ABSTRACT

Metal silicides and germanides, which are intermetallic compounds between metals and silicon (Si) or germanium (Ge), respectively, are among the best candidates for a potentially new class of nanomaterials as they have diverse electrical and magnetic properties. For example, metallic silicides and germanides such as NiSi, TiSi2, NiGe, and PtGe can be employed as interconnects and metal gates in conventional complementary metal-oxide-semiconductor (CMOS) technologies due to their low resistivity [3]. Ferromagnetic Fe3Si and Mn5Ge3 can be used in spintronic applications, while superconducting vanadium silicide (V3Si) and Nb3Ge can function as superconducting nanoelectronic components [4-6]. Herein, we present the synthesis, structural characterization, and interesting applications of various metal silicide and germanide 1D nanostructures. We elaborate on synthetic approaches and applications of them in the following two sections: i) transition metal silicide 1D nanostructures and ii) transition metal germanide 1D nanostructures. In this review, we would like to focus on the metal silicide and germanide 1D nanostructures synthesized by the chemical vapor transport (CVT) approach because of its simplicity and wide applicability, although there are several other methods to grow these nanostructures. 7.2 Metal Silicide 1D Nanostructures

7.2.1 Synthetic StrategiesRational chemical synthesis of metal silicide 1D nanostructures is challenging because of the numerous stoichiometries and complicated phase behavior appearing in many metal silicide compounds. Several research groups have sought to develop general synthetic methods for the growth of freestanding silicide 1D nanostructures. Lieber and coworkers reported NiSi NW synthesis by silicidation of Si NWs [7]. Silicidation can easily produce metal silicide/Si heterostructures; however, this method provides low throughput and it is hard to obtain single-crystalline structures and various phases. Y. L. Chueh et al. synthesized TaSi2 NWs by annealing NiSi2 thin film, NiSi2 nanodots, FeSi2 thin film, or FeSi2 nanodots on a Si substrate in a Ta ambient. However, long reaction times over several hours were required for the TaSi2 NW growth and impure

NWs were synthesized [8, 9]. To fabricate various metal silicide NWs, the Jin group employed inorganic complexes containing the transition metal and Si atoms in the metal organic chemical vapor deposition (MOCVD) system. For example, single-crystalline iron silicide (FeSi) NWs, cobalt silicide (CoSi) NWs, and Mn19Si33NWs were synthesized via MOCVD by using trans-Fe(CO)4(SiCl3)2, Co(CO)4SiCl3, and Mn(CO)5SiCl3, as precursors, respectively [10-12]. We applied anhydrous metal halides as a transition metal source and Si substrate or Si powder as a Si source in the CVT process. Using this approach, a variety of single-crystalline metal silicide NWs were synthesized, including NWs of the compositions CoSi, FeSi, Fe5Si3, manganese silicide (MnSi), and chromium silicide (CrSi2) [13-15]. Composition-tuned metal silicide NWs were also fabricated for simultaneous synthesis of CoSi, Co2Si, and Co3Si NWs. Furthermore, metal silicide NTs were synthesized for the first time [16, 17].