Achieving High Quantum Efficiency Narrow-Band β-Sialon:Eu 2+

Dec 14, 2017 - β-Sialon:Eu2+ has been reported to be the most promising narrow-band green phosphor for wide color gamut LCD backlights, but the coexi...
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Article Cite This: Chem. Mater. 2018, 30, 494−505

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Achieving High Quantum Efficiency Narrow-Band β‑Sialon:Eu2+ Phosphors for High-Brightness LCD Backlights by Reducing the Eu3+ Luminescence Killer Shuxing Li,†,‡ Le Wang,*,§ Daiming Tang,∥ Yujin Cho,⊥ Xuejian Liu,# Xingtai Zhou,∇ Lu Lu,○ Lin Zhang,◆ Takashi Takeda,‡ Naoto Hirosaki,‡ and Rong-Jun Xie*,†,‡ †

College of Materials, Xiamen University, Xiamen 361005, China Sialon Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan § College of Optical and Electronic Technology, China Jiliang University, Hangzhou, Zhejiang 310018, China ∥ Thermal Energy Materials Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan ⊥ Nano Device Characterization Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan # The State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China ∇ Center for Thorium Molten Salts Reactor System, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China ○ Beijing Yuji-Xinguang Optoelectronic Technology Company Ltd, Beijing 101111, China ◆ Xi’an Hongyu Opto-Electrical Company Ltd, Xi’an, Shanxi 710199, China ‡

S Supporting Information *

ABSTRACT: β-Sialon:Eu2+ has been reported to be the most promising narrow-band green phosphor for wide color gamut LCD backlights, but the coexistence of the Eu3+ luminescence killer with the Eu 2+ luminescence center limits its luminescence performance to a great extent. In this study, we propose a direct reduction strategy to successfully realize the reduction of Eu3+ to Eu2+ and, finally, increase the effective concentration of Eu2+ in the crystal lattice and greatly minimize the amount of Eu3+ on the particle surface. As a result, the luminescence of treated β-sialon:Eu2+ is enhanced by 2.3 times, and the internal quantum efficiency significantly increases from 52.2 to 96.5%. The mechanisms for such large enhancements in luminescence are clarified by investigating the microstructure, luminescence spectra, valence state, concentration, and distribution of Eu using a variety of chemical analyses. We find that the low efficiency is ascribed to the coexistence of the Eu3+ luminescence killer with the Eu2+ luminescence center. The white LED backlight using the treated β-sialon:Eu2+ demonstrates a high luminous efficacy of 136 lm W−1 (22.5% up) and a wide color gamut (∼96% National Television System Committee standard (NTSC)), which thus promises high brightness and energy saving. We believe that the strategy proposed in this work would also work for other luminescent materials containing mixed valence of dopants. gap”, is much lower than that of red and blue ones.2,12,13 In contrast, phosphor-converted (pc) white LED backlights, which combine a single LED chip with phosphor materials, are extensively adopted due to their high efficiency and low cost. In pc-wLED backlights, the three key technical parameters, namely, color gamut, luminous efficacy, and reliability, are dominantly determined by phosphor materi-

1. INTRODUCTION Liquid crystal displays (LCDs) are ubiquitous in our information-rich world, ranging from smartphones, tablets, and computers to large-screen TVs and data projectors.1,2 White light-emitting diodes (LEDs) have been widely applied as backlight units in modern LCD technologies to achieve larger color gamut, higher brightness, and lower power consumption.3−11 Multichip white LEDs, which comprise red, green, and blue chips, exhibit an excellent color gamut. But, the separated driving circuits for each LED are costly. Moreover, the efficiency of the green LED, known as “green © 2017 American Chemical Society

Received: November 2, 2017 Revised: December 13, 2017 Published: December 14, 2017 494

