Metal Nanowire Networks Research Articles

Ultra‐Large Two‐Dimensional Metal Nanowire Networks by Microfluidic Laminar Flow Synthesis for Formic Acid Electrooxidation

AbstractDespite the great research interest in two‐dimensional metal nanowire networks (2D MNWNs) due to their large specific surface area and abundance of unsaturated coordination atoms, their controllable synthesis still remains a significant challenge. Herein, a microfluidics laminar flow‐based approach is developed, enabling the facile preparation of large‐scale 2D structures with diverse alloy compositions, such as PtBi, AuBi, PdBi, PtPdBi, and PtAuCu alloys. Remarkably, these 2D MNWNs can reach sizes up to submillimeter scale (~220 μm), which is significantly larger than the evolution from the 1D or 3D counterparts that typically measure only tens of nanometers. The PdBi 2D MNWNs affords the highest specific activity for formic acid (2669.1 mA mg−1) among current unsupported catalysts, which is 103.5 times higher than Pt‐black, respectively. Furthermore, in situ Fourier transform infrared (FTIR) experiments provide comprehensive evidence that PdBi 2D MNWNs catalysts can effectively prevent CO* poisoning, resulting in exceptional activity and stability for the oxidation of formic acid.

Ultra-Large Two-Dimensional Metal Nanowire Networks by Microfluidic Laminar Flow Synthesis for Formic Acid Electrooxidation.

Despite the great research interest in two-dimensional metal nanowire networks (2D MNWNs) due to their large specific surface area and abundance of unsaturated coordination atoms, their controllable synthesis still remains a significant challenge. Herein, a microfluidics laminar flow-based approach is developed, enabling the facile preparation of large-scale 2D structures with diverse alloy compositions, such as PtBi, AuBi, PdBi, PtPdBi, and PtAuCu alloys. Remarkably, these 2D MNWNs can reach sizes up to submillimeter scale (~220 μm), which is significantly larger than the evolution from the 1D or 3D counterparts that typically measure only tens of nanometers. The PdBi 2D MNWNs affords the highest specific activity for formic acid (2669.1 mA mg-1) among current unsupported catalysts, which is 103.5 times higher than Pt-black, respectively. Furthermore, in situ Fourier transform infrared (FTIR) experiments provide comprehensive evidence that PdBi 2D MNWNs catalysts can effectively prevent CO* poisoning, resulting in exceptional activity and stability for the oxidation of formic acid.

Exploring the degradation of silver nanowire networks under thermal stress by coupling in situ X-ray diffraction and electrical resistance measurements.

The thermal instability of silver nanowires (AgNWs) leads to a significant increase of the electrical resistance of AgNW networks. A better understanding of the relationship between the structural and electrical properties of AgNW networks is primordial for their efficient integration as transparent electrodes (TEs) for next-generation flexible optoelectronics. Herein, we investigate the in situ evolution of the main crystallographic parameters (i.e. integrated intensity, interplanar spacing and peak broadening) of two Ag-specific Bragg peaks, (111) and (200), during a thermal ramp up to 400 °C through in situ X-ray diffraction (XRD) measurements, coupled with in situ electrical resistance measurements on the same AgNW network. First, we assign the (111) and (200) peaks of χ-scans to each five crystallites within AgNWs using a rotation matrix model. Then, we show that the thermal transition of bare AgNW networks occurs within a temperature range of about 25 °C for the electrical properties, while the structural transition spans over 200 °C. The effect of a protective tin oxide coating (SnO2) on AgNW networks is also investigated through this original in situ coupling approach. For SnO2-coated AgNW networks, the key XRD signatures from AgNWs remain constant, since the SnO2 coating prevents Ag atomic surface diffusion, and thus morphological instability (i.e. spheroidization). Moreover, the SnO2 coating does not affect the strain of both (111) and (200) planes. The thermal expansion for bare and SnO2-coated AgNW networks appears very similar to the thermal expansion of bulk Ag. Our findings provide insights into the underlying failure mechanisms of AgNW networks subjected to thermal stress, helping researchers to develop more robust and durable TEs based on metallic nanowire networks.

