A novel borate phosphor Lu₅Ba₆B₉O₂₇:Ce³⁺ codoped with Sr²⁺/Tb³⁺ for NUV-white light emitting diode application

Abstract

At present, there are still challenges in developing highly efficient and thermally stable phosphors for ultraviolet/near ultraviolet-white light emitting diodes (UV/NUV-WLEDs). Herein, we use traditional high-temperature solid-state reactions to prepare blue-emitting phosphors Lu_5−xBa₆B₉O₂₇:xCe³⁺ (0.1% ≤ x ≤ 3.0%) and Lu_4.975Ba_6−ySr_yB₉O₂₇:0.5%Ce³⁺ (0% ≤ y ≤ 20%), and green-emitting phosphors Lu_4.975−zBa₆B₉O₂₇:0.5%Ce³⁺,zTb³⁺ (0% ≤ z ≤ 16%), abbreviated as LBB:xCe³⁺, LBB:0.5%Ce³⁺,ySr²⁺ and LBB:0.5%Ce³⁺,zTb³⁺, respectively. Upon 340 nm excitation, LBB:Ce³⁺ exhibits an asymmetric blue emission ranging from 360 nm to 480 nm. Furthermore, the emission intensity of LBB:0.5%Ce³⁺ increased 4.8-fold without a spectral shift through the partial substitution of Sr²⁺ for Ba²⁺. Through constructing the Ce³⁺ → Tb³⁺ energy transfer in the LBB structure, the temperature-dependent integral emission intensity at 483 K improved from only 25% to 68% of the intensity at 303 K due to the existence of fast energy transfer from Ce³⁺ to Tb³⁺. The related results indicate that LBB:xCe³⁺, LBB:0.5%Ce³⁺,ySr²⁺ and LBB:0.5%Ce³⁺,zTb³⁺ phosphors can be used for UV/NUV-WLEDs.

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1. Introduction

Phosphor-converted white light emitting diodes (pc-WLEDs) have become the next-generation solid-state lighting because of their advantages such as environmental friendliness, high efficiency, and low cost, among other advantages.^1–4 Currently, the commercialized pc-WLEDs combine blue LEDs with the yellow phosphor (Y,Gd)₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺). However, these pc-WLED devices still present several shortcomings, such as a lower color rendering index (R_a < 80), a high correlated color temperature (CCT > 5000 K), and harmful blue light.^5,6 Alternatively, using ultraviolet (UV) or near-ultraviolet (NUV) LEDs to excite red-, green-, and blue-emitting phosphors may solve these shortcomings, since their white light is entirely generated by phosphors that contain the entire visible range with a flat spectral distribution.^7,8 Nevertheless, several commercial phosphors for UV/NUV-WLEDs also have drawbacks, such as the harsh preparation conditions of K₂SiF₆:Mn⁴⁺ (red),⁹ strong thermal quenching of (Ba,Sr)₂SiO₄:Eu²⁺ (green),¹⁰ and easy aging of BaMgAl₁₀O₁₇:Eu²⁺ (blue).¹¹ The phosphors determine the final performance of pc-WLEDs. Thus, it is still essential to develop efficient and thermally stable phosphors for UV/NUV-WLEDs.

The performance of phosphors mainly depends on two factors: the activators, which are usually Ce³⁺/Eu²⁺ due to their strong absorption from the spin-allowed transition 4f → 5d, and the host, which generally should have a large bandgap with structural rigidity.^7,8 Commercial LED phosphors not only require high efficiency and excellent thermal stability, but also must be low cost and environmentally friendly. Therefore, borates are always considered to be an optimal host due to their low synthesis temperature, high UV transparency, and expected chemical stability.¹² In recent years, several highly efficient borate phosphors (quantum efficiency, QE > 50%) have been designed by doping Ce³⁺ into the RE₂O₃-BaO-B₂O₃ (RE = Y, Lu) phase system; for instance, Y₅Ba₂B₅O₁₇:Ce³⁺ (λ_ex = 365 nm; λ_em = 443 nm; FWHM ≈ 100 nm),¹¹ Lu₅Ba₂B₅O₁₇:Ce³⁺ (λ_ex = 340 nm; λ_em = 447 nm; FWHM = 104 nm),¹³ Y₂Ba₃B₆O₁₅:Ce³⁺ (λ_ex = 340 nm; λ_em = 446 nm; FWHM = 70 nm),¹⁴ and Lu₂Ba₃B₆O₁₅:Ce³⁺ (λ_ex = 473 nm; λ_em = 446 nm; FWHM = 68 nm).^15,16 Recently, S. K. Filatov et al. developed a novel Lu₅Ba₆B₉O₂₇ structure through the RE₂O₃-BaO-B₂O₃ phase system.¹⁷ Thus, doping Ce³⁺ ions into a Lu₅Ba₆B₉O₂₇ structure to obtain novel phosphors is highly anticipated for LED applications.

