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Multiple branch growth of SnO2 nanowires by thermal evaporation process
Superlattices and Microstructures 44 (2008) 728–734
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Multiple branch growth of SnO2 nanowires by thermal evaporation process H.-W. Ra, K.J. Kim, Y.H. Im ∗ School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, South Korea
article
info
Article history: Received 12 February 2008 Received in revised form 24 July 2008 Accepted 28 August 2008 Available online 26 September 2008 Keywords: Tin oxide Branched nanowires Thermal evaporation
a b s t r a c t The non-catalytic growth of single-crystalline branched SnO2 nanowires was achieved on a silicon substrate via a simple thermal evaporation process. The morphological study by field emission scanning electron microscopy revealed that the branched nanowires grew at a high density over the whole substrate surface. It was observed that the formation of these nanostuctures is affected mainly by the growth temperature and partial Sn vapor pressure. Detailed structural characterizations by X-ray diffraction and using selected area electron diffraction patterns demonstrated that the grown nanowires consist of the tetragonal rutile phase and that the branches and stems grew along the [010] and [100] directions, respectively. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Nanostructures of controlled size, shape, crystal structure, and surface structure have become the subject of intensive research, due to their importance in understanding the fundamental roles of dimensionality and their potential applications [1,2]. A number of nanostructures have been reported for a wide range of materials, including wires [3–5], belts [6,7], tubes [8,9], cages [10] and sheets [11]. Among these materials, nanostructures of tin oxide (SnO2 ), an n-type semiconductor with a wide band gap (3.6 eV), are particularly promising materials, due to the characteristics of these feature parameters in various applications. In particular, branched SnO2 nanowires have attracted much interest in the fields of optical devices, sensors and catalysts, because of their high surface-to-volume ratio [12–17]. The growth of these nanostructures has been reported from the thermal evaporation of SnO powder, which involves either self catalytic growth or catalyst assisted growth [18–20]. However, the
∗
Corresponding author. Tel.: +82 63 2702434; fax: +82 63 2702306. E-mail address: [email protected] (Y.H. Im).
0749-6036/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2008.08.002
H.-W. Ra et al. / Superlattices and Microstructures 44 (2008) 728–734
729
growth mechanism is not fully understood yet. Further research is needed to gain better insight into the growth phenomena, so that effective methods can be developed for the large-scale production of branched SnO2 nanowires. In this letter, we present the growth of multiple branched SnO2 nanowires by the simple oxidation of elemental tin (Sn) using the thermal evaporation method. The grown branched SnO2 nanowires were studied by structural and compositional characteristics. 2. Experimental The experimental setup consisted of horizontal quartz tube furnace, a pumping system and gas controlling system. High purity Sn metal powder was thermally evaporated in the presence of oxygen gas at a particular temperature. In a typical reaction process, metallic Sn powder was loaded in an alumina boat which is placed at the center of the furnace. Pieces of Si (100) were used as substrates to deposit the branched nanowires. The Si substrates were placed on the downstream side at a distance of 20–22 cm from the centre of the furnace. After completing this arrangement, the furnace was evacuated by means of a rotary pump to a pressure of 10−3 Torr and heated to 1200 ◦ C with a heating rate of 20 ◦ C/min. Once the desired temperature was attained, high-purity nitrogen and oxygen gases with flow rates of 150 standard cubic centimeters per minute (sccm) and 10 sccm, respectively, were introduced into the furnace. The local substrate temperature was measured to be about 850–950 ◦ C. After allowing the reaction to proceed for 2 h, the flow of oxygen was stopped and the tube furnace was cooled to room-temperature under the continuous flow of nitrogen. Finally, a layer of white wool-like products was found on the substrates. The general morphologies of the as-grown products were characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Their crystal structures and crystallinity were studied by means of X-ray diffraction (XRD) with Cu-Kα radiation and selected area electron diffraction (SAED) patterns. The composition of the as-grown products was investigated using energy dispersive spectrometry (EDS) in conjunction with FESEM. For the TEM observations, the structures were dispersed in ethanol via ultrasonication and, then, a drop of ethanol, containing the branched nanostructures, was placed onto a Cu mesh grid and examined. 