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A novel solution-phase route for the synthesis of crystalline silver nanowires
Materials Research Bulletin 40 (2005) 1796–1801 www.elsevier.com/locate/matresbu
A novel solution-phase route for the synthesis of crystalline silver nanowires Yang Liu, Ying Chu *, Likun Yang, Dongxue Han, Zhongxian Lu¨ Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China Received 28 April 2003; received in revised form 7 November 2004; accepted 11 May 2005
Abstract A unique solution-phase route was devised to synthesize crystal Ag nanowires with high aspect-ratio (8–10 nm in diameter and length up to 10 mm) by the reduction of AgNO3 with Vitamin C in SDS/ethanol solution. The resultant nanoproducts were characterized by transmission electron microscope (TEM), X-ray diffraction (XRD) and electron diffraction (ED). A soft template mechanism was put forward to interpret the formation of metal Ag nanowires. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; B. Chemical synthesis; C. Electron diffraction
1. Introduction In recent years, nanoscale materials, especially one-dimensional (1D) nanomaterials have attracted extensive attention because of their unusual quantum properties and potential applications [1–4]. Nanowires, especially metal nanowires, have been the focus of many recent studies. They are expected to play a vital role as both connecters and functional components applied in fabricating electronic, optical and sensing devices [5–8] and to provide an ideal system to experimentally probe transport properties under various physical confinements (e.g., quantized conductance and localization effects) [9,10]. The electric and optical properties of the low dimensional nanomaterials profoundly depend on size and dimensionality. Many excellent quantum effects have not been experimentally observed in the wide field * Corresponding author. Tel.: +86 431 5269668; fax: +86 431 5684009. E-mail address: [email protected] (Y. Chu). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.05.005
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of nanomaterials, largely because of the inability to get highly crystalline samples with narrow size distribution and tunable crystallographic orientation [2,4]. Therefore, in a sense, shape and size controls are the key to the success of any approach of synthesizing nanomaterials. Silver nanowires seem to be particularly interesting to be synthesized and studied because bulk silver exhibits the highest electrical and thermal conductivities among all metals [11]. Silver has also been extensively exploited in a variety of applications that range from catalysis, through electronics, to photonics and photography [12–14]. The performance of silver in most of these applications could be significantly enhanced by processing silver into wire-like nanostructures with well-controlled dimensions [11]. To possess enhanced physical properties, the silver nanowires need to be of small diameter, and high aspect-ratio. Silver nanowires have been successfully synthesized by templating against various types of one-dimensional structures, such as channels in alumina or polymer membranes [15,16], mesoporous materials [17,18], carbon nanotubes [19], block co-polymers [20], DNA chains [21], calyx[4]hydrquinone nanotubes [22]. However, most of the templates are tedious to be prepared and the dissolution of the template in corrosive media is required to retrieve the nanorods/wires if separate single rods/wires are desired. The introduction of microemulsions has provided a relatively simple and powerful method for controlling the size, shape and surface texture of nanoparticles and nanorods/wires. In such a microenvironment, the nanomaterials are encapsulated into the closed shells of surfactant molecules. The size and the shape of nanomaterials could be well controlled by the size of capsules varying with the different ratios of water and oil. Using this structured reaction media, a variety of 1D nanoscale materials including Cu [23], CdS [24], BaSO4 [25], BaCrO4 [26], BaWO4 [27], TiO2 [28] nanorods or nanowires have been successfully synthesized. It is noteworthy that metal Ag nanowires with 20–30 nm in width and up to 2.5 mm in length were synthesized by UV irradiating AgNO3 in n-octanol solution containing SDS [29]. Therefore, it remains a great challenge to synthesize high aspect-ratio Ag nanowires with diameter less than 10 nm. In this paper, we describe a solution-phase route to the large-scale synthesis of the Ag nanowires with average diameters of ca. 10 nm and lengths of up to 10 mm.