DOI: 10.1021/acs.chemmater.7b04605 Chem. Mater. 2018, 30, 494−505

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Chemistry of Materials als.14−16 In general, phosphors for backlights are required to have a narrow emission band and a specific peak position, high quantum efficiency, and excellent thermal stability.12,13 To develop backlighting technology capable of displaying bright and vivid images, innovative phosphor materials with narrowband emissions in the green and red spectral regions are continuously pursed.9,14,17−21 Since human eyes are highly sensitive to green light, the development of efficient narrowband green phosphors seems to be particularly crucial to increase the maximum accessible color gamut and luminous efficacy.2,22 β-Sialon:Eu2+, which is derived from β-Si3N4 by partial substitution of Si by Al and N by O, has been well-known as a very interesting narrow-band green phosphor.23−26 In the crystal structure, the densely packed, corner-sharing (Si,Al)(N,O)4 tetrahedra form a three-dimensional network with a one-dimensional channel running along the c-axis.27 Investigations have been conducted to reveal the atomic site of Eu2+ in the β-sialon lattice.28−32 Li et al. first theoretically predicted that Eu2+ ion is interstitially situated in the channels parallel to the c-axis and directly connected to adjacent six N atoms.28 Then Kimoto et al. directly observed the interstitial Eu2+ in the large hexagonal channels with the scanning transmission electron microscopy (STEM) technique.29 Later, Brgoch et al. proposed that Eu2+ ion occupied a distorted 12-coordinate interstitial site in the hexagonal channels via extended X-ray absorption fine structure (EXAFS) measurements.30 Recently, Wang et al. suggested a highly symmetric EuN9 coordination polyhedron using first-principles calculations, and these special coordination environments were directly responsible for the electronic structure characteristic, i.e., a large splitting of >0.1 eV between the two highest Eu2+ 4f bands, which ensures the unique narrow-band emission.31,32 Cozzan et al. further corroborates changes in EuN9 polyhedra as a function of composition, and the polyhedral distortion leads to trends in the emission behavior that were predicted in calculations by Wang.33 To date, β-sialon:Eu2+ is accepted as the most promising green phosphor for wide color gamut wLED backlights.4,34,35 Xie et al. first developed a wide color gamut of 92% National Television System Committee standard (NTSC) by pumping β-sialon:Eu2+ (green) and CaAlSiN3:Eu2+ (red) phosphors with a blue LED chip.4 Wang et al. then achieved a wider color gamut of 94.2% NTSC by using βsialon:Eu2+ (green) and K2SiF6:Mn4+ (red) phosphors.34 Yoshimura et al. recently succeeded in fabricating a superwide color gamut of 106% NTSC with narrow-band sharp βsialon:Eu2+ (green) and K2SiF6:Mn4+ (red) phosphors.35 However, there is still a bottleneck to significantly improve the luminescence efficiency of β-sialon:Eu2+ because only quite a small amount of Eu2+ ions can be accommodated into the host lattice. The low dopant concentration thus leads to a smaller number of photons involved in the luminescence process. For example, Hirosaki et al. first reported the interesting β-sialon:Eu2+ phosphor with the Eu2+ doping concentration of only 0.296 mol %, and the internal and external quantum efficiencies under the 450 nm excitation are limited to be 50% and 33%, respectively.23 On the other hand, mixed Eu valences (Eu2+ and Eu3+) are inevitably present in the lattice of most luminescent materials, so that the concentration of Eu2+ contributing to the luminescence is further reduced, which is especially serious for the diluted βsialon:Eu2+ phosphor. This is another critical factor that suppresses the luminescent efficiency of β-sialon:Eu2+.

Although the valence of Eu ion is usually distinguished from the emission profile (i.e., Eu2+ gives a broadband emission attributed to the 4f65d → 4f7 transition whereas Eu3+ shows a series of sharp lines corresponding to the 5D0 → 7FJ (J = 0−4) transitions), it is not an effective way to examine the existence of Eu3+ as the luminescence of Eu3+ is usually quenched and hardly observed. Chen et al. indicated the coexistence of divalent and trivalent Eu ions in the Ca−α-sialon lattice by EELS examination.36 Moreover, the preferred Eu doping around the grain surface was observed in Ca−α-sialon, and an increase of Eu incorporation via the surface engineering strategy led to a maximum enhancement in emission intensity by 80%. Gan et al. revealed that both Ce3+ (1.01 Å CN = 6) and Ce4+ (0.87 Å CN = 6) exist in the β-sialon structure.37 According to the size effect,38,39 it is reasonable to speculate the coexistence of Eu2+ (1.17 Å CN = 6) and Eu3+ (0.95 Å CN = 6) in the channel of β-Sialon because the smaller Eu3+ can be more easily incorporated into the interstitial site. The decrease of effective luminescent centers (Eu2+) and the presence of luminescent killers (Eu3+) will definitely lead to the decrease of photoluminescence intensity.40 Therefore, in order to improve the quantum efficiency of β-sialon:Eu2+, it is of great importance to reduce the number of luminescence killers (Eu3+) and enhance the reduction of Eu3+ into Eu2+. There are basically two necessary conditions for realizing the reduction of Eu3+ to Eu2+ in a compound. First, the dopant trivalent Eu3+ ion replaces a divalent cation in the host. Second, the substituted cation has a radius similar to that of the divalent Eu2+ ion. Accordingly, the crystal-site engineering approaches, such as the site occupancy preference method and the enlargement of the substitution site strategy, have been proposed to promote the reduction of Eu3+.41−43 For instance, Li et al. transformed Eu3+ to Eu2+ by cosubstituting [Ca2+− P5+] for [La3+−Si4+] in Ca(2→8)La(8→2)(SiO4)6−x(PO4)xO2 (0 ≤ x ≤ 6) system according to the first rule.41 Huang et al. partially substituted Al3+−F− by Si4+−O2− in Ca12Al14O32F2:Eu3+ to enlarge the activator site that enables Eu3+ to be reduced.42 Similarly, Zhang et al. incorporated Si4+− Ca2+ into CaYAlO4 to replace Al4+−Y3+, which leads to the expansion of CaO9 polyhedron as well as the partial reduction of Eu3+ to Eu2+.43 Unfortunately, these strategies do not work in β-sialon:Eu2+ where Eu ions are located in the interstitial channels rather than in the substituted sites. Recently, several other specific approaches have been proposed to initiate the formation of Eu2+ from Eu3+ in inorganic luminescent materials, such as the postsynthesis reduction in a strong reducing atmosphere or by the high-energy radiation.44−47 For example, reactions between H2 and mixed-valent Eu2+/Eu3+ ions in glasses were shown to be an effective method to tailor the cooperative performance in a single-compound phosphor with well-controlled Eu3+ and Eu2+ ions.44−46 The exposure of Eu3+-doped Ga2O3 nanocrystals to X-ray radiation also leads to the reduction of Eu3+ to Eu2+, demonstrating an alternative way of manipulating the oxidation state.47 It should be noted that recent work by Cozzan et al. reports an annealing step in argon for 8 h during the preparation process of β-sialon:Eu2+ with a high photoluminescence quantum yield, but the exact mechanism is absent.33 In this contribution, we attempted to greatly improve the luminescence efficiency of β-sialon:Eu2+ by postannealing it in a N2−H2 reducing atmosphere to prompt the reduction of Eu3+ to Eu2+. This finally led to an increase of the luminescence intensity by 2.3 times and thus a high external 495

