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Growth mechanism and morphology dependent luminescence properties of ZnO nanostructures prepared in aqueous solution
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Materials Letters 62 (2008) 3099 – 3102 www.elsevier.com/locate/matlet
Growth mechanism and morphology dependent luminescence properties of ZnO nanostructures prepared in aqueous solution Bing Cheng a,⁎, Xiufeng Wang a , Liying Liu a , Litong Guo b a
School of Materials Science & Engineering, Shaanxi University of Science & Technology, Xi'an 710021, PR China State Key laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, PR China
b
Received 26 December 2007; accepted 31 January 2008 Available online 9 February 2008
Abstract ZnO nanostructures were prepared in aqueous solution assisted by sonication at low temperature of 80 °C. Morphology control of ZnO nanostructures was attempted by using triethanolamine as complexing agent and ammonia as the modifying agent. Three different morphologies of ZnO nanostructures, including rugby-, dart- and flower-like, were obtained in the different conditions. Possible mechanism for the formation of ZnO nanostructures with the different morphologies was proposed. The rugby-like ZnO nanostructures showed the various photoluminescence bands centered on UV (385 nm), green (490–530 nm), and orange (610–640 nm) region. Correspondingly, the dart-like ZnO nanostructures did not show orange emission and the flower-like ZnO nanostructures only exhibited UV emission. © 2008 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; ZnO nanostructures; Luminescence characteristics
1. Introduction As an important luminescent material, ZnO, with a wide band gap (3.37 eV) and large excitation binding energy (60 meV), has been widely investigated in the recent decades [1–6]. Many researches have been focused on the application in making of the photoelectric devices depending on its superior luminescence and photoelectric properties [7–10]. So a lot of papers reported the methods to obtain p-type ZnO semiconductor material, however, the effect of morphology on photoluminescence characteristics has not been investigated in depth [11–13]. In fact, the morphology is one of the key factors affecting photoluminescence characteristics of ZnO nanostructures (ZNs). Herein, we reported a simple and novel approach to the production of ZNs in aqueous solution. In comparison with other methods which fabricate ZNs with a single morphology, the presented approach can obtain rugby-, dart-, and flower-like ZNs. Moreover, photoluminescence (PL) measurements of ZNs
⁎ Corresponding author. Tel.: +86 029 86168131; fax: +86 029 86168135. E-mail address: [email protected] (B. Cheng). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.115
have also been performed to indicate the effect of morphology on photoluminescence characteristics. Additionally, the formation mechanism of ZNs with various morphologies has been investigated. 2. Experimental The experimental procedure was carried out as follows: zinc acetate was dissolved in 300 ml of DI water and then triethanolamine (TEA) as complexing agent was dropped with magnetic stirring at room temperature for 20 min to ensure complete mixing. And then, ammonia as the modifying agent was dropped to adjust the value of pH. Then the solution was stirred for 4 h at reaction temperature (80 °C) assisted by sonication. At last, the product was washed with DI water and then dried in a vacuum oven at 60 °C. Table 1 gave the detailed reaction conditions of these experiments and morphologies obtained. The morphology of ZNs was examined with field-emission scanning electron microscope (FESEM; JEOL JSM-6700) and high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100F). The structure was indicated by X-ray diffraction (XRD; Rigaku D/max 2200pc). And the PL
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B. Cheng et al. / Materials Letters 62 (2008) 3099–3102
Table 1 Sample numbers, reaction conditions, and obtained morphologies Sample number
Reaction media ([TEA] = 0.01 M)
Value of pH
Morphology
1 2 3
Zn(Ac)2/TEA = 1:1 Zn(Ac)2/TEA = 1:1 Zn(Ac)2/TEA = 2:1
10 12 12
Rugby-like Dart-like Flower-like
measurements were performed at room temperature by fluorometry (F-2500, Hitachi) with an excitation wavelength of 325 nm. 3. Results and discussion A representative XRD spectrum of ZNs was shown in Fig. 1. Results showed that all obtained ZNs possessed the hexagonal wurtzite structures (JCPDS no. 36-1451) and no diffraction peaks of any other minerals were detected. The strong reflections in the XRD patterns demonstrated that the obtained ZNs possessed good crystallinity. Fig. 2 showed the SEM images of the ZNs. The SEM images clearly exhibited that ZNs possessed straight shapes and uniform size. Three different morphologies of ZNs were obtained. The first one (see Fig. 2a) had a rugby-like structure obtained with molar ratio of Zn2+/ TEA = 1 at pH 10. The insert in Fig. 2a indicated that the