2. Experimental Analytically pure AgNO3, Vitamin C, sodium dodecyl sulfate (SDS), ethanol and isooctane were used in a typical experimental procedure, and used as received without further purification. The preparation of 1D Ag nanowires was achieved by mixing equal volumes of two ethanol solutions at the same molar ratio of water to SDS, one containing an aqueous solution of metal salt (AgNO3), and the other containing an aqueous solution of Vitamin C. The mixed solution was stirred for 12 h, and then the resulting solution was aged for 48 h. Subsequently, monodispersity Ag nanowires were successfully achieved in the solution. In this work, the SDS concentration based on the overall volume of solution was fixed at 0.05 mol/L, the concentration of total metal ions and Vitamin C were kept at 0.01 mol/L on the basis of the volume of aqueous solution added in the solution. The temperature was fixed at 25 8C, and the molar ratio of water to SDS was fixed at 10. The X-ray diffraction (XRD) pattern was obtained on a Rigaku D/max 2500V PC diffractometer with Cu Ka radiation. JEM-2010 transmission electron microscope (TEM) was used to examine the morphology of the crystal Ag nanowires.
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3. Result and discussion Recently, Ag nanowires were synthesized by Wang et al., using the simple UV photoreduction of AgNO3 with 254 nm UV light in the presence of sodium dodecyl sulfate, and n-octanol solvent played important roles in the formation of the Ag nanowires [29]. Using the same method, they substituted ethanol for n-octanol, and then spherical nanoparticles were obtained. In our work, ethanol was still used as solvent, Vitamin C as reductive agent for the synthesis, and we found that Ag nanowires can also be obtained in a novel SDS/ethanol/AgNO3 solution system. Fig. 1 shows the TEM images of as-made nanoproducts, and it reveals that the organized morphology of the products is not monotonous. A part of the nanowires present network, as shown in Fig. 1a. Although the alignment of them is not regular, we can still clearly observe that some bundles of Ag nanowires resemble together, forming a crossing network. The other morphology of the nanoproducts can be observed in Fig. 1b. These nanowires form bundles that
Fig. 1. TEM images of Ag nanowires obtained using Vitamin C as reduction agent.
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almost perfect side-by-side alignment between the nanowires. Similar results were also obtained in fabricating BaWO4 nanorods [27]. We believe that the side-by-side alignment is due to the higher aspect ratio (ca. 1000) of our nanowires. From Fig. 1b we can also calculate the length of the nanowires was almost up to 10 mm. Fig. 1c shows the TEM image of the tail end of the bundle. It is found that these nanowires are nearly parallel to each other, and the reasons are two-fold: first, this side-by-side ordering occurs in order to minimizing the excluded volume per particle in the array as first proved by Onsager [30]. Secondly, the higher lateral capillary forces along the length of a nanowire, as compared with its width, could be another important driving force for the side-by-side alignment of nanowires [27]. The clear image of the nanowires consisting of the network and the bundles is shown in Fig. 1d, which reveals that the nanowires is rather uniform with an average diameter of 8–10 nm. Furthermore, the highly crystalline nature of the Ag nanowires is conformed by the selected area electron diffraction measurement made on these well-dispersed wires. Fig. 2 shows the ED pattern. The XRD pattern of the Ag nanowires is shown in Fig. 3. This pattern indicates that the Ag nanowires ˚. are crystalline with cubic lattice constant of a = 4.07 A In order to investigate the influences of reduction agent on the shape of Ag nanoproducts, the morphology of Ag nanoparticles was further established by recording TEM images of Ag suspension prepared by using sodium citrate and hydrazine hydrate as reduction agents. As shown in Fig. 4a, citrate reduction produces rod-like Ag nanoparticles of width 15–20 nm and length 50–200 nm. Ag particles with irregular shape and larger size (Fig. 4b) were obtained with hydrazine hydrate as reduction agent. Neither of these reduction agents have the ability to produce Ag nanowires. Therefore, Vitamin C is necessary for the formation of Ag nanowires under the experimental conditions. To the best of our knowledge, no other nanowires were synthesized in this novel system. However, the growth mechanism of the silver nanowires is still unclear at the present time. The possible reasons for the
Fig. 2. ED pattern of Ag nanowires obtained using Vitamin C as reduction agent.
