- Email: [email protected]
Microwave-assisted synthesis and luminescent properties of pure and doped ZnS nanoparticles
Journal of Alloys and Compounds 402 (2005) 274–277
Microwave-assisted synthesis and luminescent properties of pure and doped ZnS nanoparticles Huaming Yang ∗ , Chenghuan Huang, Xiaohui Su, Aidong Tang Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China Received 28 March 2005; received in revised form 25 April 2005; accepted 26 April 2005 Available online 27 June 2005
Abstract Pure and doped ZnS nanoparticles have been successfully synthesized via microwave irradiation. X-ray diffraction (XRD) analysis shows that the crystal size of the pure ZnS particles is about 6.5 nm. The ultraviolet–visible (UV–vis) absorption spectra of the samples indicate that the irradiation time shows no significant influence on the size of ZnS nanoparticles. It is also found that the luminescent properties of ZnS nanoparticles are greatly affected by either the microwave irradiation time or dopants of various metallic ions (Ag+ , Cu2+ , Ce3+ and Sn4+ ). The intensity of photoluminescence (PL) emission reaches its maximum and then decreases with prolonging the microwave irradiation time. The luminescence intensity of ZnS nanoparticles doped with 0.2% cerium, which is stronger than that of other dopants, is about two times that of sample doped with 0.2% Ag+ and 1.6 times that of pure ZnS nanoparticles. But when the amount of doped cerium reaches 0.4%, the emission efficiency is lower than that of pure ZnS. All peak emission wavelength is 450 nm, indicating that the luminescent centers of various metallic ions are not formed within the doped ZnS nanoparticles. © 2005 Elsevier B.V. All rights reserved. Keywords: ZnS; Nanostructures; Microwave irradiation; Doping; Luminescence
1. Introduction Zinc sulfide (ZnS), a wide direct band gap of II–VI compound, is one of the most typical and important crystalline materials for both application and research [1–6]. The most notable feature of nanosized ZnS particles is that its physical and chemical properties dramatically differ from that observed from bulk solid semiconductor, i.e. nanoparticles exhibit wider energy gap and quantum size confinement. Recently, ZnS nanoparticles have been synthesized by various methods, including solid-state reaction, sol–gel process and hydrothermal method, but some of them require longer processing time or higher temperature [7–9]. Microwave is an electromagnetic radiation with the frequency range 0.3–300 GHz. A thermal gradient during microwave processing can be avoided due to the properties of internal and volumetric heating, providing a uniform environ∗
Corresponding author. Tel.: +86 731 8830549; fax: +86 731 8710804. E-mail address: [email protected] (H. Yang).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.04.150
ment for chemical reaction [10]. This method has been successfully applied for the organic and metal clusters synthesis and the preparation of a variety of nanosized inorganic materials [11–14]. Compared with conventional heating, microwave heating has the advantage of high-efficiency and rapid formation of nanoparticles with a narrow size distribution. In this work, we have employed the microwave method to prepare doped and undoped ZnS nanoparticles. The effect of microwave irradiation time and various dopants of ZnS on the PL property of the ZnS nanoparticles have also been discussed.
