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Ce3+: YF3 nanocrystals embedded transparent glass ceramics
Materials Letters 64 (2010) 824–826
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Tunable green-red luminescence in Ho3+/Yb3+/Ce3+: YF3 nanocrystals embedded transparent glass ceramics Fugui Yang a,b,c, Guitang Chen a,b, Zhenyu You a, Chaoyang Tu a,⁎ a b c
Key Laboratory of Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China Graduate School of Chinese Academy of Sciences, Beijing, 10086, China The Engineering Technical College of Chendu University of Technology, Leshan, Sichuan 629000, China
a r t i c l e
i n f o
Article history: Received 26 November 2009 Accepted 9 January 2010 Available online 15 January 2010 Keywords: Ceramics Nano-composites Nano-materials Optical materials and properties
a b s t r a c t The Ho3+/Yb3+/Ce3+:YF3 nanocrystals embedded transparent oxyfluoride glass ceramics were successfully synthesized and the emission spectra have been measured. We changed the doped concentrations of ion Ce3+ in the glass ceramics and unexpectedly found that the different concentrations brought forth different luminescence wavelengths, a tunable luminescence wave band from green to red, which might be useful to the tunable visible laser or display applications. In addition, excitated with wavelength 975 nm LD, the transitions of Ho3+: 5G4 → 5I8 at 390 nm, 5G5 → 5I8 at 420 nm and 5F3 → 5I8 at wavelength 490 nm were observed in the glass ceramic GCCe0, while in samples GCCeJ(J=1,3,5,8), the three peaks disappeared. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The up-conversion can convert pump energy from the infrared spectral region to the visible-ultraviolet (UV) region, which is an active research for a variety of applications such as detectors for the IR range, high density optical data storage or color displays. It is well known that up-conversion emission can be enhanced or quenched by nonradiative energy transfer (ET) from one rare earth (RE) ion to another ion. The ion–pair interactions, referred as energy transfer, have been studied widely in the materials doped with Er3+, Pr3+, Tb3+, Ho3+, or Tm3+ ions [1–6]. Among various RE-doped hosts, the nanostructure transparent oxyfluoride glass ceramic may combine the favorable properties such as low phonon energy, high mechanical and chemical stabilities in either the fluoride crystals or the oxide glass matrix. Since Wang and Ohwaki achieved the precipitation of PbxCd1 − xF2 nanocrystal in the oxyfluoride glass matrix [7], many oxyfluoride glass ceramics have been brought forth such as BaF2, CaF2 nanocrystal embodied in glass ceramics. Among these glass ceramics, YF3 nanocrystal embodied in glass ceramic has obtained increasing concerns due to the large fine interaction parameter of doping RE ions in YF3 crystalline phase [8]. The Ho3+/Yb3+ co-doped in oxyfluoride glass ceramics have been investigated by many other researchers [9]. The ion Yb3+ provides the pumping energy levels and the ion Ho3+ can emit green and red lights excitated with 971–980 nm wavelength. And so, in our work, we added a sort of doped ion Ce3+ to synthesize the transparent oxyfluoride glass ceramics containing of
⁎ Corresponding author. Tel./fax: + 86 59183711368. E-mail address: [email protected] (C. Tu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.01.026
Ho3+/Yb3+/Ce3+: YF3 nanocrystals embedded transparent oxyfluoride glass ceramics, which was the first time to report Ho3+/Yb3+/Ce3+ triply doped oxyfluoride glass ceramics. In the experiment, we amazingly found that the intensities of green and red luminescences relied on the concentration of Ce3+ ions doped in the glass ceramics heavily, which could be used to realize the tunable green–red luminescence. 