3. Results and discussion The general morphologies of the as-grown nanostructures on the silicon substrates were observed by FESEM, as shown in Fig. 1. The low-resolution image revealed that the branched nanowires grew over the whole substrate surface (Fig. 1(a)). Fig. 1(b) shows the high-resolution FESEM image of the branched nanostructure of an individual SnO2 nanowire. It is seen that several small diameter nanowires grew orthogonally along the stem of the SnO2 nanowire. We would expect that the stems of the branched nanowires were formed at the beginning and that the branches then grew on them. The diameters and lengths of the stems were much greater than those of the branched nanowires. Recently, the growth of these branched nanowires has been explained mainly by the self-catalytic vapor–liquid–solid (VLS) mechanism in which metallic Sn particles serve as the catalyst for the growth [20,21]. In this work, some droplet-like protrusions were also found at or close to the tips of the growth branches as shown in Fig. 1(b), which represents nearly the VLS growth. Therefore, we believe that the growth of these branched nanowires comply with the conventional VLS mechanism. The crystal structures were determined by XRD with Cu-Kα radiation. Fig. 2 shows a typical XRD pattern for the as-grown branched SnO2 nanostructures. The observed values showed good agreement with the values reported by the joint committee on powder diffraction standards (JCPDS) card (411445), indicating that a tetragonal rutile phase of SnO2 (with lattice constants of a = 0.4738 nm and c = 0.3187 nm) was formed. No other peaks related to metallic Sn were observed in the pattern. The compositional analysis of the as-grown products using EDS spectroscopy is shown in Fig. 3. The EDS spectrum indicated that the nanostructures were composed of Sn and oxygen only. Fig. 4 shows a typical TEM image of the as-grown branched SnO2 nanowire corresponding to the circled portions in the FESEM images in Fig. 1. It is clear that new branches continue to grow over the
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H.-W. Ra et al. / Superlattices and Microstructures 44 (2008) 728–734
Fig. 1. (a) Low and (b) high magnification FESEM images of branched SnO2 nanowires grown on the silicon substrates by the thermal evaporation process.
Fig. 2. Typical XRD pattern of the branched SnO2 nanowires grown on the silicon substrates by the thermal evaporation process.
H.-W. Ra et al. / Superlattices and Microstructures 44 (2008) 728–734
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Fig. 3. Typical EDS spectrum of the branched SnO2 nanowires grown on the silicon substrates by the thermal evaporation process.
Fig. 4. Typical TEM image of the branched SnO2 nanowires grown on the silicon substrates by the thermal evaporation process. The inset shows the corresponding SAED pattern of the circled portion of the nanowire shown in Fig. 4.
preformed branches, resulting in the formation of multiple branched nanowires. The inset of Fig. 4 shows a typical corresponding SAED pattern of the branched nanowires. The observed diffraction pattern clearly indicates that the branched SnO2 nanowires are single crystalline and possess a rutile structure of SnO2 . The SAED pattern further confirms that the growth directions of the branches and stems in the SnO2 nanostructures are the [010] and [100] directions, respectively, which are parallel to each other. Although the self-catalytic VLS mechanism has been proposed to understand the formation of multiple branched SnO2 nanojuctions, understanding the fundamental phenomena, such as when and why the evolution of these nanostructures occurs, still remains a significant challenge. In an attempt to address the above issues, we investigated the evolution of the synthesized SnO2 structures along the distance away from the Sn powder which was loaded in the center of the furnace. As shown in Fig. 5, the local growth temperatures were measured from the center of the furnace to 25 cm downstream. The SnO2 structure evolution along the carrier gas flow direction can be divided to three regions viz. region I (1100 ◦ C) forming 1D microstructures (Fig. 6(a)), region II (1050–950 ◦ C) where the 1D nanostructures occurs (Fig. 6(b)) and region III (950–850 ◦ C) forming multiple branched nanowires (Fig. 6(c)). The SnO2 structure changes from 1D microstructures to 1D nanostructures as the temperature decreased from 1100 to 950 ◦ C along the carrier gas flow direction. In general, the
732
H.-W. Ra et al. / Superlattices and Microstructures 44 (2008) 728–734
Fig. 5. The measured temperature profile as a function of distance from the center position along the carrier gas flow direction.