The unshielded 5d energy levels of Ce³⁺/Eu²⁺ ions (i.e., the centroid shift ε_c and the crystal field splitting ε_cfs) are sensitive to local crystal fields. Typically, alterations in the local crystal fields can be achieved through cation substitution, which can also cause a shift in the PL spectra.^7,8 In the LBB structure, a solid solution may be formed by cation substitution upon replacing Ba²⁺ (r = 1.35 Å; CN = 6) with Sr²⁺ (r = 1.18 Å; CN = 6). In most cases, as Sr²⁺ is gradually substituted for Ba²⁺, a red-shift in the PL spectrum occurs as the crystal field splitting becomes stronger, such as the emission peaks of (Ba_1−xSr_x)₉Sc₂Si₆O₂₄:Eu²⁺ tuned from 506 nm to 526 nm,¹⁸ the emission spectra of (Ba_1−xSr_x)₉Sc₂Si₆O₂₄:Ce³⁺ red-shifted by 20 nm,¹⁹ and the emission of Sr_xBa_2−xSiO₄:Eu²⁺ with a red-shift of ∼65 nm.²⁰ In contrast, the emission peaks of (Ba,Sr)₃Lu(PO₄)₃:Eu²⁺ show a significant blue-shift from 506 nm to 479 nm due to the neighboring-cation effect upon increasing the ratio of Sr/Ba.²¹ In addition, the emission color also can be adjusted by constructing Ce³⁺ → Tb³⁺ energy transfer in the host. Importantly, previous studies have shown that a fast energy transfer from Ce³⁺ to Tb³⁺ in the RE₂O₃-BaO-B₂O₃ system results in the improvement of luminescence thermal stability.^22–24 Therefore, it is also quite possible to construct a Ce³⁺ → Tb³⁺ energy transfer with a fast process in the LBB host.

To the best of our knowledge, no relevant research has yet focused on the Ce³⁺/Tb³⁺-activated Lu₅Ba₆B₉O₂₇ (LBB) structure. Herein, we prepare a series of blue phosphors Lu_5−xBa₆B₉O₂₇:xCe³⁺ (0.1% ≤ x ≤ 3.0%) and Lu_4.975Ba_6−yB₉O₂₇:0.5%Ce³⁺,ySr (0% ≤ y ≤ 20%), and green phosphors Lu_4.975−zBa₆B₉O₂₇:0.5%Ce³⁺,zTb³⁺ (0% ≤ z ≤ 16%) abbreviated as LBB:xCe³⁺, LBB:0.5%Ce³⁺,ySr²⁺ and LBB:0.5%Ce³⁺,zTb³⁺, respectively. The emission intensity of LBB:0.5% increases more than four-fold without a spectral shift and its internal QE increases from 20% to 95% when Ba²⁺ is partially replaced with Sr²⁺. Then, we improved the thermal stability of the emission by constructing energy transfer from Ce³⁺ → Tb³⁺ in the LBB structure. All results indicate that LBB:xCe³⁺, LBB:0.5%Ce³⁺,ySr²⁺ and LBB:0.5%Ce³⁺,zTb³⁺ phosphors can be used for UV/NUV-WLEDs.

