Syntheses of Nanostructure Bundles Based on Semiconducting Metal Silicides

Abstract

A variety of nanostructure bundles and arrays based on semiconducting metal silicides have been synthesized using abundant and non-toxic starting materials. Three types of fabrication techniques of the nanostructure bundles or arrays, including direct growth, template synthesis using natural nanostructured materials and template synthesis using artificially fabricated nanostructured materials are demonstrated. CrSi 2 nanowire bundles were directly grown by the exposure of Si substrates to CrCl 2 vapor at atmospheric pressure. A hexagonal MoSi 2 nanosheet, Mg 2 Si/MgO composite nanowire and Mg 2 Si nanowire bundles and MnSi 1:7 nanowire array were synthesized using a MoS 2 layered material, a SiO x nanofiber bundle, a Si nanowire array, and a Si nanowire array as the templates, respectively. Additionally, the fabrication phenomenon and structural properties of the nanostructured semiconducting metal silicides were investigated. These reactions provided the low-cost and controllable synthetic techniques to synthesize large scale and one-dimensional semiconducting metal silicides for thermoelectric applications.Semiconducting metal silicides have been extensively investigated for silicon-based semiconducting technology and considered as promising materials for the renewable energy technology in the field of thermoelectric and photovoltaic applications. 1,2 Compared to the conventional semiconducting materials, the metal silicides have many advantages, such as low-cost, natural abundance, non-toxicity and easy recycling, therefore, they are considered as environmentally friendly semiconductor materials. Recently, a variety of metal silicides, for example, β-FeSi 2 , Mg 2 Si, MnSi 1:7 , hexagonal MoSi 2 and CrSi 2 , have been successfully grown by various growth techniques. 3-7 However, few metal silicide has been applied for the practical use, which is attributed to the modest figure of merit (ZT) of the metal silicides.One-dimensional nanostructures are attracting a great deal of attention due to their unique properties and novel applications compared to those of bulk materials. A decrease in the thermal conductivity and an increase in the power factor, S 2 , of the nanostructures have been demonstrated by the theoretical calculations and proof of principle experiments, relative to the corresponding bulk structures. [8][9][10][11] Hochbaum et al. reported that the Si nanowires with diameters of about 50 nm exhibit a 100-fold reduction in thermal conductivity, yielding ZT ¼ 0.6 at room temperature. 11 Various nanostructures have been synthesized and charactered, such as wires, rods, nails, ribbons, belts, sheets and rings. It is well-known that a single-walled carbon nanotube can be metallic or semiconductive, depending on the helical angle at which the graphitic sheet is rolled up, 12 which reveals the importance of controlling its morphological and structural properties. Unfortunately, the growth window for the specific morphology is narrow, namely the morphology can be significantly changed as wires, rods, ribbons...