Nanocomposite Phosphor Consisting of CaI2:Eu2+ Single

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

Nanocomposite Phosphor Consisting of CaI2:Eu2+ Single Nanocrystals Embedded in Crystalline SiO2 Hisayoshi Daicho,*,† Takeshi Iwasaki,† Yu Shinomiya,† Akitoshi Nakano,§ Hiroshi Sawa,§ Wataru Yamada,§ Satoru Matsuishi,⊥ and Hideo Hosono⊥ †

Research & Development Department, Koito Manufacturing Co., 500, Kitawaki, Shimizu-ku, Shizuoka 424-8764, Japan Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ⊥ Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259-S2-13, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan §

S Supporting Information *

ABSTRACT: High luminescence efficiency is obtained in halide- and chalcogenide-based phosphors, but they are impractical because of their poor chemical durability. Here we report a halide-based nanocomposite phosphor with excellent luminescence efficiency and sufficient durability for practical use. Our approach was to disperse luminescent single nanocrystals of CaI2:Eu2+ in a chemically stable, translucent crystalline SiO2 matrix. Using this approach, we successfully prepared a nanocomposite phosphor by means of selforganization through a simple solid-state reaction. Single nanocrystals of 6H polytype (thr notation) CaI2:Eu2+ with diameters of about 50 nm could be generated not only in a SiO2 amorphous powder but also in a SiO2 glass plate. The nanocomposite phosphor formed upon solidification of molten CaI2 left behind in the crystalline SiO2 that formed from the amorphous SiO2 under the influence of a CaI2 flux effect. The resulting nanocomposite phosphor emitted brilliant blue luminescence with an internal quantum efficiency up to 98% upon 407 nm violet excitation. We used cathodoluminescence microscopy, scanning transmission electron microscopy, and Rietveld refinement of the X-ray diffraction patterns to confirm that the blue luminescence was generated only by the CaI2:Eu2+ single nanocrystals. The phosphor was chemically durable because the luminescence sites were embedded in the crystalline SiO2 matrix. The phosphor is suitable for use in near-ultraviolet lightemitting diodes. The concept for this nanocomposite phosphor can be expected to be effective for improvements in the practicality of poorly durable materials such as halides and chalcogenides. KEYWORDS: phosphor, nanocomposite material, self-organization, photoluminescence, halide, crystalline-SiO2

1. INTRODUCTION Phosphors are luminescent materials that are widely used for various applications, such as white light-emitting diodes (LEDs), fluorescent lights, luminous paints, and scintillators.1−4 Phosphors generally consist of a single phase of host crystals doped with rare earth ions, Mn ions, or both as luminescence centers. Oxides and nitrides, which have high durability, are often chosen as host materials. However, the development of novel, higher-luminescence phosphors based on these host materials is difficult because they have a high thermal relaxation rate due to the strong interaction between the luminescence centers and the co-ordination anions. Halide- and chalcogenidebased phosphors tend to show higher luminescence owing to their low thermal relaxation rate.5,6 However, their applicability is rather limited due to their insufficient chemical durability; in particular, they show low moisture resistance. To address this problem, we developed a novel phosphor, (Ca1−x−y,Srx,Euy)7(SiO3)6Cl2 (Cl_MS:Eu2+), with halogen and oxygen as ligands.7 This phosphor shows practical-level © XXXX American Chemical Society

performance in that it is both highly efficient and durable. For practical use, a phosphor must meet the following requirements so that its performance can be maintained in the environment in which it will be used: an internal quantum efficiency (IQE) of >70%, an absorption of >70%, and high moisture resistance. To improve the moisture resistance of halides and chalcogenides, researchers have investigated the use of these materials in nanocomposite phosphors. For example, Lehmann reported heterogeneous halide-silica phosphors.8 Although these phosphors show high moisture resistance, their luminescence efficiency is insufficient for practical use. Various glass−ceramic phosphors have also been investigated, but their luminescence efficiencies are not high enough either.9−11 CdSe quantum dots with core/shell structures were reported as another type of nanocomposite phosReceived: September 18, 2017 Accepted: November 7, 2017

