There is an ongoing drive to replace the most common transparent conductor, indium tin oxide (ITO), with a material that gives comparable performance, but can be coated from solution at speeds orders of magnitude faster than the sputtering processes used to deposit ITO. Metal nanowires are currently the only alternative to ITO that meets these requirements. This Progress Report summarizes recent advances toward understanding the relationship between the structure of metal nanowires, the electrical and optical properties of metal nanowires, and the properties of a network of metal nanowires. Using the structure-property relationship of metal nanowire networks as a roadmap, this Progress Report describes different synthetic strategies to produce metal nanowires with the desired properties. Practical aspects of processing metal nanowires into high-performance transparent conducting films are discussed, as well as the use of nanowire films in a variety of applications.

Copper nanowires grow from spherical copper seeds in an aqueous solution. Conductive films of copper nanowires have a transmittance of 65% (∼15% more than the best values reported for carbon nanotubes), and remain conductive after 1000 bending cycles or one month in air.

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This article describes how the dimensions of nanowires affect the transmittance and sheet resistance of a random nanowire network. Silver nanowires with independently controlled lengths and diameters were synthesized with a gram-scale polyol synthesis by controlling the reaction temperature and time. Characterization of films composed of nanowires of different lengths but the same diameter enabled the quantification of the effect of length on the conductance and transmittance of silver nanowire films. Finite-difference time-domain calculations were used to determine the effect of nanowire diameter, overlap, and hole size on the transmittance of a nanowire network. For individual nanowires with diameters greater than 50 nm, increasing diameter increases the electrical conductance to optical extinction ratio, but the opposite is true for nanowires with diameters less than this size. Calculations and experimental data show that for a random network of nanowires, decreasing nanowire diameter increases the number density of nanowires at a given transmittance, leading to improved connectivity and conductivity at high transmittance (>90%). This information will facilitate the design of transparent, conducting nanowire films for flexible displays, organic light emitting diodes and thin-film solar cells.Indium tin oxide (ITO) is the material of choice for transparent conducting films in flat-panel displays, organic solar cells, and organic light emitting diodes because, with a sheet resistance of 10 U sq À1 at a transmittance of 90% (l ¼ 550 nm), it is highly conductive and transparent. However, indium is a scarce element, ITO is brittle, and ITO film is expensive because it is produced with a vapor-phase coating process that is 1000 times slower than newspaper printing.1,2 These problems have motivated a search for alternatives to ITO that are flexible and can be deposited from liquids at high coating rates.3-9 As discussed in recent reviews, promising solution-processed alternatives to ITO include poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate), carbon nanotubes, graphene, ITO nanowires, and metal nanowires.10-15 Of these alternatives, films of silver nanowires currently have the highest conductance and transmittance. 4,16-18For example, Leem et al. have recently reported obtaining silver nanowire films with a sheet resistance of 10 U sq À1 at a transmittance of 89.3% (l ¼ 550 nm), nearly matching ITO. Although silver ($1000 kg À1) is more expensive than indium ($800 kg À1 ), the fact that silver nanowire films can be produced with highthroughput wet-coating processes allows them to achieve lower costs. 19,20Here we report a simple polyol synthesis that enables control over the length and diameter of silver nanowires, as well as their production on the gram scale. By measuring the properties of films composed of nanowires with distinct ranges of dimensions, we have obtained the first quantitative confirmation of theoretical predictions for the effect of nanowire length and number density on the conductance of 2D...

Metal nanowire films are among the most promising alternatives for next-generation flexible, solution-processed transparent conductors. Breakthroughs in nanowire synthesis and processing have reported low sheet resistance (Rs ≤ 100 Ω/sq) and high optical transparency (%T > 90%). Comparing the merits of the various nanowires and fabrication methods is inexact, because Rs and %T depend on a variety of independent parameters including nanowire length, nanowire diameter, areal density of the nanowires and contact resistance between nanowires. In an effort to account for these fundamental parameters of nanowire thin films, this paper integrates simulations and experimental results to build a quantitatively predictive model. First, by fitting the results from simulations of quasi-2D rod networks to experimental data from well-defined nanowire films, we obtain an effective average contact resistance, which is indicative of the nanowire chemistry and processing methods. Second, this effective contact resistance is used to simulate how the sheet resistance depends on the aspect ratio (L/D) and areal density of monodisperse rods, as well as the effect of mixtures of short and long nanowires on the sheet resistance. Third, by combining our simulations of sheet resistance and an empirical diameter-dependent expression for the optical transmittance, we produced a fully calculated plot of optical transmittance versus sheet resistance. Our predictions for silver nanowires are validated by experimental results for silver nanowire films, where nanowires of L/D > 400 are required for high performance transparent conductors. In contrast to a widely used approach that employs a single percolative figure of merit, our method integrates simulation and experimental results to enable researchers to independently explore the importance of contact resistance between nanowires, as well as nanowire area fraction and arbitrary distributions in nanowire sizes. To become competitive, metal nanowire systems require a predictive tool to accelerate their design and adoption for specific applications.