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Energy Express

  • Editor: Bernard Kippelen
  • Vol. 19, Iss. S2 — Mar. 14, 2011
  • pp: A201–A206
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Preparation, structural and luminescent properties of Ba2Gd2Si4O13:Eu3+ for white LEDs

Hai Guo, Hao Zhang, RongFei Wei, MengDi Zheng, and LiHong Zhang  »View Author Affiliations


Optics Express, Vol. 19, Issue S2, pp. A201-A206 (2011)
http://dx.doi.org/10.1364/OE.19.00A201


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Abstract

To find out efficient red phosphors used for white light-emitting diodes (LEDs), a new Ba2Gd2Si4O13:Eu3+ phosphor was prepared by conventional solid-state reaction method. The effect of Li2CO3 flux and Eu3+ doping concentrations on structural and luminescent properties of Ba2Gd2Si4O13 phosphors was studied in detail. The phosphors show intense absorption in near ultraviolet-blue region and exhibit intense red emissions with CIE coordinates of (0.66, 0.34) under 393 nm excitation. The integrated emission intensity of Ba2(Gd0.4Eu0.6)2Si4O13 excited at 393 nm, 362 nm and 464 nm is about 3.5, 4.0 and 3.1 times as that of Y2O3:Eu3+ commercial phosphors, respectively. The excellent luminescent properties and good color saturation make it a promising red phosphor for white LEDs.

© 2011 OSA

1. Introduction

Over the past few years, white light-emitting diodes (LEDs) has become an interesting field for many excellent characteristics, such as high efficiency, long lifetime, reliability, toxicity-free and energy-saving, etc [1

1. E. F. Schubert and J. K. Kim, “Solid-state light sources getting smart,” Science 308(5726), 1274–1278 (2005). [CrossRef] [PubMed]

3

3. S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma, and Q. Y. Zhang, “Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties,” Mater. Sci. Eng. Rep. 71(1), 1–34 (2010). [CrossRef]

]. In addition to three primary colors mixing emissions from three individual LED, white light can also be produced by coupling a blue or a near-ultraviolet (NUV) LED with a down-converting phosphor, much in the same way a fluorescent light bulb works. At present, the latter way has attracted more and more attention for its easy fabrication, low cost, and high brightness [3

3. S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma, and Q. Y. Zhang, “Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties,” Mater. Sci. Eng. Rep. 71(1), 1–34 (2010). [CrossRef]

5

5. H. Guo, X. Wang, X. Zhang, Y. Tang, L. Chen, and C. Ma, “Effect of NH4F flux on structural and luminescent properties of Sr2SiO4:Eu2+ phosphors prepared by solid-state reaction method,” J. Electrochem. Soc. 157(8), J310–J314 (2010). [CrossRef]

].

Nevertheless, commercial white LEDs based on the combination of a blue LED chip and a yellow-emitting phosphor Y3Al5O12:Ce3+ (YAG:Ce3+) are poor in the color rendering index (CRI) because of color deficiency in red region [6

6. T. W. Kuo, W. R. Liu, and T. M. Chen, “High color rendering white light-emitting-diode illuminator using the red-emitting Eu(2+)-activated CaZnOS phosphors excited by blue LED,” Opt. Express 18(8), 8187–8192 (2010). [CrossRef] [PubMed]

,7

7. S. Lee and S. Y. Seo, “Optimization of yttrium aluminum garnet:Ce3+ phosphors for white light-emitting diodes by combinatorial chemistry method,” J. Electrochem. Soc. 149(11), J85–J88 (2002). [CrossRef]

], which is not suitable for applications requiring high CRI properties, such as residential and medical lighting. Some red phosphors such as Y2O2S:Eu3+ and (Ca,Sr)S:Eu2+ were used to make up for this shortcoming. Unfortunately, these sulfide-based red phosphors are undesirable because of low chemical stability [8

8. Y. Zhou, J. Liu, X. Yang, X. Yu, and J. Zhuang, “A promising deep red phosphor AgLaMo2O8:Pr 3+ with blue excitation for white LED application,” J. Electrochem. Soc. 157(3), H278–H280 (2010). [CrossRef]