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DOI: 10.1021/acsami.7b14132 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Luminescence properties of (CaI2:Eu2+/SiO2). (a) PLE and PL spectra of (CaI2:Eu2+/SiO2) (blue solid curves) and BAM:Eu2+ (red dashed curves). (b) Temperature dependence of PL intensity normalized to the intensity at 303 K. The black circles and white squares indicate (CaI2:Eu2+/SiO2) and BAM:Eu2+, respectively. (c) Eu LIIIXANES spectra of phosphors before and after heat treatment at 923 K for 3 h: (CaI2:Eu2+/SiO2) before (blue dotted curve) and after (blue solid curve) heat treatment, BAM:Eu2+ before (red dotted curve) and after (red solid curve) heat treatment. BAM:Eu2+ (gray solid curves) and Eu2O3 (gray dotted curves) were used as reference compounds. (d) Relative PL intensity of (CaI2:Eu2+/SiO2) as a function of elapsed time at 358 K and 85% humidity. (e) Relative PL intensities of (CaI2:Eu2+/SiO2) (black circles) and BAM:Eu2+ (white squares) as a function of elapsed time at 358 K and 85% humidity atmosphere under irradiation with nUV light (405 nm) at a power output of 150 mW. (f) Electroluminescence spectra of white-LEDs driven at 100 mA. White-LED1 (black curve) used blue-emitting (CaI2:Eu2+/SiO2) and yellow-emitting Cl_MS:Eu2+ combined with a nUV-LED (λp = 405 nm). White-LED2 (gray curve) used blue-emitting BAM:Eu2+ and Cl_MS:Eu2+ combined with a nUV-LED. The inset shows the photograph of white-LED1.

phor.12−14 However, their durability has not reached that of rare-earth-doped phosphors, and the high toxicity of Cd makes these quantum dots impractical.15−17 Extending the work of Lehmann, Hao et al. reported that the nanocomposite phosphor (CaCl2/SiO2):Eu2+ has higher photoluminescence (PL) than a conventional Sr2P2O7:Eu2+ phosphor.18 However, the PL from (CaCl2/SiO2):Eu2+ undergoes considerable thermal quenching. In addition, neither Lehmann nor Hao et al. provided details about the crystal structures or luminescence centers of the nanocomposite phosphors that they reported.

Here we report a highly efficient nanocomposite (CaI2:Eu2+/ SiO2) phosphor that consisted of CaI2:Eu2+ single nanocrystals embedded in a crystalline SiO2 matrix and that was synthesized by a simple solid-state reaction. This phosphor, which emits intense blue luminescence with an IQE of up to 98%, meets the above-mentioned requirements, even though its Eu content was much lower than that of conventional phosphors. The phosphor is suitable as a blue phosphor for near-ultraviolet LEDs (nUV-LEDs). We used cathodoluminescence (CL) microscopy, scanning transmission electron microscopy B


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SiO2) were measured with a laser diffraction particle size analyzer (Microtrac MT3000, Nikkiso, Tokyo, Japan). The chemical composition of (CaI2:Eu2+/SiO2) was analyzed by SEM combined with energy dispersive X-ray (EDX) spectrometry, STEM combined with EDX spectrometry, inductively coupled plasma atomic emission spectroscopy (SPS3100, SII Nano Technology, Tokyo, Japan), and ion chromatography (ICS200, DIONEX, Sunnyvale, CA, USA). The valence of Eu ions was measured by X-ray absorption near-edge structure (XANES) spectroscopy on the BL11S2 beamline at Aichi Synchrotron Radiation Center. The CAS 140B-152 spectrometer was also used to obtain luminous flux, color coordinates, color temperature, and color rendering index from a pc-LED operated in an integrated sphere. 2.4. Density Functional Theory Calculations. Density functional theory calculations were performed by means of a projectoraugmented wave method implemented in VASP code with a nonlocal correlation functional vdW-DF2, which approximately accounts for dispersion interactions, that is, van der Waals interactions.21−25 Trigonal primitive and rhombohedral primitive cells containing one Ca and two I sites were used to calculate 2H and 6H structures, respectively. Brillouin zone integration to calculate the total energy was performed with a 5 × 5 × 5 Monkhorst−Pack k-mesh. The plane wave cutoff energy for the electronic wave functions was set to 500 eV. The total energy convergence criterion was set to 10−6 eV, and the maximum component of force acting on any atom in the relaxed geometry was less than 0.01 eV/Å.

