Understanding the mechanisms leading to failure in metallic nanowire-based transparent heaters, and solution for stability enhancement

Effect of anisotropy and length dispersity on electrical and optical properties of nanowire network based transparent electrodes: a computational study

We have studied the impact of nanowire alignment and measurement direction at the percolation threshold on the effective resistance (R) of two-dimensional (2D) films. This helps us to analyze the effect of anisotropy on the conductivity and transmittance of the nanowire-based network characterized by the disorder parameter (s). These optoelectronic properties are determined for systems with monodisperse and bimodal length distribution (a combination of two fixed lengths of nanowires). The 2D systems simulated using our computational approach are assumed to be transparent and conductive in which percolative transport is the primary conduction mechanism. We obtain our results numerically using a computational and geometrical approach, i.e. a Discrete (grid) method that is advantageous in algorithm speed. For a particular disorder parameters, the conductivity and transmittance increase as the length fraction (LF) increases for the bimodal distribution of the length of nanowires in networks. We have observed the maximum conductivity when the nanowires are highly aligned along the measurement direction of percolation, in contrast to the isotropic arrangement of nanowires. Significantly, alignment introduced in nanowires leads to a higher percolation threshold which leads to a decrease in the transmittance of the network. We show that the resistivity of the monodisperse network in the direction parallel (perpendicular) to the alignment decreases (increases) with the disorder parameter and scales ass(s2). This scaling holds true for the bimodal distribution of nanowires as well. For a particularLF, the electrical anisotropy increases withs. The anisotropy is maximum for nearly aligned nanowires in a bimodal network with the highest proportion of the longest wire considered. For the maximally aligned wires and highestLF, we obtained an approximately 50%enhancement in the figure of merit, denoted byφ. Hence, incorporating longer-length wires and increasing the alignment in nanowire networks can increase the conductivity, anisotropy, and figure of merit which may benefit a vast range of applications.

Wired for stability: evaluating the electrical performance of a solution-processed zinc oxide-modified silver nanowire transparent electrode

Silver nanowires (AgNWs) have gained much attention owing to their optoelectronic and mechanical properties and are therefore potential candidates to tackle intrinsic drawbacks of currently applied transparent electrodes in various (opto)electronic devices. In order for AgNWs to be justifiably considered as viable, it is necessary to address their insufficient stability by coupling them with another constituent into a nanocomposite. For this purpose, ZnO was chosen because of its low cost, solution processability and barrier properties. In this paper, a fully solution processed AgNW/ZnO TE film was investigated in order to understand the effect of ZnO coating on the electrical stability of AgNWs, including the mechanism of degradation during their exposure to high electrical current densities. The nanocomposite transparent electrode was processed with ZnO coatings to determine their effect on its optoelectronic properties and electrical stability, where the ZnO triple coated AgNW demonstrated the best combination of optoelectronic properties and stability at the highest working voltage.

One-Step Synthesis of Graphene-Covered Silver Nanowires with Enhanced Stability for Heating and Strain Sensing

Solution-processed silver nanowire (AgNW) networks have been considered as promising electrode candidates for next-generation electronic devices. However, they suffer from poor thermal and electrical stability and low mechanical properties, hindering their practical applications. In this work, graphene nanosheets are successfully introduced into AgNW via a facile one-step solvothermal process. Benefiting from increased conductive paths, the resultant AgNW/graphene films exhibit high electrical conductivity. More importantly, the interlocking NW morphology can be maintained under high temperature and applied voltage due to suppressed Ag migration, which is enabled by the introduction of graphene. This feature leads to enhanced thermal and electrical stability, making them suitable for use as transparent heaters. Furthermore, the composite films present excellent mechanical performance, and negligible resistance change is observed after 10 000 repeated bending cycles. To demonstrate their feasibility toward sensor applications, sandwiched strain sensors are designed, which can endure larger tensile strains and show higher sensitivity and repeatability compared with pure AgNW-based device. Furthermore, various hand gestures can be easily recognized by the resultant sensors based on unique combinations of sensing response. This work not only provides a low-cost method to realize large-scale synthesis of AgNW/graphene composites but also offers guidance to prepare high-performance electrodes for advanced electronics.

Haze factor of silver nanowires in variable refractive index environment: experimental and simulation approaches

While silver nanowires (Ag NWs) have been demonstrated as a highly efficient transparent conducting material, they suffer from strong light scattering, which is quantified by a large haze factor (HF) in the optical spectrum. Here we investigate the influence of the dielectric environment on the light scattering of Ag NWs by comparing experimental measurements and simulations. In air, two peaks on the HF spectra are observed experimentally at the wavelength ofλI= 350 nm andλII= 380 nm and are attributed by simulations to the influence of the Ag NWs pentagonal shape on the localized surface plasmon resonance. The relative intensity between the two peaks is found to be dependent on whether the Ag NWs are in contact with the glass substrate or not. The HF behaviour in the near IR region seems to be dominated by Rayleigh scattering following simulations results. Dielectric environments of Ag NWs with various refractive indexes were obtained experimentally by the conformal deposition of different metal oxide coatings using atomic layer deposition, including Al-doped zinc oxide, Al2O3and SiO2coatings. The HF is found to be correlated with the refractive index environment in terms of HF peaks position, intensity and broadening. This trend of HF peaks is supported by a theoretical model to understand the optical mechanism behind this phenomenon.