length of rugby-like ZNs was about 300 nm and the width was about 700 nm. The second one had a dart-like structure produced with molar ratio of Zn2+/TEA = 1 at pH 12 (see Fig. 2b). In comparison with rugby-like ZNs, dart-like ZNs grew branches in the midst part. The third one was similar to the flower structure synthesized with molar ratio of Zn2+/ TEA = 2 at pH 12 (see Fig. 2c). It can be seen that the flower-like ZNs consist of many well-aligned nanorods with 100 nm in width and about 300 nm in length. It is known that the formation mechanism has determinative effects on the morphology of ZNs [14–16]. In the current experiment, Zn (OH)2− 4 ions were growth units. The solid-state Zn(OH)2 precipitates were dissolved to yield Zn(OH)2− 4 ions and then ZnO nuclei were formed from the dehydration of Zn(OH)2− 4 ions and followed by growth with the increase of the temperature (Eq. (1)). The reaction process can be expressed by the following equation: − ZnðOHÞ2 þ 2OH− ↔ZnðOHÞ2− 4 ↔ZnO þ H2 O þ 2OH
Zn(OH)2− 4
ð1Þ
ions will As a polar crystal, the negative growth unit firstly adsorb on the positive polar planes which are rich in Zn2+ (i.e.,
Fig. 1. XRD pattern of the obtained ZNs.
Fig. 2. FESEM images of ZNs synthesized at 80 °C: (a) rugby-like ZNs; (b) dartlike ZNs; (c) flower-like ZNs. And the inserts show the details of each ZNs.
(0001) planes), so hexagonal ZNs will be easily developed [17,18]. When TEA is present in the aqueous solution, the growth of ZNs along the positive polar planes will be restricted since TEA molecules can react with Zn2+ on the positive polar planes to form Zn amino complexes. Therefore, with the growth process, the length of the ZNs increases, while the top area decreases gradually. It will result in the formation of rugby-like ZNs. And pH value is another factor affecting the morphology of ZNs. Zhao [19], Zhang [20], and Gao [21] obtained similar ZNs. And Zhang proposed that the pH value was a dominant factor for the formation of ZNs with various morphologies. In this work, both TEA and pH value have determinative effects on the morphology of ZNs. The high concentration of TEA restricts the growth of ZnO. On the contrary, the high pH value accelerates the growth of ZnO owing to the increasing of Zn(OH)2− 4 ions. When pH value is high (pH = 12), the nucleation rate will be relatively high, which will create a large number of layer defects such as micotwins during nucleation process. As those defects can induce the secondary growth, the branches of the dart-like ZNs will be formed. The HRTEM micrographs of the special sample 2 produced in a short time (30 min) to exhibit the different growth stages of ZNs have been shown in Fig. 3. The TEM micrographs clearly showed that the rugby-like ZNs were firstly formed and followed by growth and then branches were formed in the midst of the rugby-like ZNs. When the concentration of TEA is relatively low (molar ratio of Zn2+/TEA = 2) and pH value is high (pH = 12), there will not be enough Zn amino complexes to restrict the
B. Cheng et al. / Materials Letters 62 (2008) 3099–3102
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Fig. 3. TEM micrographs of dart-like ZNs produced in 30 min: (a) general morphology of ZNs; (b) high-magnification TEM image of rugby-like ZNs; (c–e) highmagnification TEM images of dart-like ZNs.
growth of ZNs. So, the stamen structures which consist of many wellaligned nanorods and petal structures will be formed in the flower-like ZNs. To investigate the effect of morphology on photoluminescence characteristics, the PL spectra of the rugby-like (a), dart-like (b), and flower-like (c) ZNs have been shown in Fig. 4. In the Fig. 4a, the rugbylike ZNs indicated UV emission at 385 nm (3.26 eV), green emission at 520 nm (2.4 eV) and orange emission at 600 nm (2.1 eV). The peak at 385 nm is usually attributed to recombination of free excitons. It is known that visible luminescence mainly originates from defect states such as Zn interstitials and oxygen vacancies [22–24]. Thus, the peak at 520 nm originates from the singly ionized oxygen vacancies [25]. And the peak at 600 nm originates from the double ionized oxygen vacancies [26]. In contrast, dart-like ZNs (Fig. 4b) showed that the peak at 550 nm, corresponding to yellow-green emission, was broader than the peaks of rugby-like ZNs (Fig. 4a). It probably resulted from the increase of layer defects during nucleation process. In the Fig. 4c, flower-like ZNs
showed the sharp and intense UV emission and no obvious visible luminescence. It proved that flower-like ZNs possessed the good crystallization quality and high stoichiometric nature. It could be conjectured that the growth of ZNs are not restricted as the decrease of the concentration of TEA. Therefore, the ZNs with different morphologies indicated different photoluminescence characteristics. It probably resulted from the concentration of TEA. TEA could restrict the growth of ZnO and create oxygen vacancies which resulted in the visible photoluminescence.
