Design of Cr³⁺-activated broadband NIR phosphors with tunable and abnormal thermal quenching behavior for NIR pc-LEDs

Abstract

Cr³⁺-activated broadband near-infrared (NIR) phosphors usually show controllable and excellent photoluminescence (PL) properties, but their poor thermal stability remains a big challenge. Herein, a series of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ garnet phosphors with tunable and abnormal thermal quenching performance have been successfully proposed. It is found that both the crystal field strength and calculated energetic difference between ⁴T₂ and ²E states decrease obviously with increasing [Ca²⁺–Si⁴⁺] co-substitution, resulting in the thermal occupation of the ⁴T₂ state and broadened PL spectra. More importantly, the Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ phosphors show improved PL thermal stability depending on the different thermal population between ⁴T₂ and ²E states, and the mechanism is investigated in detail. The PL intensity of the optimal sample reaches up to 125% and 121% at 425 K and 475 K compared with that at 300 K, respectively, which is much better than those of most Cr³⁺-activated broadband NIR phosphors. A NIR phosphor-converted light-emitting diode (NIR pc-LED) has been fabricated using the as-prepared thermally stable phosphor and its application in bio-imaging and night vision is demonstrated.

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1. Introduction

Nowadays, NIR spectroscopy is widely used in many fields of our daily life, such as food quality analysis, night vision, medical imaging, plant growth, and so on.¹ Among the various NIR sources, NIR pc-LEDs have received much attention due to the merits of high efficiency, small size, energy saving, environmentally friendly nature and tunable PL properties over the traditional ones (NIR LEDs, tungsten and halogen lamps).^2,3 As a key component of NIR pc-LEDs, the PL properties of phosphors have a great influence on the performance of LED devices.⁴ It is known that the working temperature of the LED device can reach up to above 100 °C (even 150 °C) because of accumulated heat from the chip.⁵ The PL intensity of NIR phosphors usually decreases obviously due to enhanced non-radiative transition with increasing temperature,⁶ leading to a decreased device performance. Therefore, the development of NIR phosphors with improved thermal stability is quite important for further applications.⁷

Cr³⁺-activated NIR phosphors usually show strong absorption (⁴A₂ → ⁴T₁) in the blue light region, which can match well with that of the commercial blue LED chip.⁸ In addition, the broadband NIR emission (peak position and bandwidth) of Cr³⁺ can be easily tuned depending on the crystal field strength, due to its unique 3d³ electron configuration.^4,9 Thanks to the above characteristics, Cr³⁺ is considered to be an ideal NIR emitter and has been extensively explored in recent years. For example, Na₃ScF₆:Cr³⁺ (λ_em = 774 nm, FWHM ≈ 108 nm, and I_425K ≈ 27%) shows a high Cr³⁺ doping concentration (35.96%) and the internal quantum efficiency (IQE) and external quantum efficiency (EQE) are as high as 91.5% and 40.82%, respectively.¹⁰ Through lithium ion compensation, Mg₄Ta₂O₉:Cr³⁺ (λ_em = 842 nm, FWHM ≈ 167 nm, and I_425K ≈ 25%) reaches a record EQE of 61.25%.¹¹ Compounds with a garnet structure are popular hosts for Cr³⁺ ions due to the wide bandgap, high structural rigidity and adjustable composition, such as Gd_2.4Lu_0.6Ga₄AlO₁₂:Cr³⁺ (λ_em = 728 nm, FWHM ≈ 107 nm, and I_425K ≈ 75%),¹² CaLu₂Mg₂Si₃O₁₂:Cr³⁺ (λ_em = 765 nm, FWHM ≈ 120 nm, and I_425K ≈ 82%),¹³ and Gd₃Sc₂Ga₃O₁₂:Cr³⁺ (λ_em = 760 nm, FWHM ≈ 120 nm, and I_425K ≈ 86%).¹⁴ Although the above-mentioned Cr³⁺-activated broadband NIR phosphors show excellent PL properties and high luminous efficiency, the poor thermal stability still remains a big challenge before practical application, especially for high-power devices.

In this work, a series of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ phosphors with tunable and abnormal thermal quenching performance are designed and presented. It is found that the temperature-dependent luminescence properties of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ can be controlled by the thermal population of the ⁴T₂ energy level, which is highly related to the energetic difference between ⁴T₂ and ²E states.

