Tunable luminescence via Cr³⁺–Yb³⁺/Nd³⁺ energy transfer in Cr³⁺ and Yb³⁺/Nd³⁺ coactivated NIR phosphors for non-destructive analysis

Abstract

For efficient near-infrared (NIR) emitters, tuning their luminescence properties is important for their extended optical applications. A fundamental study of garnet-type inorganics has led to the development of a new NIR phosphor Y₃ScAl₄O₁₂:Cr³⁺ (YSAO:Cr³⁺), exhibiting emission in the range of 600 to 950 nm, an internal quantum efficiency (IQE) of 84.01%, and good thermal stability at 423 K (89.7% of that at room temperature). Doping with Yb³⁺/Nd³⁺ ions helps achieve an efficient energy transfer from the Cr³⁺ to Yb³⁺/Nd³⁺ ions with excellent thermal stability. NIR pc-LED devices prepared using YSAO:Cr³⁺,Yb³⁺ and YSAO:Cr³⁺,Nd³⁺ can achieve high NIR output powers of 46.37 and 42.82 mW at 100 mA and photoconversion efficiencies of 20.27 and 18.84% at a driving current of 20 mA, respectively. Furthermore, the NIR pc-LED package demonstrated excellent capability for penetrating biological tissues, effectively distinguishing between pork, chicken, and beef. These results suggest that the phosphors can be utilized for non-destructive monitoring applications.

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1. Introduction

NIR luminescent materials have extensive applications across fields such as night vision, remote control, bio-imaging, food composition analysis, iris recognition, optical communication, targeted therapy, and tumor diagnosis due to their emission spectra spanning various NIR intervals. The emergence of high-power broadband NIR light sources based on laser-excited phosphors can enable the above-mentioned wide range of applications compared to conventional tungsten halogen and incandescent lamps.^1–3 Specifically, non-destructive detection of organic compounds can be accomplished by analyzing the distinct spectral features of C–H, O–H, and N–H bonds within the short-wave NIR range (700–1100 nm). This spectral range is enabled by NIR phosphor-converted light-emitting diodes (pc-LEDs), which offer environmentally friendly, cost-effective, long-lasting, and highly stable solutions, making them ideal NIR light sources for numerous practical applications. The essential component of these devices is the phosphor, and selecting a suitable phosphor is crucial for advancing NIR pc-LED technology.^4–6

In recent years, various near-infrared phosphors doped with different ions have been developed. Eu²⁺ has an f–d electronic configuration and exhibits broad emission properties, and Zhiguo Xia's team reported a novel Eu²⁺-activated broadband NIR-emitting phosphor BaSrGa₄O₈:Eu²⁺ with a tunable emission spectrum achieved by adjusting the Ba/Sr atomic ratio.⁷ However, its emission peaks usually fall in the deep red region, leading to insufficient emission in the longer wavelength range. In contrast, Cr³⁺ ion-doped luminescent materials have attracted much attention from researchers due to their tunable emission in the near-infrared region. Cr³⁺ ions possess a distinct structure typical of transition metal ions, in which the outermost electron layer lacks full electron occupancy, making them sensitive to the influence of the local structure. Within the crystal field environment of an intermediate state, characterized by Dq/B values close to 2.3, the energy levels of ²E and ⁴T₂ intersect, resulting in orbital mixing.^8,9 This intermediate crystal field environment induces both strong and weak crystal fields in the Cr³⁺ activated system, manifesting sharp narrowband and broadband emissions in the photoluminescence (PL) spectrum. The garnet crystal structure, known for its dense nature, stands out as an exceptional host for Cr³⁺ doping compared with other matrices. This garnet system exhibits remarkable luminescence efficiency and thermal stability, making it a preferred choice for constructing broadband near-infrared (NIR) phosphors.¹⁰ In the garnet structure (A₃B₂C₃O₁₂), the B³⁺ site offers a hexagonal octahedral crystal field environment. Substituting B³⁺ with Cr³⁺ typically yields near-infrared luminescence within the 650–950 nm range. However, there is often a necessity to develop luminous materials that emit at longer wavelengths beyond 980 nm.