10

10. W. R. Liu, C. C. Lin, Y. C. Chiu, Y. T. Yeh, S. M. Jang, and R. S. Liu, “ZnB2O4:Bi3+,Eu3+:a highly efficient, red-emitting phosphor,” Opt. Express 18(3), 2946–2951 (2010). [CrossRef] [PubMed]

]. On the other hand, with the remarkable development of NUV diodes, the combination of an NUV chip with red, green and blue phosphors pave a valid way to generate warm white light in recent years [11

11. J. S. Kim, P. E. Jeon, Y. H. Park, J. C. Choi, H. L. Park, G. C. Kim, and T. W. Kim, “White-light generation through ultraviolet-emitting diode and white-emitting phosphor,” Appl. Phys. Lett. 85(17), 3696–3698 (2004). [CrossRef]

13

13. H. Guo, H. Zhang, J. Li, and F. Li, “Blue-white-green tunable luminescence from Ba2Gd2Si4O13:Ce3+,Tb3+ phosphors excited by ultraviolet light,” Opt. Express 18(26), 27257–27262 (2010). [CrossRef]

]. But the efficiency of red phosphor is much lower than that of green and blue phosphors [14

14. G. Gundiah, Y. Shimomura, N. Kijima, and A. K. Cheetham, “Novel red phosphors based on vanadate garnets for solid state lighting applications,” Chem. Phys. Lett. 455(4-6), 279–283 (2008). [CrossRef]

16

16. W. R. Liu, C. C. Lin, Y. C. Chiu, Y. T. Yeh, S. M. Jang, R. S. Liu, and B. M. Cheng, “Versatile phosphors BaY2Si3O10:RE (RE = Ce3+, Tb3+, Eu3+) for light-emitting diodes,” Opt. Express 17(20), 18103–18109 (2009). [CrossRef] [PubMed]

]. Therefore, the problem is still open as to develop novel red phosphors with high brightness for white LEDs.

In this paper, we synthesized Eu3+-doped Ba2Gd2Si4O13 (BGSO:Eu3+) powders by solid-state reaction method using Li2CO3 as flux. The influence of Li2CO3 flux on structural and luminescent properties of BGSO:Eu3+ was studied in detail. Furthermore, the integrated emission intensity was compared with the commercial Y2O3:Eu3+ phosphors. Results indicate that BGSO:Eu3+ phosphors can be effectively excited by blue and (or) NUV LEDs and exhibit satisfactory red emissions.

2. Experimental

Powder samples of BGSO:Eu3+ were synthesized by a conventional solid-state method. Gd2O3 (99.99%) and Eu2O3 (99.99%) were purchased from Shanghai YueLong Nonferrous Metals Co., Ltd., China. BaCO3, SiO2, H3BO3, NH4F, NH4Cl, Li2CO3 (all with purity of A.R.) were purchased from Sinopharm Chemical Reagent Co., Ltd., China.

To facilitate the reaction and to improve the crystallinity of luminescent materials, flux agents or molten salts are often added to provide a more interactive medium for solid-state reaction [5

5. H. Guo, X. Wang, X. Zhang, Y. Tang, L. Chen, and C. Ma, “Effect of NH4F flux on structural and luminescent properties of Sr2SiO4:Eu2+ phosphors prepared by solid-state reaction method,” J. Electrochem. Soc. 157(8), J310–J314 (2010). [CrossRef]

,19

19. X. Li, Z. Yang, L. Guan, C. Liu, and P. Li, “Luminescent properties of Eu3+-doped La2Mo2O9 red phosphor by the flux method,” J. Cryst. Growth 310(12), 3117–3120 (2008). [CrossRef]

]. The use of an interaction medium often results in lower reaction temperatures and allows for the optimization of grain size of luminophors being synthesized.

In this process, Li2CO3 was selected as flux by comparison with NH4F, NH4Cl and H3BO3. Stoichiometric amounts of reactants were first weighed out and well ground, then fired at 1100 °C for 4 h in air atmosphere. The resulting samples were cooled to room temperature and pulverized for further characterizations. For comparison, BGSO:Eu3+ phosphors fired at 1100 °C, 1200 °C and 1300 °C were also prepared without Li2CO3 flux.

