Facile Synthesis of New Full-Color-Emitting BCNO Phosphors with High Quantum Efficiency** By Takashi Ogi, Yutaka Kaihatsu, Ferry Iskandar, Wei-Ning Wang, and Kikuo Okuyama*

In recent years, oxynitride and nitride compounds have attracted much attention as host lattices for phosphors because of their excellent properties, such as non-toxicity, outstanding thermal and chemical stability, broad available range of excitation and emission wavelengths, and high luminescence efficiency upon activation using rare-earth ions.[1–3] For example, bright yellow phosphors comprising Ca-a-SiAlON: Eu2þ and greenemitting phosphors based on b-SiAlON:Eu2þ have been synthesized and used in white light-emitting diode (LED) lamps characterized by high luminous efficiency.[4] However, the production of these materials generally requires high temperatures and pressures.[1–3] In addition, rare-earth ions, such as Eu2þ, Ce3þ, Yb2þ, and Tb3þ, which are required for use as luminescence centers, tend to be very expensive. Therefore, the development of viable methods for the production of oxynitride and nitride phosphor particles without using rare-earth ions at relatively low temperatures under ambient atmospheric pressure is desirable for white LED applications. On the other hand, much effort has been devoted to the preparation of carbon-based boron nitride (BCN) semiconductors for use as phosphors.[5–9] Theoretical studies suggest that it should be possible to use BCN materials to tune the wavelength of emitted light across the visible light spectrum by varying the composition of BCN compounds. BCN compounds are expected to behave as semiconductors with bandgap energies that are tunable by varying the atomic composition, since these materials are thought to be intermediates between graphite and hexagonal-BN (h-BN).[6] However, previous studies of BCN compounds indicate only single emission peaks in the photoluminescence (PL) spectra.[6,9,10] In addition, the PL intensity and quantum efficiency (QE) observed thus far for BCN compounds appear to be very low.[6] To the best of our knowledge, full-color-emitting BCN materials have not yet been reported. In this paper, we report for the first time, a new oxynitride phosphor, which is composed of BCNO atoms and is produced by a one-step liquid process at low temperatures (below 900 8C) under ambient atmospheric conditions. The color

[*] Prof. K. Okuyama, T. Ogi, Y. Kaihatsu, Dr. F. Iskandar, Dr. W.-N. Wang Department of Chemical Engineering Graduate School of Engineering, Hiroshima University 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527 (Japan) E-mail: [email protected] [**] T.O. was supported by a doctoral fellowship from the Japan Society for the Promotion of Science (JSPS).

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DOI: 10.1002/adma.200702551

emission of this novel BCNO phosphor has been easily tuned from the violet to the near-red regions of the photoluminescence spectrum by varying the carbon content. In addition, high external QE and a broad range of excitation wavelengths have been obtained using these BCNO phosphor particles. Furthermore, readily available, inexpensive chemicals, such as H3BO3 and (NH2)2CO have been used as the raw materials in this synthetic process. Various weight fractions of poly(ethylene glycol) (PEG) have been added to the solution to vary the carbon content. The effects of the PEG fraction and heat treatment on the PL properties of the resulting powder have been systematically investigated. Figure 1a shows a transmission electron microscopy (TEM) image of BCNO particles prepared at 800 8C. PEG, H(CH2CH2O)nOH, with a molecular weight (MW) of 20 000 has been added to the precursor solution with a PEG/boron (PEG/B) ratio of 2.0  103 (mol/mol). The prepared particles are approximately 2.5 mm in size. A typical electron diffraction (ED) pattern and high-resolution TEM (HRTEM) image taken from the prepared sample are shown in Figure 1b and c, respectively. The diffraction rings in the ED pattern have been indexed to the (002), (100), (004), and (110) reflections of h-BN (X-ray diffraction (XRD) pattern (Joint Committee on Powder Diffraction Standards (JCPDS) reference No 73-2095). As clearly apparent from the HRTEM image (Fig. 1c), the sample is polycrystalline in nature, and is composed of several distinct nanocrystals; each nanocrystal is approximately 5 nm in size. The lattice spacing of the constituent nanocrystals is 0.34 nm, as measured from the HRTEM image, which is larger than the 0.33 nm value for the (002) planes of h-BN, indicating that the prepared phosphor powder has crystals of turbostratic boron nitride (t-BN).[11] Electron energy loss spectroscopy (EELS) data for the obtained powder is shown in Figure 1d. Four ionization edges at ca. 188, 284, 400, and 532 eV are clearly apparent, corresponding to the characteristic K-edges of B, C, N, and O, respectively. The results described above confirm that the synthesized particles are composed of B, C, N, and O atoms, and contain partially crystallized t-BN regions. Figure 2 shows representative examples of excitation and emission spectra measured for the BCNO phosphor particles at room temperature. The sample shown in Figure 2 has been prepared at 800 8C using a PEG/B ratio of 2.0  103. The excitation spectrum is broad and covers the spectral region from the UV to the visible regions of the electromagnetic spectrum. In contrast, the emission spectrum is characterized by a single, intense, broad emission band at 475 nm upon

