Gas-Phase Synthesis of Rough Silicon Nanowires via the Zinc

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Gas-Phase Synthesis of Rough Silicon Nanowires via the Zinc Reduction of Silicon Tetrachloride Norihisa Uesawa,* Susumu Inasawa, Yoshiko Tsuji, and Yukio Yamaguchi Department of Chemical System Engineering, Graduate School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan ReceiVed: October 16, 2009; ReVised Manuscript ReceiVed: January 27, 2010

We report on the formation of silicon nanowires via the gas-phase reaction of silicon tetrachloride with zinc vapor at ca. 1000 °C. In this method, metal catalysts do not have to be prepared before synthesizing silicon nanowires. The obtained silicon nanowires form a “blanket” with a yellow color at the reactor wall. In contrast to the common metal catalytic CVD method, silicon nanowires with a rough surface were synthesized. The fraction of nanowires with a rough surface increased from ca. 6% to 40% when the reaction temperature was changed from 920 to 1010 °C. The average diameter of the silicon nanowires also changed from 80 to 40 nm under the same conditions. A growth direction analysis revealed that the nanowires with a smooth surface had a 〈110〉 growth direction and those with a rough surface grew along the 〈111〉 or 〈112〉 directions. This “growth direction dependent” rough surface formation can be explained by the difference in the surface energies of the sidewalls of silicon nanowires. We also observed zinc particles at one end of nanowires, indicating that the nanowires grew from these particles. Because silicon and zinc can form a liquid alloy in the reaction temperature range, we propose that vapor-liquid-solid (VLS) growth is one of the main growth mechanisms in this system. Our observation provides useful information on the formation of silicon nanowires with volatile zinc metal since no direct image of zinc particles at one end of silicon nanowires has been reported so far. A possible mechanism for rough surface formation and nanowire growth is discussed. 1. Introduction Silicon nanowires have recently attracted considerable attention because of their unique physical properties and their potential for use in device applications1,2 such as field effect transistors,3,4 chemical sensors,5 thermoelectric devices,6,7 and lithium ion batteries.8,9 Many groups have succeeded in synthesizing silicon nanowires using the laser ablation of a silicon target with iron metals,10,11 metal catalytic chemical vapor deposition (CVD),12 the oxide-assisted growth method (OAG)13-16 and a solution-based method.17 Among these methods, the metalcatalytic CVD method is a practical synthetic technique because of high growth rates and good control of silicon nanowire diameters. In general, various kinds of nanowires can be synthesized by metal catalytic CVD via vapor-liquid-solid (VLS) growth.18-20 Precursors in the gas phase decompose in the presence of a metal catalyst droplet such as Au,21,22 Al,23 and Zn,24-26 etc., and they diffuse into the catalyst to form an alloy. After the catalyst becomes supersaturated with silicon atoms, excess silicon solidifies at the catalyst’s surface and nanowires form. This catalyst-assisted growth of nanowires allows the diameters of nanowires to be changed by changing the size of metal catalysts.21,22 Many groups have improved the metal-catalytic CVD methods to control nanowire diameters and crystallinity. Recently, Hochbaum et al. reported that the thermoelectric performance of silicon nanowires largely depends on their surface roughness as well as their diameter.6 The scattering of phonons and not electrons at the rough surface of the nanowires retard their effective thermal conductivity, and this results in good thermoelectric performance for these * To whom correspondence should be addressed. E-mail: norihisa@ chemsys.t.u-tokyo.ac.jp. Telephone: +81-3-5841-2324. Fax: +81-3-58417309.

