Aaron R. Rathmell

The Growth Mechanism of Copper Nanowires and Their Properties in Flexible, Transparent Conducting Films

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

The Synthesis and Coating of Long, Thin Copper Nanowires to Make Flexible, Transparent Conducting Films on Plastic Substrates

This Communication describes the synthesis of long ( > 20 μ m), thin ( 500 mA cm − 2 ), were stable in air for over one month, and could be bent 1000 times without any degradation in their properties. Indium tin oxide (ITO) is the transparent conductor of choice for most applications because of its high transmittance and conductivity, but it is scarce, brittle, and expensive. [ 1a , 2 ] The cost of ITO fi lms is due not only to the fact that indium is a rare and costly material, but also because ITO must be deposited in an ineffi cient, low-throughput, vapor-phase coating process that, at 0.01 m s − 1 , is 1000 times slower than wet-coating processes such as newspaper printing. The limitations of ITO and other transparent conducting oxides have motivated a worldwide search for fl exible, low-cost alternatives that can be deposited from liquids at coating rates orders of magnitude greater than vapor-phase coating processes. Solution-coated fi lms of carbon nanotubes (CNTs) are one fl exible alternative to ITO, but to date they have a relatively low transmittance and sheet resistance due to their absorbance of light and the poor electrical contact between nanotubes. [ 3 ] The performances of solution-coated fiof graphene are generally inferior to that of CNT fi lms. [ 4 ] Solution-coated fi lms of silver nanowires (AgNWs) have a transmittance and sheet resistance close to ITO, but silver is also scarce and expensive. [ 5 ] Copper (resistivity ρ = 1.59 n Ω m) is nearly as conductive as silver (1.67 n Ω m), but it is 100 times less expensive and 1000 times more abundant. [ 6 ] Motivated by these fundamental advantages of copper, we have recently reported a scalable synthesis of copper nanowires (CuNWs) and fi ltered them from solution to make transparent conducting fi lms. [ 7 ] However, the properties of the fi lms were not as good as those made with AgNWs; at a sheet resistance of 15 Ω sq − 1 , the transmittance of a AgNW fi lm was about 85%, while that of a CuNW fi lm was only 65%. Three reasons why the CuNW fi lms were not as conductive as the AgNW fi lms were the relatively short lengths (10 ± 3 μ m), large diameters (90 ± 10 nm), and aggregation of the nanowires. Here we report a new synthesis that produces longer, thinner, well-dispersed CuNWs. These CuNWs were incorporated into an ink that could be coated onto a clear, plastic substrate to give a fl exible, transparent conducting fi lm with properties equivalent to fi lms of AgNWs. In contrast to ITO, these fi lms can withstand severe mechanical deformation and remain highly conductive. To grow nanowires in our previous study, we kept the reaction mixture at a constant temperature (80 ° C) throughout the nucleation and growth of the CuNWs. [ 7 ] This resulted in the formation of nanowires that were relatively short ( 90 nm in width). Furthermore, the capping agent used in the synthesis, ethylenediamine (EDA), was found to be a poor dispersant for the CuNWs. As a result, aggregation of the CuNWs made it diffi cult to fabricate uniform fi lms with high optical transmittance and low electrical resistance. In an effort to improve the properties of transparent conducting fi lms made from CuNWs, we developed a new synthesis to produce longer, thinner CuNWs that are also well dispersed. In order to grow longer, thinner wires, we heated the reaction mixture only a short time (about 3 min at 80 ° C) in order to induce reduction of copper ions (as indicated from a change in the reaction color from blue to clear). Polyvinylpyrrolidone (PVP) was then added to this mixture to prevent the CuNWs from aggregating and this mixture was quickly cooled in an ice bath. This process allowed the CuNWs to grow at a lower temperature, resulting in a longer, thinner morphology. Interestingly, if PVP was added to the reaction before heating, only copper nanoparticles formed. In contrast, the addition of PVP to the reaction after the 3 min heating stage did not prevent nanowires from forming, but instead prevented the CuNWs from forming aggregates that could not be dispersed. Once well-dispersed CuNWs were obtained, we wanted to demonstrate a scalable coating method for fabrication of transparent electrodes with CuNWs. In previous work, we fi CuNWs from solution onto a membrane and transferred them onto a glass slide coated with clear glue. Although this method enables the fabrication of fi lms from precise amounts of relatively clean nanowires, it is not a practical method for fabricating transparent electrodes over large areas. Spray coating is a potentially scalable method for fabrication of electrodes from

Metal Nanowire Networks: The Next Generation of Transparent Conductors

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.