Tri-system integration in metal-oxide nanocomposites via in-situ solution-processed method for ultrathin flexible transparent electrodes

For stable operation of ultrathin flexible transparent electrodes (uFTEs), it is critical to implement effective risk management during concurrent multi-loading operation of electrical bias and mechanical folding cycles in high-humidity environments. Despite extensive efforts in preparing solution-processed uFTEs with cost-effective and high-throughput means, achieving in-situ nano-adhesion in heterogeneous metal-oxide nanocomposites remains challenging. In this work, we observed by serendipity liquid-like behaviour of transparent metal-oxide-semiconductor zinc oxide nanoparticles (ZnONPs) onto silver nanowires (AgNWs) developed by in-situ solution processed method (iSPM). This enabled us to address the long-standing issue of vulnerability in the nanocomposite caused by the interface of dissimilar materials between AgNWs and ZnONPs, resulting in a remarkably improved multi-loading operation. Importantly, substrate-integrated uFTEs constituted of the metal-oxide nanocomposite electrode semi-embedded in the polymer matrix of greatly thin <0.5 μm thickness is successfully demonstrated with the smooth surface topography, promoted by the tri-system integration including (i) AgNW-AgNW, (ii) ZnONP-ZnONP, and (iii) AgNW-ZnONP systems. Our finding unveils the complex interfacial dynamics associated with the heterogeneous interface system between AgNWs and ZnONPs and holds great promise in understanding the in-situ nano-adhesion process and increasing the design flexibility of next generation solution-processed uFTEs.

Testing the Uniformity of Surface Resistance on Large-Format Transparent Heating Glass

The design of a glazing package containing heating glass can make a window a radiator simultaneously. For such bulky glass to act as an effective radiator simultaneously, it should be possible to provide a constant temperature over the entire surface. The continuous surface temperature of the glass depends on the uniformity of the surface resistance of the resistive layer. This paper will demonstrate the testing of heating glass parameters using a specialised apparatus. The research will mainly focus on measuring the value and distribution of the surface resistance of the transparent heating layer. A thermographic study will verify the results. As the heating glass will be subjected to a toughening process, the effect of the toughening process parameters on the degradation of the transparent heating film will be investigated.

Recent advances in conductive hydrogels: classifications, properties, and applications

Hydrogel-based conductive materials for smart wearable devices have attracted increasing attention due to their excellent flexibility, versatility, and outstanding biocompatibility. This review presents the recent advances in multifunctional conductive hydrogels for electronic devices. First, conductive hydrogels with different components are discussed, including pure single network hydrogels based on conductive polymers, single network hydrogels with additional conductive additives (i.e., nanoparticles, nanowires, and nanosheets), double network hydrogels based on conductive polymers, and double network hydrogels with additional conductive additives. Second, conductive hydrogels with a variety of functionalities, including self-healing, super toughness, self-growing, adhesive, anti-swelling, antibacterial, structural color, hydrophobic, anti-freezing, shape memory and external stimulus responsiveness are introduced in detail. Third, the applications of hydrogels in flexible devices are illustrated (i.e., strain sensors, supercapacitors, touch panels, triboelectric nanogenerator, bioelectronic devices, and robot). Next, the current challenges facing hydrogels are summarized. Finally, an imaginative but reasonable outlook is given, which aims to drive further development in the future.

Conformal Encapsulation of Silver Nanowire Transparent Electrodes by Nanosized Reduced Graphene Oxide Leading to Improved All-Round Stability

Solution-processed silver nanowire (AgNW) networks are promising as next-generation transparent conductive electrodes due to their excellent optoelectronic properties, mechanical flexibility, as well as low material and processing costs. However, AgNWs are prone to thermally induced fragmentation and chemical degradation, necessitating a conformal protective coating typically achieved by low-throughput methods such as sputtering or atomic layer deposition. Herein, we report a facile all-solution-based approach to synthesize a conformally coated AgNW network by nanosized reduced graphene oxide R(nGO). In this method, probe ultrasonication is used to obtain nanosized GO, which is coated on AgNWs by a layer-by-layer approach and then chemically treated to form R(nGO)/AgNW. We show that our transparent electrode has excellent transmittance (85-92%) and sheet resistance (17.5 Ω/sq), combined with outstanding thermal and electrothermal stability, thanks to the conformal nature of the R(nGO) film, and demonstrate its use as a transparent heater with a high maximum temperature. This, in conjunction with improved long-term chemical and mechanical bending stability of R(nGO)/AgNW, indicates that our newly developed process represents an effective and low-cost strategy to improve the overall operational resilience of metal nanowire-based transparent conductive electrodes.