2. Experimental All reagents were AR-grade and used without further purification. The precipitation of doped ZnS nanoparticles was performed from homogeneous solutions of zinc acetate (Zn(CH3 COO)2 ·2H2 O) and copper chloride (CuCl2 ·2H2 O), or cerium chloride (CeCl3 ·7H2 O), silver nitrate (AgNO3 ),
H. Yang et al. / Journal of Alloys and Compounds 402 (2005) 274–277
tin chloride (SnCl4 ·5H2 O) at 0.05 and 0.05 M sodium sulfide for each reaction. Simultaneously, the mixture filled into a 250 mL round-bottom flask was slowly stirred and ultrasonicated. Then this flask connected with a refluxing system was placed in a refitted domestic Galanz WP900 microwave oven, a water-cooled condenser outside the microwave oven cavity was connected by a glass joint to the round-bottomed flask stably set inside, which was a so-called refluxing system. In order to avoid the leaking of microwave, the glass joint was covered by an aluminium lamella. The reaction was performed under microwave irradiation (2450 MHz) for 10 min. After cooling to ambient temperature, the precipitate was centrifuged at 4000 rpm and washed three times with distilled water, and dried at 60 ◦ C for about 10 h. The crystal structure of the sample was examined by Xray diffraction (XRD) using a D/max-␥A diffractometer (Cu K␣ radiation, λ = 0.154056 nm) at a scanning rate of 4 ◦ /min in the 2θ range from 10◦ to 70◦ . The morphology of sample was observed using a JEM-200CX transmission electron microscope (TEM). The absorption spectra of the samples were measured using a 756MC UV–vis spectrophotometer (deionized water was used as reference). The photoluminescence (PL) spectra of the sample were recorded with a Hitachi M-3500 fluorescence spectrophotometer under ambient atmosphere. The slit was set at 5 nm. The samples were pressed into thin slice before measurements.
3. Results and discussion Fig. 1 shows the XRD patterns of the pure ZnS powders synthesized by microwave irradiation for 10 min. The three diffraction peaks corresponded to the (1 1 1), (2 2 0), (3 1 1) planes, indicating a zinc-blende crystal structure (JCPDS 05-0566, a = 0.5406 nm). The calculated lattice constant of
Fig. 1. XRD pattern of the pure ZnS nanocrytals synthesized by microwave irradiation for 10 min.
275
Fig. 2. TEM image of the pure ZnS nanoparticles synthesized by microwave irradiation for 10 min.
a = 0.5391 nm based on the (1 1 1) plane at 2θ = 28.657◦ is consistent with standard literature value. The average crystal size of the powder calculated by Scherrer’s formula is ca. 6.5 nm. TEM image of the pure ZnS nanoparticles is shown in Fig. 2. The particles are most round and the average particle size is ca. 5 nm. But moderate agglomeration with a size as large as ca. 20 nm can be observed. The selected area electron diffraction (SAED) pattern accounts for a relatively pseudo-crystalline state, indicating that the microstructure of materials is a mixture of orderly and disorderly regions. Fig. 3 shows that the UV–vis absorption spectra of ZnS nanoparticles obtained in different microwave irradiation time, from which little change was observed for absorption onset, indicating that the size of ZnS nanoparticles was not significantly influenced by the irradiation time. The absorption edge (λe being obtained from the intersection of the sharply decreasing region of the spectrum with the baseline) of the ZnS nanoparticles was at a shorter wavelength than 345 nm for bulk ZnS [15]. The blue shift of the absorption edge can be attributed to the quantum confinement of the ZnS nanoparticles. Fig. 4 shows the PL spectra of ZnS nanoparticles synthesized by microwave irradiation for different times. The excitation wavelength is 380 nm. The emission intensity reaches a maximum for the sample obtained at 20 min with a broad peak ranging from 400 to 500 nm and its center at 450 nm, and then sharply decreases with prolonging the irradiation time. Sun et al. reported similar phenomena for the aging effect on the luminescence properties of the doped ZnS colloid [16,17]. In this experimental condition, the nonradiation
276
H. Yang et al. / Journal of Alloys and Compounds 402 (2005) 274–277
Fig. 5. PL spectra of pure and doped ZnS nanoparticles with different metallic ions. Fig. 3. Optical absorption spectra of as-prepared ZnS nanoparticles with different processing time.