The crystalline structures of the prepared powders were investigated by X-ray diffraction (XRD) on a Philips X’Pert PRO SUPER X-ray diffraction apparatus with Cu Kα radiation (λ = 0.154056 nm) as the incident radiation. Excitation spectra and emission spectra were measured by using an FS920 spectrofluorometer (Edinburgh Instruments) with a CW Xe lamp (450 W) as the excitation light source and an RR928P photomultiplier for signal detection. All the measurements were carried out at room temperature.

3. Results and discussion

BGSO is a new silicate structure that first reported by Wierzbicka et al. in 2010 [20

20. M. Wierzbicka-Wieczorek, U. Kolitsch, and E. Tillmanns, “Ba2Gd2(Si4O13): a silicate with finite Si4O13 chains,” Acta Crystallogr. C 66(3), i29–i32 (2010). [CrossRef] [PubMed]

]. It has a monoclinic structure with space group C2/c and lattice constants of a = 12.896(3) Å, b = 5.212(1) Å, c = 17.549(4) Å, β = 104.08(3) o, V = 1144.1(5) Å3, and Z = 4. The ionic radii of Ba2+ (CN = 8), Gd3+ (CN = 6), Si4+ (CN = 4), Eu3+ (CN = 6) are 1.42, 0.94, 0.26, and 0.95 Å, respectively. In view of the effective ionic radii of cations and different coordination numbers, Eu3+ dopants were expected to replace Gd3+ sites in BGSO.

Figure 1
Fig. 1 XRD pattern of BGSO:Eu3+ (60 mol%) powders (a) fired at different temperatures (1100 °C, 1200 °C and 1300 °C) without flux, (b) fired at 1100 °C with various Li2CO3 flux amounts (1 wt%, 2 wt%, 3 wt%, 5 wt%), as well as the calculated patterns from ICSD file of BGSO.
gives the XRD patterns of BGSO:Eu3+ (60 mol%) powders (a) fired at different temperatures ranged from 1100 °C to 1300 °C without flux and (b) fired at 1100 °C with various Li2CO3 flux amounts, as well as the calculated patterns from ICSD file of BGSO as a reference.

Li2CO3 flux plays an important effect on the structure of BGSO. Without Li2CO3 flux, BaSiO3 and Gd2O3 impurities were observed in XRD patterns for samples annealed below 1200 °C. Pure BGSO can be obtained for heating temperature above 1300 °C only. More interesting, for samples prepared at relative low temperature (1100 °C) by using Li2CO3 as a flux, the diffraction peaks match quite well with the calculated patterns, indicating that Li2CO3 flux reduced the firing temperature about 200 °C successfully. Besides, XRD patterns indicate that the optimal dosage of Li2CO3 flux is about 2 wt% for the preparation of pure BGSO and excess flux leads to the formation of an unintended LiGdSiO4 impurity (weak sharp peak at 31.6 o for samples prepared with 3, 5 wt% flux) during the reaction process.

Figure 2
Fig. 2 Emission spectra (λex = 393 nm) of BGSO:Eu3+ phosphors (a) fired at 1100 °C with 2 wt% Li2CO3 flux and (b) fired at 1300 °C without flux. The inset shows the integrated emission intensity of BGSO:Eu3+ powers fired at 1100 °C with various Li2CO3 flux amounts (0 wt%, 1 wt%, 2 wt%, 3 wt%, 5 wt%), as well as fired at 1300 °C without flux.
portrays the emission spectra of BGSO:Eu3+ phosphors (a) fired at 1100 °C with Li2CO3 flux (2 wt%) and (b) fired at 1300 °C without flux. Upon a NUV irradiation of 393 nm, both samples show characteristic red emissions of Eu3+ which can be assigned to the 5D0 to 7FJ (J = 0-4) transitions. It is well-known that 590 nm emission is associated with 5D0-7F1 magnetic dipole transition and 612 nm emission corresponds to 5D0-7F2 electric dipole transition. In this work, the dominant emission peak of BGSO:Eu3+ locate at 612 nm (5D0-7F2), indicating that Eu3+ ions occupied the sites of non-inversion symmetry in BGSO matrix. Consequently, the phosphor exhibits a red light with high color purity, which can be used to improve the color rendering property of white LEDs.