The effect of nanowire length and diameter on the properties of transparent, conducting nanowire films

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.

Synthesis of oxidation-resistant cupronickel nanowires for transparent conducting nanowire networks.

Nanowires of copper can be coated from liquids to create flexible, transparent conducting films that can potentially replace the dominant transparent conductor, indium tin oxide, in displays, solar cells, organic light-emitting diodes, and electrochromic windows. One issue with these nanowire films is that copper is prone to oxidation. It was hypothesized that the resistance to oxidation could be improved by coating copper nanowires with nickel. This work demonstrates a method for synthesizing copper nanowires with nickel shells as well as the properties of cupronickel nanowires in transparent conducting films. Time- and temperature-dependent sheet resistance measurements indicate that the sheet resistance of copper and silver nanowire films will double after 3 and 36 months at room temperature, respectively. In contrast, the sheet resistance of cupronickel nanowires containing 20 mol % nickel will double in about 400 years. Coating copper nanowires to a ratio of 2:1 Cu:Ni gave them a neutral gray color, making them more suitable for use in displays and electrochromic windows. These properties, and the fact that copper and nickel are 1000 times more abundant than indium or silver, make cupronickel nanowires a promising alternative for the sustainable, efficient production of transparent conductors.

Integrating Simulations and Experiments To Predict Sheet Resistance and Optical Transmittance in Nanowire Films for Transparent Conductors

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.

Synthesis and Catalytic Properties of Au–Pd Nanoflowers

Reduction of Pd ions by hydroquinone in the presence of gold nanoparticles and polyvinylpyrrolidone resulted in the formation of nanoflowers with a Au core and Pd petals. Addition of HCl to the synthesis halted the reduction by hydroquinone and enabled the acquisition of snapshots of the nanoflowers at different stages of growth. TEM images of the reaction after 10 s show that the nanoflower morphology resulted from the homogeneous nucleation of Pd clusters in solution and their subsequent attachment to gold seeds coated with a thin (0.8 ± 0.1 nm) shell of Pd. UV-visible spectra also indicate Pd clusters formed in the early stages of the reaction and disappeared as the nanoflowers grew. The speed at which this reaction can be halted is useful not only for producing a variety of bimetallic nanostructures with precisely controlled dimensions and morphologies but also for understanding the growth mechanism of these structures. The ability of the AuPd core-shell structure to catalyze the Suzuki coupling reaction of iodobenzene to phenylboronic acid was probed and compared against the activity of Pd nanocubes and thin-shelled AuPd core-shell nanoparticles. The results of this study suggest that Suzuki coupling was not affected by the surface structure or subsurface composition of the nanoparticles, but instead was primarily catalyzed by molecular Pd species that leached from the nanostructures.

Solution-processed copper–nickel nanowire anodes for organic solar cells

This work describes a process to make anodes for organic solar cells from copper-nickel nanowires with solution-phase processing. Copper nanowire films were coated from solution onto glass and made conductive by dipping them in acetic acid. Acetic acid removes the passivating oxide from the surface of copper nanowires, thereby reducing the contact resistance between nanowires to nearly the same extent as hydrogen annealing. Films of copper nanowires were made as oxidation resistant as silver nanowires under dry and humid conditions by dipping them in an electroless nickel plating solution. Organic solar cells utilizing these completely solution-processed copper-nickel nanowire films exhibited efficiencies of 4.9%.

Reversible Sliding in Networks of Nanowires

This work demonstrates that metal nanowires in a percolating network can reversibly slide across one another. Reversible sliding allows networks of metal nanowires to maintain electrical contact while being stretched to strains greater than the fracture strain for individual nanowires. This phenomenon was demonstrated by using networks of nanowires as compliant electrodes for a dielectric elastomer actuator. Reversible nanowire sliding enabled actuation to a maximum area strain of 200% and repetitive cycling of the actuator to an area strain of 25% over 150 times. During actuation, the transmittance of the network increased 4.5 times, from 13% to 58%. Compared to carbon-based compliant electrodes, networks of metal nanowires can actuate across a broader range of optical transmittance. The widely tunable transmittance of nanowire-based actuators allows for their use as a light valve.

Optically Transparent Water Oxidation Catalysts Based on Copper Nanowires

Let the light shine through: A transparent film of copper nanowires was transformed into an electrocatalyst for water oxidation by electrodepostion of Ni or Co onto the surface of the nanowires. These core-shell nanowire networks exhibit electrocatalytic performance equivalent to metal oxide films of similar composition, but are several times more transparent.