relaxation played a prominent role. Surface passivation is realized by electrons filling the empty surface states. However, these filled surface states are metastable. As the irradiation time increases, the electrons in surface states will decay to lower energy level. This makes the nonradiation path increase and the luminescent intensity decrease. Fig. 5 indicates PL spectra of pure ZnS sample, which was synthesized by microwave irradiation for 10 min and ZnS doped with Ag+ , Cu2+ , Ce3+ and Sn4+ . Each dopant concentration is 0.2% (molar ratio). It is found that the emission peaks are at the same position (450 nm). Except for the sample of ZnS doped with cerium ion, the relative emission intensity of samples doped with other valent ions is lower than that of pure ZnS. The intensity of the sample doped
Fig. 4. PL spectra of ZnS nanoparticles synthesized by microwave irradiation for different times.
with 0.2% Ce3+ is about 2 times of that of the sample doped with 0.2% Ag+ and 1.6 times of that of pure ZnS nanoparticles. But when the amount of doped cerium reaches 0.4%, the emission efficiency is lower than that of pure ZnS, as shown in Fig. 6. According to our experimental results, the luminescent centers of Ag+ , Cu2+ , Ce3+ and Sn4+ metallic ions are not formed in the ZnS nanoparticles doped with above these ions. The high luminescent intensity of 0.2% cerium-doped ZnS is due to the enhancement of radiative recombination in luminescent process. The emission efficiency of samples doped with other valent metallic ion is lower than that of pure ZnS, which can be attributed to the increase of nonradiation recombination. Since the ion radius of Ag+ , Cu2+ , Zn2+ and Sn4+ are 0.67, ˚ respectively. Ag+ , Cu2+ and Sn4+ ions 0.62, 0.74 and 0.69 A, may be in Zn sites or as interstitial ions of ZnS nanoparticles. When these ions are doped into the ZnS nanoparticles,
Fig. 6. Emission spectra of pure ZnS nanoparticles and cerium-doped ZnS.
H. Yang et al. / Journal of Alloys and Compounds 402 (2005) 274–277
their deep centers are formed, which can inhibit more electrons (holes) to be excited and can lead to the enhancement of nonradiative recombination processes. As a result, their emission intensities become weaker than that of pure ZnS nanoparticles. ˚ larger However, since the radius of Ce3+ ion is 1.03 A, ˚ so it is difficult for the Ce3+ ion than that of Zn2+ (0.74 A), to substitute the Zn2+ sites in ZnS lattice. Hence, for the sample of cerium-doped ZnS, the position of Ce3+ ion can be classified into three types: (1) Ce3+ ions can be at the zenith of the ZnS lattice co-occupied by the multi-ZnS nanoparticle; (2) Ce3+ ions are surrounded by ZnS nanoparticles; (3) most of the Ce3+ ions are physically adsorbed on the surface of ZnS nanoparticles. The probability of the three types not only depends on the Ce3+ ion concentration during processing, but also on the micro-crystal field around Ce3+ [16]. The experimental results show that Ce3+ ion is a sensitizing agent. The high luminescent intensity of 0.2% cerium-doped ZnS is due to the enhancement of radiative recombination in the luminescence process.
4. Conclusions In summary, ZnS nanoparticles have been successfully synthesized via a simple microwave-assisted heating process. The UV–vis absorption spectra show that the size of ZnS nanoparticles was not significantly affected by the microwave irradiation time, however, the luminescence properties of ZnS nanoparticles were affected. The intensity of PL emission reaches a maximum and then decreases by prolonging the microwave irradiation time, which may be related to surface states. It has been found that the intensity of ZnS nanoparticles doped with 0.2% Ce3+ is about 2 times of that of sample doped with 0.2% Ag+ and 1.6 times of that of pure ZnS nanoparticles. The relative emission intensity of
277
ZnS nanoparticles doped with other metallic ions at 0.2% and doped with 0.4% Ce3+ is lower than that of pure ZnS, which is due to the enhancement of nonradiative recombination in luminescent process. All peak emission wavelength is 450 nm, indicating that the luminescent centers of the various metallic ions are not formed in the doped ZnS nanoparticles.
