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Growth of novel multi-trunk CdS dendrites by hydrothermal method without surfactant
Materials Letters 64 (2010) 1357–1360
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
Growth of novel multi-trunk CdS dendrites by hydrothermal method without surfactant Mei Xue, Xiaohua Zhang, Xu Wang, Bo Tang ⁎ College of Chemistry, Chemical Engineering and Materials Science, Engineering Research Center of Pesticide and Medicine Intermediate Clean Production, Ministry of Education, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, People's Republic of China
a r t i c l e
i n f o
Article history: Received 18 January 2010 Accepted 18 March 2010 Available online 25 March 2010 Keywords: CdS dendrites Single crystal Hydrothermal system
a b s t r a c t In the current paper, novel multi-trunk CdS dendrites were synthesized via a simple hydrothermal system, employing CdCl2·2H2O and KSCN as the starting materials. No extra surfactants were used. The observations from TEM and SEM showed that the product composed of a few long central trunks with secondary branches, which preferentially grew in a parallel direction with a definite angle to the trunks. Selected area electron diffraction (SAED) patterns confirmed that the dendrite was single crystalline in nature. X-ray diffraction analyses proved that the CdS dendrites were pure hexagonal structure. On the basis of the experimental results, a possible growth process has been discussed. © 2010 Published by Elsevier B.V.
1. Introduction
2. Experimental section
Semiconductor nanocrystals have been studied extensively as their optical properties are highly dependent on size and morphology. Among semiconductor materials, cadmium chalcogenides are important and have been extensively studied owing to their desired applications [1,2]. As one kind of important semiconductor material, cadmium sulfide has broad applications in light-emitting diodes, solar cells, or other optoelectronic devices. Recently, many methods were developed to fabricate CdS with novel morphologies. Much effort has been devoted to the synthesis of CdS rods [3], wires [4], and tubes [5]. Peng et al. [6] and others [7,8] reported multi-armed CdS crystals. Xie et al. have reported a kind of branch-like CdS micropatterns, using thiosemicarbazide both as a sulfur source and as a capping ligand in a methanol/water system [9,10]. Well-defined hierarchical CdS dendrites were synthesized by hydrothermal reaction of CdCl2 and thiourea with appropriate capping agent at suitable temperatures [11]. In this paper, we report a new route of controllable synthesis of multi-trunk CdS dendrites with high yields using CdCl2, KSCN and H2O as the starting materials. To the best of our knowledge, this kind of self-assembled growth of novel CdS dendrites by hydrothermal treatment of a Cd2+–SCN− complex has not been reported. Based on the experimental results, we found that growth of the novel multi-trunk-like patterns depended upon reaction time and nuclei concentration at a constant temperature. A possible mechanism of crystal growth was proposed.
In a typical experiment, 150 mg CdCl2·2H2O and 350 mg KSCN were dissolved in two beakers containing 5 mL of distilled water. The two clear solutions were mixed together slowly to yield homogeneous Cd2+–SCN− complex solution, and then it was transferred into a 20mL Teflonlined autoclave. The autoclave was maintained at 160 °C for 8 h. After the mixture cooled naturally to room temperature, the yellow precipitate was washed with distilled water for several times, and the final product was dried in a vacuum at 60 °C for 4 h. For the contrast experiments, KSCN was substituted by Na2S·9H2O, keeping the other conditions constant. The phase purity of the as-synthesized products was measured by X-ray powder diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer. Transmission electron microscopy (TEM) images and the corresponding selected area electron diffraction (SAED) patterns were carried out on a Hitachi Model H-800 instrument operating at 100 kV. Scanning electron microscopy (SEM) images were obtained on a JEOL JSM-6700F SEM with an accelerating voltage of 25 kV.
⁎ Corresponding author. Tel.: + 86 531 86180010; fax: + 86 531 86180017. E-mail address: [email protected] (B. Tang). 0167-577X/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.matlet.2010.03.045
3. Results and discussion Fig. 1 shows an XRD pattern of the as-obtained product. The strong and narrow peaks show that the material is well crystallized. By comparison with the data from JCPDS cards No.41-1049, all diffraction peaks can be indexed as a pure hexagonal structure of CdS. Compared with the standard reflection, the intensity of the (110) diffraction peak is comparatively strong, which is most probably related to the orientation of the CdS crystals. Fig. 2a shows a typical TEM image of a multi-trunk CdS dendrite. Number ① shows a long central trunk of the individual CdS dendrite.