Efficient Protection of Silver Nanowire Transparent Electrodes by All-Biorenewable Layer-by-Layer Assembled Thin Films

An efficient protection strategy for silver nanowire-based transparent electrodes (AgNW TEs) is developed to enhance their poor adhesion force on substrates and thermal, optical, chemical, and electrical stabilities. Chitin nanofibers (CNFs) and alkali lignin (AL), which possess high mechanical property, a gas/moisture barrier, and UV absorption properties, are successively assembled on AgNW TEs through layer-by-layer (LBL) assembly based on their oppositely charged surfaces. The formation of LBL-assembled CNFs and AL (CNF/AL)₁₀ bilayers, where 10 is the optimized number of bilayers, on the aldehyde-modified AgNW (Al-AgNW) TEs does not deteriorate their electrical conductivity (17.3 ± 2.1 Ω/□) and transmittance (90.1 ± 0.3% at 550 nm), and the (CNF/AL)₁₀ bilayer-coated Al-AgNW [(CNF/AL)₁₀@Al-AgNW] TEs present considerable enhancement in their adhesion force and thermal, optical, chemical, and electrical durability. In detail, their optoelectrical properties are stable over 200 cycles of the scotch peel-off test, for 10 h sonication, up to 350 °C, under UV/O₃ treatment for 100 min, in 10% HCl and 28% NH₃ for 6 and 12 h, and at an electrical potential up to 14 V, respectively. These features make (CNF/AL)₁₀@Al-AgNW TEs suitable as a durable high-performance transparent heater.

Current density at failure of twinned silver nanowires

Silver nanowires have a wide range of potential applications in stretchable and transparent electronics due to their excellent electrical, mechanical, and optical properties. For a successful application in electronic devices, evaluating the electrical reliability of these nanowires is required. We have studied experimentally the behavior of current density at failure for penta-twinned silver nanowires with diameters between 53 and 173 nm, for 93 samples. The current densities at failure are widely scattered, have an average of 9.7 × 10⁷A cm^-2, and a standard deviation of 2.96 × 10⁷A cm^-2. Heat-transfer modeling is employed to explain the results, and Weibull statistics are used to quantify failure probabilities, thus offering guidelines for future designs based on these nanowires. The scatter observed in the measurements is attributed to surface-roughness variations among samples, which lead to local hot spots of high current density. These results quantify the Joule heating electrical reliability of silver nanowires and highlight the importance of heat transfer in increasing it.

Advances in Flexible Metallic Transparent Electrodes

Transparent electrodes (TEs) are pivotal components in many modern devices such as solar cells, light-emitting diodes, touch screens, wearable electronic devices, smart windows, and transparent heaters. Recently, the high demand for flexibility and low cost in TEs requires a new class of transparent conductive materials (TCMs), serving as substitutes for the conventional indium tin oxide (ITO). So far, ITO has been the most used TCM despite its brittleness and high cost. Among the different emerging alternative materials to ITO, metallic nanomaterials have received much interest due to their remarkable optical-electrical properties, low cost, ease of manufacturing, flexibility, and widespread applicability. These involve metal grids, thin oxide/metal/oxide multilayers, metal nanowire percolating networks, or nanocomposites based on metallic nanostructures. In this review, a comparison between TCMs based on metallic nanomaterials and other TCM technologies is discussed. Next, the different types of metal-based TCMs developed so far and the fabrication technologies used are presented. Then, the challenges that these TCMs face toward integration in functional devices are discussed. Finally, the various fields in which metal-based TCMs have been successfully applied, as well as emerging and potential applications, are summarized.

Transparent and flexible passivation of MoS2/Ag nanowire with sputtered polytetrafluoroethylene film for high performance flexible heaters

We demonstrated highly transparent and flexible polytetrafluoroethylene (PTFE) passivation for the MoS2/Ag nanowire (Ag NW) electrodes used in thin film heaters (TFHs). The electrical, optical, and mechanical properties of PTFE coated MoS2/Ag NW electrode were compared to the bare MoS2/Ag NW electrode to demonstrate effective passivation of the sputtered PTFE films before and after the 85 °C-85% temperature-relative humidity environment test. In addition, we investigated the performances of TFHs with PTFE/MoS2/Ag NW as a function of PTFE thickness from 50 to 200 nm. The saturation temperature (87.3 °C) of TFHs with PTFE/MoS2/Ag NW electrode is higher than that (61.3 °C) of TFHs with bare MoS2/Ag NW, even after the 85 °C-85% temperature-relative humidity environment test, due to effective passivation of the PTFE layer. This indicates that transparent PTFE film prepared by sputtering process provides effective thin film passivation for the two-dimensional (2D) MoS2 and Ag NW hybrid electrode against harsh environment condition.