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Figure 1. (a) PL spectra, (b) photographs under 365 nm UV light and the summarized photoluminescence properties, (c) temperature-dependent PL intensity, and (d) temperature-dependent spectral emission of as-synthesized and thermally treated β-sialon:Eu2+ phosphors. were put into the boron nitride crucible again and fired in a horizontal tube furnace (GSL-1600X, Hefei Ke Jing Materials Technology Co. Ltd., China) at 1500 °C for 10 h under a high-purity N2−10% H2 mixture of 0.1 MPa. Then, they were gradually cooled to room temperature and finely ground for subsequent measurements and analyses. 2.2. Characterizations. Photoluminescence. Photoluminescence spectra were measured at room temperature using a fluorescent spectrophotometer (FL980, Edinburgh, England) equipped with visible photomultiplier tube (PMT) detector (Hamamatsu R928P). The amount of phosphor powder was controlled to be 1.0 g, and the data were collected in an integrating sphere. Diffusive Reflection Spectrum. The diffusive reflection spectrum was recorded using an UV−vis spectrophotometer with an integrating apparatus (V-560, Jasco, Tokyo, Japan). The Spectralon diffusive white standard (BaSO4) was used for calibration. The spectrum was measured between 250 and 800 nm with 1 nm step size. Quantum Efficiency. The luminescence spectra for QE measurements and the absorption efficiency were recorded using an intensified multichannel spectrometer (MCPD-7000, Otsuka Electronics, Japan). The Spectralon diffusive white standard (BaSO4) was used for calibration. Thermal Quenching. The temperature-dependent luminescence was measured by an intensified multichannel spectrometer (MCPD7000, Otsuka Electronics, Japan). The phosphor powders were heated to 300 °C in a 50 °C interval at a heating rate of 100 °C min−1 and held at each temperature for 5 min for thermal equilibrium. Decay Time. Fluorescence lifetime measurement was taken on a time-correlated single-photon-counting fluorometer (TemPro, Horiba Jobin-Yvon, Tokyo, Japan) equipped with a NanoLED-450 nm with a pulse duration full width at half-maximum of ∼1 ns. SEM-EDS Measurements. The surface morphology and the EDS measurements were performed using a high-resolution field emission

quantum efficacy of 71.3% (increased by 1.9 times) for the treated β-sialon:Eu2+. The mechanism behind the great enhancement was clarified and discussed in this work, which is closely related to the changes in the valence and distribution of Eu in phosphor particles. The luminescence properties and surface states of phosphors as well as the valence state of Eu were investigated in detail by means of a variety of chemical analysis techniques, such as photoluminescence (PL), cathodoluminescence (CL), electron probe microanalysis (EPMA), X-ray absorption near-edge structure (XANES), Xray photoelectron spectroscopy (XPS), and electron spin resonance (ESR). A high-efficiency (136 lm W−1) and wide color gamut (96% NTSC) white LED, prepared by combining β-sialon:Eu2+ (green) and K2SiF6:Mn4+ (red) phosphors with a blue LED chip, was also demonstrated in this study.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Appropriate amounts of α-Si3N4 (SNE10, UBE Ltd., Japan), AlN (F-grade, Tokuyama Chemical Co. Ltd., Japan), Al2O3 (TAIMICRON, Daimei Chemical Co. Ltd., Japan), and Eu2O3 (Shin-Etsu Chemical Co. Ltd., Japan) were weighed out and well-mixed according to the composition of Si6−zAlzOzN8−z:Eu2+ (z = 0.4, 0.4 wt % Eu). A 0.4 wt % amount of Eu was determined according to its optimized luminescence intensity, as shown in Supporting Information Figure S1. The powder mixture was then packed into a boron nitride crucible and fired in a gas pressure sintering furnace (FVPHR-R-10, FRET-40, Fuji Dempa Kogyo, Osaka) at 1900 °C for 2 h under high-purity nitrogen pressure of 1.0 MPa. After firing, the samples were gradually cooled to room temperature in the furnace. Finally, the sintered powders were finely crushed and ground for subsequent thermal treatment. The as-prepared phosphor powders 496