4. Conclusions The rugby-, dart-, and flower-like ZnO nanostructures have been successfully synthesized through a wet-chemical route at the low temperature of 80 °C assisted by sonication. The morphology can be controlled by adjusting the molar ratio of Zn2+/TEA and the pH value of the solution. Moreover, the effect of the morphology on the photoluminescence characteristics has been investigated and the results showed that the rugby-like ZnO nanostructures possessed UV (385 nm), green (490–530 nm), and orange (610–640 nm) region, the dart-like ZnO nanostructures showed UV and yellow-green emission and the flower-like ZnO nanostructures only exhibited UV emission. Acknowledgement This project is financially supported by the National Natural Science Foundation of China (No. 50372038). References
Fig. 4. Room-temperature photoluminescence spectra of different morphologies ZNs: (a) rugby-like ZNs; (b) dart-like ZNs; (c) flower-like ZNs.
[1] [2] [3] [4] [5] [6]
M.S. Mo, J.C. Yu, L.Z. Zhang, Adv. Mater. 17 (2005) 756. R.F. Service, Science 276 (1997) 895. Z.L. Wang, J.H. Song, Science 312 (2006) 242. M.H. Huang, S. Mao, H. Feick, et al., Science 292 (2001) 1897. X.F. Duan, Y. Huang, Y. Cui, et al., Nature 409 (2001) 66. G.C. Xi, Y.K. Liu, X.Y. Liu, et al., J. Phys. Chem. B 110 (2006) 14172.
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B. Cheng et al. / Materials Letters 62 (2008) 3099–3102
[7] J. Zhang, L. Sun, J. Yin, et al., Chem. Mater. 14 (2002) 4172. [8] X. Gao, X. Li, W. Yu, J. Phys. Chem. B 109 (2005) 1155. [9] H.S. Kang, J.W. Kim, S.H. Lim, et al., Superlattices Microstruct. 39 (2006) 193. [10] H.S. Kang, J.S. Kang, J.W. Kim, S.Y. Lee, J. Appl. Phys. 95 (2004) 1246. [11] P. Bhattacharya, R.R. Das, R.S. Katiyar, Thin Solid Films 447 (2004) 564. [12] D.C. Look, D.C. Reynolds, C.W. Litton, et al., Appl. Phys. Lett. 81 (2002) 1830. [13] R. Konenkamp, R.C. Word, M. Godinez, Nano Lett. 5 (2005) 2005. [14] H. Zhang, D. Yang, Y. Ji, et al., J. Phys. Chem. B 108 (2004) 3955. [15] Z. Wang, X.F. Qian, J. Yin, et al., Langmuir 20 (2004) 3441. [16] Y.G. Wang, C. Yuen, S.P. Lau, et al., Chem. Phys. Lett. 377 (2003) 329. [17] Y. Li, G.W. Meng, L.D. Zhang, et al., Appl. Phys. Lett. 76 (2000) 2011.
[18] [19] [20] [21] [22] [23] [24] [25] [26]
X. Kong, Y. Ding, Z. Wang, J. Phys. Chem. B 108 (2004) 570. B. Zhao, H. Chen, Mater. Lett. 61 (2007) 4890. Y. Zhang, J. Mu, Nanotechnology 18 (2007) 075606. H. Gao, F. Yan, J. Li, Y. Zeng, J. Wang, J. Phys. D: Appl. Phys. 40 (2007) 3654. K. Vanheusden, C.H. Seager, W.L. Warren, et al., Appl. Phys. Lett. 68 (1996) 403. P.S. Xu, Y.M. Sun, C.S. Shi, et al., Nucl. Instrum. Methods Phys. Res. B 199 (2003) 286. S.B. Zhang, S.H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 205. P. Jiang, J. Zhou, H. Fang, et al., Adv. Funct. Mater. 17 (2007) 1303. V.A. Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, et al., J. Lumin. 87 (2000) 454.