Acknowledgements This work was supported by the National Natural Science Foundation of China (50304014) and the Program for New Century 121 Excellent Talents in Hunan Province. References [1] O. Tetsuichiro, K. Kenichi, T. Tsunemasa, J. Cryst. Growth 99 (1990) 737. [2] P.I. Ekwo, C.E. Okeke, Energy Convers. Manage. 33 (1992) 159. [3] L.P. Colletti, R. Slaughter, J.L. Stickney, J. Soc. Inform. Display 5 (1997) 87. [4] S. Velumani, J.A. Ascencio, Appl. Phys. A 79 (2004) 153. [5] P. Yang, M.K. L¨u, D. X¨u, J. Phys. Chem. Solids 64 (2003) 155. [6] J.F. Chen, Y.L. Li, Y.J. Yun, Mater. Res. Bull. 39 (2004) 185. [7] V. Stani´c, T.H. Etsell, A.C. Pierre, Mater. Lett. 31 (1997) 35. [8] T.A. Guiton, C.L. Czekai, C.G. Pantano, J. Non-Cryst. Solids 121 (1990) 7. [9] T. Hanaoka, T. Taqo, M. Kishida, Bull. Chem. Soc. Jpn. 74 (2001) 1349. [10] M.J. Blandamer, A.R. Butler, D.R. Baghurst, Chem. Soc. Rev. 20 (1991) 1. [11] P. Lidstr¨om, J. Tierney, B. Wathery, Tetrahedron 57 (2001) 9225. [12] W.X. Tu, H.F. Liu, J. Mater. Chem. 10 (2000) 2207. [13] H. Wang, J.Z. Xu, J.J. Zhu, J. Cryst. Growth 244 (2002) 88. [14] Q. Liu, Y. Wei, Mater. Res. Bull. 33 (1998) 779. [15] R. He, X.F. Qian, J. Yin, Colloids Surf. A 220 (2003) 151. [16] L.D. Sun, C.H. Yan, C.H. Liu, J. Alloys Compd. 275–277 (1998) 234. [17] C.M. Jin, J.Q. Yu, L.D. Sun, J. Lumin. 66–67 (1996) 315.
Microwave-assisted synthesis and luminescent properties of pure and doped ZnS nanoparticles Huaming Yang ∗ , Chenghuan Huang, Xiaohui Su, Aidong Tang Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China Received 28 March 2005; received in revised form 25 April 2005; accepted 26 April 2005 Available online 27 June 2005
Abstract Pure and doped ZnS nanoparticles have been successfully synthesized via microwave irradiation. X-ray diffraction (XRD) analysis shows that the crystal size of the pure ZnS particles is about 6.5 nm. The ultraviolet–visible (UV–vis) absorption spectra of the samples indicate that the irradiation time shows no significant influence on the size of ZnS nanoparticles. It is also found that the luminescent properties of ZnS nanoparticles are greatly affected by either the microwave irradiation time or dopants of various metallic ions (Ag+ , Cu2+ , Ce3+ and Sn4+ ). The intensity of photoluminescence (PL) emission reaches its maximum and then decreases with prolonging the microwave irradiation time. The luminescence intensity of ZnS nanoparticles doped with 0.2% cerium, which is stronger than that of other dopants, is about two times that of sample doped with 0.2% Ag+ and 1.6 times that of pure ZnS nanoparticles. But when the amount of doped cerium reaches 0.4%, the emission efficiency is lower than that of pure ZnS. All peak emission wavelength is 450 nm, indicating that the luminescent centers of various metallic ions are not formed within the doped ZnS nanoparticles. © 2005 Elsevier B.V. All rights reserved. Keywords: ZnS; Nanostructures; Microwave irradiation; Doping; Luminescence
1. Introduction Zinc sulfide (ZnS), a wide direct band gap of II–VI compound, is one of the most typical and important crystalline materials for both application and research [1–6]. The most notable feature of nanosized ZnS particles is that its physical and chemical properties dramatically differ from that observed from bulk solid semiconductor, i.e. nanoparticles exhibit wider energy gap and quantum size confinement. Recently, ZnS nanoparticles have been synthesized by various methods, including solid-state reaction, sol–gel process and hydrothermal method, but some of them require longer processing time or higher temperature [7–9]. Microwave is an electromagnetic radiation with the frequency range 0.3–300 GHz. A thermal gradient during microwave processing can be avoided due to the properties of internal and volumetric heating, providing a uniform environ∗
Corresponding author. Tel.: +86 731 8830549; fax: +86 731 8710804. E-mail address: [email protected] (H. Yang).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.04.150
ment for chemical reaction [10]. This method has been successfully applied for the organic and metal clusters synthesis and the preparation of a variety of nanosized inorganic materials [11–14]. Compared with conventional heating, microwave heating has the advantage of high-efficiency and rapid formation of nanoparticles with a narrow size distribution. In this work, we have employed the microwave method to prepare doped and undoped ZnS nanoparticles. The effect of microwave irradiation time and various dopants of ZnS on the PL property of the ZnS nanoparticles have also been discussed.
