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Facile Atmospheric Pressure Synthesis of High Thermal Stability and Narrow-Band Red-Emitting SrLiAl3N4:Eu2+ Phosphor for High Color Rendering Index White Light-Emitting Diodes Xuejie Zhang, Yi-Ting Tsai, Shin-Mou Wu, Yin-Chih Lin, JyhFu Lee, Hwo-Shuenn Sheu, Bing-Ming Cheng, and Ru-Shi Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05485 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 15, 2016

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ACS Applied Materials & Interfaces

Facile Atmospheric Pressure Synthesis of High Thermal Stability and Narrow-Band Red-Emitting SrLiAl3N4:Eu2+ Phosphor for High Color Rendering Index White LightEmitting Diodes

Xuejie Zhang,† Yi-Ting Tsai,† Shin-Mou Wu,‡ Yin-Chih, Lin,‡ Jyh-Fu Lee,§ Hwo-Shuenn Sheu,§ Bing-Ming Cheng,§ and Ru-Shi Liu*,†,£

†

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan

‡

National Chung-Shan Institute Science & Technology, Chemical Systems Research Division,

Taoyuan 325, Taiwan §

National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan

£

Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology,

National Taipei University of Technology, Taipei 106, Taiwan

ACS Paragon Plus Environment

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ABSTRACT

Red phosphors (e.g., SrLiAl3N4:Eu2+) with high thermal stability and narrow-band properties are urgently explored to meet the next-generation high-power white light-emitting diodes (LEDs). However, to date, synthesis of such phosphors remains an arduous task. Herein, we report, for the first time, a facile method to synthesis SrLiAl3N4:Eu2+ through Sr3N2, Li3N, Al and EuN under atmospheric pressure. The as-synthesized narrow-band red-emitting phosphor exhibits excellent thermal stability, including small chromaticity shift and low thermal quenching. Intriguingly, the title phosphor shows an anomalous increase in theoretical lumen equivalent with the increase of temperature as a result of blue shift and band broadening of the emission band, which is crucial for high-power white LEDs. Utilizing the title phosphor, commercial YAG:Ce3+, and InGaN-based blue LED chip, a proof-of-concept warm white LEDs with a color rendering index (CRI) of 91.1 and R9 = 68 is achieved. Therefore, our results highlight that this method, which is based on atmospheric pressure synthesis, may open a new means to explore narrow-band-emitting nitride phosphor. In addition, the underlying requirements to design Eu2+doped narrow-band-emitting phosphors was also summarized.

KEYWORDS: narrow-band-emitting, high-CRI, white LEDs, red nitride phosphor, high thermal stability


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1. INTRODUCTION

Currently, producing an efficient illumination-grade white light with a suitable correlated color temperature (CCT: 2700–4000 K), a conspicuous color rendering index (CRI > 85) with optimized luminous efficacy (LE) remains an outstanding challenge in the field of lighting.1–3 In today’s omnipresent light sources, most commercially available phosphor-converted white lightemitting diodes (LEDs) are based on a broadband YAG:Ce3+ yellow phosphor and an InGaNbased blue-emitting LED chip. This type of white LEDs can provide a high conversion efficiency, close to the theoretical maximum. However, the absence of red light in the emission spectral region of the light source resulted in a cool-white (high CCT) with a low CRI. To attain high-quality white light (CRI > 85), nitride phosphors, such as (Ba,Sr)2Si5N8:Eu2+ or (Ca,Sr)SiAlN3:Eu2+, were employed in commercial high-CRI white LEDs. However, LE is known to be achieved at the expense of CRI, and a better balance must be obtained between the two parameters. Unfortunately, these red phosphors exhibit a broad emission band, and a significant part of light beyond 650 nm is located outside the sensitivity curve of the human eye, causing great loss in LE.4–9 To further improve the LE while maintaining CRI, a narrow-band (FWHM ≤ 60 nm) red-emitting phosphor with appropriate peak position (~630 nm) is urgently needed.

