Electrospun metal nanofiber webs as high-performance transparent electrode

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.

Incorporating carbon nanotubes in sol-gel synthesized indium tin oxide transparent conductive films

Carbon nanotubes (CNTs) are incorporated in indium tin oxide (ITO) films through a sol-gel dip coating method. CNT incorporation has no effect on lattice parameter and preferential orientation of the ITO matrix. Raman spectrum confirms the presence of CNTs in the ITO matrix. The surface roughness is decreased by CNT addition. Moreover, the optical transmittance of the ITO-CNT films is more than that of ITO films in the IR region. The higher transmittance region of the ITO-CNT films compared to that of ITO films is extended to the visible region with increasing thickness of the films. In terms of electrical resistance, the ITO film shows slightly lower sheet resistance compared to the ITO-CNT film. However, the sheet resistance of the ITO-CNT film is further reduced by thickness and reaches the minimum amount in the 15-layer deposited sample, which is lower than that of ITO samples. It seems that CNT incorporation in the ITO matrix can dramatically reduce microcrack formation, especially in higher thicknesses. Furthermore, the sensitivity of ITO films to thermal cycling is dramatically enhanced with CNT addition. In addition, the figure of merit of ITO films is strongly improved in the vis-NIR region with CNT incorporation.

Uniaxially aligned electrospun cellulose acetate nanofibers for thin layer chromatographic screening of hydroquinone and retinoic acid adulterated in cosmetics

Uniaxially aligned cellulose acetate (CA) nanofibers were successfully fabricated by electrospinning and applied to use as stationary phase for thin layer chromatography. The control of alignment was achieved by using a drum collector rotating at a high speed of 6000 rpm. Spin time of 6h was used to produce the fiber thickness of about 10 μm which was adequate for good separation. Without any chemical modification after the electrospinning process, CA nanofibers could be readily devised for screening hydroquinone (HQ) and retinoic acid (RA) adulterated in cosmetics using the mobile phase consisting of 65:35:2.5 methanol/water/acetic acid. It was found that the separation run on the aligned nanofibers over a distance of 5 cm took less than 15 min which was two to three times faster than that on the non-aligned ones. On the aligned nanofibers, the masses of HQ and RA which could be visualized were 10 and 25 ng, respectively, which were two times lower than those on the non-aligned CA fibers and five times lower than those on conventional silica plates due to the appearance of darker and sharper of spots on the aligned nanofibers. Furthermore, the proposed method efficiently resolved HQ from RA and ingredients commonly found in cosmetic creams. Due to the satisfactory analytical performance, facile and inexpensive production process, uniaxially aligned electrospun CA nanofibers are promising alternative media for planar chromatography.

High-durable AgNi nanomesh film for a transparent conducting electrode

Uniform metal nanomesh structures are promising candidates that may replace of indium-tin oxide (ITO) in transparent conducting electrodes (TCEs). However, the durability of the uniform metal mesh has not yet been studied. For this reason, a comparative analysis of the durability of TCEs based on pure Ag and AgNi nanomesh, which are fabricated by using simple transfer printing, is performed. The AgNi nanomesh shows high long-term stability to oxidation, heat, and chemicals compared with that of pure Ag nanomesh. This is because of nickel in the AgNi nanomesh. Furthermore, the AgNi nanomesh shows strong adhesion to a transparent substrate and good stability after repeated bending.

Conductive nanomaterials for printed electronics

This is a review on recent developments in the field of conductive nanomaterials and their application in printed electronics, with particular emphasis on inkjet printing of ink formulations based on metal nanoparticles, carbon nanotubes, and graphene sheets. The review describes the basic properties of conductive nanomaterials suitable for printed electronics (metal nanoparticles, carbon nanotubes, and graphene), their stabilization in dispersions, formulations of conductive inks, and obtaining conductive patterns by using various sintering methods. Applications of conductive nanomaterials for electronic devices (transparent electrodes, metallization of solar cells, RFID antennas, TFTs, and light emitting devices) are also briefly reviewed.

