Ultra-broadband shortwave infrared emission under blue light excitation of a Cr³⁺/Ni²⁺ co-doped Y₃Al₃MgSiO₁₂ garnet phosphor through effective energy transfer and its applications

Abstract

Short-wave infrared (SWIR) phosphor-converted light-emitting diodes (pc-LEDs) are promising for biomedical and nondestructive applications. Still, their progress is constrained by the lack of efficient, ultra-broadband phosphors excitable by low-cost blue LEDs. Cr³⁺-activated materials exhibit strong blue light excitation, but their emission is primarily confined to the NIR-I region. In contrast, Ni²⁺ has the potential to achieve SWIR emission, yet suffers from weak absorption in the blue-light region. In this study, Y₃Al₃MgSiO₁₂:Ni²⁺ and Y₃Al₃MgSiO₁₂:Cr³⁺–Ni²⁺ phosphors were synthesized. Compared to previous reports, the Y₃Al₃MgSiO₁₂:Cr³⁺–Ni²⁺ phosphor in this study achieved three significant advancements: (1) efficient energy transfer from Cr³⁺ to Ni²⁺ was achieved (η = 91.6%), resulting in a 10.45-fold enhancement of SWIR emission intensity upon 438 nm blue-light excitation, and the optimal excitation wavelength was shifted to the blue-light region. (2) Ultra-broadband continuous emission spanning the NIR-I to NIR-III regions, with an exceptionally wide FWHM (185 + 311 nm), was achieved in this phosphor. (3) Remarkably high thermal stability was achieved for the NIR-II–III emission in a region where strong electron–phonon coupling and poor thermal stability are typically observed. The underlying mechanism was elucidated through analysis of the crystal structure rigidity and the Huang–Rhys factor (S). A SWIR pc-LED device was further fabricated by integrating the phosphor with a 450 nm blue LED chip, confirming its application potential in covert information recognition and nondestructive detection scenarios. This study not only introduces a broadband SWIR-emitting material system excitable by blue light but also provides a novel strategy for developing efficient and thermally stable SWIR phosphors.

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1. Introduction

Shortwave infrared (SWIR) luminescence technology offers deep tissue penetration, no autofluorescence interference, and a high degree of compatibility with characteristic absorption peaks of organic molecules (such as O–H and C–H), demonstrating unique advantages in fields such as in vivo imaging, non-destructive testing, and photodynamic therapy.^1,2 The NIR region is conventionally categorized into NIR-I (700–1000 nm), NIR-II (1000–1500 nm), and NIR-III (1500–1850 nm), while the SWIR spectrum spans the 900–1700 nm range.³ Compared with traditional tungsten halogen lamps, shortwave infrared phosphor-converted light-emitting diodes (SWIR pc-LEDs) are regarded as a core technology for the next generation of SWIR light sources.⁴ They offer advantages such as low cost, miniaturization, high power efficiency, and long lifespan.⁵ Typically, SWIR pc-LEDs consist of an InGaN blue LED chip combined with broadband SWIR-emitting phosphor materials. The broadband emission can effectively span the NIR-II to NIR-III ranges (1000–1700 nm), making them suitable for applications such as high-resolution vascular imaging and deep tumor detection.^6,7 Therefore, developing novel phosphors capable of both blue-light excitation and ultra-broadband SWIR emission is key to expanding the application boundaries of SWIR pc-LEDs.

The dopant activators in SWIR phosphors are generally classified as trivalent lanthanide ions and transition metal ions. In contrast to the 4f–4f transitions of trivalent lanthanide ions, transition metal ions show 3d–3d spin-allowed transitions, thereby exhibiting excellent broadband NIR emission performance.⁸ As a result, transition metal ions are more suitable for doping into host matrices to design SWIR phosphors. Owing to its broad and strong absorption in the blue-light region, Cr³⁺ stands out among transition metal ions as a highly compatible activator for InGaN blue-light chips.^9–12 However, the emission of Cr³⁺ is predominantly confined to the NIR-I region, and extending its emission wavelength into the NIR-II/III region remains a significant challenge.¹³ Fortunately, Ni²⁺ ions with a 3d⁸ electronic configuration have shown great potential for ultra-broadband SWIR emission in the NIR-II region in weak octahedral crystal field environments (Dq/B < 1.7). This makes Ni²⁺ an ideal luminescent center, a feature already validated in phosphors such as ZnGa₂O₄:Ni²⁺, MgTi₂O₅:Ni²⁺, and Mg₃Ga₂GeO₈:Ni²⁺.^14–16 However, it should be noted that in existing studies, Ni²⁺ excitation primarily occurs in the ultraviolet region, while its absorption in the blue-light region is relatively weak, preventing efficient excitation by blue-light chips. Although some studies have developed single Ni²⁺-activated NIR phosphors that can be excited by blue light, such as Y₃Al₂Ga₃O₁₂:Ni²⁺ and Mg₂SnO₄:Ni²⁺,^17–19 their spectral overlap with the blue-light region remains low. Additionally, Ni²⁺-activated NIR phosphors typically exhibit large Stokes shifts, resulting in relatively low photoluminescence efficiency. To address this issue, a Cr³⁺–Ni²⁺ co-doping strategy has been proposed, which enhances Ni²⁺ SWIR emission through energy transfer from Cr³⁺, thereby improving photoluminescence efficiency. Cr³⁺ is chosen as a sensitizer not only because it efficiently absorbs blue light but also because its NIR emission strongly overlaps with the Ni²⁺ absorption band. This advantage has been validated in systems such as MgGa₂O₄:Cr³⁺–Ni²⁺ and MgO:Cr³⁺–Ni²⁺.^20,21 However, the development of Cr³⁺–Ni²⁺ co-doped SWIR phosphors with efficient energy transfer, excellent photoluminescence performance, and good thermal stability remains a significant challenge.

In this study, we report a novel broadband SWIR phosphor, Y₃Al₃MgSiO₁₂:Ni²⁺, exhibiting ultra-broadband emission spanning 1000–1650 nm. Notably, it is excitable by visible light (λ_ex = 408 nm), rather than UV radiation. Diverging from the previous Y₃Ga₃MgSiO₁₂:Ni²⁺ report, more stable and economically viable Al was employed to substitute Ga. This substitution not only enhances the rigidity of the crystal structure, thereby improving thermal stability, but also significantly reduces raw material costs. Furthermore, through a Cr³⁺-Ni²⁺ co-doping strategy, Y₃Al₃MgSiO₁₂:Cr³⁺–Ni²⁺ demonstrates ultra-broadband SWIR emission (FWHM = 185 + 311 nm) spanning 600–1650 nm under 438 nm blue-light excitation, covering the spectral regions from NIR-I to NIR-III. The phosphor exhibits highly efficient energy transfer from Cr³⁺ to Ni²⁺ (η = 91.6%) and excellent thermal stability, with NIR-II-III emission intensities maintaining 56% of their room temperature values at 373 K, respectively. The influence of the Huang–Rhys factor and the rigidity of the crystal structure on this thermal stability is also discussed. Finally, a SWIR pc-LED was fabricated by coupling the Y₃Al₃MgSiO₁₂:Cr³⁺–Ni²⁺ phosphor with a commercial 450 nm InGaN blue LED chip. The resulting device exhibited promising performance, demonstrating its potential for applications in covert object identification and non-contact, nondestructive detection scenarios.

2. Results and discussion

2.1. Structure and composition

Fig. 1(a and b) illustrates the powder X-ray diffraction patterns of Y₃Al₃MgSiO₁₂:xNi²⁺ (x = 1%–10%) and Y₃Al₃MgSiO₁₂:5% Ni²⁺–yCr³⁺ (y = 1%–10%) phosphors. The diffraction peaks are following the standard card of Y₃Al₅O₁₂ (PDF#97-017-0157), thereby indicating that all the synthesized samples are pure phases. To elucidate the impact of Ni²⁺ and Cr³⁺ doping, the Rietveld refinement of Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ are illustrated in Fig. 1(c). Tables S1 and S2 provide detailed information regarding the processing and refining parameters for the aforementioned samples, along with the atomic number fraction coordinates and atomic occupancies of the samples. Fig. 1(d) illustrates the crystal structure of the compound Y₃Al₃MgSiO₁₂, which exhibits a typical A₃B₅O₁₂-type garnet structure and belongs to the space group Ia3̄d. In this structure, the Y³⁺, Al₁³⁺/Mg²⁺, and Al₂³⁺/Si⁴⁺ cations are coordinated to 8, 6, and 4 O²⁻ ions, respectively, to form dodecahedral, octahedral, and tetrahedral structures.²² In a six-coordinate environment, the ionic radii of Ni²⁺, Al³⁺, Mg²⁺, and Cr³⁺ are 0.69, 0.535, 0.72, and 0.615 Å, respectively. It is predicted that when Ni²⁺ and Cr³⁺ ions are doped into the matrix Y₃Al₃MgSiO₁₂, Ni²⁺ will take the place of the Mg²⁺ ion due to the similarity of ionic radii and valence, while the Cr³⁺ ion will occupy the position of the Al³⁺ ion. Ionic substitution behavior is primarily governed by the charge compatibility and ionic radius matching between the dopant and host cations. Among these factors, the ionic radius difference parameter (D_r) serves as a critical criterion for evaluating the feasibility of substitution. It can be calculated using the following empirical equation:

Fig. 1 (a and b) X-ray diffraction patterns of Y₃Al₃MgSiO₁₂:xNi²⁺ (x = 1%–10%) and Y₃Al₃MgSiO₁₂:5% Ni²⁺–yCr³⁺ (y = 1%–10%). (c) Rietveld refinement results of Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺. (d) Crystal structure of Y₃Al₃MgSiO₁₂.