materials. Thus, surface-roughened thin silicon nanowires are promising materials for use in thermoelectric devices.27 However, while we are able to control the diameter of nanowires, little has been done to try and control their surface roughness. Surface-roughened silicon nanowires are currently prepared by the electrodeless etching method (EE method)28 using a silicon wafer. Although this method is a quick and simple way to obtain surface roughened nanowires, a lot of silicon is lost because of the silicon wafer etching process by hydrofluoric acid. Controlling the surface roughness and diameter of the nanowires during synthesis would be a more suitable material preparation method. Therefore, it is necessary to develop a new synthetic route for silicon nanowires wherein we are able to control both the diameter and surface roughness. We used the zinc reduction method29,30 wherein silicon tetrachloride is reduced by zinc vapor at ca. 1000 °C followed by the formation of solid silicon. The overall reaction is,

SiCl4(g) + 2Zn(g) f Si(s) + 2ZnCl2(g)

(1)

This method has attracted plenty of attention from industry over the last few decades since it was expected to be a costless and more efficient new route for bulk silicon production compared with the conventional Siemens method. The conversion yield from SiCl4 to silicon is thermodynamically estimated to be about 80% at high temperature. However, because of the difficulty in handling reactor systems at such high temperatures and the formation of an undesirable fine silicon powder, the development of the zinc reduction method ceased. Fine powder formation is not desirable during bulk silicon production but, from the viewpoint of nanomaterial science, fine powder formation may potentially be useful in synthesizing silicon nanomaterials.

10.1021/jp909920d  2010 American Chemical Society Published on Web 02/22/2010

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In this paper, we report on the formation of silicon nanowires via the zinc reduction method. To the best of our knowledge, this is the first report on the formation of silicon nanowires via the zinc reduction method. One of the main features in this method is that metal catalysts do not have to be prepared before the synthesis and hence this provides us with a single step synthesis method for silicon nanowires. The diameter and the surface roughness of the nanowires changed depending on the reaction temperature. From electron microscope analyses, the possible formation kinetics of these silicon nanowires is discussed. Another important thing is that although zinc metal has been reported to assist silicon nanowire growth,24-26 little is clear on how silicon nanowires form with zinc particles because no direct evidence, such as TEM images, of silicon nanowires with zinc particles at one end has been shown so far. “Volatile” nature of zinc metal could be a possible reason for the difficulty in observation. We carefully observed our samples and found silicon nanowires with zinc particles at one end. Our observation would give us useful information on the formation kinetics of silicon nanowires from volatile zinc metal. 2. Experimental Methods Chemicals: Silicon tetrachloride (SiCl4) [99.9999%] was purchased from Tri Chemical Laboratories Inc., Japan. Zinc metal [99.995%] was purchased from Mitsui Kinzoku, Japan. The argon (Ar) carrier gas used was G1 grade and purchased from Shin-ei Shoji, Japan. All the chemicals were used as received without any further purification. Synthesis: Silicon nanowires were synthesized in a horizontal quartz reactor placed within an electric furnace. Three-zone heating was used to obtain a uniform temperature. A set of thermocouples was placed on the surface of the reactor to measure the reaction temperature. The length and diameter of the reactor were 500 mm and 50 mm, respectively. Two gas lines were connected to the reactor, one for SiCl4 and the other for zinc vapor. Because SiCl4 is in a liquid phase below 56 °C at atmospheric pressure, the precursor was introduced into the reactor via an Ar carrier gas passing through a liquid SiCl4 bubbler at room temperature. Zinc vapor was also introduced using the Ar carrier gas. Zinc metal was heated to 885 °C in a quartz heating vessel, and some zinc vapor formed at the surface of the liquid zinc because of the phase equilibrium between liquid and vapor zinc. The boiling and melting points of metal zinc are 907 and 420 °C, respectively. The zinc vapor that formed at the liquid surface was transported by the Ar carrier gas to the reactor. Zinc vapor formation below its boiling point provides us with a simple and easy way to control the flow rate of zinc vapor. A preheating zone in the reactor sufficiently heated both gases to the reaction temperature, typically 950 °C. Before synthesis, the reactor was kept at the reaction temperature for at least 1 h to ensure a thermally homogeneous system was created. The total flow rate was a summation of the SiCl4, Zn, and Ar flow and was fixed at 1106 sccm. The flow rate was controlled by a mass flow controller under ambient conditions. The concentration of the introduced SiCl4 gas was measured by gas chromatography (Shimadzu GC-8A). The amount of zinc vapor introduced was measured by considering the mass change of zinc solids before and after the reaction. The ratio of the introduced gas was SiCl4:Zn:Ar ) 1:5:9. We tested four different reaction temperatures of 920, 950, 980, and 1010 °C. The total reaction pressure was 1 atm. We put quartz boats into the reactor to collect silicon products. The production rate of silicon nanowires using this method was roughly 40 mg/h. Characterization: The synthesized products were observed directly using scanning electron microscopy (SEM) (Hitachi

