- Email: [email protected]
Photoluminescence of ZnO nanoparticles prepared by a novel gel-template combustion process
ARTICLE IN PRESS
Journal of Luminescence 119–120 (2006) 248–252 www.elsevier.com/locate/jlumin
Photoluminescence of ZnO nanoparticles prepared by a novel gel-template combustion process Jianguo Zhoua,, Yali Wanga, Fengying Zhaoa,b, Yingling Wanga, Yan Zhanga, Lin Yanga, a
College of Chemistry and Environment Science, Henan Normal University, Xinxiang 453007, Henan Province, China b Institute of Science Technology and Society, Henan Normal University, China Available online 30 January 2006
Abstract ZnO nanoparticles have been synthesized by mean of a novel gel-template combustion method. The products were characterized by using TG-DTA, XRD and TEM techniques. The products appeared to be regularly spherical or elliptical and their sizes ranged from 20 to 30 nm.The average particle size increased with increasing sintering temperature. The photoluminescence spectra of the resulting ZnO nanoparticles showed a near-UV emission and a green emission. Furthermore, the dependence of the optical properties on the annealing temperatures and annealing time were also investigated. With the increase of the annealing temperature, the intensity of the green emission at 500 nm caused by oxygen vacancies in ZnO rapidly increased, but almost disappeared only at 600 1C.The probable mechanism was discussed. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 78.67.Bf; 81.07.Bc; 81.20.Ka Keywords: ZnO; Nanoparticles; Template combustion synthesis; Photoluminescence properties
1. Introduction There has been much recent interest in preparation and properties of nanoparticles, such as metal and oxides, etc. ZnO as a wide-band-gap semiconductor has attracted more and more attention Corresponding authors. Tel.: +86 373 3326990; +86 013937395640; fax : +86 373 3329115. E-mail addresses: [email protected], zhoujgwj@sina. com (J. Zhou), [email protected] (L. Yang).
over the past few years [1,2]. ZnO is also an interesting compound semiconductor for ultraviolet (UV) LED and laser applications due to its energy gap of 3.4 eV. In addition, the exciton binding energy of 60 meV exceeds the room-temperature energy of 26 meV.Therefore, ZnO can achieve excitonic UV emission, even at room temperature, thus expected to be a promising candidate for room-temperature UV LED and laser. Up to now, ZnO nanoparticles have been investigated widely as a luminescent material
0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2005.12.038
ARTICLE IN PRESS J. Zhou et al. / Journal of Luminescence 119– 120 (2006) 248–252
Exo
Fig. 1 shows TG-DTA curses of the precursor. There were two stages of weight loss lying in the temperature ranges 70–200 and 290–510 1C, respectively. The strong exothermic peak at 191 1C in DTA curve accompanying with a most significant weight loss, was attributed to the decomposition of nitrate in the precursor. Another exothermic reaction took place between 300 and 500 1C, which corresponded to the decomposition of residue gelatin and complete oxidation of carbonaceous matters. The XRD patterns are shown in Fig. 2. The precursor was amorphous. Crystallization to pure
TG 100 80
∆T
All of the chemical reagents used in the experiments were analytical grade without further purification and treatment. ZnO nanoparticles were synthesized via a procedure similar to that reported previously for YAG:Bi,Eu nanoparticles [13]. The typical procedure was depicted as follows: 6 g Zn(NO3)2 6H2O was dissolved in 50 ml distilled water and then 5 g gelatin was slowly added into the above solution under vigorous stirring in an 80 1C water bath. The mixture was then cooled to 4 1C turning to a yellowish gel. The gel was dried in a vacuum chamber at 60 1C for 12 h. The dried gel (hereafter termed as precursor) was preheated at 200 1C for
3. Results and discussion
60 Endo
2. Experimental
2 h in air. The obtained powders were subsequently annealed at various temperatures from 400 to 800 1C for 3 h in a muffle furnace in air, producing fine powders. TG-DTA analysis of the precursor was carried out on Shimadzu DT-40 thermal analyzer. Phase formation of product was identified by using a Bruker D8 Advance X-ray diffractometer. The morphology and particle size of the product was examined by transmission electronic microscope (TEM) (Jeol 200CX, Japan). The crystallite size of the prepared nanoparticles was calculated following the Debye–Sherrer formula. The photoluminescence spectra of product were performed on a Hitachi F-4500 fluorescence spectrometer (Xe lamp) at room temperature.