Fabrication of Meso-Porous Sintered Metal Thin Films by Selective Etching of Silica Based Sacrificial Template

Meso-porous metal materials have enhanced surface energies offering unique surface properties with potential applications in chemical catalysis, molecular sensing and selective separation. In this paper, commercial 20 nm diameter metal nano-particles, including silver and copper were blended with 7 nm silica nano-particles by shear mixing. The resulted powders were cold-sintered to form dense, hybrid thin films. The sacrificial silica template was then removed by selective etching in 12 wt% hydrofluoric acid solutions for 15 min to reveal a purely metallic meso-porous thin film material. The impact of the initial silica nano-particle diameter (7–20 nm) as well as the sintering pressure (5–20 ton·m⁻²) and etching conditions on the morphology and properties of the final nano-porous thin films were investigated by porometry, pyknometery, gas and liquid permeation and electron microscopy. Furthermore, the morphology of the pores and particle aggregation during shear mixing were assessed through cross-sectioning by focus ion beam milling. It is demonstrated that meso-pores ranging between 50 and 320 nm in average diameter and porosities up to 47% can be successfully formed for the range of materials tested.

A broadband plasmonic enhanced transparent conductor

We have designed a high performance transparent conductor using silver and aluminium stacked films perforated with hexagonally periodic subwavelength holes which are close to the lattice period. A broadband (λ of 400-800 nm) enhanced transmission is achieved by strong coupling of surface plasmons and propagating modes through the holes. Optical and electrical modeling predict a good combination of transparency and sheet resistance (e.g. ∼85% T and ∼2.4 Ω sq(-1)). Preliminary characterization results of the samples made via nanosphere lithography will also be presented.

Electrolessly deposited electrospun metal nanowire transparent electrodes

Metal nanowire (MNW) transparent electrodes have been widely developed for their promising sheet resistance (R(s))-transmittance (T) performance, excellent mechanical flexibility, and facile synthesis. How to lower the junction resistance without compromising optical transmittance has become the key issue in enhancing their performance. Here we combine electrospinning and electroless deposition to synthesize interconnected, ultralong MNW networks. For both silver and copper nanowire networks, the R(s) and T values reach around 10 Ω/sq and 90%, respectively. This process is scalable and takes place at ambient temperature and pressure, which opens new opportunities for flexible electronics and roll-to-roll large-scale manufacturing.

Materials and structures for stretchable energy storage and conversion devices

Stretchable energy storage and conversion devices (ESCDs) are attracting intensive attention due to their promising and potential applications in realistic consumer products, ranging from portable electronics, bio-integrated devices, space satellites, and electric vehicles to buildings with arbitrarily shaped surfaces. Material synthesis and structural design are core in the development of highly stretchable supercapacitors, batteries, and solar cells for practical applications. This review provides a brief summary of research development on the stretchable ESCDs in the past decade, from structural design strategies to novel materials synthesis. The focuses are on the fundamental insights of mechanical characteristics of materials and structures on the performance of the stretchable ESCDs, as well as challenges for their practical applications. Finally, some of the important directions in the areas of material synthesis and structural design facing the stretchable ESCDs are discussed.

A highly crystalline single Au wire network as a high temperature transparent heater

A transparent conductor which can generate high temperatures finds important applications in optoelectronics. In this article, a wire network made of Au on quartz is shown to serve as an effective high temperature transparent heater. The heater has been fabricated by depositing Au onto a cracked sacrificial template. The highly interconnected Au wire network thus formed exhibited a transmittance of ∼87% in a wide spectral range with a sheet resistance of 5.4 Ω □(-1). By passing current through the network, it could be joule heated to ∼600 °C within a few seconds. The extraordinary thermal performance and stability owe much to the seamless junctions present in the wire network. Furthermore, the wire network gets self-annealed through joule heating as seen from its increased crystallinity. Interestingly, both transmittance and sheet resistance improved following annealing to 92% and 3.2 Ω □(-1), respectively.

Relationship between material properties and transparent heater performance for both bulk-like and percolative nanostructured networks

Transparent heaters are important for many applications and in the future are likely to be fabricated from thin, conducting, nanostructured networks. However, the electrical properties of such networks are almost always controlled by percolative effects. The impact of percolation on heating effects has not been considered, and the material parameter combinations that lead to efficient performance are not known. In fact, figures of merit for transparent heaters have not been elucidated, either in bulk-like or percolative systems. Here, we develop a simple yet comprehensive model describing the operation of transparent heaters. By considering the balance of Joule heating versus power dissipated by both convection and radiation, we derive an expression for the time-dependent heater temperature as a function of both electrical and thermal parameters. This equation can be modified to describe the relationship between temperature, optical transmittance, and electrical/thermal parameters in both bulk-like and percolative systems. By performing experiments on silver nanowire networks, systems known to display both bulk-like and percolative regimes, we show the model to describe real systems extremely well. This work shows the performance of transparent heaters in the percolative regime to be significantly less efficient compared to the bulk-like regime, implying the diameter of the nanowires making up the network to be critical. The model allows the identification of figures of merit for networks in both bulk-like and percolative regimes. We show that metallic nanowire networks are most promising, closely followed by CVD graphene, with networks of solution-processed graphene and carbon nanotubes being much less efficient.