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800 8C are similar with emission peaks located at 460 and 475 nm, respectively. The emission peak of the sample prepared at 800 8C has a relatively higher intensity as compared to the sample prepared at 700 8C, which is not unexpected since photoluminescent materials prepared at higher temperatures generally exhibit higher PL intensities because of increased crystallinity.[12–14] However, the sample prepared at 900 8C shows the lowest PL intensity of this set of samples, and the PL spectra measured for this sample is very different from that of samples prepared at 700 and 800 8C. A possible explanation for this phenomenon is that the carbon content in the sample evaporates at 900 8C, thus significantly changing the composition of the sample. To further investigate the influence of the carbon content on the PL properties of the sample, BCNO phosphors with various PEG fractions have been prepared at 800 8C. The PL spectra measured at room temperature for BCNO phosphor particles prepared using various PEG fractions are shown in Figure 4. The excitation wavelength has been fixed at 365 nm for all these measurements. As shown in Figure 4, the emission intensities observed Figure 1. a) Low-magnification TEM image of a BCNO particle. b) ED pattern, c) HRTEM image, for samples prepared using PEG are very and d) electron energy loss spectroscopy data acquired for the same sample. high. Upon increasing the PEG/B ratio in the precursor solution from 2.0  103 to 3 excitation at 365 nm, which is generally used as the standard 6.0  10 , the emission spectra are red-shifted from 475 to excitation wavelength in the long UV range. 500 nm. To determine the cause of the shift in the PL spectra, The effect of the preparation temperature on the PL the chemical composition (B, C, N, and O) of the prepared properties of BCNO phosphor particles has been systemsamples has been measured. The carbon contents of samples atically investigated. Figure 3 shows the PL spectra of samples prepared using PEG/B ratios of 2.0  103 and 6.0  103 are 0.079% and 0.1415%, respectively, based on bulk analysis. This prepared at 700, 800, and 900 8C using a PEG/B ratio of result shows that the carbon concentration of the prepared 2.0  103. The PL spectra for the samples prepared at 700 and

Figure 2. Excitation and emission spectra measured for a BCNO phosphor sample prepared at 800 8C using a PEG/B ratio of 2.0  103 (mol/mol).

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Figure 3. PL spectra of BCNO samples prepared at 700, 800, and 900 8C using a PEG/B ratio of 2.0  103 (mol/mol).

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Figure 4. PL spectra of BCNO samples prepared at 800 8C using PEG/B ratios of a) 2.0  103, b) 2.8  103, c) 4.0  103, and d) 6.0  103 (mol/mol).