Materials emitting within the first (650–900 nm) and second (1000–1350 nm) biological windows garner significant attention due to their broad applications in biological imaging and instrumentation.¹¹ Ln³⁺ ions, notably Yb³⁺ and Nd³⁺, exhibit longer-wavelength near-infrared emission capabilities.¹² Yb³⁺ ions, with a single excited state energy level at ²F_5/2, emit near-infrared light around 1000 nm, while Nd³⁺ ions offer absorption spanning from UV to NIR, emitting at 1060 nm (⁴F_3/2 → ⁴I_11/2), meeting the requirements for in vitro optical imaging.¹³ However, the limitations of NIR luminescent materials stem from narrow emission bandwidths and excitation peak mismatches with blue chips. Addressing this challenge involves leveraging energy transfer between Cr³⁺ and Ln³⁺ ions. Codoping Cr³⁺ and Ln³⁺ ions enables continuous emission bands in longer wavelength ranges, thereby enhancing the NIR emission efficiency. Recent research has utilized Cr³⁺ to sensitize Yb³⁺ or Nd³⁺ ions, achieving NIR emissions within the 650–1100 nm range. He et al. reported Cr³⁺ and Yb³⁺ codoped Ca₂LuZr₂Al₃O₁₂ broadband near-infrared phosphors by the energy transfer from Cr³⁺ to Yb³⁺ ions, and Yb³⁺ doping improved its thermal stability.¹⁴ Wang et al. reported the energy transfer via the Cr³⁺ to Nd³⁺ ions in Ca₃In₂Ge₃O₁₂:0.07Cr³⁺,0.15Nd³⁺.¹⁵ These results reveal that the garnet structure matrix codoped with Cr³⁺ and Yb³⁺/Nd³⁺ is an ideal choice for investigating NIR luminescent phosphors with excellent thermal stability.

In this study, we report a YSAO:Cr³⁺ near-infrared phosphor with high luminescence efficiency (IQE/EQE = 88.4%/39.17%) and good thermal stability (89.7%/150 °C). The introduction of Yb³⁺/Nd³⁺ into YSAO:Cr³⁺ forms YSAO:Cr³⁺,Yb³⁺/YSAO:Cr³⁺,Nd³⁺, achieving an efficient energy transfer from the Cr³⁺ to Yb³⁺/Nd³⁺ ions, respectively. YSAO:Cr³⁺,Yb³⁺ and YSAO:Cr³⁺,Nd³⁺ have excellent thermal stability. The energy transfer from the Cr³⁺ to Yb³⁺/Nd³⁺ ions was analyzed by studying the luminescence properties of the material. Near-infrared pc-LEDs were obtained by encapsulating phosphors with 450 nm blue LED chips, demonstrating possible applications in non-destructive analysis and night vision.

2. Experimental

2.1. Sample synthesis

A series of YSAO:xCr³⁺, YSAO:0.10Cr³⁺,yYb³⁺ and YSAO:0.10Cr³⁺,yNd³⁺ was synthesized by high temperature solid-state reactions. The starting materials include Y₂O₃ (99.999%), Sc₂O₃ (99.99%), Al₂O₃ (99.99%), Cr₂O₃ (99.99%), Yb₂O₃ (99.99%), Nd₂O₃ (99.99%) and H₃BO₃ (A.R.). H₃BO₃ was used as a flux with a doping amount of 2% of the total mass. After weighing the ingredients according to stoichiometric proportions, the mixtures were ground for 20 minutes in an agate mortar. The resulting mixtures were transferred to alumina crucibles and sintered at 1773 K for 4 hours under nitrogen. Finally, the samples were reground for further characterization.

2.2 Characterization

X-ray diffraction (XRD) analysis of the powder samples was performed using an AXS D8 Advance instrument from Bruker (Germany) with CuKα radiation (λ = 0.15405 nm). The instrument's measurement range was set between 10° and 80° with a step size of 0.02° and a dwell time of 1 second per step. The voltage and current were maintained at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) was conducted using an AXIS SUPRA+ model spectrometer from Shimadzu to analyze the surface elements and chemical states of the materials. High-resolution field emission scanning electron microscopy (HRFE-SEM) (model JSM-IT800, JEOL, Japan) was used to obtain energy-dispersive X-ray spectroscopy (EDS) data for investigating the microstructure and morphology of the samples. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra were recorded using an FLS1000 fluorescence spectrometer from Edinburgh Instruments, which is outfitted with a continuous xenon lamp and visible/NIR photomultiplier tube (PMT) detectors (Hamamatsu P9289P and R5509), with which the thermal stability of the samples and fluorescence lifetimes were also assessed. Quantum efficiency (QE) was determined using an integrating sphere combined with a spectrometer. The optoelectronic performance of the NIR pc-LEDs was assessed and analyzed with the aid of an integrating sphere spectral radiometer system (HASS-2000, 350–1650 nm, Everfine).

