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Sm3+ codoped oxyfluoride aluminosilicate glasses and glass ceramics for white light emitting diodes
Journal of Alloys and Compounds 496 (2010) L33–L37
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Full color photoluminescence of Tb3+ /Sm3+ codoped oxyfluoride aluminosilicate glasses and glass ceramics for white light emitting diodes Zhenyu Lin ∗ , Xiaoluan Liang, Yuwen Ou, Chaxing Fan, Shuanglong Yuan, Huidan Zeng ∗ , Guorong Chen Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 3 November 2009 Received in revised form 16 February 2010 Accepted 17 February 2010 Available online 3 March 2010 Keywords: Rare-earth-doped materials Photoluminescence Oxyfluoride glasses and glass ceramics
a b s t r a c t A spectroscopic investigation of Tb3+ /Sm3+ codoped SiO2 –Al2 O3 –CaO–CaF2 glasses and glass ceramics is presented. Full color photoluminescence combining blue, green and orange-red light was realized by ultraviolet light excitation. The emission spectra showed that the intensity was enhanced with the increasing concentration of F− anions. Meanwhile, the self-quenching effect of Sm3+ ions and an energy transfer from Tb3+ to Sm3+ ions were evidenced. Significantly increased luminescent intensity was observed from glasses to glass ceramics, which was attributed to the precipitation of rare-earth ions into the lower phonon energy CaF2 crystallites. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, light emitting diodes (LEDs) have been a subject of considerable interest owing to their quick response time, environmental benefits, lower energy consumption and wide optical and electrical applications, such as flash lightings, automotive light, electroluminescent displays, etc. [1,2]. Particularly, white LEDs (WLEDs) as the new generation of solid state light source has attracted much attention, because it is expected to replace the conventional incandescent and fluorescent lamps in the near future. Currently, W-LEDs have been fabricated by encapsulating two or three different types of phosphors in an epoxy resin with either ultraviolet (UV) or blue LED chips [3,4]. The disadvantage of this fabrication is that the epoxy resin tends to deteriorate at the operating temperature, and leads to a reduction of its lifetime. In order to overcome this problem, rare-earth ions (REI) doped glasses could be a promising candidate for W-LEDs because of their high thermal stability, low cost, ease of mass production and more importantly, the epoxy resin free assembly process. REI doped oxyfluoride glasses have been widely investigated in the past few decades, because they combine the advantages of the high mechanical strength of oxide glasses and the low phonon energy of fluoride glasses [5–9]. Nowadays, some researches have been focused on the possible applications of these glasses in the fabrication of W-LEDs. For examples, Liu and Heo have studied
∗ Corresponding author. E-mail addresses: [email protected] (Z. Lin), [email protected] (H. Zeng). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.155
the white light emission through the upconversion route based on oxyfluoride glasses codoped with Ho3+ , Tm3+ and Yb3+ ions [10]. Lakshminarayana et al. has realized the white color luminescence from Tm3+ /Dy3+ ions codoped oxyfluoride glasses under UV light excitation [11]. Luo et al. investigated the luminescent properties of Ce3+ /Dy3+ codoped oxyfluoride glasses and glass ceramics under UV excitation; the material exhibited a combination of blue, green and red emission bands for white light emission [12]. So far, few studies were concerned on the effect of F− anion concentration on the luminescent properties of REI doped oxyfluoride aluminosilicate glasses. The aim of this work is therefore to investigate the F− anion concentration dependent optical and luminescent properties of Tb3+ /Sm3+ ions codoped oxyfluoride aluminosilicate glasses and glass ceramics. The spectroscopic characterizations have been carried out to evaluate the optical properties and the dependence of luminescent properties on the fluoride content and REI concentration, as well as heat treatment conditions is discussed. 2. Experimental procedures The glass compositions are listed in Table 1, in which analytical reagent grade SiO2 , Al2 O3 , CaCO3 , CaF2 and high purity Tb4 O7 and Sm2 O3 were used as starting materials for melting. The weighted raw materials (30 g) were mixed thoroughly and then transferred to an alumina crucible with a cover to melt at 1500 ◦ C for 1 h in air. The melt was poured into a preheated stainless steel mold for quenching and then annealed at their respective annealing temperatures. The glasses were cut into plates with a thickness of 1.2 mm and polished smoothly for further studies. The glass ceramics were prepared by a two-step heat treatment. The parent glasses were nucleated at 630 ◦ C for 24 h and then maintained at 700 ◦ C for 3 h (A5-3) and 5 h (A5-5) for the growth of crystallites. The temperatures of crystallization were based on the related differential scanning calorimetry (DSC) measurements (Model ZF-DSC-D3X).
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Z. Lin et al. / Journal of Alloys and Compounds 496 (2010) L33–L37
Table 1 Composition of analyzed glasses (mol%). Glass no.
