A red-emitting Sr₃La_(1−x)Eu_x(AlO)₃(BO₃)₄ phosphor with high thermal stability and color purity for near-UV-excited wLEDs

Abstract

A high efficiency red-emitting Eu³⁺ doped Sr₃La(AlO)₃(BO₃)₄ phosphor has been calcined successfully through a traditional solid-state method, and the photoluminescence and cathode-luminescence properties of samples are investigated in detail. The result of density functional theory (DFT) calculation shows that the Sr₃La(AlO)₃(BO₃)₄ host lattice has an indirect band gap. The broad excitation ranges from 200 nm to 500 nm, especially the strongest excitation located in the near ultraviolet (near-UV) region, suggests that the phosphor can match well with not only blue LED chips but also near-UV LED chips. Besides, the phosphor can emit bright red light peaking at 596, 618, 655 and 706 nm under either near-UV or blue light excitation and the internal quantum efficiency can reach 40%. The temperature-dependent PL properties and calculation of color purity show that the phosphor has an excellent thermal stability and outstanding color stability. Ultimately, by employing Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ phosphor as a red light component, a stable warm white light emission is obtained with Ra = 80, CIE = (0.3443, 0.2903) and CCT = 4680 K. These results indicate that red Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ phosphor may have a great potential for wLEDs with fantastic properties.

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1. Introduction

Of late, because of their high luminous efficiency, low power consumption, long lifetime and environmental friendliness, the phosphor-converted white light-emitting diodes (wLEDs) have attracted extensive research and commercial interest.^1–4 Conventionally, wLEDs are fabricated by a blue InGaN LED chip (λ_ex = 450–470 nm) covered by Ce³⁺ doped yellow emitting garnet (YAG:Ce³⁺) phosphor material. However, on account of the absence of sufficient red component, this type of wLED has a high color temperature (CCT ≈ 7750 K) and poor color rendering index (CRI ≈ 70–80) even though it has a high quantum efficiency and good thermal stability.^5–7 On the other hand, the blue light emitted by the InGaN chip can also cause damage to human eyes. Thus, the investigations of near ultraviolet white LEDs (n-UV-wLEDs) phosphors with excellent luminescence properties are necessary. Recently, a feasible method to fabricate wLED has been to combine a near ultraviolet chip and red, green and blue (RGB) phosphors. As a widely used red emission phosphor, Y₂O₂S:Eu³⁺ has been used in high efficiency fluorescent lamps, cathode ray tubes, field emission displays and plasma displays, but its thermal stability and chemical stability are poor.⁸ In this case, in order to improve the performances of red phosphor and wLEDs, it is an urgent task to develop novel red-emitting phosphors with excellent physical and chemical stabilities, and higher efficiencies. At present, the research of red phosphor is mainly focused on different substrates doped with Mn⁴⁺ or Eu³⁺, and lots of red emission phosphors with fantastic luminescent properties have been synthesized. For example, Wang et al. prepared a non-rare-earth-based oxide CaMg₂Al₁₆O₂₇:Mn⁴⁺ red phosphor, driven by a 350 mA current, the assembled w-LED device showed tunable Ra (70–85.5) and CCT (6674–3896 K).⁹ In addition, Guo et al. made a red-emitting Na₃Sc₂(PO₄)₃:Eu³⁺ phosphor with a high internal quantum efficiency of 49%.¹⁰ Huang et al. also synthesized a red Ca₃Lu(AlO)₃(BO₃)₄:Eu³⁺ phosphor with ultra-internal quantum efficiency of 98.5%.¹¹ The similar studies include Na_0.5Sr_0.25NbO₃:Eu³⁺,¹² Gd₃Zn₂Nb₃O₁₄:Eu³⁺,¹³ Sr[Li₂Al₂O₂N₂]:Eu²⁺,¹⁴ Na₂Gd(PO₄)(MoO₄):Eu³⁺ (ref. 15) and BaGeF₆:Mn⁴⁺,¹⁶ etc., all of them have excellent luminescence properties.

