Tailoring of silver wires and their performance as transparent conductive coatings

Characterization of Silver Nanowire Layers in the Terahertz Frequency Range

Thin layers of silver nanowires are commonly studied for transparent electronics. However, reports of their terahertz (THz) properties are scarce. Here, we present the electrical and optical properties of thin silver nanowire layers with increasing densities at THz frequencies. We demonstrate that the absorbance, transmittance and reflectance of the metal nanowire layers in the frequency range of 0.2 THz to 1.3 THz is non-monotonic and depends on the nanowire dimensions and filling factor. We also present and validate a theoretical approach describing well the experimental results and allowing the fitting of the THz response of the nanowire layers by a Drude–Smith model of conductivity. Our results pave the way toward the application of silver nanowires as a prospective material for transparent and conductive coatings, and printable antennas operating in the terahertz range—significant for future wireless communication devices.

Ultralong AgNWs-induced toxicity in A549 cells and the important roles of ROS and autophagy

Safety concerns have been raised with regard to silver nanowires (AgNWs) because of their extensive applications. Recently, ultralong AgNWs have shown physical properties superior to those of short AgNWs. However, little is known about their toxicity and potential risks. In this study, we demonstrated a series of ultralong AgNWs-induced biological effects in human lung cancer epithelial cells (A549). Ultralong AgNWs treatments induced ROS generation, mitochondria-mediated apoptosis, and self-protective autophagy at nonlethal concentrations. In contrast to some previous reports, apoptosis was found not to correlate with the reduction of intracellular ROS. Measuring the processing of ROS generation, apoptosis and autophagy, we demonstrated that ROS not only enhance mitochondrial damage, but also raise protective autophagic flux in ultralong AgNW-treated cells. Moreover, ultralong AgNWs were found to be internalized into the cytoplasm of the epithelial cells. This study not only investigates ultralong AgNWs-induced cytotoxicity but also pinpoints ROS as a key signal in mechanisms of their toxicity.

Computational Investigation of the Morphology, Efficiency, and Properties of Silver Nano Wires Networks in Transparent Conductive Film

Random networks of silver nano wires have been considered for use in transparent conductive films as an alternative to Indium Tin Oxide (ITO), which is unsuitable for flexible devices. However, the random distribution of nano wires makes such conductive films non-uniform. As electrical conductivity is achieved through a percolation process, understanding the scale-dependency of the macroscopic properties (like electrical conductivity) and the exact efficiency of the network (the proportion of nano wires that participate in electrical conduction) is essential for optimizing the design. In this paper, we propose a computational method for identifying the representative volume element (RVE) of nano wire networks. This defines the minimum pixel size in devices using such transparent electrodes. The RVE is used to compute the macroscopic properties of films and to quantify the electrically conducting efficiency of networks. Then, the sheet resistance and transparency of networks are calculated based on the predicted RVEs, in order to analyze the effects of nano wire networks on the electrical and optical properties of conductive films. The results presented in this paper provide insights that help optimizing random nano wire networks in transparent conductive films for achieving better efficiencies.

Optical Properties of β-RDX Thin Films Deposited on Gold and Stainless Steel Substrates Calculated from Reflection-Absorption Infrared Spectra

The optical properties for crystalline films of the highly energetic material (HEM) hexahydro-1,3,5-trinitro-s-triazine, which is also known as RDX, deposited on gold (Au) and stainless steel (SS) substrates are presented. RDX has two important stable conformational polymorphs at room temperature: α-RDX and β-RDX. The optical properties obtained in the present work correspond to thin film samples of predominantly β-RDX polymorph. The infrared spectroscopic intensities measured showed significant differences in the β-RDX crystalline films deposited on the two substrates with respect to the calculated real part of refractive index. The β-RDX/Au crystalline films have a high dynamic response, which is characterized by the asymmetric stretching mode of the axial nitro groups, whereas for the β-RDX/SS crystalline films, the dynamic response was mediated by the -N-NO₂ symmetric stretch mode. This result provides an idea of how the electric field vector propagates through the β-RDX crystalline films deposited on the two substrates.

Inactivation, Clearance, and Functional Effects of Lung-Instilled Short and Long Silver Nanowires in Rats

