Browsing by Author "Bulmer, John S."

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    Extreme magneto-transport of bulk carbon nanotubes in sorted electronic concentrations and aligned high performance fiber
    (Nature Publishing Group, 2017-09-22) Bulmer, John S.; Lekawa-Raus, Agnieszka; Rickel, Dwight G.; Balakirev, Fedor F.; Koziol, Krzysztof K. K.
    We explored high-field (60T) magneto-resistance (MR) with two carbon nanotube (CNT) material classes: (1) unaligned single-wall CNTs (SWCNT) films with controlled metallic SWCNT concentrations and doping degree and (2) CNT fiber with aligned, long-length microstructure. All unaligned SWCNT films showed localized hopping transport where high-field MR saturation definitively supports spin polarization instead of a more prevalent wave function shrinking mechanism. Nitric acid exposure induced an insulator to metal transition and reduced the positive MR component. Aligned CNT fiber, already on the metal side of the insulator to metal transition, had positive MR without saturation and was assigned to classical MR involving electronic mobility. Subtracting high-field fits from the aligned fiber's MR yielded an unconfounded negative MR, which was assigned to weak localization. It is concluded that fluctuation induced tunnelling, an extrinsic transport model accounting for most of the aligned fiber's room temperature resistance, appears to lack MR field dependence.
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    Photonic sorting of aligned, crystalline carbon nanotube textiles
    (Nature Publishing Group, 2017-10-11) Bulmer, John S.; Gspann, Thurid S.; Orozco, Francisco; Sparkes, Martin; Koerner, Hilmar; Di Bernardo, A.; Niemiec, Arkadiusz; Robinson, J. W. A.; Koziol, Krzysztof K. K.; Elliott, James A.; O’Neill, William
    Floating catalyst chemical vapor deposition uniquely generates aligned carbon nanotube (CNT) textiles with individual CNT lengths magnitudes longer than competing processes, though hindered by impurities and intrinsic/extrinsic defects. We present a photonic-based post-process, particularly suited for these textiles, that selectively removes defective CNTs and other carbons not forming a threshold thermal pathway. In this method, a large diameter laser beam rasters across the surface of a partly aligned CNT textile in air, suspended from its ends. This results in brilliant, localized oxidation, where remaining material is an optically transparent film comprised of few-walled CNTs with profound and unique improvement in microstructure alignment and crystallinity. Raman spectroscopy shows substantial D peak suppression while preserving radial breathing modes. This increases the undoped, specific electrical conductivity at least an order of magnitude to beyond that of single-crystal graphite. Cryogenic conductivity measurements indicate intrinsic transport enhancement, opposed to simply removing nonconductive carbons/residual catalyst.