2. Experimental section

2.1 Synthesis

A series of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.010Cr³⁺ (x = 0–0.5) phosphors were synthesized by a traditional high-temperature solid-state reaction method. Lu₂O₃ (99.995%), CaCO₃ (A.R.), SiO₂ (99.95%), Cr₂O₃ (A.R.), Ga₂O₃ (99.99%) and NH₄Cl (A. R.) were used as raw materials without further purification. The raw materials were weighed stoichiometrically (0.3 wt% of NH₄Cl as flux) and finely ground in an agate mortar for 20 min. The mixture was transferred to a corundum crucible and sintered at 1300 °C for 5 h. After cooling to room temperature, the samples were reground for further characterization. Lu_2.7Ca_0.3Ga_4.7−xSi_0.3O₁₂:xCr³⁺ (x = 0.005–0.040) phosphors were synthesized similarly according to the above procedures.

2.2 Characterization

The phase purity and crystal structure of the samples were confirmed using a Bruker D8 Advance X-ray diffractometer (XRD) operating with a Cu Kα radiation source (λ = 1.5405 Å) under 40 kV and 40 mA. Measurements were carried out at room temperature over 5–90° 2θ range. Rietveld refinement was performed using TOPAS-Academic V5 software. The PL excitation (PLE) spectra, PL spectra, and PL decay curves were measured on a FLS1000 spectrometer (Edinburgh Instruments, a 450 W Xe900 continuous xenon lamp as the excitation source) equipped with a thermo-electronic cooled (−22 °C) R928P photomultiplier (PMT) from Hamamatsu and a MercuryiTC temperature controller (Oxford).

2.3 LED fabrication

The NIR pc-LED device was fabricated by coating the prepared NIR phosphor on an InGaN blue chip (450 nm, 1 W) with a mass ratio of the phosphor to epoxy resin of 1 : 1. The current-dependent electroluminescence and output power were collected using an ATA-1000 photoelectric measuring system (EVERFINE, China).

3. Results and discussion

3.1. Crystal structure of Lu_3−xCa_xGa_5−xSi_xO₁₂

The phase purity of as-prepared Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ (x = 0–0.5) phosphors was examined by X-ray diffraction (XRD). All the diffraction peaks are in good agreement with the standard data of Lu₃Ga₅O₁₂ (PDF no. 13-0400) and no impurity peaks are observed (Fig. 1a), indicating that the co-substitution of [Lu³⁺–Ga³⁺] by [Ca²⁺–Si⁴⁺] has almost no influence on the crystal structure. The Rietveld refinement (Fig. 1b) is performed using Lu₃Ga₅O₁₂ as the starting model, and the small residual factors (R_p = 6.00%, R_wp = 10.23%, Table S1, ESI†) further confirm the good phase purity. Lu_3−xCa_xGa_5−xSi_xO₁₂ crystallize in the same space group Ia3̄d (no. 230) as the Lu₃Ga₅O₁₂ garnet and there are three kinds of polyhedra in the garnet structure: (Lu/Ca)O₈ dodecahedra, GaO₆ octahedra and (Ga/Si)O₄ tetrahedra (Fig. 1c). Cr³⁺ ions (r = 0.615 Å, CN = 6) prefer to replace the octahedral Ga³⁺ site (r = 0.62 Å, CN = 6) due to the similar radius. The (Ga/Cr)O₆ octahedra might be enlarged and distorted (Fig. 1c) due to the much smaller ionic radius of Si⁴⁺ (r = 0.26 Å and CN = 4) than Ga³⁺ (r = 0.47 Å and CN = 4), indicating a weakening crystal field strength,¹⁵ and then a broadened emission can be anticipated with increasing x values in Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ phosphors.

Fig. 1 (a) XRD patterns of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ (x = 0–0.5). (b) The Rietveld refinement of Lu_2.7Ca_0.3Ga_4.7Si_0.3O₁₂:Cr³⁺ (R_p = 6.00%, R_wp = 10.23%). (c) The crystal structure.

Fig. S1 (ESI†) presents the SEM and elemental mapping images of Lu_2.7Ca_0.3Ga_4.7Si_0.3O₁₂:Cr³⁺ phosphors, which show an irregular shape with several hundred nanometers in size. All the elements such as Lu, Ga, Ca, Si, O and Cr are found to be distributed uniformly in the matrix.

