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Photoluminescence properties of Zn0.9Mg0.1S phosphors doped by europium and manganese ions
Journal of Alloys and Compounds 551 (2013) 711–714
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Photoluminescence properties of Zn0.9Mg0.1S phosphors doped by europium and manganese ions Yujun Liang a,b,⇑, Jiamin Wu a,b, Dongyan Yu a,b, Guogang Li b a b
Engineering Research Center of Nano-Geomaterials of Ministry of Education (China University of Geosciences), Wuhan 430074, PR China Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 10 September 2012 Received in revised form 5 November 2012 Accepted 6 November 2012 Available online 16 November 2012 Keywords: Phosphor Photoluminescence properties X-ray diffraction White emitting
a b s t r a c t The Zn0.9Mg0.1S:mEu2+, nMn2+ phosphors were synthesized by solid-state reactions via double-crucible method. The obtained phosphors exhibit two broad bands, the blue 460 nm emission band is originated from the 5d–4f transition of Eu2+ ions, while the yellow 570 nm emission band is originated from the 4 T1–6A1 transition of Mn2+ ions in Zn0.9Mg0.1S host, and show white color under an ultraviolet (UV) source by adjusting the ratio of Eu2+ and Mn2+ appropriately. The energy transfer from Eu2+ to Mn2+ is confirmed by the photoluminescence properties of phosphors and the decay life time measurements. The optimal composition of Zn0.9Mg0.1S:0.02Eu2+, 0.004Mn2+ generates warm white light with chromaticity coordinates (0.350, 0.385), indicating that Zn0.9Mg0.1S:mEu2+, nMn2+ phosphors would be a good single-phased white-emitting phosphor for full color display. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, single-phased white-emitting phosphors have attracted considerable attention because of their promising applications in the optoelectronics. Furthermore, it has been well known that Eu2+ may act as an efficient sensitizer transferring energy to Mn2+ in many host lattices [1–3]. The energy transfer from Eu2+ to Mn2+ (ETEu ? Mn) in luminescent materials has been studied extensively during the past few years [2,4,5]. Accordingly, the development of new and single-phased phosphor by co-doping Eu2+ and Mn2+ ions instead of combination of red, green, and blue phosphors has become one of the main researches for white-light ultraviolet light-emitting diodes (UVLEDs). Zinc sulfide (ZnS), a typical II–VI compound semiconductor, is an important optoelectronic device material because of its wide direct band gap (3.77 eV for the hexagonal wurtzite phase [6] and 3.72 eV for the cubic phase [7] at room temperature). However, the constant band gap of ZnS has limited its application in LEDs. As we all known, the band gap of MgS is 4.65 eV and the ZnS can form solid solution with MgS to some extent, so the band gap of ZnS can be adjusted by the addition of Mg2+ ions in the range of 3.72–4.65 eV. Besides, the addition of Mg2+ ions in ZnS can also causes change in emission energy of activator ion such as Mn2+ ions due to modification of host matrix [8]. ⇑ Corresponding author at: Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, China. Tel.: +86 27 67884814; fax: +86 27 67883731. E-mail address: [email protected] (Y. Liang). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.036
It is convincible that the emission peak energy shifts from green to red emission owing to weakening of crystal field effect caused by the increase of ionic radii with the increase of Mg content [9–12]. ZnS single-host phosphor could emit more than one emission band by codoping activators with f–d or d–d electron configurations, such as Cu2+/Mn2+ [13–15], the energy transfer (ET) would occur between activator/coactivator couples by effective resonant type via a multipolar interaction and the ET–Mn2+ can be of exchange interaction. Nevertheless, to the best of our knowledge, there is no work reported about the spectroscopic properties of Zn1 xMgxS co-doped with Eu and Mn. In this study, the ternary compound involving Zn0.9Mg0.1S was chosen as the host material and Eu2+, Mn2+ as activator ions to synthesize white phosphor by a double crucible method in conventional solid-state reaction. The optical properties of the phosphors and energy transfer mechanism among luminescent centers were investigated. Meanwhile, the doping concentrations of Eu2+ and Mn2+ ions in single-phased Zn0.9Mg0.1S phosphor were also discussed.
2. Experimental 2.1. Sample preparations Two series of single-phased phosphors with compositions of Zn0.9Mg0.1S:mEu2+, 2.0 mol%Mn2+ and Zn0.9Mg0.1S: 2.0 mol%Eu2+, nMn2+ (m = 0, 0.5, 1.0, 1.5, 2.0 mol%; n = 0, 0.1, 0.2, 0.3, 0.4, 0.5 mol%) were prepared via the conventional solid-state reaction by a double-crucible method [13,16]. The raw materials ZnCO3 (Analytically Reagent), Mg(OH)24MgCO35H2O (Analytically Reagent), S (99.99%),
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MnSO4H2O (99.99%), Eu2O3 (99.99%) and NH4Cl (Analytically Reagent) were mixed thoroughly in stoichiometric proportions, and then fired at 850 °C for 3 h under a starch reducing atmosphere to get the products.
