Ultrafast growth of single crystalline si nanowires

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Ultrafast growth of single-crystalline Si nanowires J.B. Chang, J.Z. Liu, P.X. Yan ⁎ , L.F. Bai, Z.J. Yan, X.M. Yuan, Q. Yang Institute for Plasma and Metal Materials, Lanzhou University, Lanzhou 730000, China Received 25 September 2005; accepted 22 December 2005 Available online 31 January 2006 Abstract Silicon nanowires (SiNWs) have been catalytically synthesized by heat treatment of Si nanopowder at 980 °C. The SiNWs comprise crystalline Si nanoparticles interconnected with metal catalyst. The formation mechanism of nanowires generally depends on the presence of Fe catalysts in the synthesis process of solid–liquid–solid (SLS). Although gas phase of vapor–liquid–solid (VLS) method can be used to produce various of different nanowire materials, growth model based on the SLS mechanism by heat treatment is more ascendant for providing ultrafast growth of single-crystalline Si nanowires and controlling the diameter of them easily. The growth of single-crystalline SiNWs and morphology were discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: SiNWs 1. Introduction Crystalline nanostructures offer unique access to low- dimensional physics, and they can be used as nanotechnology building blocks to reach higher device integration densities than conventional fabrication methods and have more singularity character. One-dimensional (1D) structure with nanometer diameters, such as carbon nanotubers and semiconductor nanowires, has great potential for testing and understanding fundamental concepts about the roles of dimensionality and size, for example, optical, electrical, and mechanical properties and for their potential applications in research and electronic nanodevices [1]. It's known that Si nanowires (SiNWs) have the strong ability to confine photoenergy from visible light [2]. SiNWs are partic ularly attractive due to the central role of the silicon semiconductor industry, which would allow SiNWs to be implemented using existing technologies. Because silicon turns into a direct band-gap semiconductor at nanometer size due to quantum confinement [3],itcouldbeusedin optoelectronics. SiNWs can be synthesized by laser ablation [1], thermal evaporation of solid sources [4–6] and chemical vapor deposition (CVD) [7]. The various directional features of these techniques were reported and the model proposed for preferred SiNWs growth directions [8]. These methods are often based on the vapor–liquid–solid (VLS idea [9] using various metals as catalysts, such as Au, Fe, Ti and Ga, to enhance the growth of SiNWs. In this work, we demonstrate a simple method of growing SiNWs. Si nanopowder was used in our work instead of the dangerous gas of silane as the Si source. The synthesis of SiNWs was carried out using a mixture of Si nanoparticles and iron nitrate by thermal treatment at 980 °C in an evacuated sealed quartz tube. The key parameter necessary to induce nanowires formation is the tem perature and catalyst. Materials Letters 60 (2006) 2125 – 2128 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +86 931 8912661; fax: +86 931 8913554. E-mail address: pxyan@lzu.edu.cn (P.X. Yan). Fig. 1. XRD spectrum of Si nanoparticles prepared by cathode arc plasma. 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.085 2. Experiment Before the preparation of producing SiN Ws, Si nanoparticles were synthesized by cathode arc plasma which is one of the most powerful met hods because of uniform particles and high efficiency, using a mixture of SiO 2 and C with a molar proportion (1 :1) in argon atmosphere. The discharge voltage is 20 V and the current is 120 A. The flux of hydrogen as protective gas was 15 standard cubic centimeters per minute. Then the Si nanopowder was ultrasonically dispersed in the alcohol solution containing Fe(NO 3 ) 3 , which was subsequently evaporated and the dried samples were calcined in an H 2 flow at 980 °C for 1 h. X-ray diffraction (XRD) measurements and transmission electron microscope (TEM) were employed to Fig. 2. TEM image of Si nanoparticles by cathode arc plasma. Fig. 3. (a) TEM image of Si nanoparticles after heat treatment at 980 °C without Fe catalysts and (b) nanowires with Fe, (c) TEM image showing nanopartical catalysts at the end of nanowires. (d) TEM image of an individual smooth nanowire and corresponding SAED pattern. 2126 J.B. Chang et al. / Materials Letters 60 (2006) 2125–2128 investigate the structure and morphology of the Si nanoparticles and nanowires. 3. Results and discussion The phase composition and phase structure of the as-synthesized products were examined by X-ray diffraction (XRD, Siemens D-500 with Cu Ka radiation and a normal 2θ scan ). Fig. 1 shows a typical XRD spectrum of the Si nanoparticles on different crystal planes synthesized by cathode plasma discharge. It can be seen from the dominant diffraction peaks, as indexed in the spectrum, and originated from cubic-structure Si, which can be readily indexed to face-centered cell of Si (Joint Committee on Powder Diffraction Standard (JCPDS) Card, No. 05-0565). Their average diameter of 21.6 nm was calculated using Scherrer equation, which is in good accordance with the TEM observation (Fig. 2). During the heat treatment at 980 °C, Si nanoparticles couldn't be transformed into nanowires if there was no iron catalysts as shown in Fig. 3(a). When introduced iron catalysts, Si nanowires came out. Fig. 3(b) shows the TEM images of SiNWs with tens of nanometer in diameter and several hundreds of micrometers in length. Fe