Recently, Schnick and colleagues reported several narrow-band red-emitting phosphors, with representative phosphors including SrLiAl3N4:Eu2+,1 CaLiAl3N4:Eu2+,10 SrMg2Al2N4:Eu2+,11


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SrMg3AlN4:Eu2+,12 Ca18.75Li10.5Al39N55:Eu2+.13 Some of these phosphors exhibit poor luminescence efficiency, whereas, some are exceedingly red. Meanwhile, several of these phosphors show a disorder structure and more than one crystallographic site. To date, a narrowband red-emitting phosphor SrLiAl3N4:Eu2+, with an emission peak position at ~650 nm and a FWHM of ~50 nm, was regarded as the most promising red phosphor for use in next-generation lighting because of the material’s excellent luminescent properties of low thermal quenching and high external quantum efficiency (QE). SrLiAl3N4 possesses an extraordinary structure (Figure 1b) as follows: (1) highly rigid framework of edge- and corner-sharing AlN4 and LiN4 tetrahedral with a degree of condensation k (i.e., atomic ratio of (Li,Al):N) in this compound up to unity; (2) ordered distribution of LiN4 and AlN4 tetrahedral; and (3) cuboid-like SrN8 and the activator Eu2+ (1.25 Å) with almost the same size as Sr2+ (1.26 Å).1,14 All these structural characterizations contribute to the high performance of the SrLiAl3N4:Eu2+ red phosphor. Therefore, SrLiAl3N4:Eu2+ may be commercialized in the future for the fabrication of next-generation highCRI white LEDs.

The reported synthesis method of SrLiAl3N4:Eu2+ in literature1 is complex and the raw materials are not readily available, hence, limits the further study and actual application of the red phosphor. In this paper, we report a facile synthesis method using Sr3N2, Li3N, metal powder of Al, and nitride EuN as the raw materials under atmospheric pressure. The structure, morphology, luminescent properties including vacuum ultraviolet (VUV) spectrum, lifetimes, the


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valence state of Eu, thermal quenching behavior, and chromaticity stability of the title SrLiAl3N4:Eu2+ phosphor are studied in detail. A warm white LEDs with CRI of 91.1 (R9 = 68) and CCT = 2875 K was fabricated by employing red phosphor SrLiAl3N4:Eu2+, commercial yellow phosphor YAG:Ce3+, and blue LED chip.

2. EXPERIMENTAL SECTION

2.1. SAMPLE PREPARATION

The narrow-band red-emitting phosphor SrLiAl3N4:Eu2+ with 2.0 mol% Eu activator was made using solid-state reaction methods. The raw materials were Sr3N2 (Materion, 99.5%), Li3N (Alfa Assad, 99.9%), Al (Showa, 99.9%), and EuN (Cerac, 99.9%). The elemental precursors were weighed and ground using an agate mortar and pestle in an Argon-filled glove box (H2O < 1 ppm, O2 < 1 ppm). Then, the mixture was transferred into molybdenum crucibles, and sintered at 1000 °C for 4 h under a 90%N2-10%H2 gas mixture in a tube furnace. The product was slowly cooled to room temperature inside the tube furnace under N2-H2 flow, and was then ground to powder for further characterization. Commercial CaAlSiN3:Eu2+ red-emitting phosphor was used as a reference to compare chromaticity stability as a function of temperature.

