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Abstract
Nitride phosphors usually have great influence on the performance of white LEDs. However, most nitrides require synthesis under high temperature and high pressure, especially Ca18.75Li10.5[Al39N55] (CLAN), which demands preparation even under a pressure of 51.7 MPa. Herein, we applied a simple synthetic route to obtain CLAN at a low pressure of 0.85 MPa and temperature of 1250 °C. In addition, the luminescence properties of Ce3+ doped CLAN were studied and the CLAN:Ce3+ shows three excitation bands peaked at 267, 387, and 478 nm. Two emission peaks are located at 513 nm and 567 nm, corresponding to the d-f electronic transition of Ce3+, respectively. The emission intensity at 150 °C retains about 83.3% of the initial value at room temperature, indicating a good thermal stability. A prototype white LED encapsulated with a 450 nm chip and the CLAN:Ce3+ phosphor exhibits bright white emission, indicating potential applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Solid-state lighting (SSL) exceeds traditional lighting in terms of durability, high luminous efficiency, energy saving, and excellent reliability [1–4]. Currently, phosphor-converted white light-emitting diodes (pc-wLEDs) are considered as the main modus for SSL that are fabricated by combining either an ultraviolet (UV, 350-420 nm) or blue (450-470 nm) LED chip with phosphors [5–7]. Phosphor is the key material for wLEDs directly determining the luminous efficiency and color rendering index (CRI) of wLEDs [8–12]. In recent years, outside of Y3Al5O12 (YAG):Ce3+ [13,14], some nitrides with significant Ce3+ emission of spectral modulation (from blue to red spectral range) have been reported, e.g. LaSi3N5:Ce3+ (λem ∼ 465 nm) [15], (Sr, Ca)2Si5N8:Ce3+ (λem ∼ 535-556 nm) [16,17], (La, Ca)3Si6N11:Ce3+ (λem ∼ 535-595 nm) [18,19], (Sr, Ca)AlSiN3:Ce3+ (λem ∼ 550-600 nm) [20,21], CaSiN2:Ce3+ (λem ∼ 610 nm) [2], etc. In addition, Ce3+-doped nitride phosphors have high thermal stability compared to YAG:Ce3+, and exhibit high optical absorption in the near-UV and blue light region [15–21]. These nitride materials (nitridosilicates) are commonly synthesized at a harsh sintering temperature (1500-2000 °C). However, nitridoaluminate phosphors can be synthesized at moderate temperature of 1000-1300 °C, and show small Stokes shifts and narrowband emission due to the high-rigidity and symmetrical sphalerite-type frameworks [22–24].
Ca18.75Li10.5[Al39N55] (CLAN) has a cubic crystal structure and consists of two interspersed [AlN4] supertetrahedra [25–27]. Thus, CLAN is an excellent host for high-performance phosphors because of its highly condensed crystal structure and symmetrical lattice site environment. However, the reported synthesis method for rare earth doped CLAN is extremely difficult. For example, CLAN:Eu2+ need a hot isostatic press (HIP) method at nitrogen pressure of 51.7 MPa [25]. The HIP method leads to a large amount of the AlN impurity (∼40 wt%) [25], resulting in poor luminous efficiency and thermal stability. Therefore, the investigations for CLAN are limited due to the strenuous synthesis method.
Herein, we develop a simple method at a low gas pressure (0.85 MPa) to synthesize the CLAN powders. The AlN impurity is greatly depressed. Based on the red emission of CLAN:Eu2+ [25] and luminescence theoretical model between Eu2+ and Ce3+ [28–32], we speculate that CLAN:Ce3+ should emit light with a shorter wavelength. The properties of Ce3+-doped CLAN are reported for the first time. CLAN:Ce3+ can be effectively excited under blue light, and shows yellow-green emission. Based on the CLAN:Ce3+ phosphor and a 450 nm LED chip, the manufactured white LED has a luminous efficiency of 54.3 lm/W.
2. Experimental section
2.1 Synthesis
The samples with the nominal composition of Ca18.75-2xLi10.5 + x[Al39N55]:xCe3+ (x = 0-0.6) were synthesized by a high-temperature solid-state reaction. All the nitride raw materials of Ca3N2 (Aldrich, 99.99%), Li3N (Aldrich, 99.99%), AlN (Aldrich, 99.99%), and CeN (3A Chemicals, 99.95%) were weighed stoichiometrically and mixed for 20 min in the agate mortar inside an argon-filled glovebox (H2O < 1 ppm, O2 < 1 ppm). The mixtures were sintered in a gas pressure sintering furnace (ZKL-1700) at 1250 °C for 6 hours at 0.85 MPa N2 atmosphere. After that, samples were natural cooled to room temperature, and grounded to powders for characterizations.