(STEM), and Rietveld refinement of the X-ray diffraction (XRD) patterns to confirm that the blue luminescence was generated by the CaI2:Eu2+ single nanocrystals, and we suggest that the nanocomposite phosphor was generated by means of a self-organization process involving the formation of CaI2:Eu2+ single nanocrystals within the crystalline SiO2 matrix.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Powdered (CaI2:Eu2+/SiO2) was prepared from amorphous SiO2, CaI2, Eu2O3, and excess NH4I (the typical Si:Ca:Eu:I molar ratio was 6.00:0.80:0.05:2.40). In addition, to clarify the origin of the (CaI2:Eu2+/SiO2) luminescence, we used the same raw materials to prepare samples with Si:Ca:Eu:I molar ratios of 6.00:0.82:0.03:2.40, 6.00:0.835:0.015:2.40, and 6.00:0.85:0.00:2.40. The raw materials were milled and then were sintered at 1273 K for 7 h under a reducing atmosphere (5% H2 in N2). The sintered sample was washed in water to remove free iodide. A plated sample of (CaI2:Eu2+/SiO2) was prepared by the same method, except that instead of the amorphous SiO2 powder, we used a SiO2 glass plate that had been roughened on one side by sandblasting and then washed. The roughened surface was covered with a mixture of powdered CaI2, Eu2O3, and excess NH4I (Ca/Eu/I molar ratio = 0.80:0.05:2.40), and the plate was then sintered and washed. 2.2. Fabrication of LED Devices and Evaluation of the Device by Means of an nUV Irradiation Test. (CaI2:Eu2+/SiO2) and BaMgAl10O17:Eu2+ (BAM:Eu2+), a commercially available blue phosphor, were dispersed separately at 7 vol % in a transparent silicone resin. Unpackaged nUV-LEDs (λ = 405 nm) combined with each of the dispersions were used to fabricate phosphor-converted LEDs (pcLEDs). Specifically, the nUV-LEDs were coated with a silicone dispersion of (CaI2:Eu2+/SiO2) or BAM:Eu2+, and the coated nUVLEDs were then cured in an oven at 423 K for 1 h. The resulting pcLEDs were used to evaluate the durability of the phosphors under the various conditions including high temperature, high humidity, and nUV-irradiation (405 nm). Thus, the pc-LEDs were driven to emit a nUV power output of 150 mW at 358 K in air at 85% humidity. In addition we fabricated white-LEDs using (CaI2:Eu2+/SiO2) or BAM:Eu2+ combined with a yellow phosphor Cl_MS:Eu2+ and a nUV-LED (λp = 405 nm). Blue phosphor and Cl_MS:Eu2+ were mixed at the following weight ratios: (CaI2:Eu2+/SiO2)/Cl_MS:Eu2+ = 3/2 for white-LED1 and BAM:Eu2+/ Cl_MS:Eu2+ = 4/1 for whiteLED2. Then, a phosphor paste was made by dispersing each of the above phosphor mixtures in translucent silicone resin at a concentration of 1.3 vol %. Finally, a white-LED was fabricated by encapsulation of a nUV-LED (λp = 405 nm) on a white substrate with the phosphor paste, which was allowed to harden at 150 °C for 90 min. 2.3. Characterization of Phosphor Samples. PL spectra under continuous excitation and photoluminescence excitation (PLE) spectra of the phosphor samples were measured at room temperature with a multichannel optical analyzer (PMA C5966-31, Hamamatsu Photonics, Shizuoka, Japan). IQE values were determined from luminescence spectra obtained by means of 407 nm laser-diode excitation of phosphor samples mounted in an integrating sphere. For detection, we used a CAS 140B-152 spectrometer (Instrument Systems, Munich, Germany). The details of the IQE measurement method are provided in our previous report.7 XRD patterns of powdered (CaI2:Eu2+/SiO2) samples were measured on the BL02B2 beamline (wavelength 82.7 pm) at the SPring-8 synchrotron radiation facility. The Rietveld analysis was made using a software GSAS.19,20 Scanning electron microscopy (SEM) (JSM7100F, JEOL, Tokyo, Japan) and STEM (HD-2700, Hitachi, Tokyo, Japan) were used to characterize the luminescence sites of (CaI2:Eu 2+/SiO2). CL microscopy images were observed with a scanning electron microscope (DS-130, TOPCON, Tokyo, Japan) equipped with a camera for imaging and spectroscopy (LN-CCD, Nippon Roper, Tokyo, Japan). The terminal structure of SiO2 was analyzed by Fourier transform IR spectroscopy (Frontier T/V-ATR, PerkinElmer, Waltham, MA USA). The particle size distributions of the raw materials and the (CaI2:Eu2+/