Materials Letters 62 (2008) 3099 – 3102 www.elsevier.com/locate/matlet
Growth mechanism and morphology dependent luminescence properties of ZnO nanostructures prepared in aqueous solution Bing Cheng a,⁎, Xiufeng Wang a , Liying Liu a , Litong Guo b a
School of Materials Science & Engineering, Shaanxi University of Science & Technology, Xi'an 710021, PR China State Key laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, PR China
b
Received 26 December 2007; accepted 31 January 2008 Available online 9 February 2008
Abstract ZnO nanostructures were prepared in aqueous solution assisted by sonication at low temperature of 80 °C. Morphology control of ZnO nanostructures was attempted by using triethanolamine as complexing agent and ammonia as the modifying agent. Three different morphologies of ZnO nanostructures, including rugby-, dart- and flower-like, were obtained in the different conditions. Possible mechanism for the formation of ZnO nanostructures with the different morphologies was proposed. The rugby-like ZnO nanostructures showed the various photoluminescence bands centered on UV (385 nm), green (490–530 nm), and orange (610–640 nm) region. Correspondingly, the dart-like ZnO nanostructures did not show orange emission and the flower-like ZnO nanostructures only exhibited UV emission. © 2008 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; ZnO nanostructures; Luminescence characteristics
1. Introduction As an important luminescent material, ZnO, with a wide band gap (3.37 eV) and large excitation binding energy (60 meV), has been widely investigated in the recent decades [1–6]. Many researches have been focused on the application in making of the photoelectric devices depending on its superior luminescence and photoelectric properties [7–10]. So a lot of papers reported the methods to obtain p-type ZnO semiconductor material, however, the effect of morphology on photoluminescence characteristics has not been investigated in depth [11–13]. In fact, the morphology is one of the key factors affecting photoluminescence characteristics of ZnO nanostructures (ZNs). Herein, we reported a simple and novel approach to the production of ZNs in aqueous solution. In comparison with other methods which fabricate ZNs with a single morphology, the presented approach can obtain rugby-, dart-, and flower-like ZNs. Moreover, photoluminescence (PL) measurements of ZNs
⁎ Corresponding author. Tel.: +86 029 86168131; fax: +86 029 86168135. E-mail address: [email protected] (B. Cheng). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.115
have also been performed to indicate the effect of morphology on photoluminescence characteristics. Additionally, the formation mechanism of ZNs with various morphologies has been investigated. 2. Experimental The experimental procedure was carried out as follows: zinc acetate was dissolved in 300 ml of DI water and then triethanolamine (TEA) as complexing agent was dropped with magnetic stirring at room temperature for 20 min to ensure complete mixing. And then, ammonia as the modifying agent was dropped to adjust the value of pH. Then the solution was stirred for 4 h at reaction temperature (80 °C) assisted by sonication. At last, the product was washed with DI water and then dried in a vacuum oven at 60 °C. Table 1 gave the detailed reaction conditions of these experiments and morphologies obtained. The morphology of ZNs was examined with field-emission scanning electron microscope (FESEM; JEOL JSM-6700) and high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100F). The structure was indicated by X-ray diffraction (XRD; Rigaku D/max 2200pc). And the PL
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B. Cheng et al. / Materials Letters 62 (2008) 3099–3102
Table 1 Sample numbers, reaction conditions, and obtained morphologies Sample number
Reaction media ([TEA] = 0.01 M)
Value of pH
Morphology
1 2 3
Zn(Ac)2/TEA = 1:1 Zn(Ac)2/TEA = 1:1 Zn(Ac)2/TEA = 2:1
10 12 12
Rugby-like Dart-like Flower-like
measurements were performed at room temperature by fluorometry (F-2500, Hitachi) with an excitation wavelength of 325 nm. 3. Results and discussion A representative XRD spectrum of ZNs was shown in Fig. 1. Results showed that all obtained ZNs possessed the hexagonal wurtzite structures (JCPDS no. 36-1451) and no diffraction peaks of any other minerals were detected. The strong reflections in the XRD patterns demonstrated that the obtained ZNs possessed good crystallinity. Fig. 2 showed the SEM images of the ZNs. The SEM images clearly exhibited that ZNs possessed straight shapes and uniform size. Three different morphologies of ZNs were obtained. The first one (see Fig. 2a) had a rugby-like structure obtained with molar ratio of Zn2+/ TEA = 1 at pH 10. The insert in Fig. 2a indicated that the length of rugby-like ZNs was about 300 nm and the width was about 700 nm. The second