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2.2. Characterization The crystal structure of the phosphor was examined by X-ray diffractometer (XRD) with Cu Ka (k = 1.5418 Å) radiation (XRD: Japan D/Max-3B). In order to investigate its luminescent properties, photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured on a fluorescence spectrometer (Jobin– Yvon Fluoromax-4p) using a 450-W xenon lamp as the excitation source at room temperature. The spectra were measured under identical conditions so that the intensities of the emissions may be compared. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Osilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation (Contimuum Sunlite OPO).
3. Results and discussion Cubic zinc blende or cubic with different hexagonal wurtzite phase can be observed in ZnxMg1 xS phosphors [8,17]. The XRD patterns of Mn (2.0 mol%), Eu (2.0 mol%) single-doped and co-doped Zn0.9Mg0.1S powders are shown in Fig. 1. Zn0.9Mg0.1 S:0.02Mn (Fig. 1a) and Zn0.9Mg0.1S:0.02Eu (Fig. 1b) exhibit a hexagonal structure (H) with less cubic phase (C), determined by comparing the diffraction intensity of the peaks indexed as H(1 0 0) and C(2 0 0). However, the diffraction peak intensity of the hexagonal phase increases in Zn0.9Mg0.1S:0.02Mn, 0.02Eu phosphor (Fig. 1c) compared to that of Zn0.9Mg0.1S:0.02Mn and Zn0.9Mg0.1S:0.02Eu. According to Dexter’s theory [18], the energy transfer rate is proportional to the spectral overlap between the energy donor emission band and the energy acceptor absorption band. Fig. 2 shows the excitation and emission spectra of the Zn0.9Mg0.1S:0.02Eu2+ and Zn0.9Mg0.1S:0.02Mn2+ phosphors. The emission spectrum of Zn0.9Mg0.1S:0.02Eu2+ (Fig. 2c) has two groups of emission located at 448 nm and a shoulder peak at 520 nm, which correspond to the transitions of 4f65d1–4f7 of Eu2+ and the host emission, respectively. The excitation spectrum of Zn0.9Mg0.1S:0.02Eu2+ (Fig. 2b) ranges from 250 to 400 nm with two peaks at 340 and 364 nm, which is attributed to the host absorption and Eu2+4f7–4f65d1 transition. The excitation spectrum of Zn0.9Mg0.1S:0.02Mn2+ (Fig. 2a) monitored at 572 nm exhibits five broad bands centered at 340, 392, 428, 468 and 498 nm. The 340 nm excitation band corresponds to the host absorption, while the bands peaked at 392, 428, 468, 498 nm are assigned to the transitions of Mn2+ ions from 6A1(6S) to 4E(4D), 4T2(4D) and [4A1(4G), 4E(4G)] levels, respectively [19,20]. The emission band
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Fig. 2. Photoluminescence excitation and emission spectra of Zn0.9Mg0.1S:0.02Eu2+ and Zn0.9Mg0.1S:0.02Mn2+ phosphors.
of Zn0.9Mg0.1S:0.02Mn2+ (Fig. 2d) centered at 572 nm is ascribed to the luminescence of Mn2+ owing to the transition from the lowest excited state 4T1(4G) to the ground state 6A1(6S) in the Zn2+ sites crystal field. According to the Fig. 2a and c, it is obvious that there is a large spectral overlap between the blue emission band of Eu2+ and the blue–green absorption band of Mn2+. Thus, the energy transfer from the band of Eu2+ to the absorption band of Mn2+ is possible in Zn0.9Mg0.1S host. In addition, the incorporation of Mg2+ in ZnS:Mn shifts the emission band of Mn2+ ions towards blue region for 20 nm in the comparison of that in the ZnS:Mn reported previously [21], which is caused by the distorted crystal field when 10 mol% Mg2+ ions are added into the host ZnS and replace Zn2+ sites. Fig. 3 shows the emission spectra of Zn0.9Mg0.1S:mEu2+, 0.02Mn2+ samples with the changes of Eu2+ concentrations in the range of 0–2.0 mol% and the excitation wavelength is 340 nm. All samples just show one yellow emission band corresponding to Mn2+ emitting light and no emission peak belonged to Eu2+ ions is appeared in Fig. 3. This means that the energy transfer from
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2Theta Degree (2θ) Fig. 1. X-ray diffraction patterns of (a) Zn0.9Mg0.1S:0.02Mn2+, (b) Zn0.9Mg0.1S:0.02Eu2+, and (c) Zn0.9Mg0.1S:0.02Eu2+, 0.02Mn2+ phosphors.