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Fig. 1. XRD pattern of the products obtained by hydrothermal reaction of CdCl2 and KSCN at 160 °C for 8 h.
It is interesting that the secondary branches emerge at about 30° with respect to the central trunk. The onsets of tertiary branches can be obviously observed in the image of part branches. The tubers of secondary branches indicate that the whole architecture resulted from the self-growth of the CdS nucleus instead of accumulation of various crystals. From Fig. 2a we also can see the growth regularity of the dendrites. Number ② shows the second long central trunk having the length of 2.2 μm, obviously, secondary branches have grown on it. The third central trunk also has appeared, and it already possesses a considerable length (see number ③). The fourth central trunk has grown, too, although it is short and thin (see number ④). Moreover, the selected area electron diffraction (SAED) patterns (Fig. 2b) revealed that a long central trunk with secondary and tertiary branches are single crystalline in nature and the diffraction pattern can be indexed to hexagonal CdS. The SEM image in Fig. 2c results
proved the multi-trunk-shaped structure of the crystals. The image shows that the product consists almost entirely of such dendritic structures with mean length of 2–4 μm along the trunks, and this indicates the high yield and good uniformity achieved with this approach. The architectures here have many small secondary branches on the main trunks. Furthermore, the number of long central trunks of an individual dendrite varies from three to five. The SEM image in Fig. 2d exhibits the detailed configuration of the novel dendrites. We can see that the two rows of secondary branches separate by 180°, and the branches in the same row are parallel to each other emerging at about 30° with respect to the central trunk. In order to investigate the formation process of the multi-trunk CdS dendrites, time-resolved experiments were carried out and the corresponding TEM images of the as-prepared samples are illustrated in Fig. S1 (see supporting information). The TEM images in Fig. S2 are all the images of the products prepared by hydrothermal reaction of CdCl2 and KSCN at 160 °C for 8 h, keeping the other parameters constant. An individual dendrite in Fig. S2a has two trunks. In Fig. S2b, a dendrite has three trunks. In Fig. S2c, dendrites with various shapes which have two, three and four trunks are found. Please notice the dot arrow in Fig. S2c, two new trunks are growing. In Fig. S2d, we can see the flower-like dendrites with shorter trunks and more secondary branches in many directions. Based on the experimental results, we speculate that local concentration of the nuclei was necessary for the formation of the novel multi-trunk patterns. When the nuclei concentration was enough, multi-trunk CdS dendrites formed. The contrast experiments were carried out to demonstrate the great influence of KSCN on the dendritic morphology. Fig. 3 shows the SEM images of the products prepared by substituting Na2S·9H2O for KSCN, keeping the other parameters constant. From the images, we can see that the products were hexagonal CdS nanoparticles instead of elongated CdS crystals. This is because when Na2S·9H2O is used as the precursor, an equilibrium surroundings for crystal growth would be satisfied, and the crystal morphology was closer to the equilibrium conditions. In this work, the system contained only three components: CdCl2, KSCN and H2O. No other surfactant or template was needed. Before
Fig. 2. TEM and SEM images of the as-prepared products by hydrothermal reaction of CdCl2 and KSCN at 160 °C for 8 h: (a) TEM image of a individual CdS dendrite. (b) SAED pattern taken from the crystal shown in (a). (c) SEM image of multi-trunk CdS dendrites. (d) SEM image of a CdS dendrite.
