Efficiency optimization of BaY₂Al_2−ySc_yGa₂SiO₁₂:xCr³⁺ garnet phosphors with sustained anti-thermal quenching behavior

Abstract

Systematic studies on optimizing the efficiency of garnet phosphors while preserving their anti-thermal quenching behavior remain limited. In this study, we explore the garnet-structured BaY₂Al_2−ySc_yGa₂SiO₁₂:xCr³⁺ phosphor system, emphasizing the critical role of crystal field modulation in achieving this balance. For y = 0, the optimized BaY₂Al_1.95Ga₂SiO₁₂:0.05Cr³⁺ phosphor exhibits broadband deep-red and near-infrared (NIR) emission, with an internal quantum efficiency (IQE) of 73%, an external quantum efficiency (EQE) of 13%, and a luminescence intensity that reaches 108% of its room-temperature value at 150 °C, demonstrating a pronounced anti-thermal quenching behavior. This desired property arises from the thermal population shift of the dominant excited states from the ²E to ⁴T₂ state with increasing temperature. With y > 0, the systematic substitution of Al³⁺ with Sc³⁺ induces greater structural distortion around Cr³⁺ ions, optimizing the crystal field environment, broadening NIR emission, and further enhancing luminescence efficiency. The optimized composition, BaY₂Al_1.5Sc_0.5Ga₂SiO₁₂:0.05Cr³⁺, achieves an impressive IQE of 82%, an EQE of 25% and maintains 104% of its luminescence intensity at 150 °C. A NIR pc-LED with a remarkable output power of 241 mW@300 mA was fabricated using this phosphor, enabling the capture of high-quality finger vein images. This demonstration confirms the feasibility of these phosphors for biometric authentication and other advanced NIR applications.

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Introduction

Lighting accounts for about 20% of global electricity consumption,^1,2 making energy-efficient solutions critical. Phosphor-converted light-emitting diodes (pc-LEDs) have emerged as the leading technology for energy-efficient lighting.^3,4 However, their luminous efficacy decreases by 0.2–1% for every 1 °C rise in temperature,^5–7 a challenge that becomes more pronounced under high-current operation due to thermal accumulation. This performance degradation, driven by non-radiative transitions in the phosphors,⁸ leads to a phenomenon known as thermal quenching, where the luminescence decreases as the operating temperature exceeds 150 °C.^9,10 Thermal quenching not only reduces emission efficiency but also destabilizes the emission center, compromising the color rendering quality of pc-LEDs.^11,12 Addressing this limitation requires the development of thermally robust phosphors with zero- or anti-thermal quenching behavior, enabling improved performance in practical applications.

Thermal quenching originates from non-radiative relaxation, where excitation energy dissipates as heat through lattice vibrations.¹³ This process involves several deactivation pathways, including crossover, multiphonon relaxation and thermal ionization, all of which contribute to luminescence loss.^14,15 Various strategies have been developed to mitigate thermal quenching, such as defect engineering, energy transfer-based approaches and structural modulations.^16–18 Defect engineering leverages defect energy levels within the bandgap to transfer energy to the emission center, compensating for light emissions. This approach has been successfully employed in phosphor doped with ions such as Eu²⁺, Ce³⁺, Cr³⁺, and Bi³⁺.^19–22 However, designing precise defect energy levels remains a significant challenge, limiting its widespread applicability. Energy transfer strategies, commonly observed between active center pairs, such as Bi³⁺/Eu³⁺, Eu²⁺/Mn²⁺, and Cr³⁺/Yb³⁺,^22–25 aim to prevent thermal quenching of the main luminescence center. While effective, these strategies often sacrifice the luminescence of the donor ions, leading to overall efficiency loss.²⁶ In contrast, structural modulation offers a more comprehensive approach, by not only influencing defect energy levels and energy transfer processes but also tailoring structural rigidity, crystallinity, symmetry, and particularly the crystal field environment of the materials.^27,28 By improving these properties, structural modulation significantly enhances the luminescence thermal stability of phosphors, making it a more promising strategy for addressing thermal quenching and advancing high-performance materials.

Cr³⁺-doped deep-red and near-infrared phosphors are ideal candidates to achieve highly efficient NIR emission. These materials find extensive applications in advanced lighting systems, night vision, non-destructive analysis, plant growth, bioimaging, and biometrics,^29,30 owing to their high penetrability, rapid response, and low chemical damage potential.^31–34 Notably, the 3d³ electronic configuration of Cr³⁺ allows for broad-band emission spanning deep-red to NIR regions.³⁵ However, Cr³⁺-doped phosphors are also susceptible to thermal quenching, limiting their practical utility. Structural modulation has proven effective in addressing this limitation by regulating the local crystal environment to enhance luminescence thermal stability.^36,37 For example, the incorporation of Li⁺ within Ca₃Sc₂Si₃O₁₂:Cr³⁺ makes it retain 97.4% of NIR emission at 150 °C,³⁸ while co-substitution of Zn²⁺–Zr⁴⁺ in Ca₃Y₂Ge₃O₁₂:Cr³⁺ also enhances the luminescence thermal stability.³⁹ Specifically, the structural modulation of garnet phosphors can induce a change in the luminescence mechanism from the spin-forbidden ²E → ⁴A₂ to the spin-allowed ⁴T₂ → ⁴A₂ transition,^37,40 which is closely linked to the anti-thermal quenching.⁴¹ However, systematic studies on optimizing the efficiency of garnet phosphors while preserving their anti-thermal quenching behavior remain limited.