2. Experimental All reagents were AR-grade and used without further purification. The precipitation of doped ZnS nanoparticles was performed from homogeneous solutions of zinc acetate (Zn(CH3 COO)2 ·2H2 O) and copper chloride (CuCl2 ·2H2 O), or cerium chloride (CeCl3 ·7H2 O), silver nitrate (AgNO3 ),
H. Yang et al. / Journal of Alloys and Compounds 402 (2005) 274–277
tin chloride (SnCl4 ·5H2 O) at 0.05 and 0.05 M sodium sulfide for each reaction. Simultaneously, the mixture filled into a 250 mL round-bottom flask was slowly stirred and ultrasonicated. Then this flask connected with a refluxing system was placed in a refitted domestic Galanz WP900 microwave oven, a water-cooled condenser outside the microwave oven cavity was connected by a glass joint to the round-bottomed flask stably set inside, which was a so-called refluxing system. In order to avoid the leaking of microwave, the glass joint was covered by an aluminium lamella. The reaction was performed under microwave irradiation (2450 MHz) for 10 min. After cooling to ambient temperature, the precipitate was centrifuged at 4000 rpm and washed three times with distilled water, and dried at 60 ◦ C for about 10 h. The crystal structure of the sample was examined by Xray diffraction (XRD) using a D/max-␥A diffractometer (Cu K␣ radiation, λ = 0.154056 nm) at a scanning rate of 4 ◦ /min in the 2θ range from 10◦ to 70◦ . The morphology of sample was observed using a JEM-200CX transmission electron microscope (TEM). The absorption spectra of the samples were measured using a 756MC UV–vis spectrophotometer (deionized water was used as reference). The photoluminescence (PL) spectra of the sample were recorded with a Hitachi M-3500 fluorescence spectrophotometer under ambient atmosphere. The slit was set at 5 nm. The samples were pressed into thin slice before measurements.
3. Results and discussion Fig. 1 shows the XRD patterns of the pure ZnS powders synthesized by microwave irradiation for 10 min. The three diffraction peaks corresponded to the (1 1 1), (2 2 0), (3 1 1) planes, indicating a zinc-blende crystal structure (JCPDS 05-0566, a = 0.5406 nm). The calculated lattice constant of
Fig. 1. XRD pattern of the pure ZnS nanocrytals synthesized by microwave irradiation for 10 min.