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wavelength (nm) Fig. 3. Emission spectra of Zn0.9Mg0.1S:mEu2+, 0.02Mn2+. Lines 1, 2, 3, 4 and 5 represent the emission spectra of the phosphor of Zn0.9Mg0.1S:mEu2+, 0.02Mn2+, and m has the value of 0, 0.5, 1.0, 1.5 and 2.0 mol%, respectively.
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100
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λex= 340 nm λem= 460 nm 80
n = 0.1, τ = 0.535 μs n = 0.2, τ = 0.487 μs n = 0.3, τ = 0.393 μs n = 0.4, τ = 0.372 μs n = 0.5, τ = 0.316 μs
PL Intensity (a.u.)
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Fig. 5. Decay curves for the samples of Zn0.9Mg0.1S:0.02Eu2+, nMn2+ (n = 0.1, 0.2, 0.3, 0.4, 0.5 mol%).
Fig. 4. Emission spectra of Zn0.9Mg0.1S:0.02Eu2+, nMn2+. Lines 1, 2, 3, 4 and 5 represent the emission spectra of the phosphor of Zn0.9Mg0.1S:0.02Eu2+, nMn2+, and n have the value of 0.1, 0.2, 0.3, 0.4 and 0.5 mol%, respectively. Table 1 Comparison of CIE chromaticity coordinates for Zn0.9Mg0.1S:mEu2+, nMn2+ phosphors.
Eu2+ centers to Mn2+ centers do exist in the Zn0.9Mg0.1S:mEu2+, 0.02Mn2+ samples and there are sufficient Mn2+ ions to absorb the blue emission of Eu2+ ions so that the blue emission of Eu2+ ions transfer to yellow-emitting of Mn2+ ions completely in these products. The reason for energy transfer from Eu2+ to Mn2+ ions has been explained in Fig. 2 that the 6A1(6S)–4E(4D), 4T2(4D) and [4A1(4G), 4E(4G)] level bands of Mn2+ excitation spectrum have a big overlap with the broad blue emission band of Eu2+ ions. In addition, it is apparent from Fig. 3 that the relative intensity of Mn2+ emission enhances with the increase of Eu2+ doping concentrations in Zn0.9Mg0.1S:mEu2+, 0.02Mn2+. The result indicate that the Eu2+ dopant has a sensitization effect on Mn2+ emitting resulting from the energy transfer from Eu2+ centers to Mn2+ centers. Fig. 4 shows the PL spectra of Zn0.9Mg0.1S:0.02Eu2+, nMn2+ phosphors with different Mn2+ doping contents. All spectra consist of two broad bands centered at about 460 and 570 nm attributed to the transitions of 4f65d1–4f7 of Eu2+ and 4T1(4G)–6A1(6S) of Mn2+, respectively. It is obvious that the PL emission intensity at 460 nm of Eu2+ decreases with increasing Mn2+ contents, and the emission intensity of Mn2+ at 570 nm is found to increase with rising Mn2+ contents, which further supports the occurrence of energy transfer from Eu2+ to Mn2+ in Zn0.9Mg0.1S:mEu2+, nMn2+ samples. As we all known, white light can be fabricated by combination of blue and yellow light. Therefore, the typical white light emission can be achieved by changing co-doping concentrations of Eu2+and Mn2+ ions in Zn0.9Mg0.1S:mEu2+, nMn2+ specimens. Besides, compared to the mixture of Eu2+ doped phosphor and Mn2+ doped phosphor, Eu2+ and Mn2+ co-doped phosphor has some advantages in the application. Firstly, it is difficult to control the particle sizes of Eu2+ doped phosphor and Mn2+ doped phosphor in the same level, which affects the final properties of products significantly. Secondly, Eu2+ doped phosphor and Mn2+ doped phosphor has reaborption with each other leading to decreased luminous efficiency [22]. Last but not the least, the fabrication of the co-doped product has lower cost than that of the mixture of single-doped products. Fig. 5 presents the decay curves of Zn0.9Mg0.1S:0.02Eu2+, nMn2+ (n = 0.1, 0.2, 0.3, 0.4, 0.5 mol%) samples. From all these decay curves, it is clearly observed that with increasing Mn2+ concentration in all the co-doped samples the life times of Eu2+ are decreased. From these decay life time measurements, the energy transfer from Eu2+ to Mn2+ is confirmed.