2.2 Characterizations
The phases of the as-prepared samples were analyzed by the X-ray powder diffraction (XRD) of the Bruker D8 (Germany) with a graphite monochromatized Cu Kα radiation (λ = 0.15406 nm). The XRD data for crystal structure analysis were recorded in the range of 5-90 ° with a step size of 0.01 ° (2θ). The phase purity and crystallographic parameters were studied based on the Fullprof Suite package [23,33]. The morphology and elemental composition of samples were measured at room temperature by a field emission scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS) system (Hitachi, S-3400N, Japan). The diffuse reflection spectra were performed in the range of 300 to 600 nm (1nm step size) using a UV−vis spectrophotometer (Lambda 950), with BaSO4 coating layer as a standard reference in its integrating sphere. The photoluminescence (PL) spectra, photoluminescence excitation (PLE) spectra and quantum efficiency of the samples were measured at room temperature by a FS5 fluorescence spectrometer (Edinburgh Instruments, UK). The corresponding thermal quenching of emission properties and decay curves of as-synthesized phosphors were both measured by the same FS5 fluorescence spectrophotometer. The samples were heated from 25 °C to 250 °C at a constant rate of 5 °C/min through an externally connected temperature controller (Orient KOJI, China), and maintained for 5 min at each test temperature. The pc-wLED were fabricated by combining an InGaN LED chip (λem = 450 nm) with CLAN:Ce3+ phosphor. The luminescent properties were measured using an integrated sphere spectroradiometer system (LHS-1000, Ever fine Co., Hangzhou, China).
3. Results and discussion
3.1 Structure and morphology
The Rietveld refinement of CLAN:0.3Ce3+ is depicted in Fig. 1(a). The crystallographic details are listed in Table 1, and the atomic coordinates and isotropic displacement parameters are given in Supplement 1. The Rietveld refinement results of Rwp = 6.79%, Rp = 5.04%, and χ2 = 3.52 indicate that the fabricated sample mainly crystallizes into the CLAN phase (95.4 wt%), and the content of side-phase AlN is 4.6 wt%. Nitride raw materials are light or have low melting point, which are easy to be oxidized and lost in the preparation process, resulting in relative excess of AlN. Compared with the reported data (∼40 wt%) [25], the AlN impurity content is significantly reduced by 88.5%, indicating an optimized effect of our synthesis method. As shown in Fig. 1(b), within a wide range of Ce3+ doping concentration, the XRD patterns of CLAN:xCe3+ (x = 0-0.6) maintain a good correlation with COD-4002664.
Fig. 1. (a) Rietveld refinements of the XRD pattern of CLAN:0.3Ce3+. Blue and purple bars indicate positions of the Bragg reflections of CLAN and AlN, respectively. Brown line represents the difference plot (observed-calculated) on the same scale. (b) XRD patterns of CLAN:xCe3+ (x = 0-0.6). Standard data (COD-4002664) of CLAN were shown as reference. (c) Lattice structure diagram of CLAN and (d) coordination spheres of Ce/Ca sites.
Table 1. Crystallographic parameters of Rietveld refinements of CLAN.
CLAN features a supertetrahedron crystal structure (Fig. 1(c)) and belongs to the cubic crystal system, space group Fd$\bar{3}$m (no.227), with lattice parameters a = 22.3531(2) Å, V = 11168.97(3) Å3. In CLAN, condensed [AlN4] tetrahedrons form edge- and vertex-sharing anionic network structures resulting in negative formal charge that can be compensated by Ca2+ and Li+ cations. The structure of the anion network in CLAN can be regarded as the structural variant of AlN. Therefore, CLAN has a high structural stability and chemical inertness. In CLAN (Fig. 1(d), Table S2 and S3), there are three independent crystallographic Ca sites with 6-coordination N atoms, and the average Ca–N bond lengths of Ca(1)N6, Ca(2)N6, and Ca(3)N6 polyhedra are 2.449, 2.651, and 2.423 Å, respectively. Ca(1)N6 and Ca(3)N6 form a distorted octahedron (trigonal antiprismatic), and Ca(2)N6 forms a distorted pentahedron (trigonal prismatic) [25,26].
