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Iron-catalytic growth of prism-shaped single-crystal silicon nanowires by chemical vapor deposition of silane
Chemical Physics Letters 411 (2005) 198–202 www.elsevier.com/locate/cplett
Iron-catalytic growth of prism-shaped single-crystal silicon nanowires by chemical vapor deposition of silane Chen Li a
a,b,1
, Chi Gu
a,b,1
, Zengtao Liu
a,b
, Jinxiao Mi c, Yong Yang
a,b,*
State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, Fujian 361005, China b Department of Chemistry, Xiamen University, Xiamen, Fujian 361005, China c Department of Material Science and Engineering, Xiamen University, Xiamen, Fujian 361005, China Received 26 April 2005; in final form 9 May 2005 Available online 29 June 2005
Abstract Single-crystal silicon nanowires with the prism structures were synthesized by chemical vapor deposition of SiH4 gas at 450 °C. Fe particles which were located at the tip of the CNTs were employed as a catalyst for the growth of silicon nanowires (SiNWs). Transmission electron microscopy studies of the materials showed that the nanowires have a diameter of 50–70 nm and a length of several micrometers. High-resolution transmission electron microscopy demonstrated that the nanowires have excellent single-crystal characteristics. Both the CNTs and Fe play a key role in the growth process of the SiNWs. A growth mechanism was proposed for the growth of silicon nanowires under our experimental conditions. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional nanostructures are significant for nanoscale devices based on their distinctive structures and properties. Since silicon has long been the dominant material in semiconductor and microelectronics industries, thus the study of synthesis and properties of nanostructured silicon material is one of the most important subjects in the research of nanostructured material field. Nanostructured silicon such as wires [1], ribbons [2] and tubes [3] have been widely studied for the applications in nanoscale electronics and photonics [4], field-effect transistors (FETs) [5] and so on. Many efforts have been made to prepare silicon nanowires (SiNWs) by different methods, such as laser ablation [6,7], thermal evaporation deposition [8], chemical vapor deposition (CVD) [9–11], and supercritical fluid–liquid– *
Corresponding author. Fax: +86 592 2185753. E-mail addresses: [email protected] (Y. Yang), chenli@xmu. edu.cn (C. Li). 1 These authors contributed equally to this work. 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.117
solid (SFLS) synthesis [12]. The most popular growth mechanism for SiNWs is vapor–liquid–solid (VLS) growth mechanism [13,14]. In VLS process, a metal, such as gold or iron that forms a low temperature eutectic phase with silicon is used as the catalyst that has been heated to a temperature greater than the eutectic temperature (363 °C for Au–Si, 1207 °C for Fe–Si) [15]. In addition, oxide-assisted-growth (OAG) method [16] has also been developed to produce large quantities of SiNWs. There, an excimer laser is used to ablate a solid composite target of highly Si powder mixed metals (Fe, Ni or Co). The target temperature was 1100–1400 °C and the nanowires growth temperature was selected as 900–1100 °C. In general, when using pure Fe metal as a catalyst for the growth of silicon nanowires, almost all methods in the literatures [6,16] require the growth temperature higher than 1000 °C. It is quite difficult to grow single-crystal SiNWs without Au catalyst assistance at the lower temperature. In this work, the SiNWs were grown at 450 °C by using common catalyst of iron (Fe), which was previously used as a catalyst for growing CNTs. More
C. Li et al. / Chemical Physics Letters 411 (2005) 198–202
199
importantly, it should be noted that the growth of the SiNWs here is different from the conventional VLS mechanism, revealing the crystalline structures of the crystal may have influence on the structure of the grown one-dimensional nanostructures. The interface between Si and Fe prefers to take the least lattice mismatch.