Weight (%)
because nanocrystallization can change the optical properties of wide-gap semiconductors by the quantum confinement effect [3]. However, surface defects and impurity levels are easily generated during the nanoparticle formation process. Several surface treatment approaches succeeded in suppressing the green emission intensity from ZnO nanostructures [4–6]. However, these processes require multiple steps for obtaining high-intensity UV emission without green emission. The preparation of different sizes and shapes of ZnO is often studied in previous reports [7–9]. The luminescence properties of the ZnO particles [10] have been reported and discussed. In the meantime, with regard to the preparation of the ZnO particles via calcinations [11] or thermal treatment [12], though there have been a lot of reports, suggesting the luminescence properties depended on the preparation method and particle morphology. In the present study, ZnO nanoparticles were prepared by using a novel template combustion process, aiming for ZnO nanostructures with strong UV emission using a simple one-step process. The sizes, morphologies, crystallinities, and photoluminescence properties of the resulting ZnO particles were investigated. The influence of annealing temperatures and time on the sizes and photoluminescence properties of ZnO nanoparticles were also investigated.
249
DTA
40 20 20
210
400
590
Temperature (˚C) Fig. 1. TG-DTA curves of the precursor.
780
ARTICLE IN PRESS 250
J. Zhou et al. / Journal of Luminescence 119– 120 (2006) 248–252
ZnO set in around 200 1C and progressed upon further heating. These as-prepared powders had the phase composition of wurtzite ZnO with P63mc structure (JCPDS 36–1451). The average crystallite size values for the precursor heated to 200 and 500 1C were found to be basically the same (20 nm) , while the samples annealed at 600 1C was 40 nm.This observation showed that in the range of 200–500 1C only the progress of crystallization from amorphous precursor took place while further growth of crystallites occured in the above 500 1C. Thus well-crystallized ZnO formed from precursor above 500 1C. Fig. 3 shows TEM micrograph of the ZnO sample annealed at 600 1C. The ZnO powders were uniform with well-distributed spherical or elliptical particle with a sizes range from 40 to 60 nm.This was consistent with the grain sizes estimated by using XRD peak broadening method. The average particle size increased with increasing annealing temperature, thus suggesting the gradual growth of the nanoparticles during the heating process. Fig. 4 shows the room-temperature PL spectra of the ZnO nanoparticles characterized above. The characteristic two emission peaks, a narrow nearUV and a broad green peak, were observed. The near-UV emission at 380 nm agrees with the band gap of bulk ZnO [14], which comes from the recombination of free excitons [15]. The green
Fig. 3. TEM micrograph of the ZnO powders annealed at 600 1C.
Fig. 4. The PL spectra of ZnO nanoparticles annealed at different temperatures in the oxygen atmosphere.
Fig. 2. XRD patterns of the ZnO precursor annealed at different temperatures.
emission at 500 nm is related to the singly ionized oxygen vacancy, and this emission results from the recombination of a photogenerated hole with a singly ionized charge state of the specific defect [16]. The oxygen vacancies in ZnO can be modified by annealing the samples in O2 atmosphere [17]. We annealed our sample under O2 atmosphere at 400, 500, 600, 650, 700 and 800 1C, respectively. The green emission could be quenched at 600 1C, indicating that the crystalline quality of the sample improved. But when the temperature reached 650 1C, the green emission started to increase. When the annealing temperature was above
ARTICLE IN PRESS J. Zhou et al. / Journal of Luminescence 119– 120 (2006) 248–252
700 1C, the green emission eventually dominanted the PL spectrum. The intensity ratio of the nearUV emission and green emission (Ri) decreased from 4.8 at 400 1C to 0.4 at 800 1C. The UV emission peak was slightly red-shift with increasing annealing temperature, which resulted from the defect related shallow binding excitons formed during high-temperature annealing. Fig. 5 showed room-temperature PL spectra of the ZnO nanoparticles annealed at different conditions in O2 atmosphere. At 500 1C (Fig. 5a), with the increasing of annealing time, the intensity of UV peak increased, the intensity of green emission decreased, and the intensity ratio (Ri) increased, indicating that crystalline quality of the sample improved due to the oxygen entering into the
251
crystal lattice sites and improving the stoichiometric proportion of the sample and therefore enhancing its crystal quality. However, annealed at 800 1C (Fig. 5b),with the increasing of annealing time, the intensities of both UV emission and green emission improved, but, the Ri decreased, suggesting that there was a higher concentration of defects (oxygen vacancies) in the ZnO nanoparticles likely due to the reevaporation of O. We think this behavior is due to the competition between the O atoms getting into the lattice and those evaporating out of the ZnO lattice in O2 atmosphere. At lower heating temperature, the kinetic energy of atoms in the ZnO lattice is relatively low and the adsorption rate of the O atoms is faster than the escaping rate. So more O atoms can compensate the O vacancies at lower temperature (o600 1C). But at higher heating temperatures, the kinetic energy of the atoms becomes larger and larger, possibly resulting in a larger escaping rate of O atoms than the adsorption rate to make more O vacancies in the ZnO lattice. This also strongly suggests that the deeplevel emission is the result of oxygen vacancies in the ZnO nanoparticles. Things we want to point out were that the observed optimized heating temperature of 600 1C was low and strong UV emission without green emission was obtained by a simple one-step process.