3. RESULTS AND DISCUSSION The PLE and PL spectra of (CaI2:Eu2+/SiO2) at room temperature showed a broad band arising from the allowed 5d−4f transition of the Eu2+ ion (Figure 1a, blue solid curves). Specifically, the PL spectrum obtained at an excitation wavelength of 400 nm showed a single band centered at 471 nm with a full width at half-maximum of 32.4 nm. For reference, we also measured the PLE and PL spectra of BAM:Eu2+ (Figure 1a, red dashed curves). The chemical compositions of (CaI2:Eu2+/SiO2) and BAM:Eu2+ were analyzed by inductively coupled plasma atomic emission spectroscopy combined with ion chromatography (Table S1). Even though the Eu content in (CaI2:Eu2+/SiO2) was 1/6 that in BAM:Eu2+, the PL peak intensity of (CaI2:Eu2+/SiO2) was 2.7 times that of BAM:Eu2+ upon excitation at 400 nm. (CaI2:Eu2+/SiO2) had an external quantum efficiency of 83%, an IQE of 98%, and an absorption of 85% upon excitation at 407 nm, whereas the corresponding values for BAM:Eu2+ were 62, 97, and 64%, respectively. It is important that phosphors used for nUV-LEDs exhibit high PL intensity at excitation wavelengths longer than 400 nm because the quantum efficiency of InGaN LEDs with emission wavelengths shorter than 400 nm decreases drastically due to weak carrier localization by low In content in the InGaN well.26 Because of the high PL intensity at 400 nm excitation, (CaI2:Eu2+/SiO2) is a promising candidate as a blue phosphor for nUV-LEDs. In addition, (CaI2:Eu2+/SiO2) was durable at high temperature and high humidity. We compared a thermal quenching property of (CaI2:Eu2+/SiO2) with that of BAM:Eu2+. InGaN LEDs generate heat with current injection. In order to drive the InGaN LEDs stably, it is necessary to keep the junction temperature of the InGaN LEDs below 423 K. The PL intensity of (CaI2:Eu2+/SiO2) at 423 K kept at 96% of that at room temperature (Figure 1b). The thermal quenching property of (CaI2:Eu2+/SiO2) was almost the same as that of BAM:Eu2+. We also evaluated the resistance of Eu2+ ions to heat-oxidation by determining the PL intensity and Eu ion valence after heating (CaI2:Eu2+/SiO2) and BAM:Eu2+ for 3 h C