Investigations of TiO2/NanoTiO2 Bimodal Coatings Obtained by a Hybrid PVD/ALD Method on Al-Si-Cu Alloy Substrate

This study aimed to investigate the influence of bimodal TiO2/nanoTiO2 coatings obtained in the PVD/ALD hybrid process on an Al-Si-Cu-type aluminium alloy on the physicochemical properties of the investigated materials. The reference materials were uncoated substrates and samples coated with TiO2 coatings in single PVD and ALD processes. Tests were carried out on the morphology of coatings using scanning electron microscopy (SEM) and atomic force microscopy (AFM) to determine the structure of the tested coatings and their influence on physicochemical properties. The tests of physicochemical properties were carried out using the potentiodynamic and electrochemical impedance spectroscopy methods. The analysis of corrosion products obtained during the study of physicochemical properties was performed using SEM and EDS analysis. Based on the analysis of anode polarisation curves of the tested materials and Tafel analysis, it was found that a sample demonstrated the highest corrosion resistance with a bimodal coating-type TiO2/nanoTiO2, which had the lowest value of corrosion current and the highest value of polarisation resistance. The slightest corrosion resistance was characteristic for uncovered samples. Moreover, the influence of the tested coatings on the improvement of tribological contact of the surfaces of the coated materials with the counter-specimen made of cemented carbides was demonstrated.

Bioinspired Toughening Mechanisms in a Multilayer Transparent Conductor Structure

With increasing demands and interest in flexible and foldable devices, much effort has been devoted to the development of flexible transparent electrodes. An in-depth understanding of failure mechanisms in nanoscale structure is crucial in developing stable, flexible electronics with long-term durability. The present work investigated the mechanoelectric characteristics of transparent conductive electrodes in the form of dielectric/metal/dielectric (DMD) sandwich structures under bending, including one time and repeated cyclic bending test, and provides an explanation of their failure mechanism. We demonstrate how a thin metallic layer helps to enhance the mechanical robustness of the DMD as compared with that without, tune the mechanical properties of the cohesive layer, and improve the electrode fracture resistance. Abnormal crack propagation and toughening of multilayer DMD structures are analyzed, and its underlying mechanisms are explained. We consider the knowledge of the failure mechanisms of transparent conductive electrodes gained from the present study as a foundation for future design improvements.

Oxynitride-Encapsulated Silver Nanowire Transparent Electrode with Enhanced Thermal, Electrical, and Chemical Stability

Silver nanowire (AgNW) networks have been explored as a promising technology for transparent electrodes due to their solution-processability, low-cost implementation, and excellent trade-off between sheet resistance and transparency. However, their large-scale implementation in applications such as solar cells, transparent heaters, and display applications has been hindered by their poor thermal, electrical, and chemical stability. In this work, we present reactive sputtering as a method for fast deposition of metal oxynitrides as an encapsulant layer on AgNWs. Because O₂ cannot be used as a reactive gas in the presence of oxidation-sensitive materials such as Ag, N₂ is used under moderate sputtering base pressures to leverage residual H₂O on the sample and chamber to deposit Al, Ti, and Zr oxynitrides (AlO_xN_y, TiO_xN_y, and ZrO_xN_y) on Ag nanowires on glass and polymer substrates. All encapsulants improve AgNW networks' electrical, thermal, and chemical stability. In particular, AlO_xN_y-encapsulated networks present exceptional chemical stability (negligible increase in resistance over 7 days at 80% relative humidity and 80 °C) and transparency (96% for 20 nm films on AgNWs), while TiO_xN_y demonstrates exceptional thermal and electrical stability (stability up to over temperatures 100 °C more than that of bare AgNW networks, with a maximum areal power density of 1.72 W/cm², and no resistance divergence at up to 20 V), and ZrO_xN_y presents intermediate properties in all metrics. In summary, a novel method of oxynitride deposition, leveraging moderate base pressure reactive sputtering, is demonstrated for AgNW encapsulant deposition, which is compatible with roll-to-roll processes that are operated at commercial scales, and this technique can be extended to arbitrary, vacuum-compatible substrates and device architectures.