A tough and high-performance transparent electrode from a scalable and transfer-free method

Conductive metal films are patterned into transparent metal nanowire networks by using electrospun fibers as a mask. Both the transmittance and sheet resistance (6 Ω/□ at 83% transmittance and 24 Ω/□ at 92% transmittance) of the metal nanowire-based electrode out-perform commercial indium doped tin oxide (ITO) electrodes. The metal nanowire-based transparent electrodes were fabricated on both rigid glass and flexible polyethylene terephthalate (PET) substrates. In addition to state of art performance, the transparent electrodes also exhibit outstanding toughness. They can withstand repeated scotch tape peeling and various bending tests. The method for making the metal nanowire is scalable, and a touch screen on flexible substrate is demonstrated.

Low temperature growth of graphene on Cu-Ni alloy nanofibers for stable, flexible electrodes

Here, we report a facile approach to grow graphene on Cu-Ni alloy NFs at a temperature as low as 450-500 °C, in which solid polystyrene (PS) carbon source and two-temperature-zone furnace were used to prepare graphene. The graphene coated Cu-Ni (designated as G-coated Cu-Ni) NFs were fully characterized by Raman spectra, XPS, FESEM and TEM. The G-coated Cu-Ni NFs exhibited excellent anti-oxidation, anti-corrosion and flexibility properties. The anti-corrosion of G-coated Cu-Ni NFs was examined through cyclic voltammetry measurements by using sea water as the electrolyte solution. Finally, using crossed arrays of G-coated Cu-Ni NF composite electrode thin films (sheet resistance is ∼10 Ω sq(-1)) as the flexible electrode, an alternating current (AC) electroluminescent (EL) device with a configuration of G-coated Cu-Ni/active layer (ZnS : Cu phosphor)/dielectric layer (BaTiO3)/front electrode (CNT) has been fabricated. Under an AC voltage of 200 V and frequency of 1300 Hz, the ACEL device emitted blue light at 496 nm with a brightness of 103 cd m(-2).

Highly thermal conductive copper nanowire composites with ultralow loading: toward applications as thermal interface materials

Thermal interface materials (TIMs) are of ever-rising importance with the development of modern microelectronic devices. However, traditional TIMs exhibit low thermal conductivity even at high loading fractions. The use of high-aspect-ratio material is beneficial to achieve low percolation threshold for nanocomposites. In this work, single crystalline copper nanowires with large aspect ratio were used as filling materials for the first time. A thermal conductivity of 2.46 W/mK was obtained at an ultralow loading fraction, ∼0.9 vol %, which was enhanced by 1350% compared with plain matrix. Such an excellent performance makes copper nanowires attractive fillers for high-performance TIMs.

Ultrasmooth, extremely deformable and shape recoverable Ag nanowire embedded transparent electrode

Transparent electrodes have been widely used in electronic devices such as solar cells, displays, and touch screens. Highly flexible transparent electrodes are especially desired for the development of next generation flexible electronic devices. Although indium tin oxide (ITO) is the most commonly used material for the fabrication of transparent electrodes, its brittleness and growing cost limit its utility for flexible electronic devices. Therefore, the need for new transparent conductive materials with superior mechanical properties is clear and urgent. Ag nanowire (AgNW) has been attracting increasing attention because of its effective combination of electrical and optical properties. However, it still suffers from several drawbacks, including large surface roughness, instability against oxidation and moisture, and poor adhesion to substrates. These issues need to be addressed before wide spread use of metallic NW as transparent electrodes can be realized. In this study, we demonstrated the fabrication of a flexible transparent electrode with superior mechanical, electrical and optical properties by embedding a AgNW film into a transparent polymer matrix. This technique can produce electrodes with an ultrasmooth and extremely deformable transparent electrode that have sheet resistance and transmittance comparable to those of an ITO electrode.