Towards enhanced transparent conductive nanocomposites based on metallic nanowire networks coated with metal oxides: a brief review

Silver nanowire networks, coated with metal oxides, are extensively studied from a fundamental perspective. This review summarizes their integration into various applications and guides metal oxide selection based on specific device requirements.

Bridge percolation: electrical connectivity of discontinued conducting slabs by metallic nanowires.

The properties of nanostructured networks of conductive materials have been extensively studied under the lens of percolation theory. In this work, we introduce a novel type of local percolation phenomenon used to investigate the conduction properties of a new hybrid material that combines sparse metallic nanowire networks and fractured conducting thin films on flexible substrates. This original concept could potentially lead to the design of a novel composite transparent conducting material. Using a complementary approach including formal analytical derivations, Monte Carlo simulations and electrical circuit representation for the modelling of bridged-percolating nanowire networks, we unveil the key relations between linear crack density, nanowire length and network areal mass density that ensure electrical percolation through the hybrid. The proposed theoretical model provides key insights into the conduction mechanism associated with the original concept of bridge percolation in random nanowire networks.

Flexible Active Peltier Coolers Based on Interconnected Magnetic Nanowire Networks.

Thermoelectric energy conversion based on flexible materials has great potential for applications in the fields of low-power heat harvesting and solid-state cooling. Here, we show that three-dimensional networks of interconnected ferromagnetic metal nanowires embedded in a polymer film are effective flexible materials as active Peltier coolers. Thermocouples based on Co-Fe nanowires exhibit much higher power factors and thermal conductivities near room temperature than other existing flexible thermoelectric systems, with a power factor for Co-Fe nanowire-based thermocouples of about 4.7 mW/K2m at room temperature. The effective thermal conductance of our device can be strongly and rapidly increased by active Peltier-induced heat flow, especially for small temperature differences. Our investigation represents a significant advance in the fabrication of lightweight flexible thermoelectric devices, and it offers great potential for the dynamic thermal management of hot spots on complex surfaces.

Electrical conductance of two-dimensional random percolating networks based on mixtures of nanowires and nanorings: A mean-field approach along with computer simulation.

We have studied the electrical conductance of two-dimensional (2D) random percolating networks of zero-width metallic nanowires (a mixture of rings and sticks). We took into account the nanowire resistance per unit length and the junction (nanowire-nanowire contact) resistance. Using a mean-field approximation (MFA) approach, we derived the total electrical conductance of these nanowire-based networks as a function of their geometrical and physical parameters. The MFA predictions have been confirmed by our Monte Carlo (MC) numerical simulations. The MC simulations were focused on the case when the circumferences of the rings and the lengths of the wires were equal. In this case, the electrical conductance of the network was found to be almost insensitive to the relative proportions of the rings and sticks, provided that the wire resistance and the junction resistance were equal. When the junction resistance dominated over the wire resistance, a linear dependency of the electrical conductance of the network on the proportions of the rings and sticks was observed.

Thermo-electro-optical properties of seamless metallic nanowire networks for transparent conductor applications.