2.3 NIR pc-LED device fabrication

To fabricate a NIR pc-LED, YSAO:Cr³⁺,Yb³⁺ and YSAO:Cr³⁺,Nd³⁺ phosphors were thoroughly mixed with silicone resins A and B in a 1 : 1 ratio. The resulting mixture was then applied to the top surface of a 450 nm blue InGaN chip. The final optical device was cured at 100 °C for 1 hour. The fabricated near-infrared pc-LED was driven under currents ranging from 20 to 150 mA.

3. Results and discussion

As shown in Fig. 1a, the space group of YSAO is Ia3̄d (230). The crystal structure of YSAO contains three types of polyhedra: the [AlO₄] tetrahedron, the [(Al/Sc)O₆] octahedron, and the [YO₈] dodecahedron. The [YO₈] dodecahedra combine to form triangular units through edge-sharing, with these triangular units interfacing with one face of the [(Al/Sc)O₆]octahedra. The corresponding faces of the [(Al/Sc)O₆] octahedra is in contact with other triangular units of the dodecahedra. The [AlO₄] tetrahedra connect to the dodecahedra through edge-sharing in a rotational manner, thereby establishing a complex three-dimensional network.¹⁶ Considering the ionic radius and valence, Cr³⁺ ions replace the Sc³⁺/Al³⁺ sites and the Y³⁺ sites are suitable for Nd³⁺ and Yb³⁺ ions. Fig. 1b and c depict the XRD patterns of YSAO:xCr³⁺ (0.04 ≤ x ≤ 0.16), YSAO:0.10Cr³⁺,0.15Yb³⁺ and YSAO:0.10Cr³⁺,0.03Nd³⁺, respectively. The diffraction peaks are in excellent correspondence with the standard pattern of Y₃Al₅O₁₂ (JCPDS 01-079-1891), suggesting the formation of a pure phase. The lattice expansion caused by the substitution of Sc³⁺ (CN = 6, r = 0.745 Å) for Al³⁺ (CN = 8, r = 0.535 Å) results in a shift of the position of the diffraction peaks to smaller angles. To further confirm the phase purity of the samples, YSAO:0.10Cr³⁺, YSAO:0.10Cr³⁺,0.15Yb³⁺ and YSAO:0.10Cr³⁺,0.03Nd³⁺ samples were refined as shown in Fig. 1d, e and f. The fitting parameters for the samples YSAO:0.10Cr³⁺, YSAO:0.10Cr³⁺,0.15Yb³⁺, and YSAO:0.10Cr³⁺,0.03Nd³⁺ are R_wp = 7.05%, R_p = 4.725%; R_wp = 6.47%, R_p = 4.29%; and R_wp = 7.58%, R_p = 5.62%, respectively. All refinement reliability parameters are below 10%, indicating that the samples possess good phase purity. These results reveal that Cr³⁺, Yb³⁺, and Nd³⁺ ions were successfully doped into the YSAO matrix without forming any impurity phases. The corresponding results of the refinement are presented in Table S1.† A representative YSAO:0.10Cr³⁺ sample was analyzed by SEM to investigate its morphological characteristics, as illustrated in Fig. S2.† Its microstructure consists of irregular fragments of varying sizes between 2 and 5 μm. The EDS elemental maps in Fig. 1g show that elements such as Y, Sc, Al, O and Cr are evenly distributed in the phosphor particles.

Fig. 1 (a) Crystal structure of the YSAO phase; (b) XRD patterns of YSAO:xCr³⁺ (0.04 ≤ x ≤ 0.16); (c) XRD patterns of YSAO:0.10Cr³⁺, YSAO:0.10Cr³⁺,0.15Yb³⁺ and YSAO:0.10Cr³⁺,0.03Nd³⁺; Rietveld refinements of (d) YSAO:0.10Cr³⁺, (e) YSAO:0.10Cr³⁺,0.03Nd³⁺, and (f) YSAO:0.10Cr³⁺,0.15Yb³⁺; (g) SEM map of YSAO:0.10Cr³⁺ and EDS elemental distribution.