Here, R_d and R_r represent the ionic radii of the dopant and the substituted host cation, respectively. Generally, when the ionic radius difference D_r is below 30%, ionic substitution is considered feasible within the crystal structure.²³ In the case of Y₃Al₃MgSiO₁₂, the D_r values of Ni²⁺ relative to the Mg²⁺ and Al³⁺ sites are 4.17% and 28.97%, respectively, indicating a stronger preference for substitution at the Mg²⁺ site due to the better ionic radius match. In contrast, Cr³⁺ exhibits D_r values of 14.58% and 14.95% for the Mg²⁺ and Al³⁺ sites, respectively. Thermoluminescence (TL) measurements can be employed to characterize the distribution of charge traps, where the peak position reflects the trap depth and the peak area corresponds to the trap concentration.²⁴ To further investigate the site occupancy behavior of the dopant ions, TL spectra were recorded under 260 nm excitation for pre-irradiated Y₃Al₃MgSiO₁₂ host and Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ samples (Fig. S1). Both samples showed no distinct TL peaks, suggesting that no significant charge traps were introduced during the doping process. Based on the principle of charge balance, Cr³⁺ is more likely to occupy the Al³⁺ sites preferentially.

Fig. 2(a) depicts the elemental mapping images of Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ under a scanning electron microscope (SEM), and Fig. 2(b–h) demonstrates that all elements namely Y, Al, Mg, Si, O, Cr, and Ni are distributed uniformly throughout the particles, with no evidence of element aggregation or phase separation.

Fig. 2 (a) SEM image and (b–h) the elemental mapping images of the as-prepared Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ phosphor.

2.2. Luminescence properties

2.2.1. Cr³⁺/Ni²⁺ mono-doped Y₃Al₃MgSiO₁₂. Fig. 3(a) illustrates the diffuse reflectance (DR) spectra of Y₃Al₃MgSiO₁₂:xNi²⁺ (x = 0%, 5%) and Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ phosphors. Multiple absorption peaks are observed in the ultraviolet-visible-near infrared region. A comparison of the DR spectra of the Y₃Al₃MgSiO₁₂ matrix with those of Y₃Al₃MgSiO₁₂:5% Ni²⁺ is shown, in which the absorption peaks are located at 408 nm, 668 nm, 1064 nm, and 464 nm, respectively, and can be classified as three main peaks and a weak absorption peak. These transitions are derived from the spin-allowed ³A₂ → ³T₁ (³P), ³A₂ → ³T₁ (³F), and ³A₂ → ³T₂ (³F) electronic transitions and the spin-forbidden ³A₂ → ³T₁ (¹D) electronic transitions of Ni²⁺.²⁵ In comparison to the DR spectrum of Y₃Al₃MgSiO₁₂:5% Ni²⁺, the compound Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ reveals two prominent absorption peaks at 438 nm and 590 nm, which arise from the electronic transitions of Cr³⁺:⁴A₂(⁴F) → ⁴T₁(⁴F) and ⁴A₂(⁴F) → ⁴T₂(⁴F), respectively.²⁶ The value of the band gap (E_g) can be calculated according to the following formula:

[hνF(R_∞)]ⁿ = A(hν − E_g). (3)

Fig. 3 (a) The DRS of Y₃Al₃MgSiO₁₂:xNi²⁺ (x = 0, 5%) and Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺. (b) Band gap determination, where (F(R)*hν)² is a function of photon energy. (c and d) The band structure and density of states of the Y₃Al₃MgSiO₁₂ host.

Here, A is a proportionality constant, hν denotes the energy of a single photon, and F(R_∞) is the Kubelka–Munk function, which is proportional to the absorption coefficient. The exponent n reflects the nature of the band gap: n = 1/2 corresponds to indirectly allowed, n = 2 to directly allowed, n = 3/2 to indirectly forbidden, and n = 3 to directly forbidden.²⁷ In this work, the Y₃Al₃MgSiO₁₂ phosphor has a direct band gap type with an n value of 2. Fig. 3(b) shows the relationship between [hνF(R_∞)]² and hν. Through the analysis of diffuse reflectance data, we ascertain that the optical band gaps of Y₃Al₃MgSiO₁₂, Y₃Al₃MgSiO₁₂:5%Ni²⁺, and Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ are 5.12 eV, 5.10 eV, and 4.92 eV, respectively.

Fig. 3(c and d) presents the electronic band structure of Y₃Al₃MgSiO₁₂ and the projected density of states (PDOS) of its constituent elements Y, Al, Mg, Si, and O. It can be observed that the material exhibits a direct bandgap, with a calculated bandgap value of 4.55 eV. The calculated bandgap is closely related to the bond strength between the atoms and their valence electrons. A stronger covalent character implies greater overlap between valence electron orbitals, resulting in more localized electron cloud density and tighter bonding, which in turn leads to the formation of stronger chemical bonds.²⁸ Typically, an increase in covalent bond strength widens the energy gap between the valence and conduction bands, resulting in a larger bandgap.²⁹ A larger bandgap not only reflects stronger covalency but also contributes to enhanced luminescence intensity. Additionally, materials with smaller bandgaps are more susceptible to thermally activated photoionization processes, which exacerbate luminescence quenching.³⁰ Therefore, the material with relatively large bandgaps is expected to exhibit better thermal stability upon Ni²⁺/Cr³⁺ doping. Further analysis of the DOS reveals that the valence band is primarily composed of O (p) states, accompanied by minor contributions from the Al (s) and Al (p) states, while the conduction band is predominantly derived from the Y (d) states.

The emission position of dopant ions at the luminescent center largely depends on the crystal field environment of the host lattice. Therefore, assessing the crystal field strength provides valuable insights into the luminescent behavior of Ni²⁺ and Cr³⁺ ions within the Y₃Al₃MgSiO₁₂ matrix. The crystal field environment of Ni²⁺ and Cr³⁺ ions in an octahedral field can be evaluated using the Tanabe–Sugano diagrams for 3d⁸ and 3d³ configurations in Fig. 4(a and b). The crystal field strength (Dq/B) for Y₃Al₃MgSiO₁₂:5% Ni²⁺ and Y₃Al₃MgSiO₁₂:1% Cr³⁺ phosphors can be determined using the following two equations:

Fig. 4 The Tanabe–Sugano diagram of (a) Y₃Al₃MgSiO₁₂:Ni²⁺ and (b) Y₃Al₃MgSiO₁₂:Cr³⁺.

Here, Dq stands for the crystal field parameter and B refers to the Racah parameter. The terms ν₁ and ν₃ refer to the energy values of the transitions ³A₂ → ³T₂ (F) and ³A₂ → ³T₁ (P) for Ni²⁺ ions, and ⁴A₂ → ⁴T₁ (F) and ⁴A₂ → ⁴T₂ (F) for Cr³⁺ ions. According to crystal field theory, the crystal field can be classified as strong or weak based on the relative energy of the ³T₂ and ¹E levels for Ni²⁺ ions: a weak crystal field is indicated when ³T₂ is the first excited state transitioning to the ground state ³A₂ (Dq/B < 1.7). In contrast, a strong crystal field occurs when ¹E is the first excited state (Dq/B > 1.7).³¹ For Cr³⁺ ions, the field strength is determined by the relative energies of ⁴T₂ and ²E, a weak field is present when ⁴T₂ is the first excited state transitioning to the ground state ⁴A₂ (Dq/B < 2.3), and a strong field is present when ²E is the first excited state (Dq/B > 2.3).³² Thus, the calculated crystal field strengths Dq/B for Y₃Al₃MgSiO₁₂:5% Ni²⁺ and Y₃Al₃MgSiO₁₂:1% Cr³⁺ phosphors are 0.823 (< 1.7) and 1.97 (< 2.3), respectively, indicating that both Ni²⁺ and Cr³⁺ ions in these phosphors are situated in a weak crystal field.

