We have used aqueous dispersions of silver nanowires to prepare thin, flexible, transparent, conducting films. The nanowires are of length and diameter close to 6.5 µm and 85 nm respectively. At low thickness, the films consist of networks but appear to become bulk-like for mean film thicknesses above ~160 nm. These films can be very transparent with optical transmittance reaching as high as 92% for low thickness. The transmittance (550 nm) decreases with increasing thickness, consistent with an optical conductivity of 6472 S/m. The films are also very uniform; the transmittance varies spatially by typically <2%. The sheet resistance decreases with increasing thickness, falling below 1 Ω/ for thicknesses above 300 nm. The DC conductivity increases from 2×10 5 S/m for very thin films before saturating at 5×10 6 S/m for thicker films.Similarly, the ratio of DC to optical conductivity increases with increasing thickness from 25 for the thinnest films, saturating at ~500 for thicknesses above ~160 nm. We believe this is the highest conductivity ratio ever observed for nanostructured films and is matched only by doped metal oxide films. These nanowire films are electromechanically very robust, with all but the thinnest films showing no change in sheet resistance when flexed over >1000 cycles. Such results make these films ideal as replacements for indium tin oxide as transparent electrodes. We have prepared films with optical transmittance and sheet resistance of 85% and 13 Ω/ respectively. This is very close to that displayed by commercially available indium tin oxide.

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.

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, R(s). To improve our understanding of the consequences of this, we develop a model which relates T to R(s) in the percolation regime. We define a percolative figure of merit, Π, for which high values result in high T and low R(s). High values of Π are achieved for high DC conductivity and low optical conductivity. In addition, the film thickness, t(min), 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 t(min) 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 < 20 nm should display properties superior to the best ITO. We demonstrate the deficiencies of standard bulk theory and the importance of understanding percolation by measuring R(s) and T for networks of silver flakes. We measure the bulk ratio of DC to optical conductivity to be ∼35, suggesting R(s) = 100 Ω/◻ and T = 90% are attainable. However, the large flake thickness results in high t(min) and so low Π, resulting in actual values of T = 26% for R(s) = 100 Ω/◻. This makes this material completely unsuitable for transparent conductor applications.

A method to produce scalable, low-resistance, high-transparency, percolating networks of silver nanowires by spray coating is presented. By optimizing the spraying parameters, networks with a sheet resistance of R(s) ≈ 50 Ω □(-1) at a transparency of T = 90% can be produced. The critical processing parameter is shown to be the spraying pressure. Optimizing the pressure reduces the droplet size resulting in more uniform networks. High uniformity leads to a low percolation exponent, which is essential for low-resistance, high-transparency films.

We have prepared flexible, transparent, and very conducting thin composite films from poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), filled with both arc discharge and HIPCO single-walled nanotubes, at high loading level. The films are of high optical uniformity. The arc discharge nanotube-filled composites were significantly more conductive, demonstrating DC conductivities of >10(5) S/m for mass fractions >50 wt %. The ratio of DC to optical conductivity was higher for composites with mass fractions of 55-60 wt % than for nanotube-only films. For an 80 nm thick composite, filled with 60 wt % arc discharge nanotubes, this conductivity ratio was maximized at sigma(DC)/sigma(Op) = 15. This translates into transmittance (550 nm) and sheet resistance of 75 and 80 Omega/square, respectively. These composites were electromechanically very stable, showing <1% resistance change over 130 bend cycles.