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Chemistry of Materials scanning electron microscope (SEM; S4800, Hitachi, Japan) at room temperature. Powder X-ray Diffraction. The crystalline phase of the phosphor powder was identified by XRD (Ultima III, Rigaku, Japan), using Cu Kα radiation at 40 kV and 200 mA. A step size of 0.01° was used with a scanning rate of 1° min−1 from 5° to 100°. The crystal parameters were calculated with the general structure analysis system (GSAS) package48 within EXPGUI interface.49 Cathodoluminescence. Cathodoluminescence measurements were performed using a field emission SEM (S4300, Hitachi, Japan) equipped with a CL system (Horiba MP32S/M, Horiba, Japan). The accelerating voltage was 5 kV, and the beam current was fixed at 100 pA. Transmission Electron Microscopy. Transmission electron microscopy and high-resolution TEM (HRTEM) analyses were attained using a JEM-3100FEF electron microscope fitted with field emission gun at an accelerating voltage of 300 kV (JEOL, Japan). Electron Probe Microanalysis. The cross-section processing was conducted by embedding phosphor powders in an epoxy resin (Gatan G2) and cutting them with an Ar-ion cross-section polisher (SM09010, JEOL, Japan) before the EPMA measurement. The point quantitative analysis was done using a field emission gun electron probe microanalysis (JXA-8530F, JEOL, Japan) at an accelerating voltage of 15 kV and a probe current of 0.2 nA. The composition distribution was analyzed using a field emission gun electron probe microanalysis at 10 kV. X-ray Absorption Near-Edge Structure. The X-ray absorption near-edge structure (XANES) spectra at Eu L3-edge were measured with the beamline of BL14W1 at Shanghai Synchrotron Radiation Facility (SSRF).50 The IFEFFIT-1.2.11 software package was used to analyze the data by standard methods. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy was carried out with a PHI Quantera SXM (ULVAC-PHI, Japan) using Al Kα monochromatic X-ray radiation at 20 kV and 5 mA. The energies were calibrated with C 1s peak as 285.0 eV. Electron Spin Resonance. Room-temperature electron spin resonance measurements were performed to detect the electrically active defects with an X-band spectrometer (JES-FA200, JEOL, Japan) using a cryostat (ES-CT470, JEOL, Japan). The absolute magnitudes of the g value were calibrated using a Mn2+ standard sample. Fabrication of White LEDs. Phosphor-converted wLED backlights were prepared by combining a blue GaInN LED chip with βsialon:Eu2+ (green) and K2SiF6:Mn4+ (red) phosphors. The color temperature and chromaticity coordinates were targeted by controlling the ratio of green to red phosphors. The optical properties of wLED backlights were measured by a high-accuracy array spectroradiometer (HAAS-2000, EVERFINE, China) under a forward-bias current of 20 mA at room temperature.

efficiency remains almost unchanged after thermal treatment. Albeit the same absorption at 450 nm, the internal quantum efficiency (IQE) is dramatically increased from 52.2 to 96.5% and the external quantum efficiency (EQE) is improved from 38.4 to 71.3%. The temperature-dependent PL intensity shows an improvement of 11% at 300 °C after thermal treatment, implying that the postannealed sample has a smaller thermal quenching than the as-synthesized one (see Figure 1c). The reasons for the luminescent enhancement would be discussed later. Moreover, a slight red shift and a broadening of the emission width are observed as the temperature increases from 30 to 300 °C (see Figure 1d and Figure S3). This was ascribed to a larger crystal field splitting of the Eu site caused by the distortion in the Eu−N/O9 polyhedron as a function of temperature.33 In order to get deep insights into the mechanism for such a great improvement, the surface states of phosphor particles were analyzed in detail. Figure 2 displays the surface

Figure 2. SEM images of as-synthesized (a and b) and thermally treated (c and d) rod-shaped β-sialon:Eu2+ phosphors.

morphology of both samples. The as-synthesized sample shows a typical rod-shaped morphology with very smooth surfaces. By contrast, the surface of thermally treated sample seems to be etched and molten, and some small particles are observed, which are initially assumed to be the corrosion products falling off from the β-sialon:Eu2+ particles during the treatment process. The XRD patterns of as-synthesized and thermally treated βsialon:Eu2+ phosphor powders are shown in Figure 3. No other impurities are observed other than the 15R AlN polytypoid for both samples, and the impurity content decreases a little after thermal treatment. That is to say, the observed small particles in the thermally treated sample is either β-sialon:Eu2+ or newly

