Electrical Connectivity in Single-Walled Carbon Nanotube Networks

Transport in single-walled carbon nanotubes (SWCNTs) networks is shown to be dominated by resistance at network junctions which scale with the size of the interconnecting bundles. Acid treatment, known to dope individual tubes, actually produces a dramatic reduction in junction resistances, whereas annealing significantly increases this resistance. Measured junction resistances for pristine, acid-treated and annealed SWCNT bundles correlate with conductivities of the corresponding films, in excellent agreement with a model in which junctions control the overall network performance

Size Effects and the Problem with Percolation in Nanostructured Transparent Conductors

Much research is underway at present to develop nanostructured transparent conductors for use as electrodes. Transparent electrodes typically require high visible transmittances, T > 90%, and so must be very thin. We show that for most nanostructured films thin enough to display T > 90%, the conduction can be described by percolation theory. This means DC conductivities are lower than in bulk, giving correspondingly higher sheet resistances, Rs. To improve our understanding of the consequences of this, we develop a model which relates T to Rs in the percolation regime. We define a percolative figure of merit, Π, for which high values result in high T and low Rs. High values of Π are achieved for high DC conductivity and low optical conductivity. In addition, the film thickness, tmin, where the DC conductivity first deviates from its bulk value and the percolation exponent, n, must both be as low as possible. We find that this model fits extremely well to much of the data in the literature. We demonstrate that tmin scales linearly with the smallest dimension of the nanostructure in question (i.e., diameter for wires or thickness for flakes). This clearly confirms that low diameter nanowires or thin platelets are best for transparent conducting applications. We predict the properties of silver nanowire networks to improve as wire diameter is decreased. Networks of wires with D Rs and T for networks of silver flakes. We measure the bulk ratio of DC to optical conductivity to be ∼35, suggesting Rs = 100 Ω/◻ and T = 90% are attainable. However, the large flake thickness results in high tmin and so low Π, resulting in actual values of T = 26% for Rs = 100 Ω/◻. This makes this material completely unsuitable for transparent conductor applications

High Quality Dispersions of Functionalized Single Walled Nanotubes at High Concentration

Single walled nanotubes are difficult to disperse in solvents, with dispersion quality limited by nanotube bundling at high concentration. We quantitatively study dispersions of singlewall nanotubes, functionalized with the bulky molecules PABS, PEG, and ODA, in common solvents. TGA measurements coupled with AFM analysis of deposited nanotubes shows almost complete coverage of the functionalities along the nanotube body. The best solvents are characterized by Hildebrand solubility parameters that are close to those of the functional groups. At low concentration, the dispersions contain predominately individual functionalized SWNTs as evidenced by root-mean-square bundle diameters of ∼3−4 nm. This can be compared with the measured diameter of individual functionalized nanotubes of ∼3 nm. These nanotubes display very weak concentration dependent aggregation when dispersed in common solvents. Root-mean-square bundle diameters of only ∼5−6 nm were observed at concentrations as high as 1 mg/mL. This translates into >100 bundles per cubic micron of solvent, much higher than observed in other systems. These results have practical implications for the production of well dispersed polymer-nanotube composites that would be expected to display high interfacial stress transfer