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Carbon-assisted synthesis of silicon nanowires Gautam Gundiah, F.L. Deepak, A. Govindaraj, C.N.R. Rao * Chemistry and Physics of Materials Unit and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India Received 18 September 2003 Published online: 4 November 2003 Abstract Carbon-assisted synthesis of silicon nanowires has been accomplished with silicon powders as well as solid sub- strates. The method involves heating an intimate mixture of silicon powder and activated carbon or a carbon coated solid substrate in argon at 1200–1350 °C, and yields abundant quantities of crystalline nanowires. Besides being simple, the method eliminates the use of metal catalysts. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction There has been intense research activity in the area of inorganic nanowires and nanotubes in the last few years [1–3]. Thus, nanowires of a variety of inorganic materials including oxides, nitrides and chalcogenides have been synthes ized and characterized. In particular, silicon nanowires (SiNWs) have received considerable attention and several methods have been employed for their synthesis. These include thermal evaporation of Si powder [4], vapor–liquid–solid method involving liquid metal solvents with low solubility for Si [5], laser ablation [6,7], and the use of silicon oxide in mixture with Si [8,9]. SiO 2 -sheathed crystalline SiNWs have been obtained by heating Si–SiO 2 mixtures [10]. It has been recently reported that enhanced yields of SiNWs are obtained by heating a Si substrate coated with carbon nanoparticles at 1050 °C under vacuum [11]. We consider the role of carbon to be as in other carbothermal methods of synthesizing nanowires of oxides, nitrides and other materials, involving a vapor–solid mecha- nism wherein carbon reacts with the oxide proba- bly producing a suboxide-type species. As part of our program on the carbothermal synthesis of in- organic nanowires [12–14], we have been investi- gating carbon-assisted synthesis of SiNWs. In this article, we report our important findings, which are of relevance to the vapor–solid and oxide- assisted growth of SiNWs. 2. Experimental The synthesis of SiNWs has been carried out by employing the following procedures. Procedure (i) Chemical Physics Letters 381 (2003) 579–583 www.elsevier.com/locate/cplett * Corresponding author. Fax: +91-80-8462760. E-mail address: cnrrao@jncasr.ac.in (C.N.R. Rao). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.09.142 involved the solid state synthesis in which silicon powder (Aldrich Chemicals) was finely ground with activated carbon, keeping the molar ratio of Si to C at 1:1 or 1:0.5. The activated carbon was prepared by decomposing polyethylene glycol (600 units) in argon atmos phere at 700 °C for 3 h. The finely ground mixture was taken in an alumina boat and heated at 1200 °C for 3 h in a mixture of Ar (50 sccm; sccm, standard cubic centimeter per minute) and H 2 (20 sccm). The reaction was also carried out under similar conditions in the absence of carbon to verify whether carbon plays a role in the formation of the nanowires. Procedure (ii) was similar to (i), except that the reactants were heated in an Ar atmosphere (without any H 2 ). The product obtained was grey or white in color and was collected as fine powders. In procedure (iii), a silicon substrate was used as the source of silicon. The Si(1 0 0) substrates were cleaned by ultrasonication in distilled water. Amorphous carbon was sputtered on the sub- strates using a JEOL JEE-400 vacuum evaporator, with a sputtering time of 0.5–1 min. The carbon- coated Si substrates were heated to 1350 °C for 3 h in an atmosphere of Ar/H 2 (25 sccm each). The product formed as a layer on the substrate was grey or white in color. A blank run with the sili- con substrate without any sputtered carbon was carried out under similar conditions. X-ray diffraction (XRD) patterns of the prod- ucts were recorded using a Seifert instrument with Cu Ka radiation. Scanning electron microscope (SEM) images were obtained with a Leica S-440-i microscope. Transmission electron microscopic (TEM) images were obtained with a JEOL (JEM 3010) operating with an accelerating voltage of 300 kV. Powder samples for TEM were dispersed in CCl 4 using an ultrasonic bath, and a drop of the suspension placed on a copper support grid cov- ered with holey carbon film. 3. Results and discussion Heating silicon powder at 1200 °C, in the ab- sence of any activated carbon, yields a small pro- portion of SiNWs. In Fig. 1a, we show a typical SEM image of the product of such a reaction to illustrate the poor yield of SiNWs. When the re- action was carried out in the presence of activated carbon (Si:C, 1:1) by procedure (i), we obtained nanowires in a good yield, as can be visualized in the SEM image in Fig. 1b. These have diameters ranging from 75–350 nm, with lengths of a few microns. The XRD pattern of the product shown in Fig. 2a matches with that of bulk silicon of cubic structure (JCPDS file: 27-1702). There is a minor