Figure 2(a) and (b) show the morphology and size of CLAN:0.3Ce3+. The sample shows good crystallinity and uniformity without agglomeration, and the average particle size is about 10 µm, which is appropriate for the LED packaging [6,7,34]. The particle morphology of the sample has obvious edges and most of the particles are regularly octahedra. The regular morphology and smooth surface are benefit for increasing luminescence efficiency. Ca, Al, N, and Ce are uniformly distributed in the whole particle based on the EDS mapping results in Fig. 2(c). This shows that the chemical composition of the sample is homogeneous and the reaction of the raw materials is sufficient and uniform. Li cannot be detected by this method.
Fig. 2. (a) and (b) SEM images of CLAN:0.3Ce3+ phosphor. (c) Ca, Al, N, and Ce elements mapping images.
3.2 Photoluminescence properties
In Fig. 3(a), the diffuse reflectance spectra of Ce3+ doped and undoped samples indicate that the samples have an intense broad absorption from 300 to 600 nm. The optical band gap of ∼3.68 eV was determined by the Kubelka-Munk function [2]:
The suitable band gap suggests that CLAN would be a promising host candidate for phosphors with intriguing PL properties. The 5d state of Ce3+ is splitted by crystal field and, hence, there are three absorption bands in the PLE spectrum located at 267, 387, and 478 nm (see Fig. 3(b)). Therefore, CLAN:Ce3+ is suitable for both UV and blue chips to construct the LED light source. The peaks of the low energy excitation band at 387 and 478 nm are attributed to the transition of Ce3+ from the 4f ground state to the lower energy level (2T2g) of the 5d excited state. The absorption band at 267 nm is caused by the higher energy level (2Eg) of the 5d excited state [34]. A possible host excitation (band to band excitation) band at 310-370 nm is also noted, corresponding to the bandgap of ∼3.68 eV. The characteristic blue emission of AlN:Ce3+ with an asymmetric emission band centered at ∼440 nm is not found [35,36]. Consequently, the influence of AlN on the luminescent properties of target products CLAN:Ce3+ can be excluded.
Fig. 3. (a) Diffuse reflectance spectra of CLAN (blue) and CLAN:0.3Ce3+ (red). Inset shows the Tauc plot [F(R) hν]1/n (n = 1/2) for CLAN. (b) PLE and PL spectra of CLAN:0.3Ce3+ and (c) their PLE and PL spectra at different excitation wavelengths and monitoring wavelengths. (d) Schematic energy-level diagram of Ce3+ in CLAN.
CLAN:Ce3+ shows a yellow-green emission spectrum with a maximum peak at about 513 nm (∼19493 cm-1) and a shoulder at about 567 nm (∼17637 cm-1). The energy difference between the two peaks is about 1856 cm−1, attributed to the spin-orbit coupled ground states 2F5/2 and 2F7/2 of Ce3+ [37]. The composed emission bands of CLAN:Ce3+ show one high peak and one low peak, which are similar to those of CaS:Ce3+ or SrS:Ce3+ with only single site [38,39]. Besides, the position and shape of PL spectra do not change with the change of excitation wavelength (Fig. 3(c)). Furthermore, the luminescence decay curve of CLAN:Ce3+ can be fitted by a single-exponential decay model. These results suggest that the Ce3+ emission in CLAN arises from a single emission center.
In CLAN, Ce3+ ions can occupy three Ca sites and emit light as emission centers, which seems to be conflict with the PL spectrum from the single emission center. The reason may be the same as the narrow-band luminescence of CLAN:Eu2+ [25]. Due to the large energy separation between the 5d excited state and 4f ground state of Ce3+ ions, the 5d energy level of Ce2 at the non-luminescent site is in the conduction band. The electrons enter the conduction band and cannot return to the ground state through a radiation process, as shown in Fig. 3(d).