Rapid reaction time, high attainable temperatures, minimum operating voltage, excellent optical transmittance, and tunable sheet resistance are all desirable properties of transparent conductors, which are important thin-film components in numerous electronic devices. A seamless nanowire network (NWN) refers to a structure composed of nanowires that lack interwire contact junctions, resulting in a continuous and uninterrupted network arrangement. This seamless nature leads to unique properties, including high conductivity and surface area-to-volume ratios, which make it a promising candidate for a vast application range in nanotechnology. Here, we have conducted an in-depth computational investigation to study the thermo-electro-optical properties of seamless nanowire networks and understand their geometrical features using in-house computational implementations and a coupled electrothermal model built in COMSOL Multiphysics software. Sheet resistance calculations were performed using Ohm's law combined with Kirchhoff circuit laws for a random resistor network and compared with those obtained employing COMSOL. In this work, aluminium, gold, copper, and silver nanowires are the materials of choice for testing the transparent conduction performance of our systems. We have studied a wide range of tuning parameters, including the network area fraction, the width-to-depth aspect ratio, and the length of the nanowire segments. We obtained corresponding figures of merit (optical transmittance versus sheet resistance) and temperature distributions to provide a complete characterization of the performance of real-world transparent conductors idealized with seamless NWNs. Our analysis accounted for the thermo-electro-optical responses of the NWNs and the inspection of various controlling parameters depending on system design considerations to shed light on how the electrical transport, optical qualities, and thermal management of these systems can be optimized.

All-atmospheric fabrication of Ag–Cu core–shell nanowire transparent electrodes with Haacke figure of merit >600 × 10–3 Ω−1

Transparent conducting electrodes (TCEs) are essential components in devices such as touch screens, smart windows, and photovoltaics. Metal nanowire networks are promising next-generation TCEs, but best-performing examples rely on expensive metal catalysts (palladium or platinum), vacuum processing, or transfer processes that cannot be scaled. This work demonstrates a metal nanowire TCE fabrication process that focuses on high performance and simple fabrication. Here we combined direct and plating metallization processes on electrospun nanowires. We first directly metallize silver nanowires using reactive silver ink. The silver catalyzes subsequent copper plating to produce Ag–Cu core–shell nanowires and eliminates nanowire junction resistances. The process allows for tunable transmission and sheet resistance properties by adjusting electrospinning and plating time. We demonstrate state-of-the-art, low-haze TCEs using an all-atmospheric process with sheet resistances of 0.33 Ω sq−1 and visible light transmittances of 86% (including the substrate), leading to a Haacke figure of merit of 652 × 10–3 Ω−1. The core–shell nanowire electrode also demonstrates high chemical and bending durability.

Recent Progress in Transparent Conductive Materials for Photovoltaics

Transparent conducting materials (TCMs) are essential components for a variety of optoelectronic devices, such as photovoltaics, displays and touch screens. In recent years, extensive efforts have been made to develop TCMs with both high electrical conductivity and optical transmittance. Based on material types, they can be mainly categorized into the following classes: metal oxides, metal nanowire networks, carbon-material-based TCMs (graphene and carbon nanotube networks) and conjugated conductive polymers (PEDOT:PSS). This review will discuss the fundamental electrical and optical properties, typical fabrication methods and the applications in solar cells for each class of TCMs and highlight the current challenges and potential future research directions.

Random 2D nanowire networks: Finite-size effect and the effect of busbar/nanowire contact resistance on their electrical conductivity

We have studied the resistance of two-dimensional random percolating networks of zero-width metallic nanowires (rings or sticks). We took into account the nanowire resistance per unit length, the junction (nanowire/nanowire contact) resistance, and the busbar/nanowire contact resistance. Using a mean-field approximation (MFA), we derived the total resistance of the nanoring-based networks as a function of their geometrical and physical parameters. We have proposed a way of accounting for the contribution of the busbar/nanowire contact resistance toward the network resistance. The MFA predictions have been confirmed by our Monte Carlo numerical simulations. Our study evidenced that the busbar/nanowire contact resistance has a significant effect on the electrical conductivity when the junction resistance dominates over the wire resistance.

De-densifying metallic nanowire networks using betweenness centrality

Silver nanowire networks show great promise as transparent conductors, but many nanowires in these materials participate only meagerly in conduction. Using graph theoretic measures to rank the importance of different nanowires, we show that such networks can be strategically de-densified to well below the percolation threshold while maintaining low resistance. This de-densification improves the network’s figure of merit by nearly 30% and reaches 70% of the theoretical maximum performance. The mathematical methods applied here are sufficiently general to apply to any networked material in which there is a non-monotonic relationship between density and performance.