Uesawa et al.

Figure 1. (a) Digital image of the yellow blanket-like solids obtained after the reaction. (b, c) SEM images of the obtained products. They show that the blanket consists of many nanoscale wires, and the diameters of the wires are smaller than 100 nm.

S-900) and transmission electron microscopy (TEM) (JEOL 2010F) with an accelerating voltage of 200 kV and augmented with energy dispersive X-ray spectroscopy (EDS) (JEOL JED2300). TEM samples were prepared by pounding nanowires with a mortar, and they were dispersed in ethanol. A drop of the ethanol solution was placed onto a carbon-coated copper TEM grid and dried under ambient conditions. The average diameter of the silicon nanowires was obtained by measuring more than 200 nanowires in the SEM images. A crystal analysis of the silicon nanowires was carried out using X-ray diffraction (XRD) (Rigaku ATX-G) with Cu KR radiation. The purity of the nanowires was determined by X-ray fluorescence spectroscopy (XRF) (JEOL JSX-3400R). 3. Results The obtained products are represented by that shown in Figure 1a, and they are a yellow blanket-like solid. Figure 1b and 1c shows SEM images of the obtained products, and they consist of wire-like materials. Their diameters are less than 100 nm as shown in Figure 1c. We confirmed that they were composed of about 99% silicon with 1% zinc as an impurity from XRF measurements. Therefore, the product mainly consists of silicon. TEM images of the obtained products are shown in Figure 2a and 2b. We confirmed that the obtained nanowires had a crystal structure with a lattice spacing of 0.19 nm, which corresponds to a (110) silicon lattice spacing. The selected area electron diffraction (SAED) pattern of the nanowires in Figure 2b is shown in Figure 2c. The clear diffraction spots suggest that the crystallinity of the silicon nanowires is quite good. The growth direction of these silicon nanowires was also confirmed to be in the 〈110〉 direction from Figure 2b and 2c. Elemental analysis on a nanowire was conducted using EDX, and we found that the main component of the nanowires is silicon while zinc was not detected, as shown in Figure 2d. From these results, we conclude that the silicon nanowires formed via a gas-phase reaction of silicon tetrachloride and vapor zinc. As already mentioned, the blanket contains 1% zinc. Since each nanowire consists of almost pure silicon, most solid zinc is thought to precipitate between or on the silicon nanowires. Although these formed silicon nanowires have good crystallinity, most of the spots in Figure 2c are distorted, indicating that the nanowires

Gas-Phase Synthesis of Rough Silicon Nanowires

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Figure 4. Effect of temperature on the diameter of silicon nanowires. We counted more than 200 silicon nanowires at each temperature using SEM (Hitachi S-900).

Figure 2. (a) TEM image of obtained products. (b) High magnification TEM images of nanowires. The lattice spacing is 1.9 Å, suggesting that this nanowire has a growth direction of 〈110〉. (c) Selected area electron diffraction pattern of the nanowire in (b). (d) EDX spectrum of a silicon nanowire. The small peaks around 1 keV correspond to carbon and oxygen, respectively. The copper signal was from the copper grid.

