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Luminescent Properties of Sr4Si3O8Cl4:Eu2+, Bi3+ Phosphors for Near UV InGaN-Based Light-Emitting-Diodes

by 1,†, 2,† and 2,*
1
College of Chemistry and Chemical Engineering, Neijiang Normal University, Neijiang 641112, China
2
Key Laboratory of Comprehensive Utilization of Mineral Resources in Ethnic Regions, Joint Research Centre for International Cross-Border Ethnic Regions Biomass Clean Utilization in Yunnan, School of Chemistry & Environment, Yunnan Minzu University, Kunming 650500, China
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Academic Editor: Totaro Imasaka
Appl. Sci. 2015, 5(4), 1494-1502; https://doi.org/10.3390/app5041494
Received: 30 September 2015 / Revised: 27 October 2015 / Accepted: 23 November 2015 / Published: 1 December 2015
(This article belongs to the Section Optics and Lasers)

Abstract

Sr4Si3O8Cl4 co-doped with Eu2+, Bi3+ were prepared by the high temperature reaction. The structure and luminescent properties of Sr4Si3O8Cl4:Eu2+, Bi3+ were investigated. With the introduction of Bi3+, luminescent properties of these phosphors have been optimized. Compared with Sr3.90Si3O8Cl4:0.10Eu2+, the blue-green phosphor Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+ shows stronger blue-green emission with broader excitation in near-UV range. Bright blue-green light from the LED means this phosphor can be observed by the naked eye. Hence, it may have an application in near UV LED chips.
Keywords: luminescent properties; phosphors; light-emitting diodes; Sr4Si3O8Cl4 luminescent properties; phosphors; light-emitting diodes; Sr4Si3O8Cl4

1. Introduction

White light-emitting-diodes (LEDs) have more advantages, such as energy saving, long lifetime, environmental friendly and high efficiency, compared with fluorescent lamps [1,2,3]. At present, white LEDs can be obtained by combining near UV LED chips with blue/green/red tricolor phosphors, in order to obtain a higher efficiency white LED with appropriate color temperature and higher color-rendering index. This type of white LED is expected to dominate the market in the near future [4,5]. So it is important to find new tricolor phosphors for near UV LED chips.
The phosphors for near UV LED chips should exhibit intense broad emission with broad excitation band in near UV range. Compared with the phosphors activated by some trivalent rare earth ions (such as Eu3+), Eu2+ ions in many phosphors have broader emission with broad excitation band in near UV range [6,7,8,9], which are assigned to the 4f65d1 → 4f7 transitions. In order to strengthen and broaden the absorption of phosphors in near UV range, one important approach is to introduce sensitizer to strengthen the excitation band in the phosphor. We consider that Bi3+ is probably an eligible co-activator. On one hand, the introduction of Bi3+ will influence their sub-lattice structure around the luminescent center ion. On the other hand, Bi3+ is a very good sensitizer of luminescence in many hosts [10,11,12]. It can efficiently absorb the UV-light and transfer the energy to the luminescent center, then the excitation band would be broadened and the emission intensity of the luminescent center would be strengthened.
Alkaline earth halo-silicates are well known good hosts for inorganic luminescent materials, due to their low synthesis temperature and high luminescence efficiency [9]. Alkaline earth chlorosilicate Sr4Si3O8Cl4 can be served as the host of phosphors, which has an orthorhombic crystal structure [13]. The luminescent properties of Sr4Si3O8Cl4 doped with Eu2+ were firstly reported by Burrus et al. [13]. The luminescent properties of Sr4Si3O8Cl4 doped with Eu2+ were optimized by co-doping with some divalent metal ions (such as Ca2+, Mg2+, Zn2+) [14,15,16].
In this paper, Sr4Si3O8Cl4 co-doped with Eu2+, Bi3+ were prepared by the high temperature reaction in the reduction atmosphere. The luminescent properties of Sr4Si3O8Cl4:Eu2+, Bi3+ were investigated. Finally, single blue-green LED was fabricated by combining the blue-green phosphor with ~395 nm emitting LED chips.

