There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.

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Graphene photonics and optoelectronics

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We report a comprehensive study of transparent and conductive silver nanowire (Ag NW) electrodes, including a scalable fabrication process, morphologies, and optical, mechanical adhesion, and flexibility properties, and various routes to improve the performance. We utilized a synthesis specifically designed for long and thin wires for improved performance in terms of sheet resistance and optical transmittance. Twenty Ω/sq and ∼80% specular transmittance, and 8 ohms/sq and 80% diffusive transmittance in the visible range are achieved, which fall in the same range as the best indium tin oxide (ITO) samples on plastic substrates for flexible electronics and solar cells. The Ag NW electrodes show optical transparencies superior to ITO for near-infrared wavelengths (2-fold higher transmission). Owing to light scattering effects, the Ag NW network has the largest difference between diffusive transmittance and specular transmittance when compared with ITO and carbon nanotube electrodes, a property which could gr...

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Scalable Coating and Properties of Transparent, Flexible, Silver Nanowire Electrodes

Transparent electrodes are a necessary component in many modern devices such as touch screens, LCDs, OLEDs, and solar cells, all of which are growing in demand. Traditionally, this role has been well served by doped metal oxides, the most common of which is indium tin oxide, or ITO. Recently, advances in nano-materials research have opened the door for other transparent conductive materials, each with unique properties. These include CNTs, graphene, metal nanowires, and printable metal grids. This review will explore the materials properties of transparent conductors, covering traditional metal oxides and conductive polymers initially, but with a focus on current developments in nano-material coatings. Electronic, optical, and mechanical properties of each material will be discussed, as well as suitability for various applications.

Emerging Transparent Electrodes Based on Thin Films of Carbon Nanotubes, Graphene, and Metallic Nanostructures

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 × 105 S/m for very thin films before saturating at 5 × 106 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...

http://www.tara.tcd.ie/bitstream/handle/2262/64106/De%20ACS%20Nano%2009%202.pdf;sequence=1

Silver Nanowire Networks as Flexible, Transparent, Conducting Films: Extremely High DC to Optical Conductivity Ratios

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.

Electrical Connectivity in Single-Walled Carbon Nanotube Networks

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.

Spray deposition of highly transparent, low-resistance networks of silver nanowires over large areas.

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 ...

Size effects and the problem with percolation in nanostructured transparent conductors.

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.

Philip E. Lyons

Papers

Silver Nanowire Networks as Flexible, Transparent, Conducting Films: Extremely High DC to Optical Conductivity Ratios

Electrical Connectivity in Single-Walled Carbon Nanotube Networks

Spray deposition of highly transparent, low-resistance networks of silver nanowires over large areas.

Size effects and the problem with percolation in nanostructured transparent conductors.

Transparent, Flexible, and Highly Conductive Thin Films Based on Polymer−Nanotube Composites