Abstract: Synthesis and Characterization of Quasi 1D Structures

Thin ZnO nanowires with diameters around 50 nanometers are synthesized via pulsed laser assisted chemical vapor deposition. The as-grown nanowires are configured as field effect transistors and measured in a 4-K helium cryostat with quartz optical view port. The logarithmic scale conductivity as a function of reciprocal temperature shows Arrhenius behavior. For T > 50 K, the conductivity show thermally activated conduction and can be expressed as σ ~ exp(-Ea/kT), where Ea is the activation energy around 60 meV, attributed to the shallow donor levels below the conduction band edge. On the other hand, the activation energy at T < 50 K is all below 1 meV. It is found that in this regime, 3D Mott’s variable range hopping model governs the transport, with conductivity expressed as σ ~ exp(-AT-1/4). Furthermore, it is observed that the ZnO nanowires under continuous UV irradiation give rise to a metal-semiconductor transition at 210 K. This phenomenon is attributed to the significantly enhanced charge carriers due to a combined effect of thermal ionization and photo excitation. Furthermore, the nanowires have been doped to enhance the electrical conductivity. Here we present the temperature and magnetic field dependent conductance of In doped ZnO nanowires. The experimental results are modeled with weak localization theory in the quasi 1D regime.

When time permits, I will describe another project on magnetic nanostructures. Co nanowires and nanotubes are synthesized via low voltage electrodeposition method in hexagonally ordered anodic aluminum oxide templates. High resolution transmission electron microscopy and x-ray diffraction results show that the structures are uniform in size, and consist predominantly hcp structure with the magnetocrystalline easy axis (caxis) perpendicular to the channel axis. The magnetic domain structures are studied via magnetic force microscopy. In the case of Co nanowires, a strong dipole exists at the ends of the wire, together with a spatial magnetization modulation along the wire. Based on theoretical modeling, such intrinsic modulation originates from the competition between the magnetocrystalline polarization along the easy axis and the shape anisotropy along the wire axis. In contrast, Co nanotubes show weak magnetic signal from MFM imaging, manifesting in a circumferential magnetization, and a smoother magnetization reversal for field applied parallel to the tube axis.