3. RESULTS AND DISCUSSION The photoluminescence properties of β-sialon:Eu2+ phosphors before and after thermal treatment were first investigated. When excited at 450 nm, both samples show a very narrow emission band centered at 540 nm, which is attributed to the 4f65d1 → 4f7 characteristic transition of Eu2+, as shown in Figure 1a. The PL intensity of the thermally treated sample is more than 2.3 times higher than that of the as-synthesized one, indicating that the postannealing in a reducing atmosphere is an effective way to increase the luminescence of β-sialon:Eu2+. The emission band becomes slightly narrowed, with the full width at half-maximum (fwhm) being decreased from 57 to 54 nm. The body color of the thermally treated sample looks much brighter under the 365 nm UV irradiation, which is in line with the improved PL intensity (see Figure 1b). The diffuse reflection spectra of as-synthesized and thermally treated β-sialon:Eu2+ phosphors are provided in Figure S2. The two curves are similar to each other, and the absorption

Figure 3. XRD patterns of as-synthesized and thermally treated βsialon:Eu2+ phosphors. 497

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Chemistry of Materials formed amorphous impurity. Crystal parameters calculated by the XRD refinement tell us that the lattice volume (169.146(3) vs 168.965(4) Å3) shrinks slightly after thermal treatment, indicative of the variation in composition (i.e., lower z value). Cathodoluminescence was conducted to further examine the luminescence information on a microscale level, as shown in Figure 4. The CL mapping images in Figure 4c,d give the

Figure 5. TEM images (a and b), high-resolution TEM images of area 1 (c) and area 3 (d), and high-resolution TEM images of area 2 (e) and area 4 (f) for as-synthesized and thermally treated β-sialon:Eu2+ particles, respectively. The insets in panels c−e are the corresponding fast Fourier transform (FFT) patterns, and the inset in panel f is the selected area electron diffraction (SAED).

The electron probe microanalysis was conducted on the cross-sections of the particles to understand the elemental distribution within the phosphor particles. Consistent with the XRD analysis results, the as-synthesized phosphors consist of two phases, namely, the major β-sialon phase and the minor 15R phase (AlN polytypoid), as shown in Figure 6. The 15R phase is found to be rich in the Eu element because it contains a Eu layer in its crystal structure, and Eu ions can enter into its lattice easily (Figure 6c,d).51 As for the thermally treated sample, in addition to β-sialon and 15R, it also contains an oxygen-rich amorphous phase. The oxygen-rich amorphous particles are situated on the local surface areas of both β-sialon and 15R (Figure 6f). Chemical compositions of these phases were measured by the point quantitative analysis, and the result is summarized in Table 1. As can be seen, the composition of 15R remains almost unchanged after thermal treatment. However, the contents of Al and O in the thermally treated β-sialon:Eu2+ crystal decrease while that of N increases. This leads to a slight lattice shrinkage because the bond length of Si−N (1.74 Å) is shorter than that of Al−O (1.75 Å) and Al−N (1.87 Å),52 which is consistent with the XRD refinement result. The previous study addressed that the oxygen-less β-sialon:Eu2+ phosphor had a narrower bandwidth than the standard βsialon:Eu2+.53 This is the reason why the fwhm value becomes smaller after thermal treatment (57 → 54 nm). In the thermally treated sample, it is interesting to find that the amorphous phases on the local surfaces of both β-sialon and 15R particles are enriched in O and Si elements despite the low Si content in the 15R crystal. This means that the Si element is transported during the thermal treatment. Besides, the Eu content of the surface amorphous phase is reduced after thermal treatment. We further measured the Eu/O contents

Figure 4. SEM images (a and b), CL mapping images at 535 nm (c and d), and CL spectra of the selected points (e and f) for assynthesized and thermally treated β-sialon:Eu2+ particles, respectively.

luminescence intensity distribution of a whole particle, and the CL spectra in Figure 4e,f show the luminescence information on the selected points on particle surfaces. Obvious differences are seen between the as-synthesized and thermally treated samples. The particles selected from the as-synthesized sample give uniform but weak luminescence. In contrast, the thermally treated sample emits very strong luminescence, and the CL intensity of the selected points on the thermally treated sample is about 10 times higher than that of the as-synthesized sample. Interestingly, some nonluminescent areas appear on the particle surface, which are those small particles observed in Figure 2c. These “dead” areas are not considered as β-sialon particles due to their silent luminescence. The EDS results then confirm that the nonluminescence areas are oxygen-rich compounds, as shown in Figures S4 and S5. The TEM technique was adopted to examine the particle crystallinity as well as the nonluminescent area. The TEM image in Figure 5a shows that the surface of the as-synthesized phosphor particle is very smooth. The HRTEM images of areas 1 and 2 on the as-synthesized phosphor particle in Figure 5c,e indicate that the whole particle is well-crystallized, and an amorphous layer is coated on the surface. It is quite similar to the previous study.23 On the other hand, for the thermally treated phosphor particles, it is seen that area 3 is highly crystallized and free of any amorphous layers, whereas the whole area 4 is amorphous (see Figure 5d,f). The particle debris observed in the thermally treated sample is thus confirmed to be amorphous, and these amorphous particles are nonluminescent and oxygen-rich. 498

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Figure 6. EPMA analysis results. Backscattered electron images (a and b), Eu element distributions (c and d), O element distributions (e and f) for the cross-sections of as-synthesized and thermally treated phosphor particles, respectively. There are two kinds of phases: β-sialon and 15R AlN polytypoid in the as-synthesized sample while there are three kinds of phases: β-sialon, 15R AlN polytypoid, and oxygen-rich amorphous phase in thermally treated sample.

