We measure and compare the electronic transport properties of individual multiwall carbon nanotubes ͑MWNTs͒ and individual single-wall carbon nanotubes ͑SWNTs͒, and SWNT networks of varying thickness. The thinnest SWNT networks, like the individual semiconducting SWNTs, show nonlinear current-voltage ͑I-V͒ characteristics at low temperatures with a current that can be modulated by a gate-source voltage. The overall temperature dependence of conductance in the transparent networks changes systematically as the thickness of the network increases and is consistent with hopping conduction. On the other hand, the thickest SWNT networks ͑freestanding film͒ show more metallic behavior: their I-V characteristics are linear with no gate-voltage effect, and a large fraction of their conductivity is retained at very low temperatures, consistent with tunneling through thin barriers separating metallic regions. We make a comparison with individual MWNTs, which in some cases show even greater retention of conductance at very low temperatures, but ͑unlike the thickest SWNT networks͒ no changeover to metallic temperature dependence at higher temperatures. The temperature dependence of conductance in individual MWNTs is consistent with a model involving conduction in the two outer shells.
Evolution of a single graphene layer with disorder generated by remote oxygen plasma irradiation is investigated using atomic force microscopy, Raman spectroscopy and electrical measurement. Gradual changes of surface morphology from planar graphene to isolated granular structure associated with a decrease of transconductance are accounted for by two-dimensional percolative conduction by disorder and the oxygen plasma-induced doping effect. The corresponding evolution of Raman spectra of graphene shows several peculiarities such as a sudden appearance of a saturated D peak followed by a linear decrease in its intensity, a relatively inert characteristic of a D' peak and a monotonic increase of a G peak position as the exposure time to oxygen plasma increases. These are discussed in terms of a disorder-induced change of Raman spectra in the graphite system.
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