Nanoparticles meet electrospinning: recent advances and future prospects

Nanofibres can be fabricated by various methods and perhaps electrospinning is the most facile route. In past years, electrospinning has been used as a synthesis technique and the fibres have been prepared from a variety of starting materials and show various properties. Recently, incorporating functional nanoparticles (NPs) with electrospun fibres has emerged as one of most exciting research topics in the field of electrospinning. When NPs are incorporated, on the one hand the NPs endow the electrospun fibres/mats novel or better performance, on the other hand the electrospun fibres/mats could preserve the NPs from corrosion and/or oxidation, especially for NPs with anisotropic structures. More importantly, electrospinning shows potential applications in self-assembly of nanoscale building blocks for generating new functions, and has some obvious advantages that are not available by other self-assembly methods, i.e., the obtained free-standing hybrid mats are usually flexible and with large area, which is favourable for their commercial applications. In this critical review, we will focus on the fabrication and applications of NPs-electrospun fibre composites and give an overview on this emerging field combining nanoparticles and electrospinning. Firstly, two main strategies for producing NPs-electrospun fibres will be discussed, i.e., one is preparing the NPs-electrospun fibres after electrospinning process that is usually combined with other post-processing methods, and the other is fabricating the composite nanofibres during the electrospinning process. In particular, the NPs in the latter method will be classified and introduced to show the assembling effect of electrospinning on NPs with different anisotropic structures. The subsequent section describes the applications of these NPs-electrospun fibre mats and nanocomposites, and finally a conclusion and perspectives of the future research in this emerging field is given.

A solution processed flexible nanocomposite electrode with efficient light extraction for organic light emitting diodes

Highly efficient organic light emitting diodes (OLEDs) based on multiple layers of vapor evaporated small molecules, indium tin oxide transparent electrode, and glass substrate have been extensively investigated and are being commercialized. The light extraction from the exciton radiative decay is limited to less than 30% due to plasmonic quenching on the metallic cathode and the waveguide in the multi-layer sandwich structure. Here we report a flexible nanocomposite electrode comprising single-walled carbon nanotubes and silver nanowires stacked and embedded in the surface of a polymer substrate. Nanoparticles of barium strontium titanate are dispersed within the substrate to enhance light extraction efficiency. Green polymer OLED (PLEDs) fabricated on the nanocomposite electrode exhibit a maximum current efficiency of 118 cd/A at 10,000 cd/m² with the calculated external quantum efficiency being 38.9%. The efficiencies of white PLEDs are 46.7 cd/A and 30.5%, respectively. The devices can be bent to 3 mm radius repeatedly without significant loss of electroluminescent performance. The nanocomposite electrode could pave the way to high-efficiency flexible OLEDs with simplified device structure and low fabrication cost.

Uniform and ordered copper nanomeshes by microsphere lithography for transparent electrodes

We report a comprehensive simulation and experimental study on the optical and electronic properties of uniform and ordered copper nanomeshes (Cu NMs) to determine their performance for transparent conductors. Our study includes simulations to determine the role of propagating modes in transmission and experiments that demonstrate a scalable, facile microsphere-based method to fabricate NMs on rigid quartz and flexible polyethylene terephthalate substrates. The fabrication method allows for precise control over NM morphology with near-perfect uniformity and long-range order over large areas on rigid substrates. Our Cu NMs demonstrate 80% diffuse transmission at 17 Ω/square on quartz, which is comparable to indium tin oxide. We also performed durability experiments that demonstrate these Cu NMs are robust from bending, heating, and abrasion.

Preparation of PCDTBT nanofibers with a diameter of 20 nm and their application to air-processed organic solar cells

A strategy for fabricating organic photovoltaic (OPV) devices based on PCDTBT nanofibers and PC70BM is described. Electrospinning techniques are used to prepare PCDTBT nanofibers and OPV devices in ambient air. The diameters of the PCDTBT nanofibers are approximately twice the exciton diffusion length, 20 nm. The active layer exhibits 100% photoluminescence quenching due to the small nanofiber diameter, indicating that the excitons are efficiently dissociated. The electrospun PCDTBT nanofibers absorb more photons at longer wavelengths, leading to improved photon harvesting. OPV devices composed of PCDTBT nanofibers show a high short circuit current of 11.54 mA cm(-2) and a high power conversion efficiency of 5.82%. The increase in the short circuit current is attributed to enhanced photon harvesting and charge transport. This method may be applied to the fabrication, in ambient air, of large-area active layers composed of other new conjugated polymers to yield high-performance OPV devices.

Copper nanowires as fully transparent conductive electrodes

In pondering of new promising transparent conductors to replace the cost rising tin-doped indium oxide (ITO), metal nanowires have been widely concerned. Herein, we demonstrate an approach for successful synthesis of long and fine Cu nanowires (NWs) through a novel catalytic scheme involving nickel ions. Such Cu NWs in high aspect ratio (diameter of 16.2 ± 2 nm and length up to 40 μm) provide long distance for electron transport and, meanwhile, large space for light transmission. Transparent electrodes fabricated using the Cu NW ink achieve a low sheet resistance of 1.4 Ohm/sq at 14% transmittance and a high transparency of 93.1% at 51.5 Ohm/sq. The flexibility and stability were tested with 100-timebending by 180°and no resistance change occurred. Ohmic contact was achieved to the p- and n-GaN on blue light emitting diode chip and bright electroluminescence from the front face confirmed the excellent transparency.