Table 1. Chemical Compositions of Phases Measured by EPMA for the As-Synthesized and Thermally Treated Samples

15R-AlN amorphousa β-sialon amorphousb

before after after before after after

Eu

Al

Si

N

O

total (at. %)

1.8 1.8 0.4 0.1 0.1 0

30.0 30.5 10.3 2.4 1.9 2.6

15.8 15.5 22.3 42.7 42.3 29.6

48.8 48.7 1.6 52.7 53.8 1.6

3.7 3.6 65.4 2.2 1.9 66.1

100.0 100.0 100.0 100.0 100.0 100.0

a

Amorphous phase on local surface of 15R-AlN. bAmorphous phase on local surface of β-sialon.

inside the phosphor particle by line scanning the cross-sections of as-synthesized and thermally treated β-sialon:Eu2+ particles, as shown in Figure 7. Interestingly, the average Eu content inside the crystal increases from 0.34 ± 0.054 to 0.44 ± 0.048 wt %, whereas the average O content decreases from 2.06 ± 0.205 to 1.68 ± 0.186 wt % in average after thermal treatment. This is consistent with Xie et al.’s work24 that lower z values generally result in a relatively higher solubility of Eu2+; i.e., the low concentration of Al and O in β-sialon enhances the solubility of Eu2+. Importantly, the Eu concentration in the particle is increased by 29% (0.34 → 0.44 wt %) during the thermal treatment process, which absolutely contributes to the luminescence enhancement. The luminescence of 15R and the surface oxidized particles that are marked as b and c in Figure 6 was measured with the CL technique, as given in Figure 8. The CL emission band of Eu-doped 15R is centered at 495 nm. Since the content of 15R is extremely low, its blue emission is therefore not identified in the overall PL emission (Figure 1a). As for the surface oxidized β-sialon:Eu2+ particle, its inner region gives a strong green emission of β-sialon:Eu2+ peaking at 535 nm while the

Figure 7. EPMA line-scanning analysis for cross-sections of assynthesized (a) and thermally treated (b) phosphor particles, and (c) measured Eu/O contents along the scanning line. The particles were selected from Figure 6 marked a and c, respectively.

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Figure 8. SEM images (a and b), CL mapping images at 495 nm (c and d), CL mapping images at 535 nm (e and f), CL spectra of selected points (g and h), for phosphor particles marked b and c in Figure 6, respectively.

amorphous surface shows no luminescence from Eu3+ despite its higher Eu content than the inner region. This implies that there exists an extremely low content of Eu3+ on the particle surface after thermal treatment, as would be verified by the following XPS and XANES results. Furthermore, the valence state of Eu was analyzed by X-ray absorption near-edge structure to determine the charge variation caused by thermal treatment. BaMgAl10O17:Eu2+ (BAM:Eu2+) and Eu2O3 were taken as references of two Eu valence states, namely, Eu2+ and Eu3+, respectively. The corresponding results are shown in Figure 9. Two peaks at

information only. The XPS survey scans taken on the surface of both samples are shown in Figure S6a. It is found that the intensity of Eu 3d spectrum in the as-synthesized sample is much higher than that of the thermally treated sample. The calculated Eu content is 1.8 and 0.3 at. % on the particle surface of the as-synthesized and thermally treated samples, respectively (see Table S1). This is consistent with the preceding EPMA analysis in which the surface amorphous area has a very low Eu content after thermal treatment. To check the inner crystal regions, the XPS measurement was also taken with the help of argon etching to remove the surface (see Figure S6b). Unfortunately, the high-resolution XPS spectra of Eu 3d and Eu 4d with argon etching (Figure 10c,d) for the assynthesized and thermally treated samples show that most of the Eu3+ and Eu2+ ions are reduced to be zero upon argon etching process. Nevertheless, the Eu contents in the inner regions are calculated to be 0.6 and 0.8 at. % for the assynthesized and thermally treated samples, respectively. It is interesting to find that in the as-synthesized sample the Eu content shows an obvious decrease from the surface toward the inner region of the grain. This uneven distribution was also found in Ca-α-sialon:Eu grains,36 implying that it is difficult for Eu ions to enter into the crystal grains. More interestingly, after thermal treatment, the Eu content on the surface decreases obviously while that in the inner region increases correspondingly. Again, it confirms that the thermal treatment indeed promotes Eu ions to diffuse into the inner grain, as verified by the EPMA line scanning. In addition to the content analysis, the Eu valence of the surface region was analyzed by using high-resolution XPS spectra of Eu 3d and Eu 4d. The Eu 3d spectra (Figure 10a) can be divided into two groups, with one belonging to the 3d3/2 structure and the other to 3d5/2, due to a large spin−orbit interaction of the Eu 3d core level. In the Eu 3d3/2 spectrum, the peak at the binding energy of 1164 eV corresponds to the Eu3+ ions while the binding energy peak at about 1155 eV is assigned to the Eu2+ ions. Similar phenomena are also observed in the Eu 3d5/2 spectrum. The peak at the binding energy of 1134 eV is originated from the Eu3+ ions and the side peak at