There is a potential for silver nanowires (AgNWs) to be inhaled, but there is little information on their health effects and their chemical transformation inside the lungs in vivo. We studied the effects of short (S-AgNWs; 1.5 μm) and long (L-AgNWs; 10 μm) nanowires instilled into the lungs of Sprague-Dawley rats. S- and L-AgNWs were phagocytosed and degraded by macrophages; there was no frustrated phagocytosis. Interestingly, both AgNWs were internalized in alveolar epithelial cells, with precipitation of Ag₂S on their surface as secondary Ag₂S nanoparticles. Quantitative serial block face three-dimensional scanning electron microscopy showed a small, but significant, reduction of NW lengths inside alveolar epithelial cells. AgNWs were also present in the lung subpleural space where L-AgNWs exposure resulted in more Ag+ve macrophages situated within the pleura and subpleural alveoli, compared with the S-AgNWs exposure. For both AgNWs, there was lung inflammation at day 1, disappearing by day 21, but in bronchoalveolar lavage fluid (BALF), L-AgNWs caused a delayed neutrophilic and macrophagic inflammation, while S-AgNWs caused only acute transient neutrophilia. Surfactant protein D (SP-D) levels in BALF increased after S- and L-AgNWs exposure at day 7. L-AgNWs induced MIP-1α and S-AgNWs induced IL-18 at day 1. Large airway bronchial responsiveness to acetylcholine increased following L-AgNWs, but not S-AgNWs, exposure. The attenuated response to AgNW instillation may be due to silver inactivation after precipitation of Ag₂S with limited dissolution. Our findings have important consequences for the safety of silver-based technologies to human health.

Synthesis and highly effective purification of silver nanowires to enhance transmittance at low sheet resistance with simple polyol and scalable selective precipitation method

The high quality transparent conducting film (TCF) at a low sheet resistance of uniform and purified silver nanowires (AgNWs) have been successfully produced, the optoelectronic performance, which exceeds that of indium tin oxide (ITO).

Effective passivation of Ag nanowire-based flexible transparent conducting electrode by TiO2 nanoshell

Silver nanowire-based flexible transparent electrodes have critical problem, in spite of their excellent electrical and optical properties, that the electrical conductance and transparency degrade within several days in air because of oxidation of silver. To prevent the degradation of the silver nanowire, we encapsulated Ag-NWs with thin TiO₂ barrier. Bar-coated silver nanowires on flexible polymer substrate were laminated at 120 °C, followed by atomic layer deposition of TiO₂ nanoshell. With 20 nm of TiO₂ nanoshells on silver nanowires, the transparent electrode keeps its electrical and optical properties over 2 months. Moreover, the TiO₂-encapsulated silver nanowire-based transparent electrodes exhibit excellent bending durability.

Flexible transparent conductive films combining flexographic printed silver grids with CNT coating

A high-performance ITO-free transparent conductive film (TCF) has been made by combining high resolution Ag grids with a carbon nanotube (CNT) coating. Ag grids printed with flexography have a 20 μm line width at a grid interval of 400 μm. The Ag grid/CNT hybrid film exhibits excellent overall performance, with a typical sheet resistance of 14.8 Ω/□ and 82.6% light transmittance at room temperature. This means a 23.98% reduction in sheet resistance and only 2.52% loss in transmittance compared to a pure Ag grid film. Analysis indicates that filling areas between the Ag grids and interconnecting the silver nanoparticles with the CNT coating are the primary reasons for the significantly improved conductivity of the hybrid film that also exhibits excellent flexibility and mechanical strength compared to an ITO film. The hybrid film may fully satisfy the requirements of different applications, e.g. use as the anode of polymer solar cells (PSCs). The J-V curve shows that the power conversion efficiency (PCE) of the PSCs using the Ag grid/CNT hybrid anode is 0.61%, which is 24.5% higher than that of the pure Ag grids with a PCE of 0.49%. Further investigations to improve the performance of the solar cells based on the printed hybrid TCFs are ongoing.

Azeotropic distillation assisted fabrication of silver nanocages and their catalytic property for reduction of 4-nitrophenol

Monodisperse silver nanocages (AgNCs) with specific interiors were successfully synthesized by an azeotropic distillation (AD) assisted method and exhibited excellent catalytic activities for reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) due to the unique hollow morphology and small thickness of the silver shell.

Salt-mediated polyol synthesis of silver nanowires in a continuous-flow tubular reactor

Silver nanowires were successfully synthesized by a polyol reduction method in a continuous-flow reactor with a yield of 2 g h⁻¹.

Electrospinning-derived ultrafine silver–carbon composite nanofibers for flexible transparent conductive films

Hybrid TCFs with good conductivity, transparency, flexibility and environment resistance abilities were fabricated through partially substituting AgNWs with ultrafine Ag/CNFs.

Transparent conductive silver nanowire electrodes with high resistance to oxidation and thermal shock

Silver nanowire electrodes incorporated with polyethoxysiloxane reveal excellent resistance to oxidation and significantly increase conductivity by 1–2 orders of magnitude.

The role of propagating modes in silver nanowire arrays for transparent electrodes

Silver nanowires have been shown to demonstrate enhanced transmission and promising potential for next-generation transparent electrodes. In this paper, we systematically investigated the electrical and optical properties of 1D and 2D silver nanowire arrays as a function of diameter and pitch and compared their performance to that of silver thin films. Silver nanowires were found to exhibit enhanced transmission over thin films due to propagating resonance modes between nanowires. We evaluated the angular dependence and dispersion relation of these propagating modes and demonstrate that larger nanowire diameters and pitches are favored for achieving higher solar transmission at a particular sheet resistance. Silver nanowires may achieve achieve solar transmission > 90% with sheet resistances of a few Ω/sq and figure of merit σdc/σop > 1000.