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ACS Applied Materials & Interfaces at 923 K in air. We found that the PL intensity of (CaI2:Eu2+/ SiO2) after heat treatment was 84% of that before heat treatment, whereas the PL intensity of BAM:Eu2+ after heat treatment was only 24% of the original value. For each of the phosphors, the Eu ion valences before and after heat treatment were determined by XANES (Figure 1c), and the ratios of divalent to trivalent ions were estimated by linear combination fitting. Before heat treatment, 30% of the Eu ions in (CaI2:Eu2+/SiO2) were trivalent, and that percentage was not altered substantially by heat treatment. In contrast, 60% of the Eu ions in BAM:Eu2+ were converted to trivalent ions by the heat treatment. A life test of (CaI2:Eu2+/SiO2) at 358 K in air at 85% humidity showed that the PL intensity varied by less than 2% after 2000 h (Figure 1d). In addition, we prepared pc-LEDs by coating a nUV-LED separately with either (CaI2:Eu2+/SiO2) or BAM:Eu2+. The resulting pc-LEDs were used to evaluate the durability of the phosphors under various conditions including high temperature, high humidity, and nUV irradiation. Specifically, we investigated the intensity of blue luminescence from the pc-LEDs after they were driven at 358 K in air at 85% humidity, and we found that the luminescence intensity of the (CaI2:Eu2+/SiO2) phosphor varied by less than 4% after 300 h (Figure 1e). In order to evaluate the applicability to whiteLEDs of the (CaI2:Eu2+/SiO2), we fabricated white-LEDs. White-LED1 consisted of a nUV-LED (λp = 405 nm) combined with the blue emitting phosphor (CaI2:Eu2+/SiO2) and the yellow emitting phosphor Cl_MS:Eu2+ (inset Figure 1f). As a reference, we also prepared white-LED2 by replacing the blue emitting phosphor with BAM:Eu2+. Figure 1f shows the electroluminescence spectra of both white-LEDs. When these white-LEDs were driven at 100 mA, the luminous flux, color coordinates, color temperature, and color-rendering index of white-LED1 were 19.1 lm, (cx, cy) = (0.341, 0.328), 5063 K, and 79.0, respectively, whereas the corresponding values for white-LED2 were 15.8 lm, (0.336, 0.318), 5276 K ,and 86.7, respectively. A cross-sectional SEM image of a (CaI2:Eu2+/SiO2) particle cut by a focused ion beam shows that the particle has dual structure (Figure 2a): an approximately 10 μm diameter homogeneous nucleus at the center of the particle and an approximately 10 μm thick outer layer in which white spots were distributed. A CL microscopy image of the same particle obtained with a scanning electron microscope equipped with a camera for imaging and spectroscopy at an excitation voltage of 5 kV showed that blue light was emitted only from the white spots in the outer layer (Figure 2b). SEM-EDX analysis revealed that the homogeneous nucleus was composed of SiO2 containing a small amount of I ions. The matrix of the outer layer was composed of SiO2 contaminated with Ca and I ions. The white spots in the outer layer contained Si, O, Ca, I, and Eu (Table S2). To investigate the origin of the luminescence in detail, we simultaneously measured electron diffraction and chemical composition along a line across the nanocrystal. A focused ion beam was used to prepare 60 nm thick cross sections of a phosphor particle, which were then observed by STEM (Figure 3a). The average diameter of the white spots was about 50 nm. An electron diffraction image obtained from one of the spots indicated that the spots were due to single crystals (Figure 3a, inset). We used STEM-EDX to measure the composition of a spot and the surrounding matrix along the red line indicated in Figure 3a (Figure 3b). The atomic percentages of Si and O in the spot were lower than the corresponding percentages in the

Figure 2. (a) Cross-sectional SEM image of a (CaI2:Eu2+/SiO2) particle. (b) CL microscopy image of the same particle obtained with a microscope equipped with a camera for imaging and spectroscopy.