2.2 CHARACTERIZATION


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Sample structure information was measured by Synchrotron X-ray diffraction pattern (SXRD) (λ = 0.4959 Å) using the BL01C2 beamline at the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The crystal structure was refined by TOPAS 4.2 software. The powder x-ray diffraction (XRD) pattern was also studied by Bruker D2 with Cu Kα radiation (λ = 1.5405 Å) at 30 kV and 30 mA. Optical properties of the excitation and emission spectra were obtained by Edinburgh FSP920 combined fluorescence lifetime and steady-state spectrometer with a 450 W xenon lamp and thermo-electric cooled red sensitive PMT. The luminescence decay curves were measured by Edinburgh FSP920 using a 150 W nF900 ns ﬂash lamp with a pulse width of 1 ns and pulse repetition rate of 40 kHz as the excitation source. The temperature-dependent emission spectra were recorded on a FluoroMax-3 spectrophotometer set up with a 150 W Xe lamp and Hamamatsu R928 photo-multiplier tube using a THMS-600 heating equipment. The vacuum ultraviolet (VUV) photoluminescence emission (PL) and excitation (PLE) spectra was obtained using the BL03A beamline at the NSRRC, Taiwan. The PLE spectra were recorded by scanning a 6 m cylindrical grating monochromator with a grating of 450 grooves/mm, which spanned the wavelength range of 100-350 nm. A LiF plate served as a filter to remove the high-order light from the synchrotron. The emission from the phosphor was analyzed with a 0.32 m monochromator and was detected with a photomultiplier (Hamamatsu R943-02) in a photoncounting mode. For measurements of low temperature spectra, the sample holder was attached to a cryo-head of a helium closed-cycle cryostat system (APD HC-DE204S); which was mounted on a rotatable flange so that the sample could be rotated to about 45 degree with respect to both


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the incident VUV source and the entrance slit of the dispersed monochromator. The temperature control unit provided the cold head of the cryostat to better than ±1 K during the data collection period. The L3-edge of Eu was observed by X-ray absorption near-edge structure on transmission mode at BL17C beamline (NSRRC, Taiwan). Incident beam intensity (I0) was measured by gasionization chambers filled with He and N2 gases, whereas transmitted beam intensity (It) was measured using Ar and N2 gases. The particle size and morphology of the sample was measured using scaning electron microscopy (SEM, Nova NanoSEM 450, FEI, Oregon, USA), and the elemental composition was determined using an erergy-dispersive X-ray spectroscopy (EDX) was attached to the SEM. The electroluminescence was recorded using a spectrophotometer with integrated spheres (EVERFINE, PMS-80) at 20 mA forward current. 27Al solid-state MAS-NMR and 7Li solid-state MAS-NMR spectra were acquired on a wide-bore 14.1-Tesla Bruker Avance III NMR spectrometer, equipped with a 1.9 mm and 3.2 double-resonance magic-angle-spinning probehead for Al and Li, respectively. The Larmor frequency for 27Al was 156.40 MHz and 7Li was 233.3 MHz. The samples were spun at 35 kHz. A selective π/4 pulse of 0.8 µs for Al and π/6 pulse of 1.7 µs for Li were used for central-transition excitation. The recycle delays were 1 s for Al and 2 s for Li. The chemical shift was referenced to 1 M Al(NO3)3 aqueous solution.

3. RESULTS AND DISCUSSION

The powder X-ray diffraction (XRD) pattern of SrLiAl3N4:Eu2+ is shown in Figure S1. Such pattern is highly consistent with the reported pattern of SrLiAl3N4 except for some weak


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diffraction peaks from the second-phase AlN. To confirm the structure and evaluate the content of impurity, we performed Rietveld structure refinement for SrLiAl3N4:Eu2+ using the powder synchrotron X-ray diffraction (SXRD) data through the TOPAS 4.2 software. The initial parameters were obtained from the single crystal SrLiAl3N4 reported by Schnick’s group.1 The experimental, calculated, and difference Rietveld refinement XRD patterns at room temperature are shown in Figure 1a and the main parameters from the processing and refinement of the data are presented in Table S1. Almost all the diffraction peaks can be indexed to the corresponding data and gave a goodness of fit parameters of Rwp = 3.65%, Rp = 2.36%, and x2 = 1.08. The second-phase AlN was quantified to be ~5.48 wt% from Rietveld refinement.