The high-resolution spectra corresponding to the deconvoluted peaks of Y-3d, Al-2p, Sc-2p, C-1s, O-1s, and Cr-2p elements are shown in Fig. S3.† The elements Y, Sc, Al, O, and Cr were observed in the XPS spectrum of YSAO:0.10Cr³⁺ (in Fig. 2a and Fig. S4†). The peaks at 576.8 and 586.5 eV are ascribed to the 2p_3/2 and 2p_1/2 electrons of Cr³⁺, respectively, which are consistent with the reported binding energies of Cr³⁺ ions at octahedral sites.¹⁷ The PLE and PL spectra of YSAO:0.10Cr³⁺ are depicted in Fig. 2b. The two broad excitation bands are located at about 440 and 603 nm, corresponding to the ⁴A₂ → ⁴T₁(4F) and ⁴A₂ → ⁴T₂(4F) transitions of the Cr³⁺ ions, respectively. The emission spectrum contains narrowband emission from the ²E → ⁴A₂ (689 nm) transition and broadband emission from the ⁴T₂ → ⁴A₂ transition. The PL spectra and intensities of YSAO:xCr³⁺ (0.04 ≤ x ≤ 0.16) at various Cr³⁺-concentrations are presented in Fig. S5† and Fig. 2c, respectively. The optimized Cr³⁺ doping concentration is 0.10, beyond which concentration quenching occurs. Through calculation and analysis, the dipole–dipole effect dominates concentration quenching (ESI Note S1 and Fig. S6†). Fig. 2d illustrates the decay curve of YSAO:0.10Cr³⁺ excited at 440 nm and monitored at 710 nm, which can be well fitted by single-exponential functions:¹⁸

where I₀ and I(t) denote the initial emission intensity and the emission intensity at time t, respectively. A denotes the fitting constant, while t and τ indicate the time and lifetime, respectively. The fluorescence lifetime was found to be 328.20 μs by fitting. As the concentration of Cr³⁺ increases, the lifetime gradually decreases from 357.54 to 289.73 μs (in Fig. S7†), which is mainly caused by the non-radiative transition.

Fig. 2 (a) Typical XPS survey spectra of YSAO:0.10Cr³⁺ and illustration of high-resolution Cr-2p; (b) PLE and PL spectra of YSAO:0.10Cr³⁺ at room temperature; (c) emission intensity of YSAO:xCr³⁺ (x = 0.04–0.16); (d) single-exponential fitting decay curve of YSAO:0.10Cr³⁺; (e) PL and PLE spectra of YSAO:0.10Cr³⁺ at T = 7 K; (f) Tanabe–Sugano energy level diagram of Cr³⁺ in YSAO; (g) temperature dependent PL spectra of YSAO:0.10Cr³⁺ upon 440 nm excitation (T = 7–280 K); (h) contour plot of the emission spectra (λ_ex = 440 nm) of YSAO:0.10Cr³⁺ recorded within the temperature range of 303–483 K; (i) IQE and AE measurement of YSAO:0.10Cr³⁺.

In general, the crystal field strength can be assessed by calculating the crystal field splitting energy (Dq) and the Racah parameter (B) obtained from the emission and excitation spectra, as follows:^19,20

where B is the Racah parameter and Dq is the crystal field intensity. The precise energy position of E(²E)_ZPL can be identified using the spike at 1.8 eV in the PLE spectrum at 7 K, as illustrated in Fig. 2e. On the basis of the previous work of Adachi, we suggest that E(⁴T₂)_ZPL is about 1.85 eV, which is close to the onset of the PLE spectrum shown in Fig. 2e. As shown in Fig. 2f, the values of Dq/B and E(⁴T₁)_ZPL are calculated to be 2.21 and 2.66 eV according to eqn (2)–(4), indicating that the chromium ions are in the intermediate crystal field of the YASO matrix. In this case, the ⁴T₂ → ⁴A₂ spin-permissive transition and the ²E → ⁴A₂ spin-permissive transition can occur concurrently. This corresponds to the observation that the emission spectra exhibit both narrowband and broadband emission.²¹