In this work, we developed a series of garnet-structured BaY₂Al_2−x−ySc_yGa₂SiO₁₂:xCr³⁺ phosphors, leveraging the crystal field modulation approach to achieve enhanced NIR emission while maintaining the sustained anti-thermal quenching properties. For y = 0, the optimal composition BaY₂Al₂Ga₂SiO₁₂:0.05Cr³⁺ already demonstrated anti-thermal quenching behavior attributed to the thermal population shift of the dominant excited states from the ²E to ⁴T₂ state with increasing temperature. Substituting Al³⁺ with Sc³⁺, which has a larger ionic radius and weakens the crystal field, induced atomic disorder and lattice distortion. This structural modulation broke the d–d forbidden transition nature of Cr³⁺, significantly boosting the emission efficiency. The optimized composition, BaY₂Al_1.5Sc_0.5Ga₂SiO₁₂:0.05Cr³⁺ (y = 0.5), achieved a remarkable IQE of 82% and retained 104% of its luminescence intensity at 150 °C. To demonstrate its application potential, a pc-LED integrating a blue LED chip with a BaY₂Al_1.5Sc_0.5Ga₂SiO₁₂:0.05Cr³⁺ phosphor was fabricated, successfully enabling finger vein imaging, night vision and non-destructive testing. These findings highlight the critical role of crystal field modulation in optimizing luminescence efficiency while maintaining sustained anti-thermal quenching behavior.

Results and discussion

Structure characterization and phase analysis

BaY₂Al₂Ga₂SiO₁₂ (AG) adopts a typical garnet structure with a space group of Ia3̄d. The crystal structure and coordination polyhedra are depicted in Fig. 1a.^42,43 In this structure, each Al³⁺ and Ga³⁺ center coordinates with 6 oxygen atoms, forming octahedra that provide two distinct octahedral coordination environments in the phosphor. Additionally, Al³⁺, Ga³⁺ and Si⁴⁺ centers coordinate with four oxygen atoms each, forming three distinct tetrahedra: [AlO₄], [GaO₄] and [SiO₄]. Meanwhile, Ba²⁺ or Y³⁺ centers coordinate with eight oxygen atoms to form [BaO₈] and [YO₈] dodecahedra, respectively. The [Y/BaO₈] dodecahedra connect to the [Al/GaO₆] octahedra through shared edges, while the [Al/Ga/SiO₄] tetrahedra link to the [Al/GaO₆] octahedra via shared corners. With a similar ionic radius (R = 0.615 Å, CN = 6) and valence state to Al³⁺ (R = 0.535 Å, CN = 6) and Ga³⁺ (R = 0.62 Å, CN = 6), Cr³⁺ preferentially occupies octahedral sites, replacing either Al³⁺ or Ga³⁺.

Fig. 1 (a) The crystal structure of AG; (b) XRD patterns of AG:xCr³⁺ (x = 0, 0.01, 0.02, 0.04, 0.05, and 0.06); (c) XRD patterns of A_2−yS_yG:0.05Cr³⁺ (y = 0, 0.3, 0.5, 0.7, 0.9, 1.2, 1.7, and 2) and magnified XRD patterns in the region between 31 and 34°; (d) SEM and EDS elemental mapping images of A_1.5S_0.5G:0.05Cr³⁺.

Fig. 1b presents the XRD patterns of AG:xCr³⁺ phosphors over the 2θ range of 10–90°. The diffraction peaks match the standard reference card (PDF#89-6660), confirming the successful synthesis of the garnet-structured AG:xCr³⁺ phosphors. The impurity phase peak observed at 2θ = 28° is attributed to trace amounts of BaAl₂O₄ which has a minimal impact on the luminescence performance due to its negligible content. To further analyze the crystal structure of AG:xCr³⁺ phosphors, Rietveld refinement was conducted on the XRD data using Y₃Al₂Ga₃O₁₂ (PDF#89-6660) and BaAl₂O₄ (PDF#82-2001) as starting models. Fullprof software was utilized to refine both the primary and impurity phases with the results presented in Fig. S1(a–f) (ESI†). The refinement factors R_p, R_wp and R_exp, all below 6.70, 9.30 and 5.20, respectively, indicate a strong correlation between experimental and simulated data, validating the reliability of the refinements. Furthermore, as the Cr³⁺ doping concentration increases, the unit cell constants (a/b/c) and volume (V) show a consistent upward trend (Fig. S1g and h, ESI†), confirming the substitution of Al³⁺ by Cr³⁺ ions, which possess a slightly larger ionic radius.

To enhance the phosphor performance, Sc³⁺ was introduced to replace Al³⁺ in a series of BaY₂Al_2−ySc_yGa₂SiO₁₂:0.05Cr³⁺ (denoted as A_2−yS_yG:0.05Cr³⁺) phosphors. XRD patterns (Fig. 1c) reveal that at Sc³⁺ content below y = 0.9, an impurity phase peak corresponding to trace amount of BaAl₂O₄ is present at 2θ = 28°. EDS elemental mapping (Fig. 1d) confirms uniform elemental distribution and successful incorporation of Sc³⁺ into the lattice. At y = 0.9, the impurity phase peak of BaAl₂O₄ disappears, forming a pure A_1.1S_0.9G:0.05Cr³⁺ phase. However, a trace of Ba₉Sc₂(SiO₄)₆ emerge at y = 1.7. An enlarged XRD view at 31–34° shows peaks shift to lower angles with increasing Sc³⁺ content, indicating lattice expansion due to Sc³⁺ substitution for smaller Al³⁺ ions.^44,45 Rietveld refinement (Fig. S2a–g, ESI†) yields R_p, R_wp, and R_exp within an acceptable range, confirming reliable fitting. Increasing the Sc³⁺ content also causes a consistent rise in cell constants (a/b/c) and unit cell volume (V), further validating the successful substitution (Fig. S2h, ESI†).