SiO2
Al2 O3
CaO
CaF2
Tb4 O7
Sm2 O3
A1 A2 A3 A4 A5
45 45 45 45 45
20 20 20 20 20
25 20 15 15 15
10 15 20 20 20
0.125 0.125 0.125 0.125 0.125
0.5 0.5 0.3 0.5 0.7
The photoluminescence spectra and the corresponding decay curves were recorded using a fluorescent spectrometer (Model Fluorolog-3-P) with a xenon lamp as the excitation source. To determine the crystallite phases developed in the glasses ceramics, X-ray diffraction measurements (XRD) were carried out with the Cu K␣ radiation (Model D8 ADVANCE). All measurements were performed at room temperature.
3. Results and discussion Fig. 1 shows the emission spectra of glass samples with different CaF2 contents excited by a 374 nm UV light. The emission bands corresponding to Tb3+ and Sm3+ ions are in agreement with other systems [13,14]. The Tb3+ emission bands centered at 414 nm, 435 nm (purple), 487 nm (blue) and 541 nm (green) are ascribed to 5 D → 7 F , 5 D → 7 F , 5 D → 7 F and 5 D → 7 F transitions, respec5 5 3 3 4 4 6 4 tively [13]. The Sm3+ emissions peaked at 562 nm (green), 598 nm (orange-red), and 645 nm (red) can be assigned to the 4 G5/2 → 6 HJ/2 (J = 5, 7, 9) transitions [14]. The 5 D4 → 7 F4 and 5 D4 → 7 F3 transitions of Tb3+ ions are common in other studies but were not observed in the present Tb3+ /Sm3+ codoped samples probably due to the overlap with the tail of Sm3+ emissions peaked at 598 nm, respectively [15]. It can be seen that the emission intensity increases with CaO replaced by CaF2 gradually. The emission wavelengths, however, were barely affected due to the shielding effects of the outer 5s and 5p orbital on the 4f electrons. These interesting findings suggest that the fluorine anions play a key role in the structure and phonon energy (PE) of glass matrix. In alumina silicate glasses, the network consists of [AlO4 ] and [SiO4 ] tetrahedral linked by the bridging oxygen (BO), and the vibration stretching energy of Si–O band is about 1100 cm−1 . The addition of fluorine occupying the BO sites forms Si–F bands, whose vibration stretching energy is about 940 cm−1 [16]. Therefore, the overall stretching vibration energy declined gradually with CaF2 content, resulting in a lower PE. The low PE in turn leaded to low non-radiative loss and thus obtained high quantum efficiency of fluorescence. This explanation is in excellent agreement with previous studies on germanium glasses [8,17,18].
Fig. 1. Emission spectra of samples (A1, A2, A4) excited by 374 nm UV light.
Fig. 2. Decay curves of Tb3+ and Sm3+ emission in the samples (A1, A2, A4): (a) at 541 nm emission and (b) at 598 nm emission.
Fig. 2 shows the decay curves (ex = 374 nm, em = 541 nm and 598 nm) with different CaF2 contents in the glasses samples. Fluorescence lifetimes can be calculated by a curve-fitting technique based on the following equation [19]: I = A1 exp
−t 1
(1)
where I is luminescence intensity; A1 is a constant; t is time and 1 is the decay times for exponential components. The results showed that the fitted fluorescence lifetimes increased from 2.65 ms to 3.16 ms for Tb3+ emission at 541 nm and from 2.48 ms to 2.88 ms for Sm3+ emission at 598 nm with the increasing CaF2 content. The increased lifetimes indicate an increased quantum efficiency [20], which also supports the above discussion on the relationship between PE and emission intensity. Fig. 3 shows the emission spectra of glass samples with different Sm2 O3 content excited by 374 nm UV light. It can be seen that the emission intensity of the Sm3+ at 598 nm firstly increases with Sm2 O3 content and then decreases. On the other hand, the emission intensity from Tb3+ ions decreases with the increasing Sm2 O3 content in the samples. These phenomena can be explained as follows. The changes of luminescent intensity for the Sm3+ and Tb3+ ions are related to both the interaction and the energy transfer among involved REI in the samples. The decreased luminescent intensity of Sm3+ ions implies the occurrence of the self-quenching effect (SQE)
Fig. 3. Emission spectra of samples (A3, A4, A5) excited by 374 nm UV light.
Z. Lin et al. / Journal of Alloys and Compounds 496 (2010) L33–L37
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Fig. 6. Emission spectra of samples (A5, A5-3 and A5-5) excited by 374 nm UV light.
Fig. 4. Energy diagram of Tb3+ and Sm3+ ions.
due to interactions between the Sm3+ ions. Such a SQE is related to the ion-ion relaxation in Sm3+ ions and the possible mechanism for energy transfer is dipole-dipole interaction [21]. The decrease of Tb3+ ions emission can be interpreted by the energy transfer from Tb3+ to Sm3+ . A scheme of Tb3+ and Sm3+ ions energy level and relative transitions are depicted in Fig. 4. The REI have an initial absorption of UV energy that excites the electrons from the ground state to the excited state. The excited electrons relax to a lower energy level by a non-radiative relaxation (Tb3+ : 5 D3 → 5 D4 , Sm3+ : 5P 6 7/2 → G5/2 ) and then transfer back to the ground state by releasing the energy as visible lights. The energy level of Sm3+ (4 G5/2 ) is slightly lower than that of Tb3+ (5 D4 ). It is possible that part of the energy of Tb3+ at 5 D4 level transfers to the lower level of Sm3+ (4 G5/2 ) resulting in the decline of Tb3+ ions emission. The energy transfer from Tb3+ to Sm3+ ions could be achieved by the phonon assistant relaxation process due to the small energy difference between the two energy levels. Fig. 5 shows the XRD patterns of the parent glasses and glass ceramics. The diffuse band for the parent glasses indicates its amorphous structure. After heat treatment, evident diffraction peaks attributed to CaF2 (JCPDS No. 89-4794) are seen at 2 angles of 28.2◦ , 47.0◦ and 55.8◦ . The intensity of these peaks increases with
Fig. 5. XRD patterns of the samples (A5, A5-3 and A5-5).