As a popular red emission activator, Eu³⁺ can generate strong emission at around 620 nm corresponding to ⁵D₀ → ⁷F₂ transition.^17–20 It is worth noting that the characteristic emission of Eu³⁺ roots in the transitions of its intra-4f electrons, and the 4f orbit lies inside the ion and is shielded from the surrounding by the filled 5s and 5p orbits. Therefore, the matrix lattice field has little effect on the optical transition within the 4f_n configuration, and the luminescence of Eu³⁺ ion has a good color stability. So far, researchers have synthesized a variety of Eu³⁺ ion-doped phosphor materials with different hosts, such as tungstate, molybdate, fluoride, aluminate, oxide and phosphate.^21–26

In this work, we have calcined the red-emitting Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ phosphor. The relatively internal quantum efficiency (IQE) and color purity can attain to 40% and 92.40% severally with a desired doping concentration of 60 mol% Eu³⁺ ion. Under the excitation of 394 nm, the phosphor displays a strong red emission with the strongest peak at 618 nm. Finally, by employing Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ as the red phosphor, we generate a white LED with the CCT = 4680 K and Ra = 80, which presents a substantial advance towards its commercial application. All the results indicate that the present synthesized phosphor is a suitable red-emitting applicant for n-UV-based wLEDs.

2. Experimental section

2.1. Materials and preparation

A series of Sr₃La_1−x(AlO)₃(BO₃)₄:xEu³⁺ (x = 0, 10, 20, 40, 60, 80, 100 mol%) phosphor samples are calcined by the traditional solid-state reaction. The raw materials SrCO₃ (A. R.), La₂O₃ (A. R.), Al₂O₃ (A. R.), Eu₂O₃ (A. R.) and H₃BO₃ (A. R.) are weighed with the designed stoichiometric ratio. Of which, the excess of boric acid is 15 wt%. And then the raw materials are ground to mix adequately for 50 min. The uniform mixtures are calcined at 1050 °C for 4 h in air. Finally, the products are ground evenly into power in agate mortar for further measurements.

Based on the ratio of Sr₃La_0.6Eu_0.4(AlO)₃(BO₃)₄ : BAM (BaMg₂Al₁₆O₂₇:Eu²⁺) : (Ca,Sr)₂SiO₄ = 9 : 1 : 3, the three kinds of phosphors are mixed absolutely with the silicone, then the mixture is coated on the surface of a 395 nm LED chip to integrate the wLED. The wLED is drove under the current of 150 mA. The CRI, CIE chromaticity coordinates, and CCT of the wLED are measured by using the Spectrophotocolorimeter (Hongpu, Hangzhou, China).

2.2. Measurements

XRD patterns of the phosphor samples are obtained to check the phase purity by X-ray diffractometer in the 2θ range of 10–90° with a step of 0.02° min⁻¹ using Cu-K_α1 radiation (λ = 1.54056 Å) (XRD-6000, SHIMADZU). The band structures and corresponding density of states of Sr₃La(AlO)₃(BO₃)₄ host are calculated based on the density functional theory (DFT) with the Cambridge Serial Total Energy Package (CASTEP) code.²⁷ The exchange–correlation functional generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) is chosen for the theoretical basis of density function with an energy cut-off of 480 V, and the k-point mesh is set as 2 × 2 × 4. The convergence criteria are set as follows: the force on the atoms is less than 0.03 eV Å⁻¹, the stress on the atoms is less than 0.05 GPa, the atomic displacement is less than 1 × 10⁻³ Å, and the energy change per atom is less than 1 × 10⁻⁵ eV. The microstructures of the calcined samples are observed by scanning electron microscopy (SEM, JSM7500, Japan). The diffuse reflectance spectra (DRS) are recorded on a UV-vis spectrophotometer (UV-3600) using the BaSiO₄ white power as the reference. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra are measured at room temperature via a fluorescence spectrophotometer (F-4600, HITACHI, Japan), where the photomultiplier tube is operated at 500 V and a 150 W Xe lamp is used as the excitation source. High temperature luminescence intensity measurements are carried out by means of an aluminum plaque with cartridge heaters; the temperature is measured by thermocouples inside the plaque and controlled with a standard TAP-02 high temperature fluorescence controller (Orient KOJI instrument Co., Ltd.). Fluorescence lifetime is acquired on a Fluorolog-3 spectrofluorometer (Horiba JobinYvon) with a SpectraLED (355 nm, S-355) as the excitation source and a picosecond photon detection module (PPD-850) as the detector. The internal quantum efficiencies (IQE) are obtained on HITACHI F7000 spectrometer with excitation wavelength of 394 nm. With the help of Fluorolog Spectrofluorometer (HORIBA) equipped with an integrating sphere coated with barium sulfate, the internal quantum efficiency of the prepared samples is evaluated.