Zn0.9Mg0.1S:mEu2+ m
nMn2+ n
CIE (x, y)
0.02 0.02 0.02 0.02 0.02 0.02 0
0 0.001 0.002 0.003 0.004 0.005 0.02
0.150, 0.165, 0.275, 0.258, 0.350, 0.393, 0.467,
0.130 0.172 0.315 0.319 0.385 0.421 0.473
The Commission International de I’Eclairage (CIE) chromaticity coordinates for Zn0.9Mg0.1S:mEu2+, nMn2+ phosphors are measured and summarized in Table 1. It can be observed that the CIE coordinates of Zn0.9Mg0.1S:0.02Eu2+ is in the blue region of (x = 0.150, y = 0.130), and these of Zn0.9Mg0.1S:mEu2+, 0.02Mn2+ samples move to the yellow region of (x = 0.467, y = 0.473). For those of Zn0.9Mg0.1S:0.02Eu2+, nMn2+, the (x, y) coordinates vary systematically from (0.165, 0.172), (0.275, 0.315), (0.258, 0.319), (0.350, 0.385) to (0.393, 0.421). It presents directly that the color of luminescence can be changed from blue to white and yellowish white with different m and n values. The typical white light emission is shown at m = 2.0 mol% and n = 0.4 mol%, where the concentration proportion of europium and manganese is equal to 1:5. So, there is a possibility of tuning the chromaticity parameters of the Zn0.9Mg0.1S:mEu2+, nMn2+ phosphors by varying the concentration of Eu2+ or Mn2+ ions. This is very important for lighting applications where the chromaticity coordinate’s characterization in the CIE chromaticity diagram is indispensable. 4. Summary In summary, solid solutions of ZnS and MgS namely Zn0.9Mg0.1S codoped with Eu and Mn were successfully synthesized by solidstate reaction via double-crucible method at 850 °C. XRD patterns show that these phosphors are of wurtzite phase with a trace of sphalerite. The energy transfer is confirmed through the spectral overlap between energy donor and accepter. Upon excitation of 340 nm, Zn0.9Mg0.1S:Eu2+, Mn2+ phosphors show two emission bands around 460 nm (blue) ascribed to the allowed 4f–5d transition of Eu2+ ions and 570 nm (yellow) originated from 4 T1(4G)–6A1(6S) transition of Mn2+ ions. A tunable color emission
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can be obtained from Zn0.9Mg0.1S:Eu2+, Mn2+ phosphors by adjusting the dopant concentrations of Eu2+ and Mn2+ appropriately. Therefore, the white-emitting Zn0.9Mg0.1S:Eu2+, Mn2+ phosphor would be an attractive candidate as used in UV-chip white LEDs for general illumination. Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant No. 21171152), and by the Fundamental Research Funds for the Central Universities (No. CUG090108). References [1] Xiaofei Qu, Cao Lixin, Liu Wei, Ge Su, Pingping Wang, J. Alloys Comp. 487 (2009) 387. [2] Jong Su Kim, Kwon Taek Lim, Yong Seok Jeong, Pyung Eun Jeon, Jin Chul Choi, Hong Lee Park, Solid State Commun. 135 (2005) 21. [3] Jong Su Kim, Ae Kyung Kwon, Yun Hyung Park, Jin Chul Choi, Hong Lee Park, Gwang Chul Kim, J. Lumin. (2007) 122–123. 583–586. [4] W.-J. Yang, L. Luo, T.-M. Chen, N.-S. Wang, Chem. Mater. 17 (2005) 3883. [5] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931. [6] H.C. Ong, R.P.H. Chang, Appl. Phys. Lett. 79 (2001) 3612.