2. Experimental The anodic aluminum oxide (AAO) templates were formed by using a two-step anodization method as reported in our previous paper [17]. Then, a small amount of iron was deposited into the bottom of the AAO pores using an alternating current (AC) electrodeposition method. Electrodeposition was carried out in a ammonium iron (II) sulfate hexahydrate (NH4)2Fe(SO4)2 Æ 6H2O solution (30 g/L, pH 5.0) at 50 Hz. After that, the CNTs were formed in the pores of AAO template by pyrolyzing a flow of mixture of acetylene (10 sccm) and nitrogen (50 sccm) at 650 °C for 1 h. In order to expose CNTs tips containing iron catalyst, we remove the AAO template partly before deposition of silicon. Prior to the pyrolysis of silane, the quartz boat tube furnace was purged with pure nitrogen gas, while it was cooled down to 450 °C. While the flow of pure nitrogen gas (50 sccm) remains as a carrier gas, a flow of 5% silane (diluted in high purity argon) with a flow rate of 5 sccm (Si partial pressure of 4 Torr) was pyrolyzed to grow silicon nanostructure under Fe catalysis at the tip of the CNTs. Several sets of pyrolysis times were implemented in our experiments to form the SiNWs with different lengths. The sample was investigated in scanning electron microscopy (SEM) (Oxford Company, LEO-1530). Furthermore, AAO templates together with product were dissolved in a 50% H3PO4 for 1 h, then in a 1 M NaOH to remove the residue aluminum and AAO template completely. The samples were prepared for high-resolution transmission microscopy (HRTEM) (FEI Company, TECNAI-F30) observation and energy dispersive X-ray (EDX) spectra examination.
3. Results and discussion Fig. 1 is an SEM image for the cross-sectional view of the SiNWs on CNTs array after partly dissolving the AAO template. It can be seen that one-dimensional silicon nanowires were exactly located at the end of the carbon nanotubes with high density. These SiNWs tended to stick to each other. The diameter of the SiNWs is about 50–70 nm, which is well consistent with the diameter of the CNTs and AAO pores. It also can be seen that most SiNWs have a length of several microns.
Fig. 1. The SEM image of the SiNWs grown on the CNTs after removing the AAO template partly.
Figs. 2a,b are the TEM images and HRTEM image which provide an insight into the structure of the SiNWs, respectively. These SiNWs were broken down from the CNTs during ultrasonically cleaning for TEM observation. It can be found from Fig. 2a that the as-grown SiNWs are actually the prism-shaped nanostructures. Fig. 2b displays a typical HRTEM image taken from the edge area of the SiNWs (indicated by the box area in Fig. 2a). Detailed analysis on lattice images give interplanar spacing of 0.31 nm, closely corresponding to that of Si(1 1 1) plane which reveals the growth direction [1 1 1] of these SiNWs. A selected-area electron diffraction (SAED) pattern (see Fig. 2a, left inset) taken from top face (Fig. 2a, indicated by red arrow) of the tilted image was taken along [1 1 1] zone axis of single crystal Si. Another SAED pattern was taken along [2 1 1] zone axis of single crystal Si (see Fig. 2b, left inset). From the data above, we can confirm the as-grown prism of single crystal SiNWs (Fig. 1a) is a combination assembled by (1 1 1) and ( 1 1 1), (2 1 1) and ( 2 1 1), (0 1 1) and (0 1 1) three pairs of parallel planes. It belongs to cubic silicon structure with a = 0.5341 nm, space group Fd3m. Fig. 2c gives an EDX spectra taken from an individual SiNW, which further confirmed that the nanowires are compose of silicon with a minimal content of oxygen. In order to further investigate the growth process of the SiNWs, different pyrolysis time of silane were carried out in this work. It can be found from Figs. 3a–d that there are different morphologies of the SiNWs at different growth stages. With the increasing of pyrolysis time of silane, a triangle-shaped silicon tip at initial stage
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Fig. 2. (a) The TEM image of the SiNWs with a SAED image (upper left inset) taken from the face indicated by the red arrow and a schematic representation of crystal faces grown (lower left inset). (b) The HRTEM images of the enlarged box area in (a) with its SAED image (left inset). (c) EDX spectrum taken from an individual SiNWs. (For interpretation of the references to the color in this figure legend, the reader is referred to the web version of this article.)