Photoluminescence properties

To evaluate the impact of Cr³⁺ doping on the luminescence performance of the AG matrix and identify the optimal Cr³⁺ doping concentration, the absorption, excitation, and emission spectra of AG:xCr³⁺ phosphors were systematically analyzed. The absorption spectra, shown in Fig. 2a, reveal a host absorption band near 280 nm, while the absorption peaks near 370, 450 and 600 nm are attributed to the ⁴A₂ → ⁴T₁ (⁴P), ⁴A₂ → ⁴T₁ (⁴F), and ⁴A₂ → ⁴T₂ (⁴F) electron transitions of Cr³⁺, respectively. Notably, the absorption band at around 450 nm aligns with the emission wavelength of the commercially available blue LED chips, enabling their efficient and cost-efficient excitation by a 450 nm source. The excitation spectra, illustrated in Fig. 2b, display two prominent excitation peaks near 445 and 600 nm when monitored at 708 nm. These peaks are attributed to the ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ electron transitions of Cr³⁺, respectively. The emission spectra presented in Fig. 2c confirm the presence of NIR emission spanning 650–1100 nm under 445 nm excitation. A strong emission band centered at 690 nm is observed, attributed to the spin-forbidden ²E → ⁴A₂ transition of Cr³⁺.^46,47 As the Cr³⁺ content increases, the emission intensity initially rises and then declines, peaking at x = 0.05, corresponding to AG:0.05Cr³⁺. The reduction in luminous efficiency at Cr³⁺ concentrations above 0.05 is attributed to concentration quenching. As the Cr³⁺ content increases, the probability of energy transfer between adjacent Cr³⁺ ions also increases. This energy transfer mechanism is typically assessed by the critical distance (R_c). When R_c is less than 5 Å, energy transfer occurs primarily via exchange interactions; when R_c exceeds 5 Å, multipole–multipole interactions become dominant. R_c can be estimated using the following equation:^21,48,49where V represents the cell volume, x_c is the optimal doping content of Cr³⁺, and N refers to the number of activator ions per unit cell. For our system, the calculated R_c is 18.68 Å, significantly exceeds 5 Å, indicating that multipole–multipole interactions govern the energy transfer process among Cr³⁺ ions. When x = 0.05, the AG:Cr³⁺ phosphor achieves a 73% IQE, but its EQE remains at 13% due to low absorption efficiency (Fig. S3, ESI†). Note that the positions of the bands in both the excitation and emission spectra remain consistent with increasing Cr³⁺ doping content at low levels, indicating that the crystal field strength is minimally affected. However, as the Cr³⁺ doping content reaches 0.05, a distinct shoulder at around 750 nm emerges, corresponding to the ⁴T₂ → ⁴A₂ transition. This suggests that while low Cr³⁺ doping levels have negligible influence on the crystal field strength, higher doping concentrations may induce subtle modifications in the crystal field environment in AG:Cr³⁺ phosphors.

Fig. 2 (a) Absorption spectra of AG:xCr³⁺; (b) normalized excitation spectra of AG:xCr³⁺; (c) emission spectra of AG:xCr³⁺; (d) absorption spectra of A_2−yS_yG:0.05Cr³⁺; (e) normalized excitation spectra of A_2−yS_yG:0.05Cr³⁺; (f) emission spectra of A_2−yS_yG:0.05Cr³⁺; (g) Tanabe–Sugano energy level diagram for Cr³⁺ ions in an octahedral environment; (h) the decay curves of A_2−yS_yG:0.05Cr³⁺; and (i) the IQE, AE and EQE of A_2−yS_yG:0.05Cr³⁺.

The decay curves of AG:xCr³⁺ phosphors, as shown in Fig. S4 (ESI†), could not be adequately described by a single exponential function. Instead, they were well-fitted using a double exponential decay function provided below:^50,51

I_t = I₀ + A₁e^−(t/τ₁) + A₂e^−(t/τ₂) (2)

where t denotes time, I₀ and I_t stand for the emission intensity at t = 0 and time t, respectively. A₁ and A₂ are fitting constants, τ₁ and τ₂ correspond to the decay times, and τ_ave represents the average lifetime. Fig. S4 (ESI†) illustrates that under 445 nm excitation, the lifetimes of the three emission peaks located at 690, 708 and 725 nm range from 357 to 500 μs, showing a consistent decreasing trend with increasing Cr³⁺ content. This observation can be explained by the dual effects of increased Cr³⁺ doping: while higher doping levels enhance the absorption efficiency of excitation light, they also promote energy transfer between Cr³⁺ ions. This mutual energy transfer leads to non-radiative losses, ultimately reducing the fluorescence lifetime with increasing Cr³⁺ concentration.