[7] T.K. Tran, W. Park, W. Tong, M.M. Kyi, B.K. Wagner, C.J. Summers, J. Appl. Phys. 81 (1977) 2803. [8] Dong Chin Lee, Jong Hyuk Kang, Duk Young Jeon, Sun Jin Yun, J. Lumin. 126 (2007) 711. [9] M.K. Jayaraj, Aldrin Antony, P. Deneshan, Thin Solid Films 389 (2001) 284. [10] R. Inoue, M. Kitagawa, Y. Horii, S. Kinba, T. Nishigaki, K. Ichino, S. Tanaka, H. Kobayashi, in: Ninth International Conference on II–VI Compounds, Kyoto, Japan, 1999, p. 140. [11] R. Inoue, M. Kitagawa, Y. Horii, T. Nishigaki, S. Kinba, K. Ichino, S. Tanaka, H. Kobayashi, J. Lumin. 1264 (2000) 87–89. [12] Yoshinori Horii, Masahiko Kitagawa, Hiroki Taneoka, Hiroyuki Kusano, Tomoko Murakami, Yuichi Hino, Hiroshi Kobayashi, Mater. Sci. Eng. B 85 (2001) 92. [13] Wendeng Wang, Fuqiang Huang, Yujuan Xia, Anbao Wang, J. Lumin. 128 (2008) 610. [14] H. Lu, S. Chu, J. Cryst. Growth 265 (2004) 476. [15] Je Hong Park, Sung Hun Lee, Jong Su Kim, Ae Kyung Kwon, Hong Lee Park, Sang Do Han, J. Lumin. (2007) 126–566. [16] Yu-Feng Lin, Yen-Hwei Chang, Bin-Siang Tsai, J. Alloys Comp. 377 (2004) 277. [17] H. Kimijima, M. Kitagawa, R. Inoue, N. Shiraishi, M. Hoashi, K. Ichino, H. Kobayashi, Appl. Surf. Sci. 432 (1997) 113–114. [18] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [19] K.-S. Sohn, B. Cho, H.D. Park, Mater. Lett. 41 (1999) 303. [20] D.T. Palumbo, J.J. Brown, J. Electrochem. Soc.: Solid State Sci. 117 (1970) 1184. [21] Hua Yang, Jingzhe Zhao, Lizhu Song, Lianchun Shen, Zichen Wang, Li Wang, Dong Zhang, Mater. Lett. 57 (2003) 2287. [22] J.S. Kim, J.Y. Kang, P.E. Jeon, J.C. Chol, H.L. Park, T.W. Kim, Jpn. J. Appl. Phys. 43 (2004) 989.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Photoluminescence properties of Zn0.9Mg0.1S phosphors doped by europium and manganese ions Yujun Liang a,b,⇑, Jiamin Wu a,b, Dongyan Yu a,b, Guogang Li b a b
Engineering Research Center of Nano-Geomaterials of Ministry of Education (China University of Geosciences), Wuhan 430074, PR China Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 10 September 2012 Received in revised form 5 November 2012 Accepted 6 November 2012 Available online 16 November 2012 Keywords: Phosphor Photoluminescence properties X-ray diffraction White emitting
a b s t r a c t The Zn0.9Mg0.1S:mEu2+, nMn2+ phosphors were synthesized by solid-state reactions via double-crucible method. The obtained phosphors exhibit two broad bands, the blue 460 nm emission band is originated from the 5d–4f transition of Eu2+ ions, while the yellow 570 nm emission band is originated from the 4 T1–6A1 transition of Mn2+ ions in Zn0.9Mg0.1S host, and show white color under an ultraviolet (UV) source by adjusting the ratio of Eu2+ and Mn2+ appropriately. The energy transfer from Eu2+ to Mn2+ is confirmed by the photoluminescence properties of phosphors and the decay life time measurements. The optimal composition of Zn0.9Mg0.1S:0.02Eu2+, 0.004Mn2+ generates warm white light with chromaticity coordinates (0.350, 0.385), indicating that Zn0.9Mg0.1S:mEu2+, nMn2+ phosphors would be a good single-phased white-emitting phosphor for full color display. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, single-phased white-emitting phosphors have attracted considerable attention because of their promising applications in the optoelectronics. Furthermore, it has been well known that Eu2+ may act as an efficient sensitizer transferring energy to Mn2+ in many host lattices [1–3]. The energy transfer from Eu2+ to Mn2+ (ETEu ? Mn) in luminescent materials has been studied extensively during the past few years [2,4,5]. Accordingly, the development of new and single-phased phosphor by co-doping Eu2+ and Mn2+ ions instead of combination of red, green, and blue phosphors has become one of the main researches for white-light ultraviolet light-emitting diodes (UVLEDs). Zinc sulfide (ZnS), a typical II–VI compound semiconductor, is an important optoelectronic device material because of its wide direct band gap (3.77 eV for the hexagonal wurtzite phase [6] and 3.72 eV for the cubic phase [7] at room temperature). However, the constant band gap of ZnS has limited its application in LEDs. As we all known, the band gap of MgS is 4.65 eV and the ZnS can form solid solution with MgS to some extent, so the band gap of ZnS can be adjusted by the addition of Mg2+ ions in the range of 3.72–4.65 eV. Besides, the addition of Mg2+ ions in ZnS can also causes change in emission energy of activator ion such as Mn2+ ions due to modification of host matrix [8]. ⇑ Corresponding author at: Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, China. Tel.: +86 27 67884814; fax: +86 27 67883731. E-mail address: [email protected] (Y. Liang). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.036
It is convincible that the emission peak energy shifts from green to red emission owing to weakening of crystal field effect caused by the increase of ionic radii with the increase of Mg content [9–12]. ZnS single-host phosphor could emit more than one emission band by codoping activators with f–d or d–d electron configurations, such as Cu2+/Mn2+ [13–15], the energy transfer (ET) would occur between activator/coactivator couples by effective resonant type via a multipolar interaction and the ET–Mn2+ can be of exchange interaction. Nevertheless, to the best of our knowledge, there is no work reported about the spectroscopic properties of Zn1 xMgxS co-doped with Eu and Mn. In this study, the ternary compound involving Zn0.9Mg0.1S was chosen as the host material and Eu2+, Mn2+ as activator ions to synthesize white phosphor by a double crucible method in conventional solid-state reaction. The optical properties of the phosphors and energy transfer mechanism among luminescent centers were investigated. Meanwhile, the doping concentrations of Eu2+ and Mn2+ ions in single-phased Zn0.9Mg0.1S phosphor were also discussed.