The low-temperature spectra demonstrate that the spectrum of YSAO:0.10Cr³⁺ consists solely of the narrow-band ²E → ⁴A₂ transitions, accompanied by the N lines and the corresponding phonon sidebands (in Fig. 2g). Fig. 2e shows that the R line produced by the ²E → ⁴A₂ transitions splits into two: R1 (14 584.70 cm⁻¹) and R2 (14 559.22 cm⁻¹), with a difference of 25.48 cm⁻¹ in wavenumber. The R-line splitting results from the influence of the non-cubic crystal field and spin–orbit coupling, which leads to the splitting of the ²E energy levels.²² With increasing temperature, the emission intensity of the R and N lines decreases rapidly relative to the broadband, and the emission band of YSAO:0.10Cr³⁺ gradually broadens. In an intermediate crystal field environment of six-coordinated octahedra, Cr³⁺ ions exhibit ²E → ⁴A₂ spike emission at low temperatures (e.g., 7 K), whereas the ⁴T₂ → ⁴A₂ gradually dominates as the temperature increases. Since the ²E energy level is situated below the ⁴T₂ energy level, the latter can be thermally occupied at higher temperatures. According to the spin-selection rule, the probability of the ⁴T₂ → ⁴A₂ transition is higher than that of the ²E → ⁴A₂ transition. Consequently, the broadband emission from the ⁴T₂ → ⁴A₂ transition can rapidly dominate the PL spectrum. Under 440 nm excitation, the emission spectra of YSAO:0.10Cr³⁺ were obtained at various temperatures ranging from 303 to 483 K, as illustrated in Fig. 2h. At 423 K, the emission intensity is 89.7% of that at room temperature, suggesting that YSAO:Cr³⁺ shows excellent thermal stability. By calculation, the activation energy is found to be 0.25 eV (ESI Note S2 and Fig. S8†). The YSAO:0.10Cr³⁺ phosphor endows an internal quantum efficiency of 84.01% under 440 nm blue light excitation, as depicted in Fig. 2i (ESI Note S3†). These values suggest that the YSAO:0.10Cr³⁺ phosphor possesses exceptional photoluminescence properties, which are advantageous for the development of NIR luminescence applications, particularly in high-performance PC-LEDs.¹⁶

The energy transfer from the Cr³⁺ to Nd³⁺/Yb³⁺ ions was established by introducing Nd³⁺/Yb³⁺ into YSAO:0.10Cr³⁺. Fig. 3a presents the PLE spectra monitored at emission wavelengths of the Yb³⁺ (1030 nm) and Nd³⁺ (1062 nm) ions, respectively. They consist of the main excitation bands of the ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ transitions of the Cr³⁺ ions, indicating that the main excitation energy comes from the absorption of the Cr³⁺ ions. Under excitation at 440 nm, the PL spectra of YSA0:Cr³⁺,Nd³⁺ and YSA0:Cr³⁺,Yb³⁺ show the characteristic peaks of Cr³⁺, Yb³⁺, and Nb³⁺ ions (Fig. 3b). The emission peaks in the range of 900–1050 nm originate from the ²F_5/2 → ²F_7/2 transitions of the Yb³⁺ ions in YSAO:Cr³⁺,Yb³⁺, while the emission bands in the range of 850–1100 nm of YSAO:Cr³⁺,Nd³⁺ correspond to the ⁴F_3/2 → ⁴I_9/2 and ⁴F_3/2 → ⁴I_11/2 transitions of the Nd³⁺ ions, respectively.²³ These results indicate that Cr³⁺ ions can effectively transfer energy to the Yb³⁺ and Nd³⁺ ions. The thermal stability of phosphors is an essential index for assessing the high-temperature luminescence properties. To further investigate the effect of Yb³⁺ and Nd³⁺ on the thermal stability of the YSAO:0.10Cr³⁺ phosphor, the temperature-dependent PL spectra of YSAO:0.10Cr³⁺,0.03Nd³⁺ and YSAO:0.10Cr³⁺,0.15Yb³⁺ phosphors under 440 nm excitation were recorded, as shown in Fig. 3c and e. At a temperature of 423 K, the luminous intensity of YSAO:0.10Cr³⁺,0.03Nd³⁺ and YSAO:0.10Cr³⁺,0.15Yb³⁺ can still be maintained at 99.73% and 94.65% at room temperature, respectively (Fig. 3d and f). As the temperature increases, the rise in non-radiative transitions results in a decrease in the PL intensity of Cr³⁺ ions (89.7% at 423 K). However, the introduction of Nd³⁺ ions enhances the energy transfer efficiency between Cr³⁺ and Nd³⁺ ions with increasing temperature, enabling Cr³⁺ ions to transfer more absorbed energy to Nd³⁺ ions. This further reduces thermal quenching, with the characteristic emission of Nd³⁺ ions, exhibiting excellent thermal stability. Consequently, this enhances the overall thermal stability of the sample. Yb³⁺ possesses a straightforward ground state (²F_7/2) and an excited state (²F_5/2), minimizing energy loss due to cross-relaxation between distinct energy levels. A competitive relationship exists between the thermal quenching of the Cr³⁺-emission and the energy transfer from the Cr³⁺ to Yb³⁺ ions.^24–26 Therefore, the introduction of Nd³⁺ and Yb³⁺ controls the energy transfer pathway from the luminescence quenching center (Cr³⁺) to the thermally stable centers (Nd³⁺and Yb³⁺), inhibiting the non-radiative energy dissipation of Cr³⁺ and enhancing the thermal stability of luminescence.