Time of Failure of Metallic Nanowire Networks under Coupled Electrical and Thermal Stress: Implications for Transparent Electrodes Lifetime

Time of Failure of Metallic Nanowire Networks under Coupled Electrical and Thermal Stress: Implications for Transparent Electrodes Lifetime

Transparent film heater based on a microgrid-patterned metal nanowire network prepared by using the bilayer lift-off technique

Transparent film heater based on a microgrid-patterned metal nanowire network prepared by using the bilayer lift-off technique

Electroplated core\u2013shell nanowire network electrodes for highly efficient organic light-emitting diodes

In this study, we performed metal (Ag, Ni, Cu, or Pd) electroplating of core–shell metallic Ag nanowire (AgNW) networks intended for use as the anode electrode in organic light-emitting diodes (OLEDs) to modify the work function (WF) and conductivity of the AgNW networks. This low-cost and facile electroplating method enabled the precise deposition of metal onto the AgNW surface and at the nanowire (NW) junctions. AgNWs coated onto a transparent glass substrate were immersed in four different metal electroplating baths: those containing AgNO3 for Ag electroplating, NiSO4 for Ni electroplating, Cu2P2O7 for Cu electroplating, and PdCl2 for Pd electroplating. The solvated metal ions (Ag+, Ni2+, Cu2+, and Pd2+) in the respective electroplating baths were reduced to the corresponding metals on the AgNW surface in the galvanostatic mode under a constant electric current achieved by linear sweep voltammetry via an external circuit between the AgNW networks (cathode) and a Pt mesh (anode). The amount of electroplated metal was systematically controlled by varying the electroplating time. Scanning electron microscopy images showed that the four different metals (shells) were successfully electroplated on the AgNWs (core), and the nanosize-controlled electroplating process produced metal NWs with varying diameters, conductivities, optical transmittances, and WFs. The metal-electroplated AgNWs were successfully employed as the anode electrodes of the OLEDs. This facile and low-cost method of metal electroplating of AgNWs to increase their WFs and conductivities is a promising development for the fabrication of next-generation OLEDs.

Electrical conductivity of random metallic nanowire networks: an analytical consideration along with computer simulation.

The current interest in the study of the 2D systems of randomly deposited metallic nanowires is inspired by a combination of their high electrical conductivity with excellent optical transparency. Metallic nanowire networks show great potential for use in numerous technological applications. Although there are models that describe the electrical conductivity of the random nanowire networks through wire resistance, junction resistance, and number density of nanowires, they are either not rigorously justified or contain fitting parameters. We have proposed a model for the electrical conductivity in random metallic nanowire networks. We have mimicked such random nanowire networks as random resistor networks (RRN) produced by the homogeneous, isotropic, and random deposition of conductive zero-width sticks onto an insulating substrate. We studied the electrical conductivity of these RRNs using a mean-field approximation. An analytical dependency of the electrical conductivity on the main physical parameters (the number density and electrical resistances of these wires and of the junctions between them) has been derived. Computer simulations have been performed to validate our theoretical predictions. We computed the electrical conductivity of the RRNs against the number density of the conductive fillers for the junction-resistance-dominated case and for the case where the wire resistance and the junction resistance were equal. The results of the computations were compared with this mean-field approximation. Our computations demonstrated that our analytical expression correctly predicts the electrical conductivity across a wide range of number densities.

Patterning of Metal Nanowire Networks: Methods and Applications.