2. Experimental 2.1. Sample preparations Two series of single-phased phosphors with compositions of Zn0.9Mg0.1S:mEu2+, 2.0 mol%Mn2+ and Zn0.9Mg0.1S: 2.0 mol%Eu2+, nMn2+ (m = 0, 0.5, 1.0, 1.5, 2.0 mol%; n = 0, 0.1, 0.2, 0.3, 0.4, 0.5 mol%) were prepared via the conventional solid-state reaction by a double-crucible method [13,16]. The raw materials ZnCO3 (Analytically Reagent), Mg(OH)24MgCO35H2O (Analytically Reagent), S (99.99%),
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Y. Liang et al. / Journal of Alloys and Compounds 551 (2013) 711–714
MnSO4H2O (99.99%), Eu2O3 (99.99%) and NH4Cl (Analytically Reagent) were mixed thoroughly in stoichiometric proportions, and then fired at 850 °C for 3 h under a starch reducing atmosphere to get the products.
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2.2. Characterization The crystal structure of the phosphor was examined by X-ray diffractometer (XRD) with Cu Ka (k = 1.5418 Å) radiation (XRD: Japan D/Max-3B). In order to investigate its luminescent properties, photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured on a fluorescence spectrometer (Jobin– Yvon Fluoromax-4p) using a 450-W xenon lamp as the excitation source at room temperature. The spectra were measured under identical conditions so that the intensities of the emissions may be compared. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Osilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation (Contimuum Sunlite OPO).
3. Results and discussion Cubic zinc blende or cubic with different hexagonal wurtzite phase can be observed in ZnxMg1 xS phosphors [8,17]. The XRD patterns of Mn (2.0 mol%), Eu (2.0 mol%) single-doped and co-doped Zn0.9Mg0.1S powders are shown in Fig. 1. Zn0.9Mg0.1 S:0.02Mn (Fig. 1a) and Zn0.9Mg0.1S:0.02Eu (Fig. 1b) exhibit a hexagonal structure (H) with less cubic phase (C), determined by comparing the diffraction intensity of the peaks indexed as H(1 0 0) and C(2 0 0). However, the diffraction peak intensity of the hexagonal phase increases in Zn0.9Mg0.1S:0.02Mn, 0.02Eu phosphor (Fig. 1c) compared to that of Zn0.9Mg0.1S:0.02Mn and Zn0.9Mg0.1S:0.02Eu. According to Dexter’s theory [18], the energy transfer rate is proportional to the spectral overlap between the energy donor emission band and the energy acceptor absorption band. Fig. 2 shows the excitation and emission spectra of the Zn0.9Mg0.1S:0.02Eu2+ and Zn0.9Mg0.1S:0.02Mn2+ phosphors. The emission spectrum of Zn0.9Mg0.1S:0.02Eu2+ (Fig. 2c) has two groups of emission located at 448 nm and a shoulder peak at 520 nm, which correspond to the transitions of 4f65d1–4f7 of Eu2+ and the host emission, respectively. The excitation spectrum of Zn0.9Mg0.1S:0.02Eu2+ (Fig. 2b) ranges from 250 to 400 nm with two peaks at 340 and 364 nm, which is attributed to the host absorption and Eu2+4f7–4f65d1 transition. The excitation spectrum of Zn0.9Mg0.1S:0.02Mn2+ (Fig. 2a) monitored at 572 nm exhibits five broad bands centered at 340, 392, 428, 468 and 498 nm. The 340 nm excitation band corresponds to the host absorption, while the bands peaked at 392, 428, 468, 498 nm are assigned to the transitions of Mn2+ ions from 6A1(6S) to 4E(4D), 4T2(4D) and [4A1(4G), 4E(4G)] levels, respectively [19,20]. The emission band
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Fig. 2. Photoluminescence excitation and emission spectra of Zn0.9Mg0.1S:0.02Eu2+ and Zn0.9Mg0.1S:0.02Mn2+ phosphors.