treatment time, indicating an increased degree of crystallization of the glass ceramics. The emission spectra of REI (Tb3+ and Sm3+ ) in the parent glasses and glass ceramics are presented in Fig. 6. It is evident that the REI emission is significantly enhanced from the glasses to glass ceramics with prolonged heat treatment. This is because REI could precipitate into the CaF2 crystallites and occupy the Ca2+ sites during the heat treatment process, considering their similar ionic radii (Ca2+ : 0.099 nm, Sm3+ : 0.096 nm and Tb3+ : 0.118 nm) [22,23], thus, the surrounding environment of REI in the thermally treated samples is believed to be different from that in parent glasses because crystallites offer new sites for Tb3+ /Sm3+ ions where most REI ions are surrounded by fluorine rather than oxygen. Because the fluoride crystallites have a lower vibration energy of about 300–400 cm−1 [24], the non-radiative relaxation assisted by phonon is suppressed and instead more energy is relaxed by light emission. On the other hand, heat treatments have little effect on the emission wavelengths due to the f–f inner shell transition, but influence the emission intensity ratio of 4 G5/2 → 6 H9/2 (645 nm) to 4 G5/2 → 6 H5/2 (562 nm) to some extent. The transition 4 G5/2 → 6 H7/2 with J = ±1 is not only a magnetic dipole (MD) allowed transition, but also an electric dipole (ED) dominated, but the transition 4 G5/2 → 6 H9/2 is purely an ED one [25]. The emission intensity ratio of ED and MD (ED/MD) is commonly used as a measurement of rare-earth site symmetry. The lower ED/MD intensity ratio corresponds to a higher symmetric local environment for REI. In the current case, the 4 G5/2 → 6 H5/2 (MD) transition of Sm3+ ions is slightly intense than the 4 G5/2 → 6 H9/2 (ED) transition. A comparison of the emission spectra among A5, A5-3 and A5-5 samples shows that the ratio of the intensities of the ED (4 G5/2 → 6 H9/2 ) and MD (4 G5/2 → 6 H5/2 ) decreases from glass to glass ceramics. The ED/MD ratio for A5, A53 and A5-5 are 0.95, 0.86 and 0.87, respectively, implying that the REI in the CaF2 crystallites have a higher symmetric environment than that in the glass. Similar phenomena were also observed in previously reported work on Eu3+ doped glasses, where the Eu3+ ions were incorporated into BaF2 crystallites, obtaining a higher symmetric local rare-earth environment in glass ceramics [26]. The fluorescence lifetime for the Tb3+ and Sm3+ ions has been further compared between the glass (A5) and glass ceramics (A53, A5-5). An expected increase of lifetime with thermal treatment time is shown in Fig. 7. After the heat treatment, the CaF2 crystallites are distributed in the glass matrices. As the crystal field strength around each CaF2 crystal is site dependent, the average rates are used as the decay rate of the system. The fluorescence following a selected excitation wavelength for a particular emis-
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Z. Lin et al. / Journal of Alloys and Compounds 496 (2010) L33–L37 Table 2 Chromaticity coordinates calculated from emission spectra excited by 374 nm UV light. Glass no. Chromaticity Coordinates
Fig. 7. Decay curves of Tb3+ and Sm3+ emission in the samples (A5, A5-3, A5-5): (a) at 541 nm emission and (b) at 598 nm emission.
sion band will have a single-exponential time dependent [27]. The lifetimes calculated for the 5 D4 → 7 F5 transition of Tb3+ by excitation at 374 nm are 2.84 ms, 3.12 ms, and 3.25 ms for the A5, A5-3 and A5-5 samples, respectively. Also, the lifetimes calculated for the 5 G5/2 → 6 H7/2 transition of Sm3+ under the same excitation are 2.55 ms, 2.88 ms, and 2.99 ms for the A5, A5-3 and A5-5 samples, respectively. The effect of crystallization on the lighting improvement could also be reflected through the lifetime variation. When the Tb3+ /Sm3+ ions precipitate into the CaF2 crystallites, they all coordinate with F− anions with lower vibration energy. The excited electrons could stay longer and then transfer back to the ground state with a lower decay rate [28]. Excitation spectra of samples exhibit similar characteristics. For example, Fig. 8 shows the excitation spectra of the sample A5. The excitation peak at 377 nm, monitored at 541 nm, is due to the transition from the ground level 7 F6 to 5 D3 of Tb3+ ions, while the peak at 372 nm, monitored at 598 nm, corresponds to the 6 H5/2 to 4 K13/2 electronic transition of the Sm3+ ions. Based on these excitation spectra, it is demonstrated that the excitation wavelength around 374 nm for these codoped glasses can exactly match the requirements of UV chip for W-LEDs. In addition, a broad excitation band for Sm3+ ions peaked at 400 nm is observed. It is overlapping with the blue emission of Tb3+ ions, thus the energy migration from Tb3+ to Sm3+ most likely occurs.