3. Results and discussion

3.1. Structure and morphology of the samples

Fig. 1 exhibits the powder X-ray reflection diffraction (PXRD) patterns for Sr₃La_(1−x)Eu_x(AlO)₃(BO₃)₄ (x = 0, 10, 20, 40, 60, 80 and 100 mol%) phosphor samples. Although the phase of Sr₃La (AlO)₃(BO₃)₄ cannot be indexed from ICSD files, it is found that the diffraction peaks of samples can match well with the reference diffraction lines based on Ca₃Y(AlO)₃(BO₃)₄ (ICSD file no. 172154). Therefore, it can be inferred that the structure of the sample is the same as that of Ca₃Y(AlO)₃(BO₃)₄. The Rietveld structure refinement employed the in GSAS of Sr₃La (AlO)₃(BO₃)₄ is shown in Fig. S1.†

Fig. 1 PXRD patterns of Sr₃La_1−xEu_x(AlO)₃(BO₃)₄ (x = 0, 10, 20, 40, 60, 80 and 100 mol%) phosphors calcined at 1050 °C for 4 h. The diffraction peaks of the samples are basically indexed to the pure phase of Ca₃Y(AlO)₃(BO₃)₄ (ICSD: 172154).

The obtained profile factors are R_p = 12.60% and R_wp = 9.82%. The refined lattice parameters of Sr₃La (AlO)₃(BO₃)₄ are a = 10.388 Å, c = 5.852 Å, V = 546.947 ų, and space group P6₃/m.

With the help of typical crystal structure diagram of Sr₃La(AlO)₃(BO₃)₄ based on the structure refinement, it is easy to comprehend the coordination environment of cations and structure character in this matrix. As shown in Fig. 2, the Sr₃La(AlO)₃(BO₃)₄ belong to tetragonal crystal system with the space group of P6₃/m, and the coordinate numbers of La₁ and La₂ are 7 and 9, respectively. For Eu³⁺, its radius (∼0.96 Å) is closer to that of La₁ (∼1.06 Å), so the Eu³⁺ prefers to replace La₁ when it enters the crystal lattice of Sr₃La(AlO)₃(BO₃)₄.

Fig. 2 The crystal structure for Sr₃La(AlO)₃(BO₃)₄ host showing the connectivity of Sr, La, Al, B and O, and illustration of different coordination environment.

The crystalline morphology of the Sr₃La_0.7Eu_0.3(AlO)₃(BO₃)₄ phosphor is checked by scanning electron microscopy (shown in Fig. 3a), and the accordingly energy dispersive X-ray analysis (EDX) is employed to identify the content of different elements as exhibited in Fig. 3b. In Fig. 3c, it is clearly seen from the images that the samples are composed of agminated particles with sizes ranging from 5 to 15 μm and the average size is about ∼9 μm.

Fig. 3 (a) SEM image of the as-prepared Sr₃La_0.7Eu_0.3(AlO)₃(BO₃)₄ phosphor with magnification of 500; (b) EDX; (c) distribution of size.