2. Results and Discussion

The XRD patterns of Sr3.90Si3O8Cl4:0.10Eu2+ and Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+ are shown in Figure 1. They are almost consistent with the JCPDS card 40-0074 [Sr4Si3O8Cl4]. This indicates that the phosphor Sr4Si3O8Cl4:Eu2+ almost shares the same phase as Sr4Si3O8Cl4, except for a few peaks of SrSiO3 (marked by star shape). This result is in accordance with the references [15,16]. The impurity phase may be due to loss of chlorine content during the firing process. Eu2+ and Bi3+ maybe occupy the site of Sr2+ in an octahedral site, because the ion radii of Bi3+ (103 pm) and Eu2+ (117 pm) are close to that of Sr2+ (118 pm). Meanwhile, with the doping of Bi3+/Eu2+, a slight shift in the diffraction peaks toward higher angles can be found in Figure 1. This shift is due to the difference of ionic radius between Bi3+/Eu2+ and Sr2+.
Figure 2a is the excitation spectra of the phosphors Sr4−4xSi3O8Cl4:4xEu2+ (x = 0.015, 0.020, 0.025, 0.030) by monitoring emission at 490 nm. These four curves are of similar shapes. The broad excitation band from 230 nm to 430 nm is due to the 4f–5d transitions of Eu2+. These broad and intense excitation bands in near UV range match well with near-UV LED chip. When the content of Eu2+ is 0.10, the excitation intensity is the strongest.
The emission spectra of Sr4−4xSi3O8Cl4:4xEu2+ (x = 0.015, 0.020, 0.025, 0.030) under 320 nm excitation are shown in Figure 2b. The blue-green emission band is due to the 4f65d–4f7 transition of Eu2+. With the increase of the Eu2+ content, the intensity of the blue-green emission becomes higher. When the content of Eu2+ is 0.10, the emission intensity is the strongest.
Figure 1. X-ray powder diffraction (XRD) patterns of Sr3.90Si3O8Cl4:0.10Eu2+ and Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+. The Asterisk represents the peak of SrSiO3.
Figure 1. X-ray powder diffraction (XRD) patterns of Sr3.90Si3O8Cl4:0.10Eu2+ and Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+. The Asterisk represents the peak of SrSiO3.
Applsci 05 01494 g001
Figure 2. (a) Excitation spectra and (b) emission spectra of Sr4–4xSi3O8Cl4:4xEu2+ (x = 0.015, 0.020, 0.025, 0.030).
Figure 2. (a) Excitation spectra and (b) emission spectra of Sr4–4xSi3O8Cl4:4xEu2+ (x = 0.015, 0.020, 0.025, 0.030).
Applsci 05 01494 g002
Since the luminescent mechanism of Eu2+ is the 4f–5d allowed electric-dipole transition, the energy transfer process of Eu2+ in the Sr4Si3O8Cl4 phosphors would be attributed to an electric multipole-mutipole interaction. When the energy transfer occurs between the same sites of activators, the intensity of multipole interaction can be determined by the change of the emission intensity from the emitting level. According to the report of Van Uitert [17], the emission intensity (I) of per activator ion follows the equation [18,19]:
I χ = K [ 1 + β χ Q 3 ] 1
where I is the emission intensity, χ is the Eu2+ concentration, K and β are constants for the same excitation condition for a given host crystal. Q is a constant of multipole interaction equals to 3, 6, 8 or 10 for energy transfer among the nearest-neighbor ions, dipole-dipole (d–d), dipole-quadruple (d–q) or quadruple-quadruple (q–q) interaction, respectively. To get a Q value for the emission center, the plot of log (I/χ) as a function of log (χ) for the Sr4−4xSi3O8Cl4:4xEu2+ phosphors is plotted, as shown in Figure 3. It can be seen that a linear relation between log (I/χ) and log (χ) is found and the slope is −0.5299. The Q value can be obtained as 1.5897. That means the quenching is directly proportional to the activator ion concentration, which indicates that the concentration quenching is caused by the energy transfer among the nearest-neighbor Eu2+ ions in the Sr4−4xSi3O8Cl4: 4xEu2+ phosphors.
Figure 3. The plot of log (I/x) as a function of log (x) for the Sr4−4xSi3O8Cl4:4xEu2+phosphors (λex = 320 nm).
Figure 3. The plot of log (I/x) as a function of log (x) for the Sr4−4xSi3O8Cl4:4xEu2+phosphors (λex = 320 nm).
Applsci 05 01494 g003