To further enhance the luminescence efficiency of the optimal AG:0.05Cr³⁺ phosphor, Sc³⁺ was introduced to modulate the crystal field at a fixed Cr³⁺ content, resulting in a series of A_2−yS_yG:0.05Cr³⁺ phosphors. The absorption spectra (Fig. 2d) reveal a consistent enhancement in absorption intensity with the incorporation of Sc³⁺. This improvement arises from the substitution of Al³⁺ with Sc³⁺, a cation with a larger radius, which disrupts the inversion symmetry of the octahedral sites occupied by Cr³⁺. Such disruption enhances the electric dipole transition of Cr³⁺, as indicated by our density functional theory (DFT) calculations shown in Fig. S5 (ESI†), thereby increasing both the absorptivity and luminescence efficiency of the phosphors. Additionally, the incorporation of Sc³⁺ decreases the crystal field strength of AG:0.05Cr³⁺ phosphors, resulting in a luminescence mechanism shift from the spin-forbidden ²E → ⁴A₂ transition to the spin-allowed ⁴T₂ → ⁴A₂ transition, thus enhancing the luminescence intensity. The normalized excitation spectra (Fig. 2e) reveal the ⁴A₂ → ⁴T₁ transition within the range of 350–500 nm and the ⁴A₂ → ⁴T₂ transition in the range of 500–675 nm. The corresponding emission spectra (Fig. 2f) demonstrate that the luminescence in the range of 650–1100 nm arises from the spin-allowed transition ⁴T₂ → ⁴A₂ and the partially spin-forbidden transition ²E → ⁴A₂, indicating an intermediate crystal field environment. In addition, a very sharp peak at ∼690 nm is observed, which is typically attributed to the ZPL emission of the ²E → ⁴A₂ transitions. As the Sc³⁺ content increases, the integrated emission intensity of A_2−yS_yG:0.05Cr³⁺ phosphors initially increases, peaking at y = 0.5, before declining with further doping. Furthermore, the relative intensity ratio of emission from ⁴T₂ → ⁴A₂ and ²E → ⁴A₂ transitions progressively increased with higher Sc³⁺ content, indicating a reduction in the crystal field strength of A_2−yS_yG:0.05Cr³⁺ phosphors.

The above conclusion is further supported by calculations of the crystal field strength for A_2−yS_yG:0.05Cr³⁺ phosphors, which can be determined using the following equations:^52–54

10D_q = E(⁴A₂ → ⁴T₂) − ΔS/2 (4)

where D_q represents the crystal field constant, B is the Racah parameter, E(⁴A₂ → ⁴T₁) and E(⁴A₂ → ⁴T₂) denote the energies corresponding to the ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ electronic transitions of Cr³⁺, respectively, while ΔS represents the difference between the excitation peak energy (⁴A₂ → ⁴T₂) and the emission peak energy (⁴T₂ → ⁴A₂). Based on the excitation spectra (Fig. 2e), the crystal field strength (D_q/B) for all A_2−yS_yG:0.05Cr³⁺ samples was calculated, and the results are summarized in Table S1 (ESI†). To visually illustrate the effect of Sc³⁺ incorporation on crystal field strength, Fig. 2g shows the variation of D_q/B values within the Tanabe–Sugano plot. As the Sc³⁺ content increases, the crystal field strength decreases from 2.65 to 2.26. At the same time, the FWHM of the emission spectra (Fig. 2f) broadens significantly, increasing from 75 to a maximum of 138 nm (at y = 2), reflecting the modulation of the luminescence properties by the weakened crystal field strength. It should be noted that the Tanabe–Sugano plot in Fig. 2g does not fully reproduce the experimentally obtained excited-state energies expressed as E/B, particularly for the ²E state, on its vertical axis. For instance, for D_q/B = 2.65 (y = 0), the experimental E/B value for ²E is ∼25.2, while the diagram yields a value of ∼21.0. When calculating the excited-state energies for Cr³⁺ ions, strong phonon coupling effects must be considered,⁵⁵ as the equilibrium energy levels are not simply given by the zero-phonon line (ZPL) energies of the PL or PLE bands, especially at elevated temperatures.⁵⁶ Consequently, accurately determining these ZPL energies from experimental spectra is very challenging,^56,57 even when the Stokes shift term (ΔS/2) is included, as in eqn (4).^52–54,58 In the present study, the peak energies of the ⁴T₂ and ⁴T₁ were used as approximations for their ZPL energies in the Racah parameter calculations (eqn (4)–(6)).^55–57 This approach, however, leaves an issue that must be addressed in future work.

Fig. 2h presents the decay curves of A_2−yS_yG:0.05Cr³⁺ phosphors monitored at their strongest emission peaks under 445 nm excitation. The average photoluminescence values calculated using eqn (2) and (3) are 325.68, 260.61, 202.54, 164.50, 143.54, 120.40, 109.40, and 104.34 μs, for increasing Sc³⁺ content, as summarized in Table S2 (ESI†). This decreasing trend corresponds to a luminescence mechanism shift from ²E → ⁴A₂ to ⁴T₂ → ⁴A₂ as the crystal field strength decreases. Since the ⁴T₂ → ⁴A₂ transition has a lower lifetime compared to the ²E → ⁴A₂ transition,^59–61 the weakening crystal field strength induced by Sc³⁺ substitution promotes the dominance of the spin-allowed ⁴T₂ → ⁴A₂ transition, leading to a significant reduction in the photoluminescence lifetimes.

