Phase-Controlled Growth of Metastable Fe 5 Si 3 Nanowires by a Vapor Transport Method Kumar S. K. Varadwaj, ² Kwanyong Seo, ² Juneho In, ² Paritosh Mohanty, ² Jeunghee Park, ‡ and Bongsoo Kim* ,² Contribution from the Department of Chemistry, KAIST, Daejeon 305-701, Korea, and Department of Chemistry, Korea UniVersity, Jochiwon 339-700, Korea Received March 4, 2007; E-mail: bongsoo@kaist.ac.kr Abstract: We report the synthesis of single-crystalline nanowires (NWs) of metastable Fe5Si3 phase via an iodide vapor transport method. Free-standing Fe5Si3 NWs are grown on a sapphire substrate placed on a Si wafer without the use of any catalyst. The typical size of the Fe5Si3 nanowires is 5-15 µm in length and 100-300 nm in diameter. Synthesis of the metastable phase is induced by composition-dependent nucleation from the gas-phase reaction. Depending on the concentration ratio of FeI2(g) to SiI4(g), different phases of iron silicides are formed. The growth of nanowires is facilitated by the initial nucleation of silicide particles on the substrate and further self-seeded growth of the NWs. The present work not only provides a method for the synthesis of metastable Fe 5Si3 nanowires but also suggests that the phase controlled synthesis can be further optimized to produce other metal-rich silicide nanostructures for future spintronic devices. Introduction Iron and silicon in their solid-solution series produce a rich variety of binary compounds with a wide range of magnetic, electrical, and optical properties. 1 FeSi has attracted much attention because of its anomalous temperature-dependent electrical, optical, and magnetic properties, similar to those of Kondo insulators. 2 Co substitution for Fe in FeSi, moreover, produces unusual positive magnetoresistance and a large anomalous Hall Effect, making Fe 1-x Co x Si a potential candidate for spintronics applications. 3 The silicon-rich phase in the solid- solution series such as -FeSi 2 is a direct band gap material and can be used as a light emitting diode (LED) in silicon. 4 Ferromagnetic properties are observed in iron-rich phases such as Fe 3 Si and Fe 5 Si 3 . 5 Fe 5 Si 3 is a high-temperature phase, which is metastable with respect to a mixture of FeSi and Fe 3 Si below 825 °C. 6 Fe 5 Si 3 has a Curie temperature of 110 °C and giant magnetoresistance (GMR) has been observed in nanogranular Fe 5 Si 3 in a silicon matrix. 5 The traditional solid-state reactions involve the mixing of bulk reactant solids and annealing at elevated temperatures. The mechanisms of these reactions are primarily based on three steps: interdiffusion of the reactant elements in solid state, nucleation, and growth of the crystalline product. High tem- perature and long reaction time are necessary to overcome the high activation energies for long-range diffusion in extended solids. 6 In these diffusion-limited solid-state reactions, only thermodynamically stable phases in the phase diagram nucleate. The vapor deposition method has been shown to be a potentially effective approach to synthesize metastable solids. Jansen et al. synthesized metastable solids by a vapor deposition method in which the desired components undergo atomic level mixing on the substrate and react at very mild temperatures to form metastable solids. 7 This method circumvents the high activation energy necessary for transport in the bulk materials and leads to the formation of metastable materials. A few approaches to synthesize metastable phases in the solid state have also been reported. The Fe 5 Si 3 phase which is stable at high temperature in bulk can be formed by rapid cooling of the melt. 5b T. Novet et al. developed a method for the synthesis of Fe 5 Si 3 thin films in which the ultrathin amorphous elemental layers diffuse at a low temperature and nucleation becomes the rate-determining step in the formation of crystalline materials. 6 Using this approach they demonstrated composition dependent crystalline ² KAIST. ‡ Korea University. (1) Massalski, T. B. Binary Alloy Phase Diagrams; American Society for Metals: Ohio, 1986; Vol. 1, p 1100. (2) (a) Paschen, S.; Felder, E.; Chernikov, M. A.; Degiorgi, L.; Schwer, H.; Ott, H. R.; Young, D. P.; Sarrao, J. L.; Fisk, Z. Phy. ReV.B 1997, 56, 12916. (b) Aeppli, G.; Fisk, Z. Comments Condens. Matter Phys. 1992, 16, 155. (c) Schlesinger, Z.; Fisk, Z.; Zhang, H. -T.; Maple, M. B.; DiTusa, J. F.; Aeppli, G. Phys. ReV. 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SOC. 2007, 129, 8594-8599 10.1021/ja071439v CCC: $37.00 © 2007 American Chemical Society