Flexible Transparent Heater Fabricated from Spray-Coated In:ZnO/Ag-NWs/In:ZnO Multilayers on Polyimide Foil

A flexible transparent heater is presented, based on an all-sprayed composite architecture of indium-doped zinc oxide (IZO) layers that sandwich a network of silver nanowires, on a polyimide-foil substrate. This architecture could be materialized through the development of a low-temperature (240 °C) spray-pyrolysis process for the IZO layers, which is compatible with the thermal stability of the transparent polyimide substrate and allows for the formation of compact and transparent layers, without precipitates. The IZO layers entirely embed the silver nanowires, offering protection against environmental degradation and decreasing the junction resistance of the nanowire network. The resulting transparent heaters have a high mean transmittance of 0.76 (including the substrate) and sheet resistance of 7.5 Ω/sq. A steady-state temperature of ~130 °C is achieved at an applied bias of 3.5 V, with fast heater response times, with a time constant of ~4 s The heater is mechanically stable, reaching or surpassing 100 °C (at 3.5 V), under tensile, respectively, compressive-bending stress. This work shows that high-performance transparent heaters can be fabricated using all-sprayed oxide/silver-nanowire composite coatings, that are compatible with large-scale and low-cost production.

Effects of non-homogeneity and oxide coating on silver nanowire networks under electrical stress: comparison between experiment and modeling

Silver nanowire (AgNW) networks are among the most promising indium-free, flexible transparent electrodes for energy, lighting and heating devices. However, the lack of stability of such networks is a key factor that limits their industrial application. While applications require homogeneous networks, non-homogeneous AgNW networks are intentionally prepared in the present work to probe the mechanisms leading to failure under electrical stress. We show that induced non-homogeneities have a strong impact both on the spatial distribution of temperature (measured by IR imaging) and the current density throughout the electrode (as deduced from modeling). Regions with higher current density under elevated electrical stress are correlated to the origin of degradation. Furthermore, the influence of a zinc oxide (ZnO) layer on electrical performances of non-homogeneous specimens is studied. Thanks to ZnO coating, the tortuosity of electrical potential lines measured by the one-probe mapping technique is much lower than for bare networks. Additionally, coated network electrical failure occurs at 40% higher voltage compared to bare network, over 18 V, while reaching superior power-induced heating of 360 °C. The results presented here will contribute to the design and fabrication of more robust nanowire networks, particularly for application in transparent heaters.

Planar and Transparent Memristive Devices Based on Titanium Oxide Coated Silver Nanowire Networks with Tunable Switching Voltage

Threshold switching devices are fundamental active elements in more than Moore approaches, integrating the new generation of non-volatile memory devices. Here, the authors report an in-plane threshold resistive switching device with an on/off ratio above 10⁶ , a low resistance state of 10 to 100 kΩ and a high resistance state of 10 to 100 GΩ. Our devices are based on nanocomposites of silver nanowire networks and titanium oxide, where volatile unipolar threshold switching takes place across the gap left by partially spheroidized nanowires. Device reversibility depends on the titanium oxide thickness, while nanowire network density determines the threshold voltage, which can reach as low as 0.16 V. The switching mechanism is explained through percolation between metal-semiconductor islands, in a combined tunneling conduction mechanism, followed by a Schottky emission generated via Joule heating. The devices are prepared by low-cost, atmospheric pressure, and scalable techniques, enabling their application in printable, flexible, and transparent electronics.

Dynamic degradation of metallic nanowire networks under electrical stress: a comparison between experiments and simulations

Metallic nanowire networks represent a promising solution for a new generation of transparent and flexible devices, including touch screens, solar cells and transparent heaters. They, however, lack stability under thermal and electrical stresses, often leading to the degradation of nanowires, which results in the loss of electrical percolation paths. We propose a comprehensive description of the degradation mechanism in a metallic nanowire network subjected to electrical stress. The nanowire network degradation is ascribed, at a very local scale, to the hot-spot formation and the subsequent propagation of a spatially correlated disruptive crack. We compare the behaviour of actual networks under electrical and thermal stresses to dynamic simulations of randomly deposited sticks on a 2D surface, and a thermal phenomenon simulated in a metal thin film. On one hand, such comparison allows us to deduce an average junction resistance between nanowires. On the other hand, we observed that initial flaws in a discrete network result in a local current density increase in the surrounding area, further leading to an amplified Joule effect. This phenomenon promotes the spatial correlation in the damage of the percolating network. Such non-reversible failure of the transparent electrode is in good agreement with experimental observations.

Failing Forward: Stability of Transparent Electrodes Based on Metal Nanowire Networks

Metal nanowire (MNW)-based transparent electrode technologies have significantly matured over the last decade to become a prominent low-cost alternative to indium tin oxide (ITO). Beyond reaching the same level of performance as ITO, MNW networks offer additional advantages including flexibility and low materials cost. To facilitate adoption of MNW networks as a replacement to ITO, they must overcome their inherent stability issues while maintaining their properties and cost-effectiveness. Herein, the fundamental failure mechanisms of MNW networks are discussed in detail. Recent strategies to computationally model MNWs from the nano- to macroscale and suggest future work to capture dynamic failure to unravel mechanisms that account for convolution of the failure modes are highlighted. Strategies to characterize MNW network failure in situ and postmortem are also discussed. In addition, recent work about improving the stability of MNW networks via encapsulation is discussed. Lastly, a perspective is given on how to frame the requirements of MNW-encapsulant hybrids with reference to their target applications, namely: solar cells, transparent film heaters, sensors, and displays. A cost analysis to comment on the feasibility of implementing MNW hybrids is provided, and critical areas to focus on for future work on MNW networks are suggested.