M. Xue et al. / Materials Letters 64 (2010) 1357–1360
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along the [001] direction rather than the [110] direction due to its high surface energy. Finally, the branch of the dendrite grows along the normal growth direction [001] [11]. After the first long central trunk grew up, the second central trunk, the third central trunk, and even more central trunks would appear and grow up if the concentration of nuclei is enough. The schematic view of the growth process of the CdS dendrites can be simply described in Fig. 4. In principle, fractal and dendritic growth are diffusion-controlled growth, and nonequilibrium growth and molecular anisotropy are the prerequisites for the formation of dendritic structures [12–14]. Herein, anisotropy comes from the intrinsical anisotropy of the hexagonal structure CdS [15,16]. Besides, KSCN may act as not only the sulfur source but also a ligand to form relatively stable Cd2+–NCS− complexes in the initial solution. While the complex ions of SCN− with Cd2+ lead to a high remaining monomer concentration after the nucleation stage, thus a nonequilibrium growth for the elongated crystals is facilitated [10,17]. 4. Conclusion
Fig. 3. (a) SEM images of the as-prepared products obtained by hydrothermal reaction of CdCl2 and Na2S at 160 °C for 8 h. (b) High-magnification SEM image of the CdS crystals in (a).
In summary, novel multi-trunk CdS dendrites in a pure single hexagonal phase were successfully synthesized by a simple hydrothermal method, employing CdCl2 and KSCN as the reactants. No other surfactant was needed during the formation of the dendrite shaped structures. This interesting morphology of CdS is expected to have novel properties and may have potential applications in the semiconductor industry. The simple and convenient method can probably be expanded to synthesize other inorganic materials with novel shape. Acknowledgements
heating, the system was a clean and transparent solution. On the basis of the experimental results, a possible growth process of CdS dendrites can be simply described as follows. In the initial solution, Cd2+ ions choose sp3 hybridization, SCN− can bind Cd2+ to generate the corresponding complex ion with tetrahedron configuration. When the system was heated, the interaction between Cd2+ and SCN− will be weakened, and Cd2+ will be released gradually. On the other hand, SCN− is attacked by the strong nucleophilic O atoms of H2O molecules leading to the weakening of the C = S double bonds, which will be broken to release S2− anions slowly. Then the active S2− reacts with Cd2+ to generate CdS nuclei. Owing to the slow release of reaction ions, elongated growth along the [001] direction of rodlike crystals is favored. Subsequently, some tubers emerge on the side surface, which are symmetrically separate between each other and initially grow along the [110] direction. However, the CdS crystal prefers to grow
This work is supported by the National Natural Science Funds for Distinguished Young Scholar (No. 20725518), Major Program of National Natural Science Foundation of China (No. 90713019). M. Xue thanks Prof. Jiechao Ge and Dr. Yan Geng for SEM measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2010.03.045. References [1] [2] [3] [4]
Weller H. Angew Chem Int Ed 1993;32:41–53. Alivizatos AP. J Phys Chem 1996;100:13226–39. Peng ZA, Peng XG. J Am Chem Soc 2001;123:1389–95. Zhang X, Xie Y, Zhao Q, Tian Y. New J Chem 2003;27:827–30.
Fig. 4. Schematic view of growth process of the CdS dendrites.
1360 [5] [6] [7] [8]
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Rao CNR, Govindaraj A, Deepak FL, Gunari NA. Appl Phys Lett 2001;78:1853–5. Peng ZA, Peng XG. J Am Chem Soc 2002;124:3343–53. Jun Y, Lee SM, Kang NJ, Cheon J. J Am Chem Soc 2001;123:5150–1. Chen M, Xie Y, Lu J, Xiang YJ, Zhang SY, Qian YT, et al. J Mater Chem 2002;12: 748–53. [9] Gao F, Lu QY, Xie SH, Zhao DY. Adv Mater 2002;14:1537–40. [10] Zhang XJ, Zhao QR, Tian YP, Xie Y. Cryst Growth Des 2004;4:355–9. [11] Wang QQ, Xu G, Han GR. Cryst Growth Des 2006;6:1776–80.
[12] Dick KA, Depper K, Larsson MW, Martensson T, Seifert W, Wallenberg LR, et al. Nat Mater 2004;3:380–4. [13] Fleury V. Nature 1997;390:145–8. [14] He R, Qian XF, Yin J, Zhu ZK. Chem Phys Lett 2003;369:454–8. [15] Peng XG, Manna L, Yang WD. Nature 2000;404:59–61. [16] Manna L, Milliron DJ, Meisel A, Scher EC, Alivisatos AP. Nat Mater 2003;2:382–5. [17] Xiao ZL, Han CY, Kwok WK, Wang HH, Welp U, Wang J, et al. J Am Chem Soc 2004;126:2316–7.