Figure 9. Normalized Eu L3-edge XANES spectra of as-synthesized, thermally treated, and reference samples.

about 6978 and 6985 eV are observed, which are attributed to the divalent and trivalent oxidation states of Eu ions, respectively.54,55 After thermal treatment, the relative intensity of Eu3+ absorption reduces significantly. The Eu2+ and Eu3+ are further calculated to be 79 ± 3% and 21 ± 3% for the assynthesized sample and 98 ± 3% and 2 ± 3% for the thermally treated sample, respectively, which confirm the effective reduction of Eu3+ to Eu2+. Since XANES results provide the Eu valence information on the whole particles, the X-ray photoelectron spectroscopy analysis was further investigated to obtain the surface valence 500

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Figure 10. High-resolution XPS spectra of Eu 3d and Eu 4d regions of as-synthesized (before) and thermally treated (after) samples without (a and b) and with (c and d) argon etching, respectively. Schematic distribution of Eu ions in β-sialon matrix and surface region for as-synthesized (e) and thermally treated (f) samples.

During the reduction process, hydrogen will first react with the surface oxide layer, and the released electrons from hydrogen would successfully complete the reduction process of Eu3+ to Eu2+ (reactions 1 and 2).63,64 As a result, the thermally treated sample has a sharp surface with extremely low content of Eu3+, as verified by TEM and XPS results. Then, O2− and Eu3+ in the lattice will diffuse into the surface region to enhance reactions 1 and 2. The ESR spectrum in Figure S8 indicates the existence of native oxygen vacancy in the as-synthesized sample. The diffusion of O2− in the lattice was supposed to be promoted by the migration of native VO••.65−68 On the other hand, the reduced Eu2+ and the generated N3− (reaction 3) diffuse the other way around from the surface to the lattice. The substitution of O2− by N3− results in a high N/O ratio and a reduced VO•• signal in the thermally treated β-sialon lattice (Figure S8). This is also consistent with the XPS results wherein the surface Eu decreased dramatically while the lattice Eu increased correspondingly after thermal treatment. The whole reduction process is initially surface-controlled and then shifts to be diffusion-controlled. To give a better understanding of the whole process, a schematic mechanism was depicted in Figure 11.

1124 eV is attributed to the Eu2+ ions.56 Figure 10b shows the XPS spectrum of Eu 4d electrons. The two peaks around 142 and 135 eV are labeled as Eu3+, due to the multiplet structure of trivalent 4d4f6 configurations. The other small peaks around 127 and 124 eV, labeled as Eu2+, are due to divalent 4d4f7 configurations.57,58 For the as-synthesized sample, Eu3+ ions are dominate, and their absolute amount is large as a very high Eu content (1.8 at. %) is detected on the surface. In contrast, although in the surface region there is a low Eu content (0.3 at. %) after thermal treatment, it is still easy to find that the relative intensity of Eu2+ is much stronger than that of Eu3+, indicative of the extremely low content of Eu3+. The significant decrease of Eu3+ efficiently prevents the energy transfer from Eu2+ to Eu3+ and thus reduces the possibility of nonradiative relaxation. It can be confirmed by both the enhanced IQE (52.2 → 96.5%) and the prolonged decay time (830 → 954 ns; see Figure S7). Moreover, the improved thermal stability (11% at 300 °C) also corroborates the decreased nonradiative relaxation. Many studies have shown that PL properties would be enhanced by modifying the surface state to reduce the luminescence quenching.59−61 Here, the dramatic reduction of Eu3+ ions on the surface finally contributes to the substantial enhancement in PL properties. Based on the above analyses, the distribution of Eu ions in both the β-sialon matrix and the surface region before and after thermal treatment is schematically depicted in Figure 10e,f. By conducting the analyses mentioned above, the reduction of Eu3+ to Eu2+ ions and the increase of the Eu2+ content in the phosphor grain can be concluded. On the other hand, the amorphous particles on local surfaces after thermal treatment are not welcome, as will be discussed in the following. Similar phenomenon was previously observed for Sr2Si5N8:Eu2+ phosphor; i.e., a passivation SrSiO3 layer was formed at the phosphor surface to help prevent the thermal degradation when thermally treated in a N2−H2 mixed atmosphere.62

O2 −(surface) + H 2(g) F H 2O(g) + 2e

(1)

Eu 3 +[Xe] 4f 5 (surface) + e F Eu 2 +[Xe] 4f 6

(2)

N2(g) + 3H 2(g) + 3O2 − F N3 − + 3H 2O(g)

(3)