3.2. Energy gap, electronic structure and density of state (DOS)

To get insight into the electronic properties of Sr₃La(AlO)₃(BO₃)₄ host matrix, the band structure and density of states (DOS) are computed and plotted in Fig. 4 after optimizing the Sr₃La(AlO)₃(BO₃)₄ structure. In Fig. 4a, the band gap between conduction bands minimum and valence bands maximum is about 4.57 eV, and they are located at G and K point in Brillouin zone severally, indicating that this host presents an indirect band gap. Therefore, the structure of Sr₃La(AlO)₃(BO₃)₄ is influential to luminescence since the transition probability of the indirect band gap is lower than that of the direct band gap due to the phonons involved in the transition process. In addition, the valence band of Sr₃La(AlO)₃(BO₃)₄ is remarkably flat, which reveals the very low mobility of the holes. By contrast, the dispersion of electronic states forming the conduction band is pronounced. With the help of density of states (DOS) and atomic partial density of states (PDOS) in Fig. 4b, we can acquire more information. The valence bands are dominated by the 2s orbit of O, 5p orbit of La and 4p of Sr, while the conduction bands comprise the 2p orbit of O, 2s and 2p orbits of B, 6s of La and 5s of Sr.

Fig. 4 Band structure and density of states for Sr₃La(AlO)₃(BO₃)₄ crystal.

In order to analyze the absorption spectroscopy, the diffuse reflection spectrum (DRS) of Sr₃La(AlO)₃(BO₃)₄ host and Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ phosphor are measured, as shown in Fig. 5a. Obviously, compared with the Sr₃La(AlO)₃(BO₃)₄ host, the diffuse reflectance spectrum of Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ shows an intense broad absorption band in the 200–300 nm range deriving from the Eu³⁺ → O²⁻ charge transfer band (CTB). In addition, a battery of relatively sharp peaks at 362 nm, 380 nm, 395 nm, 416 nm, 465 nm, 532 nm and 591 nm can be ascribed to 4f–4f transitions of Eu³⁺ ions (⁷F₀ → ⁵D₄, ⁷F₀ → ⁵L₇, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃, ⁷F₀ → ⁵D₂, ⁷F₁ → ⁵D₁ and ⁷F₁ → ⁵D₀, respectively). To further discuss the energy levels, the energy gap can be calculated according to the following function:²⁸

[F(R_∞)hν]ⁿ = c(hν − E_g) (1)

where hν is the photon energy, c is a proportional constant, E_g is the value of the band gap, n = 2 for a direct transition or 1/2 for an indirect transition. The F(R_∞) can be described by the Kubelka–Munk function:²⁹Here, R, K and S are the reflection, the absorption and the scattering coefficient, respectively. From the linear extrapolation of [F(R_∞)hν]^1/2 = 0, the E_g value of Sr₃La(AlO)₃(BO₃)₄ host and Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ are estimated to be about 5.29 and 3.46 eV (see Fig. 5b) severally, indicating that the energy gap is reduced after doping the Eu³⁺. In general, the real band gap is higher than theoretical value sometimes since DFT-calculated band gaps are always underestimated (The unmodified exchange correlation function is applied in DFT calculation).³⁰

Fig. 5 (a) DRS of Sr₃La(AlO)₃(BO₃)₄ host and Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄; (b) absorption spectrum of Sr₃La(AlO)₃(BO₃)₄ host and Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ as calculated by the Kubelka–Munk formula.

3.3. Photoluminescence properties

The room-temperature photoluminescence excitation spectra (λ_em = 618 nm) and emission (λ_ex = 356 nm, 365 nm, 377 nm, 404 nm, 449 nm, 488 nm) of Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ calcined at 1050 °C are shown in Fig. 6. The PLE spectrum of Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ phosphor consists of a broad band in the 250–300 nm region peaking at about 272 nm and several intense sharp lines in the 300–470 nm region, indicating that these phosphors can be effectively excited by UV LED chip (350–400 nm). The PLE spectra exhibit several peaks at 319 nm, 362 nm, 382 nm, 394 nm, 414 nm and 465 nm, which can be attributed to the ⁷F₀ → ⁵F₂, ⁷F₀ → ⁵D₄, ⁷F₀ → ⁵L₇, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃, ⁷F₀ → ⁵D₂ transitions of Eu³⁺ ions separately. This result corresponds well to that of DRS. The broad band is attributed to the charge transfer band (CTB) transition from the fully filled 2p orbits of O²⁻ ions to the partially filled 4f orbits of Eu³⁺.^8,17,18,31 Besides, the PL spectrum shows a red emission with the peaks at 596 nm, 618 nm, 655 nm and 706 nm, which result from the transition of ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃, ⁵D₀ → ⁷F₄.