x y
A1
A2
A3
A4
A5
0.419 0.410
0.422 0.412
0.436 0.427
0.422 0.413
0.415 0.407
The luminescence properties of the glasses for white LEDs need to be evaluated by those of the International Commission on Illumination (CIE) chromaticity diagram for lighting application. According to CIE, each color is represented by three coordinates in a three-dimensional diagram. The coordinates of sample are calculated by a normalizing emission spectrum and a photoluminescence spectrum of sample [29]. The chromaticity coordinates of these glasses are calculated from the emission spectra, as shown in Table 2. It can be seen that for the samples (A1, A2, A4), the chromaticity coordinates are slightly changed with variation of fluorine anion, which implies that the glass composition has little effect on the chromaticity coordinates. By contrast, the chromaticity could change effectively by adjusting the Sm3+ ions concentration for the samples (A3, A4, A5). 4. Conclusions Tb3+ /Sm3+ codoped oxyfluoride aluminosilicate glasses and glass ceramics have been prepared by the melt-quenching method and subsequent heat treatment. The phonon energy decreased with the increasing of F− anions content in the glass matrix, leading to enhance the emission intensity and the fluorescence lifetimes. The SQE of Sm3+ ions was observed in the glasses, and also the energy transfer from Tb3+ (5 D4 ) to Sm3+ (4 G5/2 ) occurred through a nonradiative process. The incorporation of low phonon energy CaF2 crystallites can significantly increase the luminescent intensity of Tb3+ /Sm3+ ions after heat treatment, which implied that Tb3+ /Sm3+ ions were incorporated into low phonon energy CaF2 crystallites. As expected, the fluorescence lifetimes of Tb3+ and Sm3+ increase from glass to glass ceramics regularly. The chromaticity coordinates can be tuned by varying the concentration of Sm3+ ions. In conclusion, Tb3+ /Sm3+ codoped oxyfluoride aluminosilicate glasses and glass ceramics are promising candidates for W-LEDs. Further work is underway to fully realize a white light emission. Acknowledgements The authors wish to thank Dr. M. Peng (University of ErlangenNürnberg, Germany) and Dr. F. Xia (University of Adelaide, Australia) for helpful discussion. The present work is financially supported by the National Natural Science Foundation of China (NSFC 50702021), the Research Funds for Young Teachers for the Doctoral Program by Ministry of Education of China (20070251013) and the Shanghai Leading Academic Discipline Project, project B502. References [1] [2] [3] [4] [5] [6] [7]
Fig. 8. Excitation spectra for sample A5 at 541 and 598 nm emission.
[8] [9] [10] [11]
C. Guo, F. Gao, L. Liang, B.C. Choi, J.-H. Jeong, J. Alloys Compd. 479 (2009) 607. L. Zhou, J. Wei, J. Wu, F. Gong, L. Yi, J. Huang, J. Alloys Compd. 476 (2009) 390. S.C. Allen, A.J. Steckl, Appl. Phys. Lett. 92 (2008) 143309. P. Li, L. Pang, Z. Wang, Z. Yang, Q. Guo, X. Li, J. Alloys Compd. 478 (2009) 813. ˜ S. León-Luis, J. Abreu-Afonso, J. Pena-Martínez, J. Méndez-Ramos, A. Yanes, J. del-Castillo, V. Rodríguez, J. Alloys Compd. 479 (2009) 557. L. Lin, G. Ren, M. Chen, Y. Liu, J. Alloys Compd. 486 (2009) 261. V. Lavin, P. Babu, C.K. Jayasankar, I.R. Martin, V.D. Rodriguez, J. Chem. Phys. 115 (2001) 10935. Z. Pan, A. Ueda, R. Mu, S. Morgan, J. Lumin. 126 (2007) 251. H. Ping, D. Chen, Y. Yu, Y. Wang, J. Alloys Compd. 490 (2010) 74. C. Liu, J. Heo, Mater. Lett. 61 (2007) 3751. G. Lakshminarayana, H. Yang, J. Qiu, J. Solid State Chem. 182 (2009) 669.