one had a dart-like structure produced with molar ratio of Zn2+/TEA = 1 at pH 12 (see Fig. 2b). In comparison with rugby-like ZNs, dart-like ZNs grew branches in the midst part. The third one was similar to the flower structure synthesized with molar ratio of Zn2+/ TEA = 2 at pH 12 (see Fig. 2c). It can be seen that the flower-like ZNs consist of many well-aligned nanorods with 100 nm in width and about 300 nm in length. It is known that the formation mechanism has determinative effects on the morphology of ZNs [14–16]. In the current experiment, Zn (OH)2− 4 ions were growth units. The solid-state Zn(OH)2 precipitates were dissolved to yield Zn(OH)2− 4 ions and then ZnO nuclei were formed from the dehydration of Zn(OH)2− 4 ions and followed by growth with the increase of the temperature (Eq. (1)). The reaction process can be expressed by the following equation: − ZnðOHÞ2 þ 2OH− ↔ZnðOHÞ2− 4 ↔ZnO þ H2 O þ 2OH
Zn(OH)2− 4
ð1Þ
ions will As a polar crystal, the negative growth unit firstly adsorb on the positive polar planes which are rich in Zn2+ (i.e.,
Fig. 1. XRD pattern of the obtained ZNs.
Fig. 2. FESEM images of ZNs synthesized at 80 °C: (a) rugby-like ZNs; (b) dartlike ZNs; (c) flower-like ZNs. And the inserts show the details of each ZNs.
(0001) planes), so hexagonal ZNs will be easily developed [17,18]. When TEA is present in the aqueous solution, the growth of ZNs along the positive polar planes will be restricted since TEA molecules can react with Zn2+ on the positive polar planes to form Zn amino complexes. Therefore, with the growth process, the length of the ZNs increases, while the top area decreases gradually. It will result in the formation of rugby-like ZNs. And pH value is another factor affecting the morphology of ZNs. Zhao [19], Zhang [20], and Gao [21] obtained similar ZNs. And Zhang proposed that the pH value was a dominant factor for the formation of ZNs with various morphologies. In this work, both TEA and pH value have determinative effects on the morphology of ZNs. The high concentration of TEA restricts the growth of ZnO. On the contrary, the high pH value accelerates the growth of ZnO owing to the increasing of Zn(OH)2− 4 ions. When pH value is high (pH = 12), the nucleation rate will be relatively high, which will create a large number of layer defects such as micotwins during nucleation process. As those defects can induce the secondary growth, the branches of the dart-like ZNs will be formed. The HRTEM micrographs of the special sample 2 produced in a short time (30 min) to exhibit the different growth stages of ZNs have been shown in Fig. 3. The TEM micrographs clearly showed that the rugby-like ZNs were firstly formed and followed by growth and then branches were formed in the midst of the rugby-like ZNs. When the concentration of TEA is relatively low (molar ratio of Zn2+/TEA = 2) and pH value is high (pH = 12), there will not be enough Zn amino complexes to restrict the
B. Cheng et al. / Materials Letters 62 (2008) 3099–3102
3101
Fig. 3. TEM micrographs of dart-like ZNs produced in 30 min: (a) general morphology of ZNs; (b) high-magnification TEM image of rugby-like ZNs; (c–e) highmagnification TEM images of dart-like ZNs.
growth of ZNs. So, the stamen structures which consist of many wellaligned nanorods and petal structures will be formed in the flower-like ZNs. To investigate the effect of morphology on photoluminescence characteristics, the PL spectra of the rugby-like (a), dart-like (b), and flower-like (c) ZNs have been shown in Fig. 4. In the Fig. 4a, the rugbylike ZNs indicated UV emission at 385 nm (3.26 eV), green emission at 520 nm (2.4 eV) and orange emission at 600 nm (2.1 eV). The peak at 385 nm is usually attributed to recombination of free excitons. It is known that visible luminescence mainly originates from defect states such as Zn interstitials and oxygen vacancies [22–24]. Thus, the peak at 520 nm originates from the singly ionized oxygen vacancies [25]. And the peak at 600 nm originates from the double ionized oxygen vacancies [26]. In contrast, dart-like ZNs (Fig. 4b) showed that the peak at 550 nm, corresponding to yellow-green emission, was broader than the peaks of rugby-like ZNs (Fig. 4a). It probably resulted from the increase of layer defects during nucleation process. In the Fig. 4c, flower-like ZNs
showed the sharp and intense UV emission and no obvious visible luminescence. It proved that flower-like ZNs possessed the good crystallization quality and high stoichiometric nature. It could be conjectured that the growth of ZNs are not restricted as the decrease of the concentration of TEA. Therefore, the ZNs with different morphologies indicated different photoluminescence characteristics. It probably resulted from the concentration of TEA. TEA could restrict the growth of ZnO and create oxygen vacancies which resulted in the visible photoluminescence.