transforms gradually to a square, ultimately to SiNWs. It also can be revealed that in growth process of SiNWs, the CNTs have a size-confined effect on the growth of SiNWs through confining the size of metal catalyst. In our approach, no SiNWs were formed at 450 °C just with Fe catalysis. This implied that the CNTs play an important role on growth of the SiNWs. It has been reported that carbothermal reduction of the CNTs can help silicon powder to form nanostructures [18,19]. Thus, it is believed that the CNTs facilitate the pyrolysis of silane and growth of the SiNWs. That is, the CNTs, as an assistance of the Fe catalyst, help to greatly decrease the reaction temperature. During the whole
CVD process, the CNTs always keep Fe particles active state and maintain highly reductive atmosphere around them when silane diffuses into them. It can be clearly seen from Fig. 4 that Fe particle is located between C and Si and joints them for each other (see Fig. 4b), which is different from the conventional vapor–liquid–solid (VLS) growth mechanism reported before [9]. In addition, the temperature of growing silicon is just 450 °C which is far below the eutectic temperature of Fe–Si alloy (>1200 °C). The 450 °C just reach the temperature where SiH4 start a decomposing reaction. The normal VLS mechanism is not applicable to explain our results. Fig. 4a shows
C. Li et al. / Chemical Physics Letters 411 (2005) 198–202
201
Fig. 3. TEM images of silicon nanostructures growth by different pyrolysis time of silane. (a) a triangle-shaped silicon tip grown on a CNT by 30-min pyrolysis time. (b) A rectangle silicon grown on a CNT by 40-min pyrolysis time and white arrow indicates transformation direction from triangleshaped silicon to rectangle silicon. (c) 150 min. (d) 240 min.
a HRTEM image of triangle-shaped area of the CNTs–Fe–Si junction which provides the information of the starting nucleation. The reciprocal lattice peaks, which were obtained from fast fourier transforms (FFT) of the lattice-resolved images of outer layer area (Fig. 4a, left inset) shows the interplanar spacing values of about 0.38, 0.22 and 0.31 nm which are indexed to the (1 1 0), (2 1 1) and (1 1 1) silicon planes, respectively. Fourier transforms image of an inner catalyst area (Fig. 4, right inset) solely exhibits Fe(1 1 0) of an identified spacing value of 0.20 nm. It should be noted that the spacing values of Si(2 2 0) (d2 2 0 = ˚ ) and Si(2 1 1) (d2 1 1 = 2.2172 A ˚ ), which help 1.9201 A to form the prism shaped SiNWs, are very close to ˚ ). one of Fe(1 1 0) plane (d1 1 0 = 2.0268 A It has been reported that the crystallographic lattice structure at the interface is important in defining the structural characteristics of the grown nanowires. The interface prefers to take the least lattice mismatch [20]. Thus, we think the preferred growth direction and the 3D-prism shape should be determined by the Fe(1 1 0) as is discussed above. It is confirmed by an elemental mapping (Fig. 4b) of another developing junction
specimen at early stage. The Si(2 2 0) and Si(2 1 1) which are the side planes of the prism shaped SiNWs, are determined by the Fe(1 1 0) as these planes are well matched to Fe(1 1 0). Thus, the specific plane of Fe particles play an important role in initiating nucleation and growth of silicon nanostructures, resulting in morphology and orientation control. The interface between Si and Fe prefers to take the least lattice mismatch. Besides, the Si{1 1 1} has the lowest surface energy among the Si simple index surfaces. In a word, with the assistance of the CNTs and the catalysis of Fe particles, preference of taking the least lattice mismatch between Si and Fe leads to the growth of the single-crystal SiNWs with well-defined structures at the lower temperature.
4. Summary In conclusion, by chemical vapor deposition of silane, the large-scale SiNWs were grown under the catalysis of Fe particles at the lower temperature – 450 °C. These nanowires have a diameter of 50–70 nm and a length
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C. Li et al. / Chemical Physics Letters 411 (2005) 198–202
not only have a size-confined effect on the growth of SiNWs, but also facilitate the pyrolysis and deposition of silane on iron and growth of the SiNWs because of their carbothermal reduction. The growth process proposed here which is different from the conventional VLS process should be ascribe to preference of taking the least lattice mismatch between Si and Fe. The preference leads to the control of morphology and crystalline orientation of the SiNWs.