Z. Lin et al. / Journal of Alloys and Compounds 496 (2010) L33–L37 [12] Q. Luo, X. Qiao, X. Fan, H. Yang, X. Zhang, S. Cui, L. Wang, G. Wang, J. Appl. Phys. 105 (2009) 043506. [13] G. Lakshminarayana, R. Yang, J. Qiu, M. Brik, G. Kumar, I. Kityk, J. Phys. D: Appl. Phys. 42 (2009) 015414. [14] K.K. Mahato, D.K. Rai, S.B. Rai, Opt. Mater. 29 (2007) 1429. [15] H. Lin, E. Pun, X. Wang, X. Liu, J. Alloys Compd. 390 (2005) 197. [16] P.-F. Wang, S.-J. Ding, D.W. Zhang, J.-T. Wang, W.W. Lee, Thin Solid Films 385 (2001) 115. [17] W.S. Tsang, W.M. Yu, C.L. Mak, W.L. Tsui, K.H. Wong, H.K. Hui, J. Appl. Phys. 91 (2002) 1871. [18] M. Wachtler, A. Speghini, K. Gatterer, H.P. Fritzer, D. Ajò, M. Bettinelli, J. Am. Ceram. Soc. 81 (1998) 2045. [19] G. Lakshminarayana, J. Qiu, J. Alloys Compd. 476 (2009) 720. [20] D. Chen, Y. Wang, Y. Yu, E. Ma, J. Solid State Chem. 179 (2006) 1445.
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[21] X. Liang, Y. Yang, C. Zhu, S. Yuan, G. Chen, A. Pring, F. Xia, Appl. Phys. Lett. 91 (2007) 091104. [22] A. Lucca, M. Jacquemet, F. Druon, F. Balembois, P. Georges, P. Camy, J.L. Doualan, R. Moncorg, Opt. Lett. 29 (2004) 1879. ˜ J. Phys.: Condens. Matter 15 (2003) 7127. [23] U.G. Caldino, [24] S. Tanabe, H. Hayashi, T. Hanada, N. Onodera, Opt. Mater. 19 (2002) 343. [25] P.S. May, D.H. Metcalf, F.S. Richardson, R.C. Carter, C.E. Miller, R.A. Palmer, J. Lumin. 51 (1992) 249. [26] Y. Dwivedi, S.B. Rai, Opt. Mater. 31 (2008) 87. [27] G. Lakshminarayana, R. Yang, M. Mao, J. Qiu, I.V. Kityk, J. Non-Cryst. Solids 355 (2009) 2668. [28] S. Todoroki, S. Inoue, Appl. Surf. Sci. 223 (2004) 39. [29] M. Godlewski, M. Skrobot, E. Guziewicz, M.R. Phillips, J. Lumin. 125 (2007) 85.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Full color photoluminescence of Tb3+ /Sm3+ codoped oxyfluoride aluminosilicate glasses and glass ceramics for white light emitting diodes Zhenyu Lin ∗ , Xiaoluan Liang, Yuwen Ou, Chaxing Fan, Shuanglong Yuan, Huidan Zeng ∗ , Guorong Chen Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 3 November 2009 Received in revised form 16 February 2010 Accepted 17 February 2010 Available online 3 March 2010 Keywords: Rare-earth-doped materials Photoluminescence Oxyfluoride glasses and glass ceramics
a b s t r a c t A spectroscopic investigation of Tb3+ /Sm3+ codoped SiO2 –Al2 O3 –CaO–CaF2 glasses and glass ceramics is presented. Full color photoluminescence combining blue, green and orange-red light was realized by ultraviolet light excitation. The emission spectra showed that the intensity was enhanced with the increasing concentration of F− anions. Meanwhile, the self-quenching effect of Sm3+ ions and an energy transfer from Tb3+ to Sm3+ ions were evidenced. Significantly increased luminescent intensity was observed from glasses to glass ceramics, which was attributed to the precipitation of rare-earth ions into the lower phonon energy CaF2 crystallites. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, light emitting diodes (LEDs) have been a subject of considerable interest owing to their quick response time, environmental benefits, lower energy consumption and wide optical and electrical applications, such as flash lightings, automotive light, electroluminescent displays, etc. [1,2]. Particularly, white LEDs (WLEDs) as the new generation of solid state light source has attracted much attention, because it is expected to replace the conventional incandescent and fluorescent lamps in the near future. Currently, W-LEDs have been fabricated by encapsulating two or three different types of phosphors in an epoxy resin with either ultraviolet (UV) or blue LED chips [3,4]. The disadvantage of this fabrication is that the epoxy resin tends to deteriorate at the operating temperature, and leads to a reduction of its lifetime. In order to overcome this problem, rare-earth ions (REI) doped glasses could be a promising candidate for W-LEDs because of their high thermal stability, low cost, ease of mass production and more importantly, the epoxy resin free assembly process. REI doped oxyfluoride glasses have been widely investigated in the past few decades, because they combine the advantages of the high mechanical strength of oxide glasses and the low phonon energy of fluoride glasses [5–9]. Nowadays, some researches have been focused on the possible applications of these glasses in the fabrication of W-LEDs. For examples, Liu and Heo have studied
∗ Corresponding author. E-mail addresses: [email protected] (Z. Lin), [email protected] (H. Zeng). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.155