2. Experiment and discussion The oxyfluoride glasses with composition of 44SiO2–28Al2O3– 17NaF–11YF3–0.1HoF3–0.5YbF3–xCeF3 (x = 0, 0.1, 0.3, 0.5, and 0.8) were prepared by melt quenching from purity SiO2, Al2O3, NaF, YF3, and HoF3, YbF3, CeF3 raw materials. The X-ray diffraction (XRD), a powder diffractometer (DMAX2500 Rigaku) using Cu–Kα radiation (λ = 0.154 nm), was carried out to identify the crystallization phases and the mean size of the crystallites. The microstructures of the samples were studied by a high resolution transmission electron microscope (HRTEM, JEM-2010). The up-conversion emission spectra were measured by the Hamamatsu R943-02 photomultiplier tube and the Spex 1000 M monochromator excitated with 975 nm Ti sapphire laser. All the measurements were carried out at room temperature. The emission spectra of the precursor glass and the glass ceramics excitated with 975 nm were presented, as shown in Fig. 1(a). Here we named the precursor glass with code GCe0 and the glass ceramics with code GCCeJ (J = 0, 1, 3, 5, 8), where J was related to the different CeF3 contents: x = 0, 0.1, 0.3, 0.5, and 0.8 referred above. In the sample GCe0, three peaks with the centers at 545, 650, and 750 nm were observed, which were corresponding to the transitions of Ho3+ energy levels: 5F4, 5S2 → 5I8, 5F5 → 5I8, and 5F4, 5S2 → 5I7,
F. Yang et al. / Materials Letters 64 (2010) 824–826
825
Fig. 1. (a) Emission spectra of precursor glass (code GCe0) and glass ceramics (GCCeJ (J = 0, 1, 3, 5, 8)) excitated with 975 nm LD, the inset at the left side is the blue region for GCCe0 and the right side describes the intensities versus CeF3 contents at wavelengths 545 nm, 650 nm, and 750 nm, respectively;(b) the visible photos corresponding to the glass ceramics excitated with 975 nm LD. Here we named the precursor glass with code GCe0 and the glass ceramics with code GCCeJ (J = 0, 1, 3, 5, 8), where J was related to the different CeF3 contents: x = 0, 0.1, 0.3, 0.5, and 0.8 referred above.
respectively. While in sample GCCe0, the other three bands in blue legion with the centers at 390, 420, and 490 nm were also observed (see the inset at the left side of Fig. 1(a)). Which were corresponding to the transitions of Ho3+ energy levels: 5G4 → 5I8, 5G5 → 5I8, and 5 F3 → 5I8, respectively. From the emission spectra of GCe0 and GCCe0, we could obtain: in sample GCe0, the red light (650 nm) dominated the emission bands; whereas in sample GCCe0, the green one (545 nm) dominated the bands. The intensity of green one (545 nm) in sample GCCe0 was about 500 times stronger than that of GCe0, Moreover, in sample GCCe0, the intensity of green one (545 nm) was 24 and 1000 times stronger than those of red light and infrared light (750 nm), respectively. In addition, the fine splitting emission bands also confirmed that the Ho3+ and Yb3+ ions were just incorporated into the YF3 crystalline phase after thermal treatment. Obviously, it was the addition of ion Ce3+ that dramatically changed the luminescence properties. Furthermore, we changed the concentration of Ce3+ and found that the emission intensity of 545 nm decreased from 505 (arbitrary unit) in sample GCCe0 to 130 in the GCCe1; the intensity of 650 nm decreased from 85 to 70 and the intensity of 750 nm decreased from 200 to 40, respectively. In samples GCCeJ (J = 1, 3, 5, 8), the emissions centered at 390, 420, and 490 nm disappeared completely, which was different from GCCe0. From sample GCCe0 to GCCe8, namely with the increasing concentration of Ce3+, the intensities in the region of 500–800 nm wavelength band changed in diversity. Just as shown in Fig. 1, the intensities of 545 nm and 750 nm decreased heavily at all times, whereas the intensity of 650 nm decreased slowly and even increased inversely. Moreover, in the high doped concentration of Ce3+ samples, the intensity of 650 nm was higher than those of 545 and 750 nm, dominating in the region of 500–800 nm wavelength bands. The intensities of 545, 650, and 750 nm versus the content of CeF3 were shown in the inset at the right side of Fig. 1(a), which indicated that the addition of CeF3 resulted in tunable green–red up-conversion
luminescence in the glass ceramics. This phenomenon was quite different from other Er3+/Yb3+/Ce3+ triply doped oxyfluoride glass ceramics, where the intensities of the green light (540 nm, 520 nm) and the red light (652 nm) decreased heavily with the introduction of ion Ce3+[10,11]. Fig. 1(b) presented the visible photographs for the corresponding glass ceramics excitated with 975 nm, which might be utilized in tunable visible solid-state laser or color display. According to the Ce3+/Ho3+ relative concentration, the most probable cross-relaxation process could occur between Ho3+ and Ce3+ ions with the assistance of phonon. The energy levels were shown in Fig. 2. The cross-relaxation process could take place more easily with the increasing concentration of the Ce3+. After a series of cross-relaxation processes many electrons populated in the 5I7 level of Ho3+. We
Fig. 2. The energy levels diagram of Ho3+, Yb3+, and Ce3+.
