Benjamin J. Wiley

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.

Chemical Synthesis of Novel Plasmonic Nanoparticles

Under the irradiation of light, the free electrons in a plasmonic nanoparticle are driven by the alternating electric field to collectively oscillate at a resonant frequency in a phenomenon known as surface plasmon resonance. Both calculations and measurements have shown that the frequency and amplitude of the resonance are sensitive to particle shape, which determines how the free electrons are polarized and distributed on the surface. As a result, controlling the shape of a plasmonic nanoparticle represents the most powerful means of tailoring and fine-tuning its optical resonance properties. In a solution-phase synthesis, the shape displayed by a nanoparticle is determined by the crystalline structure of the initial seed produced and the interaction of different seed facets with capping agents. Using polyol synthesis as a typical example, we illustrate how oxidative etching and kinetic control can be employed to manipulate the shapes and optical responses of plasmonic nanoparticles made of either Ag or Pd. We conclude by highlighting a few fundamental studies and applications enabled by plasmonic nanoparticles having well-defined and controllable shapes.

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

Controlled-reflectance surfaces with film-coupled colloidal nanoantennas

Efficient and tunable absorption is essential for a variety of applications, such as designing controlled-emissivity surfaces for thermophotovoltaic devices, tailoring an infrared spectrum for controlled thermal dissipation and producing detector elements for imaging. Metamaterials based on metallic elements are particularly efficient as absorbing media, because both the electrical and the magnetic properties of a metamaterial can be tuned by structured design. So far, metamaterial absorbers in the infrared or visible range have been fabricated using lithographically patterned metallic structures, making them inherently difficult to produce over large areas and hence reducing their applicability. Here we demonstrate a simple method to create a metamaterial absorber by randomly adsorbing chemically synthesized silver nanocubes onto a nanoscale-thick polymer spacer layer on a gold film, making no effort to control the spatial arrangement of the cubes on the film. We show that the film-coupled nanocubes provide a reflectance spectrum that can be tailored by varying the geometry (the size of the cubes and/or the thickness of the spacer). Each nanocube is the optical analogue of a grounded patch antenna, with a nearly identical local field structure that is modified by the plasmonic response of the metal’s dielectric function, and with an anomalously large absorption efficiency that can be partly attributed to an interferometric effect. The absorptivity of large surface areas can be controlled using this method, at scales out of reach of lithographic approaches (such as electron-beam lithography) that are otherwise required to manipulate matter on the nanoscale.

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.

Solution-Processed Flexible Polymer Solar Cells with Silver Nanowire Electrodes

The conventional anode for organic photovoltaics (OPVs), indium tin oxide (ITO), is expensive and brittle, and thus is not suitable for use in roll-to-roll manufacturing of OPVs. In this study, fully solution-processed polymer bulk heterojunction (BHJ) solar cells with anodes made from silver nanowires (Ag NWs) have been successfully fabricated with a configuration of Ag NWs/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/polymer:phenyl-C(61)-butyric acid methyl ester (PCBM)/Ca/Al. Efficiencies of 2.8 and 2.5% are obtained for devices with Ag NW network on glass and on poly(ethylene terephthalate) (PET), respectively. The efficiency of the devices is limited by the low work function of the Ag NWs/PEDOT:PSS film and the non-ideal ohmic contact between the Ag NW anode and the active layer. Compared with devices based on the ITO anode, the open-circuit voltage (V(oc)) of solar cells based on the Ag NW anode is lower by ~0.3 V. More importantly, highly flexible BHJ solar cells have been firstly fabricated on Ag NWs/PET anode with recoverable efficiency of 2.5% under large deformation up to 120°. This study indicates that, with improved engineering of the nanowires/polymer interface, Ag NW electrodes can serve as a low-cost, flexible alternative to ITO, and thereby improve the economic viability and mechanical stability of OPVs.