Atomic force microscopy visualization of hard segment alignment in stretched polyurethane nanofibers prepared by electrospinning

Molecular-level orientation within nanofibers has been attracting attention as a tool for controlling and designing highly functional nanofibers. In this study, we used atomic force microscopy to visualize the phase separation between soft and hard segments on a polyurethane (PU) nanofiber surface prepared by electrospinning. Furthermore, the stretched nanofibers prepared with a high-speed rotating collector were found to have a different phase distribution in the phase-separated structure, with the hard segment domains aligned to the fiber axis. In contrast, unstretched PU nanofibers prepared without rotation were observed to have nonuniformly distributed segments. These results indicate that the application of an intense elongation force along the nanofiber axis with a rotating mandrel collector changed the distribution of segment alignments.

Reconfigurable topography for rapid solution processing of transparent conductors

Cost-effective materials for transparent conducting electrodes are essential for many devices used in clean energy production and consumer electronics. Here we report a technique for non-lithographic patterning of silver nanowires on flexible substrates from solution via microcontact transfer printing using donor substrates with reconfigurable topography. This approach is a highly scalable fabrication strategy for high performance transparent conductors.

Laser patterning of transparent conductive metal nanowire coatings: simulation and experiment

Transparent and electrically conductive metal nanowire networks are possible replacements for costly indium tin oxide (ITO) films in many optoelectronic devices. ITO films are regularly patterned using pulsed lasers so similar technologies could be used for nanowire coatings to define electrode structures. Here, the effects of laser irradiation on conducting silver nanowire coatings are simulated and then investigated experimentally for networks formed by spray deposition onto transparent substrates. The ablation threshold fluence is found experimentally for such nanowire networks and is then related to film thickness. An effective model using finite-element heat transfer analysis is examined to look at energy dissipation through these nanowire networks and used to understand mechanisms at play in the laser-material interactions. It is demonstrated that the three-dimensional nature of these coatings and the relative ratios of the rates of lateral to vertical heat diffusion are important controlling parameter affecting the ablation threshold.

Photoresist-free patterning by mechanical abrasion of water-soluble lift-off resists and bare substrates: toward green fabrication of transparent electrodes

This paper describes the fabrication of transparent electrodes based on grids of copper microwires using a non-photolithographic process. The process--"abrasion lithography"--takes two forms. In the first implementation (Method I), a water-soluble commodity polymer film is abraded with a sharp tool, coated with a conductive film, and developed by immersion in water. Water dissolves the polymer film and lifts off the conductive film in the unabraded areas. In the second implementation (Method II), the substrate is abraded directly by scratching with a sharp tool (i.e., no polymer film necessary). The abraded regions of the substrate are recessed and roughened. Following deposition of a conductive film, the lower profile and roughened topography in the abraded regions prevents mechanical exfoliation of the conductive film using adhesive tape, and thus the conductive film remains only where the substrate is scratched. As an application, conductive grids exhibit average sheet resistances of 17 Ω sq(-1) and transparencies of 86% are fabricated and used as the anode in organic photovoltaic cells in concert with the conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Compared to devices in which PEDOT:PSS alone serves as an anode, devices comprising grids of copper/nickel microwires and PEDOT:PSS exhibit lowered series resistance, which manifests in greater fill factor and power conversion efficiency. This simple method of forming micropatterns could find use in applications where cost and environmental impact should be minimized, especially as a potential replacement for the transparent electrode indium tin oxide (ITO) in thin-film electronics over large areas (i.e., solar cells) or as a method of rapid prototyping for laboratory-scale devices.