melting point of nanoclusters decreases with decreasing size in the nanometer region. Therefore, the evolution from 1D microstructures to 1D nanostructures can be explained by a self-catalytic VLS mechanism in which the size of the liquid droplet acting as the nucleation sites decreases with decreasing temperature [22]. With further increase in distance between the source and substrate, the substrate temperature decreased to 900 ◦ C in region III. The diameter of the SnO2 nanowires is about few hundred nm, but their shape changes to multiple branched nanowires. The stemmed nanowires served as the building blocks for the formation of the final products. Then, the Sn vapor can be condensed onto the stemmed nanowires for the nucleation process. The branched SnO2 nanowires can be attributed to the condensed Sn droplets on the outer walls of the stems or branches of the SnO2 nanowires. In this region, the size of the molten Sn droplets decreases with temperature as compared to other region. In addition, it is expected that the partial pressure of the Sn vapor is decreased toward downstream due to the amount of the Sn vapor consumed on the substrate or reactor wall. Although it is still not completely understood why the multiple branched nanostructures occurs, lower temperature and Sn partial vapor pressure of region III are responsible for the formation of multiple branched nanowires. We believe that our efforts will provide the basic insights required for further experimental and theoretical work to explain the growth phenomena of the various nanostructures. 4. Conclusions Single-crystalline branched SnO2 nanowires were grown at a high density on a silicon substrate via a simple thermal evaporation process without the use of any metal catalyst or additives. Detailed structural studies revealed that the branched SnO2 nanowires were crystalline with the tetragonal rutile phase and that the branches and stems grew along the [010] and [100] directions, respectively. It was observed that the growth temperature and partial Sn vapor pressure are the critical factors to determine the formation of multiple branched nanowires. The obtained branched SnO2 nanostructures with a large surface area can be useful for the fabrication of various high-efficiency nanodevices and catalysts in the near future. Acknowledgements This work was supported in part by a Korea Research Foundation Grant (KRF-2006-D00411I02959), and by Korea Science and Engineering Foundation Grant (R01-2006-000-11306-0) funded by the Korean Government.
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Fig. 6. FESEM images of SnO2 samples along the carrier gas flow direction, corresponding to region I–III in Fig. 5, respectively. The insets show the magnified view.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
S. Iijima, Nature 354 (1991) 56. A.M. Marales, C.M. Lieber, Science 279 (1998) 208. T.J. Trentler, K.M. Hickman, S.C. Goel, A.M. Viano, P.C. Gibbons, W.E. Buhro, Science 270 (1995) 1791. S.T. Lee, N. Wang, Y.F. Zhang, Y.H. Tang, MRS Bull. 24 (1999) 36. X.F. Duan, C.M. Lieber, Adv. Mater. 12 (2000) 298. Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. W. Shi, H. Peng, N. Wang, C.P. Li, L. Xu, C.S. Lee, R. Kalish, S.T. Lee, J. Amer. Chem. Soc. 123 (2001) 11095. N.G. Chopra, R.J. Luyke, K. Cherry, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettle, Science 269 (1995) 966. E. Bengu, L.D. Marks, Phys. Rev. Lett. 86 (2001) 2385. Z.R. Dai, Z.W. Pan, Z.L. Wang, J. Amer. Chem. Soc. 124 (2002) 8673. Z.R. Dai, Z.W. Pan, Z.L. Wang, J. Phys. Chem. B 106 (2002) 902. E.R. Leite, I.T. Weber, E. Longo, J.A. Varela, Adv. Mater. 12 (2000) 966. S. Ferrere, A. Zaban, B.A. Gsegg, J. Phys. Chem. B 101 (1997) 4490. Y.S. He, J.C. Campbell, R.C. Murphy, M.F. Arendt, J.S. Swinnea, J. Mater. Res. 8 (1993) 3131. W. Dazhi, W. Shulin, C. Jun, Z. Suyuan, L. Fangqing, Phys. Rev. B 49 (1994) 282. J.K. Jian, X.L. Chen, W.J. Wang, L. Dai, Y.P. Xu, Appl. Phys. A 76 (2003) 291. Y.J. Ma, F. Zhou, L. Li, Z. Zhang, Solid State Commun. 130 (2004) 313. Z.L. Wang, Z. Pan, Adv. Mater. 14 (2002) 1029. Y.X. Chen, L.J. Campbell, W.L. Zhou, J. Crystal Growth 270 (2004) 505. Z. Ying, Q. Wan, Z.T. Song, S.L. Feng, Mater. Lett. 59 (2006) 1670. D.-W. Yuan, R.-F. Yan, G. Simkovich, J. Mater. Sci. 34 (1999) 2911. H.Y. Peng, Z.W. Pan, L. Xu, X.H. Fan, N. Wang, C.S. Lee, S.T. Lee, Adv. Mater. 13 (2001) 317.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Multiple branch growth of SnO2 nanowires by thermal evaporation process H.-W. Ra, K.J. Kim, Y.H. Im ∗ School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, South Korea
article
info