275
Fig. 2. TEM image of the pure ZnS nanoparticles synthesized by microwave irradiation for 10 min.
a = 0.5391 nm based on the (1 1 1) plane at 2θ = 28.657◦ is consistent with standard literature value. The average crystal size of the powder calculated by Scherrer’s formula is ca. 6.5 nm. TEM image of the pure ZnS nanoparticles is shown in Fig. 2. The particles are most round and the average particle size is ca. 5 nm. But moderate agglomeration with a size as large as ca. 20 nm can be observed. The selected area electron diffraction (SAED) pattern accounts for a relatively pseudo-crystalline state, indicating that the microstructure of materials is a mixture of orderly and disorderly regions. Fig. 3 shows that the UV–vis absorption spectra of ZnS nanoparticles obtained in different microwave irradiation time, from which little change was observed for absorption onset, indicating that the size of ZnS nanoparticles was not significantly influenced by the irradiation time. The absorption edge (λe being obtained from the intersection of the sharply decreasing region of the spectrum with the baseline) of the ZnS nanoparticles was at a shorter wavelength than 345 nm for bulk ZnS [15]. The blue shift of the absorption edge can be attributed to the quantum confinement of the ZnS nanoparticles. Fig. 4 shows the PL spectra of ZnS nanoparticles synthesized by microwave irradiation for different times. The excitation wavelength is 380 nm. The emission intensity reaches a maximum for the sample obtained at 20 min with a broad peak ranging from 400 to 500 nm and its center at 450 nm, and then sharply decreases with prolonging the irradiation time. Sun et al. reported similar phenomena for the aging effect on the luminescence properties of the doped ZnS colloid [16,17]. In this experimental condition, the nonradiation
276
H. Yang et al. / Journal of Alloys and Compounds 402 (2005) 274–277
Fig. 5. PL spectra of pure and doped ZnS nanoparticles with different metallic ions. Fig. 3. Optical absorption spectra of as-prepared ZnS nanoparticles with different processing time.
relaxation played a prominent role. Surface passivation is realized by electrons filling the empty surface states. However, these filled surface states are metastable. As the irradiation time increases, the electrons in surface states will decay to lower energy level. This makes the nonradiation path increase and the luminescent intensity decrease. Fig. 5 indicates PL spectra of pure ZnS sample, which was synthesized by microwave irradiation for 10 min and ZnS doped with Ag+ , Cu2+ , Ce3+ and Sn4+ . Each dopant concentration is 0.2% (molar ratio). It is found that the emission peaks are at the same position (450 nm). Except for the sample of ZnS doped with cerium ion, the relative emission intensity of samples doped with other valent ions is lower than that of pure ZnS. The intensity of the sample doped
Fig. 4. PL spectra of ZnS nanoparticles synthesized by microwave irradiation for different times.
with 0.2% Ce3+ is about 2 times of that of the sample doped with 0.2% Ag+ and 1.6 times of that of pure ZnS nanoparticles. But when the amount of doped cerium reaches 0.4%, the emission efficiency is lower than that of pure ZnS, as shown in Fig. 6. According to our experimental results, the luminescent centers of Ag+ , Cu2+ , Ce3+ and Sn4+ metallic ions are not formed in the ZnS nanoparticles doped with above these ions. The high luminescent intensity of 0.2% cerium-doped ZnS is due to the enhancement of radiative recombination in luminescent process. The emission efficiency of samples doped with other valent metallic ion is lower than that of pure ZnS, which can be attributed to the increase of nonradiation recombination. Since the ion radius of Ag+ , Cu2+ , Zn2+ and Sn4+ are 0.67, ˚ respectively. Ag+ , Cu2+ and Sn4+ ions 0.62, 0.74 and 0.69 A, may be in Zn sites or as interstitial ions of ZnS nanoparticles. When these ions are doped into the ZnS nanoparticles,
Fig. 6. Emission spectra of pure ZnS nanoparticles and cerium-doped ZnS.