Fig. 6 The PL (λ_ex = 394 nm) and PLE (λ_em = 618 nm) spectra of Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ phosphor.

To obtain the best doping content of Eu³⁺, the emission spectra of Sr₃La_(1−x)Eu_x(AlO)₃(BO₃)₄ have been investigated. As shown in Fig. 7a, under the excitation of 394 nm, the emission intensity keeps increasing until the value of x = 60 mol%, beyond which the concentration quenching occurs and the emission intensity begins to decrease. Therefore, the optimal doped concentration is about 60 mol%, it is also displayed by the corresponding tendency chart in Fig. 7b. In general, when the concentration of activator ions increases, the distance between the activators decreases, leading to the enhancement in energy transfer between the activators. If the ions concentration continues to increase, it will cause fluorescence quenching.

Fig. 7 Room-temperature (a) emission and (b) fluorescence decay curves (excited at 394 nm and monitored at 618 nm), (c) normalized integrated intensity and lifetimes of Eu³⁺ in Sr₃La_1−xEu_x(AlO)₃(BO₃)₄ with various Eu³⁺ concentrations, and (d) lg(I/x) vs. lg(x).

The photoluminescence decay curves of the samples also reveal the dependence of photoluminescence properties on the activator concentration and can provide important information of the concentration quenching in the energy transfer process. The fluorescence decay lifetime measurements for Eu³⁺ emission (λ_ex = 394 nm, λ_em = 618 nm) with Eu³⁺ concentration increasing are shown in Fig. 7c and the corresponding tendency is shown in Fig. 7b. All the fluorescence intensities show the trend of fast decay first and then slow decrease. And the Eu³⁺ decay curves of Sr₃La_1−xEu_x(AlO)₃(BO₃)₄ (x = 10, 20, 40, 60, 80, 100 mol%) samples can be well fitted with a second-order exponential decay equation (defects in the samples might act as the channels of non-radiative relaxation) as:^32,33

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (4)

where I(t) is the fluorescence intensity at time t and I₀ is the background luminescence intensity, A₁ and A₂ are constants while τ₁ and τ₂ are the decay times for the double-exponential components, respectively. The fitting parameters are listed in Table 1, showing that the lifetime of Eu³⁺ in Sr₃La_1−xEu_x(AlO)₃(BO₃)₄ is in the range of microsecond, which is reasonable for the ⁵D₀ → ⁷F₂ transitions of Eu³⁺. With increasing Eu³⁺ concentration, the lifetime becomes shorter gradually from 1.991 ms to 1.910 ms, it can be attributed to the increased non-radiative energy transfer. At the same time, it is worth noting that the decrease in fluorescence decay lifetime is very small (0.081 ms), this is an interesting phenomenon.

Table 1 The fitting results and average lifetimes of fluorescence decay curves of Sr₃La_1−xEu_x(AlO)₃(BO₃)₄

Concentration of Eu^3+ (mol%)	τ1 (ms)	τ2 (ms)	A1	A2	τ* (ms)
10	2.63	1.93	790	8872	1.991
20	2.96	3.79	9644	117	1.979
40	1.68	2.12	3100	6550	1.976
60	1.65	2.27	4808	4882	1.964
80	1.88	2.64	9276	488	1.914
100	2.58	1.86	707	9053	1.910