3.2. PL properties of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺

To investigate the influence of [Ca²⁺–Si⁴⁺] co-substitution on the PL properties, the emission spectra of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ are studied in detail. It can be seen that the room-temperature PL spectrum of Lu₃Ga₅O₁₂:0.01Cr³⁺ is dominated by a sharp emission peaked at 705 (R₁ line) and 691 nm (R₂ line) corresponding to ²E → ⁴A₂ transition of Cr³⁺ (Fig. 2a). Meanwhile, the broadband emission from 620 to 830 nm comes from ⁴T₂ → ⁴A₂ transition.¹⁶ With increasing [Ca²⁺–Si⁴⁺] co-substitution, the PL intensity of the broadband ⁴T₂ → ⁴A₂ emission increases significantly (Fig. 2b), leading to an increased value (from 43 to 150 nm, Fig. S2, ESI†) of full width at half maximum (FWHM).

Fig. 2 (a) Emission spectrum of Lu₃Ga₅O₁₂:0.01Cr³⁺ (λ_ex = 430 nm). (b) Emission spectra (λ_ex = 430 nm) of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (x = 0–0.5). (c) Tanabe–Sugano diagram of Cr³⁺ ions in an octahedral coordination. (d) Fluorescence intensity ratio (FIR) of ⁴T₂ → ⁴A₂ to ²E → ⁴A₂ [I(⁴T₂)/I(²E)] transitions of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (x = 0–0.5).

The spectral broadening of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ is probably related to the decrease of crystal field strength with x value. As is well known, the energy of the ⁴T₂ level of Cr³⁺ is lower than that of the ²E level when exposed to a weak crystal field, according to the Tanabe–Sugano diagram for the 3d³ electron configuration (Fig. 2c), and the PL spectra consist of an intense broadband emission band. The energy of the ²E level is lower as a result of a strong crystal field, and the PL spectra consist of a sharp emission line. When Cr³⁺ ions are exposed to intermediate crystal fields, both emission bands are visible in the PL spectra. Therefore, to further explore the influence of [Ca²⁺–Si⁴⁺] co-substitution on the PL properties, the crystal field splitting parameters (Dq and B) of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (x = 0–0.5) are calculated from the PL excitation spectra (PLE, Fig. S3, ESI†) with the following equations:^17,18

10Dq = E(⁴T₂) = E(⁴A₂→⁴T₂) (1)

ΔE = E(⁴T₁) − E(⁴T₂) = E(⁴A₂→⁴T₁) − E(⁴A₂→⁴T₂) (3)

Here, Dq and B represent the crystal field strength and repulsive force between 3d electrons, respectively.¹⁶ The calculated results are shown in Table 1. It can be seen that the Dq/B values of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ decrease from 2.52 to 2.23 with increasing x, indicating that a decreased crystal field strength is obtained with increasing [Ca²⁺–Si⁴⁺] co-substitution. The weakened crystal field strength downshifts the ⁴T₂ energy level (Fig. 2c), due to which the possibility of thermal population at the ⁴T₂ state will increase compared with that at the ²E state,¹⁵ leading to an increased fluorescence intensity ratio (FIR) of ⁴T₂ → ⁴A₂ to ²E → ⁴A₂ [I(⁴T₂)/I(²E)] transitions (Fig. 2d). The decreased lifetime of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (Fig. S4, ESI†) is another evidence for the thermally populated ⁴T₂ energy level.¹⁹

Table 1 Crystal field splitting parameter Dq, Racah parameter B and values of energy levels of Lu_3−xCa_xGa_4.99−xSi_xO₁₂:0.01Cr³⁺ (x = 0–0.5)

x	Dq (cm^−1)	B (cm^−1)	Dq/B	Δ 1 (cm^−1)	Sℏω (cm^−1)	Δ 2 (cm^−1)
0	1652.89	655.07	2.52	2344.53	1551.25	793.28
0.05	1639.34	667.91	2.45	2209.05	1592.36	616.69
0.1	1631.32	680.03	2.40	2148.91	1648.02	500.89
0.15	1626.02	680.84	2.39	2115.89	1674.10	441.79
0.2	1623.38	684.91	2.37	2109.47	1709.93	399.54
0.25	1618.12	685.71	2.36	2116.53	1742.04	374.49
0.3	1612.90	693.89	2.32	2064.33	1749.85	314.48
0.4	1602.56	702.92	2.28	1960.94	1733.48	227.46
0.5	1594.90	715.45	2.23	1923.72	1706.64	217.08