In addition, the detailed integrated emission intensity of BGSO:Eu3+ powers fired at 1100 °C with various Li2CO3 flux amounts (0 wt% to 5 wt%.), as well as the sample fired at 1300 °C without flux are shown directly in the inset of Fig. 2. The luminescent intensity increases with the content of Li2CO3 and reaches a maximum when the content of Li2CO3 is 2 wt%. With the help of Li2CO3 flux, compared with the sample synthesized at 1300 °C, the sample prepared at 1100 °C has a higher luminescent intensity. Such phenomena indicate that Li2CO3 flux not only reduced the firing temperature about 200 °C, but also improved the luminescent performance. The fluorescence enhancement also proves that Li2CO3 flux is helpful to enhance the crystallization degree and to decrease surface defects, agreeing well with the results obtained from XRD patterns.

To investigate the contribution of Eu3+ ions towards the luminescent properties of BGSO:Eu3+ phosphors, we synthesized BGSO powders with 2 wt% Li2CO3 flux by varying the concentration of Eu3+. The doping content of Eu3+ is labeled as x (x = 0.1-0.8). Luminescent properties are highly dependent on the activator concentration, and brightness tends to increase with increasing activator concentration. Figure 3
Fig. 3 Integrated emission intensity of BGSO:xEu3+ with various Eu3+ concentrations (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8).
presents the emission intensity as a function of Eu3+ concentration. When amount of Li2CO3 flux is fixed, with Eu3+ increasing, the intensity of Eu3+ emission increases rapidly first and reaches a maximum (x = 0.6) and then remarkably decreases when Eu3+ content is further increased.

This phenomenon is due to pairing or aggregation of activators at high concentration led to efficient resonant energy transfer between Eu3+ ions and a fraction of energy migration to distant luminescent killer or quencher followed by the appearance of quenching effect. According to Blasse’s energy transfer mechanism in oxide phosphors [21

21. G. Blasse, “Energy transfer in oxidic phosphors,” Phys. Lett. A 28(6), 444–445 (1968). [CrossRef]

], the critical transfer distance (Rc) can be calculated from the concentration quenching data by using the following equation:
Rc2(3V4πxcN)1/3,
(1)
where xc is the critical concentration, N is the number of host cations in the unit cell and V is the volume of the unit cell. In this case, V = 1144.1 Å3, N = 4 and xc = 0.6. Therefore, the Rc calculated was about 9.7 Å.

To probe the luminescent properties of BGSO:Eu3+ phosphors, optimized composition Ba2(Gd0.4Eu0.6)2Si4O13 phosphor was compared with commercial Y2O3:Eu3+ red phosphors by excitation and emission spectra.

Figure 5
Fig. 5 Emission spectra of BGSO:Eu3+ and commercial Y2O3:Eu3+, the inset shows the integrated emission intensity of BGSO:Eu3+ and commercial Y2O3:Eu3+.
shows the emission spectra of BGSO:Eu3+ and commercial Y2O3:Eu3+ phosphors (λex = 393 nm). Both samples show characteristic red emissions of Eu3+ which can be assigned to the 5D0 to 7FJ (J = 0-4) transitions. Obviously, the emission intensity of BGSO:Eu3+ phosphor is much stronger than that of Y2O3:Eu3+ phosphor. The integrated emission intensity of BGSO:Eu3+ phosphor is about 3.5 times stronger than that of Y2O3 excited at 393 nm, whereas 4 and 3.1 times better than that of Y2O3:Eu3+ excited at 362 and 464 nm, respectively (as shown in inset of Fig. 5). Hence, it is believed that the emission efficiency of BGSO:Eu3+ have meet the commercial requirement in solid-state lighting.

The luminescence colors of as-prepared BGSO:Eu3+ powers are characterized by Commission International de I’Eclairage (CIE) chromaticity coordinates. The chromaticity coordinate of the BGSO:Eu3+ and commercial Y2O3:Eu3+ phosphors are (0.66, 0.34) and (0.65, 0.35), respectively. The chromaticity coordinates of BGSO:Eu3+ phosphor are even closer to the standard of the National Television System Committee (NTSC) for red phosphor (0.67, 0.33) than that of Y2O3:Eu3+, indicating that the phosphor has a high color purity. The result proves that this phosphor can effectively improve the CRI of white LEDs.