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In order to further investigate the quenching mechanism, it is necessary to calculate the critical transfer distance (R_c). According to Blasse's theory, the critical distance R_c can be calculated approximately by this equation:³⁴

where the V is the volume of the unit cell, x_c is the critical concentration from Fig. 7a, and N is the number of available sites for the dopant (Eu³⁺) in the unit cell. In this case, V = 531.92 Å^3,35 x_c = 0.6, N = 2, so the critical distance for Eu³⁺ in Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ is calculated to be ∼9.16 Å. It can be inferred that the concentration quenching mainly takes place via electric multipolar interaction between Eu³⁺ ions because the exchange interaction is dominant only for short critical transfer distance (R_c < 5 Å) based on the Dexter's theory.³⁶ Hence concentration quenching process must be controlled by electric multipole interactions. Generally, there are three multipole interactions, namely, dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions. In order to further investigate the concentration quenching mechanism in the phosphor, the following equation is used:³⁷where I represents the emission intensity of Eu³⁺ with different content, x is the concentration of Eu³⁺, β and κ are the constants at some condition, and θ equals to 6, 8, 10 for dipole–dipole, dipole–quadrupole, quadrupole–quadrupole, respectively.

The relationship between lg(I/x) and lg(x) has been plotted as shown in Fig. 7d. The slope of line is about −2.18, and the calculated θ ≈ 6, meaning that the concentration quenching mechanism between the closer Eu³⁺ ions in the Sr₃La_1−xEu_x(AlO)₃(BO₃)₄ is dominated by dipole–dipole interaction.

Furthermore, the luminescence color of Sr₃La_1−xEu_x(AlO)₃(BO₃)₄ has been characterized using CIE 1931 chromaticity diagram. The CIE coordinate value of the Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ phosphor is calculated to be (0.6524, 0.3473), situated in the red region and at the extreme corner of the CIE diagram as shown in Fig. 8.

Fig. 8 CIE (x, y) coordinate diagram showing chromaticity points of Eu³⁺ luminescence in Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄; insets are the corresponding luminescent photos under 394 nm UV lamp illumination.

To get a deep insight into the chromatic behaviors of the obtained red emission, its color purity is calculated with the help of the following formula:^38,39

In the expression, the (x, y), (x_i, y_i) and (x_d, y_d) refer to the CIE coordinates of the synthesized compounds, white illumination and dominant wavelength, respectively. In this present work, (x, y) = (0.6524, 0.3466), (x_i, y_i) = (0.310, 0.316) and (x_d, y_d) = (0.679, 0.317). As a result, the color purity of the achieved red emission is demonstrated to be about 92.40%, which is superior to some previous developed red-emitting phosphors, such as Na₃Sc₂(PO₄)₃:Eu³⁺ (87%)¹⁰ and Na₂Gd(PO₄)(MoO₄):Eu³⁺ (92%).¹⁵

3.4. Quantum efficiency

The internal quantum efficiency (IQE) of Sr₃La_1−xEu_x(AlO)₃(BO₃)₄ phosphors are calculated by using the following equations:⁴⁰

In which L_S stands for the emission spectrum of the phosphor, and E_R represents the spectrum of excitation light without phosphor in integrating sphere. The IQE of Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ is calculated to be about 40%, as shown in Fig. 9. And the IQEs of Sr₃La_1−xEu_x(AlO)₃(BO₃)₄ are displayed in Table 2.

Fig. 9 Room-temperature excitation of BaSiO₄ and reflection and emission of Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄Eu³⁺.

Table 2 IQE of Sr₃La_1−xEu_x(AlO)₃(BO₃)₄ (λ_ex = 394 nm)

Sample no.	Concentration of Eu^3+ (mol%)	IQE (%)
1	10	27
2	20	33
3	40	38
4	60	40
5	80	37
6	100	33