of Zn0.9Mg0.1S:0.02Mn2+ (Fig. 2d) centered at 572 nm is ascribed to the luminescence of Mn2+ owing to the transition from the lowest excited state 4T1(4G) to the ground state 6A1(6S) in the Zn2+ sites crystal field. According to the Fig. 2a and c, it is obvious that there is a large spectral overlap between the blue emission band of Eu2+ and the blue–green absorption band of Mn2+. Thus, the energy transfer from the band of Eu2+ to the absorption band of Mn2+ is possible in Zn0.9Mg0.1S host. In addition, the incorporation of Mg2+ in ZnS:Mn shifts the emission band of Mn2+ ions towards blue region for 20 nm in the comparison of that in the ZnS:Mn reported previously [21], which is caused by the distorted crystal field when 10 mol% Mg2+ ions are added into the host ZnS and replace Zn2+ sites. Fig. 3 shows the emission spectra of Zn0.9Mg0.1S:mEu2+, 0.02Mn2+ samples with the changes of Eu2+ concentrations in the range of 0–2.0 mol% and the excitation wavelength is 340 nm. All samples just show one yellow emission band corresponding to Mn2+ emitting light and no emission peak belonged to Eu2+ ions is appeared in Fig. 3. This means that the energy transfer from
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2Theta Degree (2θ) Fig. 1. X-ray diffraction patterns of (a) Zn0.9Mg0.1S:0.02Mn2+, (b) Zn0.9Mg0.1S:0.02Eu2+, and (c) Zn0.9Mg0.1S:0.02Eu2+, 0.02Mn2+ phosphors.
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wavelength (nm) Fig. 3. Emission spectra of Zn0.9Mg0.1S:mEu2+, 0.02Mn2+. Lines 1, 2, 3, 4 and 5 represent the emission spectra of the phosphor of Zn0.9Mg0.1S:mEu2+, 0.02Mn2+, and m has the value of 0, 0.5, 1.0, 1.5 and 2.0 mol%, respectively.
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n = 0.1, τ = 0.535 μs n = 0.2, τ = 0.487 μs n = 0.3, τ = 0.393 μs n = 0.4, τ = 0.372 μs n = 0.5, τ = 0.316 μs
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Fig. 5. Decay curves for the samples of Zn0.9Mg0.1S:0.02Eu2+, nMn2+ (n = 0.1, 0.2, 0.3, 0.4, 0.5 mol%).
Fig. 4. Emission spectra of Zn0.9Mg0.1S:0.02Eu2+, nMn2+. Lines 1, 2, 3, 4 and 5 represent the emission spectra of the phosphor of Zn0.9Mg0.1S:0.02Eu2+, nMn2+, and n have the value of 0.1, 0.2, 0.3, 0.4 and 0.5 mol%, respectively. Table 1 Comparison of CIE chromaticity coordinates for Zn0.9Mg0.1S:mEu2+, nMn2+ phosphors.
Eu2+ centers to Mn2+ centers do exist in the Zn0.9Mg0.1S:mEu2+, 0.02Mn2+ samples and there are sufficient Mn2+ ions to absorb the blue emission of Eu2+ ions so that the blue emission of Eu2+ ions transfer to yellow-emitting of Mn2+ ions completely in these products. The reason for energy transfer from Eu2+ to Mn2+ ions has been explained in Fig. 2 that the 6A1(6S)–4E(4D), 4T2(4D) and [4A1(4G), 4E(4G)] level bands of Mn2+ excitation spectrum have a big overlap with the broad blue emission band of Eu2+ ions. In addition, it is apparent from Fig. 3 that the relative intensity of Mn2+ emission enhances with the increase of Eu2+ doping concentrations in Zn0.9Mg0.1S:mEu2+, 0.02Mn2+. The result indicate that the Eu2+ dopant has a sensitization effect on Mn2+ emitting resulting from the energy transfer from Eu2+ centers to Mn2+ centers. Fig. 4 shows the PL spectra of Zn0.9Mg0.1S:0.02Eu2+, nMn2+ phosphors with different Mn2+ doping contents. All spectra consist of two broad bands centered at about 460 and 570 nm attributed to the transitions of 4f65d1–4f7 of Eu2+ and 4T1(4G)–6A1(6S) of Mn2+, respectively. It is obvious that the PL emission intensity at 460 nm of Eu2+ decreases with increasing Mn2+ contents, and the emission intensity of Mn2+ at 570 nm is found to increase with rising Mn2+ contents, which further supports the occurrence of energy transfer from Eu2+ to Mn2+ in Zn0.9Mg0.1S:mEu2+, nMn2+ samples. As we all known, white light can be fabricated by combination of blue and yellow light. Therefore, the typical white light emission can be achieved by changing co-doping concentrations of Eu2+and Mn2+ ions in Zn0.9Mg0.1S:mEu2+, nMn2+ specimens. Besides, compared to the mixture of Eu2+ doped phosphor and Mn2+ doped phosphor, Eu2+ and Mn2+ co-doped phosphor has some advantages in the application. Firstly, it is difficult to control the particle sizes of Eu2+ doped phosphor and Mn2+ doped phosphor in the same level, which affects the final properties of products significantly. Secondly, Eu2+ doped phosphor and Mn2+ doped phosphor has reaborption with each other leading to decreased luminous efficiency [22]. Last but not the least, the fabrication of the co-doped product has lower cost than that of the mixture of single-doped products. Fig. 5 presents the decay curves of Zn0.9Mg0.1S:0.02Eu2+, nMn2+ (n = 0.1, 0.2, 0.3, 0.4, 0.5 mol%) samples. From all these decay curves, it is clearly observed that with increasing Mn2+ concentration in all the co-doped samples the life times of Eu2+ are decreased. From these decay life time measurements, the energy transfer from Eu2+ to Mn2+ is confirmed.