4. Conclusion

A red-emitting phosphor Ba2Gd2Si4O13:Eu3+ was successfully prepared by solid-state reaction method by using Li2CO3 as flux. Li2CO3 flux can reduce the firing temperature about 200 °C, maintain the pure phase structure and enhance the luminescent performance. The integrated emission intensity of Ba2Gd2Si4O13:Eu3+ excited at 393 nm, 362 nm and 464 nm is about 3.5, 4.0 and 3.1 times as that of commercial red phosphor (Y2O3:Eu3+), respectively. The Ba2Gd2Si4O13:Eu3+ phosphors, with relative low synthesis temperature, good color saturation, strong absorption in both NUV and blue region, and excellent luminescent properties, are promising red emitting candidates for phosphor-converted white LEDs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 10904131).

References and Links

1.

E. F. Schubert and J. K. Kim, “Solid-state light sources getting smart,” Science 308(5726), 1274–1278 (2005). [CrossRef] [PubMed]

2.

S. Nakamura, T. Mukai, and M. Senoh, “Candela-class high-brightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes,” Appl. Phys. Lett. 64(13), 1687–1689 (1994). [CrossRef]

3.

S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma, and Q. Y. Zhang, “Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties,” Mater. Sci. Eng. Rep. 71(1), 1–34 (2010). [CrossRef]

4.

X. Piao, T. Horikawa, H. Hanzawa, and K. I. MacHida, “Characterization and luminescence properties of Sr2Si5N8:Eu2+ phosphor for white light-emitting-diode illumination,” Appl. Phys. Lett. 88(16), 161908 (2006). [CrossRef]

5.

H. Guo, X. Wang, X. Zhang, Y. Tang, L. Chen, and C. Ma, “Effect of NH4F flux on structural and luminescent properties of Sr2SiO4:Eu2+ phosphors prepared by solid-state reaction method,” J. Electrochem. Soc. 157(8), J310–J314 (2010). [CrossRef]

6.

T. W. Kuo, W. R. Liu, and T. M. Chen, “High color rendering white light-emitting-diode illuminator using the red-emitting Eu(2+)-activated CaZnOS phosphors excited by blue LED,” Opt. Express 18(8), 8187–8192 (2010). [CrossRef] [PubMed]

7.

S. Lee and S. Y. Seo, “Optimization of yttrium aluminum garnet:Ce3+ phosphors for white light-emitting diodes by combinatorial chemistry method,” J. Electrochem. Soc. 149(11), J85–J88 (2002). [CrossRef]

8.

Y. Zhou, J. Liu, X. Yang, X. Yu, and J. Zhuang, “A promising deep red phosphor AgLaMo2O8:Pr 3+ with blue excitation for white LED application,” J. Electrochem. Soc. 157(3), H278–H280 (2010). [CrossRef]

9.

X. Yang, J. Liu, H. Yang, X. Yu, Y. Guo, Y. Zhou, and J. Liu, “Synthesis and characterization of new red phosphors for white LED applications,” J. Mater. Chem. 19(22), 3771–3774 (2009). [CrossRef]

10.

W. R. Liu, C. C. Lin, Y. C. Chiu, Y. T. Yeh, S. M. Jang, and R. S. Liu, “ZnB2O4:Bi3+,Eu3+:a highly efficient, red-emitting phosphor,” Opt. Express 18(3), 2946–2951 (2010). [CrossRef] [PubMed]

11.

J. S. Kim, P. E. Jeon, Y. H. Park, J. C. Choi, H. L. Park, G. C. Kim, and T. W. Kim, “White-light generation through ultraviolet-emitting diode and white-emitting phosphor,” Appl. Phys. Lett. 85(17), 3696–3698 (2004). [CrossRef]

12.

H. Guo, X. Wang, J. Chen, and F. Li, “Ultraviolet light induced white light emission in Ag and Eu3+ co-doped oxyfluoride glasses,” Opt. Express 18(18), 18900–18905 (2010). [CrossRef] [PubMed]

13.