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Fig. 3. XRD pattern of Ag nanowires.
formation of Ag nanowires are two-fold: first, the cooperation of ethanol solvent with micelles of 0.05 mol/L SDS in ethanol solution, leads to the directional aggregation of the originally produced clusters and further directional growing. It is well known that the surfactant in the solution will aggregate into micelles when its concentration exceeds CMC. Moreover, depending on the concentration and surrounding medium of the surfactant, the micelles display different shapes, such as spherical, lay-like and rod-like shapes. To our knowledge, the concentration of 0.05 mol/L SDS in ethanol is higher than CMC, so there must be organized micelles in the as-prepared system. During the reaction, the micelles play the roles of ‘‘seeds’’, the originally produced Ag cluster aggregates on the micelles, and further grow to nanoproducts, which can be called a special soft template technique to synthesize nanowires [29]. Second, the reaction speed between AgNO3 and Vitamin C is slow, which is an important factor for the formation of Ag nanowires. When the reaction speed is fast, the produced Ag clusters are too excessive to grow according to a certain orientation, which will lead to the formation of larger and irregular particles. The further mechanism should be studied in detail.
Fig. 4. TEM images of Ag nanoparticles obtained using (a) sodium citrate and (b) hydrazine hydrate as reduction agents.
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4. Conclusion In summary, high aspect-ratio, crystal Ag nanowires with diameter as small as 8–10 nm and length up to 10 mm have been successfully prepared in a novel SDS/ethanol/AgNO3 solution by using Vitamin C as the reductive agent. It was noticed that the cooperation between ethanol and micelle and the slow reaction speed between AgNO3 and Vitamin C can be used to illustrate the formation of Ag nanowires. It is expected to exploit this mild solution-phase method to the preparation of other one-dimensional nanostructured materials.
Acknowledgement This work was supported by the National Nature Science Foundation of China, No. 20173008.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
A.M. Morales, C.M. Lieber, Science 279 (1998) 208. J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1471–1473. Z.H. Kang, E.B. Wang, M. Jiang, S.Y. Lian, Y.G. Li, C.W. Hu, Eur. J. Inorg. Chem. (2003) 370. H. Yorikawa, H. Uchida, S. Muramatsu, J. Appl. Phys. 79 (1996) 3619. M.A. El-Sayed, Acc. Chem. Res. 34 (2001) 257. J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. S.-W. Chung, J.-Y. Yu, J.R. Heath, Appl. Phys. Lett. 76 (2000) 2068. F. Favier, E.C. Walter, M.P. Zach, T. Benter, R.M. Penner, Science 293 (2001) 2227. Z. Zhang, X. Sun, M.S. Dresselhaus, J.Y. Ying, Phys. Rev. B 61 (2000) 4850. M.P. Zach, K.H. Ng, R.M. Penner, Science 290 (2000) 2120. Y.G. Sun, Y.N. Xia, Adv. Mater. 14 (2002) 833. W.F. Hoelderich, Catal. Today 62 (2000) 115. W.A. Challener, R.R. Ollmann, K.K. Kam, Sens. Actuators B 56 (1999) 254. R. Jin, Y.W. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Science 294 (2001) 1901. C.R. Martin, Science 266 (1994) 1961. Z. Zhang, D. Gekhtman, M.S. Dresselhaus, J.Y. Ying, Chem. Mater. 11 (1999) 1659. Y.J. Han, J.M. Kim, G.D. Stucky, Chem. Mater. 12 (2000) 2068. M.H. Huang, A. Choudrey, P. Yang, Chem. Commun. (2000) 1063. D. Ugarte, A. Chatelain, W.A. de Heer, Science 274 (1996) 1897. D. Zhang, L. Qi, J. Ma, H. Cheng, Chem. Mater. 13 (2001) 2753. E. Braun, Y. Eichen, U. Sivan, G. Ben-Yoseph, Nature 391 (1998) 775. B.H. Hong, S.C. Bae, C.-W. Lee, S. Jeong, K.S. Kim, Science 294 (2001) 348. M.P. Pileni, Langmuir 17 (2001) 7476. B.A. Simmons, S. Li, V.T. John, G.L. McPherson, A. Bose, W. Zhou, J. He, Nano Lett. 2 (2002) 263. M. Li, S. Mann, Langmuir 16 (2000) 7088. F. Kim, S. Kwan, J. Akana, P. Yang, J. Am. Chem. Soc. 123 (2001) 4360. S. Kwan, F. Kim, J. Akana, P. Yang, Chem. Commun. (2001) 447. D.B. Zhang, L.M. Qi, J.M. Ma, H.M. Cheng, J. Mater. Chem. 12 (2002) 3677. C.Y. Wang, M.H. Chen, G.M. Zhu, Z.G. Lin, J. Colloid Interface Sci. 243 (2001) 362. L. Onsager, Phys. Rev. 62 (1942) 558.
