- Email: [email protected]
New red phosphor for near-ultraviolet light-emitting diodes with high color-purity
Materials Research Bulletin 45 (2010) 240–242
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
New red phosphor for near-ultraviolet light-emitting diodes with high color-purity Zhengliang Wang a,*, Pei He b, Rui Wang b, Jishou Zhao a, Menglian Gong b a b
School of Chemistry and Biotechnology, Yunnan Nationalities University, Kunming, Yunnan 650031, PR China School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 August 2009 Received in revised form 30 September 2009 Accepted 14 October 2009 Available online 23 October 2009
New red phosphors, Na5Eu(MoO4)4 doped with boron oxide were prepared by the solid-state reaction. Their structure and photo-luminescent properties were investigated. With the introduction of boron oxide, the red emission intensity of the phosphors under 395 nm excitation is strengthened, with high color-purity (x = 0.673, y = 0.327). The single red light-emitting diode was obtained by combining InGaN chip with the red phosphor, bright red light can be observed by naked eyes from the red light-emitting diodes under a forward bias of 20 mA. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds C. X-ray diffraction D. Luminescence D. Optical properties
Recently growing interest was focused on semiconductor white-light-emitting diodes (W-LEDs), which have many advantages, such as high efficiency, long lifetime, low power consumption and environment-friendly characteristics [1,2]. In order to obtain a higher efficiency white LED with an appropriate color temperature and a higher color-rendering index, a new approach using near-ultraviolet (nUV) InGaN-based LED chip coated with blue/green/red tricolor phosphors was introduced [3,4]. Because of larger stocks shift, red phosphors exhibit lower efficiency, compared with blue and green phosphors. At present, the commonly used red-emitting phosphors for nUV InGaN-based white LEDs are mainly some sulfide phosphors (Y2O2S:Eu3+, CaS:Eu2+, SrY2S4:Eu2+) [5–7]. However, there are some disadvantages to use those sulfide-based materials, such as chemical instability, and low efficiency [8]. Therefore, it is important to explore new red phosphors that can be efficiently excited in the near-UV range around 400 nm with high efficiency, stability, and without environmental hazards. In our previous work [9], the luminescence of Na5La(MoO4)4:xEu3+ was reported, and the phosphor Na5Eu(MoO4)4 shows intense red emission, which maybe find application on near-UV white LEDs. It is well known that boron oxide is not only a good flux, but also able to replace some competent of the phosphors, and enhance the luminescent properties [10,11]. In this article, red
* Corresponding author. Tel.: +86 871 5910017; fax: +86 871 5910017. E-mail address: [email protected] (Z. Wang). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.10.005
phosphors Na5Eu(MoO4)4 doped with different content of boron oxide was prepared, and their photo-luminescent properties were investigated. At last, single red LED was fabricated by combining the red phosphor with 400 nm-emitting InGaN chips. The samples Na5Eu(MoO4)4 doped with different content of boron oxide were prepared with a solid-state reaction technique at high temperature. The starting materials are (NH4)6Mo7O244H2O (A.R. grade), NaHCO3 (A.R. grade), HBO3 (A.R. grade), and Eu2O3 (99.99% purity). The stoichiometric mixtures were ground and fired at 600 8C for 8 h. The samples with 0, 2 and 4 mol% boric acid additions (compared with four times of MoO42 content) were named as NEMB0, NEMB2 and NEMB4, respectively The structure of these phosphors was recorded by X-ray powder diffraction (XRD) using Cu Ka radiation on a RIGAKU D/max 2200 vpc X-Ray Diffractometer. Their photo-luminescent spectra and luminescence decay curve were recorded on an FLS920 fluorescence spectrophotometer and a 450 W xenon lamp was used as excitation source. The emission spectrum of the red LED was recorded on Labsphere Inc., LED-1100. The XRD patterns of Na5Eu(MoO4)4 doped with different content of boron oxide are shown in Fig. 1.The XRD patterns are similar with JCPDS card 72-2158 [Na5La(MoO4)4, space group I41/a]. This reveals that the phosphors share the scheelite-related isostructure as Na5La(MoO4)4. The diffraction peaks of the phosphors show slight shift, compared with the JCPDS card. This is due to the distinct ionic radii between Eu3+ and La3+. The photo-luminescent spectra of NEMB0, NEMB2 and NEMB4 are shown in Fig. 2. The broad band from 200 to 310 nm is
Z. Wang et al. / Materials Research Bulletin 45 (2010) 240–242
Fig. 1. The XRD patterns of EMB0, NEMB2 and NEMB4.
