Browsing by Author "Nishidome, Hiroyuki"

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    Hall effect in gated single-wall carbon nanotube films
    (Springer Nature, 2022) Yomogida, Yohei; Horiuchi, Kanako; Okada, Ryotaro; Kawai, Hideki; Ichinose, Yota; Nishidome, Hiroyuki; Ueji, Kan; Komatsu, Natsumi; Gao, Weilu; Kono, Junichiro; Yanagi, Kazuhiro
    The presence of hopping carriers and grain boundaries can sometimes lead to anomalous carrier types and density overestimation in Hall-effect measurements. Previous Hall-effect studies on carbon nanotube films reported unreasonably large carrier densities without independent assessments of the carrier types and densities. Here, we have systematically investigated the validity of Hall-effect results for a series of metallic, semiconducting, and metal–semiconductor-mixed single-wall carbon nanotube films. With carrier densities controlled through applied gate voltages, we were able to observe the Hall effect both in the n- and p-type regions, detecting opposite signs in the Hall coefficient. By comparing the obtained carrier types and densities against values derived from simultaneous field-effect-transistor measurements, we found that, while the Hall carrier types were always correct, the Hall carrier densities were overestimated by up to four orders of magnitude. This significant overestimation indicates that thin films of one-dimensional SWCNTs are quite different from conventional hopping transport systems.
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    Isotropic Seebeck coefficient of aligned single-wall carbon nanotube films
    (AIP Publishing LLC, 2018) Fukuhara, Kengo; Ichinose, Yota; Nishidome, Hiroyuki; Yomogida, Yohei; Katsutani, Fumiya; Komatsu, Natsumi; Gao, Weilu; Kono, Junichiro; Yanagi, Kazuhiro
    How the morphology of a macroscopic assembly of nanoobjects affects its properties is a long-standing question in nanomaterials science and engineering. Here, we examine how the thermoelectric properties of a flexible thin film of carbon nanotubes depend on macroscopic nanotube alignment. Specifically, we have investigated the anisotropy of the Seebeck coefficient of aligned and gated single-wall carbon nanotube thin films. We varied the Fermi level in a wide range, covering both theﾠp-type andﾠn-type regimes, using electrolyte gating. While we found the electrical conductivity along the nanotube alignment direction to be several times larger than that in the perpendicular direction, the Seebeck coefficient was found to be fully isotropic, irrespective of the Fermi level position. We provide an explanation for this striking difference in anisotropy between the conductivity and the Seebeck coefficient using Mott's theory of hopping conduction. Our experimental evidence for an isotropic Seebeck coefficient in an anisotropic nanotube assembly suggests a route toward controlling the thermoelectric performance of carbon nanotube thin films through morphology control.