Article history: Received 12 February 2008 Received in revised form 24 July 2008 Accepted 28 August 2008 Available online 26 September 2008 Keywords: Tin oxide Branched nanowires Thermal evaporation
a b s t r a c t The non-catalytic growth of single-crystalline branched SnO2 nanowires was achieved on a silicon substrate via a simple thermal evaporation process. The morphological study by field emission scanning electron microscopy revealed that the branched nanowires grew at a high density over the whole substrate surface. It was observed that the formation of these nanostuctures is affected mainly by the growth temperature and partial Sn vapor pressure. Detailed structural characterizations by X-ray diffraction and using selected area electron diffraction patterns demonstrated that the grown nanowires consist of the tetragonal rutile phase and that the branches and stems grew along the [010] and [100] directions, respectively. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Nanostructures of controlled size, shape, crystal structure, and surface structure have become the subject of intensive research, due to their importance in understanding the fundamental roles of dimensionality and their potential applications [1,2]. A number of nanostructures have been reported for a wide range of materials, including wires [3–5], belts [6,7], tubes [8,9], cages [10] and sheets [11]. Among these materials, nanostructures of tin oxide (SnO2 ), an n-type semiconductor with a wide band gap (3.6 eV), are particularly promising materials, due to the characteristics of these feature parameters in various applications. In particular, branched SnO2 nanowires have attracted much interest in the fields of optical devices, sensors and catalysts, because of their high surface-to-volume ratio [12–17]. The growth of these nanostructures has been reported from the thermal evaporation of SnO powder, which involves either self catalytic growth or catalyst assisted growth [18–20]. However, the
∗
Corresponding author. Tel.: +82 63 2702434; fax: +82 63 2702306. E-mail address: [email protected] (Y.H. Im).
0749-6036/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2008.08.002
H.-W. Ra et al. / Superlattices and Microstructures 44 (2008) 728–734
729
growth mechanism is not fully understood yet. Further research is needed to gain better insight into the growth phenomena, so that effective methods can be developed for the large-scale production of branched SnO2 nanowires. In this letter, we present the growth of multiple branched SnO2 nanowires by the simple oxidation of elemental tin (Sn) using the thermal evaporation method. The grown branched SnO2 nanowires were studied by structural and compositional characteristics. 2. Experimental The experimental setup consisted of horizontal quartz tube furnace, a pumping system and gas controlling system. High purity Sn metal powder was thermally evaporated in the presence of oxygen gas at a particular temperature. In a typical reaction process, metallic Sn powder was loaded in an alumina boat which is placed at the center of the furnace. Pieces of Si (100) were used as substrates to deposit the branched nanowires. The Si substrates were placed on the downstream side at a distance of 20–22 cm from the centre of the furnace. After completing this arrangement, the furnace was evacuated by means of a rotary pump to a pressure of 10−3 Torr and heated to 1200 ◦ C with a heating rate of 20 ◦ C/min. Once the desired temperature was attained, high-purity nitrogen and oxygen gases with flow rates of 150 standard cubic centimeters per minute (sccm) and 10 sccm, respectively, were introduced into the furnace. The local substrate temperature was measured to be about 850–950 ◦ C. After allowing the reaction to proceed for 2 h, the flow of oxygen was stopped and the tube furnace was cooled to room-temperature under the continuous flow of nitrogen. Finally, a layer of white wool-like products was found on the substrates. The general morphologies of the as-grown products were characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Their crystal structures and crystallinity were studied by means of X-ray diffraction (XRD) with Cu-Kα radiation and selected area electron diffraction (SAED) patterns. The composition of the as-grown products was investigated using energy dispersive spectrometry (EDS) in conjunction with FESEM. For the TEM observations, the structures were dispersed in ethanol via ultrasonication and, then, a drop of ethanol, containing the branched nanostructures, was placed onto a Cu mesh grid and examined. 