With the advance in flexible and stretchable electronics, one-dimensional nanomaterials such as metal nanowires have drawn much attention in the past 10 years or so. Metal nanowires, especially silver nanowires, have been recognized as promising candidate materials for flexible and stretchable electronics. Owing to their high electrical conductivity and high aspect ratio, metal nanowires can form electrical percolation networks, maintaining high electrical conductivity under deformation (e.g., bending and stretching). Apart from coating metal nanowires for making large-area transparent conductive films, many applications require patterned metal nanowires as electrodes and interconnects. Precise patterning of metal nanowire networks is crucial to achieve high device performances. Therefore, a high-resolution, designable, and scalable patterning of metal nanowire networks is important but remains a critical challenge for fabricating high-performance electronic devices. This review summarizes recent advances in patterning of metal nanowire networks, using subtractive methods, additive methods of nanowire dispersions, and printing methods. Representative device applications of the patterned metal nanowire networks are presented. Finally, challenges and important directions in the area of the patterning of metal nanowire networks for device applications are discussed.

A Hierarchical Metal Nanowire Network Structure for Durable, Cost-Effective, Stretchable, and Breathable Electronics.

Polymer nanofiber-based porous structures ("breathable devices") have been developed for breathable epidermal electrodes, piezoelectric nanogenerators, temperature sensors, and strain sensors, but their applications are limited because increasing the porosity reduces device robustness. Herein, we report an approach to produce ultradurable, cost-effective breathable electronics using a hierarchical metal nanowire network and an optimized photonic sintering process. Photonic sintering significantly reduces the sheet resistance (16.25 to 6.32 Ω sq-1) and is 40% more effective than conventional thermal annealing (sheet resistance: 12.99 Ω sq-1). The mechanical durability of the sintered (648.9 Ω sq-1) sample is notably improved compared to that of the untreated (disconnected) and annealed (19.1 kΩ sq-1) samples after 10,000 deformation cycles at 40% tensile strain. The sintered sample exhibits ∼29 times less change in electrical performance compared to the thermally annealed sample. This approach will lead to the development of affordable and ultradurable commercial breathable electronics.

Fabrication of junction-free Cu nanowire networks via Ru-catalyzed electroless deposition and their application to transparent conducting electrodes

Over the past few years, metal nanowire networks have attracted attention as an alternative to transparent conducting oxide materials such as indium tin oxide for transparent conducting electrode applications. Recently, electrodeposition of metal on nanoscale template is widely used for formation of metal network. In the present work, junctionless Cu nanowire networks were simply fabricated on a substrate by forming a nanostructured Ru with 80 nm width as a seed layer, followed by direct electroless deposition of Cu. By controlling the density of Ru nanowires or the electroless deposition time, we readily achieve desired transmittance and sheet resistance values ranging from ∼1 kΩ sq−1 at 99% to 9 Ω sq−1 at 89%. After being transferred to flexible substrates, the nanowire networks exhibited no obvious increase in resistance during 8000 cycles of a bending test to a radius of 2.5 mm. The durability was verified by evaluation of its heating performance. The maximum temperature was greater than 180 °C at 3 V and remained constant after three repeated cycles and for 10 min. Transmission electron microscopy and x-ray diffraction studies revealed that the adhesion between the electrolessly deposited Cu and the seed Ru nanowires strongly influenced the durability of the core–shell structured nanowire-based heaters.

Three-dimensional porous electrode based on silver nanowires for hydrogen sulfide detection

A new type of 3D material that allows gas circulation through it composed of polyether sponge, silver nanowires and silver dendrites was built. The material was bonded inside a graphite tube. The silver dendrites were electrochemically deposited on the metallic nanowire network during the recirculation of the electrolyte through the electrode. Air containing 0.89 ÷ 100 ppm H2S was circulated through the porous electrode. Subsequently, the electrode was characterized by cyclic voltammetry. It exhibited a linear response for concentrations between 3.6 ÷ 28.5 ppm H2S in air and the detection limit was below 1 ppm. Due to the large electroactive surface and innovative design it was possible to easily obtain sensitivities of 3.9·104 nA/ppmv at concentrations lower than 1 ppm H2S in air.