Figure 3. XRD spectrum of the yellow blanket-like solid. The broad peak around 23° is attributed to the halo from a glass substrate on which a blanket-like solid was fixed for the XRD measurement.

have some lattice defects. We measured the crystallinity of the yellow blanket using XRD as shown in Figure 3. Before the measurement, the blanket was washed with HCl to remove the zinc metal. Five distinct diffraction peaks at 28.4°, 47.3°, 56.1°, 69.1°, and 76.4° were observed, and they correspond to the (111), (220), (311), (400), and (331) lattice spacings of silicon crystals, respectively. The ratio of observed peak intensities agrees well with the results from the powder silicon crystals,31 suggesting that this blanket does not have any specific orientation. Figure 4 shows that the reaction temperature determines the diameter of the silicon nanowires. The diameters of the silicon nanowires decrease as the reaction temperature increases, suggesting that the diameter of silicon nanowires may be controlled by changing the reaction temperature. Additionally, the reaction temperature also affects the fraction of silicon nanowires with a rough surface. Figure 5a shows a TEM image of a silicon nanowire with a rough surface that was obtained at 1010 °C. A high resolution TEM image of Figure 5a and its corresponding SAED pattern are shown in Figure 5b and 5c, respectively. Rough surface nanowires also have good crystallinity, but their surface morphology is different from those of smooth nanowires as shown in Figure 2. The growth direction

Figure 5. (a) TEM images of silicon nanowires with rough surfaces. (b) High magnification TEM images of silicon nanowires with rough surfaces. The lattice spacing is 3.1 Å, suggesting that this nanowire has a growth direction of 〈111〉. (c) Selected area electron diffraction pattern of the nanowire in b. (d) SEM image of a rough surface silicon nanowire. The inset represents the enlarged view of the undulate structure on the nanowire. A variation in distance d between a side edge and a parallel line to the growth direction of each nanowire was used to evaluate the roughness.

Figure 6. Temperature effect on the fraction of silicon nanowires with a rough surface. More than 50 nanowires were counted at each temperature using SEM (Hitachi S-900).

of the rough nanowire is the 〈111〉 direction. We also observed the surface structure of a rough silicon nanowire by SEM (Figure 5d), and the rough surface structure was clear. Figure 6 shows the fraction of nanowires with a rough surface at each reaction temperature. Here we measured a variation in distance d between a side edge of nanowire and a line parallel to each growth