3. Results and discussion The general morphologies of the as-grown nanostructures on the silicon substrates were observed by FESEM, as shown in Fig. 1. The low-resolution image revealed that the branched nanowires grew over the whole substrate surface (Fig. 1(a)). Fig. 1(b) shows the high-resolution FESEM image of the branched nanostructure of an individual SnO2 nanowire. It is seen that several small diameter nanowires grew orthogonally along the stem of the SnO2 nanowire. We would expect that the stems of the branched nanowires were formed at the beginning and that the branches then grew on them. The diameters and lengths of the stems were much greater than those of the branched nanowires. Recently, the growth of these branched nanowires has been explained mainly by the self-catalytic vapor–liquid–solid (VLS) mechanism in which metallic Sn particles serve as the catalyst for the growth [20,21]. In this work, some droplet-like protrusions were also found at or close to the tips of the growth branches as shown in Fig. 1(b), which represents nearly the VLS growth. Therefore, we believe that the growth of these branched nanowires comply with the conventional VLS mechanism. The crystal structures were determined by XRD with Cu-Kα radiation. Fig. 2 shows a typical XRD pattern for the as-grown branched SnO2 nanostructures. The observed values showed good agreement with the values reported by the joint committee on powder diffraction standards (JCPDS) card (411445), indicating that a tetragonal rutile phase of SnO2 (with lattice constants of a = 0.4738 nm and c = 0.3187 nm) was formed. No other peaks related to metallic Sn were observed in the pattern. The compositional analysis of the as-grown products using EDS spectroscopy is shown in Fig. 3. The EDS spectrum indicated that the nanostructures were composed of Sn and oxygen only. Fig. 4 shows a typical TEM image of the as-grown branched SnO2 nanowire corresponding to the circled portions in the FESEM images in Fig. 1. It is clear that new branches continue to grow over the
730
H.-W. Ra et al. / Superlattices and Microstructures 44 (2008) 728–734
Fig. 1. (a) Low and (b) high magnification FESEM images of branched SnO2 nanowires grown on the silicon substrates by the thermal evaporation process.
Fig. 2. Typical XRD pattern of the branched SnO2 nanowires grown on the silicon substrates by the thermal evaporation process.
H.-W. Ra et al. / Superlattices and Microstructures 44 (2008) 728–734
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Fig. 3. Typical EDS spectrum of the branched SnO2 nanowires grown on the silicon substrates by the thermal evaporation process.
Fig. 4. Typical TEM image of the branched SnO2 nanowires grown on the silicon substrates by the thermal evaporation process. The inset shows the corresponding SAED pattern of the circled portion of the nanowire shown in Fig. 4.
preformed branches, resulting in the formation of multiple branched nanowires. The inset of Fig. 4 shows a typical corresponding SAED pattern of the branched nanowires. The observed diffraction pattern clearly indicates that the branched SnO2 nanowires are single crystalline and possess a rutile structure of SnO2 . The SAED pattern further confirms that the growth directions of the branches and stems in the SnO2 nanostructures are the [010] and [100] directions, respectively, which are parallel to each other. Although the self-catalytic VLS mechanism has been proposed to understand the formation of multiple branched SnO2 nanojuctions, understanding the fundamental phenomena, such as when and why the evolution of these nanostructures occurs, still remains a significant challenge. In an attempt to address the above issues, we investigated the evolution of the synthesized SnO2 structures along the distance away from the Sn powder which was loaded in the center of the furnace. As shown in Fig. 5, the local growth temperatures were measured from the center of the furnace to 25 cm downstream. The SnO2 structure evolution along the carrier gas flow direction can be divided to three regions viz. region I (1100 ◦ C) forming 1D microstructures (Fig. 6(a)), region II (1050–950 ◦ C) where the 1D nanostructures occurs (Fig. 6(b)) and region III (950–850 ◦ C) forming multiple branched nanowires (Fig. 6(c)). The SnO2 structure changes from 1D microstructures to 1D nanostructures as the temperature decreased from 1100 to 950 ◦ C along the carrier gas flow direction. In general, the
732
H.-W. Ra et al. / Superlattices and Microstructures 44 (2008) 728–734
Fig. 5. The measured temperature profile as a function of distance from the center position along the carrier gas flow direction.