H. Yang et al. / Journal of Alloys and Compounds 402 (2005) 274–277
their deep centers are formed, which can inhibit more electrons (holes) to be excited and can lead to the enhancement of nonradiative recombination processes. As a result, their emission intensities become weaker than that of pure ZnS nanoparticles. ˚ larger However, since the radius of Ce3+ ion is 1.03 A, ˚ so it is difficult for the Ce3+ ion than that of Zn2+ (0.74 A), to substitute the Zn2+ sites in ZnS lattice. Hence, for the sample of cerium-doped ZnS, the position of Ce3+ ion can be classified into three types: (1) Ce3+ ions can be at the zenith of the ZnS lattice co-occupied by the multi-ZnS nanoparticle; (2) Ce3+ ions are surrounded by ZnS nanoparticles; (3) most of the Ce3+ ions are physically adsorbed on the surface of ZnS nanoparticles. The probability of the three types not only depends on the Ce3+ ion concentration during processing, but also on the micro-crystal field around Ce3+ [16]. The experimental results show that Ce3+ ion is a sensitizing agent. The high luminescent intensity of 0.2% cerium-doped ZnS is due to the enhancement of radiative recombination in the luminescence process.
4. Conclusions In summary, ZnS nanoparticles have been successfully synthesized via a simple microwave-assisted heating process. The UV–vis absorption spectra show that the size of ZnS nanoparticles was not significantly affected by the microwave irradiation time, however, the luminescence properties of ZnS nanoparticles were affected. The intensity of PL emission reaches a maximum and then decreases by prolonging the microwave irradiation time, which may be related to surface states. It has been found that the intensity of ZnS nanoparticles doped with 0.2% Ce3+ is about 2 times of that of sample doped with 0.2% Ag+ and 1.6 times of that of pure ZnS nanoparticles. The relative emission intensity of
277
ZnS nanoparticles doped with other metallic ions at 0.2% and doped with 0.4% Ce3+ is lower than that of pure ZnS, which is due to the enhancement of nonradiative recombination in luminescent process. All peak emission wavelength is 450 nm, indicating that the luminescent centers of the various metallic ions are not formed in the doped ZnS nanoparticles.
Acknowledgements This work was supported by the National Natural Science Foundation of China (50304014) and the Program for New Century 121 Excellent Talents in Hunan Province. References [1] O. Tetsuichiro, K. Kenichi, T. Tsunemasa, J. Cryst. Growth 99 (1990) 737. [2] P.I. Ekwo, C.E. Okeke, Energy Convers. Manage. 33 (1992) 159. [3] L.P. Colletti, R. Slaughter, J.L. Stickney, J. Soc. Inform. Display 5 (1997) 87. [4] S. Velumani, J.A. Ascencio, Appl. Phys. A 79 (2004) 153. [5] P. Yang, M.K. L¨u, D. X¨u, J. Phys. Chem. Solids 64 (2003) 155. [6] J.F. Chen, Y.L. Li, Y.J. Yun, Mater. Res. Bull. 39 (2004) 185. [7] V. Stani´c, T.H. Etsell, A.C. Pierre, Mater. Lett. 31 (1997) 35. [8] T.A. Guiton, C.L. Czekai, C.G. Pantano, J. Non-Cryst. Solids 121 (1990) 7. [9] T. Hanaoka, T. Taqo, M. Kishida, Bull. Chem. Soc. Jpn. 74 (2001) 1349. [10] M.J. Blandamer, A.R. Butler, D.R. Baghurst, Chem. Soc. Rev. 20 (1991) 1. [11] P. Lidstr¨om, J. Tierney, B. Wathery, Tetrahedron 57 (2001) 9225. [12] W.X. Tu, H.F. Liu, J. Mater. Chem. 10 (2000) 2207. [13] H. Wang, J.Z. Xu, J.J. Zhu, J. Cryst. Growth 244 (2002) 88. [14] Q. Liu, Y. Wei, Mater. Res. Bull. 33 (1998) 779. [15] R. He, X.F. Qian, J. Yin, Colloids Surf. A 220 (2003) 151. [16] L.D. Sun, C.H. Yan, C.H. Liu, J. Alloys Compd. 275–277 (1998) 234. [17] C.M. Jin, J.Q. Yu, L.D. Sun, J. Lumin. 66–67 (1996) 315.