The calculated AE, IQE, and EQE values for all the samples are plotted in Fig. 2i. The results show a consistent trend of first increasing and then decreasing as the Sc³⁺ content increases. The optimal quantum efficiency is achieved when the Sc³⁺ doping content reaches y = 0.5. Specifically, compared to the unsubstituted AG:0.05Cr³⁺ phosphor, the Sc³⁺-substituted sample shows significant improvements, with an IQE increase from 73% to 82%, AE from 18% to 31%, and EQE from 13% to 25%. This enhancement can be attributed to the increased atomic disorder and modulation of the crystal field strength of A_2−yS_yG:0.05Cr³⁺ phosphors by incorporating Sc³⁺. These changes optimize the local coordination environment, thereby improving the luminescence efficiency of the phosphors.¹⁸

Anti-thermal quenching

The operating temperature of the blue LED chip often exceeds 100 °C, highlighting the importance of luminescence thermal stability of the phosphors in determining the luminescence performance of pc-LEDs. For Cr³⁺ doped phosphors, the unshielded nature of 3d³ electrons leads to strong interactions between the lattice vibration and the crystal field. Significantly influencing their photoluminescence properties, including thermal stability. Upon excitation, the valence electrons of Cr³⁺ transition from the ground state to the excited state and subsequently return to the ground state, and release energy either as photons through radiative transitions or as thermal energy via non-radiative transitions. As the temperature increases, enhanced lattice vibrations amplify non-radiative relaxation processes, thereby reducing the luminescence efficiency of Cr³⁺.^19,62 Notably, materials with higher structural rigidity tend to exhibit higher-energy phonon modes, minimizing the probability of non-radiative transitions. Consequently, such materials demonstrate superior luminescence thermal stability.^14,15,63,64 Regarding anti-thermal quenching, Adachi^65–67 established a theoretical model that combines phonon-assisted luminescence enhancement effects with thermal quenching effects, and they validated the model with experimental data. In their framework, increased phonon populations at elevated temperatures actively facilitates radiative transitions by breaking the parity-forbidden selection rules, thereby explaining the anti-thermal quenching phenomenon. Their model acknowledges that thermal quenching is an inherent process in all phosphor materials. In contrast, our interpretation focuses how structural rigidity reduces non-radiative losses, thereby mitigating (not eliminating) thermal quenching. Thus, while enhanced structural rigidity can improve luminescence thermal stability by limiting non-radiative decay, it alone cannot fully account for the anti-thermal quenching behavior observed; a comprehensive understanding must also incorporate the phonon-assisted radiative processes described by Adachi et al.

Remarkably, as illustrated in Fig. 3a, AG:0.05Cr³⁺ demonstrates stable anti-thermal quenching behavior, with the integrated emission intensity increasing as the temperature rises. Specifically, at 423 K, the total emission intensity reaches 108% of its room temperature value and remains constant until 473 K. Similarly, as shown in Fig. 3b, A_1.5S_0.5G:0.05Cr³⁺ also demonstrates anti-thermal quenching behavior, achieving 104% of its room temperature emission intensity at 423 K and maintaining a value higher than room temperature until 473 K. Moreover, Fig. 3a and b reveal a clear trend of spectral broadening and red-shift of the emission center as the temperature rises. This behavior is further analyzed in Fig. 3c and d, where the emission intensity of the zero-phonon line (ZPL) from the ²E → ⁴A₂ transition gradually decreases, while the intensity of the ⁴T₂ → ⁴A₂ transition increases significantly as the temperature rises. To quantify these changes, the photoluminescence spectra were deconvoluted into sharp line (²E → ⁴A₂) and broadband (⁴T₂ → ⁴A₂) components, where their integrated emission intensities were labeled as I(²E) and I(⁴A₂), respectively. As shown in Fig. 3e and f, the ratio I(⁴A₂)/I(²E) increases progressively with temperature, resulting in a noticeable broadening of the FWHM and the red-shifts in the emission peaks. Although the anti-thermal quenching behavior of A_1.5S_0.5G:0.05Cr³⁺ is slightly less pronounced than that of AG:0.05Cr³⁺, the incorporation of Sc³⁺ significantly enhances its luminescence efficiency while retaining a satisfactory level of anti-thermal quenching. This highlights the potential of structural modulation as an effective strategy for optimizing the performance of Cr³⁺-doped phosphors.

Fig. 3 Temperature-dependent emission spectra of (a) AG:0.05Cr³⁺ and (b) A_1.5S_0.5G:0.05Cr³⁺; temperature dependent emission intensity from (c) ²E → ⁴A₂ and (d) ⁴T₂ → ⁴A₂ transitions and emission intensity ratio of ⁴T₂ → ⁴A₂ and ²E → ⁴A₂ transition from (e) AG:0.05Cr³⁺ and (f) A_1.5S_0.5G:0.05Cr³⁺.

