3O9(RE = Y, Lu, Sc) silicate broadband near-infrared phosphors for luminescence towards NIR-II region via a multi-site occupancy strategy, J. Rare Earths, 2024, 42, 1447–1457.
30. C. Wang, X. Zhang, C. Zhong, X. Wu, Y. Xu, S. Yin, Q. Yang, L. Zhou and H. You, Enhanced quantum efficiency and thermal stability by crystal-field engineering in a Y(Ga,Al)₃(BO₃)₄:Cr³⁺,Yb³⁺phosphor for diverse short-wave infrared applications, J. Mater. Chem. C, 2024, 12, 3515–3525.
31. G. Zheng, C. Lou, Z. Yuan, W. Xiao, L. Shang, J. Zhong, M. Tang and J. Qiu, Rare-Metal-Free Ultrabroadband Near-Infrared Phosphors, Adv. Mater., 2025, 37, 2415791.
32. A. Balhara, S. K. Gupta, B. Modak, M. Abraham, A. K. Yadav, H. V. Annadata, S. Das, N. S. Rawat and K. Sudarshan, Synergy between structural rigidity and cluster defects in a bright near-infrared Cr³⁺-based phosphor for excellent thermal stability and long afterglow, J. Mater. Chem. C, 2024, 12, 9716–9732.
33. H. Zhang, J. Zhong, F. Du, L. Chen, X. Zhang, Z. Mu and W. Zhao, Efficient and Thermally Stable Broad-Band Near-Infrared Emission in a KAlP₂O₇:Cr³⁺ Phosphor for Nondestructive Examination, ACS Appl. Mater. Interfaces, 2022, 14, 11663–11671.
34. W. Zhang, C. Wang, X. Qiao and X. Fan, Simultaneous enhancement of red luminescence and thermal stability of a novel red phosphor: A Li⁺-Eu³⁺co-occupied site strategy in BaAl₂Ge₂O₈lattices, J. Rare Earths, 2024, 42, 1846–1854.
35. W. Chen, W. Li, X. Zhang, C. Ma, Z. Xia and B. Lei, Carbon dots embedded in lead-free luminescent metal halide crystals toward single-component white emitters, Sci. China Mater., 2022, 65, 2802–2808.
36. S. Liu, J. Du, Z. Song, C. Ma and Q. Liu, Intervalence charge transfer of Cr³⁺-Cr³⁺aggregation for NIR-II luminescence, Light: Sci. Appl., 2023, 12, 181.
37. C. Zhong, Y. Xu, X. Wu, S. Yin, X. Zhang, L. Zhou and H. You, High Output Power and High Quantum Efficiency in Novel NIR Phosphor MgAlGa_0.7B_0.3O₄:Cr³⁺with Profound FWHM Variation, Adv. Mater., 2024, 36, 2309500.
38. L. Yao, Q. Shao, S. Han, C. Liang, J. He and J. Jiang, Enhancing Near-Infrared Photoluminescence Intensity and Spectral Properties in Yb³⁺Codoped LiScP₂O₇:Cr³⁺, Chem. Mater., 2020, 32, 2430–2439.
39. Y. Zhang, S. Miao, Y. Liang, C. Liang, D. Chen, X. Shan, K. Sun and X.-J. Wang, Blue LED-pumped intense short-wave infrared luminescence based on Cr³⁺-Yb³⁺-co-doped phosphors, Light: Sci. Appl., 2022, 11, 136.
40. M. U. Dumesso, W. Xiao, G. Zheng, E. T. Basore, M. Tang, X. Liu and J. Qiu, Efficient, Stable, and Ultra-Broadband Near-Infrared Garnet Phosphors for Miniaturized Optical Applications, Adv. Opt. Mater., 2022, 10, 2200676.
41. D. Xu, Q. Zhang, X. Wu, W. Li and J. Meng, Synthesis, luminescence properties and energy transfer of Ca₂MgWO₆:Cr³⁺,Yb³⁺phosphors, Mater. Res. Bull., 2019, 110, 135–140.
42. Z. Lu, S. Chen, Y. Liu, C. Yuan, R. Li, P. Sun, Z. Luo, Z. Liu and J. Jiang, LiGaP₂O₇:Cr³⁺, Yb³⁺phosphors for broadband NIR LEDs toward multiple applications, J. Alloys Compd., 2023, 956, 170311.
43. Y. Yang, W. Lü, X. Kang, Z. Zhu, Q. Pan and Q. Zeng, Broadening near-infrared emission and enhancing thermal stability of Cr³⁺-activated SrLaGa₃O₇ phosphors via Yb³⁺co-doping, J. Lumin., 2025, 280, 121098.
