Fabrication and assessment of structure, composition, and electronic properties of nanowire arrays

Nanocomposite materials consisting of arrays of parallel, uniform-diameter nanowires within a supporting matrix have a variety of potential applications. The focus of this work is on two nanowire array systems, bismuth and bismuth telluride nanowires in alumina templates. These systems are both promising for thermoelectric applications due to an expected increase in thermoelectric efficiency with reduced dimensionality.; Bismuth telluride nanowire arrays were fabricated by electrochemical deposition of Bi₂Te₃ into porous anodic alumina templates. A process has been developed that allows for the production of high density (∼5 × 109/cm2), high aspect-ratio (>1000), ordered nanowire arrays over large areas (>1mm2), which will enable routine assessment of the array properties as well as potential incorporation into existing device structures. High spatial resolution characterization techniques, including imaging, diffraction, and energy-dispersive spectroscopy in the transmission electron microscope (TEM), have been employed to assess the structure and composition in the arrays. The nanowires are dense, polycrystalline Bi₂Te₃ with strong texturing along the wire axis. A short (<5 μm) Te-rich composition gradient was identified at the base of the pores.; In addition, the composition, structure, and electronic properties of pressure-injected bismuth nanowire arrays have been assessed at high spatial resolution by employing imaging, diffraction, and electron energy loss spectrometry (EELS) in the TEM. The nanowires are polycrystalline with high aspect-ratio grains, and there is evidence of internal localized strain fields. The Bi-Al ₂O₃ interface in the arrays is compositionally abrupt, with a narrow interphase region dominated by Bi-O bonding. Low-loss EELS studies indicate that the volume plasmon loss peak in individual Bi nanowires shifts to higher energy and broadens as the wire diameter decreases from 90 to 35nm. A low-loss excitation is present in spectra from the Bi-Al₂O ₃ interface that is consistent with an interfacial plasmon excitation. Energy-filtered imaging reveals that the excitation is strongly localized at the interface.; This investigation reveals that nanowire arrays represent a promising path forward for thermoelectric and other potential applications. These results enable an understanding of the relationship between fabrication parameters and the local structure, composition, and electronic excitations in nanowire arrays and will allow for correlation of this information with nanowire array properties.