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To further analyze the PL properties of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ in detail, a configurational coordinate diagram (Fig. 3a) is constructed based on the obtained spectral characteristics. Δ₁ is the energetic difference between the R₁ line and the maximum of the ⁴A₂ → ⁴T₂ PLE band.²⁰ The energetic difference Δ₂ between ⁴T₂ and ²E electronic manifolds, which determines the thermal occupation of the ⁴T₂ energy level, can be obtained from the following relationship:^21,22

Δ₂ = Δ₁ − Sℏω (4)

where S and ℏω are the Huang–Rhys factor and effective phonon energy, respectively. The values of S and ℏω can be calculated using the following equations:²³

ΔE = (2S − 1)ℏω (5)

where ΔE is the Stokes shift (energy difference between the first absorption and corresponding emission bands) and Γ(T) is the FWHM of the emission band at temperature T. The Δ₂ values of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ are calculated according to the above equations and the results are shown in Fig. 3b. It can be seen that the values of Δ₂ decrease with increasing x, indicating a higher probability of ²E → ⁴T₂ transition (i.e. thermally populated ⁴T₂ state) which is consistent with the enhanced ⁴T₂ → ⁴A₂ broadband emission and increased FIR of I(⁴T₂)/I(²E) of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (as shown in Fig. 2b and d).

Fig. 3 (a) Schematic configurational coordinate diagram and (b) energy difference between ⁴T₂ and ²E states of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (x = 0–0.5).

3.3. Thermal stability of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺

The PL intensity of phosphors usually shows a decreasing trend due to enhanced non-radiative transition with increasing temperature, which is a major obstacle for their application. Therefore, it is of great importance to develop NIR phosphors with improved thermal stability. In previous work,²⁴ it is verified that the PL thermal stability of Cr³⁺ can be optimized by the electron occupation between ⁴T₂ and ²E states. As discussed above, the energetic difference Δ₂ and crystal field strength (Table 1) between ⁴T₂ and ²E states (thermal population of the ⁴T₂ state) of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ can be easily tuned by co-substitution of [Lu³⁺–Ga³⁺] by [Ca²⁺–Si⁴⁺], which may provide an alternative way for improving the thermal stability.

In order to evaluate the influence of [Ca²⁺–Si⁴⁺] co-substitution on thermal stability, the temperature-dependent PL spectra of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (x = 0, 0.15 and 0.30) are studied in detail (Fig. 4 and Fig. S5, ESI†). On the one hand, it is noted that the integrated PL intensity of all Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (x = 0, 0.15 and 0.30) samples increases first and then decreases with increasing temperature (Fig. 4a–c). On the other hand, the thermal stability of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ becomes better and better with increasing x and the PL intensity at x = 0.30 reaches up to 125% and 121% at 425 K and 475 K compared with that at 300 K (Fig. 4c), respectively, which is much better than those of most Cr³⁺-activated broadband NIR phosphors (Table 2).

Fig. 4 Normalized PL intensity of (a)–(c) ⁴T₂ + ²E →⁴A₂, (d)–(f) ⁴T₂ → ⁴A₂ and (g)–(i) ²E → ⁴A₂ transitions of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ as a function of temperature (300–475 K) with x = 0, 0.15 and 0.30, respectively.

Table 2 Thermal stability of Cr³⁺-activated broadband NIR phosphors

Phosphors	λ em (nm)	I 423K (%)	Ref.
Lu2.7Ca0.3Ga4.7Si0.3O12:Cr^3+	748	125	This work
Y3In2Ga3O12:Cr^3+	760	100	25
Ca3Sc2Si3O12:Cr^3+	770	97.4	26
GaTaO4:Cr^3+	840	60	27
ScBO3:Cr^3+	800	51	28
Na3Al2Li3F12:Cr^3+	750	99	29
K2NaInF6:Cr^3+	774	78.3	30
K2NaIScF6:Cr^3+	765	89.6	31
ScF3:Cr^3+	853	85.5	32