attributable to the O ! Mo charge transfer (CT) transition. The sharp lines in 310–550 nm range are intra-configurational 4f–4f transitions of Eu3+ ions in the host lattices. The strongest excitation peak is at 395 nm, which is due to the 7F0 ! 5L6 transitions of Eu3+ ion. Curves b, d and f are the emission spectra under 395 nm excitation. The strongest emission peak is at 617 nm, which is due to the 5D0 ! 7F2 transition of Eu3+. This result implies that Eu3+ ions occupy the lattice sites without inversion symmetry, which is in good agreement with the results of its crystal structure [12]. Other transitions from the 5D0 excited level to 7FJ ground states are very weak, which is advantageous to obtain good CIE (Commission Internationale de l’Eclairage) chromaticity coordinates for phosphors. The sample NEMB2 shows the strongest emission, and the CIE values for the phosphor are calculated to be x = 0.673, y = 0.327 in terms of its emission spectrum, which is very close to the NTSC (National Television Standard Committee) standard values for red (x = 0.67, y = 0.33). The decay curve for 5D0 ! 7F2 (616 nm) of the Eu3+ of NEMB2 is shown in Fig. 3. The decay curve can be well fitted by a singleexponential function as I = A exp(t/t), and the values of lifetime of this phosphor is 0.549 ms. Fig. 4 shows the emission spectrum of the red light-emitting diode of near-UV InGaN-based the red phosphor NEMB2. The emission band at 395 nm is attributed to the emission of InGaN
241
Fig. 3. Decay curve of the Eu3+ emission of NEMB2 (lex = 395 nm, lem = 617 nm).
Fig. 4. The EL spectra of the single red LEDs-based NEMB2 under 20 mA current excitation.
chip, and the emission peaks at 617 and 702 nm are due to the emissions of red phosphor. Bright red light from the LED is observed by naked eyes. Its CIE chromaticity coordinates are calculated to be (x = 0.465, y = 0.222). The emission of InGaN chip (395 nm) can still be observed in Fig. 4, this is advantageous to obtain a white-emitting LED by combining this phosphor with appropriate blue and green phosphors. We think that each proper mono-color LED phosphor must meet the following necessary conditions from the standpoint of application. (1) The phosphor must efficiently be excited in nearUV range. But any mono-color phosphor cannot absorb all this energy; otherwise, other phosphor probably cannot be efficiently excited. (2) The phosphor exhibits higher luminescent intensity under 400 nm excitation. (3) The chromaticity coordinates of the phosphor are close to the NTSC standard values. In summary, red phosphor Na5Eu(MoO4)4 doped with d boron oxide, shows intense red emission under near-ultraviolet light, with high color-purity, it is considered to be a good candidate for the red component of near-ultraviolet white-light-emitting diodes. Acknowledgements
Fig. 2. The excitation (lem = 617 nm) and emission (lex = 395 nm) spectra of EMB0, NEMB2 and NEMB4.
This work was financially supported by a research grant from the Guangdong Province Government (ZB2003A07), and by the
242
Z. Wang et al. / Materials Research Bulletin 45 (2010) 240–242
Scientific Research Foundation for Young Teachers of Yunnan Nationalities University (0310-0229). References [1] S. Dalmasso, B. Damilano, C. Pernot, A. Dussaigne, D. Byrne, N. Grandjean, M. Leroux, J. Massies, Phys. Stat. Solidi A 192 (2002) 139. [2] J.H. Yum, S.Y. Seo, S. Lee, Y.E. Sung, J. Electrochem. Soc. 150 (2003) H47. [3] J.K. Sheu, S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, Y.C. Lin, W.C. Lai, J.M. Tsai, G.C. Chi, R.K. Wu, IEEE Photonics Technol. Lett. 15 (2003) 18. [4] C.H. Chiu, C.H. Liu, S.B. Huang, T.M. Chen, J. Electrochem. Soc. 155 (2008) J71.