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F. Yang et al. / Materials Letters 64 (2010) 824–826
noticed that the energy gap between 5I7 and 5I8 (about 5200 cm− 1) was wide, and so the cross radiation process: 5I7 (Ho3+) + 2F5/2 (Ce3+) → 5I8(Ho3+) + 2F7/2(Ce3+) could only take place slightly between one Ho3+ and one or two Ce3+ ions by virtue of the phonon assistance (the maximum phonon energy of the lattice was about 500 cm− 1 for YF3 crystal [12]). As a result, there were still many electrons in the 5I7 level, even in the high doped Ce3+ concentration samples. After receiving enough photons transferred from Yb3+ ions, these electrons could be pumped to the 5F5 level of Ho3+ and then emitted 650 nm light. This was the reason that the intensity of 5F5 → 5I8 (Ho3+) at 650 nm emission decreased slowly and even increased in the high CeF3 concentration doped in samples.
controlled red–green luminescence. According to the adjustment of concentration of Ce3+, we could obtain the needed wavelength luminescence, which might be a potential application to the tunable visible laser or display. Acknowledgments This project was supported by the Science & Technology Plan Project of Fujian Province of China (Grant Nos. 2005HZ1026 and 2007H0037), the major project from FJIRSM (SZD08001-2) and FJIRSM (SZD09001), the open fund project of Photo-electricity Material Chemistry and Physics Lab (2009KL004). References
3. Conclusions The Ho3+/Yb3+/Ce3+: YF3 nanocrystals embedded transparent oxyfluoride glass ceramics were synthesized and discussed. Excitated with wavelength 975 nm LD, the transitions of Ho3+:5G4 → 5I8 at 390 nm, 5G5 → 5I8 at 420 nm and 5F3 → 5I8 at wavelength 490 nm were observed in the glass ceramic GCCe0, while in samples GCCeJ (J = 1,3,5,8), the three peaks disappeared. Furthermore, we found that the emission intensities of transitions: 5F4, 5S2 → 5I8 at 545 nm, 5 F5 → 5I8 at 650 nm, and 5F4, 5S2 → 5I7 at 750 nm in the glass ceramic GCCe0 were much higher than those of in the precursor glass GCe0. Particularly, the introduction of Ce3+ into the Ho3+/Yb3+ co-doped glass ceramics brought about unexpected phenomenon of tunable
[1] Oliveira AS, de Araujo M, Gouveia-Neto AS, Medeiros Neto JA, Sombra ASB, Messaddeq Y. Appl Phys Lett 1998;72:753–5. [2] Shan G, Andrews MP, Gonzalez T, Djeghelian H. Mater Lett 2008;6:4187–90. [3] Zhao J, Sun Y, Kong X, Tian L, Wang Y, Tu L, et al. J Phys Chem B 2008;112(49): 15666–72. [4] Wei Y, Lu F, Zhang X, Chen D. Mater Lett 2007;61:1337–40. [5] Chen X. Mater Lett 2009;63:1023–6. [6] Chen GY, Somesfalean G, Zhang ZG, Sun Q, Wang FP. Opt Lett 2007;32:87–9. [7] Wang Y, Ohwaki J. Appl Phys Lett 1993;63:3268–70. [8] Chen D, Wang Y, Yu Y, Huang P. Appl Phys Lett 2007;91:051920. [9] Qiu J, Makishima A. Sci Technol Adv Mater 2004;5:313–7. [10] Dantelle G, Mortier M, Vivien D, Patriarche G. Chem Mater 2005;17:2216–22. [11] Yu XC, Song F, Wang WT, Luo LJ, Ming CG, Cheng ZZ, et al. Opt Commun 2009;282: 2045–8. [12] Chen DQ, Wang YS, Zheng KL, Guo TL, Yu YL, Huang P. Appl Phys Lett 2007;91: 251903.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Tunable green-red luminescence in Ho3+/Yb3+/Ce3+: YF3 nanocrystals embedded transparent glass ceramics Fugui Yang a,b,c, Guitang Chen a,b, Zhenyu You a, Chaoyang Tu a,⁎ a b c
Key Laboratory of Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China Graduate School of Chinese Academy of Sciences, Beijing, 10086, China The Engineering Technical College of Chendu University of Technology, Leshan, Sichuan 629000, China
a r t i c l e
i n f o
Article history: Received 26 November 2009 Accepted 9 January 2010 Available online 15 January 2010 Keywords: Ceramics Nano-composites Nano-materials Optical materials and properties