nanoparticles as catalysts are embedded in SiNWs as shown in Fig. 3 (c). Selected area electron diffraction (SAED) indicates that the nanowires are made of crystalline silicon in Fig. 3(d). After heat treatment, when H 2 flow was closed and air was introduced into the quartz tube during the cooling procedure, SiO x nanotubes would be produced deriving from SiNWs oxidated (Fig. 4). The ring in the SAED image inserted in Fig. 4(b) is from the reflection of SiO 2 (400). The mechanism of SiNWs growth is explained now. At high temperature, Fe(NO 3 ) 3 deposited into iron oxide and momentarily deoxidized into iron nanodroplets in H 2 atmosphere. In the metal- catalyzed SLS technique, a liquid metal cluster or catalyst acts as the energetically favored site for the adsorption fusing Si nanoparticles of solid-phase reactants, to function as Si reservoir by eutectic liquid formation, and to become supersaturated with Si. The present of a nanopartical catalyst at one end of the nanowires is the essential feature of SLS growth. As shown by the arrow in Fig. 3(c), SiNWs terminated at one end in a nanoparticle with a diameter 1∼1.2 times that of the connected nanowire. Fig. 3(d) shows the TEM image of an individual smooth nanowire and the corresponding selected area diffraction (SAED) pattern. The d-spacings of the nanocrystals calculated from the two diffraction dots of the SAED pattern are consistent with those of Si (200) and (400). It is well established that Si nanowires grown by the metal-catalyzed SLS technique usually have a growth direction along (200) and are single-crystalline [10]. Therefore, the (200) growth direction may be regarded as a typical feature of the metal-catalyzed SLS process. We surprisedly find that nanotubes appear in this experiment in Fig. 4(a). It is obvious that nanopartical catalysts are at the end of nanotubes as shown by the arrow. Fig. 4(b) shows TEM image of an individual nanotube and the corresponding SAED pattern (diffraction ring), in which the ring is from reflection of SiO 2 (400). When inpouring air into the evacuated sealed quartz tube at high temperature, the surface of SiNWs was immediately oxided [11] and meanwhile the Si inside was melted and subsequently evaporated leaving a SiO 2 tube [2]. 4. Propose mechanism The formation mechanism of nanowires generally depends on the presence or absence of metal catalysts in the synthesis process, i.e., SLS and VLS except for oxide-assisted growth [12]. Unlike the well-developed VLS, the detail of SLS process for silicon nanowires is not expatiated. In this paper, we Fig. 4. (a) TEM image showing catalysts at the end of nanotubes, (b) TEM image of an individual nanotube and SAED pattern. Fig. 5. Schematic figure of nanowires growth process. 2127J.B. Chang et al. / Materials Letters 60 (2006) 2125–2128 expound SiNWs formati on mechanism. The solid–liquid– solid (SLS) nanowires growth mechanism is illustrated in the case of nanowires growth process in Fig. 5. The SiNWs have been synthesized by heat treatment of Si nanoparticles. In the metal- catalyzed SLS technique, a liquid metal cluster or catalyst acts as the energetically favored site for the adsorption of liquid- phase reactants. The heat treatment can be acted as a kind of sinter. In this experiment, the prim ary growth mode is that agglomeration incorporates particles into nanowires. In heat treatment process, particles begin to melt and the adsorbability gradually augments with the temperature increasing. Because of the driving force of thermodynamics, the Si atoms dissolve in the Fe nanocrystal to form a liquid FeSi seed droplet. Thereupon, a tiny cervi x between Fe particle and Si particle comes into being. The cervix is filled and leveled up through diffusion of the surface. The seed droplet reaches the eutectic composition. Si diffuses from the liquid molten alloy phase ball and grows epitaxially at the liquid/solid interface. Simultaneity, Fe catalysts remove ahead and continue to absorb other Si nanoparticles resulting in the production of long SiNWs. The nanowires are able to grow when the FeSi alloy eutectic temperature and the concentration of crystallizing material can be exceeded. The current interest in the physics and possible applications of Si nanostructures and the need to develop techniques to fabricate such structures made it appropriate to take another look at the SLS technique as a means of Si nanowires ultrafast growth of fabrication. 5. Conclusion In summary, Si nanowires were catalytically synthesized by calcining Si nanopowder containing Fe(NO 3 ) 3 in an H 2 .A pathway of the growth of SiNWs was presented based on SLS mechanism. Under the conditions used to grow the nanowires, diffusion of Si through or around a solid FeSi nanoparticle appears to be rapid enough to transport Si away from the surface to the growing wires. In addition, rapid oxidation of SiNWs could lead to SiO x nanotubes. We further discussed the growth mechanism. Acknowledgements We thank for Engineer Shuang Wang and Youxiang Li, Testing and analytic center, Gansu Academy of Science, who give a lot of help. References [1] Alfredo M. Morales, Charles M. Lieber, Science 279 (1998) 208. [2] N. Wang, B.D. Yao, Y.F. Chan, X.Y. Zhang, Nano Lett. 3 (2003) 475. [3] Xin yuan Zhao, M. Wei, L. Yang, M.Y. Choul, Phys. Rev. Lett. 92 (2004) 236805. [4] N. Wang, Y.H. Tang, Y.F. Zhang, D.P. Yu, C.S. Lee, I. Bello, S.T. Lee, Chem. Phys. Lett. 283 (1998) 368. 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Conclusion In summary, Si nanowires were catalytically synthesized by calcining Si nanopowder