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direction and defined nanowires which have a variation in the distance larger than 5 nm as “rough” ones as shown in the inset of Figure 5d. The average distance d in rough silicon nanowires is 9 nm (standard deviation 3 nm). The fraction of rough silicon nanowires was ca. 6% at 920 °C, and it increased to ca. 40% at 1010 °C. These results imply that we can control both the diameter and the surface roughness of silicon nanowires by changing the reaction temperature. 4. Discussion 4.1. Mechanism of Silicon Nanowire Formation. As already mentioned, because no direct image of zinc particles at one end of the nanowires has been reported so far, the formation mechanism of silicon nanowires in this system should be discussed. We consider two different growth modes to explain the growth mechanism. One is a metal catalyst relevant growth and the other is an oxide-assisted growth mode. The latter growth mechanism has been reported for systems in which silicon nanowires were grown via the evaporation of silicon oxide materials such as SiO. Some oxide or oxygen is required for this growth mode. We can preclude the possibility of an oxygen related mechanism because we carefully replaced the air inside the reactor with Ar before the reaction started, and we do not use any oxide materials as a source. If the former growth mechanism is dominant, catalyst particles are expected to be observed at one end of silicon nanowires. Figure 7a shows a typical TEM image of a “three-arm” product with a particle at the center. Some arms were broken and did not connect to the silicon nanowires because of the pounding procedure for TEM sample preparation. High resolution TEM images of Figure 7a showed that each arm had a lattice spacing of 0.31 nm, which corresponds to silicon (111) lattice spacing as shown in Figure 7c. Furthermore, EDX analysis revealed that the particle at the center mainly consisted of zinc (Figure 7e). We propose that this zinc particle acted as a catalyst for silicon nanowire growth. According to the phase diagram of Si-Zn,32 silicon can dissolve in liquid zinc in the reaction temperature region. The amount of dissolved silicon is 5 wt % at 920 °C and 9 wt % at 1010 °C, respectively. During the reaction, vapor zinc and silicon atoms that formed via the reduction of silicon tetrachloride produce liquid alloy drops. Once the alloy liquid drops are formed, silicon atoms dissolve into these drops. When these drops are supersaturated with silicon, excess silicon atoms are excluded from the liquid drops, and nanowire formation via VLS mechanism occurs. Zinc, therefore, acts as a reducing agent for silicon tetrachloride as well as being an active point for nanowire growth in this mechanism. We note that not all active points contain a zinc element in our TEM observation. In Figure 7b, four silicon nanowires grew from an active point. They have a lattice spacing of 0.19 nm in the growth direction (Figure 7d), indicating 〈110〉 oriented nanowire growth. The active point does not contain any detectable zinc element at the center (Figure 7f). This result seems to contradict the zinc-related formation mechanism proposed above. However, this discrepancy can be explained as follows: Zinc atoms can be incorporated into formed nanowires during reaction, because the solubility of zinc in solid silicon is almost the same as that of Au.1 This results in consumption of zinc catalyst, which is similar to a size decrease of the gold catalysts due to silicon nanowire formation.33-35 In addition to this, an evaporation effect due to the high vapor pressure of zinc at a reaction temperature would be another cause for consumption of zinc catalyst. Several nanowires grew from a common foothold, as shown in Figure 7a and 7b. This kind

Figure 7. (a, b) TEM images of three- and four-arm products in our samples. (c, d) High magnification TEM images of a and b, respectively. The insets represent high resolution TEM images of the white square in each image. Each lattice spacing is 0.31 nm in c and 0.19 nm in d. (e, f) EDX spectra of the black particle in c and the center of four arms in d.

of growth has been reported for oxide-assisted growth36 and during the zinc catalyst-assisted formation of silicon nanowires.25 The number of silicon nanowires from one common site is thought to be affected by surface energies of silicon nanowires36 and the ratio of the number density of zinc catalyst particles to the concentration of source gas.25 4.2. Growth Direction of Nanowires and Rough Surface Formation. Silicon nanowires with rough surface structures mainly have a 〈111〉 growth direction as shown in Figure 5b

Figure 8. Growth direction analysis of silicon nanowires with rough/ smooth surfaces. Red, blue and green represents growth directions of 〈110〉, 〈111〉, and 〈112〉, respectively. Silicon nanowires synthesized at 920, 950, 980, and 1010 °C were counted in this figure.

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TABLE 1: Values Used for the Calculation of ∆G in Reactions 2 and 3 chemical species

∆fG°1200 [kJ/mol]

SiCl4 (g) Cl2 (g) Zn (g)

-505.3a 0a 0a

ZnCl2 (g)

-234.5b

Cp [J/mol K]

S°298 [J/mol K]

S°1200 [J/mol K] 273.6a 189.9a

71.3 (s)c 102.1 (l)d,e 57.7 (g)d,f

111.5a

∆fH°298 [kJ/mol] 0a 130.4a

∆fH°1200 [kJ/mol]

∆fusH [kJ/mol]

∆vapH [kJ/mol]

Tmelt [K]

Tboil [K]

23d

126b

548d

1029d

0a 0a

-415.1a

a Reference 42. b The Gibbs energy at 1200 K is estimated using basic thermodynamic data on the Table 1. The value agrees with the obtained value by Gaussian. c Reference 43. d Reference 44. e Use the value of FeCl2. f Use the value of SiCl2.