To explore the mechanism of the anti-thermal quenching behavior observed in these phosphors, XRD patterns of the optimal AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺ samples were measured at 423 K to evaluate lattice expansion. As shown in Fig. 4a, the diffraction peaks at 423 K remain consistent with those at room temperature, with negligible peak shifts as the temperature increases. This consistency suggests that both AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺ exhibit excellent structural rigidity, a crucial characteristic in minimizing non-radiative transitions.^20,62 To further assess lattice parameter variations, Rietveld refinement of the XRD patterns for AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺ was conducted at both room temperature and 423 K using Fullprof software. The refinement results, presented in Fig. S6 and Table S3 (ESI†), show only minimal increases in lattice constants with temperature elevation. This slight lattice expansion reflects the robust structural stability of these phosphors. Additionally, the minor increase in lattice parameters leads to a corresponding elongation of the ligand bond lengths for the octahedral central atom, as illustrated in Fig. 4b. Specifically, the bond lengths increase from 1.946 Å and 1.984 Å at room temperature to 1.955 Å and 1.998 Å at 423 K for AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺, respectively. Since the crystal field constant (D_q) is inversely proportional to the ligand bond length, these changes slightly weaken the crystal field strength. This relationship is described by the following equation:⁶⁸

where z is the anion valence, e represents the elementary charge, r is the radius of the d wave function, and R is the average bond length between the central ion and its ligands. A decrease in D_q, as a consequence of the elongation of the ligand bond lengths, leads to a red-shift in the emission center, as observed for AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺ phosphors. Furthermore, the bond angles also change with rising temperature, reducing the distortion of the octahedron. Specifically, the bond angle, as shown in Fig. 4b, changes from 84.2 to 85.0° in AG:0.05Cr³⁺, and from 84.3 to 85.7° in A_1.5S_0.5G:0.05Cr³⁺ as the temperature rises from room temperature to 423 K. This indicates a decrease in the degree of octahedral distortion with rising temperature.

Fig. 4 (a) Temperature-dependent XRD patterns of AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺ and magnified XRD patterns in the region between 32 and 32.5°; (b) octahedral atom of AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺ at room temperature and 423 K; (c) temperature-dependent Raman spectra of AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺; (d) configurational coordinate diagram of the thermal quenching mechanism of the Cr³⁺.

To explore the relationship between octahedral distortion and luminescence properties, we performed DFT calculations to evaluate transition dipole moments (TDMs) for both undistorted and distorted octahedral environments (Fig. S7a and b, ESI†). Since the square of the TDM correlates with radiative transition probability, these calculations provide insight into luminescence efficiency. For AG:0.05Cr³⁺, the TDM increases from 0.07 in the undistorted structure to 0.60 in the distorted one, while in A_1.5S_0.5G:0.05Cr³⁺, it increases from 0.11 to 0.78. These results suggest that local distortions enhance radiative transitions by relaxing parity-forbidden selection rules. However, experimentally, the luminescence intensity increases at higher temperatures despite reduced octahedral distortion. This discrepancy can be reconciled by considering that experimentally refined structures represent statistical averages over thermally fluctuating configurations. At elevated temperatures, local distortions persist but become more uniformly distributed, making the average structure appear closer to an undistorted octahedron while still allowing enhanced transitions. Thus, while DFT results indicate that increased local distortion enhances radiative transitions, the experimental trend is influenced by the dynamic nature of thermal vibrations, which must be considered when interpreting structure–property relationships in luminescent materials.

Debye temperature (Θ_D) is commonly employed to evaluate the structural rigidity of phosphors. In general, a higher Debye temperature corresponds to a superior structural rigidity. The Θ_D values of AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺ phosphors are thus calculated as follows:^64,69

where h and k_B are the Planck constant and Boltzmann constant, V is the volume of a unit cell, N is the number of atoms per unit cell, B_H represents the adiabatic bulk modulus of the crystal, M stands the molecular mass of the compound, and ν is the Poisson ratio. The calculated Θ_D values for AG:0.05Cr³⁺ are 624.5 and 623.7 K at 298 and 423 K, respectively. Similarly, the Θ_D values for A_1.5S_0.5G:0.05Cr³⁺ are 615.3 and 615.0 K at these temperatures. Detailed calculation parameters are shown in Table S4 (ESI†). These results demonstrate significantly higher Θ_D values compared to those of many other phosphors (Table S5, ESI†), highlighting the exceptional structural rigidity of A_2−yS_yG:0.05Cr³⁺. For a fixed composition, the Debye temperature remains nearly constant across the measured temperature range (e.g., 624.5 K at 298 K vs. 623.7 K at 423 K), with variations well within computational uncertainty. This confirms that the lattice stiffness is essentially unaffected by temperature, consistent with its nature as an intrinsic property of the crystal structure under the quasi-harmonic approximation.

To further evaluate the impact of phonon vibrations on photoluminescence quenching, temperature-dependent Raman spectra of AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺ were measured from room temperature to 473 K, as shown in Fig. 4c. Notably, the positions of the vibrational peaks remain almost unchanged across the temperature range, reflecting the high structural rigidity of AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺ from another perspective. Notably, the vibrational peaks exhibit negligible shifts in the position across the temperature range, underscoring the high structural rigidity of these phosphors. A closer examination reveals slight leftward shifts of the vibrational peaks at 473 K, attributable to phonon softening, which suggests a minor reduction in polyhedral distortion.⁷⁰ Additionally, the Raman spectra indicate a max phonon energy of approximately 760 cm⁻¹, significantly lower than the transition energy (∼13 300 cm⁻¹) between the ⁴T₂ and ⁴A₂ states. This substantial energy gap implies that non-radiative transitions are unlikely to occur, thereby reinforcing the exceptional thermal stability and luminescence efficiency of AG:0.05Cr³⁺ and A_1.5S_0.5G:0.05Cr³⁺ phosphors.