A laser metallurgy route for the batch preparation of mm-scale 3D silver/graphite heteronanoclusters in air

We report a batch preparation of mm-scale 3D Ag hetero-nanoclusters which exhibit an excellent surface plasmon resonance ability via facile laser metallurgy. Under laser irradiation, the porous AgI-based coordination network crystals were instantly converted into 3D graphite-encapsulated Ag hetero-nanoclusters with uniform sizes and gaps in several seconds. The obtained hetero-nanoclusters exhibited superior 3D confocal laser energy utilization compared with the other 0D, 1D and 2D SERS substrates, solving the bottleneck caused by laser focusing deviation in the SERS active depth. The mass-produced SERS devices were ultra-sensitive for the detection of life and industrial organic pollutants in terms of low detection and enriched capacity.

Double-Sided Graphene Oxide Encapsulated Silver Nanowire Transparent Electrode with Improved Chemical and Electrical Stability

Owing to their high conductivity, transparency, flexibility, and compatibility with solution processes, silver nanowire (AgNW) networks have been widely explored as a promising alternative to indium tin oxide (ITO). However, their susceptibility to corrosion and thermal instability still remain limiting factors for widespread adoption in a range of devices including solar cells, transparent heaters, and light-emitting diodes. In this study, we report a scalable and economically viable process involving electrophoretic deposition (EPD) to fabricate a highly stable hybrid transparent electrode with a sandwich-like structure, where a AgNW network is covered by graphene oxide (GO) films on both sides. The newly developed all solution process allows the conductive transparent film to be transferred to an arbitrary surface after deposition and demonstrates excellent sheet resistance (15 Ω/sq) and tunable transmittance (70-87% at 550 nm). Unlike bare AgNW networks, the hybrid electrode retains its original conductivity under long-term storage at up to 80% relative humidity. This chemical resilience is explained by the absence of silver corrosion products for the AgNW encapsulated by GO as indicated by X-ray photoelectron spectroscopy. In situ voltage ramping and resistance measurements up to 20 V indicate a novel stabilization mechanism enabled by the presence of GO which delays the failure onset and prevents abrupt divergence of the resistance to the megaohm range experienced by bare AgNW networks. The double-sided nature of the GO coating offers combined stability and performance to the AgNW network, which adds unique versatility of our electrodes to be used toward applications that require a wide range of thermal and chemical stabilities.

Improving thermal and electrical stability of silver nanowire network electrodes through integrating graphene oxide intermediate layers

Silver nanowire (Ag NW)-based flexible and transparent electrodes are a promising candidate for various electronic and optoelectronic applications. However, thermal and electrical instabilities of Ag NW networks during operation and post treatments need to be improved for practical applications. In this work, Ag NW/Graphene Oxide (GO) hybrid films with a multilayer structure were developed, in which transparent GO sheets were inserted between Ag NWs. For the pristine Ag NW networks, contacted NWs exhibited poorer thermal stability than individual NWs as faster Ag diffusion between NWs led to the breakage of the junctions at working temperatures, hence leading to the overall device failure. In contrast, the GO intermediate layers hindered the Ag diffusion between NWs in the Ag NW/Graphene Oxide hybrid films and maintained the junction structure, giving rise to enhanced thermal stability compared to the pristine networks and the GO-covered samples. For electrical tests, unlike the network degradation under annealing treatments, a local deterioration perpendicular to the current flow was directly observed after electrical breakdown, which was attributed to high local temperature under large applied voltage. The electrical failure of the devices was related to the network structure and defects. Furthermore, the pristine devices showed notable variation of failure voltage, which in the hybrid devices is more uniform and improved in general.

Flexible and Transparent Ferroferric Oxide-Modified Silver Nanowire Film for Efficient Electromagnetic Interference Shielding

Transparent and flexible electromagnetic interference (EMI) shielding film is highly desirable due to the fast-growing flexible electronics. A silver nanowire (Ag NW) film is considered to be an ideal candidate for a transparent and flexible EMI shielding film but suffers low EMI shielding effectiveness (SE) at high transparency and poor bending durability. Herein, we introduce ferroferric oxide (Fe₃O₄) into a Ag NW film and demonstrate a robust EMI shielding film, which exhibits SE of 24.9 dB at 8.2 GHz and optical transparency of 90%. Fe₃O₄ exhibits roles of the improved absorption loss for electromagnetic radiation due to its high permeability, the enhanced reflection loss for electromagnetic radiation by increasing the conductivity of Ag NWs film, and the improved stability for the enhanced adhesion of the Ag NW EMI shielding film. Our work provides a facile method for high-performance transparent EMI shielding film, which exhibits great potential for protection for electronic devices.