while smooth nanowires mainly grew along the 〈110〉 direction, as shown in Figure 2b. This trend is also clearly shown in Figure 8. Independent of the temperature difference, smooth nanowires have a growth direction of 〈110〉 while most surface roughened silicon nanowires have a growth direction of 〈111〉. In addition, the number of rough nanowires increases as the reaction temperature increases as shown in Figure 6. The growth direction dependence on reaction temperature can be explained by the thermodynamic stability of the crystal planes. Qin et al. reported that the total surface energy of silicon nanowires with a growth direction of 〈111〉 becomes smaller than those of 〈110〉 growth direction nanowires at temperatures higher than 850 °C, and the unusual 〈111〉 growth direction becomes dominant at high temperatures.36 Considering the total surface energy of silicon nanowires, more nanowires with a 〈111〉 growth direction were formed at 1010 °C than at 920 °C, as shown in Figure 6. In catalytic CVD, the catalyst size affects the growth direction of silicon nanowires.37 The 〈110〉 growth direction is dominant when using gold catalysts smaller than 10 nm,21 the 〈111〉 direction becomes dominant for catalysts larger than 20 nm, and a transient 〈112〉 direction is dominant for intermediate catalyst sizes.22 The 〈112〉 direction is dominant for a diameter range of 20 to 120 nm when indium catalysts were used.38 Our results are contrary to these findings because the 〈110〉 growth direction was dominant for nanowires thicker than 50 nm, the 〈111〉 direction was evident in thin nanowires, and the diameter depended on the reaction temperature. These results indicate that the preference of growth direction depends on the temperature, catalyst types, and sizes. To date, several papers have reported on the faceting of sidewalls, which results in a rough “sawtooth” sidewall structure for nanowires.39,40 Li et al. reported that this faceting phenomenon was caused by the direct deposition of a source gas onto boron-doped silicon nanowire sidewalls.40 On the other hand, Ross et al. concluded that the self-oscillation of catalyst liquid drops was one of the main reasons for the sawtooth faceting of sidewalls.41 Since the latter observation was restricted to growth under ultrahigh vacuum conditions at 10-8 to 10-5 torr and at reaction temperatures from 500 to 600 °C, the self-oscillation mechanism is less likely to occur in our case. We propose that the deposition of silicon onto the formed nanowires is one of the main reasons for the formation of rough silicon nanowires. The sidewall of silicon nanowires with smooth and rough surfaces consists of different crystal facets because of the difference in growth direction, as shown in Figure 8. Therefore, the smooth and rough silicon nanowires have different surface stabilities or sidewall surface energies. The deposition rate is thought to depend on the surface energy of sidewalls resulting in the “growth-direction-dependent” formation of rough silicon nanowires. Sidewall etching during the reaction is another possible route for rough surface formation. Chlorine could be considered to

be an etching species since we used silicon tetrachloride as a source material. However, the etching hypothesis can be precluded by considering that if the thermal decomposition of SiCl4 or ZnCl2 was the main cause of chlorine formation, the reactions may be described as follows:

SiCl4(g) f Si(s) + 2Cl2(g)

(2)

ZnCl2(g) f Zn(g) + Cl2(g)

(3)