The excitation and emission spectra of Y₃Al₃MgSiO₁₂:xNi²⁺ (x = 1%–10%) phosphors are illustrated in Fig. 5(a and b). Upon monitoring at a wavelength of 1450 nm, three excitation bands at 408 nm, 464 nm, and 668 nm are observed to align with the absorption bands evident in the DR spectrum (Fig. 3(a)). These transitions are attributed to the ³A₂ (F) → ³T₁ (³P), ³A₂ (F) → ¹T₂ (D), and ³A₂ (F) → ³T₁ (F) transitions of Ni²⁺.³³ Due to the spin-allowed ³T₂ (³F) to ³A₂ (³F) transition of Ni²⁺ ions, Y₃Al₃MgSiO₁₂:Ni²⁺ has a wide emission band in the wavelength range of 1000–1700 nm under the excitation of 408 nm, and its FWHM value reaches 311 nm. The modulation of crystal field splitting induced by increasing Ni²⁺ concentration has a substantial effect on the luminescence properties. The emission in Fig. 5(b) undergoes a noticeable redshift, with the peak wavelength shifting from 1422 nm to 1460 nm as the Ni²⁺ content increases. This phenomenon stems from the changes in the crystal field splitting around the Ni²⁺ ions, and can be analyzed using the following formula:

Fig. 5 (a and b) Excitation and emission spectra of Y₃Al₃MgSiO₁₂:xNi²⁺ (x = 1%–10%) phosphors. (c) The dependence of emission intensity on Ni²⁺ concentration. (d) The fitting diagram of the concentration quenching mechanism. (e) Decay curve for the main emission of the phosphors. (f) Dependence of average lifetime on Ni²⁺ concentration.

In this formula, Z and e denote the charge numbers of the anion and electron, respectively, r and R represent the radius of the d-orbital wave function and the distance between the central ion and the ligand. As the concentration of Ni²⁺ doping increases, the crystal field splitting around it intensifies. This not only reduces the influence of Ni²⁺ on the lowest 3d states but also weakens the crystal field strength surrounding Ni²⁺. Consequently, this alters the energy levels of ³T₁ (³P) and ³T₁ (³F), leading to the observed redshift in the normalized emission spectrum.³⁴

Fig. 5(c) reveals a strong concentration-dependent trend in the emission intensity of Y₃Al₃MgSiO₁₂:Ni²⁺. The emission intensity increases with the Ni²⁺ content, reaches a maximum at x = 5%, and then decreases due to concentration quenching. As the Ni²⁺ concentration increases, the probability of energy transfer between Ni²⁺ ions in the Y₃Al₃MgSiO₁₂ host correspondingly increases. Given that the excitation and emission spectra of Y₃Al₃MgSiO₁₂:Ni²⁺ phosphors do not overlap, the primary form of the energy transfer mechanism is determined by the critical distance (R_c) between Ni²⁺ ions.³⁵ When the value of R_c is equal to or less than 0.5 nm, the primary form of energy transfer is exchanged interaction. However, when R_c exceeds 0.5 nm, the dominant mechanism shifts to multipolar interaction. To comprehend the concentration quenching process of Y₃Al₃MgSiO₁₂:Ni²⁺, it is essential to employ the following formula to calculate R_c:

In this equation, N represents the number of cations in the cell, V represents the cell volume, and x_c represents the optimal nickel ion concentration in Y₃Al₃MgSiO₁₂. For Y₃Al₃MgSiO₁₂:5% Ni²⁺, R_c was calculated using N = 8, V = 1727.212 ų, and x_c = 0.05. According to the formula, it can be estimated that the R_c value is approximately 2.02 nm greater than 0.5 nm, indicating that the primary mechanism of concentration quenching is multipolar interaction. To analyze the mechanism and type of multipolar interaction between Ni²⁺ ions in Y₃Al₃MgSiO₁₂, the following equation can be used to analyze according to Dexter's theoretical hypothesis:^36,37

In this equation, the variable I represents the photoluminescence intensity detected in the sample, and x represents the concentration of Ni²⁺ ion. For a given host lattice, the constants β and k are fixed, while the parameter θ determines the type of multipolar interactions. Specifically, θ = 3, 6, 8, and 10 correspond to the non-radiative energy transfer between neighboring ions, dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively.³⁸ The log–log plot of log(I/x) versus log(x) is shown in Fig. 5(d), and this relationship exhibits a linear dependence, with a fitted slope of approximately −1.70799. The calculation result of θ is 5.12, which can be approximated as 6. Therefore, the quenching process is primarily dominated by the non-radiative energy transfer caused by the dipole–dipole multipole interaction.

Fig. 5(e) shows the decay curves of the 1450 nm SWIR emission of Y₃Al₃MgSiO₁₂:xNi²⁺ (x = 1%–10%) phosphors under 408 nm excitation. The decay curves can be fitted by a double exponential function.³⁹

In eqn (9), A₁ and A₂ are fixed constants, and τ₁ and τ₂ represent the lifetime of the exponential component, the fluorescence intensity at time t is denoted by I. Based on eqn (10), the corresponding average lifetimes were calculated to be 492.21, 324.90, 226.08, 176.53, and 134.12 μs, respectively. The PL intensity of the Ni²⁺-doped sample reaches its maximum at a doping concentration of 5%. However, the luminescence lifetime monotonously decreases with an increase in Ni²⁺ concentration. This contrasting phenomenon can be attributed to the enhancement of non-radiative relaxation processes.⁴⁰ The relationship between fluorescence lifetime (τ), radiative relaxation rate (W_R), and non-radiative relaxation rate (W_NR) can be expressed as follows:

With an increase in Ni²⁺ concentration, the probability of energy transfer between adjacent Ni²⁺ ions as well as between Ni²⁺ ions and luminescence quenching centers increases, which significantly enhances the occurrence of non-radiative relaxation processes and consequently shortens the luminescence lifetime.⁴¹ The specific values of the fitting results are shown in Table 1. Fig. 5(f) shows the correlation between the fluorescence lifetime and the change of x doping concentration.

Table 1 The fitted fluorescence decay results for the 1450 nm emission (λ_ex = 408 nm) of Y₃Al₃MgSiO₁₂:xNi²⁺ (x = 1%–10%) phosphors

x (mol%)	Lifetime τ1 (μs)	Lifetime τ2 (μs)	A 1	A 2	χ ^2	Lifetime τ* (μs)
1%	131.77	555.51	880.48	1189.25	1.006	492.21
3%	115.09	393.73	1269.30	1131.06	0.978	324.90
5%	77.61	276.81	1509.88	1238.86	0.941	226.08
7%	65.12	224.01	1524.95	1040.09	0.999	176.53
10%	60.91	186.60	1921.94	875.30	0.958	134.12

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2.2.2. Cr³⁺/Ni²⁺ co-doped Y₃Al₃MgSiO₁₂. Although the Ni²⁺ doped Y₃Al₃MgSiO₁₂ phosphor can be excited by 408 nm light and exhibit broadband SWIR luminescence, its excitation efficiency for commercial blue LED chips is still not efficient enough, and the luminescence performance can be further improved. Therefore, we introduce the co-doping of Cr³⁺ and Ni²⁺ to act as sensitizers to adjust the excitation band and improve the luminescence properties of the phosphors. As shown in Fig. 6(a), compared with Y₃Al₃MgSiO₁₂:Ni²⁺, the excitation band of Y₃Al₃MgSiO₁₂:Cr³⁺ is dominant in the blue region. A clear overlap is observed between the emission spectrum of Y₃Al₃MgSiO₁₂:1% Cr³⁺ and the excitation spectrum of Y₃Al₃MgSiO₁₂:1% Ni²⁺ within the 600–850 nm wavelength range. This indicates that there is an energy transfer phenomenon between Cr³⁺ and Ni²⁺ ions.⁴²

Fig. 6 (a) Emission spectra of Y₃Al₃MgSiO₁₂:1% Cr³⁺ and excitation spectra of Y₃Al₃MgSiO₁₂:1% Ni²⁺. (b and c) Excitation and emission spectra of Y₃Al₃MgSiO₁₂:5% Ni²⁺–yCr³⁺ (y = 1%–10%) phosphors. (d) Dependence of integrated emission intensity in 600–1100 nm and 1100–1650 nm spectral ranges on Cr³⁺ concentration. (e) The energy transfer efficiency of Y₃Al₃MgSiO₁₂:5% Ni²⁺–1% Cr³⁺. The inset shows the SWIR intensity of Y₃Al₃MgSiO₁₂:5% Ni²⁺, Y₃Al₃MgSiO₁₂ 5% Cr³⁺–5% Ni²⁺. (f) Energy transfer (ET) process diagram of Cr³⁺ and Ni²⁺ in Y₃Al₃MgSiO₁₂.