4. Conclusion
Fig. 5. The PL spectra of ZnO nanoparticles annealed at different conditions: (a) at 500 1C, (b) at 800 1C.
ZnO nanoparticles were prepared by a novel template combustion process, aiming for strong UV emission without green luminescence, by a simple one-step process. We investigated the influence of heating conditions on size and photoluminescence of ZnO nanoparticles. The average sizes of nanoparticles increased with the increasing annealing temperature, and this increase was prominent when the annealing temperature exceeded 600 1C. The resulting ZnO nanoparticles demonstrated a near-UV emission and a green emission, and the photoluminescence properties depended on heating temperatures and annealing time. With the increasing of annealing temperature, the green emission intensity caused
ARTICLE IN PRESS 252
J. Zhou et al. / Journal of Luminescence 119– 120 (2006) 248–252
by oxygen defects of ZnO increased remarkably. The photoluminescence properties of ZnO nanoparticles were thus controlled by employing appropriate heating condition, without changing their morphology. Acknowledgements Financial supports from the Henan Train Project for University Innovation Talents, National Natural Science Foundation of China (No. 20371016), Natural Science Foundation of Henan (No. 0111030200), Henan Education Department (No. 2004150005) and National Program on Key Basic Research Projects of China (973 Program, No. 2005CB724306). References [1] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289. [2] M.H. Huang, S. Mao, H. Feick, J.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897.
[3] Y. Wang, N. Herron, J. Phys. Chem 95 (1991) 525. [4] J.S. Kang, H.S. Kang, S.S. Pang, E.S. Shim, S.Y. Lee, Thin Solid Films 443 (2003) 5. [5] C.L. Yang, J.N. Wang, W.K. Ge, L. Guo, S.H. Yang, D.Z. Shen, J. Appl. Phys 90 (2001) 4489. [6] Y.G. Wang, S.P. Lau, X.H. Zhang, H.H. Hng, H.W. Lee, S.F. Yu, B.K. Tay, J. Cryst. Growth 259 (2003) 335. [7] J.Q. Hu, X.L. Ma, Z.Y. Xie, N.B. Wong, C.S. Lee, S.T. Lee, Chem. Phys. Lett 334 (2001) 97. [8] Y. Li, G.W. Meng, L.D. Zhang, F. Phillipp, Appl. Phys. Lett 76 (2000) 2011. [9] X.S. Fang, C.H. Ye, X.S. Peng, Y.H. Wang, Y.C. Wu, L.D. Zhang, J .Mater. Chem 3 (2003) 3040. [10] X.T. Zhang, Y.C. Liu, Z.Z. Zhi, J.Y. Zhang, Y.M. Lu, W. Xu, D.Z. Shen, G.Z. Zhong, X.W. Fan, X.G. Kong, J. Cryst. Growth 240 (2002) 463. [11] J.Q. Hu, Q. Li, N.B. Wong, C.S. Lee, S.T. Lee, Chem. Mater 14 (2002) 1216. [12] L. Guo, Y.L. Ji, H. Xu, P. Simon, Z. Wu, J. Am. Chem. Soc 124 (14) (2002) 864. [13] J.G. Zhou, F.Y. Zhao, Z.Q. Li, S.P. Xia, L. Yang, S.Y. Gao, J. Mater. Sci 39 (2004) 4711. [14] E.M. Wong, P.C. Searson, Appl. Phys. Lett 74 (1999) 2939. [15] V. Stikant, D.R. Clarke, J. Appl. Phys 83 (1998) 5447. [16] Y. Li, G.S. Cheng, L.D. Zhang, J. Mater. Res 15 (2000) 2305. [17] J. Lim, K. Shin, H.W. Kim, C. Lee, Mater. Sci. Eng B 107 (2004) 301.