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To understand the mechanism behind the abnormal thermal stability, the PL spectra are deconvoluted into separate ⁴T₂ → ⁴A₂ broadband and ²E → ⁴A₂ sharp-line emissions for all temperatures.²² The PL intensity of the ⁴T₂ → ⁴A₂ broadband emission (Fig. 4d–f) exhibits a similar trend to the corresponding total PL intensity (Fig. 4a–c), while the ²E → ⁴A₂ sharp-line emission (Fig. 4g–i) decreases monotonically with x. In the Lu_3−xCa_xGa_5−xSi_xO₁₂ system, the ²E state can serve as a reservoir for the higher lying ⁴T₂ state.¹⁹ The electrons will migrate from the ²E to ⁴T₂ energy level with increasing temperature and reach a new thermal equilibrium, leading to a thermally populated ⁴T₂ state (which can be further verified by the decay curves in Fig. S6, ESI†). This explains why Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ phosphors exhibit an abnormal thermal stability before 425 K. At higher temperatures, the ⁴T₂→⁴A₂ broadband emission dominates the PL spectra and then a traditional thermal quenching phenomenon is observed due to the increased non-radiative rate of the ⁴T₂ level.

The improved thermal stability of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (x = 0, 0.15 and 0.30) with increasing x is due to the different energetic difference Δ₂ between ⁴T₂ and ²E states which has a great influence on the initial thermal equilibrium between ⁴T₂ and ²E energy levels (i.e. FIR of I(⁴T₂)/I(²E)) and determines the thermal quenching performance at elevated temperatures. The PL intensity of ²E → ⁴A₂ sharp-line emission is strong at large Δ₂ values (small x), whose decrease has a great influence on the whole PL intensity with increasing temperature. Then the ²E → ⁴T₂ transition strengthens with decreasing Δ₂ (increasing x). Finally, the ²E → ⁴T₂ transition has little influence on the thermal quenching performance due to the insufficient ²E electrons for smaller Δ₂ values. That is why the thermal stability of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ decreases when x > 0.30 (Fig. S7, ESI†). In this work, it is obvious that Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ phosphors have optimal Δ₂ values and the best thermal stability when x = 0.30.

Considering the higher thermal stability (Fig. 3c) and PL intensity (Fig. S8, ESI†) of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ at x = 0.30, a series of Lu_2.7Ca_0.3Ga_4.7Si_0.3O₁₂:Cr³⁺ with various Cr³⁺ concentrations were prepared (Fig. S9, ESI†) to further optimize the PL properties (Fig. 5a). The PL intensity of Lu_2.7Ca_0.3Ga_4.7−xSi_0.3O₁₂:xCr³⁺ reaches its maximum value at x = 0.02 (Fig. S10, ESI†) and then decreases owing to the concentration quenching effect. Moreover, it is found that both the FWHM (from to 140 to 182 nm, Fig. 5b) and FIR (Fig. 5c) of I(⁴T₂)/I(²E) show an increasing trend as the Cr³⁺ concentration increases, just like that of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (Fig. 2b, d). But the reason for the increased ⁴T₂ → ⁴A₂ broadband emission with increasing Cr³⁺ content is different from that of Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (which is due to a decreased crystal field strength). In this case, the normalized PLE spectra of Lu_2.7Ca_0.3Ga_4.7−xSi_0.3O₁₂:xCr³⁺ almost overlap with each other (Fig. S11 (ESI†), implying an unchanged crystal field strength). It is considered that the distance between Cr³⁺ and Cr³⁺ ions in Lu_2.7Ca_0.3Ga_4.7−xSi_0.3O₁₂:xCr³⁺ decreases with increasing Cr³⁺ concentration, which is conducive to the electron migration from the ²E to ⁴T₂ state, thus resulting in a thermally populated ⁴T₂ state (i.e. enhanced ⁴T₂ → ⁴A₂ broadband emission) at higher Cr³⁺ content.²⁴

Fig. 5 (a) Emission spectra, (b) FWHM and (c) FIR of ⁴T₂⁴A₂ to ²E → ⁴A₂ transitions of Lu_2.7Ca_0.3Ga_4.7−xSi_0.3O₁₂:xCr³⁺ (x = 0.005–0.040). (d) Normalized PL intensity of Lu_2.7Ca_0.3Ga_4.7−xSi_0.3O₁₂:xCr³⁺ as a function of temperature (300–425 K).

