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Nanostructured carbons and their electrical transport characteristics

Authors Shi, Wu
Issue Date 2011
Summary This thesis is about the fabrication, characterization, device preparation and electrical transport properties measurements of three types of nanostructured carbons. In particular, the foci are on the 4 Angstrom carbon nanotubes embedded in zeolite crystals, bundles of double-wall carbon nanotubes, and disordered graphene. The 4 Angstrom single-wall carbon nanotubes (SWCNTs) embedded in zeolite crystals are fabricated by a new heating process which introduces ethylene gas as the carbon source. Raman characterization indicates the sample quality to be improved compared to that fabricated by the original heating process that involved converting the precursor tripropylamine. Transport measurements carried out on these newly fabricated 4 Angstrom SWCNT samples show two types of superconducting resistive transitions. The first type is one-dimensional (1D) crossover to three-dimensional (3D) superconducting transition, which was observed to initiate at 15 K, followed by a sharp, order of magnitude resistance drop at 7.5 K. The sharp transition exhibits anisotropic magnetic field dependence. And differential resistance versus current curves indicate that the establishment of coherence proceeded in stages as the temperature is lowered below 15K. In particular, the sharp resistance drop and its attendant nonlinear IV characteristics are consistent with the manifestations of a Berezinskii-Kosterlitz-Thouless (BKT) transition that establishes a quasi-long range order in the plane transverse to the c-axis of the nanotubes. The second type is quasi 1D superconducting transition, which was also observed to initiate at 15 K. But the resistance drop exhibits a smooth feature and magnetic field independence up to 11 Tesla as temperature decreases. And differential resistance increases smoothly with bias current. Specific heat and new Meissner effect measurements carried out by Prof. Rolf Lortz‘s group provide strong support of the superconductivity in 4 Angstrom CNTs, with detailed features that are consistent with the transport results. The double-wall carbon nanotubes (DWCNTs) are characterized by HRTEM and Raman spectroscopy. The diameters of the outer and inner tubes are measured to be around 1.54 nm and 0.83 nm, respectively. The DWCNTs are packed closely together in each bundle, forming a nearly crystal-like structure. Resonant Raman measurements reveal the chirality information of the inner and outer tubes. By using the electron-beam lithography (EBL) techniques, both single DWCNT bundle and multiple DWCNT bundles were fabricated into devices for transport measurements. Clear evidences, comprising a resistance drop as a function of temperature, magnetoresistance and a differential resistance signature of the supercurrent, indicate a range of superconducting transition temperatures within 3-18K, with a peak in the occurrences around 5-7 K. Raman characterization for one of the selective samples shows the inner tube to be metallic and the outer tube to be semiconducting in character. A broad superconducting anomaly is also observed in specific heat data of a bulk DWCNT sample, which yields a Tc distribution that correlates well with the distribution obtained from the electrical data. Pristine graphene can have a high mobility that ensures ballistic transport on submicron distances. However, the presence of disorder would limit the carrier mean free path and prevent ballistic transport. Two types of disordered graphene were fabricated and studied. One is substitutionally boron-doped grapheme, and the other is nanostructured by plasma etching. They exhibit large negative magnetoresistance behavior that differs from the pristine graphene. The preliminary graphene work presented in this thesis is for the purpose of setting the stage for the experiments to be performed in the near future.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2011
Language English
Format Thesis
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