Structure and Composition of Bismuth Nanowire Arrays
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"Departmentof Chemistry, University of California, Berkeley "DepartmentofofComputer Sciences & Electrical Engineering, M.I.T ""Departments Physics and Computer Sciences & Electrical Engineering, M.I.T Department of Materials Science & Engineering, University of California, Berkeley
ABSTRACT Arrays of nanowires have attracted considerable recent attention due to their unique electronic and optical properties. While much effort has been directed at fabricating arrays and measuring their properties, much less has been done to characterize these materials. Understanding the structure and composition of the constituents in these arrays is crucial in order to control their properties. In this work, arrays with wire diameters from 35-90nm were fabricated by pressure injecting liquid bismuth into porous alumina templates. Transmission electron microscopy (TEM) and analytical electron microscopy (AEM) were used to characterize the arrays. INTRODUCTION Metal and semiconductor nanowire arrays exhibit properties that differ from the corresponding bulk materials as a result of the two-dimensional quantum confinement of the charge carriers.' Quantum confinement effects become significant in bismuth at larger, more easily accessible sizes than in other materials because of bismuth's relatively small effective mass. These effects have been observed in magnetotransport 2 and optical3 measurements of bismuth nanowire arrays. In addition, nanowire arrays have been predicted to demonstrate enhanced thermoelectric properties compared to bulk bismuth.4 Because of the small wire diameters and large interfacial areas associated with nanowire arrays, the array properties are strongly dependent upon the wire and interface structure and composition. The effects of structure variation have been studied in bismuth thin films, where carrier mobility is found to vary by approximately an order of magnitude depending on whether the films are single or polycrystalline.' This effect will be exaggerated in nanowires because
transport is further confined to a single dimension. Compositional effects due to impurities or reaction products will also play a significant role in determining the properties of such wires. When nanowires are configured in arrays, the interface morphology enters as another determinant in carrier scattering. To understand the effect of these parameters, it is necessary to determine the local structure and composition of the wires and at the interfaces in the arrays. In this work, TEM and AEM, including energy dispersive x-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS), have been applied to characterize bismuth nanowire arrays. EXPERIMENTAL PROCEDURES The arrays of bismuth nanowires were fabricated by a pressure injection process. A basic overview of the method is given here; the detailed procedure has been described elsewhere.' Templates of alumina with an ordered array of nanochannels were made by anodization of aluminum foils. Pieces of pure bismuth were placed on top of the alumina template in an iconel
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