Stretchable Microfluidic Radiofrequency Antennas

www.MaterialsViews.com C O M Stretchable Microfl uidic Radiofrequency Antennas M U N I By Masahiro Kubo , Xiaofeng Li , Choongik Kim , Michinao Hashimoto , Benjamin J. Wiley , Donhee Ham , and George M. Whitesides * C A IO N This paper describes a new method for fabricating stretchable radiofrequency antennas. The antennas consist of liquid metal (eutectic gallium indium alloy, EGaIn [ 1 , 2 ] ) enclosed in elastomeric microfl uidic channels. In particular, a microfl uidic structure made of two types of elastomers (polydimethylsiloxane (PDMS) and Ecofl ex (type 0030, Reynolds Advanced Materials)) with different stiffness has been developed to improve the stretchability and mechanical stability of the antennas. These antennas can be stretched up to a strain [defi ned as the percentage change in length or ( l – l 0 )/ l 0 ] of 120 %. This high stretchability allows the resonance frequencies of the antennas to be mechanically tuned over a wide range of frequencies. The antennas can also be repeatedly stretched, while retaining a high effi ciency (> 95 %) in radiation. “Stretchability” in electronics has the potential to open new opportunities, particularly for large-area devices and systems, and in systems that require the device to conform to a nonplanar surface, or to bend and stretch while in use. [ 3–5 ] Compared to “fl exible” electronics built on nonstretchable polymer or paper substrates, [ 6 , 7 ] stretchable electronics can cover almost arbitrarily curved surfaces and movable parts. Mechanical compliance may increase the comfort of the user for wearable electronics or implantable medical devices, and simplify the integration for a range of applications. [ 3–5 , 8 ] New approaches to stretchable electronics are now being developed. In a recent advance, Rogers et al. [ 4 , 5 ] described stretchable integrated circuits with elongation of up to 100 % using wavy, thin silicon ribbons on pre-stretched elastic substrates. Antennas offer new, attractive applications for stretchable electronics; these applications might include reconfi gurable antennas, [ 9 ] antennas for limited and nonplanar spaces, [ 10 ] and wearable sensors. Two methods are commonly used to build antennas for commercial applications. The most common method uses sheet-metal processing; in this method, a metal sheet is punched, bent, and welded into the desired structure. A second method uses chemical etching and plating to make small patterns of metal. This method can make fl exible antennas by patterning metal on a fl exible substrate. Neither

Integration of photonic and silver nanowire plasmonic waveguides.

Future optical data transmission modules will require the integration of more than 10,000 x 10,000 input and output channels to increase data transmission rates and capacity. This level of integration, which greatly exceeds that of a conventional diffraction-limited photonic integrated circuit, will require the use of waveguides with a mode confinement below the diffraction limit, and also the integration of these waveguides with diffraction-limited components. We propose to integrate multiple silver nanowire plasmonic waveguides with polymer optical waveguides for the nanoscale confinement and guiding of light on a chip. In our device, the nanowires lay perpendicular to the polymer waveguide with one end inside the polymer. We theoretically predict and experimentally demonstrate coupling of light into multiple nanowires from the same waveguide, and also demonstrate control over the degree of coupling by changing the light polarization.

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.

On the polyol synthesis of silver nanostructures: glycolaldehyde as a reducing agent.

The polyol synthesis is a popular method of preparing metal nanostructures, yet the mechanism by which metal ions are reduced is poorly understood. Using a spectrophotometric method, we show, for the first time, that heating ethylene glycol (EG) in air results in its oxidation to glycolaldehyde (GA), a reductant capable of reducing most noble metal ions. The dependence of reducing power on temperature for EG can be explained by this temperature-dependent oxidation, and the factors influencing GA production can have a profound impact on the nucleation and growth kinetics. These new findings provide critical insight into how the polyol synthesis can be used to generate metal nanostructures with well-controlled shapes. For example, with the primary reductant identified, it becomes possible to evaluate and understand its explicit role in generating nanostructures of a specific shape to the exclusion of others.