Fig. 3 (a) PLE spectra of YSAO:0.10Cr³⁺, YSAO:Cr³⁺,Yb³⁺ and YSAO:Cr³⁺,Nd³⁺; (b) PL spectra of YSAO:0.10Cr³⁺, YSAO:Cr³⁺,Yb³⁺ and YSAO:Cr³⁺,Nd³⁺ under 440 nm excitation; (c) PL spectra of the YSAO:0.10Cr³⁺,0.03Nd³⁺ phosphor at different temperatures; (d) emission intensity of YSAO:0.10Cr³⁺,0.03Nd³⁺ (T = 303–483 K); (e) PL spectra of the YSAO:0.10Cr³⁺,0.15Yb³⁺ phosphor at different temperatures; (f) emission intensity of YSAO:0.10Cr³⁺,0.15Yb³⁺ (T = 303–483 K).

Fig. 4a and d show the PL spectra (λ_ex = 440 nm) of YSAO:0.10Cr³⁺,yNd³⁺ (y = 0.01–0.18) and YASO:0.10Cr³⁺,yYb³⁺ (y = 0.03–0.60), respectively. It is evident that the emission intensity of the Nd³⁺/Yb³⁺ ions reaches a maximum at y = 0.03 or y = 0.15, while the emission intensity of the Cr³⁺ ions decreases monotonically. These phenomena further indicate the energy transfer from the Cr³⁺ to Nd³⁺/Yb³⁺ ions in YSAO:0.10Cr³⁺,Nd³⁺ and YASO:0.10Cr³⁺,Yb³⁺. Fig. 4b and e present the decay curves of YSAO:0.10Cr³⁺,yNd³⁺ and YSAO:0.10Cr³⁺,yYb³⁺ under 440 nm excitation and 707 nm monitoring. The fluorescence lifetimes of the Cr³⁺ ions at varying y values are determined as follows:^27,28

Fig. 4 (a) Concentration-dependent PL spectra of YSAO:0.10Cr³⁺,yNd³⁺ (y = 0.01–0.18); (b) PL decay curves of YSAO:0.10Cr³⁺,yNd³⁺ (y = 0.01–0.18) at 707 nm; (c) energy transfer efficiency in YSAO:0.10Cr³⁺,yNd³⁺ (y = 0.01–0.18); (d) concentration-dependent PL spectra of YSAO:0.10Cr³⁺,yYb³⁺ (y = 0.03–0.60); (e) PL decay curves of YASO:0.10Cr³⁺,yYb³⁺ (y = 0.03–0.60) at 707 nm; (f) energy transfer efficiency in YSAO:0.10Cr³⁺,yYb³⁺ (y = 0.03–0.60).

As the concentration of Nd³⁺/Yb³⁺ increases, the photoluminescence lifetimes of the Cr³⁺ ions decrease from 252.38 to 19.02 μs or 234.11 to 13.22 μs, respectively. The shortened lifetime further demonstrates the energy transfer from the Cr³⁺ to Yb³⁺/Nd³⁺ ions. The energy transfer efficiency was calculated using the following expression:²⁹

Here, τ and τ₀ represent the corresponding decay times of the Cr³⁺ ions in the presence and absence of Yb³⁺/Nd³⁺ ions, respectively. Fig. 4c and f show the luminescence lifetimes and energy transfer efficiencies. The maximum transfer efficiencies from the Cr³⁺ to Nd³⁺/Yb³⁺ ions are 94.13/95.92%. These results suggest that the energy transfer from the Cr³⁺ to Nd³⁺/Yb³⁺ ions is highly efficient. Typically, energy transfer between the sensitizer and the activator may occur through resonance interactions, which encompasses exchange interactions and multipolar interactions. On the basis of the refinement results, the unit cell volumes (V) of YSAO:Cr³⁺,Yb³⁺ and YSAO:Cr³⁺,Nd³⁺ are 1792.55 ų and 1793.034 ų, respectively. Here, N is 16, and x_c represents the sum of the concentrations of Cr³⁺ and Yb³⁺ ions, and Cr³⁺ and Nd³⁺ ions, at which the emission intensity of Cr³⁺ ions is reduced by half. Through calculations, we obtained R_c values of 11.81 and 11.02 Å, respectively, both of which are significantly greater than 5 Å. Therefore, we can infer that the energy transfer mechanism from the Cr³⁺ to Nd³⁺/Yb³⁺ ions in YSAO is primarily due to multipolar interactions. According to the approximation of Reisfeld and the theory of multipolar interaction of Dexter, the nature of the multipolar interaction between donor and acceptor ions can be determined using the following equation:^15,30