samples is increased upon the addition of more PEG to the samples. Based on this analysis, the PL spectral shifts can be attributed to variations in the chemical composition of the BCNO phosphor particles. Upon increasing the PEG concentration, carbon atom impurities are able to substitute for boron or nitrogen atoms in the t-BN crystallites. As a result, allowed electronic transitions of the t-BN-type matrix are modified and the states generated by carbon are changed.[5] In other words, the shifts of the PL peaks arise from variations in the BCNO bandgap, which accompany changes in the chemical composition of BCNO particles. Previous studies have reported that the bandgaps of BCN compounds depend strongly on the atomic arrangement within the compounds.[15–17] As mentioned above, the effects of the heating conditions and PEG fraction on the PL properties have been investigated, and the emission peak of the BCNO phosphors has been tuned over the blue region of the photoluminescent spectrum. In addition, the possibility of full-color emission has been explored by changing various parameters, such as the synthesis temperature, PEG fraction, and heating time. Figure 5 shows the PL spectra of multi-color-emitting BCNO samples prepared under various conditions. The measured PL peaks for the BCNO samples shift from 387 (violet emission) to 571 nm (near-red emission), as shown in the inset of Figure 5. The peak positions tend to shift to longer wavelengths with increasing PEG fraction, possibly because of the increased carbon content incorporated within the BCNO particles. The results described above reveal that the combination of a low synthesis temperature and a high PEG fraction leads to red-shifting of the BCNO phosphors. Furthermore, in the case of green-emission BCNO phosphors, the PL spectra have been measured at room temperature with an excitation wavelength of 450 nm. The results show that the produced BCNO phosphor has a relatively sharp emission peak at around 520 nm. From these results, it can be reasonably concluded that the green-emission phosphor can be excited at 450 nm.

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Figure 5. PL spectra and digital photographs of BCNO samples prepared under various conditions. The PEG/B ratio, operating temperature, and heating time are specified in each case. a) 2.0  103, 900 8C, 30 min; b) 2.0  103, 800 8C, 30 min; c) 4.0  103, 700 8C, 60 min; d) 4.0  103, 7008C, 45 min; and e) 4.4  103, 700 8C, 30 min.

Figure 6 shows the Commission Internationale de l’Eclairage (CIE) diagram of BCNO phosphor particles prepared under the conditions described in Figure 5. The CIE (x, y) coordinates of the particles are (0.174, 0.107), (0.181, 0.137), (0.259, 0.36), (0.318, 0.476) and, (0.455, 0.398), indicating that

Figure 6. Commission Internationale de l’Eclairage diagram and external quantum efficiencies of BCNO powders prepared under various conditions. The PEG/B ratio, operating temperature, and heating time are specified in each case. a) 2.0  103, 900 8C, 30 min; b) 2.0  103, 800 8C, 30 min; c) 4.0  103, 700 8C, 60 min; d) 4.0  103, 700 8C, 45 min; and e) 4.4  103, 700 8C, 30 min.

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the phosphors change in color from dark-blue to white, green, yellow, and near-red. As shown in Figure 6, the external quantum efficiencies (EQEs) of the resulting particles are as follows: 34% (Fig. 6a), 79% (Fig. 6b), 76% (Fig. 6c), 53% (Fig. 6d), and 10% (Fig. 6e). Notably, the sample exhibiting an EQE of 79% (Figs. 5b and 6b) essentially has a considerably higher EQE as compared to BCN materials prepared using the chemical vapor deposition (CVD) method,[6] even though CVD operates at higher temperatures (typically above 1500 8C). Furthermore, the above BCNO sample (Figs. 5b and 6b) represents a new blue-emitting phosphor material, which may be an attractive alternative to commercially used BaMgAl10O17:Eu2þ (BAM) phosphors. Although the EQE of the BCNO phosphor reported in the present study is slightly lower than that of the commercial BAM phosphor (EQE of about 95%, from Kasei Optonix, Japan), these BCNO phosphor particles are very promising because they can be synthesized by a facile method at low temperatures (800 8C, which is much lower than the 1300–1500 8C temperatures required to prepare BAM), and normal pressures from relatively inexpensive precursors.[18] Additional studies are required to understand the origin of the PL shift and to improve the EQEs of BCNO phosphors. In summary, BCNO phosphor particles with tunable emissions ranging from 387 to 571 nm have been prepared using a facile liquid-phase process at relatively low temperatures. The experimental results obtained in the present study reveal that control of the carbon content by optimizing the operating temperature and reaction time are important for the production of BCNO phosphor particles with emission wavelengths tunable over the entire visible light spectrum. Additional studies will be performed in the near future to elucidate the effects of the carbon content on the PL properties of BCNO phosphor particles. The results of the present study suggest that the BCNO powders prepared here are attractive candidates for use in the preparation of materials for multicolor LEDs. The BCNO phosphors exhibit high EQE values, and are relatively easy to prepare from cheap raw materials at low preparation temperatures and normal pressure conditions. Therefore, this method can readily be scaled up for industrial applications.