Highly accurate particulate matter detection assisted by an air heater based on a silver nanowire film

Recently, many studies have been carried out to solve the particulate matter pollution problem. However, the detection accuracy for particulate matter in the atmosphere remains unsatisfactory due to the influence of the air relative humidity. Herein, we report a Ag nanowire film air heater to enhance the detection accuracy through internal heating. From air temperature and air relative humidity relationship analysis, it has been found that the Ag nanowire film air heater can form the most suitable air relative humidity in the detection system, thereby enhancing the detection accuracy. Consequently, the Ag nanowire film air heater-assisted light scattering particulate matter detector has achieved tremendous enhancement in its detection accuracy, which is comparable with the data obtained by the beta gauge method. Film resistance plays a key role in internal air temperature distribution and the resultant air relative humidity at given voltages. To achieve the most suitable air relative humidity for continuous online monitoring, response time and power consumption should be balanced. Therefore, guidance for designing Ag nanowire films with proper resistance used in an optional-sized detector has been given for quick response, high accuracy and low power consumption. This work is of significance for providing insight for future studies in particulate matter detection and pollution remediation.

Versatility of bilayer metal oxide coatings on silver nanowire networks for enhanced stability with minimal transparency loss

Silver nanowire (AgNW) networks have been lately much investigated thanks to their physical properties and are therefore foreseen to play a key role in many industrial devices as transparent electrodes, but their stability can be an issue. Although it has been shown that thin metal oxide coatings enhance the stability of AgNW networks, such stabilization is achieved at the expense of transparency. We demonstrate that by depositing a second oxide coating, which acts as an antireflective layer, it is possible to obtain highly stable and transparent composite electrodes. AgNW networks were deposited by the airbrush method, and zinc oxide (ZnO) and aluminum oxide (Al₂O₃) coatings were deposited, by Atmospheric Pressure Spatial Atomic Layer Deposition (AP-SALD), using both glass and plastic substrates; therefore, the proposed fabrication method is low-cost and compatible with high-throughput scalable fabrication. The mechanical stability of bare, ZnO and ZnO/Al₂O₃-coated AgNWs upon bending is also presented. The obtained nanocomposites exhibit highly homogeneous and conformal oxide coatings with average thicknesses of a few tens of nanometers. Samples with bilayer coatings of 70 nm ZnO/70 nm Al₂O₃ still exhibit very good stability after annealing in air up to 450 °C for 6 repetitive cycles.

Al2O3, Al doped ZnO and SnO2 encapsulation of randomly oriented ZnO nanowire networks for high performance and stable electrical devices

Two-dimensional randomly oriented nanowire (NW) networks, also called nanonets (NNs), have remarkable advantages including low-cost integration, good reproducibility and high sensitivity, which make them a promising material for electronic devices. With this work, we focus on the study of ZnO NNs as channel materials in field effect transistors (FETs). In our process, ZnO NWs were assembled in NNs by the liquid filtration method and were integrated in transistors, with the bottom-gate configuration, using simple technological steps. Non-encapsulated devices exhibited state of the art performances but their stability toward air exposure was poor. Using a proper encapsulation of the nanonets, with cheap, abundant and non-toxic oxides, we demonstrate our ability not only to stabilize their electrical properties, but also to enhance performance to values never reach before for ZnO NW-based transistors. Our best FETs exhibit a low Off-current while maintaining a very good On-current, which results in a I _on/I _off ratio exceeding 10⁶ for a drain voltage of 5 V. The behavior of these ZnO NN-based FETs was studied for three different encapsulation materials, alumina (Al₂O₃), tin oxide (SnO₂) and Al-doped ZnO (AZO). These results prove that ZnO NNs are highly promising materials for an easy and low-cost integration into FETs.

The Electro-Optical Performance of Silver Nanowire Networks

Networks of metallic nanowires have the potential to meet the needs of next-generation device technologies that require flexible transparent conductors. At present, there does not exist a first principles model capable of predicting the electro-optical performance of a nanowire network. Here we combine an electrical model derived from fundamental material properties and electrical equations with an optical model based on Mie theory scattering of light by small particles. This approach enables the generation of analogues for any nanowire network and then accurately predicts, without the use of fitting factors, the optical transmittance and sheet resistance of the transparent electrode. Predictions are validated using experimental data from the literature of networks comprised of a wide range of aspect ratios (nanowire length/diameter). The separation of the contributions of the material resistance and the junction resistance allows the effectiveness of post-deposition processing methods to be evaluated and provides a benchmark for the minimum attainable sheet resistance. The predictive power of this model enables a material-by-design approach, whereby suitable systems can be prescribed for targeted technology applications.