In order to analyze the relationship between the emission of multiple activator Ce3+ sites and the local structure, the polyhedron sites Ca(1)N6, Ca(2)N6, and Ca(3)N6 were estimated based on the centroid shift and crystal field splitting (CFS). The nephelauxetic effect may cause centroid shift εc, which can be calculated from the Dorenbos model [2,40]:
The CFS of 5d states of Ce3+ is greatly affected by the local environment. On the basis of the point charge model [28], the CFS can be determined by the equation below:
where Dq represents the magnitude of energy level separation, z is the anion charge, e is the electron charge, r is the radius of the d wavefunction, and R is the bond length [28]. Thus, the energy level separation Dq is inversely proportional to R5. The average bond lengths of Ce1 and Ce3 sites are similar and significantly shorter than that of Ce2 site, so Ce1 and Ce3 sites should belong to high CFS sites. A comprehensive analysis shows that Ce2 site has a lower centroid shift and CFS, and its 5d level is all located in the conduction band due to the larger energetic separation of the Ce3+ 4f and 5d states, leading to a nonluminous Ce2. Due to the CFS and the nephelauxetic effects, the 5d levels of Ce1 and Ce3 are partially located in the forbidden band of CLAN, and thus their excited electrons return to the ground state and glow.In order to further explore the influence of the coordinated polyhedra Ca(1)N6 and Ca(3)N6, their distortion index (D) also be calculated:
where n is the coordination number, Ri is the bond length between cation and coordinated anions, and Rav is the average bond length [20,37]. According to the calculation, the coordinated polyhedra Ca(1)N6 and Ca(3)N6 shows very small distortion indices of D1 = 0.015 and D3 = 0, respectively. While the distortion index of the coordinated polyhedral in common Ce-doped materials is usually greater than 0.03 [20,37], as shown in Table 2. Generally, the overall peak shape of the PL spectrum of Ce-activated luminescent materials consists of the spectra of each luminescent center. Because there is only single site and the distortion index is 0. CaS:Ce3+ presents typical emission of Ce3+, with two peaks corresponding to the lowest 2T2g level of the 5d state to the 2F5/2 and 2F7/2. However, Sr[LiAl3N4]:Ce3+ has two luminescent centers with high distortion at both sites, so the two peaks come from the luminescence of two sites [34,37]. The local structures of the two luminescent centers of CLAN:Ce3+ are very similar, resulting in indistinguishable spectral differences between the two centers. As a result, the overall shape of the PL spectrum of CLAN:Ce3+ is identical from the spectrum of each luminescent center. Therefore, it shows the characteristic of the single emission center.Table 2. Polyhedral distortion index (D) of several Ce-activated materials.
The luminescence decay curve (Fig. S1) of CLAN:0.3Ce3+ could be well-fitted (χ2 = 1.17) with a single-exponential function as follows equation:
where I(t) represents the luminescence intensity at the time t, I0 is the initial luminescence intensity, A is the value for different fitting, and τ is the decay time for the exponential components [41]. Figure 4(a) shows decay curves of CLAN:0.3Ce3+ monitored at 500, 513, 567, and 600 nm. The fluorescence lifetimes (χ2 = 1.07-1.20) of CLAN:0.3Ce3+ increases slightly when the monitored wavelength increases. But the change is not significant, indicating that the properties of the two luminescent centers Ce1 and Ce3 are very similar, which is consistent with the above spectral performance. This is distinguished from Sr[LiAl3N4]:Ce3+, which has two distinct fluorescence lifetimes. The PL spectra under different excitation wavelengths remain basically unchanged, the difference of fluorescence lifetime at different monitoring wavelengths is very slight, and the degree of distortion of the two sites is very small and close. These results indicate that luminescence of CLAN:Ce3+ comes from two almost indistinguishable Ce1 and Ce3 sites, and shows the characteristic of the single emission center.Fig. 4. Decay curves of CLAN:0.3Ce3+ monitored at (a) different wavelengths and (b) different temperature.
As shown in Fig. 4(b), it is calculated that the fluorescence lifetimes (χ2 = 1.05-1.18) decrease from 27.8 ns at 25 °C to 27.2 ns, 26.1 ns, 25.0 ns at 50 °C, 100 °C, and 150 °C, respectively. The nanosecond fluorescence lifetime is very short and does not change much with the temperature increase, indicating that the luminescence comes from Ce3+ and there is no defect luminescence or other processes.