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
Growth of novel multi-trunk CdS dendrites by hydrothermal method without surfactant Mei Xue, Xiaohua Zhang, Xu Wang, Bo Tang ⁎ College of Chemistry, Chemical Engineering and Materials Science, Engineering Research Center of Pesticide and Medicine Intermediate Clean Production, Ministry of Education, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, People's Republic of China
a r t i c l e
i n f o
Article history: Received 18 January 2010 Accepted 18 March 2010 Available online 25 March 2010 Keywords: CdS dendrites Single crystal Hydrothermal system
a b s t r a c t In the current paper, novel multi-trunk CdS dendrites were synthesized via a simple hydrothermal system, employing CdCl2·2H2O and KSCN as the starting materials. No extra surfactants were used. The observations from TEM and SEM showed that the product composed of a few long central trunks with secondary branches, which preferentially grew in a parallel direction with a definite angle to the trunks. Selected area electron diffraction (SAED) patterns confirmed that the dendrite was single crystalline in nature. X-ray diffraction analyses proved that the CdS dendrites were pure hexagonal structure. On the basis of the experimental results, a possible growth process has been discussed. © 2010 Published by Elsevier B.V.
1. Introduction
2. Experimental section
Semiconductor nanocrystals have been studied extensively as their optical properties are highly dependent on size and morphology. Among semiconductor materials, cadmium chalcogenides are important and have been extensively studied owing to their desired applications [1,2]. As one kind of important semiconductor material, cadmium sulfide has broad applications in light-emitting diodes, solar cells, or other optoelectronic devices. Recently, many methods were developed to fabricate CdS with novel morphologies. Much effort has been devoted to the synthesis of CdS rods [3], wires [4], and tubes [5]. Peng et al. [6] and others [7,8] reported multi-armed CdS crystals. Xie et al. have reported a kind of branch-like CdS micropatterns, using thiosemicarbazide both as a sulfur source and as a capping ligand in a methanol/water system [9,10]. Well-defined hierarchical CdS dendrites were synthesized by hydrothermal reaction of CdCl2 and thiourea with appropriate capping agent at suitable temperatures [11]. In this paper, we report a new route of controllable synthesis of multi-trunk CdS dendrites with high yields using CdCl2, KSCN and H2O as the starting materials. To the best of our knowledge, this kind of self-assembled growth of novel CdS dendrites by hydrothermal treatment of a Cd2+–SCN− complex has not been reported. Based on the experimental results, we found that growth of the novel multi-trunk-like patterns depended upon reaction time and nuclei concentration at a constant temperature. A possible mechanism of crystal growth was proposed.
In a typical experiment, 150 mg CdCl2·2H2O and 350 mg KSCN were dissolved in two beakers containing 5 mL of distilled water. The two clear solutions were mixed together slowly to yield homogeneous Cd2+–SCN− complex solution, and then it was transferred into a 20mL Teflonlined autoclave. The autoclave was maintained at 160 °C for 8 h. After the mixture cooled naturally to room temperature, the yellow precipitate was washed with distilled water for several times, and the final product was dried in a vacuum at 60 °C for 4 h. For the contrast experiments, KSCN was substituted by Na2S·9H2O, keeping the other conditions constant. The phase purity of the as-synthesized products was measured by X-ray powder diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer. Transmission electron microscopy (TEM) images and the corresponding selected area electron diffraction (SAED) patterns were carried out on a Hitachi Model H-800 instrument operating at 100 kV. Scanning electron microscopy (SEM) images were obtained on a JEOL JSM-6700F SEM with an accelerating voltage of 25 kV.
⁎ Corresponding author. Tel.: + 86 531 86180010; fax: + 86 531 86180017. E-mail address: [email protected] (B. Tang). 0167-577X/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.matlet.2010.03.045
3. Results and discussion Fig. 1 shows an XRD pattern of the as-obtained product. The strong and narrow peaks show that the material is well crystallized. By comparison with the data from JCPDS cards No.41-1049, all diffraction peaks can be indexed as a pure hexagonal structure of CdS. Compared with the standard reflection, the intensity of the (110) diffraction peak is comparatively strong, which is most probably related to the orientation of the CdS crystals. Fig. 2a shows a typical TEM image of a multi-trunk CdS dendrite. Number ① shows a long central trunk of the individual CdS dendrite.