The excitation spectra of the phosphors Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ (y = 0.00, 0.05, 0.10, 0.15, 0.20) by monitoring emission 490 nm is shown in Figure 4. With the doping of Bi3+, the shoulder peak around 400 nm is broadened with a little red shift, and the excitation intensity is enhanced. When the content of Bi3+ is 0.10, the excitation intensity is the strongest.
Figure 4. Excitation spectra of Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ (y = 0.00, 0.05, 0.10, 0.15, 0.20) by monitoring emission at 490 nm.
Figure 4. Excitation spectra of Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ (y = 0.00, 0.05, 0.10, 0.15, 0.20) by monitoring emission at 490 nm.
Applsci 05 01494 g004
Figure 5 is the emission spectra of Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ (y = 0.00, 0.05, 0.10, 0.15, 0.20) under 320 nm excitation. The broad emission from 400 nm to 600 nm is due to the 4f65d1 → 4f7 transition of Eu2+ ions. The strongest peak is located at 490 nm. The concentration dependence of the relative integral emission intensity of Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ is shown in the inserted figure of Figure 5. When the content of Bi3+ is at 0.10, its emission intensity is the strongest, and its intensity is about 1.3 times higher than that of Sr3.90Si3O8Cl4:0.10Eu2+ without Bi3+. This result is consistent with their excitation spectra. Bright blue-green light can be observed from Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+ under 365 nm light excitation (seeing the inset of Figure 5). The compositions Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ are not charge balanced and have a slight excess of positive charge because of Bi3+ substituting Sr2+. The excess charge could be compensated by a number of mechanisms, for example, cation vacancies, varying slightly O contents, etc. [20]. In this series of phosphors, this kind of substitution did not influence the emission spectra shapes of Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ (seeing Figure 5).
Figure 6 shows the electroluminescence (EL) spectra of the intense blue-green LED fabricated with Sr3.90Si3O8Cl4:0.10Eu2+ and Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+ under 20 mA current excitation. The emission band at ~400 nm is attributed to the emission of LED chip and the broad band from 430 nm to 600 nm is due to the emission of the phosphor. Comparing with curve a, the ratio of LED chip emission to phosphor emission in curve b is smaller. The result indicates phosphor Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+ can more efficiently absorb the emission of LED chip, and exhibit stronger blue-green emission, compared with Sr3.90Si3O8Cl4:0.10Eu2+. Bright blue-green light is observed from these two LEDs by the naked eye. This result is in accordance with their emission spectra. Their CIE chromaticity coordinates are calculated to be x = 0.169, y = 0.356, and x = 0.170, y = 0.362, respectively.
Figure 5. Emission spectra of Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ (y = 0.00, 0.05, 0.10, 0.15, 0.20) under 320 nm excitation, the inserted figures are the concentration dependence of the relative integral emission intensity of Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ and the image of Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+ under 365 nm excitation.
Figure 5. Emission spectra of Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ (y = 0.00, 0.05, 0.10, 0.15, 0.20) under 320 nm excitation, the inserted figures are the concentration dependence of the relative integral emission intensity of Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+ and the image of Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+ under 365 nm excitation.
Applsci 05 01494 g005
Figure 6. Electroluminescence (EL) spectra of the blue-green light-emitting-diodes (LEDs) based on (a) Sr3.90Si3O8Cl4:0.10Eu2+ and (b) Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+ under 20 mA current excitation; the inserted figures are the images of these two LEDs.
Figure 6. Electroluminescence (EL) spectra of the blue-green light-emitting-diodes (LEDs) based on (a) Sr3.90Si3O8Cl4:0.10Eu2+ and (b) Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+ under 20 mA current excitation; the inserted figures are the images of these two LEDs.
Applsci 05 01494 g006