the white light emission through the upconversion route based on oxyfluoride glasses codoped with Ho3+ , Tm3+ and Yb3+ ions [10]. Lakshminarayana et al. has realized the white color luminescence from Tm3+ /Dy3+ ions codoped oxyfluoride glasses under UV light excitation [11]. Luo et al. investigated the luminescent properties of Ce3+ /Dy3+ codoped oxyfluoride glasses and glass ceramics under UV excitation; the material exhibited a combination of blue, green and red emission bands for white light emission [12]. So far, few studies were concerned on the effect of F− anion concentration on the luminescent properties of REI doped oxyfluoride aluminosilicate glasses. The aim of this work is therefore to investigate the F− anion concentration dependent optical and luminescent properties of Tb3+ /Sm3+ ions codoped oxyfluoride aluminosilicate glasses and glass ceramics. The spectroscopic characterizations have been carried out to evaluate the optical properties and the dependence of luminescent properties on the fluoride content and REI concentration, as well as heat treatment conditions is discussed. 2. Experimental procedures The glass compositions are listed in Table 1, in which analytical reagent grade SiO2 , Al2 O3 , CaCO3 , CaF2 and high purity Tb4 O7 and Sm2 O3 were used as starting materials for melting. The weighted raw materials (30 g) were mixed thoroughly and then transferred to an alumina crucible with a cover to melt at 1500 ◦ C for 1 h in air. The melt was poured into a preheated stainless steel mold for quenching and then annealed at their respective annealing temperatures. The glasses were cut into plates with a thickness of 1.2 mm and polished smoothly for further studies. The glass ceramics were prepared by a two-step heat treatment. The parent glasses were nucleated at 630 ◦ C for 24 h and then maintained at 700 ◦ C for 3 h (A5-3) and 5 h (A5-5) for the growth of crystallites. The temperatures of crystallization were based on the related differential scanning calorimetry (DSC) measurements (Model ZF-DSC-D3X).
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Z. Lin et al. / Journal of Alloys and Compounds 496 (2010) L33–L37
Table 1 Composition of analyzed glasses (mol%). Glass no.
SiO2
Al2 O3
CaO
CaF2
Tb4 O7
Sm2 O3
A1 A2 A3 A4 A5
45 45 45 45 45
20 20 20 20 20
25 20 15 15 15
10 15 20 20 20
0.125 0.125 0.125 0.125 0.125
0.5 0.5 0.3 0.5 0.7
The photoluminescence spectra and the corresponding decay curves were recorded using a fluorescent spectrometer (Model Fluorolog-3-P) with a xenon lamp as the excitation source. To determine the crystallite phases developed in the glasses ceramics, X-ray diffraction measurements (XRD) were carried out with the Cu K␣ radiation (Model D8 ADVANCE). All measurements were performed at room temperature.
3. Results and discussion Fig. 1 shows the emission spectra of glass samples with different CaF2 contents excited by a 374 nm UV light. The emission bands corresponding to Tb3+ and Sm3+ ions are in agreement with other systems [13,14]. The Tb3+ emission bands centered at 414 nm, 435 nm (purple), 487 nm (blue) and 541 nm (green) are ascribed to 5 D → 7 F , 5 D → 7 F , 5 D → 7 F and 5 D → 7 F transitions, respec5 5 3 3 4 4 6 4 tively [13]. The Sm3+ emissions peaked at 562 nm (green), 598 nm (orange-red), and 645 nm (red) can be assigned to the 4 G5/2 → 6 HJ/2 (J = 5, 7, 9) transitions [14]. The 5 D4 → 7 F4 and 5 D4 → 7 F3 transitions of Tb3+ ions are common in other studies but were not observed in the present Tb3+ /Sm3+ codoped samples probably due to the overlap with the tail of Sm3+ emissions peaked at 598 nm, respectively [15]. It can be seen that the emission intensity increases with CaO replaced by CaF2 gradually. The emission wavelengths, however, were barely affected due to the shielding effects of the outer 5s and 5p orbital on the 4f electrons. These interesting findings suggest that the fluorine anions play a key role in the structure and phonon energy (PE) of glass matrix. In alumina silicate glasses, the network consists of [AlO4 ] and [SiO4 ] tetrahedral linked by the bridging oxygen (BO), and the vibration stretching energy of Si–O band is about 1100 cm−1 . The addition of fluorine occupying the BO sites forms Si–F bands, whose vibration stretching energy is about 940 cm−1 [16]. Therefore, the overall stretching vibration energy declined gradually with CaF2 content, resulting in a lower PE. The low PE in turn leaded to low non-radiative loss and thus obtained high quantum efficiency of fluorescence. This explanation is in excellent agreement with previous studies on germanium glasses [8,17,18].
Fig. 1. Emission spectra of samples (A1, A2, A4) excited by 374 nm UV light.
Fig. 2. Decay curves of Tb3+ and Sm3+ emission in the samples (A1, A2, A4): (a) at 541 nm emission and (b) at 598 nm emission.