Experimental The BCNO phosphors were synthesized using the following precursors: H3BO3 (Wako Chemicals, Japan) as the boron source, (NH2)2CO (Wako Chemicals, Japan) as the nitrogen source, and PEG with a MW of 20 000 (Wako Pure Chemicals, Japan) as the carbon

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source. The precursors were mixed and the resulting solution was heated at 700–900 8C for 30–60 min to obtain BCNO particles. The prepared products were characterized by HRTEM (JEM-3000F, JEOL, Tokyo, Japan) at an accelerating voltage of 300 kV. The chemical composition of the BCNO phosphors was analyzed by EELS using a spectrometer attached to the transmission electron microscope. Inductively coupled plasma (ICP) measurements were also used to determine the chemical composition of the phosphors. The PL spectra of the prepared BCNO phosphors were recorded at room temperature using a spectrofluorophotometer (RF-5300PC, Shimadzu, Kyoto, Japan) equipped with a xenon laser source. The EQE of the prepared particles was determined at an excitation wavelength of 365 nm produced by a 150 W Xe lamp using an absolute PL quantum yield measurement system (C9920-02, Hamamatsu Phtonics, Shizuoka, Japan) with an A-10095-01 powder sample holder. Received: October 10, 2007 Revised: December 22, 2007 Published online: July 14, 2008

[1] N. Hirosaki, R. J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, T. Suehiro, M. Mitomo, Appl. Phys. Lett. 2005, 86, 1. [2] R. J. Xie, N. Hirosaki, T. Suehiro, F. F. Xu, M. Mitomo, Chem. Mater. 2006, 18, 5578. [3] Y. Q. Li, A. C. A. Delsing, G. de With, H. T. Hintzen, Chem. Mater. 2005, 17, 3242. [4] K. Sakuma, K. Omichi, N. Kimura, M. Ohashi, D. Tanaka, N. Hirosaki, Y. Yamamoto, R. J. Xie, T. Suehiro, Opt. Lett. 2004, 29, 2001. [5] M. Kawaguchi, K. Nozaki, Y. Kita, M. Doi, J. Mater. Sci. 1991, 26, 3926. [6] M. O. Watanabe, S. Itoh, T. Sasaki, K. Mizushima, Phys. Rev. Lett. 1996, 77, 187. [7] M. Kawaguchi, Adv. Mater. 1997, 9, 615. [8] J. Wu, W. Q. Han, W. Walukiewicz, J. W. Ager, W. Shan, E. E. Haller, A. Zettl, Nano Lett. 2004, 4, 647. [9] X. D. Bai, E. G. Wang, J. Yu, H. Yang, Appl. Phys. Lett. 2000, 77, 67. [10] L. W. Yin, Y. Bando, D. Golberg, A. Gloter, M. S. Li, X. L. Yuan, T. Sekiguchi, J. Am. Chem. Soc. 2005, 127(16), 354. [11] Q. X. Guo, Y. Xie, C. Q. Yi, L. Zhu, P. Gao, J. Solid State Chem. 2005, 178, 1925. [12] W. N. Wang, W. Widiyastuti, T. Ogi, I. W. Lenggoro, K. Okuyama, Chem. Mater. 2007, 19, 1723. [13] T. Ogi, Y. Itoh, M. Abdullah, F. Iskandar, Y. Azuma, K. Okuyama, J. Cryst. Growth 2005, 281, 234. [14] F. Iskandar, T. Ogi, K. Okuyama, Mater. Lett. 2006, 60, 73. [15] A. Y. Liu, R. M. Wentzcovitch, M. L. Cohen, Phys. Rev. Lett. 1989, 39, 1760. [16] H. Nozaki, S. Itoh, Phys. B 1996, 220, 487. [17] Y. Miyamoto, M. L. Cohen, S. G. Louie, Phys. Rev. B: Condens. Matter 1995, 52, 14 971. [18] C. Panatarani, I. W. Lenggoro, N. Itoh, H. Yoden, K. Okuyama, Mater. Sci. Eng. B 2005, 122, 188.

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Adv. Mater. 2008, 20, 3235–3238

Facile Synthesis of New Full-Color-Emitting BCNO ...

(EELS) data for the obtained powder is shown in Figure 1d. ... PL spectra measured at room temperature for BCNO phosphor .... Published online: July 14, 2008.

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