3. Experimental Section

The phosphors Sr4Si3O8Cl4 doped with Eu2+ and Bi3+ were synthesized by a solid-state reaction. The raw materials SrCO3 (A.R. grade, Aladdin, Shanghai, China), SiO2 (A.R. grade, Aladdin, Shanghai, China), SrCl2·6H2O (A.R. grade, Aladdin, Shanghai, China), Bi2O3 (A.R. grade, Aladdin, Shanghai, China) and Eu2O3 (99.99% purity, Aladdin, Shanghai, China) were thoroughly mixed and ground together according to the given stoichiometric ratio. The mixture was first pre-fired at 400 °C for 2 h, and heated at 950 °C for 4 h. The process reduced CO atmosphere. The blue-green LED was fabricated by combing an LED chip (~395 nm) with the mixture of blue-green phosphor and epoxy resin (the ratio of mass is 1:1).
The structure of these phosphors was recorded by X-ray powder diffraction (XRD) using Cu Kα radiation on a RIGAKU D/max 2200 vpc X-ray Diffractometer (Rigaku Corporation, Osaka, Japan). Their photoluminescent spectra were recorded on a Cary Eclipse FL1011M003 (Varian Palo Alto, CA, USA) spectrofluorometer and the xenon lamp was used as excitation source. The electro-luminescence of blue-green LEDs was recorded on a high-accuracy array spectrometer (HSP6000, HongPu Optoelectronics Technology Co. Ltd., Hangzhou, China). All the measurements were performed at room temperature.

4. Conclusions

A series of phosphors, Sr4−4xSi3O8Cl4:4xEu2+, Sr3.90−4ySi3O8Cl4:0.10Eu2+, 4yBi3+, were prepared by solid state reaction at high temperature. Their structure and photo-luminescent properties were investigated. The doping of Bi3+ enhanced the excitation and emission intensity of these phosphors. The blue-green phosphor Sr3.50Si3O8Cl4:0.10Eu2+, 0.40Bi3+ exhibited intense blue-green emission with broader excitation in near-UV range. The LED based on this phosphor can emit intense blue-green light, which can be observed by the naked eye. Hence, it is considered to be a good candidate for the blue-green component of a white LED.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (21261027).