a b s t r a c t The Ho3+/Yb3+/Ce3+:YF3 nanocrystals embedded transparent oxyfluoride glass ceramics were successfully synthesized and the emission spectra have been measured. We changed the doped concentrations of ion Ce3+ in the glass ceramics and unexpectedly found that the different concentrations brought forth different luminescence wavelengths, a tunable luminescence wave band from green to red, which might be useful to the tunable visible laser or display applications. In addition, excitated with wavelength 975 nm LD, the transitions of Ho3+: 5G4 → 5I8 at 390 nm, 5G5 → 5I8 at 420 nm and 5F3 → 5I8 at wavelength 490 nm were observed in the glass ceramic GCCe0, while in samples GCCeJ(J=1,3,5,8), the three peaks disappeared. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The up-conversion can convert pump energy from the infrared spectral region to the visible-ultraviolet (UV) region, which is an active research for a variety of applications such as detectors for the IR range, high density optical data storage or color displays. It is well known that up-conversion emission can be enhanced or quenched by nonradiative energy transfer (ET) from one rare earth (RE) ion to another ion. The ion–pair interactions, referred as energy transfer, have been studied widely in the materials doped with Er3+, Pr3+, Tb3+, Ho3+, or Tm3+ ions [1–6]. Among various RE-doped hosts, the nanostructure transparent oxyfluoride glass ceramic may combine the favorable properties such as low phonon energy, high mechanical and chemical stabilities in either the fluoride crystals or the oxide glass matrix. Since Wang and Ohwaki achieved the precipitation of PbxCd1 − xF2 nanocrystal in the oxyfluoride glass matrix [7], many oxyfluoride glass ceramics have been brought forth such as BaF2, CaF2 nanocrystal embodied in glass ceramics. Among these glass ceramics, YF3 nanocrystal embodied in glass ceramic has obtained increasing concerns due to the large fine interaction parameter of doping RE ions in YF3 crystalline phase [8]. The Ho3+/Yb3+ co-doped in oxyfluoride glass ceramics have been investigated by many other researchers [9]. The ion Yb3+ provides the pumping energy levels and the ion Ho3+ can emit green and red lights excitated with 971–980 nm wavelength. And so, in our work, we added a sort of doped ion Ce3+ to synthesize the transparent oxyfluoride glass ceramics containing of
⁎ Corresponding author. Tel./fax: + 86 59183711368. E-mail address: [email protected] (C. Tu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.01.026
Ho3+/Yb3+/Ce3+: YF3 nanocrystals embedded transparent oxyfluoride glass ceramics, which was the first time to report Ho3+/Yb3+/Ce3+ triply doped oxyfluoride glass ceramics. In the experiment, we amazingly found that the intensities of green and red luminescences relied on the concentration of Ce3+ ions doped in the glass ceramics heavily, which could be used to realize the tunable green–red luminescence. 2. Experiment and discussion The oxyfluoride glasses with composition of 44SiO2–28Al2O3– 17NaF–11YF3–0.1HoF3–0.5YbF3–xCeF3 (x = 0, 0.1, 0.3, 0.5, and 0.8) were prepared by melt quenching from purity SiO2, Al2O3, NaF, YF3, and HoF3, YbF3, CeF3 raw materials. The X-ray diffraction (XRD), a powder diffractometer (DMAX2500 Rigaku) using Cu–Kα radiation (λ = 0.154 nm), was carried out to identify the crystallization phases and the mean size of the crystallites. The microstructures of the samples were studied by a high resolution transmission electron microscope (HRTEM, JEM-2010). The up-conversion emission spectra were measured by the Hamamatsu R943-02 photomultiplier tube and the Spex 1000 M monochromator excitated with 975 nm Ti sapphire laser. All the measurements were carried out at room temperature. The emission spectra of the precursor glass and the glass ceramics excitated with 975 nm were presented, as shown in Fig. 1(a). Here we named the precursor glass with code GCe0 and the glass ceramics with code GCCeJ (J = 0, 1, 3, 5, 8), where J was related to the different CeF3 contents: x = 0, 0.1, 0.3, 0.5, and 0.8 referred above. In the sample GCe0, three peaks with the centers at 545, 650, and 750 nm were observed, which were corresponding to the transitions of Ho3+ energy levels: 5F4, 5S2 → 5I8, 5F5 → 5I8, and 5F4, 5S2 → 5I7,