[5] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [6] Y. Huh, J. Shim, Y. Kim, Y. Do, J. Electrochem. Soc. 150 (2003) H57. [7] Y. Huh, J. Park, S. Kweon, J. Kim, J. Kim, Y.R. Do, Bull. Korean Chem. Soc. 25 (2004) 1585. [8] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 52 (2005) H168. [9] Z.L. Wang, H.B. Liang, J. Wang, M.L. Gong, Q. Su, Appl. Phys. Lett. 89 (2006) 071921. [10] J. Niittykoski, T. Aitasalo, J. Ho¨lsa¨, H. Jungner, M. Lastusaari, M. Parkkinen, M. Tukia, J. Alloy Compd. 374 (2004) 108. [11] Z.L. Wang, H.B. Liang, Q.H. Zeng, M.L. Gong, Q. Su, J. Electrochem. Soc. 155 (2008) J375. [12] N.J. Stedman, A.K. Cheetham, P.D. Battle, J. Mater. Chem. 4 (1994) 707.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
New red phosphor for near-ultraviolet light-emitting diodes with high color-purity Zhengliang Wang a,*, Pei He b, Rui Wang b, Jishou Zhao a, Menglian Gong b a b
School of Chemistry and Biotechnology, Yunnan Nationalities University, Kunming, Yunnan 650031, PR China School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 August 2009 Received in revised form 30 September 2009 Accepted 14 October 2009 Available online 23 October 2009
New red phosphors, Na5Eu(MoO4)4 doped with boron oxide were prepared by the solid-state reaction. Their structure and photo-luminescent properties were investigated. With the introduction of boron oxide, the red emission intensity of the phosphors under 395 nm excitation is strengthened, with high color-purity (x = 0.673, y = 0.327). The single red light-emitting diode was obtained by combining InGaN chip with the red phosphor, bright red light can be observed by naked eyes from the red light-emitting diodes under a forward bias of 20 mA. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds C. X-ray diffraction D. Luminescence D. Optical properties
Recently growing interest was focused on semiconductor white-light-emitting diodes (W-LEDs), which have many advantages, such as high efficiency, long lifetime, low power consumption and environment-friendly characteristics [1,2]. In order to obtain a higher efficiency white LED with an appropriate color temperature and a higher color-rendering index, a new approach using near-ultraviolet (nUV) InGaN-based LED chip coated with blue/green/red tricolor phosphors was introduced [3,4]. Because of larger stocks shift, red phosphors exhibit lower efficiency, compared with blue and green phosphors. At present, the commonly used red-emitting phosphors for nUV InGaN-based white LEDs are mainly some sulfide phosphors (Y2O2S:Eu3+, CaS:Eu2+, SrY2S4:Eu2+) [5–7]. However, there are some disadvantages to use those sulfide-based materials, such as chemical instability, and low efficiency [8]. Therefore, it is important to explore new red phosphors that can be efficiently excited in the near-UV range around 400 nm with high efficiency, stability, and without environmental hazards. In our previous work [9], the luminescence of Na5La(MoO4)4:xEu3+ was reported, and the phosphor Na5Eu(MoO4)4 shows intense red emission, which maybe find application on near-UV white LEDs. It is well known that boron oxide is not only a good flux, but also able to replace some competent of the phosphors, and enhance the luminescent properties [10,11]. In this article, red
* Corresponding author. Tel.: +86 871 5910017; fax: +86 871 5910017. E-mail address: [email protected] (Z. Wang). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.10.005
phosphors Na5Eu(MoO4)4 doped with different content of boron oxide was prepared, and their photo-luminescent properties were investigated. At last, single red LED was fabricated by combining the red phosphor with 400 nm-emitting InGaN chips. The samples Na5Eu(MoO4)4 doped with different content of boron oxide were prepared with a solid-state reaction technique at high temperature. The starting materials are (NH4)6Mo7O244H2O (A.R. grade), NaHCO3 (A.R. grade), HBO3 (A.R. grade), and Eu2O3 (99.99% purity). The stoichiometric mixtures were ground and fired at 600 8C for 8 h. The samples with 0, 2 and 4 mol% boric acid additions (compared with four times of MoO42 content) were named as NEMB0, NEMB2 and NEMB4, respectively The structure of these phosphors was recorded by X-ray powder diffraction (XRD) using Cu Ka radiation on a RIGAKU D/max 2200 vpc X-Ray Diffractometer. Their photo-luminescent spectra and luminescence decay curve were recorded on an FLS920 fluorescence spectrophotometer and a 450 W xenon lamp was used as excitation source. The emission spectrum of the red LED was recorded on Labsphere Inc., LED-1100. The XRD patterns of Na5Eu(MoO4)4 doped with different content of boron oxide are shown in Fig. 1.The XRD patterns are similar with JCPDS card 72-2158 [Na5La(MoO4)4, space group I41/a]. This reveals that the phosphors share the scheelite-related isostructure as Na5La(MoO4)4. The diffraction peaks of the phosphors show slight shift, compared with the JCPDS card. This is due to the distinct ionic radii between Eu3+ and La3+. The photo-luminescent spectra of NEMB0, NEMB2 and NEMB4 are shown in Fig. 2. The broad band from 200 to 310 nm is
Z. Wang et al. / Materials Research Bulletin 45 (2010) 240–242
Fig. 1. The XRD patterns of EMB0, NEMB2 and NEMB4.