Journal of Luminescence 119–120 (2006) 248–252 www.elsevier.com/locate/jlumin
Photoluminescence of ZnO nanoparticles prepared by a novel gel-template combustion process Jianguo Zhoua,, Yali Wanga, Fengying Zhaoa,b, Yingling Wanga, Yan Zhanga, Lin Yanga, a
College of Chemistry and Environment Science, Henan Normal University, Xinxiang 453007, Henan Province, China b Institute of Science Technology and Society, Henan Normal University, China Available online 30 January 2006
Abstract ZnO nanoparticles have been synthesized by mean of a novel gel-template combustion method. The products were characterized by using TG-DTA, XRD and TEM techniques. The products appeared to be regularly spherical or elliptical and their sizes ranged from 20 to 30 nm.The average particle size increased with increasing sintering temperature. The photoluminescence spectra of the resulting ZnO nanoparticles showed a near-UV emission and a green emission. Furthermore, the dependence of the optical properties on the annealing temperatures and annealing time were also investigated. With the increase of the annealing temperature, the intensity of the green emission at 500 nm caused by oxygen vacancies in ZnO rapidly increased, but almost disappeared only at 600 1C.The probable mechanism was discussed. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 78.67.Bf; 81.07.Bc; 81.20.Ka Keywords: ZnO; Nanoparticles; Template combustion synthesis; Photoluminescence properties
1. Introduction There has been much recent interest in preparation and properties of nanoparticles, such as metal and oxides, etc. ZnO as a wide-band-gap semiconductor has attracted more and more attention Corresponding authors. Tel.: +86 373 3326990; +86 013937395640; fax : +86 373 3329115. E-mail addresses: [email protected], zhoujgwj@sina. com (J. Zhou), [email protected] (L. Yang).
over the past few years [1,2]. ZnO is also an interesting compound semiconductor for ultraviolet (UV) LED and laser applications due to its energy gap of 3.4 eV. In addition, the exciton binding energy of 60 meV exceeds the room-temperature energy of 26 meV.Therefore, ZnO can achieve excitonic UV emission, even at room temperature, thus expected to be a promising candidate for room-temperature UV LED and laser. Up to now, ZnO nanoparticles have been investigated widely as a luminescent material
0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2005.12.038
ARTICLE IN PRESS J. Zhou et al. / Journal of Luminescence 119– 120 (2006) 248–252
Exo
Fig. 1 shows TG-DTA curses of the precursor. There were two stages of weight loss lying in the temperature ranges 70–200 and 290–510 1C, respectively. The strong exothermic peak at 191 1C in DTA curve accompanying with a most significant weight loss, was attributed to the decomposition of nitrate in the precursor. Another exothermic reaction took place between 300 and 500 1C, which corresponded to the decomposition of residue gelatin and complete oxidation of carbonaceous matters. The XRD patterns are shown in Fig. 2. The precursor was amorphous. Crystallization to pure
TG 100 80
∆T
All of the chemical reagents used in the experiments were analytical grade without further purification and treatment. ZnO nanoparticles were synthesized via a procedure similar to that reported previously for YAG:Bi,Eu nanoparticles [13]. The typical procedure was depicted as follows: 6 g Zn(NO3)2 6H2O was dissolved in 50 ml distilled water and then 5 g gelatin was slowly added into the above solution under vigorous stirring in an 80 1C water bath. The mixture was then cooled to 4 1C turning to a yellowish gel. The gel was dried in a vacuum chamber at 60 1C for 12 h. The dried gel (hereafter termed as precursor) was preheated at 200 1C for
3. Results and discussion
60 Endo
2. Experimental
2 h in air. The obtained powders were subsequently annealed at various temperatures from 400 to 800 1C for 3 h in a muffle furnace in air, producing fine powders. TG-DTA analysis of the precursor was carried out on Shimadzu DT-40 thermal analyzer. Phase formation of product was identified by using a Bruker D8 Advance X-ray diffractometer. The morphology and particle size of the product was examined by transmission electronic microscope (TEM) (Jeol 200CX, Japan). The crystallite size of the prepared nanoparticles was calculated following the Debye–Sherrer formula. The photoluminescence spectra of product were performed on a Hitachi F-4500 fluorescence spectrometer (Xe lamp) at room temperature.