I₀ denotes the integral intensity of the Cr³⁺ ion emission in YSAO without Yb³⁺/Nd³⁺ doping. I is the integral intensity of the Cr³⁺ ions in the presence of the Nd³⁺/Yb³⁺ ions, and C signifies the combined concentration of the Cr³⁺ and Nd³⁺/Yb³⁺ ions. The relationships between I₀/I and C^θ/3 are given in Fig. S9.† The data of YSAO:0.10Cr³⁺,yNd³⁺ and YSAO:0.10Cr³⁺,yYb³⁺ samples all fit best at θ = 10, corresponding to the quadrupole–quadrupole interaction. Fig. S10† presents the energy transfer model for YSAO:Cr³⁺,Yb³⁺/Nd³⁺. Upon excitation at 450 nm, the electrons in the Cr³⁺ ions transition from the ground state ⁴A₂ to an excited state and then relax to the lowest excited state ⁴T₂. Subsequently, a portion of these electrons return to the ground state ⁴A₂, emitting near-infrared radiation that covers the 650–900 nm range. Some of the energy is transferred to Yb³⁺ ions, causing electrons to be excited from the ²F_7/2 state to the ²F_5/2 state, and finally returning to the ²F_7/2 state, resulting in emission around 1000 nm in the long wavelength region.³¹ As for the case of the Cr³⁺ and Nd³⁺ ions, the energy is transferred to the ⁴F_5/2 or ⁴F_7/2 levels of Nd³⁺. Subsequently, electrons are released to the ⁴F_3/2 level, thereby enhancing the emission bands at 885 nm (11 441 cm⁻¹, ⁴F_3/2 → ⁴I_9/2) and 1062 nm (9697 cm⁻¹, ⁴F_3/2 → ⁴I_11/2).³²

In order to evaluate the practical application potential of YSAO:Cr³⁺,Yb³⁺ and YSAO:Cr³⁺,Nd³⁺ phosphors, we coated these phosphors on a 450 nm blue chip to obtain NIR pc-LEDs. As shown in Fig. 5a and c, the emission intensity gradually increases as the drive current increases from 20 to 150 mA. Fig. 5b and d illustrate that the NIR emission output power also increases with increasing current, while the NIR photoelectric conversion efficiency gradually decreases. At a driving current of 100 mA, the near-infrared output powers of YSAO:Cr³⁺,Yb³⁺ and YSAO:Cr³⁺,Nd³⁺ reach 46.37 and 42.82 mW, respectively, and NIR photoelectric conversion efficiencies are 15.47 and 14.30%, respectively. Relative to the previously reported NIR emission phosphors (in Table 1), the prepared YSAO:Cr³⁺,Yb³⁺ and YSAO:Cr³⁺,Nd³⁺ have excellent performances, demonstrating their potential for versatile applications.

Fig. 5 (a) PL spectra of YSAO:Cr³⁺,Nd³⁺ pc-LEDs at different drive currents; (b) the output power and photoelectric conversion efficiency; (c) PL spectra of YSAO:Cr³⁺,Yb³⁺ pc-LEDs at different drive currents; (d) the output power and photoelectric conversion efficiency.

Table 1 Optoelectronic performance of Cr³⁺/Ln³⁺ doped long-wave broadband near-infrared phosphors for pc-LED devices

Phosphors	Current	Output power	Electro-optical conversion efficiencies	Ref.
YSAO:0.10Cr^3+,0.06Nd^3+	100 mA	42.82 mW	14.30%	This work
YSAO:0.10Cr^3+,0.15Yb^3+	100 mA	46.37 mW	15.47%	This work
Gd3MgScGa2SiO12:0.04Cr^3+,0.007Yb^3+	100 mA	19.5 mW	7.52%	33
LiScP2O7:0.06Cr^3+,0.03Yb^3+.	100 mA	∼36 mW	12%	26
Ca2LaHf2Al3O12:0.01Cr^3+,0.01Yb^3+	200 mA	33.24 mW	10%	34
Ca2LuZr2Al3O12:0.08Cr^3+,0.01Yb^3+	100 mA	41.8 mW	14.3%	14
Ca2YHf2Al3O12:0.02Cr^3+,0.03Yb^3+	100 mA	18 mW	6%	35
Y3Ga5O12:Cr^3+,Yb^3+	100 mA	43.2 mW	14.3	36
Sr3SiAl10O20:0.02Cr^3+,0.01Yb^3+	100 mA	32.29 mW	∼10%	37
Gd2GaSb0.9Ta0.1O7:0.03Cr^3+,0.04Yb^3+	100 mA	∼22 mW	∼8.5%	38