attributable to the O ! Mo charge transfer (CT) transition. The sharp lines in 310–550 nm range are intra-configurational 4f–4f transitions of Eu3+ ions in the host lattices. The strongest excitation peak is at 395 nm, which is due to the 7F0 ! 5L6 transitions of Eu3+ ion. Curves b, d and f are the emission spectra under 395 nm excitation. The strongest emission peak is at 617 nm, which is due to the 5D0 ! 7F2 transition of Eu3+. This result implies that Eu3+ ions occupy the lattice sites without inversion symmetry, which is in good agreement with the results of its crystal structure [12]. Other transitions from the 5D0 excited level to 7FJ ground states are very weak, which is advantageous to obtain good CIE (Commission Internationale de l’Eclairage) chromaticity coordinates for phosphors. The sample NEMB2 shows the strongest emission, and the CIE values for the phosphor are calculated to be x = 0.673, y = 0.327 in terms of its emission spectrum, which is very close to the NTSC (National Television Standard Committee) standard values for red (x = 0.67, y = 0.33). The decay curve for 5D0 ! 7F2 (616 nm) of the Eu3+ of NEMB2 is shown in Fig. 3. The decay curve can be well fitted by a singleexponential function as I = A exp(t/t), and the values of lifetime of this phosphor is 0.549 ms. Fig. 4 shows the emission spectrum of the red light-emitting diode of near-UV InGaN-based the red phosphor NEMB2. The emission band at 395 nm is attributed to the emission of InGaN
241
Fig. 3. Decay curve of the Eu3+ emission of NEMB2 (lex = 395 nm, lem = 617 nm).
Fig. 4. The EL spectra of the single red LEDs-based NEMB2 under 20 mA current excitation.
chip, and the emission peaks at 617 and 702 nm are due to the emissions of red phosphor. Bright red light from the LED is observed by naked eyes. Its CIE chromaticity coordinates are calculated to be (x = 0.465, y = 0.222). The emission of InGaN chip (395 nm) can still be observed in Fig. 4, this is advantageous to obtain a white-emitting LED by combining this phosphor with appropriate blue and green phosphors. We think that each proper mono-color LED phosphor must meet the following necessary conditions from the standpoint of application. (1) The phosphor must efficiently be excited in nearUV range. But any mono-color phosphor cannot absorb all this energy; otherwise, other phosphor probably cannot be efficiently excited. (2) The phosphor exhibits higher luminescent intensity under 400 nm excitation. (3) The chromaticity coordinates of the phosphor are close to the NTSC standard values. In summary, red phosphor Na5Eu(MoO4)4 doped with d boron oxide, shows intense red emission under near-ultraviolet light, with high color-purity, it is considered to be a good candidate for the red component of near-ultraviolet white-light-emitting diodes. Acknowledgements
Fig. 2. The excitation (lem = 617 nm) and emission (lex = 395 nm) spectra of EMB0, NEMB2 and NEMB4.
This work was financially supported by a research grant from the Guangdong Province Government (ZB2003A07), and by the
242
Z. Wang et al. / Materials Research Bulletin 45 (2010) 240–242
Scientific Research Foundation for Young Teachers of Yunnan Nationalities University (0310-0229). References [1] S. Dalmasso, B. Damilano, C. Pernot, A. Dussaigne, D. Byrne, N. Grandjean, M. Leroux, J. Massies, Phys. Stat. Solidi A 192 (2002) 139. [2] J.H. Yum, S.Y. Seo, S. Lee, Y.E. Sung, J. Electrochem. Soc. 150 (2003) H47. [3] J.K. Sheu, S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, Y.C. Lin, W.C. Lai, J.M. Tsai, G.C. Chi, R.K. Wu, IEEE Photonics Technol. Lett. 15 (2003) 18. [4] C.H. Chiu, C.H. Liu, S.B. Huang, T.M. Chen, J. Electrochem. Soc. 155 (2008) J71.
[5] S. Neeraj, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2. [6] Y. Huh, J. Shim, Y. Kim, Y. Do, J. Electrochem. Soc. 150 (2003) H57. [7] Y. Huh, J. Park, S. Kweon, J. Kim, J. Kim, Y.R. Do, Bull. Korean Chem. Soc. 25 (2004) 1585. [8] V. Sivakumar, U.V. Varadaraju, J. Electrochem. Soc. 52 (2005) H168. [9] Z.L. Wang, H.B. Liang, J. Wang, M.L. Gong, Q. Su, Appl. Phys. Lett. 89 (2006) 071921. [10] J. Niittykoski, T. Aitasalo, J. Ho¨lsa¨, H. Jungner, M. Lastusaari, M. Parkkinen, M. Tukia, J. Alloy Compd. 374 (2004) 108. [11] Z.L. Wang, H.B. Liang, Q.H. Zeng, M.L. Gong, Q. Su, J. Electrochem. Soc. 155 (2008) J375. [12] N.J. Stedman, A.K. Cheetham, P.D. Battle, J. Mater. Chem. 4 (1994) 707.