4. Conclusions The rugby-, dart-, and flower-like ZnO nanostructures have been successfully synthesized through a wet-chemical route at the low temperature of 80 °C assisted by sonication. The morphology can be controlled by adjusting the molar ratio of Zn2+/TEA and the pH value of the solution. Moreover, the effect of the morphology on the photoluminescence characteristics has been investigated and the results showed that the rugby-like ZnO nanostructures possessed UV (385 nm), green (490–530 nm), and orange (610–640 nm) region, the dart-like ZnO nanostructures showed UV and yellow-green emission and the flower-like ZnO nanostructures only exhibited UV emission. Acknowledgement This project is financially supported by the National Natural Science Foundation of China (No. 50372038). References
Fig. 4. Room-temperature photoluminescence spectra of different morphologies ZNs: (a) rugby-like ZNs; (b) dart-like ZNs; (c) flower-like ZNs.
[1] [2] [3] [4] [5] [6]
M.S. Mo, J.C. Yu, L.Z. Zhang, Adv. Mater. 17 (2005) 756. R.F. Service, Science 276 (1997) 895. Z.L. Wang, J.H. Song, Science 312 (2006) 242. M.H. Huang, S. Mao, H. Feick, et al., Science 292 (2001) 1897. X.F. Duan, Y. Huang, Y. Cui, et al., Nature 409 (2001) 66. G.C. Xi, Y.K. Liu, X.Y. Liu, et al., J. Phys. Chem. B 110 (2006) 14172.
3102
B. Cheng et al. / Materials Letters 62 (2008) 3099–3102
[7] J. Zhang, L. Sun, J. Yin, et al., Chem. Mater. 14 (2002) 4172. [8] X. Gao, X. Li, W. Yu, J. Phys. Chem. B 109 (2005) 1155. [9] H.S. Kang, J.W. Kim, S.H. Lim, et al., Superlattices Microstruct. 39 (2006) 193. [10] H.S. Kang, J.S. Kang, J.W. Kim, S.Y. Lee, J. Appl. Phys. 95 (2004) 1246. [11] P. Bhattacharya, R.R. Das, R.S. Katiyar, Thin Solid Films 447 (2004) 564. [12] D.C. Look, D.C. Reynolds, C.W. Litton, et al., Appl. Phys. Lett. 81 (2002) 1830. [13] R. Konenkamp, R.C. Word, M. Godinez, Nano Lett. 5 (2005) 2005. [14] H. Zhang, D. Yang, Y. Ji, et al., J. Phys. Chem. B 108 (2004) 3955. [15] Z. Wang, X.F. Qian, J. Yin, et al., Langmuir 20 (2004) 3441. [16] Y.G. Wang, C. Yuen, S.P. Lau, et al., Chem. Phys. Lett. 377 (2003) 329. [17] Y. Li, G.W. Meng, L.D. Zhang, et al., Appl. Phys. Lett. 76 (2000) 2011.
[18] [19] [20] [21] [22] [23] [24] [25] [26]
X. Kong, Y. Ding, Z. Wang, J. Phys. Chem. B 108 (2004) 570. B. Zhao, H. Chen, Mater. Lett. 61 (2007) 4890. Y. Zhang, J. Mu, Nanotechnology 18 (2007) 075606. H. Gao, F. Yan, J. Li, Y. Zeng, J. Wang, J. Phys. D: Appl. Phys. 40 (2007) 3654. K. Vanheusden, C.H. Seager, W.L. Warren, et al., Appl. Phys. Lett. 68 (1996) 403. P.S. Xu, Y.M. Sun, C.S. Shi, et al., Nucl. Instrum. Methods Phys. Res. B 199 (2003) 286. S.B. Zhang, S.H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 205. P. Jiang, J. Zhou, H. Fang, et al., Adv. Funct. Mater. 17 (2007) 1303. V.A. Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, et al., J. Lumin. 87 (2000) 454.