Figure 3. (a) Cross-sectional STEM image of a (CaI2:Eu2+/SiO2) particle. The inset image indicates the electron diffraction pattern of the white-spot region. (b) Compositional profile along a line (indicated in red in panel a) crossing an emitting spot and the surrounding matrix.

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ACS Applied Materials & Interfaces matrix, whereas the percentages of I, Ca, and Eu were higher in the spot than in the matrix. Note that both SEM-EDX and STEM-EDX indicated that the ratio of divalent metal ions (Ca plus Eu) to iodide ions was 1:2 in the white spots (Table S2 and Figure 3b). The XRD pattern of the (CaI2:Eu2+/SiO2) composite showed that it consisted mainly of α-cristobalite (a hightemperature polymorph of SiO2), along with small amounts of α- and β-CaSiO3 (Figure 4a). For the unindexed peaks in the searching for constituent elements of (CaI2:Eu2+/SiO2), we were unable to find any substances registered in the Inorganic Crystal Structure Database. Rietveld refinement of the XRD pattern indicated that the unindexed peaks originated from a CaI2 polytype that had a CdCl2-type structure (thr notation: 6H) and was different from the previously reported polytype with a CdI2-type structure (2H).27,28 The formation of a 6H polytype has previously been achieved by substitution of Br at some of the I sites (CaI1.3Br0.7).29 The lattice parameters and space groups of 2H-CaI2 and 6H-CaI2:Eu2+ (Eu atom % = 0, 0.03, and 0.06 consisted in (CaI2:Eu2+/SiO2)) are listed in Table S3. Eu atom % in (CaI2:Eu2+/SiO2) were determined with inductively coupled plasma atomic emission spectroscopy. Although both structures are composed of the same layers formed by edge-sharing of CaI6 octahedra, the layer-stacking sequence is AABB··· for 2H and ABCABC··· for 6H (Figure 4b,c). We verified the Eu-occupied sites of (CaI2:Eu2+/SiO2) by varying the Eu content. The lattice parameters of α-cristobalite and α-CaSiO3 were unaffected by changes in the Eu content (Figure S1). The amount of β-CaSiO3 in (CaI2:Eu2+/SiO2) was too small for us to evaluate the effect of Eu content on the lattice parameters. However, we found that the lattice parameter of 6H-CaI2:Eu2+ lengthened along the direction of the a-axis as the Eu content increased, according to Vegard’s law (Figure 4d). In contrast, in the interlayer direction (along the c-axis), the lattice parameters of 6H-CaI2:Eu2+ were unaffected by variation of the Eu content. This result revealed that the Eu-occupied sites were not located in the interlayer but instead were Ca sites in the layers. On the basis of the CL microscopy, SEM-EDX, STEM-EDX, and XRD results, we concluded that the luminescence of (CaI2:Eu2+/SiO2) originated not from α-cristobalite, α-CaSiO3, or β-CaSiO3 but from 6H-CaI2:Eu2+. CaI2:Eu2+ has been reported to emit blue luminescence and to have a PL spectrum similar to that of (CaI2:Eu2+/SiO2).29−31 However, CaI2:Eu2+ lacks moisture resistance and deliquesces readily in the atmosphere. Importantly, two different crystalline materials, namely, the luminescent material and the matrix, were present in a single particle. We propose that the nanocomposite phosphor (CaI2:Eu2+/SiO2) was synthesized by means of self-organization through a simple solid-phase reaction process at 1273 K (Figure 5). As the mixed raw materials were heated to 1273 K, they underwent two changes. First, when the temperature exceeded the melting point of CaI2 (1052 K), a molten CaI2 phase formed around the SiO2 particles (Figure 5b). However, at this temperature, the main raw material, SiO2, remained amorphous (Figure S2). Second, when the temperature reached 1273 K, the molten CaI2 acted as a flux on the surface of the SiO2 particles, and SiO2 began to crystallize (Figure 5c). The increase in temperature from 1052 to 1273 K improved the dissolving capacity of the molten CaI2 and activated the SiO2 surface by removing the OH groups, as confirmed by Fourier transform IR (Figure S3). Crystallization of the SiO2 resulted in the aggregation of several SiO2 particles into a single particle,