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Fig. 1. XRD pattern of the products obtained by hydrothermal reaction of CdCl2 and KSCN at 160 °C for 8 h.
It is interesting that the secondary branches emerge at about 30° with respect to the central trunk. The onsets of tertiary branches can be obviously observed in the image of part branches. The tubers of secondary branches indicate that the whole architecture resulted from the self-growth of the CdS nucleus instead of accumulation of various crystals. From Fig. 2a we also can see the growth regularity of the dendrites. Number ② shows the second long central trunk having the length of 2.2 μm, obviously, secondary branches have grown on it. The third central trunk also has appeared, and it already possesses a considerable length (see number ③). The fourth central trunk has grown, too, although it is short and thin (see number ④). Moreover, the selected area electron diffraction (SAED) patterns (Fig. 2b) revealed that a long central trunk with secondary and tertiary branches are single crystalline in nature and the diffraction pattern can be indexed to hexagonal CdS. The SEM image in Fig. 2c results
proved the multi-trunk-shaped structure of the crystals. The image shows that the product consists almost entirely of such dendritic structures with mean length of 2–4 μm along the trunks, and this indicates the high yield and good uniformity achieved with this approach. The architectures here have many small secondary branches on the main trunks. Furthermore, the number of long central trunks of an individual dendrite varies from three to five. The SEM image in Fig. 2d exhibits the detailed configuration of the novel dendrites. We can see that the two rows of secondary branches separate by 180°, and the branches in the same row are parallel to each other emerging at about 30° with respect to the central trunk. In order to investigate the formation process of the multi-trunk CdS dendrites, time-resolved experiments were carried out and the corresponding TEM images of the as-prepared samples are illustrated in Fig. S1 (see supporting information). The TEM images in Fig. S2 are all the images of the products prepared by hydrothermal reaction of CdCl2 and KSCN at 160 °C for 8 h, keeping the other parameters constant. An individual dendrite in Fig. S2a has two trunks. In Fig. S2b, a dendrite has three trunks. In Fig. S2c, dendrites with various shapes which have two, three and four trunks are found. Please notice the dot arrow in Fig. S2c, two new trunks are growing. In Fig. S2d, we can see the flower-like dendrites with shorter trunks and more secondary branches in many directions. Based on the experimental results, we speculate that local concentration of the nuclei was necessary for the formation of the novel multi-trunk patterns. When the nuclei concentration was enough, multi-trunk CdS dendrites formed. The contrast experiments were carried out to demonstrate the great influence of KSCN on the dendritic morphology. Fig. 3 shows the SEM images of the products prepared by substituting Na2S·9H2O for KSCN, keeping the other parameters constant. From the images, we can see that the products were hexagonal CdS nanoparticles instead of elongated CdS crystals. This is because when Na2S·9H2O is used as the precursor, an equilibrium surroundings for crystal growth would be satisfied, and the crystal morphology was closer to the equilibrium conditions. In this work, the system contained only three components: CdCl2, KSCN and H2O. No other surfactant or template was needed. Before
Fig. 2. TEM and SEM images of the as-prepared products by hydrothermal reaction of CdCl2 and KSCN at 160 °C for 8 h: (a) TEM image of a individual CdS dendrite. (b) SAED pattern taken from the crystal shown in (a). (c) SEM image of multi-trunk CdS dendrites. (d) SEM image of a CdS dendrite.
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along the [001] direction rather than the [110] direction due to its high surface energy. Finally, the branch of the dendrite grows along the normal growth direction [001] [11]. After the first long central trunk grew up, the second central trunk, the third central trunk, and even more central trunks would appear and grow up if the concentration of nuclei is enough. The schematic view of the growth process of the CdS dendrites can be simply described in Fig. 4. In principle, fractal and dendritic growth are diffusion-controlled growth, and nonequilibrium growth and molecular anisotropy are the prerequisites for the formation of dendritic structures [12–14]. Herein, anisotropy comes from the intrinsical anisotropy of the hexagonal structure CdS [15,16]. Besides, KSCN may act as not only the sulfur source but also a ligand to form relatively stable Cd2+–NCS− complexes in the initial solution. While the complex ions of SCN− with Cd2+ lead to a high remaining monomer concentration after the nucleation stage, thus a nonequilibrium growth for the elongated crystals is facilitated [10,17]. 4. Conclusion
Fig. 3. (a) SEM images of the as-prepared products obtained by hydrothermal reaction of CdCl2 and Na2S at 160 °C for 8 h. (b) High-magnification SEM image of the CdS crystals in (a).