Fig. 6(b) shows the excitation spectra of Y₃Al₃MgSiO₁₂:5% Ni²⁺–yCr³⁺(y = 1%–10%) phosphors monitored at 1450 nm. With the gradual increase of Cr³⁺ concentration, the excitation band intensities of ³A₂ (³F) → ³T₁ (³P) and ³A₂ (³F) → ³T₁ (³F) transitions of Ni²⁺ ions gradually decrease. At the same time, the excitation bands of ⁴A₂ (⁴F) → ⁴T₁ (⁴F) and ⁴A₂ (⁴F) → ⁴T₂ (⁴F) transitions of Cr³⁺ ions gradually dominate the spectrum, which corresponds to the distribution of Y₃Al₃MgSiO₁₂:5% Ni²⁺–5%Cr³⁺ absorption bands in DR spectrum (Fig. 3(a)). The change in the excitation spectrum further supports the occurrence of energy transfer from Cr³⁺ to Ni²⁺, thereby enabling more efficient excitation of Ni²⁺-doped phosphors using blue LED chips.

As shown in Fig. 6(c), under the excitation of 438 nm blue light, Y₃Al₃MgSiO₁₂:5% Ni²⁺–yCr³⁺ (y = 1%–10%) phosphors exhibit two broad emission bands at 600–1100 nm and 1100–1650 nm, which are Cr³⁺ ion emission in the NIR-I region and Cr³⁺ emission and Ni²⁺ emission in the SWIR region, respectively. Compared with the sample without Cr³⁺, the introduction of Cr³⁺ significantly improves the emission intensity of Ni²⁺ in the SWIR region. This phenomenon is attributed to the energy transfer from Cr³⁺ to Ni²⁺, which greatly enhances the Ni²⁺ emission with low emission efficiency under blue light excitation.⁴³ As the Cr³⁺ concentration increases, the integrated emission intensity in the 600–1650 nm regions and only in the 1100–1650 nm region showed a trend of increasing first and then decreasing. Among them, Y₃Al₃MgSiO₁₂:5% Cr³⁺–5% Ni²⁺ has the highest luminous intensity, as shown in Fig. 6(d). The emission intensity of Y₃Al₃MgSiO₁₂:5% Cr³⁺–5% Ni²⁺ sample is 10.45 times higher than that of Y₃Al₃MgSiO₁₂:5% Ni²⁺ sample monitored at 438 nm. In the process of energy transfer, the resonance ET from sensitizer to activator is mainly realized by two interaction modes: exchange interaction and multipole interaction. According to G. Blasse's theory, R_c between the sensitizer (Cr³⁺) and the activator (Ni²⁺) is a key parameter to determine the dominant energy transfer mechanism in Y₃Al₃MgSiO₁₂:Ni²⁺–Cr³⁺ phosphors. When R_c is less than 0.5 nm, the exchange interaction becomes the main mode. When R_c is greater than 0.5 nm, the energy transfer process mainly depends on the multipole interaction.^44,45 The specific value of R_c can be calculated by eqn (6). In Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ phosphor, the total concentration x_c of dopants (Cr³⁺ and Ni²⁺) is 0.1, the unit cell volume V is 1731.242 ų. According to these parameters, the R_c between the sensitizer (Cr³⁺) and the activator (Ni²⁺) is calculated to be 1.61 nm, which is significantly greater than 0.5 nm. This result shows that the energy transfer from Cr³⁺ to Ni²⁺ mainly depends on the multipole interaction.

Fig. 6(e) compares the emission intensities of Y₃Al₃MgSiO₁₂:1% Cr³⁺ and Y₃Al₃MgSiO₁₂:5% Ni²⁺–1% Cr³⁺. The emission intensity of Cr³⁺ in the NIR-I region diminished following the incorporation of Ni²⁺, whereas the emission intensity of Ni²⁺ in the SWIR region augmented. This substantiates the hypothesis that Cr³⁺ ions transfer energy to Ni²⁺. The energy transfer efficiency can be calculated using the following formula:⁴⁶

Among them, η is the energy transfer efficiency of Cr³⁺ ions to Ni²⁺ ions, I₁ and I₂ represent the PL intensity of Cr³⁺ in the Ni²⁺-doped and Ni²⁺-free phosphors. The calculated η value is 91.6%, indicating that Y₃Al₃MgSiO₁₂:Ni²⁺–Cr³⁺ has an excellent energy transfer efficiency. The energy level diagram of Y₃Al₃MgSiO₁₂:Ni²⁺–Cr³⁺ (Fig. 6(f)) provides a clear explanation of the energy transfer mechanism between Cr³⁺ and Ni²⁺. Upon excitation by 438 nm blue light, electrons initially at the ⁴A₂ ground state of Cr³⁺ are promoted to the ⁴T₁ (F) energy level, then undergo non-radiative relaxation down to the ⁴T₂ (F) level. Some electrons then return to the ⁴A₂ ground state via radiative transitions, resulting in 780 nm Cr³⁺ emission, while part of the energy is transferred to the nearby Ni²⁺ ions, exciting them to the ³T₁ (F) level. The excited electrons then undergo non-radiative relaxation to the ³T₂ (F) level and eventually return to the ³A₂ (F) ground state, producing 1450 nm Ni²⁺ emission. After excitation, the electrons relax non-radiatively to the ³T₂ (F) level and eventually return to the ³A₂ (F) ground state, producing 1450 nm Ni²⁺ emission. This energy transfer from Cr³⁺ to Ni²⁺ significantly enhances the emission intensity of Ni²⁺ under blue light excitation.⁴⁷

The internal quantum efficiency (IQE) of Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ phosphor was measured at room temperature under 438 nm excitation using an integrating sphere. IQE is defined as the ratio of the number of emitted photons to the number of absorbed photons, as illustrated in Fig. S2. The calculation formula is given as follows:

Here, L_S represents the number of photons emitted by the sample, while E_R and E_S denote the number of reflected excitation photons from the BaSO₄ reference and the sample, respectively. Under 438 nm excitation and within the detection range of 200–1650 nm, the IQE of the sample was measured to be 16.8%. The external quantum efficiency (EQE) is defined as the ratio of the number of emitted photons to the number of incident excitation photons, while the absorption efficiency (AE) is the ratio of the number of absorbed photons to the number of incident excitation photons. Accordingly, EQE can be calculated using the following equation:⁴⁸

EQE = IQE × AE (16)

The experimentally measured AE was 41.6%, based on which the EQE of the sample was calculated to be 6.99% within the 200–1650 nm wavelength range. Fig. S3(a) shows the attenuation curve of near-infrared light emitted by Y₃Al₃MgSiO₁₂:5% Ni²⁺–yCr³⁺ (y = 1%–10%) phosphors near 1450 nm under 438 nm laser excitation. The average lifetimes calculated by eqn (9) and (10) are 285.01 μs, 271.95 μs, 253.76 μs, 242.55 μs and 218.13 μs, respectively. The detailed data of the fitting results are shown in Table S3. Fig. S4(b) reveals the relationship between fluorescence lifetime and y doping content.

2.2.3. Luminous thermal stability. Fig. 7(a and b) illustrates the temperature-dependent luminescence spectra of Y₃Al₃MgSiO₁₂:5% Ni²⁺ and Y₃Ga₃MgSiO₁₂:3% Ni²⁺. As the temperature rises, the luminescence intensity declines gradually, while the morphology and peak position of the luminescence spectrum remain largely unaltered. The primary reason for this phenomenon is that at room temperature, the majority of electrons return to the ground state via the radiation process. With the increase in temperature, the coupling between electron and phonon is enhanced, which makes it more likely for electrons to return from the excited state to the ground state in a non-radiative manner, resulting in a decrease in luminescence intensity.⁴⁹Fig. 7(c) shows the thermal stability of NIR-II-III emission for Y₃Ga₃MgSiO₁₂:3% Ni²⁺ and Y₃Al₃MgSiO₁₂:5% Ni²⁺ at 373 K, respectively, with the corresponding emission intensities retaining 41% and 53% of their values at room temperature. It is evident from the results that the latter exhibits significantly superior thermal stability, which is attributed to the enhanced rigidity of the crystal structure resulting from the substitution of Ga by Al. The rigidity of the host lattice is a critical factor in determining the thermal stability of the material, as a robust framework can effectively suppress lattice vibrations and energy dissipation induced by thermal excitation.²³ A comparison of bond lengths reveals that the average Al₁/Mg–O bond length in Y₃Al₃MgSiO₁₂ is 1.869 Å, which is markedly shorter than the Ga₁/Mg–O bond length of 2.016 Å in Y₃Ga₃MgSiO₁₂, indicating a higher structural rigidity in the former.

Fig. 7 (a and b) Emission spectra of Y₃Al₃MgSiO₁₂:5% Ni²⁺ and Y₃Ga₃MgSiO₁₂:3% Ni²⁺ phosphors at different temperatures. The inset shows the dependence of integrated emission intensity on temperature. (c) The 2D color plot of SWIR emission as a function of temperature. (d) Raman spectra of Y₃Ga₃MgSiO₁₂:Ni²⁺ and Y₃Al₃MgSiO₁₂:Ni²⁺. (eand f) The in situ XRD patterns in the temperature range of 298–523 K.