2. Experimental and methodology

2.1 Materials and synthesis

The phosphors LBB:xCe³⁺ (0.1% ≤ x ≤ 3.0%), LBB:0.5%Ce³⁺,ySr²⁺ (0% ≤ y ≤ 20%), and LBB:0.5%Ce³⁺,zTb³⁺ (0% ≤ z ≤ 16%) were synthesized by traditional high-temperature solid-state reactions. The starting materials Ba₂CO₃ (A.R.), Sr₂CO₃ (A.R.), H₃BO₃ (A.R.), CeO₂ (4N), and Tb₄O₇ (4N) were mixed and ground according to the given stoichiometric ratio. The weighed powders were mixed in an agate mortar and placed in an alumina crucible, and then fired at 450 °C for 4 h in air. Finally, the mixtures were heated at 1200 °C for 8 h in a reducing atmosphere (10% H₂–90% N₂).

2.2 Measurements and characterization

The X-ray diffraction (XRD) patterns were collected from a multifunctional horizontal X-ray diffractometer (Ultima IV, Rigaku, Japan) with Cu Kα radiation (λ = 1.5418 Å). The high-resolution transmission electron microscopy (HRTEM) images and component elemental maps were obtained using an FEI Tecnai G2 S-Twin transmission electron microscope with a field emission gun operating at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were measured with an XPS spectrometer (Thermo Kalpha). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded on an Edinburg FLS-980 fluorescence spectrophotometer. A pulsed laser from an optical parametric oscillator and the electric signal detected using a Tektronix digital oscilloscope TDS 3052 were used to measure the fluorescence decay. The quantum efficiency (QE) was obtained using an absolute PL quantum yield measurement system (Quantaurus-QY Plus C13534-12, Hamamatsu Photonics).

3. Results and discussion

3.1 Optimizing the doping concentration of Ce³⁺ in the LBB structure

The LBB structure belongs to the space group C2/c and consists of edge- and corner-sharing Lu-, Ba-, and B-centered polyhedra, which form a three-dimensional net, as depicted in Fig. 1(a) and S1.† The [LuO₆] octahedra are connected to each other by their vertices. The B atoms are surrounded by three O atoms, which form [BO₃] triangles. There are three configurations of Ba atoms surrounded by O atoms, and they form distorted polyhedra [Ba1O₉], [Ba2O₈], and [Ba2O₇]. All the Ba–O polyhedra are connected to each other by their common edges and to the [BO₃] triangles and [LuO₆] octahedra by their common vertices and edges. As shown in Fig. 1(b) and S2,† the XRD patterns of LBB:xCe³⁺ (0.1% ≤ x ≤ 3.0%) phosphors match well with the standard card published by S. K. Filatov, et al.,¹⁷ indicating that Ce³⁺ ions were completely dissolved in the LBB structure.

Fig. 1 Crystal structure of LBB (a); XRD patterns of LBB:xCe³⁺ (x = 0.1% and 3%) phosphors (b).

Fig. 2(a) shows the PLE and PL spectra of LBB:0.1%Ce³⁺. The PLE spectrum consists of two bands that range from 250 nm to 380 nm in the ultraviolet (UV) region, which are ascribed to the allowed transitions 5d–4f of Ce³⁺ ions. Upon 340 nm excitation, LBB:0.01Ce³⁺ exhibits an asymmetric blue emission band that can be well fit with two Gaussian bands (dashed) peaking at 383 nm (26 109 cm⁻¹) and 411 nm (24 330 cm⁻¹). Their energy difference is about 2000 cm⁻¹, being consistent with the energy separation between the sub-states ²F_7/2 and ²F_5/2 of Ce³⁺. Fig. 2(b) shows the PL spectra depending on the Ce³⁺ concentration of LBB:xCe³⁺ (0.1% ≤ x ≤ 3.0%). The maximal PL intensity occurs around 0.5% Ce³⁺, and then the PL intensity gradually decreases with the increase of Ce³⁺ content due to the well-known concentration quenching effect.²⁵

Fig. 2 PLE and PL spectra of LBB:0.1Ce³⁺ (a); PL spectra (b) and fluorescence decay curves (c) depending on the Ce³⁺ concentration of LBB:xCe³⁺ (0.1% ≤ x ≤ 3.0%); linear fitting of log(I/x) versus log(x) for LBB:xCe³⁺ (0.5% ≤ x ≤ 3.0%) (d).