In summary, novel multi-trunk CdS dendrites in a pure single hexagonal phase were successfully synthesized by a simple hydrothermal method, employing CdCl2 and KSCN as the reactants. No other surfactant was needed during the formation of the dendrite shaped structures. This interesting morphology of CdS is expected to have novel properties and may have potential applications in the semiconductor industry. The simple and convenient method can probably be expanded to synthesize other inorganic materials with novel shape. Acknowledgements
heating, the system was a clean and transparent solution. On the basis of the experimental results, a possible growth process of CdS dendrites can be simply described as follows. In the initial solution, Cd2+ ions choose sp3 hybridization, SCN− can bind Cd2+ to generate the corresponding complex ion with tetrahedron configuration. When the system was heated, the interaction between Cd2+ and SCN− will be weakened, and Cd2+ will be released gradually. On the other hand, SCN− is attacked by the strong nucleophilic O atoms of H2O molecules leading to the weakening of the C = S double bonds, which will be broken to release S2− anions slowly. Then the active S2− reacts with Cd2+ to generate CdS nuclei. Owing to the slow release of reaction ions, elongated growth along the [001] direction of rodlike crystals is favored. Subsequently, some tubers emerge on the side surface, which are symmetrically separate between each other and initially grow along the [110] direction. However, the CdS crystal prefers to grow
This work is supported by the National Natural Science Funds for Distinguished Young Scholar (No. 20725518), Major Program of National Natural Science Foundation of China (No. 90713019). M. Xue thanks Prof. Jiechao Ge and Dr. Yan Geng for SEM measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2010.03.045. References [1] [2] [3] [4]
Weller H. Angew Chem Int Ed 1993;32:41–53. Alivizatos AP. J Phys Chem 1996;100:13226–39. Peng ZA, Peng XG. J Am Chem Soc 2001;123:1389–95. Zhang X, Xie Y, Zhao Q, Tian Y. New J Chem 2003;27:827–30.
Fig. 4. Schematic view of growth process of the CdS dendrites.
1360 [5] [6] [7] [8]
M. Xue et al. / Materials Letters 64 (2010) 1357–1360
Rao CNR, Govindaraj A, Deepak FL, Gunari NA. Appl Phys Lett 2001;78:1853–5. Peng ZA, Peng XG. J Am Chem Soc 2002;124:3343–53. Jun Y, Lee SM, Kang NJ, Cheon J. J Am Chem Soc 2001;123:5150–1. Chen M, Xie Y, Lu J, Xiang YJ, Zhang SY, Qian YT, et al. J Mater Chem 2002;12: 748–53. [9] Gao F, Lu QY, Xie SH, Zhao DY. Adv Mater 2002;14:1537–40. [10] Zhang XJ, Zhao QR, Tian YP, Xie Y. Cryst Growth Des 2004;4:355–9. [11] Wang QQ, Xu G, Han GR. Cryst Growth Des 2006;6:1776–80.
[12] Dick KA, Depper K, Larsson MW, Martensson T, Seifert W, Wallenberg LR, et al. Nat Mater 2004;3:380–4. [13] Fleury V. Nature 1997;390:145–8. [14] He R, Qian XF, Yin J, Zhu ZK. Chem Phys Lett 2003;369:454–8. [15] Peng XG, Manna L, Yang WD. Nature 2000;404:59–61. [16] Manna L, Milliron DJ, Meisel A, Scher EC, Alivisatos AP. Nat Mater 2003;2:382–5. [17] Xiao ZL, Han CY, Kwok WK, Wang HH, Welp U, Wang J, et al. J Am Chem Soc 2004;126:2316–7.