Figure 4. (a) Observed (red circles), calculated (black curve), and difference (green curve) synchrotron XRD profiles for (CaI2:Eu2+/ SiO2) by Rietveld refinement. The blue arrows indicate the peaks for 6H-CaI2. (b) Crystal structures of 6H-CaI2 viewed along the b-axis. The green and purple spheres represent Ca2+ and I−, respectively. (c) Crystal structures of 2H-CaI2 viewed along the b-axis. (d) Normalized lattice parameters against 6H-CaI2 in (CaI2:Eu2+/SiO2) (Eu = 0) along the a-axis (blue circles) and c-axis (red squares) of 6H-CaI2:Eu2+ as a function of Eu content. E


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Figure 5. Formation of the nanocomposite phosphor (CaI2:Eu2+/ SiO2) by means of a self-organization process. (a) Mixed raw materials before sintering: SiO2 (white), CaI2 (violet), Eu2O3 (red), and NH4I (green). (b) At temperatures above 1052 K, CaI2 crystals melted, forming a molten phase (violet) around the SiO2 particles. (c) When the temperature reached 1273 K, crystalline SiO2 (gray) particles joined together owing to the flux effect of the molten CaI2. (d) Molten CaI2 precipitated at the boundaries of the amorphous and crystalline regions of the coagulated SiO2 particles. (e) 6H-CaI2:Eu2+ single nanocrystals (blue) were generated in the SiO2 particles owing to solidification of CaI2 during the cooling step. Electron diffraction patterns obtained from the SiO2 matrix (insets) show amorphous nature in the homogeneous nucleus region and tetragonal crystalline SiO2 in the outer layer region.

Next, we take note of the crystallization process of the SiO2. The cross-sectional SEM image of a (CaI2:Eu2+/SiO2) particle showed a ring-shaped region in which the density of single nanocrystals was particularly high (Figure 2). We investigated the difference between the SiO2 crystal structures inside and outside this ring (insets in Figure 5e). The electron diffraction patterns indicated the presence of amorphous SiO2 inside the ring (i.e., in the homogeneous nucleus) and tetragonal crystalline SiO2 outside the ring (i.e., in the matrix of the outer layer). Many 6H-CaI2:Eu2+ single nanocrystals were generated at the boundary between the amorphous and crystalline SiO2 regions. Crystallization of the SiO2 raw material required a sintering temperature of >1623 K (Figure S6), and during sintering at 1273 K, crystallization occurred only in the region where the CaI2 flux penetrated into the SiO2 particle. The self-organization process we have proposed reasonably explain the experimental data, including SEM images, electron diffraction patterns, XRD patterns, particle size distributions, and the Fourier transform IR spectrum. The nanocomposite phosphor (CaI2:Eu2+/SiO2) formed by the above-described process exhibited excellent durability because 6H-CaI2:Eu2+ single nanocrystals, the origin of the luminescence and ordinarily not resistant to moisture, were protected from the atmosphere by the crystalline SiO2 matrix. We obtained additional evidence for the proposed process by fabricating a nanocomposite phosphor plate by using a SiO2 glass plate as a raw material instead of SiO2 powder. The resulting nanocomposite phosphor plate emitted brilliant blue light upon excitation with 405 nm violet light (Figure 6a). Figure 6b shows a cross-sectional micrograph of the plate. As a result of penetration of the molten CaI2 flux into the roughened, raw-material-covered side of the SiO2 glass plate, that side of the plate became translucent and emitted brilliant