Hybrid transparent conductive film on flexible glass formed by hot-pressing graphene on a silver nanowire mesh

Polycrystalline graphene and metallic nanowires (NWs) have been proposed to replace indium tin oxide (ITO), the most widely used transparent electrode (TE) film on the market. However, the trade-off between optical transparency (Topt) and electrical sheet resistance (Rs) of these materials taken alone makes them difficult to compete with ITO. In this paper, we show that, by hot-press transfer of graphene monolayer on Ag NWs, the resulting combined structure benefits from the synergy of the two materials, giving a Topt-Rs trade-off better than that expected by simply adding the single material contributions Ag NWs bridge any interruption in transferred graphene, while graphene lowers the contact resistance among neighboring NWs and provides local conductivity in the uncovered regions in-between NWs. The hot-pressing not only allows graphene transfer but also compacts the NWs joints, thus reducing contact resistance. The dependence on the initial NW concentration of the effects produced by the hot press process on its own and the graphene transfer using hot press was investigated and indicates that a low concentration is more suitable for the proposed geometry. A TE film with Topt of 90% and Rs of 14 Ω/sq is demonstrated, also on a flexible glass substrate about 140 μm thick, a very attractive platform for efficient flexible electronic and photonic devices.

Flexible transparent conductive materials based on silver nanowire networks: a review

The class of materials combining high electrical or thermal conductivity, optical transparency and flexibility is crucial for the development of many future electronic and optoelectronic devices. Silver nanowire networks show very promising results and represent a viable alternative to the commonly used, scarce and brittle indium tin oxide. The science and technology research of such networks are reviewed to provide a better understanding of the physical and chemical properties of this nanowire-based material while opening attractive new applications.

Copper/parylene core/shell nanowire surface fastener used for room-temperature electrical bonding

The traditional bonding technology in electronic assembly relies on high-temperature processes, such as reflow soldering or curing of adhesives, which result in undesired thermal excursions and residual stress at the bonding interface. Therefore, there is an urgent need to attach electronic components on the circuit board with good mechanical and electrical properties at room temperature. In this paper, a room-temperature electrical surface fastener consisting of copper/parylene core/shell nanowire (NW) arrays were prepared, and van der Waals (VDW) forces were utilized to interconnect the core/shell NWs. Interestingly, the Parylene C film becomes conductive due to dielectric breakdown when the thickness of it is miniaturized to nanoscale. Our electrical surface fastener exhibits high macroscopic adhesion strength (∼25 N/cm(2)) and low electrical resistance (∼4.22 × 10(-2) Ω·cm(2)). Meanwhile, a new theoretical model based on VDW forces between the NWs is proposed to explain the adhesion mechanism of the core/shell structure.

Selective-area van der Waals epitaxy of topological insulator grid nanostructures for broadband transparent flexible electrodes

Broadband transparent electrodes based on a two-dimensional grid of topological insulator Bi2Se3 are synthesized by a facile selective-area van der Waals epitaxy method. These two-dimensional grid electrodes exhibit high uniformity over large area, outstanding mechanical durability, and excellent chemical resistance to environmental perturbations. Remarkably, the topological grid electrode has high transmittance of more than 85% from the visible to the near-infrared region.

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.

Micropatterned stretchable circuit and strain sensor fabricated by lithography on an electrospun nanofiber mat

This paper describes a novel approach for composite nanofiber mats and its application to fabricate a strain sensor. Electrospun poly(4-vinylpyridine) (P4VP) nanofiber mats are micropatterned by a lithographic approach that includes selective oxidation of the nanofibers and removal of unreacted fibers. The P4VP/HAuCl4 complex is converted to P4VP/Au composites by chemical reduction. We investigate the electrical resistivity of the composite mats according to the number of complexation-and-reduction cycles, the thickness of the fiber mats, and the annealing temperatures which control the percolation of the Au nanoparticles in the fiber mats. Nozzle printing of a polymeric solution on the patterned nanofiber mats simply produces an array of strain-sensitive and strain-invariant units. The patterns demonstrate high strain-sensing performance without any mechanical and electrical failure over 200 bending cycles in the strain range of ε<0.17.

Efficient welding of silver nanowire networks without post-processing

Silver nanowire (AgNW) random meshes have attracted considerable attention as flexible and high-performance transparent electrodes. Notably, post-treatment of the AgNW random meshes, such as thermal annealing, is usually required to guarantee comparable optical transparency and electrical conductivity to commercial indium tin oxide (ITO). Here, the integral elements of preparing a high-performance, large-area AgNW random mesh network are discussed. High-performance nanostructured transparent electrodes can be obtained without any post-treatment, thereby relieving the restrictions related to the substrate. Solvent washing and a large-area spray-coating method effectively reduce the wire-wire contact resistances, thus reducing or eliminating the requirement for post-treatment.