A novel solution-phase route for the synthesis of crystalline silver nanowires Yang Liu, Ying Chu *, Likun Yang, Dongxue Han, Zhongxian Lu¨ Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China Received 28 April 2003; received in revised form 7 November 2004; accepted 11 May 2005
Abstract A unique solution-phase route was devised to synthesize crystal Ag nanowires with high aspect-ratio (8–10 nm in diameter and length up to 10 mm) by the reduction of AgNO3 with Vitamin C in SDS/ethanol solution. The resultant nanoproducts were characterized by transmission electron microscope (TEM), X-ray diffraction (XRD) and electron diffraction (ED). A soft template mechanism was put forward to interpret the formation of metal Ag nanowires. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; B. Chemical synthesis; C. Electron diffraction
1. Introduction In recent years, nanoscale materials, especially one-dimensional (1D) nanomaterials have attracted extensive attention because of their unusual quantum properties and potential applications [1–4]. Nanowires, especially metal nanowires, have been the focus of many recent studies. They are expected to play a vital role as both connecters and functional components applied in fabricating electronic, optical and sensing devices [5–8] and to provide an ideal system to experimentally probe transport properties under various physical confinements (e.g., quantized conductance and localization effects) [9,10]. The electric and optical properties of the low dimensional nanomaterials profoundly depend on size and dimensionality. Many excellent quantum effects have not been experimentally observed in the wide field * Corresponding author. Tel.: +86 431 5269668; fax: +86 431 5684009. E-mail address: [email protected] (Y. Chu). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.05.005
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of nanomaterials, largely because of the inability to get highly crystalline samples with narrow size distribution and tunable crystallographic orientation [2,4]. Therefore, in a sense, shape and size controls are the key to the success of any approach of synthesizing nanomaterials. Silver nanowires seem to be particularly interesting to be synthesized and studied because bulk silver exhibits the highest electrical and thermal conductivities among all metals [11]. Silver has also been extensively exploited in a variety of applications that range from catalysis, through electronics, to photonics and photography [12–14]. The performance of silver in most of these applications could be significantly enhanced by processing silver into wire-like nanostructures with well-controlled dimensions [11]. To possess enhanced physical properties, the silver nanowires need to be of small diameter, and high aspect-ratio. Silver nanowires have been successfully synthesized by templating against various types of one-dimensional structures, such as channels in alumina or polymer membranes [15,16], mesoporous materials [17,18], carbon nanotubes [19], block co-polymers [20], DNA chains [21], calyx[4]hydrquinone nanotubes [22]. However, most of the templates are tedious to be prepared and the dissolution of the template in corrosive media is required to retrieve the nanorods/wires if separate single rods/wires are desired. The introduction of microemulsions has provided a relatively simple and powerful method for controlling the size, shape and surface texture of nanoparticles and nanorods/wires. In such a microenvironment, the nanomaterials are encapsulated into the closed shells of surfactant molecules. The size and the shape of nanomaterials could be well controlled by the size of capsules varying with the different ratios of water and oil. Using this structured reaction media, a variety of 1D nanoscale materials including Cu [23], CdS [24], BaSO4 [25], BaCrO4 [26], BaWO4 [27], TiO2 [28] nanorods or nanowires have been successfully synthesized. It is noteworthy that metal Ag nanowires with 20–30 nm in width and up to 2.5 mm in length were synthesized by UV irradiating AgNO3 in n-octanol solution containing SDS [29]. Therefore, it remains a great challenge to synthesize high aspect-ratio Ag nanowires with diameter less than 10 nm. In this paper, we describe a solution-phase route to the large-scale synthesis of the Ag nanowires with average diameters of ca. 10 nm and lengths of up to 10 mm.