Here, the critical distance (R_c) is defined as the average distance between Ce³⁺ ions when luminescence quenching occurs. R_c can be estimated from geometrical consideration by following eqn (1):²⁶

where V represents the volume of the unit cell, X_c represents the critical concentration of Ce³⁺ for concentration quenching, and N represents the number of total Ce³⁺ sites in the unit cell. In this study, V = 2566.9 ų, N = 20, and X_c = 0.05 for LBB:Ce³⁺. Thus, the R_c of Ce³⁺ ions was calculated to be 36.6 ų.

The fluorescence decay curves of LBB:xCe³⁺ (0.1% ≤ x ≤ 3.0%) are depicted in Fig. 2(c). The fluorescence lifetime of LBB:0.1%Ce³⁺ is calculated to be 23.2 ns, probably reflecting intrinsic decay of Ce³⁺ in the LBB structure. The lifetime of Ce³⁺ gradually decreases with increasing Ce³⁺ content (%), which is probably caused by the enhanced nonradiative rates and energy transfer among Ce³⁺ ions in the LBB structure. The energy transfer mechanism of Ce³⁺ in the LBB structure is governed by the Dexter theory.²⁷ The activator (Ce³⁺) concentration satisfies eqn (2):^28,29

where I is the PL intensity, y is the Ce³⁺ concentration larger than X_c = 0.05, k and β are constants, and θ is an indication of the type of electric multipolar interaction. The values of θ are 6, 8, and 10, standing for the energy transfer mechanism of electric dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. As shown in Fig. 2(d), the relationship of log(I/x) versus log(x) is well fit by a linear function with a slope –(θ/3) of 1.7. Thus, the value of θ is determined to be 5.1, implying that the concentration quenching of Ce³⁺ ions in the LBB structure mainly results from the electric dipole–dipole interactions since the θ = 5.1 approximates to 6.

3.2 Substituting Sr²⁺ for Ba²⁺ in LBB:Ce³⁺ increases the emission intensity by 4.8-fold

The XRD patterns of LBB:0.5%Ce³⁺,ySr²⁺ (0% ≤ y ≤ 20%) phosphors are shown in Fig. 3(a). With an increase in the Sr²⁺ content (x), the diffraction peaks gradually shift to higher angles as shown in the magnified XRD patterns (29°–31°), which indicates that Sr²⁺ replaces Ba²⁺ ions in the LBB structure with contraction of the unit cell. The analysis results of HRTEM and XPS are presented in Fig. 3(b) and S3–S5.† The HRTEM image shows a clear lattice fringe, which indicates that the samples possess fine crystallization (Fig. S3†). The elemental mapping and XPS results (Fig. S4 and S5†) indicate that the LBB:0.5%Ce³⁺,20%Sr²⁺ compound contains strontium (Sr), barium (Ba), lutetium (Lu), cerium (Ce), boron (B) and oxygen (O) with a homogeneous distribution.

Fig. 3 The XRD patterns of LBB:0.5%Ce³⁺,ySr²⁺ (0% ≤ y ≤ 20%) phosphors, with magnification around 29°–31° (a); elemental mapping images of LBB:0.5%Ce³⁺,20%Sr²⁺ (b).

