Preparation of Tungsten Bronze Nanowires
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Preparation of Tungsten Bronze Nanowires Hang Qi, Cuiying Wang, Jie Liu Department of Chemistry, Duke University, Durham, NC
Abstract Two simple techniques to prepare tungsten bronze nanowires are reported in this paper. Tungsten bronze nanowires with a diameter of less than 100nm and a length of more than 10 µm have been successfully prepared by employing these techniques. A Vapor-Liquid-Solid (VLS) mechanism has been proposed to explain the growth of these tungsten bronze nanowires. Introduction One-dimensional (1D) nanostructure materials, such as nanotubes and nanowires have attracted much attention due to their unique properties that originate from their high surface areas and low dimensionality [1]. Many efforts have been taken on the synthesis of various 1D nanoscale materials, especially those of semiconductor materials for the potential applications as building blocks for nanoelectronic devices [2,3]. Homogenous semiconductor nanowires have already been used as devices such as field effect transistor (FET). For more sophisticated devices, heterogeneous semiconductor nanowires (or heterogeneous one-dimensional structures) are required [4,5]. The key to prepare these sophisticated devices is the achievement of heterojunction, the junction of two kinds of materials [6,7]. Heterojunction obtained by two crossed nanowires has been reported [7-10]. In these reports, crossed nanowire junctions were made by using a layer-by-layer fluidic alignment strategy. However, junctions obtained by this strategy were not very solid for the two nanowires were merely put together crosswise. The goal of our work is to prepare heterojunction in an individual nanowire, which consists of two different kinds of material. Our strategy is modifying the chemical composition of a section of a given nanowire. To change the chemical composition of a section of a given nanowire, chemical reaction must be localized in a nanometer-sized region. We achieve this by using Atomic Force Microscopy (AFM) Based Electrochemically “Dip-Pen” Nanolithography (E-DPN) technique, which is developed from “Dip-Pen” Nanolithography (DPN) technique [11,12]. However, nanowires of some kinds of material may be broken into a series of nanoparticles after modification by E-DPN, for the crystal structure of these material changes a lot during the modification. Thus it is critical to choose a proper system. The system we have chosen for such a nanowire is tungsten oxide and tungsten bronze. Tungsten oxide and tungsten bronze have been studied intensively for more than one century for electrochromic and other interesting properties [13,14]. The word “bronze” has been applied to nonstoichiometric crystalline of transitional metal oxide, especially those ternary systems. The crystalline structure of tungsten trioxide is based on corner-shared W ─ O octahedra [15]. Within the structure, the tunnels of various sizes and shapes are generated by different arrangements of the octahedra. Tungsten bronze can be achieved by partially reducing
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W(VI) to W(V)
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