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3.5. Thermal stability

Thermal stability is another important performance factor for phosphors because LED chips will produce a large amount of heat in operation. To further verify the thermal stability characteristic of Sr₃La(AlO)₃(BO₃)₄:Eu³⁺, thermal quenching of the typical Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ sample is investigated in detail. The temperature-dependent emission spectra and emission intensity are also given in Fig. 10a. With temperature increasing, the peak positions and band shape of this phosphor do not vary, whereas the emission intensity changes obviously. In order to better compare the thermal stability, Fig. 10b exhibits the emission intensities of Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ and commercial Y₂O₃S:Eu³⁺ phosphors depend on temperature. Apparently, as the temperature increases, the emission intensities of these two kinds of phosphors have a similar trend to decrease. Specifically, at 225 °C, the emission intensity of Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ can still remain at ∼91% of the initial value (at ∼25 °C), while that of the Y₂O₃S:Eu³⁺ reduces to ∼59%. When the temperature further rises to 300 °C, the emission intensity of Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ decreases to 59% of the initial value, while that of the Y₂O₃S:Eu³⁺ decreases to 51%. The fact suggests that the Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ phosphor has a better thermal stability. Usually, the activation energy can be calculated using the Arrhenius equation:⁴¹where I₀ and I_T represent the emission intensity at room temperature and testing temperature, respectively, c is a constant, ΔE is the activation energy for thermal quenching, and k is Boltzmann's constant (8.617105 × 10⁻⁵ eV K⁻¹). The dependence of ln[(I₀/I_T) − 1)] on 1/kT is shown in Fig. 10c. From the straight slope, ΔE is calculated as 0.29 eV. The high activation energy causes a good thermal stability of phosphor. As shown in Fig. 10d, it is noteworthy that the peak position and CIE color coordinates of the Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ phosphor keep almost constant during the continuous heating from 25 °C to 300 °C, which implies that the Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ phosphor has an excellent color stability. The above facts strongly suggest that red Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ phosphor has a great potential to be applied in w-LEDs.

Fig. 10 (a) Temperature-dependent emission spectra of Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ in the range ∼25–300 °C, (b) comparisons of corresponding integrated emission intensity between Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ and Y₂O₂S:Eu³⁺ as a function of temperature, (c) the linear fitting curve of ln(I₀/I_T − 1) versus 1/kT for the Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ phosphor, (d) the color stability with different temperature and (e) configurational coordinate diagram of the ground state and excited state of Eu³⁺.

The thermal quenching can be explained by configurational coordinate diagram. In Fig. 10e, the symbols of A and B represent the ground state and excited state of Eu³⁺ separately. As the temperature increases, the lattice vibration of the matrix increases, and the electrons in the excited state can be thermally activated to reach the intersection of the ground state and the excited state coordinate curve (process ①), causing the excited state electrons to transit to the ground state non-radioactively and pass through the lattice relaxation to reach the equilibrium state of the ground state (process ②). As a result, the two processes are enhanced as the temperature continues to rise, and the luminescence intensity becomes weaker at higher temperatures.

Encapsulation experiment is actualized to evaluate the potential application possibility for Sr₃La(AlO)₃(BO₃)₄:Eu³⁺. By employing the Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ phosphor as a red component, the white light can be generated with Ra = 80, CCT = 4680 K and CIE of (0.3443, 0.2903) under the driven current of 150 mA. The corresponding electroluminescence (EL) spectrum of the fabricated white LED is shown in Fig. 11. The obtained warm white light has a high color index, indicating that the Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ phosphor has a great potential to be applied in w-LED.

Fig. 11 EL spectrum of the fabricated white LED lamp by 150 mA current. The insets are the photos of LED when not working and working.

4. Conclusion

In summary, a series of highly efficient Eu³⁺-activated Sr₃La(AlO)₃(BO₃)₄ phosphors are successfully prepared by the solid-state reaction method. The resultant phosphor could be efficiently excited by the commercial NUV chip and emitted brightly visible red emission with a high color purity of approximately 92.40%. The optimal doping concentration of Eu³⁺ ions in the Sr₃La (AlO)₃(BO₃)₄ host material is determined to be as high as 60 mol% and the dipole–dipole interaction contributes to the concentration quenching. Furthermore, the internal quantum efficiency of the Sr₃La_0.4Eu_0.6(AlO)₃(BO₃)₄ phosphor is found to be 40%. The temperature-dependent PL emission spectra revealed that the studied phosphor possessed excellent thermal stability with an activation energy of 0.29 eV. By employing Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ as a red light component, a stable wLED with proper CCT (4680 K) and CRI (Ra = 80) values is obtained. The above results indicate that red phosphor Sr₃La(AlO)₃(BO₃)₄:Eu³⁺ may be a promising candidate for fabricating near ultraviolet converted white-light LEDs.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJQN201801332) and The Fundamental Research Funds for Central Universities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra05017e

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