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Figure 1. (a) Rietveld refinement of powder SXRD pattern of SrLiAl3N4:Eu2+. Observed (crosses) and calculated (red line) SXRD patterns. Blue and green vertical lines indicate positions of the Bragg reflections of SrLiAl3N4 and AlN, respectively. Grey line represents the difference plot (observed−calculated) on the same scale. The inset shows the crystal structure of 1 × 1 × 1 unit cells of SrLiAl3N4. (b) Structural overview of SrLiAl3N4. The Yellow spheres denote Sr; the blue spheres represent N; the blue tetrahedrals signify AlN4; the orange tetrahedrals are LiN4; the pink polyhedral refers to cuboid-like SrN8 coordinated by the AlN4 and LiN4 tetrahedral.

The as-synthesized phosphor shows a pink colored powder in Figure 2a, the powder gave an intense red light under 254 nm lamp irradiation. The excitation and emission spectra of SrLiAl3N4:Eu2+ at 298 K are depicted in Figure 2c. SrLiAl3N4:Eu2+ shows a peak at 650 nm with a FWHM of 55 nm (1279 cm-1) under 460 nm excitation. The CIE chromaticity coordinates of SrLiAl3N4:Eu2+ was determined to be (0.705, 0.295), which was close to the chromaticity coordinates (0.713, 0.286) of 650 nm pure color. To compare with the commercial red phosphor, CaAlSiN3:Eu2+ (650 nm; (0.665, 0.334)) was chosen as a reference. Using the equation of reference (equation from Supporting Information),15 the color purities of SrLiAl3N4:Eu2+ and CaAlSiN3:Eu2+ were determined to be 98% and 87%. The result indicates that SrLiAl3N4:Eu2+ exhibits a higher color purity compared with commercial red phosphor. The excitation spectrum


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(λem = 650 nm) includes two parts: the former is the VUV spectral range and the latter is ultraviolet-blue-green-red spectral range. According to diffusion reflection spectrum of SrLiAl3N4,1 the high-energy absorption bands (below 375 nm) are mainly ascribed to the absorption of the host lattice and the low-energy absorption band was assigned to the allowed electronic transition from 4f7 (8S7/2) to 4f6(7FJ)5d1 (J = 0–6) within Eu2+. The robust absorption band from 400 to 600 nm indicates that this phosphor can be effectively excited by blue LED. The crystalline morphology of SrLiAl3N4:Eu2+ was checked by scanning electron microscopy (SEM) and is shown in Figure 2e. The observed particle size was below 5 µm. The atomic ratios Sr:Al:N of 1:3.08(2):4.45(6) determined by energy-dispersive X-ray (EDX) are in good accordance with the formula SrLiAl3N4:Eu2+. However, a slight overestimation of nitrogen was observed, and Eu was not detected under SEM/EDX as a result of lower dopant content. To further verify the incorporation of Li, we performed 7Li solid state magic angle spinning nuclear magnetic resonance (MAS-NMR) of SrLiAl3N4:Eu2+ (Figure 2b). One sharp isotropic signal with chemical shift value of δ = 2.55 ppm was observed in Figure 2b, similar to that reported in the reference (δ = 2.60).1 Such data also confirmed the existence of Li in the SrLiAl3N4:Eu2+ phosphor. We also could observed two splitting peaks located at 2.55 and -0.10 ppm in the magnifying NMR spectrum (Figure S2), these peaks were ascribed to two different Li sites in the SrLiAl3N4 structure. The

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Al solid-state NMR spectrum was also measured and presented in

Figure S3. Three peaks centered at 114, 100, and 75 ppm were noted: the former two peaks were attributed to the group of AlN4, and the latter was ascribed to the cluster of AlN4–xOx.16 Combing