To analyze the structural changes induced by the substitution of Ga³⁺ with Al³⁺, the Raman spectra of Y₃Ga₃MgSiO₁₂:3% Ni²⁺ and Y₃Al₃MgSiO₁₂:5% Ni²⁺ were compared, as shown in Fig. 7(d). In a manner similar to other garnet-type phosphors, the Raman peaks located at 366 and 533 cm⁻¹ correspond to the internal vibrational modes of the [YO₈] dodecahedra and [GaO₆] octahedra, respectively, while the peak at 751 and 840 cm⁻¹ is assigned to the internal vibrational mode of the [GaO₄] tetrahedra, respectively.⁵⁰ Upon substitution of Ga³⁺ by Al³⁺, the original Raman peaks shift to 393, 553, 775 and 853 cm⁻¹, corresponding to the characteristic vibrations of [YO₈] dodecahedra, [AlO₆] octahedra and [AlO₄] tetrahedra, respectively. In addition, the original broad peaks in the octahedral and tetrahedral regions became sharper after substitution, indicating a transition from structural disorder to order.⁹ This enhancement in structural ordering is attributed to the stronger Al–O bonding compared to Ga–O.⁵¹

To further evaluate the influence of Al substitution on lattice rigidity, the Debye temperature (Θ_D,i) was estimated using the following equation based on the isotropic atomic displacement parameter (U_iso,i):

Here, ħ is Planck's constant, A_i is the atomic mass, k_B is Boltzmann's constant, and U_iso,i is the atomic displacement parameter of the corresponding atom. The value of Θ_D,i is inversely proportional to U_iso,i. According to the Rietveld refinement results shown in Fig. S4, the Al₁ site of Y₃Al₃MgSiO₁₂:5% Ni²⁺ exhibits lower U_iso,i (U_iso,i = 0.0165) values compared to the Ga₁ site of Y₃Ga₃MgSiO₁₂:3% Ni²⁺ (U_iso,i = 0.0264). In addition, based on the DFT-PBE elastic constant calculations, the Debye temperatures of Y₃Ga₃MgSiO₁₂ and Y₃Al₃MgSiO₁₂ were determined to be 649 K and 725 K, respectively, which is consistent with the experimental trend.^27,52 These results confirm a positive correlation between Debye temperature and lattice rigidity, further demonstrating that Al substitution enhances the lattice stiffness.⁴⁸

Fig. 7(e and f) presents the in situ XRD results of Y₃Ga₃MgSiO₁₂:3% Ni²⁺ and Y₃Al₃MgSiO₁₂:5% Ni²⁺ samples within the range of 298–573 K. With an increase in temperature, no significant changes in diffraction peak shapes were observed, indicating that both phosphors exhibit good thermal stability. To further clarify the phase information at different temperatures, Rietveld refinements of the variable-temperature XRD data were performed in Fig. S5 and S6, and the corresponding results are summarized in Tables S4–S7. All parameters met the reliability criteria, and the diffraction data of the samples at different temperatures were well fitted. The variation of unit-cell volume (V) with temperature, as obtained from refinement, is shown in Fig. S7. The results reveal that the unit-cell volumes of Y₃Ga₃MgSiO₁₂:3% Ni²⁺ and Y₃Al₃MgSiO₁₂:5% Ni²⁺ both expand with an increase in temperature. The thermal expansion coefficient (α) can be calculated using the following equation:⁵³

Here, V₀ represents the material volume at the initial temperature, ΔV denotes the corresponding volume change, and ΔT is the temperature variation. The calculated thermal expansion coefficients are α₁ = 9.287 × 10⁻⁶ K⁻¹ and α₂ = 4.559 × 10⁻⁶ K⁻¹, respectively. The lower thermal expansion coefficient further indicates that lattice rigidity is enhanced when Al substitutes Ga, thereby providing a structural basis for the superior thermal quenching resistance of Y₃Al₃MgSiO₁₂:Cr³⁺–Ni²⁺ at elevated temperatures.⁵⁴

Fig. 8(a) presents the emission spectra of Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ phosphor recorded from 80 K to room temperature. As the temperature increase, the emission bands become progressively broader, primarily due to the enhanced electron–phonon interactions.²⁶ The vibrational amplitude of luminescent ions around their equilibrium positions in the host lattice increases with heating, thereby broadening the energy distribution of the ⁴T₂ → ⁴A₂ transition of Cr³⁺ in the 600–1100 nm region and the ³T₂ → ³A₂ transition of Ni²⁺ in the 1100–1650 nm region, which leads to an increased bandwidth of the near-infrared emission.⁴⁷

Fig. 8 (a and b) Emission spectra of the Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ phosphor at different temperatures. (c) Dependence of integrated emission intensity in the wavelength range of 600–1100 nm and 1100–1650 nm on temperature. The inset shows the 2D color plot of SWIR emission as a function of temperature. (d) The plot of ln(I₀/I_T)⁻¹versus 1/kT of the phosphor. (e) Configurational coordinate diagram of Ni²⁺. (f) FWHM versus 1/2kT plot. (g) The decay curves of Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ for 1450 nm emission at different temperatures. The inset shows the temperature-dependent fluorescence lifetime. (h) Survey of emission wavelength, thermal stability, FWHM, and energy transfer efficiency of Ni²⁺ and Cr³⁺–Ni²⁺ activated SWIR phosphors. (i) Comparison of thermal stability for Ni²⁺ and Cr³⁺–Ni²⁺ SWIR phosphors across different emission positions.

In Fig. 8(b and c), the emission spectra of Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ phosphor from room temperature to 473 K indicate that Cr³⁺ ions exhibit better thermal stability compared to Ni²⁺ ions. At 373 K, the phosphor retains 70% and 56% of its room-temperature emission intensity in the NIR-I and NIR-II/III regions, respectively. This is because Ni²⁺ ions typically exhibit a larger Stokes shift than Cr³⁺ ions, resulting in a smaller energy gap between the excited and ground states, which increases the probability of non-radiative processes. As a result, Ni²⁺ ions are more susceptible to thermal quenching at elevated temperatures, whereas Cr³⁺ ions demonstrate superior thermal stability in comparison.^55,56 The activation energy (ΔE) of Ni²⁺, as illustrated in Fig. 8(d), is determined through the application of the following formula:⁵⁷

k is the Boltzmann constant, and A is a constant value, I_T indicates that the specific temperature is the integral strength of Ni²⁺, and I₀ is the emission intensity at the initial temperature. By estimating the slope of the linear fitting curve, the thermal activation energy is 0.23511 eV. Fig. 8(e) shows the schematic diagram of the SWIR spectral deformation of Ni²⁺ caused by thermal quenching. When the excitation is 438 nm, the electron transitions from the ³A_2g (F) ground state to the ³T_1g (P) excited state. In this process, the near-infrared emission of Ni²⁺ is accompanied by the transfer of electrons from the ³T_1g (P) excited state to ³T_2g (F) and then returns to the ground state ³A_2g (F) through non-radiative relaxation. As the temperature increases, the electron vibration of the excited state of ³T_2g (F) increases, thereby increasing the probability of Ni²⁺ electrons returning to the ground state or high energy state after excitation, which leads to the expansion of the spectral range.⁴⁷

The Huang–Kun factor (S) and the average phonon energy (ħω) are key parameters for characterizing the electron–phonon coupling (EPC) strength in Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ phosphors.⁵⁶ By fitting the temperature dependence of the FWHM of the SWIR emission spectrum, their values can be determined by the following equation:

Here, k is the Boltzmann constant. As shown in Fig. 8(f), by fitting the FWHM² with 1/(2kT), S = 1.50 and ħω = 0.035 eV were obtained. The relatively low value of S suggests that there is weak electron–phonon coupling in the system, which can effectively suppress the probability of non-radiative transitions as the temperature increases.⁵⁸Fig. 8(g) and Fig. S8(a, b) respectively present the temperature-dependent luminescence decay curves at 1450 nm for Y₃Al₃MgSiO₁₂:5% Ni²⁺ and Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ phosphors under 408 nm and 438 nm excitation. The decay profiles were well-fitted using a double-exponential decay model (eqn (8)), and the average lifetimes were calculated using eqn (9).⁵⁹ Detailed fitting parameters are listed in Tables S8 and S9. With an increase in temperature, the fluorescence lifetime of the phosphor gradually decreases, which is primarily attributed to thermal quenching resulting from enhanced non-radiative relaxation processes. Under thermal excitation, intensified lattice vibrations increase the rate of non-radiative transitions, thereby significantly reducing the fluorescence lifetime. The non-radiative transition rate p can be estimated using the following equation:^24,59

Here, τ₀ and τ_T represent the fluorescence lifetimes at room temperature and a given elevated temperature, respectively. ΔE can be obtained by fitting the experimental data using the Arrhenius equation, and was calculated to be 0.23511 eV in eqn (19). Since the non-radiative transition rate increases positively with temperature, the observed decrease in fluorescence lifetime can be reasonably explained.