H. Guo, H. Zhang, J. Li, and F. Li, “Blue-white-green tunable luminescence from Ba2Gd2Si4O13:Ce3+,Tb3+ phosphors excited by ultraviolet light,” Opt. Express 18(26), 27257–27262 (2010). [CrossRef]

14.

G. Gundiah, Y. Shimomura, N. Kijima, and A. K. Cheetham, “Novel red phosphors based on vanadate garnets for solid state lighting applications,” Chem. Phys. Lett. 455(4-6), 279–283 (2008). [CrossRef]

15.

J. Zhang and Y. Wang, “Eu3+-doped Ba3Bi(PO4)3: A red phosphor for white light-emitting diodes,” Electrochem. Solid-State Lett. 13(4), J35–J37 (2010). [CrossRef]

16.

W. R. Liu, C. C. Lin, Y. C. Chiu, Y. T. Yeh, S. M. Jang, R. S. Liu, and B. M. Cheng, “Versatile phosphors BaY2Si3O10:RE (RE = Ce3+, Tb3+, Eu3+) for light-emitting diodes,” Opt. Express 17(20), 18103–18109 (2009). [CrossRef] [PubMed]

17.

H. Yang, G. Lakshminarayana, S. Zhou, Y. Teng, and J. Qiu, “Cyan-white-red luminescence from europium doped Al2O3-La2O3-SiO2 glasses,” Opt. Express 16(9), 6731–6735 (2008). [CrossRef] [PubMed]

18.

G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin) (1994).

19.

X. Li, Z. Yang, L. Guan, C. Liu, and P. Li, “Luminescent properties of Eu3+-doped La2Mo2O9 red phosphor by the flux method,” J. Cryst. Growth 310(12), 3117–3120 (2008). [CrossRef]

20.

M. Wierzbicka-Wieczorek, U. Kolitsch, and E. Tillmanns, “Ba2Gd2(Si4O13): a silicate with finite Si4O13 chains,” Acta Crystallogr. C 66(3), i29–i32 (2010). [CrossRef] [PubMed]

21.

G. Blasse, “Energy transfer in oxidic phosphors,” Phys. Lett. A 28(6), 444–445 (1968). [CrossRef]

OCIS Codes
(160.2540) Materials : Fluorescent and luminescent materials
(160.5690) Materials : Rare-earth-doped materials

ToC Category:
Spectroscopy

History
Original Manuscript: January 4, 2011
Revised Manuscript: February 28, 2011
Manuscript Accepted: March 1, 2011
Published: March 11, 2011

Citation
Hai Guo, Hao Zhang, RongFei Wei, MengDi Zheng, and LiHong Zhang, "Preparation, structural and luminescent properties of Ba2Gd2Si4O13:Eu3+ for white LEDs," Opt. Express 19, A201-A206 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-S2-A201