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the weak peak of AlO6–xNx at 8 ppm,16 one can conclude the presence of low oxygen content in the SrLiAl3N4:Eu2+ phosphor, indicating that SrLiAl3N4:Eu2+ was not stable in air. To determine the valence of the doped Eu, X-ray absorption near-edge structure (XANES) was carried out (Figure 2d). By referencing the standard BaMgAl10O17:Eu2+ (BAM:Eu2+) and Eu2O3, we can observe the coexistence of Eu2+ and Eu3+, with the content of Eu3+ higher than that of Eu2+. To check the absence of Eu3+, we chose 230, 393, and 464 nm as the characteristic excitation wavelengths of Eu3+ to excite this phosphor. However, we failed to observe the sharp red emission of the Eu3+ f-f transition in the emission spectra. This result may be due to the possibility that the electron located at the excited state of Eu3+ underwent a nonradiative transition to ground state because of lattice relaxation diminished the energy of charge transfer state below the energy of 5D0 state regardless of whether higher or lower energy was chosen as the excitation wavelength. The low-temperature photoluminescence spectrum of SrLiAl3N4:Eu2+ is shown in Figure 2f. The zero-phonon line peaks at 15792 cm-1 (633 nm) was observed under 230 nm excitation, which is close to the value of 15797 cm-1 under 470 nm excitation reported by Schnick’s group.1 We further determined the Stokes shift (SS) is 816 cm-1 by taking the SS as twice the energy difference between the zero-phonon line and the maximum of the emission band. The origin of this small SS was the location of Eu2+ in the ordered and rigid network structure of (LiAl3N4)2-. This structure limits geometrical relaxations and finally reduces the interaction between electron and phonon. A weak electron–phonon coupling effect is beneficial in reducing SS and obtaining narrow band emission. The average energy of the local and


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effective phonon modes was ~202 cm-1,1 according to the equation of SS = (2S-1)ћw, the HuangRhys factor S is calculated to be ~2.52, which is lower than that of YAG:Ce3+ (S = 6).17,18 The decay curves of SrLiAl3N4:Eu2+ (Figure S4) are different each other when monitoring different emission wavelengths (630, 650, and 670 nm) under 450 nm excitation. The calculated different lifetimes indicated that the emissions arise from Eu2+ in different lattice sites. The close lifetime also demonstrated that the sites are quite similar in the lattice, further confirming the result of SrLiAl3N4 has two similar Sr sites (Figure 1b).

Figure 2. (a) Photographs of SrLiAl3N4:Eu2+ phosphor under fluorescent lamp (top) and 254 nm UV lamp (down). (b) 7Li solid-state MAS-NMR spectrum of SrLiAl3N4:Eu2+. (c) Excitation and emission spectra of SrLiAl3N4:Eu2+ at 298 K. (d) Eu L3 XANES spectra of SrLiAl3N4:Eu2+, standard BAM:Eu2+, and standard Eu2O3. (e) SEM image of SrLiAl3N4:Eu2+ bulk phosphor.


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Scale bar, 5 µm. (f) Photoluminescence spectrum of SrLiAl3N4:Eu2+ under 230 nm excitation at 10 K.

For general lighting application, high-power blue LEDs (blue light output ＞ 100 W cm-2) are necessary. However, such a high-power inevitably results in the high junction-temperature of the chip, which finally leads to the decline in emission intensity and the shift in the maximum emission wavelength of the phosphor. Therefore, systematically evaluating the thermal quenching behavior and chromaticity stability of the as-synthesized red phosphor SrLiAl3N4:Eu2+ is an important task. First, we focused on the temperature-dependent emission intensity. The relative integrated intensity of SrLiAl3N4:Eu2+ dropped from an initial 100% at 298 K to 93% at 473 K and 88% at 573 K, respectively (Figure 3b). While at 473 and 573 K, the intensities of the peak wavelength (650 nm) were 81% and 71%, respectively. The origin of difference between the integrated intensity and the intensity of 650 nm is the blue-shift in the maximum emission wavelength and the broadening of the emission band with the increase in temperature. In fact, in 298–573 K, the peak wavelength shifted from 654 to 646 nm, meanwhile, the spectrum broadened from 62 to 75 nm. This change can also be observed in the temperature dependent spectral emission for SrLiAl3N4:Eu2+ (Figure 3a). Notably, the peak wavelength and FWHM of temperature-dependent emissions slightly differed from the emission observed at room temperature; the difference is attributed to the measurement of two emissions on different