Fig. 8(h) summarizes the luminescence performance parameters of Ni²⁺ singly doped and Cr³⁺–Ni²⁺ co-doped SWIR phosphors reported in the previous studies, including emission position, FWHM, energy transfer efficiency, and thermal stability. Fig. 8(i) compares the thermal stability (373 K) at different emission positions. In general, the emission bands of most previously reported Ni²⁺/Cr³⁺–Ni²⁺ doped materials are primarily concentrated in the NIR-II region, making it difficult to achieve coverage in the longer-wavelength NIR-III region. Moreover, their relatively low energy transfer efficiency and limited thermal stability restrict their potential for applications in the SWIR region. In the NIR-II/III region, especially NIR-III emission, it exhibits strong electron–phonon coupling and poor thermal stability. In contrast, the Y₃Al₃MgSiO₁₂:Cr³⁺–Ni²⁺ phosphor synthesized in this work can achieve ultra-broadband emission spanning the NIR I–III regions under blue-light excitation, with a combined FWHM of up to 496 (185 + 311) nm. Simultaneously, this material exhibits outstanding energy transfer efficiency and favorable thermal stability, surpassing those of previously reported systems and demonstrating great potential for use in high-performance SWIR light sources.

2.4 Performance of the SWIR pc-LED

The SWIR pc-LED device was successfully fabricated by integrating the ultra-broadband NIR-emitting Y₃Al₃MgSiO₁₂:5% Ni²⁺–5% Cr³⁺ phosphor with a commercial 450 nm blue LED chip. Due to the strong penetration capability of SWIR light, the emission from the LED prototype can pass through various materials and enable clear imaging when captured by a SWIR camera. Fig. 9(a) shows the word “YGMS” obscured by a dark filter, making it almost indistinguishable under normal indoor lighting. However, when SWIR LEDs are used in conjunction with commercial SWIR cameras, this information becomes visible, showing the potential of SWIR technology in hidden information detection. Fig. 9(b) shows the relationship between the electroluminescence (EL) spectrum of the device with the driving current (I), respectively. It can be seen that as the driving current increases from 20 mA to 320 mA, the emission intensity of Cr³⁺ in the NIR-I region and Ni²⁺ in the NIR-II-III region increases with the increase of the driving current.

Fig. 9 (a) The visible light images were captured using a standard digital camera under white LED illumination, and the SWIR images were captured using an InGaAs SWIR camera under SWIR pc-LED illumination. (b) PL spectra of an SWIR pc-LED at drive currents of 20 to 320 mA. (c) PL spectra of an SWIR pc-LED at drive currents of 20–100 mA and 20 mA from 1 to 60 minutes, as observed using a thermal imager.

Fig. 9(c) shows the temperature variation of SWIR LED devices at different input currents and for different durations under the same current condition. As the driving current increases from 20 mA to over 100 mA, the device temperature rises by only 4.1 °C (from 35.4 °C to 39.5 °C). In addition, under continuous operation at 20 mA from 1 to 60 minutes, the device exhibited only a minor temperature rise, from 35.4 °C to 36.8 °C, indicating excellent long-term thermal stability. This suggests that the synthesized Y₃Al₃MgSiO₁₂:Cr³⁺–Ni²⁺ phosphor as a luminescence conversion material is capable of efficiently converting blue light into SWIR radiation, giving it the potential to be used in specific technological applications, especially in areas where thermal stability is required.

2.5. Application in chemical compound identification

Due to the specific scattering and absorption characteristics of different substances in SWIR region, particularly the overtone and vibrational absorption of O–H and C–H bonds within the NIR-II–III range.^16,18 These features can be clearly observed in the transmission spectra. The Y₃Al₃MgSiO₁₂:Cr³⁺–Ni²⁺ phosphor exhibits stable emission in the NIR-I region (700–1000 nm) and high sensitivity to vibrational absorption in the NIR-II–III region (1000–1700 nm), thereby providing a solid spectral foundation for both qualitative and quantitative analysis of organic solvents. To evaluate the potential of this phosphor in nondestructive testing, a SWIR spectral analysis setup was constructed, as shown in Fig. 10(a). The setup comprises a phosphor-based light source excited by a xenon lamp (λ_ex = 438 nm) and an NIR detector. Various organic solvents were placed between the phosphor light source and the detector. As illustrated in Fig. 10(b), by comparing the original PL spectrum of the phosphor with its SWIR emission spectra after transmission through different solvents, distinct solvent-specific absorption bands were observed:H₂O (∼1340 nm), CH₃OH (∼1460 nm), C₂H₅OH (∼1480 nm), (CH₂OH)₂ (∼1400 and 1505 nm), and C₃H₈O₃ (∼1345 and 1390 nm). These differences clearly demonstrate the phosphor's capability to distinguish between different solvents. As shown in Fig. 10(c), based on the varying absorption intensities of C₂H₅OH in the NIR-I and NIR-II–III regions, the intensity ratio (I_{800 nm}/I_{1480 nm}) was employed to quantitatively detect the ethanol concentration in C₂H₅OH/H₂O mixtures. As the C₂H₅OH concentration increased from 10 to 90 vol%, the absorption intensities at 800 nm and 1480 nm varied accordingly, and the intensity ratio exhibited a strong linear correlation with concentration, R² = 0.98528 (Fig. 10(d)). These results verify the excellent ethanol recognition capability of the SWIR emission from Y₃Al₃MgSiO₁₂:Cr³⁺–Ni²⁺, further highlighting the potential of SWIR spectroscopy for quantitative analysis of organic compounds. Owing to its stable near-infrared emission and good compatibility with NIR detectors, this phosphor exhibits promising potential for practical applications in non-contact, nondestructive detection scenarios.

Fig. 10 (a) Schematic diagram of the SWIR spectroscopy analytical device. (b) The original spectrum of Y₃Al₃MgSiO₁₂:5%Cr³⁺–5%Ni²⁺ (dashed line) and the spectra after penetrating different solvents. (c) Spectra of SWIR light from the phosphor after penetrating C₂H₅OH/H₂O solution with different C₂H₅OH concentrations. (d) Relationships between the relative PL intensity ratio of the 800 to 1480 nm band with the C₂H₅OH concentration in C₂H₅OH/H₂O solution.

3. Conclusion

In this study, a series of Y₃Al₃MgSiO₁₂:Ni²⁺ and Y₃Al₃MgSiO₁₂:Cr³⁺–Ni²⁺ SWIR phosphors were synthesized via a conventional solid-state reaction method. The Y₃Al₃MgSiO₁₂:Ni²⁺ phosphor exhibited ultra-broadband SWIR emission covering the range of 1100–1650 nm under 408 nm excitation. Compared to the Ni²⁺ doped sample, the co-doped phosphor exhibited a 10.45-fold increase in emission intensity and maintained 56% of its NIR-II-III emission intensity at 373 K relative to room temperature, demonstrating excellent thermal stability. This can be attributed to the increased crystal structure rigidity following Ga/Al substitution and the smaller Huang–Rhys factor, which allows for weaker electron–phonon coupling. Compared to other Cr³⁺–Ni²⁺ co-doped phosphors reported in the literature, this work achieves efficient energy transfer from Cr³⁺ to Ni²⁺, with an energy transfer efficiency (η) of up to 91.6%, significantly enhancing the blue light absorption intensity. The Cr³⁺–Ni²⁺ co-doped Y₃Al₃MgSiO₁₂ phosphor displayed ultra-broadband emission spanning 600–1650 nm under 438 nm excitation, with a FWHM of (185 + 311) nm. Furthermore, a SWIR phosphor-converted LED (pc-LED) device was successfully fabricated by integrating the phosphor with a 450 nm blue LED chip, confirming its potential for SWIR applications such as covert information recognition and non-contact, nondestructive detection scenarios. This work not only introduces a high-performance ultra-broadband SWIR phosphor but also provides valuable guidance for the rational design of efficient Cr³⁺/Ni²⁺ co-doped SWIR materials excitable by blue light.

Author contributions

Qian Zhang: investigation, data curation, and writing – original draft. Xinyu Li: methodology. Ziying Wang: data curation. Qi Zhu: resources. Xuejiao Wang: funding acquisition and supervision. Jiguang Li: writing – review & editing.

Conflicts of interest

The authors of the study report no conflicting interests.

Data availability

Data will be made available on request.

Supplementary data to this article can be found online at  https://doi.org/10.1039/d5qi01841b.

Acknowledgements

This work was supported by the Liaoning Revitalization Talents Program (Grant No. XLYC2403017), the Natural Science Foundation of the Education Department of Liaoning Province (Grant No. JYTMS20231627), and the Young Talents Program of Jinzhou (Grant No. JXYC230103). The authors would also like to acknowledge the support from Scientific Compass (https://www.shiyanjia.com).