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The absorption spectrum of food in the near-infrared region primarily arises from the overtones and combination bands of hydrogen-containing groups, such as –CH, –NH, and –OH. These absorption bands are predominantly distributed within the range of 700 to 1500 nm.^39,40 The absorption of near-infrared light varies due to the different types and amounts of hydrogen-containing groups. Consequently, the types of meat can be distinguished by the intensity ratio of different emission peaks. The experimental device used the YSAO:Cr³⁺,Nd³⁺ near-infrared phosphor. As shown in Fig. 6a, the thickness of chicken, beef and pork was 5 mm. After near-infrared light passes through the different meat components, the relative emission intensity changes significantly (Fig. 6b and c). Fig. 6d shows that the luminous intensity of chicken at 1060 nm is the lowest, while that of beef is the highest (using the luminous intensity at 710 nm as a reference). Additionally, the percentage of luminescence intensity of each meat component varies significantly in different areas (Fig. 6e). This makes meat analysis more convenient and intelligent. Therefore, multi-band NIR LEDs have potential applications in non-destructive testing analysis. Furthermore, we utilized the YSAO:Cr³⁺,Yb³⁺ NIR phosphor to validate the effectiveness of the NIR light source in night vision monitoring applications. As depicted in Fig. 6f, the portion of the object obscured by a 645 nm filter (an opaque plastic bottle) remains invisible under fluorescent lighting. However, when the fabricated NIR pc-LED device is activated in a dark environment, the NIR camera is able to clearly capture the objects, including those concealed by the 645 nm filter (as shown in Fig. 6i). Additionally, Fig. 6g and h exhibit photographs taken with an ordinary digital camera and a commercial short-wave infrared camera, respectively. Under indoor white light illumination, an ordinary digital camera cannot capture the internal structure of the orange. However, the pulp, seeds, and complex structure of the orange are clearly visible, when the near-infrared LED device is used for illumination.

Fig. 6 (a) Spectrum of the NIR pc-LED; (b) spectrum after penetration of chicken, beef and pork; (c) display of chicken, beef and pork used in the experiment; (d) the intensity ratio between the emission peaks at 709 nm was normalized to 1; (e) percentage between normalized emission peak intensities; (f) visible and (i) near-infrared images of opaque plastic bottles illuminated by fluorescent lamps and manufactured near-infrared pc-LED lamps; and (g) visible and (h) near-infrared images of orange petals illuminated by fluorescent and fabricated near-infrared pc-LED lamps.

4. Conclusion

In summary, a series of Y₃ScAl₄O₁₂ garnet NIR-emitting materials were synthesized, and codoping with Nd³⁺ and Yb³⁺ ions not only effectively broadened the NIR emission bandwidth of the materials but also improved their thermal stability, due to the efficient energy transfer among Cr³⁺ and Nd³⁺/Yb³⁺ ions. These findings offer a pathway to enhance the thermal stability and energy transfer of Cr³⁺-activated NIR emitters for use in NIR light-emitting diodes. Additionally, YSAO:Cr³⁺,Nd³⁺ and YSAO:Cr³⁺,Yb³⁺ phosphors were encapsulated with a blue LED chip to obtain NIR pc-LED devices, achieving maximum NIR light output powers of 42.82 and 46.37 mW, respectively, at a driving current of 100 mA. The results indicate that YSAO:Cr³⁺,Nd³⁺ and YSAO:Cr³⁺,Yb³⁺ phosphors have promising applications in the NIR spectrum, including potential uses in night vision imaging and non-destructive analysis techniques.

Data availability

The data that support the findings of this study are available in the ESI† of this article.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This study was financially supported by the National Key Research and Development Program (grant no. 2022YFC2905201), the National Natural Science Foundation of China (grant no. 52072363 and 22305250), and the research fund of the Key Laboratory of Rare Earths, Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02260b

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