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spectrofluorometers, with a slight discrepancy in the spectral response. For this part of the present work, we focused on the trend of the temperature-dependent emission behavior temporarily regard the difference as unimportant. Furthermore, even tiny variations in emission peak position and width will result in an obvious change in the spectral theoretical lumen equivalent because the upper limit sensitivity curve of the human eye exhibits a steep grade in the saturated red spectral range. The temperature-dependent emission spectra overlapped with the human eye response curve (Figure 3d). According to the modified equation of the reference (equation shown in Supporting Information),19 we further calculated the theoretical luminous efficiency (Figure3e). Interestingly, the lumen equivalent monotonously increased with increasing temperature. The broadened width and blue-shift of emission are beneficial in increasing the theoretical luminous efficiency because emissions closer to green light at 555 nm result in higher lumen equivalents. All the above-mentioned results indicate that SrLiAl3N4:Eu2+ has a low thermal quenching behavior. To further assess the chromaticity stability of SrLiAl3N4:Eu2+, we present the change in chromaticity coordinates in Figure 3c. The chromaticity coordinates shifted to the short wavelength when the temperature changed from room temperature to 373 to 473 and finally 573 K. This trend is in agreement with the results observed in Figure 3a. To further quantitatively describe the variations, we calculated the CIE shift of SrLiAl3N4:Eu2+ and commercial CaAlSiN3:Eu2+ red phosphor (Figure 3f).20–22 Results indicate that the SrLiAl3N4:Eu2+ has a better chromaticity stability than that of commercial red phosphor.


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Figure 3. (a) Temperature-dependent spectral emission for SrLiAl3N4:Eu2+. (b) Temperature dependence of the relative integrated and peak intensity for SrLiAl3N4:Eu2+ (λex = 460 nm). (c) Temperature-dependent chromaticity coordinate variation for SrLiAl3N4:Eu2+. (d) Temperaturedependent emission spectra. The left lines denote the human eye response curve. (e) Peak wavelength, FWHM, and theoretical luminous efficiency varies as a result of increasing temperature. (f) Chromaticity shift of SrLiAl3N4:Eu2+ and commercial CaAlSiN3:Eu2+ as a function of temperature.

To demonstrate the potential application of the title phosphor, we fabricated the white LEDs using the “red phosphor (SrLiAl3N4:Eu2+) + yellow phosphor (YAG:Ce3+) + blue LED chip” method. Figures 4a and b show the electroluminescence spectra of the as-fabricated white LEDs (1, 2). Both spectra included three bands peaking at 450, 550, and 650 nm, ascribed to the blue


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LED chip, commercial YAG:Ce3+ phosphor, and the title phosphor, respectively. By employing SrLiAl3N4:Eu2+ phosphor, we easily fabricated high-CRI (> 85) white LEDs (Figure 4a). By appropriately adjusting the ratio of yellow phosphor to red phosphor, a warm white light (CCT = 2875 K) with high CRI (Ra = 91.1) was achieved. Furthermore, the R9 index was as high as 68 indicating that the red spectral part was enhanced obviously. As seen in the inset of Figure 4c, the white LEDs (2) emitted warm white light. In addition, the LE was 48.8 and 24.9 lm/W for high-CCT white LEDs and low-CCT white LEDs, respectively. These results validate the SrLiAl3N4:Eu2+ red phosphor as a promising prospect for use in the fabrication of high-power white LEDs, especially for high-CRI white LEDs.