Highly transparent low resistance ZnO/Ag nanowire/ZnO composite electrode for thin film solar cells

We present an indium-free transparent conducting composite electrode composed of silver nanowires (AgNWs) and ZnO bilayers. The AgNWs form a random percolating network embedded between the ZnO layers. The unique structural features of our ZnO/AgNW/ZnO multilayered composite allow for a novel transparent conducting electrode with unprecedented excellent thermal stability (∼375 °C), adhesiveness, and flexibility as well as high electrical conductivity (∼8.0 Ω/sq) and good optical transparency (>91% at 550 nm). Cu(In,Ga)(S,Se)₂ (CIGSSe) thin film solar cells incorporating this composite electrode exhibited a 20% increase of the power conversion efficiency compared to a conventional sputtered indium tin oxide-based CIGSSe solar cell. The ZnO/AgNW/ZnO composite structure enables effective light transmission and current collection as well as a reduced leakage current, all of which lead to better cell performance.

Fabrication of Transparent Conductive Thin Film Electrodes Based on Ag Nanowire on Transparent Substrates Using the Spray Method for Photovoltaic Applications

Materials with a remarkable combination of high electrical conductivity as well as optical transparency are playing a key role for opto-electronic devices. In addition to these specific electro-optical properties, transparent conductive materials should, for many applications, be lightweight, flexible, low-cost, non toxic and compatible with mass production techniques. In these regards, the use of Ag nanowire (Ag NW) networks appears to be a promising approach. In this study, Ag NW electrodes were fabricated by a novel spray injection method. The number of pulses was varied resulting in different network morphologies. Coatings were systematically characterised structurally, electrically and optically via SEM, four-point probe measurements and spectrophotometry, respectively. Semi uniform layers of nanowires with large haze coefficients have been obtained over large areas. Thermal annealing was shown to increase the nanowire film conductance resulting in 16 Ω/sq surface resistance and up to 73% maximal total transmittance. Films showed average optical transparencies superior to that of ITO over the 250-2500 nm range. Finally, encapsulation of Ag NWs within a matrix of ZnO nanoparticles greatly enhanced the thermal stability of these networks.

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.

Solution processing of transparent conductors: from flask to film

This critical review focuses on the solution deposition of transparent conductors with a particular focus on transparent conducting oxide (TCO) thin-films. TCOs play a critical role in many current and emerging opto-electronic devices due to their unique combination of electronic conductivity and transparency in the visible region of the spectrum. Atmospheric-pressure solution processing is an attractive alternative to conventional vacuum-based deposition methods due to its ease of fabrication, scalability, and potential to lower device manufacturing costs. An introduction into the applications of and material criteria for TCOs will be presented first, followed by a discussion of solution routes to these systems. Recent studies in the field will be reviewed according to their materials system. Finally, the challenges and opportunities for further enabling research will be discussed in terms of emerging oxide systems and non-oxide based transparent conductors (341 references).

Direct printing of silver nanoparticles by an agarose stamp on planar and patterned substrates

In this study, we have used an agarose stamp to conduct direct printing of silver nanoparticles, nanowires and nanoplates on both planar and structured substrates. Nanoparticle solution could be first coated on an agarose stamp, and then transferred to a planar substrate. Micro-patterns comprising metal nanoparticles could be printed on planar substrates without the formation of residual layers. Thus a three-dimensional metal microstructure could be easily fabricated. The patterning of electrodes by printing Ag nanowires directly on TiO(2) was also demonstrated to fabricate resistive random access memory (RRAM) devices by all-solution-processing methods. By using a flat agarose stamp, the patterns printed on the microstructured substrates were quite different from those on the nanostructured substrates. On the microstructured substrates, direct printing could print silver nanoparticles onto the protrusion surface, and could print silver layers as thick as several microns, useful for high conductivity electrodes. On the substrates with nanostructures such as photonic crystals or nano-gratings, direct printing could transfer nanoparticles into the grooves or cavities only due to the contact of the agarose stamp with the groove or concavity surface. A new approach to fabricate metal wire grid polarizers was further demonstrated. A nanoporous agarose stamp has a good potential for printing using nanoparticle suspension.

Silver nanowire-based transparent, flexible, and conductive thin film

The fabrication of transparent, conductive, and uniform silver nanowire films using the scalable rod-coating technique is described in this study. Properties of the transparent conductive thin films are investigated, as well as the approaches to improve the performance of transparent silver nanowire electrodes. It is found that silver nanowires are oxidized during the coating process. Incubation in hydrogen chloride (HCl) vapor can eliminate oxidized surface, and consequently, reduce largely the resistivity of silver nanowire thin films. After HCl treatment, 175 Ω/sq and approximately 75% transmittance are achieved. The sheet resistivity drops remarkably with the rise of the film thickness or with the decrease of transparency. The thin film electrodes also demonstrated excellent flexible stability, showing < 2% resistance change after over 100 bending cycles.