Author Contributions

Wangqing Shen and Yiwen Zhu performed the experiments. Zhengliang Wang designed and supervised the project, reviewed and contributed to the final manuscript. All authors contributed to the analysis and conclusion, and read the final paper.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Höppe, H.A. Recent developments in the field of inorganic phosphors. Angew. Chem. Int. Ed. 2009, 48, 3572–3582. [Google Scholar] [CrossRef] [PubMed]
  2. Tang, Y.S.; Hu, S.F.; Ke, W.C.; Lin, C.C.; Bagkar, N.C.; Liu, R.S. Near-ultraviolet excitable orange-yellow Sr3Al2O5Cl2:Eu2+ phosphor for potential application in light-emitting diodes. Appl. Phys. Lett. 2008, 93, 131114. [Google Scholar] [CrossRef]
  3. Zhang, R.; Lin, H.; Yu, Y.L.; Chen, D.Q.; Xu, J.; Wang, Y.S. A new-generation color converter for high-power white LED: Transparent Ce3+:YAG phosphor-in-glass. Laser Photonics Rev. 2014, 8, 158–164. [Google Scholar] [CrossRef]
  4. Zhou, J.; Xia, Z.G.; Yang, M.X.; Shen, K. High efficiency blue-emitting phosphor: Ce3+-doped Ca5.45Li3.55(SiO4)3O0.45F1.55 for near UV-pumped light-emitting diodes. J. Mater. Chem. 2012, 22, 21935–21941. [Google Scholar] [CrossRef]
  5. Mao, Z.Y.; Zhu, Y.C.; Gan, L.; Zeng, Y.; Xu, F.F.; Wang, Y.; Tian, H.; Li, J.; Wang, D.J. Tricolor emission Ca3Si2O7:Ln (Ln = Ce, Tb, Eu) phosphors for near-UV white light-emitting-diode. J. Lumin. 2013, 134, 148–153. [Google Scholar] [CrossRef]
  6. Dorenbos, P. Energy of the first 4f7→ 4f65d transition of Eu2+ in inorganic compounds. J. Lumin. 2003, 104, 239–260. [Google Scholar] [CrossRef]
  7. Sun, J.Y.; Zhang, X.Y.; Xia, Z.G.; Du, H.Y. Luminescent properties of LiBaPO4:RE (RE = Eu2+, Tb3+, Sm3+) phosphors for white light-emitting diodes. J. Appl. Phys. 2012, 111, 013101. [Google Scholar] [CrossRef]
  8. Liu, C.M.; Qi, Z.M.; Ma, C.G.; Dorenbos, P.; Hou, D.J.; Zhang, S.; Kuang, X.J.; Zhang, J.H.; Liang, H.B. High light yield of Sr8(Si4O12)Cl8:Eu2+ under X-ray excitation and its temperature-dependent luminescence characteristics. Chem. Mater. 2014, 26, 3709–3715. [Google Scholar] [CrossRef]
  9. Hao, Z.D.; Zhang, X.; Luo, Y.S.; Zhang, J.H. Preparation and photoluminescence properties of single-phase Ca2SiO3Cl2:Eu2+ bluish-green emitting phosphor. Mater. Lett. 2013, 93, 272–274. [Google Scholar] [CrossRef]
  10. Jia, W.; Perez-Andújar, A.; Rivera, I. Energy transfer between Bi3+ and Pr3+ in doped CaTiO3. J. Electrochem. Soc. 2003, 150, H161–H164. [Google Scholar] [CrossRef]
  11. Ye, S.; Wang, C.H.; Jing, X.P. Long wavelength extension of the Excitation band of LiEuMo2O8 phosphor with Bi3+ doping. J. Electrochem. Soc. 2009, 156, J121–J124. [Google Scholar] [CrossRef]
  12. Tian, L.H.; Yang, P.; Wu, H.; Li, F.Y. Luminescence properties of Y2WO6:Eu3+ incorporated with Mo6+ or Bi3+ ions as red phosphors for light-emitting diode applications. J. Lumin. 2010, 130, 717–721. [Google Scholar] [CrossRef]
  13. Burrus, H.L.; Nicholson, K.P. Fluorescence of Eu2+-activated alkaline earth halosilicates. J. Lumin. 1971, 3, 467–476. [Google Scholar] [CrossRef]
  14. Xia, Z.; Sun, J.; Du, H. Study on luminescence properties and crystal-lattice environment of Eu2+ in Sr4−xMgxSi3O8Cl4:Eu2+ phosphor. J. Rare Earths 2004, 22, 370–374. [Google Scholar]
  15. Zhang, X.G.; Zhou, F.X.; Shi, J.X.; Gong, M.L. Sr3.5Mg0.5Si3O8Cl4:Eu2+ bluish-green-emitting phosphor for NUV-based LED. Mater. Lett. 2009, 63, 852–854. [Google Scholar] [CrossRef]
  16. Zhang, S.H.; Hu, J.F.; Wang, J.J.; Li, Y.Y. Effects of Zn2+ doping on the structural and luminescent properties of Sr4Si3O8Cl4:Eu2+ phosphors. Mater. Lett. 2010, 64, 1376–1378. [Google Scholar] [CrossRef]
  17. Van Uitert, L.G. Characterization of energy transfer interactions between rare Eearth ions. J. Electrochem. Soc. 1967, 114, 1048–1053. [Google Scholar] [CrossRef]
  18. Lv, W.; Hao, Z.D.; Zhang, X.; Wang, X.J.; Zhang, J.H. Near UV and blue-based LED fabricated with Ca8Zn(SiO4)4Cl2:Eu2+ as green-emitting phosphor. Opt. Mater. 2011, 34, 261–264. [Google Scholar]
  19. Kim, S.W.; Zuo, Y.; Masui, T.; Imanaka, N. Synthesis of red-emitting Ca3−xEuxZrSi2O9 phosphors. ECS Solid State Lett. 2013, 2, R34–R36. [Google Scholar] [CrossRef]
  20. Park, S.H.; Yoon, H.S.; Boo, H.M.; Jang, H.G.; Lee, K.H.; Im, W.B. Efficiency and thermal stability enhancements of Sr2SiO4:Eu2+ phosphor via Bi3+ codoping for solid-state white lighting. J. Appl. Phys. 2012, 51, 022602. [Google Scholar] [CrossRef]
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