Due to the varying FIR values of I(⁴T₂)/I(²E) of Lu_2.7Ca_0.3Ga_4.7−xSi_0.3O₁₂:xCr³⁺, it is necessary to investigate the thermal stability. The temperature-dependent PL spectra of Lu_2.7Ca_0.3Ga_4.7−xSi_0.3O₁₂:xCr³⁺ are measured and shown in Fig. 5d and Fig. S12 (ESI†). As expected, the thermal stability of Lu_2.7Ca_0.3Ga_4.7−xSi_0.3O₁₂:xCr³⁺ becomes better before x = 0.010 and then decreases due to the gradually exhausting ²E electrons at higher Cr³⁺ concentration. From the obtained results, it can be inferred that both the Δ₂ value and Cr³⁺ concentration have a significant influence on the thermal stability of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺.

3.4. NIR pc-LED applications

Considering the excellent thermal quenching behavior of the as-prepared samples, a NIR pc-LED was fabricated by combining a Lu_2.7Ca_0.3Ga_4.7Si_0.3O₁₂:Cr³⁺ phosphor with a commercial blue InGaN chip and the electroluminescence spectrum is shown in Fig. S13 (ESI†). The NIR output power (Fig. 6a) increases monotonously from 50 to 350 mA, while the photoelectric conversion efficiency decreases slightly (Fig. 6b). The NIR light from the LED device can penetrate human fingers easily and the blood vessels can be observed clearly (Fig. 6c). In addition, with the aid of a NIR camera, the words on the bottle surface are clearly distinguished by using the packaged NIR pc-LED as the light source (Fig. 6d). The above results indicate that the as-prepared sample can be potentially used in the fields of bio-imaging and night vision.

Fig. 6 (a) Output power and (b) photoelectric conversion efficiency of the fabricated NIR-LED device as a function of driven current. (c) Photograph of fingers taken with the NIR camera under irradiation of the LED device. (d) Photographs of a bottle under natural light and NIR LED light captured with a visible camera (left and middle) and the NIR camera (right).

4. Conclusions

In summary, Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ phosphors with tunable PL properties and abnormal thermal stability were synthesized by a high-temperature solid-state method. The FWHM of the PL spectra of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ broadens from 43 to 150 nm due to the enhanced ⁴T₂ → ⁴A₂ emission, resulting from a decreased crystal field strength. The calculated energetic difference (Δ₂) between ⁴T₂ and ²E states also shows a decreasing trend with increasing [Ca²⁺–Si⁴⁺] co-substitution, leading to a thermally populated ⁴T₂ state. Based on the different thermal population of the ⁴T₂ state, the thermal stability of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ can be easily tuned. With the optimal [Ca²⁺–Si⁴⁺] co-substitution, Lu_3−xCa_xGa_5−xSi_xO₁₂:0.01Cr³⁺ (x = 0.30) shows an abnormal thermal quenching performance and the PL intensity can reach up to 125% and 121% at 425 K and 475 K compared with that at 300 K, respectively. In addition, it is found that the ⁴T₂/²E thermal population and PL thermal stability of Cr³⁺ can be further optimized by the Cr³⁺ concentration. Finally, a NIR pc-LED based on the Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺ phosphor shows excellent performance in bio-imaging and night vision.

Data availability

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

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work has been financially supported by the National Natural Science Foundation of China (No. 52272143), the Major Science and Technology Project of Zhongshan City (No. 2022A1007), the Guangdong Basic and Applied Basic Research Foundation (No. 2023A1515010866 and 2021A1515110404), the Science and Technology Project of Guangzhou City (No. 2023A04J0299), the GDAS’ Project of Science and Technology Development (No. 2022GDASZH-2022010104, 2023GDASZH-2023010104 and 2024GDASZH-2024010102), and Young Talent Project of GDAS (No. 2023GDASQNRC-0302).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: The excitation spectra, FWHM, decay curves and temperature-dependent spectra of Lu_3−xCa_xGa_5−xSi_xO₁₂:Cr³⁺; XRD patterns, PL intensity, PLE and temperature-dependent spectra of Lu_2.7Ca_0.3Ga_4.7−xSi_0.3O₁₂:xCr³⁺ (x = 0.005–0.040); electroluminescence spectrum of the LED device. See DOI: https://doi.org/10.1039/d4qm00395k

‡ Q. J. Zhu and J. S. Huo contributed equally to this work.

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