Weight (%)
because nanocrystallization can change the optical properties of wide-gap semiconductors by the quantum confinement effect [3]. However, surface defects and impurity levels are easily generated during the nanoparticle formation process. Several surface treatment approaches succeeded in suppressing the green emission intensity from ZnO nanostructures [4–6]. However, these processes require multiple steps for obtaining high-intensity UV emission without green emission. The preparation of different sizes and shapes of ZnO is often studied in previous reports [7–9]. The luminescence properties of the ZnO particles [10] have been reported and discussed. In the meantime, with regard to the preparation of the ZnO particles via calcinations [11] or thermal treatment [12], though there have been a lot of reports, suggesting the luminescence properties depended on the preparation method and particle morphology. In the present study, ZnO nanoparticles were prepared by using a novel template combustion process, aiming for ZnO nanostructures with strong UV emission using a simple one-step process. The sizes, morphologies, crystallinities, and photoluminescence properties of the resulting ZnO particles were investigated. The influence of annealing temperatures and time on the sizes and photoluminescence properties of ZnO nanoparticles were also investigated.
249
DTA
40 20 20
210
400
590
Temperature (˚C) Fig. 1. TG-DTA curves of the precursor.
780
ARTICLE IN PRESS 250
J. Zhou et al. / Journal of Luminescence 119– 120 (2006) 248–252
ZnO set in around 200 1C and progressed upon further heating. These as-prepared powders had the phase composition of wurtzite ZnO with P63mc structure (JCPDS 36–1451). The average crystallite size values for the precursor heated to 200 and 500 1C were found to be basically the same (20 nm) , while the samples annealed at 600 1C was 40 nm.This observation showed that in the range of 200–500 1C only the progress of crystallization from amorphous precursor took place while further growth of crystallites occured in the above 500 1C. Thus well-crystallized ZnO formed from precursor above 500 1C. Fig. 3 shows TEM micrograph of the ZnO sample annealed at 600 1C. The ZnO powders were uniform with well-distributed spherical or elliptical particle with a sizes range from 40 to 60 nm.This was consistent with the grain sizes estimated by using XRD peak broadening method. The average particle size increased with increasing annealing temperature, thus suggesting the gradual growth of the nanoparticles during the heating process. Fig. 4 shows the room-temperature PL spectra of the ZnO nanoparticles characterized above. The characteristic two emission peaks, a narrow nearUV and a broad green peak, were observed. The near-UV emission at 380 nm agrees with the band gap of bulk ZnO [14], which comes from the recombination of free excitons [15]. The green
Fig. 3. TEM micrograph of the ZnO powders annealed at 600 1C.
Fig. 4. The PL spectra of ZnO nanoparticles annealed at different temperatures in the oxygen atmosphere.
Fig. 2. XRD patterns of the ZnO precursor annealed at different temperatures.
emission at 500 nm is related to the singly ionized oxygen vacancy, and this emission results from the recombination of a photogenerated hole with a singly ionized charge state of the specific defect [16]. The oxygen vacancies in ZnO can be modified by annealing the samples in O2 atmosphere [17]. We annealed our sample under O2 atmosphere at 400, 500, 600, 650, 700 and 800 1C, respectively. The green emission could be quenched at 600 1C, indicating that the crystalline quality of the sample improved. But when the temperature reached 650 1C, the green emission started to increase. When the annealing temperature was above
ARTICLE IN PRESS J. Zhou et al. / Journal of Luminescence 119– 120 (2006) 248–252
700 1C, the green emission eventually dominanted the PL spectrum. The intensity ratio of the nearUV emission and green emission (Ri) decreased from 4.8 at 400 1C to 0.4 at 800 1C. The UV emission peak was slightly red-shift with increasing annealing temperature, which resulted from the defect related shallow binding excitons formed during high-temperature annealing. Fig. 5 showed room-temperature PL spectra of the ZnO nanoparticles annealed at different conditions in O2 atmosphere. At 500 1C (Fig. 5a), with the increasing of annealing time, the intensity of UV peak increased, the intensity of green emission decreased, and the intensity ratio (Ri) increased, indicating that crystalline quality of the sample improved due to the oxygen entering into the
251
crystal lattice sites and improving the stoichiometric proportion of the sample and therefore enhancing its crystal quality. However, annealed at 800 1C (Fig. 5b),with the increasing of annealing time, the intensities of both UV emission and green emission improved, but, the Ri decreased, suggesting that there was a higher concentration of defects (oxygen vacancies) in the ZnO nanoparticles likely due to the reevaporation of O. We think this behavior is due to the competition between the O atoms getting into the lattice and those evaporating out of the ZnO lattice in O2 atmosphere. At lower heating temperature, the kinetic energy of atoms in the ZnO lattice is relatively low and the adsorption rate of the O atoms is faster than the escaping rate. So more O atoms can compensate the O vacancies at lower temperature (o600 1C). But at higher heating temperatures, the kinetic energy of the atoms becomes larger and larger, possibly resulting in a larger escaping rate of O atoms than the adsorption rate to make more O vacancies in the ZnO lattice. This also strongly suggests that the deeplevel emission is the result of oxygen vacancies in the ZnO nanoparticles. Things we want to point out were that the observed optimized heating temperature of 600 1C was low and strong UV emission without green emission was obtained by a simple one-step process.