We calculated the Gibbs free energy of the above two reactions to determine if these reactions are thermodynamically preferred or not. From classical thermodynamics, the values of the Gibbs free energy of formation at a reaction temperature of 927 °C were calculated using the standard Gibbs free energy of formation and other thermodynamic properties such as their heat capacity, their entropy of fusion/evaporation, and their boiling/melting points. The values used are summarized in Table 1. For our calculation, the heat capacity was assumed to be constant, and this assumption hardly affects the calculation results. The values of the calculated ∆G in reactions 2 and 3 are positive, 505.3 and 234.5 kJ/mol, respectively, suggesting that the formation of chlorine by thermal decomposition is thermodynamically unlikely. 5. Conclusions Silicon nanowires with rough surfaces formed via the gasphase reaction of silicon tetrachloride with zinc vapor at ca. 1000 °C. We successfully obtained silicon nanowires without preparing metal catalysts on a substrate before the synthesis. Depending on the reaction temperature, both the fraction of rough surface nanowires and their diameters changed. Nanowires with smooth surfaces had a 〈110〉 growth direction, and rough nanowires grew along the 〈111〉 or 〈112〉 directions. We propose that this “growth direction dependent” formation of rough silicon nanowires can be explained by the difference in the surface stability of nanowires and the direct deposition of silicon onto sidewalls. Zinc metal particles were observed at one end of silicon nanowires. Silicon nanowires are thought to form from these zinc particles via a VLS growth mechanism. Although further investigation is required to further clarify the details of the mechanism for silicon nanowire formation, our results provide a new synthetic route for novel, surface-roughened silicon nanowires using zinc metal catalyst. Acknowledgment. The authors are grateful to H. Tsunakawa and K. Ibe for TEM technical support, S. Sakae for gas chromatography technical support, and Prof. A. Miyoshi for useful discussion for thermodynamic calculation using Gaussian. We are also grateful to H. Tanaka and E. Mizuno for technical

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support when using the reactor system. A part of this work was conducted at the Center for Nano Lithography & Analysis, The University of Tokyo, and supported by the Ministry of Education, Culture, Sports, Science and Technology (MXET), Japan. N. Uesawa thanks the GCOE program for mechanical system innovation and The University of Tokyo for financial support. Finally, this work was supported by the Industrial Technology Research Grant Program in 2009 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References and Notes (1) Schmidt, V.; Wittemann, J. V.; Senz, S.; Go¨sele, U. AdV. Mater. 2009, 21, 1. (2) Teo, B. K.; Sun, X. H. Chem. ReV. 2007, 107, 1454. (3) Cui, Y.; Zhong, Z.; Wang, D.; Wang, W. U.; Lieber, C. M. Nano Lett. 2003, 3, 149. (4) Hochbaum, A. I.; Fan, R.; He, R.; Yang, P. Nano Lett. 2005, 5, 457. (5) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (6) Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najarian, M.; Majumdar, A.; Yang, P. Nature 2008, 45, 1–163. (7) Boukai, A. I.; Bunimovich, Y.; Tahir-Kheli, J.; Yu, J. K.; Goddard, W. A., III; Heath, J. R. Nature 2008, 451, 168. (8) Chan, C. K.; Peng, H.; Liu, G.; Mcilwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nat. Nanotechnol. 2008, 3, 31. (9) Peng, K.; Jie, J.; Zhang, W.; Lee, S. T. Appl. Phys. Lett. 2008, 93, 033105. (10) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (11) Zhang, Y. F.; Tang, Y. H.; Wang, N.; Yu, D. P.; Lee, C. S.; Bello, I.; Lee, S. T. Appl. Phys. Lett. 1998, 72, 1835. (12) Westwater, J.; Gosain, D. P.; Tomiya, S.; Usui, S.; Ruda, H. J. Vac. Sci. Technol. B 1997, 15, 554. (13) Yu, D. P.; Bai, Z. G.; Ding, Y.; Hang, Q. L.; Zhang, H. Z.; Wang, J. J.; Zou, Y. H.; Qian, W.; Xiong, G. C.; Zhou, H. T.; Feng, S. Q. Appl. Phys. Lett. 1998, 72, 3458. (14) Wang, N.; Tang, Y. H.; Zhang, Y. H.; Lee, C. S.; Bello, I.; Lee, S. T. Chem. Phys. Lett. 1999, 299, 237. (15) Zhang, R. Q.; Lifshitz, Y.; Lee, S. T. AdV. Mater. 2003, 15, 635. (16) Ma, D. D. D.; Lee, C. S.; Au, F. C. K.; Tong, S. Y.; Lee, S. T. Science 2003, 299, 1874. (17) Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471. (18) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (19) Wu, Y.; Yang, P. J. Am. Chem. Soc. 2001, 123, 3165.