Fig. 2 shows the decay curves (ex = 374 nm, em = 541 nm and 598 nm) with different CaF2 contents in the glasses samples. Fluorescence lifetimes can be calculated by a curve-fitting technique based on the following equation [19]: I = A1 exp
−t 1
(1)
where I is luminescence intensity; A1 is a constant; t is time and 1 is the decay times for exponential components. The results showed that the fitted fluorescence lifetimes increased from 2.65 ms to 3.16 ms for Tb3+ emission at 541 nm and from 2.48 ms to 2.88 ms for Sm3+ emission at 598 nm with the increasing CaF2 content. The increased lifetimes indicate an increased quantum efficiency [20], which also supports the above discussion on the relationship between PE and emission intensity. Fig. 3 shows the emission spectra of glass samples with different Sm2 O3 content excited by 374 nm UV light. It can be seen that the emission intensity of the Sm3+ at 598 nm firstly increases with Sm2 O3 content and then decreases. On the other hand, the emission intensity from Tb3+ ions decreases with the increasing Sm2 O3 content in the samples. These phenomena can be explained as follows. The changes of luminescent intensity for the Sm3+ and Tb3+ ions are related to both the interaction and the energy transfer among involved REI in the samples. The decreased luminescent intensity of Sm3+ ions implies the occurrence of the self-quenching effect (SQE)
Fig. 3. Emission spectra of samples (A3, A4, A5) excited by 374 nm UV light.
Z. Lin et al. / Journal of Alloys and Compounds 496 (2010) L33–L37
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Fig. 6. Emission spectra of samples (A5, A5-3 and A5-5) excited by 374 nm UV light.
Fig. 4. Energy diagram of Tb3+ and Sm3+ ions.
due to interactions between the Sm3+ ions. Such a SQE is related to the ion-ion relaxation in Sm3+ ions and the possible mechanism for energy transfer is dipole-dipole interaction [21]. The decrease of Tb3+ ions emission can be interpreted by the energy transfer from Tb3+ to Sm3+ . A scheme of Tb3+ and Sm3+ ions energy level and relative transitions are depicted in Fig. 4. The REI have an initial absorption of UV energy that excites the electrons from the ground state to the excited state. The excited electrons relax to a lower energy level by a non-radiative relaxation (Tb3+ : 5 D3 → 5 D4 , Sm3+ : 5P 6 7/2 → G5/2 ) and then transfer back to the ground state by releasing the energy as visible lights. The energy level of Sm3+ (4 G5/2 ) is slightly lower than that of Tb3+ (5 D4 ). It is possible that part of the energy of Tb3+ at 5 D4 level transfers to the lower level of Sm3+ (4 G5/2 ) resulting in the decline of Tb3+ ions emission. The energy transfer from Tb3+ to Sm3+ ions could be achieved by the phonon assistant relaxation process due to the small energy difference between the two energy levels. Fig. 5 shows the XRD patterns of the parent glasses and glass ceramics. The diffuse band for the parent glasses indicates its amorphous structure. After heat treatment, evident diffraction peaks attributed to CaF2 (JCPDS No. 89-4794) are seen at 2 angles of 28.2◦ , 47.0◦ and 55.8◦ . The intensity of these peaks increases with
Fig. 5. XRD patterns of the samples (A5, A5-3 and A5-5).
treatment time, indicating an increased degree of crystallization of the glass ceramics. The emission spectra of REI (Tb3+ and Sm3+ ) in the parent glasses and glass ceramics are presented in Fig. 6. It is evident that the REI emission is significantly enhanced from the glasses to glass ceramics with prolonged heat treatment. This is because REI could precipitate into the CaF2 crystallites and occupy the Ca2+ sites during the heat treatment process, considering their similar ionic radii (Ca2+ : 0.099 nm, Sm3+ : 0.096 nm and Tb3+ : 0.118 nm) [22,23], thus, the surrounding environment of REI in the thermally treated samples is believed to be different from that in parent glasses because crystallites offer new sites for Tb3+ /Sm3+ ions where most REI ions are surrounded by fluorine rather than oxygen. Because the fluoride crystallites have a lower vibration energy of about 300–400 cm−1 [24], the non-radiative relaxation assisted by phonon is suppressed and instead more energy is relaxed by light emission. On the other hand, heat treatments have little effect on the emission wavelengths due to the f–f inner shell transition, but influence the emission intensity ratio of 4 G5/2 → 6 H9/2 (645 nm) to 4 G5/2 → 6 H5/2 (562 nm) to some extent. The transition 4 G5/2 → 6 H7/2 with J = ±1 is not only a magnetic dipole (MD) allowed transition, but also an electric dipole (ED) dominated, but the transition 4 G5/2 → 6 H9/2 is purely an ED one [25]. The emission intensity ratio of ED and MD (ED/MD) is commonly used as a measurement of rare-earth site symmetry. The lower ED/MD intensity ratio corresponds to a higher symmetric local environment for REI. In the current case, the 4 G5/2 → 6 H5/2 (MD) transition of Sm3+ ions is slightly intense than the 4 G5/2 → 6 H9/2 (ED) transition. A comparison of the emission spectra among A5, A5-3 and A5-5 samples shows that the ratio of the intensities of the ED (4 G5/2 → 6 H9/2 ) and MD (4 G5/2 → 6 H5/2 ) decreases from glass to glass ceramics. The ED/MD ratio for A5, A53 and A5-5 are 0.95, 0.86 and 0.87, respectively, implying that the REI in the CaF2 crystallites have a higher symmetric environment than that in the glass. Similar phenomena were also observed in previously reported work on Eu3+ doped glasses, where the Eu3+ ions were incorporated into BaF2 crystallites, obtaining a higher symmetric local rare-earth environment in glass ceramics [26]. The fluorescence lifetime for the Tb3+ and Sm3+ ions has been further compared between the glass (A5) and glass ceramics (A53, A5-5). An expected increase of lifetime with thermal treatment time is shown in Fig. 7. After the heat treatment, the CaF2 crystallites are distributed in the glass matrices. As the crystal field strength around each CaF2 crystal is site dependent, the average rates are used as the decay rate of the system. The fluorescence following a selected excitation wavelength for a particular emis-
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Z. Lin et al. / Journal of Alloys and Compounds 496 (2010) L33–L37 Table 2 Chromaticity coordinates calculated from emission spectra excited by 374 nm UV light. Glass no. Chromaticity Coordinates
Fig. 7. Decay curves of Tb3+ and Sm3+ emission in the samples (A5, A5-3, A5-5): (a) at 541 nm emission and (b) at 598 nm emission.