While structural rigidity, high phonon energy, and a wide bandgap are generally recognized as key factors for achieving high thermal stability, they do not fully account for the observed anti-thermal quenching behavior in A_2−yS_yG:0.05Cr³⁺ phosphors, highlighting the need for a more comprehensive understanding. To address this, the thermal quenching processes at both room temperature and elevated temperature (423 K) are further elucidated using the configurational coordinate diagram of Cr³⁺ (Fig. 4d). At low Cr³⁺ concentrations, AG:xCr³⁺ phosphors exhibit photoluminescence characteristic of strong crystal fields, as shown in Fig. 2c. As illustrated in the left panel of Fig. 4d, electrons excited by 445 nm light populate the ²E energy level and subsequently return to the ground state ⁴A₂via radiative transition (Process I). As Cr³⁺ concentration increases, the weakening of the crystal field strength enables partial thermal activation of electrons from ²E to ⁴T₂ (Process II), requiring an activation energy ΔE₁. Radiative transitions from ⁴T₂ to ⁴A₂ (Process III) then contribute to the emission spectrum, introducing a spectral shoulder due to the relatively low thermal activation energy ΔE₁. Additionally, electrons in the ⁴T₂ level may also reach the intersection of the ⁴T₂ and ⁴A₂ potential curves (Process IV), which requires a higher activation energy ΔE₂. From this intersection, non-radiative transitions to the ground state ⁴A₂ (Process V) depend on the magnitude of ΔE₂. In AG:0.05Cr³⁺ phosphors, the high structural rigidity results in a significantly larger ΔE₂ compared to ΔE₁ (ΔE₂ ≫ ΔE₁), suppressing non-radiative quenching via Process V. Consequently, as the temperature increases, the emission intensity from the ⁴T₂ → ⁴A₂ transition increases, compensating for potential non-radiative losses and giving rise to anti-thermal quenching behavior. When Al³⁺ is partially substituted with Sc³⁺, the crystal field strength weakens further, transitioning from strong to intermediate and ultimately to weak fields. This reduction in crystal field strength lowers ΔE₁, facilitating thermal activation of electrons from ²E to ⁴T₂. Consequently, broadband emission from ⁴T₂ → ⁴A₂ becomes dominant, while the spin-forbidden ²E → ⁴A₂ transition is further suppressed. Thermal population from ²E to ⁴T₂ enhances the ⁴T₂ → ⁴A₂ emission, compensating for non-radiative transitions and sustaining anti-thermal quenching. However, as the Sc³⁺ content increases further, ΔE₂ begins to decrease, making non-radiative quenching through Process V more likely. This explains the slightly reduced anti-thermal quenching effect in A_1.5S_0.5G:0.05Cr³⁺ compared to AG:0.05Cr³⁺. Importantly, because spin-allowed transitions (⁴T₂ → ⁴A₂) are far more probable than spin-forbidden transitions (²E → ⁴A₂), the integrated emission intensity from ⁴T₂ → ⁴A₂ transitions increases with temperature between room temperature and 423 K. These findings illustrate the critical interplay between structural rigidity, crystal field strength, and thermal population processes in governing the anti-thermal quenching behavior of Cr³⁺-doped phosphors, offering valuable insights for designing high-performance phosphors with exceptional thermal stability.

Photoelectric efficiency and pc-LED application

A pc-LED was prepared by combining a A_1.5S_0.5G:0.05Cr³⁺ phosphor with a commercial 450 nm blue chip to evaluate its photoelectric conversion performance. Fig. 5a presents the emission spectra of the devices under different driving currents. As the driving current increases, the luminous intensity also increases while the shape of the emission spectrum remains unchanged. Fig. 5b shows the NIR output power and photoelectric efficiency profiles across varying drive currents. At a driving current of 300 mA, the pc-LED device achieves an NIR output power of 241 mW, demonstrating its capability of high NIR output. However, the photoelectric efficiency decreases from 11 to 9% as the driving current increases from 50 mA to 300 mA, primarily due to the efficiency loss caused by the temperature rise of the blue LED chip at high power levels.

Fig. 5 (a) The electroluminescence spectra under different currents; (b) NIR output power and photoelectric efficiency under different driving currents; (c) the structure of the vein image acquisition device; (d) finger vein imaging; (e)–(g) images of the leaf, grapefruit, opaque bottle containing drug under visible light and NIR pc-LED irradiation.

Intravenous blood vessels are rich in deoxyhemoglobin and oxyhemoglobin, which are known to effectively absorb NIR light within the 700–1000 nm range.⁷¹ Leveraging this characteristic, we fabricated an NIR pc-LED by packaging the A_1.5S_0.5G:0.05Cr³⁺ phosphor onto a 450 nm blue LED chip, which serves as the light source of a vein image acquisition system, as schematically shown in Fig. 5c. As shown in Fig. 5(d), the captured images clearly differentiate veins (dark areas) from other tissues (light area). Each finger exhibits a unique vein distribution, emphasizing the individuality of vein patterns across different fingers. These distinct and clear finger vein images acquired using the NIR pc-LED device highlight its potential application in biometric recognition systems. Additionally, the technology can serve as a core component for portable and integrated NIR spectroscopy devices with expanded functionalities. Furthermore, we explored other potential application scenarios, as illustrated in Fig. 5(e–g). Under NIR pc-LED irradiation, the leaf and grapefruit veins are distinctly visible, and the content of the drug in the opaque bottle can be observed. This demonstrates the potential application of the NIR pc-LED device in night vision and non-destructive testing.