4. Conclusion
Fig. 5. The PL spectra of ZnO nanoparticles annealed at different conditions: (a) at 500 1C, (b) at 800 1C.
ZnO nanoparticles were prepared by a novel template combustion process, aiming for strong UV emission without green luminescence, by a simple one-step process. We investigated the influence of heating conditions on size and photoluminescence of ZnO nanoparticles. The average sizes of nanoparticles increased with the increasing annealing temperature, and this increase was prominent when the annealing temperature exceeded 600 1C. The resulting ZnO nanoparticles demonstrated a near-UV emission and a green emission, and the photoluminescence properties depended on heating temperatures and annealing time. With the increasing of annealing temperature, the green emission intensity caused
ARTICLE IN PRESS 252
J. Zhou et al. / Journal of Luminescence 119– 120 (2006) 248–252
by oxygen defects of ZnO increased remarkably. The photoluminescence properties of ZnO nanoparticles were thus controlled by employing appropriate heating condition, without changing their morphology. Acknowledgements Financial supports from the Henan Train Project for University Innovation Talents, National Natural Science Foundation of China (No. 20371016), Natural Science Foundation of Henan (No. 0111030200), Henan Education Department (No. 2004150005) and National Program on Key Basic Research Projects of China (973 Program, No. 2005CB724306). References [1] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289. [2] M.H. Huang, S. Mao, H. Feick, J.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897.
[3] Y. Wang, N. Herron, J. Phys. Chem 95 (1991) 525. [4] J.S. Kang, H.S. Kang, S.S. Pang, E.S. Shim, S.Y. Lee, Thin Solid Films 443 (2003) 5. [5] C.L. Yang, J.N. Wang, W.K. Ge, L. Guo, S.H. Yang, D.Z. Shen, J. Appl. Phys 90 (2001) 4489. [6] Y.G. Wang, S.P. Lau, X.H. Zhang, H.H. Hng, H.W. Lee, S.F. Yu, B.K. Tay, J. Cryst. Growth 259 (2003) 335. [7] J.Q. Hu, X.L. Ma, Z.Y. Xie, N.B. Wong, C.S. Lee, S.T. Lee, Chem. Phys. Lett 334 (2001) 97. [8] Y. Li, G.W. Meng, L.D. Zhang, F. Phillipp, Appl. Phys. Lett 76 (2000) 2011. [9] X.S. Fang, C.H. Ye, X.S. Peng, Y.H. Wang, Y.C. Wu, L.D. Zhang, J .Mater. Chem 3 (2003) 3040. [10] X.T. Zhang, Y.C. Liu, Z.Z. Zhi, J.Y. Zhang, Y.M. Lu, W. Xu, D.Z. Shen, G.Z. Zhong, X.W. Fan, X.G. Kong, J. Cryst. Growth 240 (2002) 463. [11] J.Q. Hu, Q. Li, N.B. Wong, C.S. Lee, S.T. Lee, Chem. Mater 14 (2002) 1216. [12] L. Guo, Y.L. Ji, H. Xu, P. Simon, Z. Wu, J. Am. Chem. Soc 124 (14) (2002) 864. [13] J.G. Zhou, F.Y. Zhao, Z.Q. Li, S.P. Xia, L. Yang, S.Y. Gao, J. Mater. Sci 39 (2004) 4711. [14] E.M. Wong, P.C. Searson, Appl. Phys. Lett 74 (1999) 2939. [15] V. Stikant, D.R. Clarke, J. Appl. Phys 83 (1998) 5447. [16] Y. Li, G.S. Cheng, L.D. Zhang, J. Mater. Res 15 (2000) 2305. [17] J. Lim, K. Shin, H.W. Kim, C. Lee, Mater. Sci. Eng B 107 (2004) 301.