2. Experimental Analytically pure AgNO3, Vitamin C, sodium dodecyl sulfate (SDS), ethanol and isooctane were used in a typical experimental procedure, and used as received without further purification. The preparation of 1D Ag nanowires was achieved by mixing equal volumes of two ethanol solutions at the same molar ratio of water to SDS, one containing an aqueous solution of metal salt (AgNO3), and the other containing an aqueous solution of Vitamin C. The mixed solution was stirred for 12 h, and then the resulting solution was aged for 48 h. Subsequently, monodispersity Ag nanowires were successfully achieved in the solution. In this work, the SDS concentration based on the overall volume of solution was fixed at 0.05 mol/L, the concentration of total metal ions and Vitamin C were kept at 0.01 mol/L on the basis of the volume of aqueous solution added in the solution. The temperature was fixed at 25 8C, and the molar ratio of water to SDS was fixed at 10. The X-ray diffraction (XRD) pattern was obtained on a Rigaku D/max 2500V PC diffractometer with Cu Ka radiation. JEM-2010 transmission electron microscope (TEM) was used to examine the morphology of the crystal Ag nanowires.
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3. Result and discussion Recently, Ag nanowires were synthesized by Wang et al., using the simple UV photoreduction of AgNO3 with 254 nm UV light in the presence of sodium dodecyl sulfate, and n-octanol solvent played important roles in the formation of the Ag nanowires [29]. Using the same method, they substituted ethanol for n-octanol, and then spherical nanoparticles were obtained. In our work, ethanol was still used as solvent, Vitamin C as reductive agent for the synthesis, and we found that Ag nanowires can also be obtained in a novel SDS/ethanol/AgNO3 solution system. Fig. 1 shows the TEM images of as-made nanoproducts, and it reveals that the organized morphology of the products is not monotonous. A part of the nanowires present network, as shown in Fig. 1a. Although the alignment of them is not regular, we can still clearly observe that some bundles of Ag nanowires resemble together, forming a crossing network. The other morphology of the nanoproducts can be observed in Fig. 1b. These nanowires form bundles that
Fig. 1. TEM images of Ag nanowires obtained using Vitamin C as reduction agent.
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almost perfect side-by-side alignment between the nanowires. Similar results were also obtained in fabricating BaWO4 nanorods [27]. We believe that the side-by-side alignment is due to the higher aspect ratio (ca. 1000) of our nanowires. From Fig. 1b we can also calculate the length of the nanowires was almost up to 10 mm. Fig. 1c shows the TEM image of the tail end of the bundle. It is found that these nanowires are nearly parallel to each other, and the reasons are two-fold: first, this side-by-side ordering occurs in order to minimizing the excluded volume per particle in the array as first proved by Onsager [30]. Secondly, the higher lateral capillary forces along the length of a nanowire, as compared with its width, could be another important driving force for the side-by-side alignment of nanowires [27]. The clear image of the nanowires consisting of the network and the bundles is shown in Fig. 1d, which reveals that the nanowires is rather uniform with an average diameter of 8–10 nm. Furthermore, the highly crystalline nature of the Ag nanowires is conformed by the selected area electron diffraction measurement made on these well-dispersed wires. Fig. 2 shows the ED pattern. The XRD pattern of the Ag nanowires is shown in Fig. 3. This pattern indicates that the Ag nanowires ˚. are crystalline with cubic lattice constant of a = 4.07 A In order to investigate the influences of reduction agent on the shape of Ag nanoproducts, the morphology of Ag nanoparticles was further established by recording TEM images of Ag suspension prepared by using sodium citrate and hydrazine hydrate as reduction agents. As shown in Fig. 4a, citrate reduction produces rod-like Ag nanoparticles of width 15–20 nm and length 50–200 nm. Ag particles with irregular shape and larger size (Fig. 4b) were obtained with hydrazine hydrate as reduction agent. Neither of these reduction agents have the ability to produce Ag nanowires. Therefore, Vitamin C is necessary for the formation of Ag nanowires under the experimental conditions. To the best of our knowledge, no other nanowires were synthesized in this novel system. However, the growth mechanism of the silver nanowires is still unclear at the present time. The possible reasons for the
Fig. 2. ED pattern of Ag nanowires obtained using Vitamin C as reduction agent.
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Fig. 3. XRD pattern of Ag nanowires.
formation of Ag nanowires are two-fold: first, the cooperation of ethanol solvent with micelles of 0.05 mol/L SDS in ethanol solution, leads to the directional aggregation of the originally produced clusters and further directional growing. It is well known that the surfactant in the solution will aggregate into micelles when its concentration exceeds CMC. Moreover, depending on the concentration and surrounding medium of the surfactant, the micelles display different shapes, such as spherical, lay-like and rod-like shapes. To our knowledge, the concentration of 0.05 mol/L SDS in ethanol is higher than CMC, so there must be organized micelles in the as-prepared system. During the reaction, the micelles play the roles of ‘‘seeds’’, the originally produced Ag cluster aggregates on the micelles, and further grow to nanoproducts, which can be called a special soft template technique to synthesize nanowires [29]. Second, the reaction speed between AgNO3 and Vitamin C is slow, which is an important factor for the formation of Ag nanowires. When the reaction speed is fast, the produced Ag clusters are too excessive to grow according to a certain orientation, which will lead to the formation of larger and irregular particles. The further mechanism should be studied in detail.