Fig. 4 shows the PLE, PL spectra, and decay curves of LBB:0.5%Ce³⁺,ySr²⁺ (0% ≤ y ≤ 20%) and luminescence thermal stability of LBB:0.5%Ce³⁺,20%Sr²⁺. In general, Sr²⁺ is substituted for Ba²⁺ to form a solid solution resulting in a red-shift or blue-shift of PL spectra, due to the variation in crystal field splitting (ε_cfs) and centroid shift (ε_c) of the activators (Ce³⁺/Eu²⁺). However, in the LBB structure, as Sr²⁺ ions gradually replace Ba²⁺ ions, there is no obvious shift in the PLE or PL spectra, but the emission intensity of LBB:0.5%Ce³⁺,20%Sr²⁺ increased by 4.8-fold compared with that of LBB:0.5%Ce³⁺. Fig. 4(c) exhibits the decay curves of the phosphor LBB:0.5%Ce³⁺,ySr²⁺ (0% ≤ y ≤ 20%), which are monitored at 410 nm and upon pulse excitation at 340 nm. The lifetime slightly increased from 23.5 ns (y = 0) to 25.3 ns (y = 20%), probably due to the decrease in the nonradiative transition probability with the increase in Sr²⁺ content (y). We consider that the structural rigidity is probably enhanced by the substitution of Sr²⁺ for Ba²⁺ in the LBB structure, which results in an increase in the lifetime.³⁰ To verify the hypothesis, we measured the QEs of LBB:0.5%Ce³⁺,ySr²⁺ (0% ≤ y ≤ 20%). The IQE, absorption efficiency (AE), and EQE values are calculated using eqn (3)–(5):^31,32

η_EQE = η_IQE × ε_AE, (5)

where L_s refers to integration of the emission spectrum of LBB:0.5%Ce³⁺,ySr²⁺ (0% ≤ y ≤ 20%) and E_R and E_S refer to spectra of excitation light with and without the samples in the integrated sphere, respectively. As shown in Fig. 5, the internal QE increased from 20% (y = 0%) to 95% (y = 20%), but there is no significant change in the absorption efficiency (AE) values. In addition, the luminescence thermal stability is another key parameter for LED phosphors. Thus, we measured the PL spectra of the LBB:0.5%Ce³⁺,20%Sr²⁺ phosphor and the corresponding relative integrated emission intensities at different temperatures. As depicted in Fig. 4(d), increasing the temperature from 303 K to 483 K causes small changes in the profile of the spectra; however, the luminescence intensity at 483 K is only 25% of the intensity at room temperature (303 K). To better understand luminescence thermal stability of LBB:0.5%Ce³⁺,20%Sr²⁺, the activation energy was calculated using the Arrhenius equation:³³where I₀ is the PL intensity at 0 K, I(T) is the PL intensity at a given temperature T, A is a constant, E_a is the activation energy for thermal quenching, and k_B is the Boltzmann constant. The experimental data are well fit using eqn (6). The value of E_a is determined to be 0.31 eV for LBB:0.5%Ce³⁺,20%Sr²⁺ as shown in Fig. 4(d).

Fig. 4 PLE and PL spectra (a and b); decay curves of LBB:0.5%Ce³⁺,ySr²⁺ (0% ≤ y ≤ 20%) phosphors (c); the temperature dependent PL spectra and relative integrated PL intensities of the LBB:0.5%Ce³⁺,20%Sr²⁺ phosphor (d).

Fig. 5 Emission spectra of LBB:0.5%Ce³⁺,ySr²⁺ (0% ≤ y ≤ 20%) for QE measurements in the absence and presence of the phosphor under 340 nm excitation (a–d).

3.3 Tb³⁺-doping into LBB:Ce³⁺ phosphors improves the luminescence thermal stability

Constructing the energy transfer from Ce³⁺ to Tb³⁺ is one of the potential solutions to improve the poor thermal stability of LBB:Ce³⁺ phosphors.^22–24,34 Thus, we prepared a series of LBB:0.5%Ce³⁺,_ZTb³⁺ (0% ≤ z ≤ 16%) phosphors. LBB:0.5%Ce³⁺,_ZTb³⁺ were characterized by XRD and no obvious impurity was detected (see Fig. 6(a) and S6†). As shown in Fig. 6(b, c) and S7–S9,† the elemental mapping, HRTEM images, and XPS results indicate that the dopants were homogeneously dissolved in the LBB host with fine crystallization.

Fig. 6 XRD patterns of LBB:0.5%Ce³⁺,zTb³⁺ (z = 1%, 8%, and 16%) (a); elemental mapping images (b); and the XPS spectrum of the LBB:0.5%Ce³⁺,16%Tb³⁺ phosphor (c).