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References

  1. E. F. Schubert and J. K. Kim, “Solid-state light sources getting smart,” Science 308(5726), 1274–1278 (2005). [CrossRef] [PubMed]
  2. S. Nakamura, T. Mukai, and M. Senoh, “Candela-class high-brightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes,” Appl. Phys. Lett. 64(13), 1687–1689 (1994). [CrossRef]
  3. S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma, and Q. Y. Zhang, “Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties,” Mater. Sci. Eng. Rep. 71(1), 1–34 (2010). [CrossRef]
  4. X. Piao, T. Horikawa, H. Hanzawa, and K. I. MacHida, “Characterization and luminescence properties of Sr2Si5N8:Eu2+ phosphor for white light-emitting-diode illumination,” Appl. Phys. Lett. 88(16), 161908 (2006). [CrossRef]
  5. H. Guo, X. Wang, X. Zhang, Y. Tang, L. Chen, and C. Ma, “Effect of NH4F flux on structural and luminescent properties of Sr2SiO4:Eu2+ phosphors prepared by solid-state reaction method,” J. Electrochem. Soc. 157(8), J310–J314 (2010). [CrossRef]
  6. T. W. Kuo, W. R. Liu, and T. M. Chen, “High color rendering white light-emitting-diode illuminator using the red-emitting Eu(2+)-activated CaZnOS phosphors excited by blue LED,” Opt. Express 18(8), 8187–8192 (2010). [CrossRef] [PubMed]
  7. S. Lee and S. Y. Seo, “Optimization of yttrium aluminum garnet:Ce3+ phosphors for white light-emitting diodes by combinatorial chemistry method,” J. Electrochem. Soc. 149(11), J85–J88 (2002). [CrossRef]
  8. Y. Zhou, J. Liu, X. Yang, X. Yu, and J. Zhuang, “A promising deep red phosphor AgLaMo2O8:Pr 3+ with blue excitation for white LED application,” J. Electrochem. Soc. 157(3), H278–H280 (2010). [CrossRef]
  9. X. Yang, J. Liu, H. Yang, X. Yu, Y. Guo, Y. Zhou, and J. Liu, “Synthesis and characterization of new red phosphors for white LED applications,” J. Mater. Chem. 19(22), 3771–3774 (2009). [CrossRef]
  10. W. R. Liu, C. C. Lin, Y. C. Chiu, Y. T. Yeh, S. M. Jang, and R. S. Liu, “ZnB2O4:Bi3+,Eu3+:a highly efficient, red-emitting phosphor,” Opt. Express 18(3), 2946–2951 (2010). [CrossRef] [PubMed]
  11. J. S. Kim, P. E. Jeon, Y. H. Park, J. C. Choi, H. L. Park, G. C. Kim, and T. W. Kim, “White-light generation through ultraviolet-emitting diode and white-emitting phosphor,” Appl. Phys. Lett. 85(17), 3696–3698 (2004). [CrossRef]
  12. H. Guo, X. Wang, J. Chen, and F. Li, “Ultraviolet light induced white light emission in Ag and Eu3+ co-doped oxyfluoride glasses,” Opt. Express 18(18), 18900–18905 (2010). [CrossRef] [PubMed]
  13. H. Guo, H. Zhang, J. Li, and F. Li, “Blue-white-green tunable luminescence from Ba2Gd2Si4O13:Ce3+,Tb3+ phosphors excited by ultraviolet light,” Opt. Express 18(26), 27257–27262 (2010). [CrossRef]
  14. G. Gundiah, Y. Shimomura, N. Kijima, and A. K. Cheetham, “Novel red phosphors based on vanadate garnets for solid state lighting applications,” Chem. Phys. Lett. 455(4-6), 279–283 (2008). [CrossRef]
  15. J. Zhang and Y. Wang, “Eu3+-doped Ba3Bi(PO4)3: A red phosphor for white light-emitting diodes,” Electrochem. Solid-State Lett. 13(4), J35–J37 (2010). [CrossRef]
  16. W. R. Liu, C. C. Lin, Y. C. Chiu, Y. T. Yeh, S. M. Jang, R. S. Liu, and B. M. Cheng, “Versatile phosphors BaY2Si3O10:RE (RE = Ce3+, Tb3+, Eu3+) for light-emitting diodes,” Opt. Express 17(20), 18103–18109 (2009). [CrossRef] [PubMed]
  17. H. Yang, G. Lakshminarayana, S. Zhou, Y. Teng, and J. Qiu, “Cyan-white-red luminescence from europium doped Al2O3-La2O3-SiO2 glasses,” Opt. Express 16(9), 6731–6735 (2008). [CrossRef] [PubMed]
  18. G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin) (1994).
  19. X. Li, Z. Yang, L. Guan, C. Liu, and P. Li, “Luminescent properties of Eu3+-doped La2Mo2O9 red phosphor by the flux method,” J. Cryst. Growth 310(12), 3117–3120 (2008). [CrossRef]
  20. M. Wierzbicka-Wieczorek, U. Kolitsch, and E. Tillmanns, “Ba2Gd2(Si4O13): a silicate with finite Si4O13 chains,” Acta Crystallogr. C 66(3), i29–i32 (2010). [CrossRef] [PubMed]
  21. G. Blasse, “Energy transfer in oxidic phosphors,” Phys. Lett. A 28(6), 444–445 (1968). [CrossRef]

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