and consequently, the grain size of the nanocomposite phosphor (CaI2:Eu2+/SiO2) became larger than that of the SiO2 raw material (Figure S4). The molten CaI2 precipitated at boundaries between the amorphous and crystalline SiO2 regions of the combined particle (Figure 5d). During the cooling step, 6H-CaI2:Eu2+ single nanocrystals were generated by solidification of the CaI2 in the SiO2 particles (Figure 5e). We performed density functional theory calculations to evaluate the stability of the 2H- and 6H-CaI2 structures. By calculating variable-cell structure relaxation under external pressures ranging from −0.5 to 10.0 GPa, we obtained plots of lattice parameter versus external pressure and plots of total electron energy versus cell volume (Figure S5). At ambient pressure, the differences between the calculated and experimental lattice parameters were less than ±0.6% for both polytypes, and the value of 1/3 of lattice parameter c (c6H′) for the 6H phase was 0.6% larger than the value of c for 2H (c2H). The calculated value of c6H′/ c2H was almost equal to the observed value. Over the entire range of volumes and pressures, however, the total energy per formula unit for 2H was always lower than that for 6H (−5.6 meV at 0 GPa). This result means that conversion of 2H to 6H was not induced by external pressure, which was caused by the squeezing of the CaI2 phase into the grain boundary in SiO2. In the case of CdI2, polytype conversion is reportedly driven by spiral growth around a suitable screw dislocation.32 Therefore, we speculate that the formation of 6H-CaI2:Eu2+ was caused by spiral growth at the SiO2 surface.

Figure 6. (a) Photograph of the (CaI2:Eu2+/SiO2) plate (15 mm square) under irradiation at 405 nm. The upper side of the micrograph is the roughened, raw-material-covered side of the plate. (b) Crosssectional micrograph of the (CaI2:Eu2+/SiO2) plate under irradiation by visible light (left) and 405 nm light (right). F


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blue luminescence. In contrast, the smooth, bare side of the plate remained transparent. This clear difference validates our contention that penetration of the CaI2 flux was directly linked to the formation of the nanocomposite phosphor.

4. CONCLUSION We prepared a nanocomposite phosphor, (CaI2:Eu2+/SiO2), by means of a simple solid-state reaction. The phosphor consisted of 6H-CaI2:Eu2+ single nanocrystals in a crystalline SiO2 matrix. Even though the Eu content in (CaI2:Eu2+/SiO2) was lower than that in commercially used BAM:Eu2+ the intensity of the PL peak of (CaI2:Eu2+/SiO2) was 2.7 times that of BAM:Eu2+ under 400 nm excitation. Although the luminescence sites of (CaI2:Eu2+/SiO2) contained iodide, the phosphor was nevertheless chemically durable enough for practical use because the luminescence sites were embedded in the crystalline SiO2, where they were protected from moisture. We expect that the concept for this nanocomposite phosphor would be applicable to other poorly durable host materials such as halides and chalcogenides.

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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/acsami.7b14132. Plots of the normalized lattice parameters of αcristobalite and α-CaSiO3 as a function of Eu content; XRD patterns of (CaI2:Eu2+/SiO2) at various sintering temperatures; Fourier transform spectra; particle size distribution curves; plots of lattice parameters versus external pressure; plots of total electron energy versus cell volume for 6H-CaI2 and 2H-CaI2; XRD patterns of SiO2 powders; composition (atom %) of (CaI2:Eu2+/ SiO2) and BAM:Eu2+; and crystallographic data for 6HCaI2:Eu2+ by Rietveld refinement, along with literature data for 2H-CaI2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone:+81-54-345-2386. ORCID

Hisayoshi Daicho: 0000-0001-5526-233X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research at Tokyo Tech was supported by the Element Strategy Initiative to Form a Core Research Center. The synchrotron radiation experiments were performed at the BL02B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2016A1620).

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REFERENCES

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Nanocomposite Phosphor Consisting of CaI2:Eu2+ Single

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