Acknowledgments We are grateful for financial support from National Natural Science Foundation of China (Grant Nos. 29925310 and 20021002) and Ministry of Science and Technology of China (Grant No. 2001CB10506).
References
Fig. 4. (a) TEM image of a triangle shaped silicon, the upper-left inset is the two-dimensional fourier transform of the outer layer of the triangle (indicated by arrow) depicting lattice spacing of Si(1 1 1) of 0.31 nm, Si(2 1 1) of 0.22 nm and Si(2 2 0) of 0.38 nm, respectively, the bottom-right inset is the two-dimensional fourier transform of central part of the triangle image (also indicated by arrow) depicting lattice spacing of 0.207 nm, which is indexed to Fe(1 1 0). (b) TEM image and its element-mapping of triangle-shaped region. The insets are the distribution of C, Fe and Si, respectively.
of several microns. Through the HRTEM and SAED observation, the nanowires exhibit the excellent single crystal nature. At the whole growth process, the CNTs
[1] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [2] W. Shi, H. Peng, N. Wang, C.P. Li, L. Xu, C.S. Lee, R. Kalish, S.T. Lee, J. Am. Chem. Soc. 123 (2001) 11095. [3] J. Sha, J. Niu, X. Ma, J. Xu, X. Zhang, Q. Yang, D. Yang, Adv. Mater. 14 (2002) 1219. [4] C.M. Lieber, Sci. Am. 285 (2001) 58. [5] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289. [6] Y.F. Zhang, Y.H. Tang, H.Y. Peng, N. Wang, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 75 (1999) 1842. [7] X. Duan, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 188. [8] S.T. Lee, N. Wang, C.S. Lee, Mater. Sci. Eng. A 286 (2000) 16. [9] Y. Cui, L.J. Lauhon, M.S. Gudiksen, J. Wang, C.M. Lieber, Appl. Phys. Lett. 78 (2001) 2214. [10] X.Y. Zhang, L.D. Zhang, G.W. Meng, G.H. Li, N.Y. Phillipp, F. Phillipp, Adv. Mater. 13 (2001) 1238. [11] J.T. Hu, O.Y. Min, P. Yang, C.M. Lieber, Nature 399 (1999) 48. [12] T. Hanrath, B.A. Korgel, Adv. Mater. 15 (2003) 437. [13] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [14] C.M. Lieber, Solid State Commun. 107 (1998) 106. [15] J. Hu, T.W. Odom, C.M. Lieber, Accounts Chem. Res. 32 (1999) 435. [16] R.Q. Zhang, Y. Lifshitz, S.T. Lee, Adv. Mater. 7–8 (2003) 635. [17] J. Zhao, Q.Y. Gao, C. Gu, Y. Yang, Chem. Phys. Lett. 358 (2002) 77. [18] G. Gundiah, F.L. Deepak, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 381 (2003) 579. [19] P. Scheier, B. Marson, M. Lonfat, W. Schneider, K. Sattler, Surf. Sci. 458 (2000) 113. [20] Y. Ding, P.X. Gao, Z.L. Wang, J. Am. Chem. Soc. 126 (2004) 2066.