44. H. Zheng, Z. Shao, F. Ling, L. Li, G. Xiang, Y. Wang and X. Zhou, Broadband near-infrared phosphor SrGe₄O₉:Cr³⁺ for NIR-pc-LED applications, Ceram. Int., 2025 , S0272884225006959.
45. X. Wu, D. Huang, Q. Lin, L. Han, W. You, J. Zhu and X. Ye, Thermally Stable Broadband Cr³⁺-Doped RbAl₃P₆O₂₀ Phosphor for Near-Infrared Spectroscopy Applications, Inorg. Chem., 2025, 64, 3476–3484.
46. S. Wang, Z. Lyu, D. Sun, S. Shen and H. You, Tailoring the Cr³⁺Emission via Combinatorial Management of Crystal-Field and Energy Transfer for Multiple NIR LEDs, Adv. Opt. Mater., 2024, 12, 2303140.
47. A. D. Sontakke, A. J. Van Bunningen, F. T. Rabouw, S. Meijers and A. Meijerink, Unraveling the Eu²⁺→ Mn²⁺Energy Transfer Mechanism in w-LED Phosphors, J. Phys. Chem. C, 2020, 124, 13902–13911.
48. Y. Xiao, Z. Hao, L. Zhang, W. Xiao, D. Wu, X. Zhang, G.-H. Pan, Y. Luo and J. Zhang, Highly Efficient Green-Emitting Phosphors Ba₂Y₅B₅O₁₇with Low Thermal Quenching Due to Fast Energy Transfer from Ce³⁺to Tb³⁺, Inorg. Chem., 2017, 56, 4538–4544.
49. M. U. Dumesso, W. Xiao, G. Zheng, E. T. Basore, M. Tang, X. Liu and J. Qiu, Efficient, Stable, and Ultra-Broadband Near-Infrared Garnet Phosphors for Miniaturized Optical Applications, Adv. Opt. Mater., 2022, 10, 2200676.
50. Y. Liu, S. He, D. Wu, X. Dong and W. Zhou, Broadband NIR Garnet Phosphors with Improved Thermal Stability via Energy Transfer, ACS Appl. Electron. Mater., 2022, 4, 643–650.
51. S. Miao, Y. Liang, R. Shi, W. Wang, Y. Li and X.-J. Wang, Broadband Short-Wave Infrared-Emitting MgGa₂O₄:Cr³⁺, Ni²⁺Phosphor with Near-Unity Internal Quantum Efficiency and High Thermal Stability for Light-Emitting Diode Applications, ACS Appl. Mater. Interfaces, 2023, 15, 32580–32588.
52. A. A. Melentsova, O. A. Lipina, A. Yu. Chufarov, A. P. Tyutyunnik and V. G. Zubkov, Energy transfer mechanism in infrared-emitting NaYGeO₄:Tm³⁺, Ho³⁺phosphors, Ceram. Int., 2024, 50, 49545–49551.
53. S. Wang, Z. Lyu, D. Sun, S. Shen and H. You, Tailoring the Cr³⁺Emission via Combinatorial Management of Crystal-Field and Energy Transfer for Multiple NIR LEDs, Adv. Opt. Mater., 2024, 12, 2303140.
54. S. Zhao, Z. Mu, L. Lou, S. Yuan, M. Liao, Q. Lin, D. Zhu and F. Wu, Broadening and enhancing emission of Cr³⁺simultaneously by co-doping Yb³⁺in Ga_1.4In_0.6SnO₅, J. Rare Earths, 2023, 41, 1895–1903.
55. C. Jin, R. Li, Y. Liu, L. Zhang, J. Zhang, P. Sun, Z. Luo and J. Jiang, Efficient and Stable Gd₃Ga₅O₁₂:Cr³⁺Phosphors for High-Performance NIR LEDs, Adv. Opt. Mater., 2023, 11, 2300772.
56. Y. Zhang, Z. Gao, Y. Li, S. Chen, M. Han, J. Li, Q. Zhang, Y. Shen, D. Deng and S. Xu, Broadband La₂LiSbO₆: Cr³⁺Phosphors with Double Luminescent Centers for NIR pc-LEDs, Inorg. Chem., 2023, 62, 17371–17381.
57. D. Huang, H. Zhu, Z. Deng, H. Yang, J. Hu, S. Liang, D. Chen, E. Ma and W. Guo, A highly efficient and thermally stable broadband Cr³⁺-activated double borate phosphor for near-infrared light-emitting diodes, J. Mater. Chem. C, 2021, 9, 164–172.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00215j

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