Electrical percolation in quasi-two-dimensional metal nanowire networks for transparent conductors

We simulate the conductivity of quasi-two-dimensional mono- and polydisperse rod networks having rods of various aspect ratios (L/D = 25-800) and rod densities up to 100 times the critical density and assuming contact-resistance dominated transport. We report the rod-size dependence of the percolation threshold and the density dependence of the conductivity exponent over the entire L/D range studied. Our findings clarify the range of applicability for the popular widthless-stick description for physical networks of rodlike objects with modest aspect ratios and confirm predictions for the high-density dependence of the conductivity exponent obtained from modest-density systems. We also propose a heuristic extension to the finite-width excluded area percolation model to account for arbitrary distributions in rod length and validate this solution with numerical results from our simulations. These results are relevant to nanowire films that are among the most promising candidates for high performance flexible transparent electrodes.

Graphite-nanoplatelet-decorated polymer nanofiber with improved thermal, electrical, and mechanical properties

Graphite-nanoplatelet (GNP)-decorated polymer nanofiber composites with hierarchical structures were fabricated by the combination of electrospinning and ultrasonication. It was found that GNPs could be well attached or embedded onto the nanofibers when their size was comparable to the nanofiber diameter. X-ray diffraction results indicated that ultrasonic treatment exerted no influence on the carbon crystal layer spacing. Fourier transform infrared spectra and Raman spectroscopy revealed the existence of interfacial interaction between GNPs and polyurethane nanofibers. The prepared nanofiber composite showed enhanced thermal stability and hardness, which originated from uniform dispersion of GNPs as well as strong interaction between GNPs and the nanofibers. The electrical conductivity was significantly improved, derived from the formation of a conductive percolation network in the nanofiber composite. During ultrasonication, cavitation bubbles may be formed in liquid, and microjets and shock waves were created near the GNP surface after collapse of the bubbles. These jets, causing sintering of GNPs, pushed GNPs toward the nanofiber surface at very high speeds. When the fast-moving GNPs hit the nanofiber surface, interfacial collision between GNPs and the nanofibers occurs, the nanofiber may experience partial softening or even melting at the impact sites, and then GNPs could be uniformly anchored onto the nanofibers. This method opens a new door for harvesting GNP-based nanofiber composites with improved material properties.

Color in the corners: ITO-free white OLEDs with angular color stability

High-efficiency white OLEDs fabricated on silver nanowire-based composite transparent electrodes show almost perfectly Lambertian emission and superior angular color stability, imparted by electrode light scattering. The OLED efficiencies are comparable to those fabricated using indium tin oxide. The transparent electrodes are fully solution-processable, thin-film compatible, and have a figure of merit suitable for large-area devices.

A transparent electrode based on a metal nanotrough network

Transparent conducting electrodes are essential components for numerous flexible optoelectronic devices, including touch screens and interactive electronics. Thin films of indium tin oxide-the prototypical transparent electrode material-demonstrate excellent electronic performances, but film brittleness, low infrared transmittance and low abundance limit suitability for certain industrial applications. Alternatives to indium tin oxide have recently been reported and include conducting polymers, carbon nanotubes and graphene. However, although flexibility is greatly improved, the optoelectronic performance of these carbon-based materials is limited by low conductivity. Other examples include metal nanowire-based electrodes, which can achieve sheet resistances of less than 10Ω □(-1) at 90% transmission because of the high conductivity of the metals. To achieve these performances, however, metal nanowires must be defect-free, have conductivities close to their values in bulk, be as long as possible to minimize the number of wire-to-wire junctions, and exhibit small junction resistance. Here, we present a facile fabrication process that allows us to satisfy all these requirements and fabricate a new kind of transparent conducting electrode that exhibits both superior optoelectronic performances (sheet resistance of ~2Ω □(-1) at 90% transmission) and remarkable mechanical flexibility under both stretching and bending stresses. The electrode is composed of a free-standing metallic nanotrough network and is produced with a process involving electrospinning and metal deposition. We demonstrate the practical suitability of our transparent conducting electrode by fabricating a flexible touch-screen device and a transparent conducting tape.

Films of Carbon Nanomaterials for Transparent Conductors

The demand for transparent conductors is expected to grow rapidly as electronic devices, such as touch screens, displays, solid state lighting and photovoltaics become ubiquitous in our lives. Doped metal oxides, especially indium tin oxide, are the commonly used materials for transparent conductors. As there are some drawbacks to this class of materials, exploration of alternative materials has been conducted. There is an interest in films of carbon nanomaterials such as, carbon nanotubes and graphene as they exhibit outstanding properties. This article reviews the synthesis and assembly of these films and their post-treatment. These processes determine the film performance and understanding of this platform will be useful for future work to improve the film performance.