The emission intensity of the CLAN:xCe3+ (x = 0.1-0.6) increases initially and then decreases with the increase of Ce doping concentration under 478 nm excitation and reaches the maximum value at x = 0.3 (Fig. 5(a) and (b)). This result reveals that the critical concentration is about 1.6 mol%. As the concentration of the activator increases, the concentration quenching effect occurs when the ion spacing between adjacent emission centers is small enough. The critical distance (Rc) is a necessary parameter to further characterize this process and can be calculated using the formula proposed by Blasse et al. [42]:
where V is the volume of the unit cell, xc is the critical concentration, and N is the number of sites that Ce3+ ions occupy in unit cell [42]. For the CLAN host, xc = 0.016, N = 150, V = 11168.97(3) Å3, the estimated value Rc is approximately 20.7 Å. Hence, the energy transfer mechanism for the CLAN:Ce3+ system should be the electrical multipolar interaction. What’s more, radiation reabsorption occurs when the PL and PLE spectra of the luminescence center strongly overlap. The PLE and PL spectra of CLAN:Ce3+ have obvious overlap, suggesting that the radiation reabsorption mechanism may also have an influence. Introducing Ce3+ into lattice to substitute Ca2+ does not significantly change the distance (R) between the Ca2+/Ce3+ ions and the ligands (N3-). This causes little change of the crystal field strength and 5d energy level splitting of Ce3+. So, it does not cause significant shift and broadening of the PL spectrum.Fig. 5. PL spectra (a) and IQEs (b) of Ca18.75-2xLi10.5 + x[Al39N55]:xCe3+ (x = 0.1-0.6) under 478 nm excitation.
The lowest-energy excitation band peak emerged at 478 nm (∼20920 cm-1) and the highest-energy emission band peak located at 513 nm (∼19493 cm-1) bring about an estimated Stokes shift of 1427 cm-1. As shown in Table 3, the internal quantum efficiencies (IQEs) of CLAN:0.3Ce3+ are determined to be 38.3% and 30.5% under 478 nm and 387 nm excitation, respectively.
Table 3. IQEs of the CLAN:xCe3+ (x = 0.1-0.6) samples under different excitation wavelengths.
3.3 Thermal quenching properties
The thermal stability of CLAN:Ce3+ was measured and displayed in Fig. 6(a). With the temperature increases, the overall peak shape and peak position of the CLAN:0.3Ce3+ sample are hardly change or shift. Figure 6(b) shows the variation of relative integrated intensity and the sample can maintain about 83.3% at 423 K (150 °C) of the initial emission intensity at 298 K (25 °C). Compared to other Ce3+-activated phosphors, the thermal stability of CLAN:Ce3+ is notable. These should be attributed to the optimization of the preparation process, which leads to better crystallization of the target product as well as the reduced AlN impurity. The rigid and ordered network structure reduces the non-radiative relaxation of excited electrons at high temperatures, which is another possible factor [43–45].
Fig. 6. (a) Temperature-dependent PL spectra of CLAN:0.3Ce3+ under 387 nm excitation, and variations (b) of their integral intensity with temperature.
3.3 Applications in wLEDs
To demonstrate the potential application of CLAN:Ce3+, a white LED was fabricated using the optimized CLAN:0.3Ce3+ phosphor and a blue LED chip. As in Fig. 7, the white LED shows bright white emission driven at 3.0 V and 20 mA. The inset is a physical diagram of the device in the on and off state. The CIE chromaticity coordinates are (0.324, 0.382). The luminous efficiency is 54.3 lm/W. The color rendering index (Ra) is 67.7. These results indicate the potential application of CLAN:Ce3+ in white LEDs.
Fig. 7. Electroluminescence spectrum of the fabricated white LED based on CLAN:Ce3+ and a 450 nm chip. Inset shows the CIE chromaticity coordinate and photos of the fabricated white LED.
4. Conclusion
In this work, a novel yellow-green phosphor CLAN:Ce3+ was synthesized by a gas pressure sintering method. Compared to the reported HIP method, the purity of the CLAN product is greatly increased and the AlN impurity is depressed only 4.6 wt%. Therefore, the luminous efficiency is improved. CLAN:Ce3+ shows yellow-green emission peaked at 513 nm and 567 nm from the Ca1 and Ca3 sites. CLAN:Ce3+ shows a good luminescent thermal stability. A fabricated white LED based on the CLAN:Ce3+ phosphor displays a luminous of 54.3 lm/W, indicating potential applications. We claim that the CLAN:Ce3+ can be obtained by a simple and effective synthetic route, which is of great help to the research of the other activators in the CLAN host.
Funding
Science and Technology Major Project of Inner Mongolia (2019GG263); Natural Science Foundation of Inner Mongolia (2019MS05030); National Natural Science Foundation of China (12164034).
Acknowledgments
Many thanks for the financial supports from the National Natural Science Foundation of China (Grant No.12164034), the National Natural Science Foundation of Inner Mongolia (Grant No. 2019MS05030), and the Science and Technology Plan of the Inner Mongolia (Grant No. 2019GG263).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the corresponding authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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