melting point of nanoclusters decreases with decreasing size in the nanometer region. Therefore, the evolution from 1D microstructures to 1D nanostructures can be explained by a self-catalytic VLS mechanism in which the size of the liquid droplet acting as the nucleation sites decreases with decreasing temperature [22]. With further increase in distance between the source and substrate, the substrate temperature decreased to 900 ◦ C in region III. The diameter of the SnO2 nanowires is about few hundred nm, but their shape changes to multiple branched nanowires. The stemmed nanowires served as the building blocks for the formation of the final products. Then, the Sn vapor can be condensed onto the stemmed nanowires for the nucleation process. The branched SnO2 nanowires can be attributed to the condensed Sn droplets on the outer walls of the stems or branches of the SnO2 nanowires. In this region, the size of the molten Sn droplets decreases with temperature as compared to other region. In addition, it is expected that the partial pressure of the Sn vapor is decreased toward downstream due to the amount of the Sn vapor consumed on the substrate or reactor wall. Although it is still not completely understood why the multiple branched nanostructures occurs, lower temperature and Sn partial vapor pressure of region III are responsible for the formation of multiple branched nanowires. We believe that our efforts will provide the basic insights required for further experimental and theoretical work to explain the growth phenomena of the various nanostructures. 4. Conclusions Single-crystalline branched SnO2 nanowires were grown at a high density on a silicon substrate via a simple thermal evaporation process without the use of any metal catalyst or additives. Detailed structural studies revealed that the branched SnO2 nanowires were crystalline with the tetragonal rutile phase and that the branches and stems grew along the [010] and [100] directions, respectively. It was observed that the growth temperature and partial Sn vapor pressure are the critical factors to determine the formation of multiple branched nanowires. The obtained branched SnO2 nanostructures with a large surface area can be useful for the fabrication of various high-efficiency nanodevices and catalysts in the near future. Acknowledgements This work was supported in part by a Korea Research Foundation Grant (KRF-2006-D00411I02959), and by Korea Science and Engineering Foundation Grant (R01-2006-000-11306-0) funded by the Korean Government.
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Fig. 6. FESEM images of SnO2 samples along the carrier gas flow direction, corresponding to region I–III in Fig. 5, respectively. The insets show the magnified view.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
S. Iijima, Nature 354 (1991) 56. A.M. Marales, C.M. Lieber, Science 279 (1998) 208. T.J. Trentler, K.M. Hickman, S.C. Goel, A.M. Viano, P.C. Gibbons, W.E. Buhro, Science 270 (1995) 1791. S.T. Lee, N. Wang, Y.F. Zhang, Y.H. Tang, MRS Bull. 24 (1999) 36. X.F. Duan, C.M. Lieber, Adv. Mater. 12 (2000) 298. Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. W. Shi, H. Peng, N. Wang, C.P. Li, L. Xu, C.S. Lee, R. Kalish, S.T. Lee, J. Amer. Chem. Soc. 123 (2001) 11095. N.G. Chopra, R.J. Luyke, K. Cherry, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettle, Science 269 (1995) 966. E. Bengu, L.D. Marks, Phys. Rev. Lett. 86 (2001) 2385. Z.R. Dai, Z.W. Pan, Z.L. Wang, J. Amer. Chem. Soc. 124 (2002) 8673. Z.R. Dai, Z.W. Pan, Z.L. Wang, J. Phys. Chem. B 106 (2002) 902. E.R. Leite, I.T. Weber, E. Longo, J.A. Varela, Adv. Mater. 12 (2000) 966. S. Ferrere, A. Zaban, B.A. Gsegg, J. Phys. Chem. B 101 (1997) 4490. Y.S. He, J.C. Campbell, R.C. Murphy, M.F. Arendt, J.S. Swinnea, J. Mater. Res. 8 (1993) 3131. W. Dazhi, W. Shulin, C. Jun, Z. Suyuan, L. Fangqing, Phys. Rev. B 49 (1994) 282. J.K. Jian, X.L. Chen, W.J. Wang, L. Dai, Y.P. Xu, Appl. Phys. A 76 (2003) 291. Y.J. Ma, F. Zhou, L. Li, Z. Zhang, Solid State Commun. 130 (2004) 313. Z.L. Wang, Z. Pan, Adv. Mater. 14 (2002) 1029. Y.X. Chen, L.J. Campbell, W.L. Zhou, J. Crystal Growth 270 (2004) 505. Z. Ying, Q. Wan, Z.T. Song, S.L. Feng, Mater. Lett. 59 (2006) 1670. D.-W. Yuan, R.-F. Yan, G. Simkovich, J. Mater. Sci. 34 (1999) 2911. H.Y. Peng, Z.W. Pan, L. Xu, X.H. Fan, N. Wang, C.S. Lee, S.T. Lee, Adv. Mater. 13 (2001) 317.