Uesawa et al. (20) Kawashima, T.; Mizutani, T.; Masuda, H.; Saitou, T.; Fujii, M. J. Phys. Chem. C 2008, 112, 17121. (21) Cui, Y.; Lauhon, J.; Gudiksen, M. S.; Wang, J.; Lieber, C. M. Appl. Phys. Lett. 2001, 78, 2214. (22) Wu, Y.; Cui, Y.; Huynh, L.; Barrelet, C. J.; Bell, D. C.; Lieber, C. M. Nano Lett. 2004, 4, 433. (23) Wang, Y.; Schmidt, V.; Senz, S.; Go¨sele, U. Nat. Nanotechnol. 2006, 1, 186. (24) Chung, S. W.; Yu, J. Y.; Heath, J. R. Appl. Phys. Lett. 2000, 76, 2068. (25) Yu, J. Y.; Chung, S. W.; Heath, J. R. J. Phys. Chem. B 2000, 104, 11864. (26) Miyamoto, Y.; Hirata, M. Jpn. J. Appl. Phys. 1976, 15, 1159. (27) Donadio, D.; Galli, G. Phys. ReV. Lett. 2009, 102, 195901. (28) Zhang, M. L.; Peng, K. Q.; Fan, X.; Jie, J. S.; Zhang, R. Q.; Lee, S. T.; Wong, N. B. J. Phys. Chem. C 2008, 112, 4444. (29) Banerjee, H. D.; Acharya, H. N.; Dutta, S. K. Indian J. Technol. 1980, 18, 84. (30) Banerjee, H. D.; Acharya, H. N.; Dutta, S. K. Mater. Lett. 1984, 2, 265. (31) International Center for Diffraction Data, File No. 27-1402. (32) Binary Alloy Phase Diagrams; American Society for Metals: Metals Park, OH, 1986; Vol. 1 (ISBN: 0-87170-261-4). (33) Hannon, J. B.; Kodambaka, S.; Ross, F. M.; Tromp, R. M. Nature 2006, 440, 69. (34) Oh, S. H.; Benthem, K. V.; Molina, S. I.; Borisevich, A. Y.; Luo, W.; Werner, P.; Zakharov, N. D.; Kumar, D.; Pantelides, S. T.; Pennycook, S. J. Nano Lett. 2008, 8, 1016. (35) Allen, J. E.; Hemesath, E. R.; Perea, D. E.; Lensch-falk, J. L.; Li, Z. Y.; Yin, F.; Gass, M. H.; Wang, P.; Bleloch, A. L.; Palmer, R. E.; Lauhon, L. J. Nat. Nanotechnol. 2008, 3, 168. (36) Qin, Y.; Zhang, X. N.; Zheng, K.; Li, H.; Han, X. D.; Zhang, Z. Appl. Phys. Lett. 2008, 93, 063104. (37) Schmidt, V.; Senz, S.; Go¨sele, U. Nano Lett. 2005, 5, 931. (38) Wang, Z. W.; Li, Z. Y. Nano Lett. 2009, 9, 1467. (39) Wei, D. P.; Chen, Q. J. Phys. Chem. C 2008, 112, 15129. (40) Li, F.; Nellist, P. D.; Cockayne, D. J. H. Appl. Phys. Lett. 2009, 94, 263111. (41) Ross, F. M.; Tersoff, J.; Reuter, M. C. Phys. ReV. Lett. 2005, 95, 146104. (42) Chase, M. W.; Davies, C. A., Jr; Downey, J. R.; Frurip, D. J., Jr.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Ref. Data 1985, 14, 1. (43) Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002. (44) Kagaku Binran (Kisohen 2), 4th ed.; Maruzen: The Chemical Society of Japan, Tokyo, 1993.

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