Iron-catalytic growth of prism-shaped single-crystal silicon nanowires by chemical vapor deposition of silane Chen Li a
a,b,1
, Chi Gu
a,b,1
, Zengtao Liu
a,b
, Jinxiao Mi c, Yong Yang
a,b,*
State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, Fujian 361005, China b Department of Chemistry, Xiamen University, Xiamen, Fujian 361005, China c Department of Material Science and Engineering, Xiamen University, Xiamen, Fujian 361005, China Received 26 April 2005; in final form 9 May 2005 Available online 29 June 2005
Abstract Single-crystal silicon nanowires with the prism structures were synthesized by chemical vapor deposition of SiH4 gas at 450 °C. Fe particles which were located at the tip of the CNTs were employed as a catalyst for the growth of silicon nanowires (SiNWs). Transmission electron microscopy studies of the materials showed that the nanowires have a diameter of 50–70 nm and a length of several micrometers. High-resolution transmission electron microscopy demonstrated that the nanowires have excellent single-crystal characteristics. Both the CNTs and Fe play a key role in the growth process of the SiNWs. A growth mechanism was proposed for the growth of silicon nanowires under our experimental conditions. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional nanostructures are significant for nanoscale devices based on their distinctive structures and properties. Since silicon has long been the dominant material in semiconductor and microelectronics industries, thus the study of synthesis and properties of nanostructured silicon material is one of the most important subjects in the research of nanostructured material field. Nanostructured silicon such as wires [1], ribbons [2] and tubes [3] have been widely studied for the applications in nanoscale electronics and photonics [4], field-effect transistors (FETs) [5] and so on. Many efforts have been made to prepare silicon nanowires (SiNWs) by different methods, such as laser ablation [6,7], thermal evaporation deposition [8], chemical vapor deposition (CVD) [9–11], and supercritical fluid–liquid– *
Corresponding author. Fax: +86 592 2185753. E-mail addresses: [email protected] (Y. Yang), chenli@xmu. edu.cn (C. Li). 1 These authors contributed equally to this work. 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.117
solid (SFLS) synthesis [12]. The most popular growth mechanism for SiNWs is vapor–liquid–solid (VLS) growth mechanism [13,14]. In VLS process, a metal, such as gold or iron that forms a low temperature eutectic phase with silicon is used as the catalyst that has been heated to a temperature greater than the eutectic temperature (363 °C for Au–Si, 1207 °C for Fe–Si) [15]. In addition, oxide-assisted-growth (OAG) method [16] has also been developed to produce large quantities of SiNWs. There, an excimer laser is used to ablate a solid composite target of highly Si powder mixed metals (Fe, Ni or Co). The target temperature was 1100–1400 °C and the nanowires growth temperature was selected as 900–1100 °C. In general, when using pure Fe metal as a catalyst for the growth of silicon nanowires, almost all methods in the literatures [6,16] require the growth temperature higher than 1000 °C. It is quite difficult to grow single-crystal SiNWs without Au catalyst assistance at the lower temperature. In this work, the SiNWs were grown at 450 °C by using common catalyst of iron (Fe), which was previously used as a catalyst for growing CNTs. More
C. Li et al. / Chemical Physics Letters 411 (2005) 198–202
199
importantly, it should be noted that the growth of the SiNWs here is different from the conventional VLS mechanism, revealing the crystalline structures of the crystal may have influence on the structure of the grown one-dimensional nanostructures. The interface between Si and Fe prefers to take the least lattice mismatch.
2. Experimental The anodic aluminum oxide (AAO) templates were formed by using a two-step anodization method as reported in our previous paper [17]. Then, a small amount of iron was deposited into the bottom of the AAO pores using an alternating current (AC) electrodeposition method. Electrodeposition was carried out in a ammonium iron (II) sulfate hexahydrate (NH4)2Fe(SO4)2 Æ 6H2O solution (30 g/L, pH 5.0) at 50 Hz. After that, the CNTs were formed in the pores of AAO template by pyrolyzing a flow of mixture of acetylene (10 sccm) and nitrogen (50 sccm) at 650 °C for 1 h. In order to expose CNTs tips containing iron catalyst, we remove the AAO template partly before deposition of silicon. Prior to the pyrolysis of silane, the quartz boat tube furnace was purged with pure nitrogen gas, while it was cooled down to 450 °C. While the flow of pure nitrogen gas (50 sccm) remains as a carrier gas, a flow of 5% silane (diluted in high purity argon) with a flow rate of 5 sccm (Si partial pressure of 4 Torr) was pyrolyzed to grow silicon nanostructure under Fe catalysis at the tip of the CNTs. Several sets of pyrolysis times were implemented in our experiments to form the SiNWs with different lengths. The sample was investigated in scanning electron microscopy (SEM) (Oxford Company, LEO-1530). Furthermore, AAO templates together with product were dissolved in a 50% H3PO4 for 1 h, then in a 1 M NaOH to remove the residue aluminum and AAO template completely. The samples were prepared for high-resolution transmission microscopy (HRTEM) (FEI Company, TECNAI-F30) observation and energy dispersive X-ray (EDX) spectra examination.