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0.150, 0.165, 0.275, 0.258, 0.350, 0.393, 0.467,
0.130 0.172 0.315 0.319 0.385 0.421 0.473
The Commission International de I’Eclairage (CIE) chromaticity coordinates for Zn0.9Mg0.1S:mEu2+, nMn2+ phosphors are measured and summarized in Table 1. It can be observed that the CIE coordinates of Zn0.9Mg0.1S:0.02Eu2+ is in the blue region of (x = 0.150, y = 0.130), and these of Zn0.9Mg0.1S:mEu2+, 0.02Mn2+ samples move to the yellow region of (x = 0.467, y = 0.473). For those of Zn0.9Mg0.1S:0.02Eu2+, nMn2+, the (x, y) coordinates vary systematically from (0.165, 0.172), (0.275, 0.315), (0.258, 0.319), (0.350, 0.385) to (0.393, 0.421). It presents directly that the color of luminescence can be changed from blue to white and yellowish white with different m and n values. The typical white light emission is shown at m = 2.0 mol% and n = 0.4 mol%, where the concentration proportion of europium and manganese is equal to 1:5. So, there is a possibility of tuning the chromaticity parameters of the Zn0.9Mg0.1S:mEu2+, nMn2+ phosphors by varying the concentration of Eu2+ or Mn2+ ions. This is very important for lighting applications where the chromaticity coordinate’s characterization in the CIE chromaticity diagram is indispensable. 4. Summary In summary, solid solutions of ZnS and MgS namely Zn0.9Mg0.1S codoped with Eu and Mn were successfully synthesized by solidstate reaction via double-crucible method at 850 °C. XRD patterns show that these phosphors are of wurtzite phase with a trace of sphalerite. The energy transfer is confirmed through the spectral overlap between energy donor and accepter. Upon excitation of 340 nm, Zn0.9Mg0.1S:Eu2+, Mn2+ phosphors show two emission bands around 460 nm (blue) ascribed to the allowed 4f–5d transition of Eu2+ ions and 570 nm (yellow) originated from 4 T1(4G)–6A1(6S) transition of Mn2+ ions. A tunable color emission
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can be obtained from Zn0.9Mg0.1S:Eu2+, Mn2+ phosphors by adjusting the dopant concentrations of Eu2+ and Mn2+ appropriately. Therefore, the white-emitting Zn0.9Mg0.1S:Eu2+, Mn2+ phosphor would be an attractive candidate as used in UV-chip white LEDs for general illumination. Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant No. 21171152), and by the Fundamental Research Funds for the Central Universities (No. CUG090108). References [1] Xiaofei Qu, Cao Lixin, Liu Wei, Ge Su, Pingping Wang, J. Alloys Comp. 487 (2009) 387. [2] Jong Su Kim, Kwon Taek Lim, Yong Seok Jeong, Pyung Eun Jeon, Jin Chul Choi, Hong Lee Park, Solid State Commun. 135 (2005) 21. [3] Jong Su Kim, Ae Kyung Kwon, Yun Hyung Park, Jin Chul Choi, Hong Lee Park, Gwang Chul Kim, J. Lumin. (2007) 122–123. 583–586. [4] W.-J. Yang, L. Luo, T.-M. Chen, N.-S. Wang, Chem. Mater. 17 (2005) 3883. [5] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, S.I. Mho, G.C. Kim, Appl. Phys. Lett. 84 (2004) 2931. [6] H.C. Ong, R.P.H. Chang, Appl. Phys. Lett. 79 (2001) 3612.
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