peak (with asterisk) which is attributed to the surface silicon oxide, since SiNWs undergo oxidation upon exposure in air. Due to the high surface-to-volume ratio of the nanowires, a prominent surface oxide layer is general ly present. We, however, see no reflections due to carbide and other impurity phases. Along with the nanowires, we also obtain Si nanojunctions, as shown in the low-magnification TEM image in Fig. 3a. The junction has a Y-shape, with arms of a uniform width of 200 nm, and a length of a few microns. Careful studies of the TEM images and electron diffraction data may unravel the nature of the junction. In Fig. 1c, we show the SEM image of the SiNWs obtained by procedure (i) with Si:C ratio of 1:0.5. The nanowires have diameters between 75 and 600 nm with lengths up to tens of microns. The TEM image presented in Fig. 3b reveals that the nanowires have a crystalline core and an amorphous sheath. The diameter of the cryst alline core is 40 nm and the thickness of the sheath is around 17 nm. The amorphous sheath serves as a protective layer to the underlying crystalline sili- con core. The amorphous sheath is of silica, formed by surface oxidation. The selected area electron diffraction, given in the inset of Fig. 3b, indicates the core to be of cubic silicon. The XRD pattern of the product, given in Fig. 2b, is char- acteristic of cubic silicon with a small impurity of silica. Reaction of silicon powder with activated car- bon in the absence of H 2 , by procedure (ii), yielded abundant quantities of SiNWs. The product ob- tained consisted of grey and white portions. The grey portion comprised SiNWs with diameters of $50 nm as shown in the SEM image in Fig. 1d. The length of the nanowires was several tens of microns. Shown in the inset of Fig. 1d is the SEM 580 G. Gundiah et al. / Chemical Physics Letters 381 (2003) 579–583 image of the white portion of the product. These nanowires have diameters ranging from 50 to 700 nm, with several tens of microns in length. A low- magnification TEM image of the nanowires is shown in Fig. 3c. The nanowires are highly crys- talline as can be seen from the high-resolution transmission electron microscope (HREM) image in Fig. 3d. The lattice spacing between the fringes is 0.31 nm, corresponding to the (1 1 1) planes of silicon. The crystallinity of the nanowires is con- siderably higher when only argon was used instead of a mixture of argon and hydrogen. The role of hydrogen in promoting the amorphization of sili- con is well-known [15,16]. In order to show the versatility of this method, we have investigated the formation of SiNWs by heating silicon substrates coated with carbon, by procedure (iii). In the absence of carbon, we ob- tained very few SiNWs, as shown in the SEM Fig. 1. SEM images of (a) the product of the reaction of silicon powder obtained by procedure (i) in the absence of carbon, (b) SiNWs obtained by procedure (i) with a Si:C ratio of 1:1, (c) SiNWs obtained by procedure (i) with Si:C ratio of 1:0.5 and (d) SiNWs obtained in the grey portion of the sample synthesized by procedure (ii). Inset shows the nanowires obtained in the white portion. Fig. 2. XRD patterns of SiNWs obtained by procedure (i) with a Si:C ratio of (a) 1:1 and (b) 1:0.5. G. Gundiah et al. / Chemical Physics Letters 381 (2003) 579–583 581 image in Fig. 4a. On carrying out the reaction with sputtered carbon, the yield of SiNWs impro ves considerably, as can be seen from the SEM image in Fig. 4b. The nanowires have diameters in the range of 50–300 nm. The formation of SiNWs in the presence of carbon can be explained as follows. Silicon is generally covered by an oxide layer. The oxide layer gets reduced by carbon into silicon monoxide by the reaction Si x O 2 þ C ! Si x O þ CO ðx > 1Þð1Þ Si x O ! Si xÀ1 þ SiO ð2Þ 2SiO ! Si þ SiO 2 ð3Þ Crystalline silicon, formed in step (3), nucleates and grows perpendicular to the (1 1 1) direction to form the nanowires. Similar reactions have been proposed for the oxide-assisted synthesis of SiNWs [7], although the monoxide type species is generated by other means. 4. Conclusions SiNWs have been obtained by reacting silicon powder or silicon substrates with carbon in an inert atmosphere. Carbothermal reduction of the silica layer covering Si generates crystalline SiNWs with high aspect ratios. The method is convenient and inexpensive for the synthesis of Si nanowires, devoid of metallic impurities. Fig. 3. (a) TEM image of a Si nanojunction obtained by procedure (i) with Si:C ratio of 1:1. (b) TEM image of a nanowire obtained by procedure (i) with Si:C ratio of 1:0.5. Inset is the SAED pattern. (c) TEM image of the white portion of the sample obtained by procedure (ii). 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As part of our program on the carbothermal synthesis of in- organic nanowires [12–14], we have been investi- gating carbon-assisted synthesis of SiNWs of silicon. The crystallinity of the nanowires is con- siderably higher when only argon was used instead of a mixture of argon and hydrogen. The role of hydrogen