References

1. R. J. Xie, Light-emitting diodes: brighter NIR-emitting phosphor making light sources smarter, Light: Sci. Appl., 2020, 9, 1.
2. Z. Hu, C. Fang, B. Li, Z. Zhang, C. Cao, M. Cai, S. Su, X. Sun, X. Shi, C. Li, T. Zhou, Y. Zhang, C. Chi, P. He, X. Xia, Y. Chen, S. S. Gambhir and Z. Cheng, First-in-human liver-tumour surgery guided by multispectral fluorescence imaging in the visible and near-infrared-I/II windows, Nat. Biomed. Eng., 2019, 4, 259–271.
3. F. Zhu, Y. Gao, B. Zhu, L. Huang and J. Qiu, Ni²⁺-doped MgTa₂O₆ phosphors capable of near-infrared II and III emission under blue-light excitation, Chem. Eng. J., 2024, 479, 147568.
4. J. Huang and K. Pu, Activatable Molecular Probes for Second Near–Infrared Fluorescence, Chemiluminescence, and Photoacoustic Imaging, Angew. Chem., Int. Ed., 2020, 59, 11717–11731.
5. S. Diao, G. Hong, A. L. Antaris, J. L. Blackburn, K. Cheng, Z. Cheng and H. Dai, Biological imaging without autofluorescence in the second near-infrared region, Nano Res., 2015, 8, 3027–3034.
6. C. Li, G. Chen, Y. Zhang, F. Wu and Q. Wang, Advanced Fluorescence Imaging Technology in the Near-Infrared-II Window for Biomedical Applications, J. Am. Chem. Soc., 2020, 142, 14789–14804.
7. V. Rajendran, H. Chang, R. Liu and R. S. Liu, Recent progress on broadband near-infrared phosphors-converted light emitting diodes for future miniature spectrometers, Opt. Mater., 2019, 1, 100011.
8. H. Chang, F. Q. He, E. H. Songa and Q. Y. Zhang, Site-selective occupancy of Cr³⁺ enabling tunable emission from near-infrared I to II in fluoride LiInF₄:Cr³⁺, J. Mater. Chem. C, 2024, 12, 3175–3184.
9. L. Yan, G. Zhu, S. Ma, S. Li, Z. Li, X. Luo and B. Dong, Emerging Fe³⁺ Doped Broad NIR–Emitting Phosphor Ca_2.5Hf_2.5(Ga, Al)₃O₁₂: Fe³⁺ for LWUV Pumped NIR LED, Laser Photonics Rev., 2024, 18, 2301200.
10. X. Zhu, S. Zhang and S. Ye, Does Mn²⁺-Mn²⁺ Spin-Exchange Interaction Involve Mn²⁺ Luminescence of Mn²⁺-Doped/Concentrated Materials, J. Phys. Chem. Lett., 2024, 15, 2804–2814.
11. M. H. Fang, G. N. A. De Guzman, Z. Bao, N. Majewska, S. Mahlik, M. Grinberg, G. Leniec, S. M. Kaczmarek, C. W. Yang, K. M. Lu, H. S. Sheu, S. F. Hu and R. S. Liu, Ultra-high-efficiency near-infrared Ga₂O₃: Cr³⁺ phosphor and controlling of phytochrome, J. Mater. Chem. C, 2020, 8, 11013–11017.
12. Y. Zhao, X. Wang, Q. Wang, T. Zhou, Y. Chen, J. Xu, X. Tang and R. J. Xie, Abnormal spectral broadening of ordered-structure near-infrared phosphor La₂CaHfO₆: Cr³⁺, J. Mater. Chem. C, 2024, 12, 10532–10539.
13. H. Cai, H. Chen, H. Zhou, J. Zhao, Z. Song and Q. L. Liu, Controlling Cr³⁺/Cr⁴⁺ concentration in single-phase host toward tailored super-broad near-infrared luminescence for multifunctional applications, Mater. Today Chem., 2021, 22, 100555.
14. M. Jin, F. Li, J. Xia, L. Zhu, Q. Zhu and J. G. Li, A new persistent luminescence phosphor of ZnGa₂O₄: Ni²⁺ for the second near-infrared transparency window, J. Alloys Compd., 2023, 931, 167491.
15. C. J. Tang, B. M. Liu, L. Huang, J. Wang and Q. Tang, Ni²⁺-activated MgTi₂O₅ with broadband emission beyond 1200 nm for NIR-II light source applications, J. Mater. Chem. C, 2022, 10, 18234–18240.
16. C. Wang, J. Lin, X. Zhang, H. Dong, M. Wen, S. Zhao, S. Yuan, D. Zhu, F. Wu and Z. Mu, Efficient ultra-broadband NIR-II emission achieved by multi-site occupancy in Mg₃Ga₂GeO₈: Ni²⁺ phosphor, J. Alloys Compd., 2023, 942, 168893.
17. L. Yuan, Y. Jin, D. Zhu, Z. Mou, G. Xie and Y. Hu, Ni²⁺-Doped Yttrium Aluminum Gallium Garnet Phosphors: Bandgap Engineering for Broad-Band Wavelength-Tunable Shortwave-Infrared Long-Persistent Luminescence and Photochromism, ACS Sustainable Chem. Eng., 2020, 8, 6543–6550.
18. B. M. Liu, X. X. Guo, L. Huang, R. F. Zhou, R. Zou, C. G. Ma and J. Wang, A Super–Broadband NIR Dual–Emitting Mg₂SnO₄: Cr³⁺, Ni²⁺ Phosphor for Ratiometric Phosphor–Converted NIR Light Source Applications, Adv. Mater. Technol., 2022, 8, 2201181.
19. F. Zhu, Y. Gao, J. Ding and J. Qiu, Synergistic enhancement of the near-infrared luminescence properties of Ni²⁺-doped SrTiO₃ perovskite phosphors and their application, J. Mater. Chem. C, 2023, 11, 10236–10246.
20. 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.
21. B. M. Liu, X. X. Guo, L. Y. Cao, L. Huang, R. Zou, Z. Zhou and J. Wang, A High-efficiency blue-LED-excitable NIR-II-emitting MgO: Cr³⁺, Ni²⁺ phosphor for future broadband light source toward multifunctional NIR spectroscopy applications, Chem. Eng. J., 2023, 452, 139313.
22. G. Merkininkaite, A. Zabiliūtė-Karaliūnė, T. Jüstel, V. Klimkevicius, S. Sakirzanovas and A. Katelnikovas, Ce³⁺ → Cr³⁺ energy transfer in Y₃Al₃MgSiO₁₂ garnet host and application in horticultural lighting, Ceram. Int., 2023, 49, 16796–16808.
23. Y. Wang, Z. Xu and M. Shang, Simultaneous NIR emission and thermal stability enhancement in garnet–type NIR phosphors through the synergistic effect of lattice distortion and enhanced rigidity, Laser Photonics Rev., 2024, 18, 2400295.
24. T. Liu, Z. Liu, J. Wu, K. Zhang, H. An, Z. Hu and H. Li, Broadband near-infrared persistent luminescence in Ni²⁺-doped transparent glass-ceramic ZnGa₂O₄, New J. Chem., 2022, 46, 851–856.
25. L. Fang, L. Lu, L. Zhang, H. Wu, G. Pan, Z. Hao and J. Zhang, Ni²⁺ activated broadband near-infrared II germanate phosphor for pc-LED applications, J. Alloys Compd., 2024, 1003, 175743.
26. J. Wang, L. Jiang, R. Pang, S. Zhang, D. Li, K. Li, C. Li and H. Zhang, Cr³⁺-doped borate phosphors for broadband near-infrared LED applications, Inorg. Chem. Front., 2022, 9, 2240–2251.
27. Z. Liao, M. Sójka, J. Zhong and J. Brgoch, Developing the Na₂CaZr₂Ge₃O₁₂: Cr³⁺ garnet phosphor for advanced NIR pc-LEDs in night vision and bioimaging, Chem. Mater., 2024, 36, 4654–4663.
28. J. Wang, L. Jiang, R. Pang, S. Zhang, D. Li, K. Li and H. Zhang, Cr³⁺-doped borate phosphors for broadband near-infrared LED applications, Inorg. Chem. Front., 2022, 9, 2240–2251.
29. Z. Fu, S. Zhou, T. Pan and S. Zhang, Band structure calculations on the monoclinic bulk and nano-SrAl₂O₄ crystals, J. Solid State Chem., 2005, 178, 230–233.
30. S. Wu, G. Zhou, Y. Wu, P. Xiong, B. Xiao and Z. Zhou, Multiple Defect–Induced High–Resolution Near–Infrared Mechanoluminescent Materials for Non–Destructive Detection of Blood Glucose and Lipids, Adv. Mater., 2024, 36, 2408508.