Conclusion

In conclusion, we successfully synthesized a series of garnet-structured BaY₂Al_2−ySc_yGa₂SiO₁₂:xCr³⁺ phosphors using a high-temperature solid-state reaction method. These phosphors exhibit tunable NIR emission spanning 650–1100 nm under 445 nm excitation, with emission characteristics strongly influenced by the crystal field environment. For y = 0, the optimal composition, BaY₂Al₂Ga₂SiO₁₂:0.05Cr³⁺, demonstrated an IQE of 73%, an EQE of 13%, and remarkable anti-thermal quenching, retaining 108% luminescence intensity at 150 °C. Substituting Al³⁺ with Sc³⁺ introduced significant structural modulation, inducing lattice distortion and red-shifting of the emission center from 708 nm to 752 nm while broadening the FWHM from 75 nm to 138 nm. At an optimal Sc³⁺ content (y = 0.5), the BaY₂Al_1.5Sc_0.5Ga₂SiO₁₂:0.05Cr³⁺ phosphor achieved enhanced quantum efficiencies, with the IQE increasing to 82% and the EQE to 25%, while retaining 104% of its luminescence intensity at 423 K. DFT calculations and experimental analyses revealed that octahedral distortion plays a crucial role in enhancing TDMs, contributing to the superior luminescence efficiency. Structural rigidity, low phonon energy, and the broad bandgap of the host material explain their outstanding thermal stability. The sustained anti-thermal quenching behavior is attributed to the well-preserved thermal repopulation from the ²E to ⁴T₂ state during structural modulation. To demonstrate the application potential of the optimized A_1.5S_0.5G:0.05Cr³⁺ phosphor, an NIR pc-LED was fabricated by integrating it with a commercially available 450 nm blue LED chip. The device achieved an impressive NIR output power of 241 mW at a driving current of 300 mA. Clear finger vein images were successfully captured using the device as an irradiation source, demonstrating its potential for biometric identification and other NIR applications. These findings underscore the pivotal role of crystal field modulation in optimizing luminescence efficiency while maintaining sustained anti-thermal quenching behavior. Moreover, they highlight the potential of Cr³⁺-doped garnet phosphors as advanced materials for high-performance NIR pc-LEDs in next-generation optoelectronic applications.

Experimental

Materials synthesis

BaY₂Al₂Ga₂SiO₁₂:xCr³⁺ and BaY₂Al_2−ySc_yGa₂SiO₁₂:0.05Cr³⁺ (x = 0, 0.01, 0.02, 0.04, 0.05, and 0.06; y = 0, 0.3, 0.5, 0.7, 0.9, 1.2, 1.7, and 2) phosphors were synthesized via a high-temperature solid-state reaction. The raw materials included BaCO₃ (Aladdin, 99.95%), Y₂O₃ (Aladdin, 99.90%), Al₂O₃ (Aladdin, 99.90%), Ga₂O₃ (Aladdin, 99.99%), SiO₂ (Aladdin, 99.90%), Sc₂O₃ (Aladdin, 99.99%), and Cr₂O₃ (Rhawn, 99.99%). All compounds were weighed based on their stoichiometric ratios. 1 wt% Li₂CO₃ was added as a flux. The weighed materials were mixed and ground for 40 minutes before being transferred into corundum crucibles. The samples were sintered at 1350 °C for 6 hours in a furnace. After the sintering process, the samples were allowed to cool to room temperature before collection.

For clarity, BaY₂Al₂Ga₂SiO₁₂:xCr³⁺ and BaY₂Al_2−ySc_yGa₂SiO₁₂:0.05Cr³⁺ are referred to as AG:xCr³⁺ and A_2−yS_yG:0.05Cr³⁺, respectively, where “A” represents Al, “S” denotes Sc, and G represents the garnet structure.

Materials characterization

The phase structures of the synthesized materials were analyzed using an X-ray powder diffractometer Cu K_α1 radiation (λ = 0.15406 Å), operating at 40 kV and 40 mA. Diffraction patterns were collected over a 2θ range of 10–90° at a scanning speed of 10° min⁻¹ with a step size of 0.02°. Morphological and compositional analyses were conducted using a scanning electron microscope (SEM, EVO-MA10) equipped with an energy dispersive spectrometer (EDS). UV-vis absorption spectra were measured using a UV-2550 spectrophotometer, covering a wavelength range of 200–800 nm. Photoluminescence spectra, excitation spectra, and decay curves were obtained using an FLS1000 (Edinburgh Instruments) transient steady-state fluorescence spectrometer. Absolute photoluminescence quantum yield and absorption rates were measured with a Hamastu C9920 Quanturus-QY spectrometer. Raman spectra were recorded using an alpha300R-Raman Imaging Microscope.

NIR pc-LED fabrication

To fabricate the NIR pc-LED device, the optimized near-infrared phosphor (A_1.5S_0.5G:0.05Cr³⁺) was integrated with commercially available 450 nm blue LED chips. The phosphor was thoroughly mixed with epoxy resin and stirred for 10 minutes to ensure homogeneity. A precise amount of the mixture was carefully applied onto the LED chip using a plastic dropper, left to settle for 20 minutes, and then cured in a drying oven at 100 °C for 2 hours. The emission spectra, near-infrared output power, and photoelectric conversion efficiency of the device were evaluated using a high-precision fast spectroradiometer (HASS 2000).

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was financially supported by the Hunan Provincial Natural Science Foundation of China (2021JJ30686 and 2023JJ40621) and the Education Department of Hunan Province (24A0096).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc00698h

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