Figure 4. Electroluminescence spectra of (a) high-CCT white LEDs (1) and (b) low-CCT white LEDs (2) fabricated using SrLiAl3N4:Eu2+ and YAG:Ce3+; (c) Chromaticity coordinates of the two white LEDs (1, 2); The inset of c shows the photographs of the two white LEDs (1, 2).


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4. CONCLUSION

In this paper, we reported the synthesis of a highly stressed narrow-band red-emitting nitride phosphor, SrLiAl3N4:Eu2+, using Sr3N2, Li3N, Al, and EuN under atmospheric pressure for the first time. This phosphor emitted a red light that peaked at 650 nm with a narrow FWHM of 55 nm under 460 nm excitation. The high thermal stability of the title phosphor, including the thermal quenching behavior and chromaticity stability, was studied in detail. By employing commercial YAG:Ce3+ yellow phosphor, SrLiAl3N4:Eu2+ red phosphor, and blue LED, we successfully fabricated a warm white LEDs with CRI of 91.1 and CCT of 2875 K. All of the results suggested that this narrow-band red-emitting phosphor is a promising candidate for highpower and high-CRI white LEDs. Furthermore, this facile synthesis method is expected to facilitate further investigation of SrLiAl3N4:Eu2+ towards its actual application and exploring other narrow-band-emitting phosphor. In addition, the analysis results also help reveal the underlying requirements to design Eu2+-doped narrow-band emitting phosphors: first, a rigid and ordering network structure is very important to decrease nonradiative relaxation; second, the replaced cation should have only one crystallographic site and almost the same size as the activator Eu2+ to reduce emission overlap and the distortion of coordination polyhedron; third, a high symmetry around the activator site with high coordination number, producing the same distance between the activator and ligand, is favorable for astricting inhomogeneous emission broadening. Last but not least, the long distance between activators and a large distance between


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the lowest 4f65d1excited state of Eu2+ and the bottom of the conduction band can effectively avoid the concentration quenching and photoionization effects at high temperature.

ASSOCIATED CONTENT

Supporting Information. The methods to calculate the color purity, lumen equivalent, chromaticity shift, and average lifetime were introduced in detail in the supporting information. Structural main parameters of the red phosphor SrLiAl3N4:Eu2+ (Table S1). Powder XRD pattern of SrLiAl3N4:Eu2+ phosphor (Figure S1). 7Li and

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Al solid-state MAS-NMR spectra of

SrLiAl3N4:Eu2+ phosphor (Figure S2 and S3). Decay curves of SrLiAl3N4:Eu2+ phosphor (Figure S4). This material is available free of charge via the Internet at http: //pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: +886-2-33661169. Fax: +886-2-23636359.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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This work was supported by the Ministry of Science and Technology of Taiwan (Contract No. MOST 104-2113-M-002-012-MY3 and MOST 104-2119-M-002-027-MY3).

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(20) Zhang, X.; Huang, L.; Pan, F.; Wu, M.; Wang, J.; Chen, Y.; Su, Q. Highly Thermally Stable Single-Component White-Emitting Silicate Glass for Organic-Resin-Free White-Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2014, 6, 2709–2717. (21) Zhang, X.; Wang, J.; Huang, L.; Pan, F.; Chen, Y.; Lei, B.; Peng, M.; Wu, M. Tunable Luminescent Properties and Concentration-Dependent, Site-Preferable Distribution of Eu2+ Ions in Silicate Glass for White LEDs Applications. ACS Appl. Mater. Interfaces 2015, 7, 10044– 10054. (22) Huang, L.; Zhu, Y.; Zhang, X.; Zou, R.; Pan, F.; Wang, J.; Wu, M. HF-Free Hydrothermal Route for Synthesis of Highly Efficient Narrow-Band Red Emitting Phosphor K2Si1–xF6:xMn4+ for Warm White Light-Emitting Diodes. Chem. Mater. 2016, 28, 1495–1502.

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TOC-image (8.4 cm × 3.6 cm)


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