F. Yang et al. / Materials Letters 64 (2010) 824–826
825
Fig. 1. (a) Emission spectra of precursor glass (code GCe0) and glass ceramics (GCCeJ (J = 0, 1, 3, 5, 8)) excitated with 975 nm LD, the inset at the left side is the blue region for GCCe0 and the right side describes the intensities versus CeF3 contents at wavelengths 545 nm, 650 nm, and 750 nm, respectively;(b) the visible photos corresponding to the glass ceramics excitated with 975 nm LD. Here we named the precursor glass with code GCe0 and the glass ceramics with code GCCeJ (J = 0, 1, 3, 5, 8), where J was related to the different CeF3 contents: x = 0, 0.1, 0.3, 0.5, and 0.8 referred above.
respectively. While in sample GCCe0, the other three bands in blue legion with the centers at 390, 420, and 490 nm were also observed (see the inset at the left side of Fig. 1(a)). Which were corresponding to the transitions of Ho3+ energy levels: 5G4 → 5I8, 5G5 → 5I8, and 5 F3 → 5I8, respectively. From the emission spectra of GCe0 and GCCe0, we could obtain: in sample GCe0, the red light (650 nm) dominated the emission bands; whereas in sample GCCe0, the green one (545 nm) dominated the bands. The intensity of green one (545 nm) in sample GCCe0 was about 500 times stronger than that of GCe0, Moreover, in sample GCCe0, the intensity of green one (545 nm) was 24 and 1000 times stronger than those of red light and infrared light (750 nm), respectively. In addition, the fine splitting emission bands also confirmed that the Ho3+ and Yb3+ ions were just incorporated into the YF3 crystalline phase after thermal treatment. Obviously, it was the addition of ion Ce3+ that dramatically changed the luminescence properties. Furthermore, we changed the concentration of Ce3+ and found that the emission intensity of 545 nm decreased from 505 (arbitrary unit) in sample GCCe0 to 130 in the GCCe1; the intensity of 650 nm decreased from 85 to 70 and the intensity of 750 nm decreased from 200 to 40, respectively. In samples GCCeJ (J = 1, 3, 5, 8), the emissions centered at 390, 420, and 490 nm disappeared completely, which was different from GCCe0. From sample GCCe0 to GCCe8, namely with the increasing concentration of Ce3+, the intensities in the region of 500–800 nm wavelength band changed in diversity. Just as shown in Fig. 1, the intensities of 545 nm and 750 nm decreased heavily at all times, whereas the intensity of 650 nm decreased slowly and even increased inversely. Moreover, in the high doped concentration of Ce3+ samples, the intensity of 650 nm was higher than those of 545 and 750 nm, dominating in the region of 500–800 nm wavelength bands. The intensities of 545, 650, and 750 nm versus the content of CeF3 were shown in the inset at the right side of Fig. 1(a), which indicated that the addition of CeF3 resulted in tunable green–red up-conversion
luminescence in the glass ceramics. This phenomenon was quite different from other Er3+/Yb3+/Ce3+ triply doped oxyfluoride glass ceramics, where the intensities of the green light (540 nm, 520 nm) and the red light (652 nm) decreased heavily with the introduction of ion Ce3+[10,11]. Fig. 1(b) presented the visible photographs for the corresponding glass ceramics excitated with 975 nm, which might be utilized in tunable visible solid-state laser or color display. According to the Ce3+/Ho3+ relative concentration, the most probable cross-relaxation process could occur between Ho3+ and Ce3+ ions with the assistance of phonon. The energy levels were shown in Fig. 2. The cross-relaxation process could take place more easily with the increasing concentration of the Ce3+. After a series of cross-relaxation processes many electrons populated in the 5I7 level of Ho3+. We
Fig. 2. The energy levels diagram of Ho3+, Yb3+, and Ce3+.