sion band will have a single-exponential time dependent [27]. The lifetimes calculated for the 5 D4 → 7 F5 transition of Tb3+ by excitation at 374 nm are 2.84 ms, 3.12 ms, and 3.25 ms for the A5, A5-3 and A5-5 samples, respectively. Also, the lifetimes calculated for the 5 G5/2 → 6 H7/2 transition of Sm3+ under the same excitation are 2.55 ms, 2.88 ms, and 2.99 ms for the A5, A5-3 and A5-5 samples, respectively. The effect of crystallization on the lighting improvement could also be reflected through the lifetime variation. When the Tb3+ /Sm3+ ions precipitate into the CaF2 crystallites, they all coordinate with F− anions with lower vibration energy. The excited electrons could stay longer and then transfer back to the ground state with a lower decay rate [28]. Excitation spectra of samples exhibit similar characteristics. For example, Fig. 8 shows the excitation spectra of the sample A5. The excitation peak at 377 nm, monitored at 541 nm, is due to the transition from the ground level 7 F6 to 5 D3 of Tb3+ ions, while the peak at 372 nm, monitored at 598 nm, corresponds to the 6 H5/2 to 4 K13/2 electronic transition of the Sm3+ ions. Based on these excitation spectra, it is demonstrated that the excitation wavelength around 374 nm for these codoped glasses can exactly match the requirements of UV chip for W-LEDs. In addition, a broad excitation band for Sm3+ ions peaked at 400 nm is observed. It is overlapping with the blue emission of Tb3+ ions, thus the energy migration from Tb3+ to Sm3+ most likely occurs.
x y
A1
A2
A3
A4
A5
0.419 0.410
0.422 0.412
0.436 0.427
0.422 0.413
0.415 0.407
The luminescence properties of the glasses for white LEDs need to be evaluated by those of the International Commission on Illumination (CIE) chromaticity diagram for lighting application. According to CIE, each color is represented by three coordinates in a three-dimensional diagram. The coordinates of sample are calculated by a normalizing emission spectrum and a photoluminescence spectrum of sample [29]. The chromaticity coordinates of these glasses are calculated from the emission spectra, as shown in Table 2. It can be seen that for the samples (A1, A2, A4), the chromaticity coordinates are slightly changed with variation of fluorine anion, which implies that the glass composition has little effect on the chromaticity coordinates. By contrast, the chromaticity could change effectively by adjusting the Sm3+ ions concentration for the samples (A3, A4, A5). 4. Conclusions Tb3+ /Sm3+ codoped oxyfluoride aluminosilicate glasses and glass ceramics have been prepared by the melt-quenching method and subsequent heat treatment. The phonon energy decreased with the increasing of F− anions content in the glass matrix, leading to enhance the emission intensity and the fluorescence lifetimes. The SQE of Sm3+ ions was observed in the glasses, and also the energy transfer from Tb3+ (5 D4 ) to Sm3+ (4 G5/2 ) occurred through a nonradiative process. The incorporation of low phonon energy CaF2 crystallites can significantly increase the luminescent intensity of Tb3+ /Sm3+ ions after heat treatment, which implied that Tb3+ /Sm3+ ions were incorporated into low phonon energy CaF2 crystallites. As expected, the fluorescence lifetimes of Tb3+ and Sm3+ increase from glass to glass ceramics regularly. The chromaticity coordinates can be tuned by varying the concentration of Sm3+ ions. In conclusion, Tb3+ /Sm3+ codoped oxyfluoride aluminosilicate glasses and glass ceramics are promising candidates for W-LEDs. Further work is underway to fully realize a white light emission. Acknowledgements The authors wish to thank Dr. M. Peng (University of ErlangenNürnberg, Germany) and Dr. F. Xia (University of Adelaide, Australia) for helpful discussion. The present work is financially supported by the National Natural Science Foundation of China (NSFC 50702021), the Research Funds for Young Teachers for the Doctoral Program by Ministry of Education of China (20070251013) and the Shanghai Leading Academic Discipline Project, project B502. References [1] [2] [3] [4] [5] [6] [7]
Fig. 8. Excitation spectra for sample A5 at 541 and 598 nm emission.
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