31. C. Y. Chang, M. H. Huang, K. C. Chen, W. T. Huang, M. Kamiński, N. Majewska, T. Klimczuk, J. H. Chen, D. H. Cherng, K. M. Lu, W. K. Pang, V. K. Peterson, S. Mahlik, G. Leniec and R. S. Liu, Ultrahigh Quantum Efficiency Near-Infrared-II Emission Achieved by Cr³⁺ Clusters to Ni²⁺ Energy Transfer, Chem. Mater., 2024, 36, 3941–3948.
32. Y. Duan, Y. Liu, G. Zhang, L. Yao and Q. Shao, Broadband Cr³⁺-sensitized upconversion luminescence of LiScSi₂O₆:Cr³⁺/Er³⁺, J. Rare Earths, 2021, 39, 1181–1186.
33. Z. Gao, Y. Zhang, Y. Li, S. Zhao, P. Zhang, X. Dong, D. Deng and S. Xu, Blue light-excited broadband NIR-II-emitting Li₂ZnSn₃O₈: Cr³⁺, Ni²⁺ phosphor for multifunctional optical applications, Inorg. Chem. Front., 2024, 11, 4711–4720.
34. S. Miao, Y. Liang, Y. Zhang, D. Chen and X. J. Wang, Blue LED–Pumped Broadband Short–Wave Infrared Emitter Based on LiMgPO₄:Cr³⁺, Ni²⁺ Phosphor, Adv. Mater. Technol., 2022, 7, 2200320.
35. L. Jiang, L. Zhang, X. Jiang, J. Xie, G. Lv and Y. Su, Cr³⁺–Yb³⁺–Ni²⁺ Tri–Doped NIR Phosphors with Spectral Output Covering NIR–I and NIR–II Regions, Adv. Mater. Technol., 2024, 9, 2301495.
36. F. Zhao, Y. Shao, Q. Liu and J. Zhong, Blue–Light–Excitable Broadband Ca₃Ga₂Ge₃O₁₂: Cr³⁺, Ni²⁺ Phosphor for the Applications in NIR–II Window, Laser Photonics Rev., 2024, 18, 2400447.
37. L. Zheng, H. Wu, K. Zhao, Y. Liu, S. Wen, G. H. Pan, H. Wu, Z. Hao, L. Zhang and J. Zhang, Ultrabroadband Near–Infrared Zn₃Ga₂GeO₈: Cr³⁺, Ni²⁺ Phosphor for PiG pc–LED, Adv. Opt. Mater., 2024, 12, 2401521.
38. 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.
39. C. Wang, Y. Zhang, X. Han, D. Hu, D. He, X. Wang and H. Jiao, Energy transfer enhanced broadband near-infrared phosphors: Cr³⁺/Ni²⁺ activated ZnGa₂O₄-Zn₂SnO₄ solid solutions for the second NIR window imaging, J. Mater. Chem. C, 2021, 9, 4583–4590.
40. 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.
41. Y. Zhuo, F. Wu, Y. Niu, Y. Wang, Q. Zhang, Y. Teng and Z. Mu, Super broadband emission across NIR–I and NIR–II under blue light excitation of Cr³⁺, Ni²⁺ Co–Doped Sr₂GaTaO₆ phosphor achieved by two–site occupation and effective energy transfer, Laser Photonics Rev., 2024, 18, 2400105.
42. V. Rajendran, K. C. Chen, W. T. Huang, M. Kamiński, M. Grzegorczyk, S. Mahlik, G. Leniec, K. M. Lu, D. H. Wei, H. Chang and R. S. Liu, Unraveling Luminescent Energy Transfer Pathways: Futuristic Approach of Miniature Shortwave Infrared Light-Emitting Diode Design, ACS Energy Lett., 2023, 8, 2395–2400.
43. A. Satpathy, W. T. Huang, M. H. Chan, T. Y. Su, M. Kamiński, N. Majewska, S. Mahlik, G. Leniec, S. M. Kaczmarek, M. Hsiao and R. S. Liu, Near–Infrared I/II Nanophosphors with Cr³⁺/Ni²⁺ Energy Transfer for Bioimaging, Adv. Opt. Mater., 2023, 11, 2300321.
44. 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.
45. Y. Zhuo, F. Wu, Y. Niu, Y. Wang, Q. Zhang, Y. Teng, H. Dong and Z. Mu, Super Broadband Emission Across NIR–I and NIR–II Under Blue Light Excitation of Cr³⁺, Ni²⁺ Co–Doped Sr₂GaTaO₆ Phosphor Achieved by Two–Site Occupation and Effective Energy, Laser Photonics Rev., 2024, 18, 2400105.
46. Z. Chen, Y. Xiang, X. Qin, L. Zhong, H. Liao, S. Liu and M. Wu, Revealing energy transfer mechanisms and accelerating intelligent detection: Cr³⁺ and Ni²⁺ co-doped Lu₂ CaMg₂Si₃O₁₂ phosphors for NIR applications, J. Mater. Chem. C, 2025 10.1039/D5TC00476D.
47. G. Cai, T. Naillon, J. Seguin, D. Scherman, N. Mignet, C. Chaneac and B. Viana, ZGSO: Cr³⁺, Ni²⁺ Persistent Phosphors with Dual Emission in NIR–I and SWIR Ranges for Bio–Imaging Applications, Small, 2024, 20, 2406507.
48. Q. Zhang, G. Li, D. Liu, P. Dang, L. Qiu, H. Lian, M. S. Molokeev and J. Lin, Optical Thermometer Based on Efficient Near–Infrared Dual–Emission of Cr³⁺ and Ni²⁺ in Magnetoplumbite Structure, Adv. Opt. Mater., 2023, 12, 2301429.
49. M. Tan, Y. Gao, J. Chen, X. Lu, B. Zhu, L. Huan and J. Qiu, A Nickel-doped, lanthanum gallium germanate-based phosphor as an ultra-broadband short-wavelength infrared region emitter for optical imaging, Ceram. Int., 2024, 50, 23685–23693.
50. D. Wang, L. Yan, G. Zhu, S. Ma, N. Zhou, S. Li 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.
51. Y. Zhao, X. Wang, Q. Wang, T. Zhou, Y. Chen, J. Xu and R. J. Xie, Abnormal spectral broadening of ordered-structure near-infrared phosphor La₂CaHfO₆: Cr³⁺, J. Mater. Chem. C, 2024, 12, 10532–10539.
52. M. D. Dramićanin, Ł. Marciniak, S. Kuzman, W. Piotrowski, Z. Ristić, J. Periša and C. G. Ma, Mn⁵⁺-activated Ca₆Ba(PO₄)₄O near-infrared phosphor and its application in luminescence thermometry, Light: Sci. Appl., 2022, 11, 279.
53. F. Zhu, Y. Gao and J. Qiu, Cr³⁺-Doped LiAlO₂ NIR-I emitting phosphors with superior resistance to thermal quenching for night vision monitoring and bioimaging, ACS Appl. Mater. Interfaces, 2024, 16, 60599–60607.
54. J. Sun, M. Jia, W. Xu, M. Wang and Z. Sun, Designing high thermally stable deep red phosphors based on low thermal expansion coefficients for optical applications, Opt. Lett., 2024, 49, 1504–1507.
55. H. Yu, Z. Gao, H. Wang, S. Zhao, D. Deng and S. Xu, Zero-thermal-quenching broadband NIR-II emitting Mg₄Nb₂O₉: Ni²⁺ phosphor via defective engineering and multi-site occupation, J. Alloys Compd., 2025, 1020, 179519.
56. A. Balhara, S. K. Gupta, B. Modak, A. K. Yadav, B. S. Naidu and K. Sudarshan, Broadband Shortwave Infrared-Emitting Cr³⁺-and Ni²⁺-Codoped Y₃Al₂Ga₃O₁₂ Phosphor with Excellent Thermal Stability for Multifunctional Applications, ACS Appl. Mater. Interfaces, 2024, 17, 1509–1521.
57. Y. Li, Y. Lu, H. Ju, W. Wang, D. Tu, H. Lu and H. Zou, A High-Efficiency NIR-II Emitting LiAlSiO₄: Cr/Ni Phosphors through Violet Excitation for NIR pc-LED Application, ACS Appl. Opt. Mater., 2024, 2, 1933–1938.
58. C. Gong, Z. Wang and J. Wang, A novel broadband near-infrared phosphor NaBaScSi₂O₇: Fe³⁺ and its application in pc-LED and night vision, Sci. China Mater., 2025, 1, 8.
59. S. Yang, X. Xue and F. Jiang, A novel KSrScSi₂O₇: xCr silicate phosphor, luminescence/chromaticity, and applications in NIR pc-LED and stable green ceramic pigments, J. Rare Earths, 2025 DOI:10.1016/j.jre.2024.12004.

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