Next discussed in detail is the abnormal local oxidation process. The presence of gasous H2O in H2 gas was found to cause an active oxidation in the Si3N4, SiC ,and sialon ceramics at high temperature of ∼1400 °C.69−71 Similarly, in our study it is the generated H2O during the reduction process (reactions 1 and 3) and the H2O carried out by the mixture gas that causes the reoxidation of local surfaces on β-sialon and 15R501

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K2SiF6:Mn4+ red phosphor with a blue LED chip.34 A cool white light (high color temperature of ∼8000−12000 K) is usually required in the LCD backlight to ensure high brightness. The representative electroluminescence spectra and the corresponding CIE 1931 color coordinates are presented in Figure 12. Importantly, the white LED fabricated by using the thermally treated sample as the green component has a luminous efficacy of 136 lm W−1. The luminous efficacy is increased by 22.5% when compared to that by using the assynthesized sample (110 lm W−1). Moreover, the white LED also has a higher luminous efficacy than that using Sr2GaS4:Eu2+ (green) and K2SiF6:Mn4+ (red) at a comparable CCT of 8330 K (105 lm W−1).8 The calculated color gamut defined in the CIE 1931 color space is ∼96% of NTSC, enabling one to obtain vivid pictures. These results demonstrate that the postannealed β-sialon:Eu2+ can be used to fabricate high brightness and large color gamut white LED backlights, which promises energy saving and brilliant displays.

Figure 11. Schematic mechanism that accounts for the reduction and oxidation process.

AlN particles. The corresponding chemical reactions are shown in reactions 4−6). Si5.6Al 0.4O0.4 N7.6(s) + 11.4H 2O(g) F 5.6SiO(g) + 0.2Al 2O3(s) + 3.8N2(g) + 11.4H 2(g)

SiO(g) + H 2O(g) F SiO2 (s) + H 2(g)

(4)

4. CONCLUSIONS

(5)

A direct reduction strategy (i.e., postannealing in a reducing N2/H2 atmosphere at high temperature) was successfully used to greatly enhance the photoluminescence properties of the technically important β-sialon:Eu2+ green phosphor. The luminescence intensity of β-sialon:Eu2+ was increased by more than 2.3 times, and the external quantum efficiency was increased from 38.4 to 71.3% associated with the postannealing. The mechanism for such a great luminescence enhancement was attributed to (i) the reduction of Eu3+ to Eu2+, (ii) the enhanced incorporation of Eu2+ into the crystal lattice, and (iii) very diluted concentration of Eu3+ on the surface region. By using the postannealed β-sialon:Eu2+, the white LED backlight showed an increase of 22.5% in luminous efficacy with a high value of 136 lm W−1. This strategy is believed to be effective not only for β-sialon:Eu2+ phosphors but also for other luminescent materials containing mixed valence of dopants (e.g., Ce3+/Ce4+; Yb2+/Yb3+).

2AlN(s) + 3H 2O(g) F Al 2O3(s) + N2(g) + 3H 2(g) (6)

The gaseous reaction product, SiO, will be formed at the very beginning (reaction 4), and later SiO will be oxidized to solid SiO2 as the partial pressure of H2O becomes greater (reaction 5). This is the reason why Si elements are transported after thermal treatment, as indicated by EPMA measurements. Likewise, amorphous Al2O3 particles are also formed. Since the annealing temperature is higher than the eutectic point of Al2O3 and SiO2 (∼1260 °C),72 an eutectic amorphous mixture is formed on the particle surface, and the local surface area looks molten, as indicated by SEM images. White LED backlights with three different correlated color temperatures (CCTs) of 8379, 10412, and 11770 K were fabricated by combining β-sialon:Eu 2+ with chromatic coordinates located at (0.3479, 0.6284) and the commercial

Figure 12. (a) Electroluminescence spectra and (b) CIE 1931 color coordinates of as-fabricated white LED backlights with different color temperatures of 8379, 10412, and 11770 K, and color space of NTSC standard (white dotted line) and white LED device (yellow dotted line); (c) Luminous efficacies of white LED backlights fabricated with as-synthesized and thermally treated β-sialon:Eu2+ phosphors. The insets in panel b are the photographs of as-fabricated and lightened white LEDs with color temperature of 8379 K. 502

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04605.

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Emission intensity; diffuse reflection spectra; temperature-dependent emission spectra; EDS mappings; XPS survey scans; decay curves; ESR spectra; and quantitative analysis results by XPS (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.-J.X). *E-mail: [email protected] (L.W.). ORCID

Shuxing Li: 0000-0001-8086-7154 Rong-Jun Xie: 0000-0002-8387-1316 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (Grant Nos. 5157223, 51561135015, and 51472087), the National Key Research and Development Program (MOST; Grant No. 2017YFB0404301), the National Postdoctoral Program for Innovative Talents (Grant No. BX201700138), and JSPS KAKENHI (Grant No. 15K06448). Furthermore, we also give special thanks to the Shanghai Synchrotron Radiation Facility (SSRF) and Prof. Zheng Jiang and Mr. Ruo-Ou Yang for providing the XANES measurements.

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REFERENCES

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