3. Results and discussion Fig. 1 is an SEM image for the cross-sectional view of the SiNWs on CNTs array after partly dissolving the AAO template. It can be seen that one-dimensional silicon nanowires were exactly located at the end of the carbon nanotubes with high density. These SiNWs tended to stick to each other. The diameter of the SiNWs is about 50–70 nm, which is well consistent with the diameter of the CNTs and AAO pores. It also can be seen that most SiNWs have a length of several microns.
Fig. 1. The SEM image of the SiNWs grown on the CNTs after removing the AAO template partly.
Figs. 2a,b are the TEM images and HRTEM image which provide an insight into the structure of the SiNWs, respectively. These SiNWs were broken down from the CNTs during ultrasonically cleaning for TEM observation. It can be found from Fig. 2a that the as-grown SiNWs are actually the prism-shaped nanostructures. Fig. 2b displays a typical HRTEM image taken from the edge area of the SiNWs (indicated by the box area in Fig. 2a). Detailed analysis on lattice images give interplanar spacing of 0.31 nm, closely corresponding to that of Si(1 1 1) plane which reveals the growth direction [1 1 1] of these SiNWs. A selected-area electron diffraction (SAED) pattern (see Fig. 2a, left inset) taken from top face (Fig. 2a, indicated by red arrow) of the tilted image was taken along [1 1 1] zone axis of single crystal Si. Another SAED pattern was taken along [2 1 1] zone axis of single crystal Si (see Fig. 2b, left inset). From the data above, we can confirm the as-grown prism of single crystal SiNWs (Fig. 1a) is a combination assembled by (1 1 1) and ( 1 1 1), (2 1 1) and ( 2 1 1), (0 1 1) and (0 1 1) three pairs of parallel planes. It belongs to cubic silicon structure with a = 0.5341 nm, space group Fd3m. Fig. 2c gives an EDX spectra taken from an individual SiNW, which further confirmed that the nanowires are compose of silicon with a minimal content of oxygen. In order to further investigate the growth process of the SiNWs, different pyrolysis time of silane were carried out in this work. It can be found from Figs. 3a–d that there are different morphologies of the SiNWs at different growth stages. With the increasing of pyrolysis time of silane, a triangle-shaped silicon tip at initial stage
200
C. Li et al. / Chemical Physics Letters 411 (2005) 198–202
Fig. 2. (a) The TEM image of the SiNWs with a SAED image (upper left inset) taken from the face indicated by the red arrow and a schematic representation of crystal faces grown (lower left inset). (b) The HRTEM images of the enlarged box area in (a) with its SAED image (left inset). (c) EDX spectrum taken from an individual SiNWs. (For interpretation of the references to the color in this figure legend, the reader is referred to the web version of this article.)
transforms gradually to a square, ultimately to SiNWs. It also can be revealed that in growth process of SiNWs, the CNTs have a size-confined effect on the growth of SiNWs through confining the size of metal catalyst. In our approach, no SiNWs were formed at 450 °C just with Fe catalysis. This implied that the CNTs play an important role on growth of the SiNWs. It has been reported that carbothermal reduction of the CNTs can help silicon powder to form nanostructures [18,19]. Thus, it is believed that the CNTs facilitate the pyrolysis of silane and growth of the SiNWs. That is, the CNTs, as an assistance of the Fe catalyst, help to greatly decrease the reaction temperature. During the whole
CVD process, the CNTs always keep Fe particles active state and maintain highly reductive atmosphere around them when silane diffuses into them. It can be clearly seen from Fig. 4 that Fe particle is located between C and Si and joints them for each other (see Fig. 4b), which is different from the conventional vapor–liquid–solid (VLS) growth mechanism reported before [9]. In addition, the temperature of growing silicon is just 450 °C which is far below the eutectic temperature of Fe–Si alloy (>1200 °C). The 450 °C just reach the temperature where SiH4 start a decomposing reaction. The normal VLS mechanism is not applicable to explain our results. Fig. 4a shows
C. Li et al. / Chemical Physics Letters 411 (2005) 198–202
201
Fig. 3. TEM images of silicon nanostructures growth by different pyrolysis time of silane. (a) a triangle-shaped silicon tip grown on a CNT by 30-min pyrolysis time. (b) A rectangle silicon grown on a CNT by 40-min pyrolysis time and white arrow indicates transformation direction from triangleshaped silicon to rectangle silicon. (c) 150 min. (d) 240 min.