The PLE and PL spectra of LBB:0.5%Ce³⁺, LBB:8%Tb³⁺, and LBB:0.5%Ce³⁺,8%Tb³⁺ phosphors are depicted in Fig. 7(a). Clearly, there is a spectral overlap between the PL spectrum of LBB:0.5%Ce³⁺ and the PLE spectrum corresponding to the ⁷F₆ → ⁵D₃ absorption of LBB:8%Tb³⁺, indicating the possibility of energy transfer from Ce³⁺ to Tb³⁺. The energy transfer was confirmed by the luminescence properties of LBB:0.5%Ce³⁺,8%Tb³⁺. The PLE spectrum of Tb³⁺ emission at 543 nm exhibits the characteristic 4f–5d transition band of the Ce³⁺ ion. Moreover, upon Ce³⁺ excitation at 340 nm, strong Tb³⁺ emission peaks at 488, 543, 585 and 625 nm corresponding to the ⁵D₄ → ⁷F_J (J = 6, 5, 4, and 3) transitions appear in the PL spectrum. To investigate the effect of energy transfer, the spectra with their corresponding CIE diagram and the fluorescence decay curves of LBB:0.5%Ce³⁺,zTb³⁺ (0% ≤ z ≤ 16%) phosphors are depicted in Fig. 7(b–d) and S10.† With the increase in Tb³⁺ content (z), the Ce³⁺ emission continuously decreases with the enhancement of the Tb³⁺ emission. Thus, the color tuning from blue to green occurs and is shown in the Commission Internationale de L'Eclairage (CIE) chromaticity diagram (Fig. 7(c)). In the LBB structure, there is also a fast energy transfer from Ce³⁺ to Tb³⁺, which can be confirmed by the Ce³⁺ lifetime of LBB:0.5%Ce³⁺,zTb³⁺. In general, the energy transfer efficiency (η_ET) of Ce³⁺ → Tb³⁺ can be calculated as follows:^35–38

where I_S0 and I_S denote the luminescence intensity of Ce³⁺ in the absence and in the presence of Tb³⁺, respectively. The energy transfer efficiencies listed in Table. S1† gradually increase with the increase of Tb³⁺ concentration in LBB:0.5%Ce³⁺,zTb³⁺ (0% ≤ z ≤ 16%). As the doping concentration (z) of Tb³⁺ reaches 16%, the energy transfer efficiency (η_ET) of Ce³⁺ → Tb³⁺ is determined to be 77%. We also use Ce³⁺ fluorescence lifetimes to calculate the energy transfer efficiency (η_ET):where τ_S and τ_S0 are the lifetimes of the sensitizer (Ce³⁺) with and without the activator (Tb³⁺), respectively. However, the Ce³⁺ lifetime only decreased from 23.5 ns in LBB:0.5%Ce³⁺ to 14.2 ns in LBB:0.5%Ce³⁺,16%Tb³⁺. Thus, the energy transfer efficiency (η_ET) of Ce³⁺ to Tb³⁺ in LBB:0.5%Ce³⁺,16%Tb³⁺ is calculated to be 40%. The reduction of emission intensity of Ce³⁺ is larger than that of lifetime, indicating the existence of fast energy transfer among some Ce³⁺ ions in LBB:Ce³⁺,Tb³⁺. Such a small decrease implies that there is a fast decay of Ce³⁺ that is too rapid to detect (see Fig. S11†). A similar fast energy transfer may occur in the nearest Ce³⁺–Tb³⁺ pairs, which has been demonstrated in previous work.^{22–24,34,38}

Fig. 7 PLE and PL spectra of LBB:0.5%Ce³⁺, LBB:8%Tb³⁺, and LBB:0.5%Ce³⁺,8%Tb³⁺ (a); PL spectra (b); emission color (c); and Ce³⁺ lifetimes of LBB:0.5%Ce³⁺,zTb³⁺ (0% ≤ z ≤ 16%) (d).