826
F. Yang et al. / Materials Letters 64 (2010) 824–826
noticed that the energy gap between 5I7 and 5I8 (about 5200 cm− 1) was wide, and so the cross radiation process: 5I7 (Ho3+) + 2F5/2 (Ce3+) → 5I8(Ho3+) + 2F7/2(Ce3+) could only take place slightly between one Ho3+ and one or two Ce3+ ions by virtue of the phonon assistance (the maximum phonon energy of the lattice was about 500 cm− 1 for YF3 crystal [12]). As a result, there were still many electrons in the 5I7 level, even in the high doped Ce3+ concentration samples. After receiving enough photons transferred from Yb3+ ions, these electrons could be pumped to the 5F5 level of Ho3+ and then emitted 650 nm light. This was the reason that the intensity of 5F5 → 5I8 (Ho3+) at 650 nm emission decreased slowly and even increased in the high CeF3 concentration doped in samples.
controlled red–green luminescence. According to the adjustment of concentration of Ce3+, we could obtain the needed wavelength luminescence, which might be a potential application to the tunable visible laser or display. Acknowledgments This project was supported by the Science & Technology Plan Project of Fujian Province of China (Grant Nos. 2005HZ1026 and 2007H0037), the major project from FJIRSM (SZD08001-2) and FJIRSM (SZD09001), the open fund project of Photo-electricity Material Chemistry and Physics Lab (2009KL004). References
3. Conclusions The Ho3+/Yb3+/Ce3+: YF3 nanocrystals embedded transparent oxyfluoride glass ceramics were synthesized and discussed. Excitated with wavelength 975 nm LD, the transitions of Ho3+:5G4 → 5I8 at 390 nm, 5G5 → 5I8 at 420 nm and 5F3 → 5I8 at wavelength 490 nm were observed in the glass ceramic GCCe0, while in samples GCCeJ (J = 1,3,5,8), the three peaks disappeared. Furthermore, we found that the emission intensities of transitions: 5F4, 5S2 → 5I8 at 545 nm, 5 F5 → 5I8 at 650 nm, and 5F4, 5S2 → 5I7 at 750 nm in the glass ceramic GCCe0 were much higher than those of in the precursor glass GCe0. Particularly, the introduction of Ce3+ into the Ho3+/Yb3+ co-doped glass ceramics brought about unexpected phenomenon of tunable
[1] Oliveira AS, de Araujo M, Gouveia-Neto AS, Medeiros Neto JA, Sombra ASB, Messaddeq Y. Appl Phys Lett 1998;72:753–5. [2] Shan G, Andrews MP, Gonzalez T, Djeghelian H. Mater Lett 2008;6:4187–90. [3] Zhao J, Sun Y, Kong X, Tian L, Wang Y, Tu L, et al. J Phys Chem B 2008;112(49): 15666–72. [4] Wei Y, Lu F, Zhang X, Chen D. Mater Lett 2007;61:1337–40. [5] Chen X. Mater Lett 2009;63:1023–6. [6] Chen GY, Somesfalean G, Zhang ZG, Sun Q, Wang FP. Opt Lett 2007;32:87–9. [7] Wang Y, Ohwaki J. Appl Phys Lett 1993;63:3268–70. [8] Chen D, Wang Y, Yu Y, Huang P. Appl Phys Lett 2007;91:051920. [9] Qiu J, Makishima A. Sci Technol Adv Mater 2004;5:313–7. [10] Dantelle G, Mortier M, Vivien D, Patriarche G. Chem Mater 2005;17:2216–22. [11] Yu XC, Song F, Wang WT, Luo LJ, Ming CG, Cheng ZZ, et al. Opt Commun 2009;282: 2045–8. [12] Chen DQ, Wang YS, Zheng KL, Guo TL, Yu YL, Huang P. Appl Phys Lett 2007;91: 251903.