a HRTEM image of triangle-shaped area of the CNTs–Fe–Si junction which provides the information of the starting nucleation. The reciprocal lattice peaks, which were obtained from fast fourier transforms (FFT) of the lattice-resolved images of outer layer area (Fig. 4a, left inset) shows the interplanar spacing values of about 0.38, 0.22 and 0.31 nm which are indexed to the (1 1 0), (2 1 1) and (1 1 1) silicon planes, respectively. Fourier transforms image of an inner catalyst area (Fig. 4, right inset) solely exhibits Fe(1 1 0) of an identified spacing value of 0.20 nm. It should be noted that the spacing values of Si(2 2 0) (d2 2 0 = ˚ ) and Si(2 1 1) (d2 1 1 = 2.2172 A ˚ ), which help 1.9201 A to form the prism shaped SiNWs, are very close to ˚ ). one of Fe(1 1 0) plane (d1 1 0 = 2.0268 A It has been reported that the crystallographic lattice structure at the interface is important in defining the structural characteristics of the grown nanowires. The interface prefers to take the least lattice mismatch [20]. Thus, we think the preferred growth direction and the 3D-prism shape should be determined by the Fe(1 1 0) as is discussed above. It is confirmed by an elemental mapping (Fig. 4b) of another developing junction
specimen at early stage. The Si(2 2 0) and Si(2 1 1) which are the side planes of the prism shaped SiNWs, are determined by the Fe(1 1 0) as these planes are well matched to Fe(1 1 0). Thus, the specific plane of Fe particles play an important role in initiating nucleation and growth of silicon nanostructures, resulting in morphology and orientation control. The interface between Si and Fe prefers to take the least lattice mismatch. Besides, the Si{1 1 1} has the lowest surface energy among the Si simple index surfaces. In a word, with the assistance of the CNTs and the catalysis of Fe particles, preference of taking the least lattice mismatch between Si and Fe leads to the growth of the single-crystal SiNWs with well-defined structures at the lower temperature.
4. Summary In conclusion, by chemical vapor deposition of silane, the large-scale SiNWs were grown under the catalysis of Fe particles at the lower temperature – 450 °C. These nanowires have a diameter of 50–70 nm and a length
202
C. Li et al. / Chemical Physics Letters 411 (2005) 198–202
not only have a size-confined effect on the growth of SiNWs, but also facilitate the pyrolysis and deposition of silane on iron and growth of the SiNWs because of their carbothermal reduction. The growth process proposed here which is different from the conventional VLS process should be ascribe to preference of taking the least lattice mismatch between Si and Fe. The preference leads to the control of morphology and crystalline orientation of the SiNWs.
Acknowledgments We are grateful for financial support from National Natural Science Foundation of China (Grant Nos. 29925310 and 20021002) and Ministry of Science and Technology of China (Grant No. 2001CB10506).
References
Fig. 4. (a) TEM image of a triangle shaped silicon, the upper-left inset is the two-dimensional fourier transform of the outer layer of the triangle (indicated by arrow) depicting lattice spacing of Si(1 1 1) of 0.31 nm, Si(2 1 1) of 0.22 nm and Si(2 2 0) of 0.38 nm, respectively, the bottom-right inset is the two-dimensional fourier transform of central part of the triangle image (also indicated by arrow) depicting lattice spacing of 0.207 nm, which is indexed to Fe(1 1 0). (b) TEM image and its element-mapping of triangle-shaped region. The insets are the distribution of C, Fe and Si, respectively.
of several microns. Through the HRTEM and SAED observation, the nanowires exhibit the excellent single crystal nature. At the whole growth process, the CNTs
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