As depicted in Fig. 8(a), the energy transfer rate (W_ET) is closely related to the distance between Ce³⁺ and Tb³⁺. Thus, we use f (W_ET) representing the distribution function of the energy transfer rate W_ET, and the energy transfer efficiency can be written as follows:²⁴

Fig. 8 The energy transfer model for Ce³⁺ → Tb³⁺ (a and b); temperature-dependent PL spectra (c) and relative integrated PL intensities (d) of LBB:0.5%Ce³⁺,16%Tb³⁺; schematic diagram of the preparation of a pc-WLED (e); emission spectrum of a NUV-chip (370 nm)-based pc-WLED using BaMgAl₁₀O₁₇:Eu²⁺ (blue), LBB:Ce³⁺,Tb³⁺ (green), and K₂SiF₆:Mn⁴⁺ (red). The inset shows an image of the WLED package (f).

From eqn (9), we can find that there is a competition among energy transfer (W_ET), intrinsic decay (γ), and thermal activation (ΔE) after exciting Ce³⁺ ions. For simplicity, as depicted in Fig. 8(b), the process of energy transfer from Ce³⁺ to Tb³⁺ can be roughly classified into two groups in terms of energy transfer rates (the fast and the slow). The slow energy transfer (process ①) cannot compete with the thermal activation (ΔE), but the fast energy transfer (process ②) of Ce³⁺ → Tb³⁺ is so rapid that it cannot be detected in the fluorescence decay pattern (Fig. 7(d) and S11†). Owing to the fast energy transfer, there is a significant improvement in the luminescence thermal stability of LBB:Ce³⁺,Tb³⁺ due to the intrinsic properties of f–f transitions of Tb³⁺ with a large energy gap (about 15 000 cm⁻¹) between the emitting level ⁵D₄ and its next lower level ⁷F₁.^22–24,39 The luminescence intensity of LBB:0.5%Ce³⁺,16%Tb³⁺ at 483 K is improved to 68% of the intensity at room temperature (303 K), as shown in Fig. 8(c and d). Finally, we use LBB:0.5%Ce³⁺,16%Tb³⁺ as a green phosphor and the commercial phosphors BaMgAl₁₀O₁₇:Eu²⁺ (BAM:Eu²⁺; blue) and K₂Si₄F₆:Mn⁴⁺ (KSF:Mn⁴⁺; red) to fabricate a NUV-WLED. As depicted in Fig. 8(e and f), the RGB phosphors are fully mixed with epoxy resin and covered on a NUV-LED chip (365 nm). Then, the prototype NUV-WLED should be solidified at 200 °C for 20 min. The correlated color temperature (CCT) and color rendering index (R_a) of the as-fabricated NUV-WLED are found to be R_a = 82 and CCT = 4082 K, indicating the potential of LBB:Ce³⁺,Tb³⁺ phosphors for LED application.

4. Conclusion

In summary, we prepared a series of blue phosphors LBB:xCe³⁺ (0.1% ≤ x ≤ 3.0%) and LBB:0.5%Ce³⁺,ySr²⁺ (0% ≤ y ≤ 20%), and green phosphors LBB:0.5%Ce³⁺,zTb³⁺ (0% ≤ z ≤ 16%) via a high-temperature solid-state reaction. In the LBB structure, the luminescence intensity increased by 4.8-fold with no spectral shift upon the substitution of Ba²⁺ with Sr²⁺. The emission intensity of LBB:0.5%Ce³⁺ at 483 K is only 25% of the intensity at 303 K, which improves to 68% when the Ce³⁺ → Tb³⁺ energy transfer is designed in the LBB structure. The doping amount of Sr²⁺ and Tb³⁺ ions should not be too large and they cannot be simultaneously doped into the LBB structure, otherwise the purity of the crystal structure will be lost. In the future, we will explore ways to stabilize the LBB structure and establish structure–property relationships between the local crystal structure and PL properties using characterization methods such as Rietveld refinement, synchrotron radiation, and low-temperature spectroscopy, which may provide new ideas for the development of efficient and thermally stable phosphors for LED applications.

Data availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (Grant No. 12104231), the Shandong Postdoctoral Fund (Grant No. SDCX-ZG-202203064), and the Innovation Training Program for College Students in Nanjing Forestry University (Grant No. 202410298184Y).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4dt01843e

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