Solution processed zinc oxide nanopyramid/silver nanowire transparent network films with highly tunable light scattering properties

Metal nanowire transparent networks are promising replacements to indium tin oxide (ITO) transparent electrodes for optoelectronic devices. While the transparency and sheet resistance are key metrics for transparent electrode performance, independent control of the film light scattering properties is important to developing multifunctional electrodes for improved photovoltaic absorption. Here we show that controlled incorporation of ZnO nanopyramids into a metal nanowire network film affords independent, highly tunable control of the scattering properties (haze) with minimal effects on the transparency and sheet resistance. Varying the zinc oxide/silver nanostructure ratios prior to spray deposition results in sheet resistances, transmission (600 nm), and haze (600 nm) of 6-30 Ω □(-1), 68-86%, and 34-66%, respectively. Incorporation of zinc oxide nanopyramid scattering agents into the conducting nanowire mesh has a negligible effect on mesh connectivity, providing a straightforward method of controlling electrode scattering properties. The decoupling of the film scattering power and electrical characteristics makes these films promising candidates for highly scattering transparent electrodes in optoelectronic devices and can be generalized to other metal nanowire films as well as carbon nanotube transparent electrodes.

Highly stretchable patterned gold electrodes made of Au nanosheets

Multilayered Au nanosheets are suggested as a novel class of material for fabricating stretchable electrodes suitable for organic-based electronic devices. The electrodes show no difference in resistivity during repeated stretching cycles of up to ϵ = 40%.

Catalytic, conductive, and transparent platinum nanofiber webs for FTO-free dye-sensitized solar cells

We report a multifunctional platinium nanofiber (PtNF) web that can act as a catalyst layer in dye-sensitized solar cell (DSSC) to simultaneously function as a transparent counter electrode (CE), i.e., without the presence of an indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO) glass. This PtNF web can be easily produced by electrospinning, which is highly cost-effective and suitable for large-area industrial-scale production. Electrospun PtNFs are straight and have a length of a few micrometers, with a common diameter of 40-70 nm. Each nanofiber is composed of compact, crystalline Pt grains and they are well-fused and highly interconnected, which should be helpful to provide an efficient conductive network for free electron transport and a large surface area for electrocatalytic behavior. A PtNF web is served as a counter electrode in DSSC and the photovoltaic performance increases up to a power efficiency of 6.0%. It reaches up to 83% of that in a conventional DSSC using a Pt-coated FTO glass as a counter electrode. Newly designed DSSCs containing PtNF webs display highly stable photoelectric conversion efficiencies, and excellent catalytic, conductive, and transparent properties, as well as long-term stability. Also, while the DSSC function is retained, the fabrication cost is reduced by eliminating the transparent conducting layer on the counter electrode. The presented method of fabricating DSSCs based on a PtNF web can be extended to other electrocatalytic optoelectronic devices that combine superior catalytic activity with high conductivity and transparency.

Forest of gold nanowires: a new type of nanocrystal growth

We report a nanowire growth that is highly unconventional: (1) nanowires can grow from substrate-bound seeds but cannot from colloidal seeds under otherwise the same conditions; (2) the nanowires grow from only one side of the seeds, with their diameter independent of the size of the seeds; and (3) vertically aligned ultrathin nanowires are obtained on substrates, using aqueous solution and ambient conditions. With carefully designed experiments, we propose and test a new mechanism that can explain these unusual phenonmena. It turns out that the strong binding of ligands in this system forces selective deposition of Au at the ligand-deficient interface between Au seeds and oxide substrates. This means of promoting anisotropic growth of nanocrystals into nanowires is previously unknown in the literature. We are able to pinpoint the site of active growth and explain the control of nanowire width. The sustained growth at the active site and the inhibited growth at its parameter push the nanocrystals upward into wires; their diameter is dependent on the dynamic competition of the two processes. The site-specific growth from substrate-anchored seeds provides a rare means to create substrate-nanowire hierarchical structures in aqueous solution under ambient conditions. Rendering a surface conductive, particularly one with complex surface morphology, is now made easy.

Reduced graphene oxide/copper nanowire hybrid films as high-performance transparent electrodes

Hybrid films composed of reduced graphene oxide (RG-O) and Cu nanowires (NWs) were prepared. Compared to Cu NW films, the RG-O/Cu NW hybrid films have improved electrical conductivity, oxidation resistance, substrate adhesion, and stability in harsh environments. The RG-O/Cu NW films were used as transparent electrodes in Prussian blue (PB)-based electrochromic devices where they performed significantly better than pure Cu NW films.