Fig. 4. TEM images of Ag nanoparticles obtained using (a) sodium citrate and (b) hydrazine hydrate as reduction agents.
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4. Conclusion In summary, high aspect-ratio, crystal Ag nanowires with diameter as small as 8–10 nm and length up to 10 mm have been successfully prepared in a novel SDS/ethanol/AgNO3 solution by using Vitamin C as the reductive agent. It was noticed that the cooperation between ethanol and micelle and the slow reaction speed between AgNO3 and Vitamin C can be used to illustrate the formation of Ag nanowires. It is expected to exploit this mild solution-phase method to the preparation of other one-dimensional nanostructured materials.
Acknowledgement This work was supported by the National Nature Science Foundation of China, No. 20173008.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
A.M. Morales, C.M. Lieber, Science 279 (1998) 208. J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1471–1473. Z.H. Kang, E.B. Wang, M. Jiang, S.Y. Lian, Y.G. Li, C.W. Hu, Eur. J. Inorg. Chem. (2003) 370. H. Yorikawa, H. Uchida, S. Muramatsu, J. Appl. Phys. 79 (1996) 3619. M.A. El-Sayed, Acc. Chem. Res. 34 (2001) 257. J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. S.-W. Chung, J.-Y. Yu, J.R. Heath, Appl. Phys. Lett. 76 (2000) 2068. F. Favier, E.C. Walter, M.P. Zach, T. Benter, R.M. Penner, Science 293 (2001) 2227. Z. Zhang, X. Sun, M.S. Dresselhaus, J.Y. Ying, Phys. Rev. B 61 (2000) 4850. M.P. Zach, K.H. Ng, R.M. Penner, Science 290 (2000) 2120. Y.G. Sun, Y.N. Xia, Adv. Mater. 14 (2002) 833. W.F. Hoelderich, Catal. Today 62 (2000) 115. W.A. Challener, R.R. Ollmann, K.K. Kam, Sens. Actuators B 56 (1999) 254. R. Jin, Y.W. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Science 294 (2001) 1901. C.R. Martin, Science 266 (1994) 1961. Z. Zhang, D. Gekhtman, M.S. Dresselhaus, J.Y. Ying, Chem. Mater. 11 (1999) 1659. Y.J. Han, J.M. Kim, G.D. Stucky, Chem. Mater. 12 (2000) 2068. M.H. Huang, A. Choudrey, P. Yang, Chem. Commun. (2000) 1063. D. Ugarte, A. Chatelain, W.A. de Heer, Science 274 (1996) 1897. D. Zhang, L. Qi, J. Ma, H. Cheng, Chem. Mater. 13 (2001) 2753. E. Braun, Y. Eichen, U. Sivan, G. Ben-Yoseph, Nature 391 (1998) 775. B.H. Hong, S.C. Bae, C.-W. Lee, S. Jeong, K.S. Kim, Science 294 (2001) 348. M.P. Pileni, Langmuir 17 (2001) 7476. B.A. Simmons, S. Li, V.T. John, G.L. McPherson, A. Bose, W. Zhou, J. He, Nano Lett. 2 (2002) 263. M. Li, S. Mann, Langmuir 16 (2000) 7088. F. Kim, S. Kwan, J. Akana, P. Yang, J. Am. Chem. Soc. 123 (2001) 4360. S. Kwan, F. Kim, J. Akana, P. Yang, Chem. Commun. (2001) 447. D.B. Zhang, L.M. Qi, J.M. Ma, H.M. Cheng, J. Mater. Chem. 12 (2002) 3677. C.Y. Wang, M.H. Chen, G.M. Zhu, Z.G. Lin, J. Colloid Interface Sci. 243 (2001) 362. L. Onsager, Phys. Rev. 62 (1942) 558.