Low-cost fabrication of flexible transparent electrodes based on Al doped ZnO and silver nanowire nanocomposites: impact of the network density

We report the study of nanocomposite transparent electrodes based on aluminium doped zinc oxide (ZnO : Al) thin films and silver nanowire (AgNW) networks. The electrodes are fully fabricated by low-cost, open-air techniques, namely, atmospheric pressure spatial atomic layer deposition and spray coating. We show that the transparency and the electrical conductivity of the ZnO : Al/AgNW nanocomposites can be tuned by controlling the AgNW network density. We also demonstrate that the thermal, electrical and mechanical stabilities of the nanocomposites are drastically enhanced compared to those of AgNW networks or ZnO : Al thin films separately. Interestingly, we report a clear continuous decrease of the electrical resistance of the nanocomposites for network densities even below the percolation threshold. We propose a model to explain the relationship between the conductivity of the nanocomposites and the AgNW network density. Our physical model is based on the non-negligible contribution of percolating clusters of AgNWs for network densities below the percolation threshold. Our results provide a means to predicting the physical properties of such nanocomposites for applications in solar cells and other optoelectronic devices. Finally, the deposition methods used open the way towards stable, low-cost and flexible transparent electrodes for industrial applications.

Transparent conductive film based on silver nanowires and single-wall carbon nanotubes for transparent heating films

In this paper, hybrid transparent conductive films (TCFs) are designed by combining silver nanowires with the single-wall carbon nanotubes (SWCNTs) and the transparent heating films (THFs) based on the TCFs are evaluated for possible vehicle applications. By comparing the properties, including the transmittance, sheet resistance, microstructure and heating curves, we found that the SWCNTs/AgNWs are considerably suitable for making THFs. The after-treatment methods, such as physical method (hot roll pressing) and chemical method (nitric acid and Poly (diallydimethylammonium chloride) solution, (PDAC)) were researched in detail to optimize the sheet resistance and transparency to fit the THF requirements. A careful study of the different after-treatment methods revealed that hot roll pressing can quickly and efficiently improve the properties, while the nitric acid is more helpful than PDAC for the long-term stability. The results showed that a small amount of SWCNTs addition can promote the endurable maximum electric current by spreading the heat fast and efficiently, and the maximum current flow can be as high as 4 A. The thermal stability of the THFs and the de-frog performance were tested, indicating that the hybrid film had an advantage in resisting current shock and good thermal efficiency was obtained. The fabricated TCFs of stable thermal properties are qualified as a windshield-glass heater.

Flexible transparent film heaters using a ternary composite of silver nanowire, conducting polymer, and conductive oxide

Scientific and technological advances in transparent conductive electrodes improve the heating performance of flexible transparent film heaters (TFHs), which can be utilized for various applications as defrosters and heaters. To achieve high performance as well as practical TFHs, several conditions, such as high optical transmittance, low electrical resistance, heating uniformity, and operational stability in various environmental conditions should be satisfied. However, due to the trade-offs between optical transmittance and electrical resistance, it is not easy to fulfill all the requirements concurrently. Here we report flexible TFHs using a ternary composite of silver nanowire (AgNW), conducting polymer (i.e., poly[3,4-ethylenedioxythiophene]:polystyrene sulfonate [PEDOT:PSS]), and a thin conductive oxide (i.e., indium tin oxide [ITO]) layer, exhibiting higher performance in terms of the maximum heating temperature (>110 °C), operational stability, mechanical flexibility, and optical transmittance (95% at 550 nm), compared to pristine AgNW-based TFHs. We also demonstrated the stable operation of the AgNW–PEDOT:PSS/ITO TFHs soaked in water, showing excellent environmental stability. To analyse the fundamental mechanisms for the improved performance of the AgNW–PEDOT:PSS/ITO TFHs, we investigated the progress of joule heating using a device simulator, and found that the improvement originated not only from reduced electrical resistance but also from enhanced heat dissipation with PEDOT:PSS and ITO. We anticipate that our analysis and results will be helpful for further development of practical flexible TFHs.

A high-performance flexible thin film heater using AgNW–PEDOT:PSS/ITO, exhibiting stable operation in water.

Comprehensive Stability Improvement of Silver Nanowire Networks via Self-Assembled Mercapto Inhibitors

Instability of silver nanowire (AgNW) has been regarded as a major obstacle to its practical applications in optoelectrical devices as transparent electrodes. Physical barrier layers such as polymer, metal, and graphene have been generally employed to improve the stability of AgNW in previous study. Herein, we first report self-assembled organothiols as an inhibitor for AgNW to achieve an overall enhancement in antioxidation, antisulfidation, thermal stability, and anti-electromigration. The self-assembled monolayers (SAMs) of phenyl azoles, methoxy silane, and methyl alkane were formed on the surface of AgNW via Ag-S covalent bond as barrier layers which provided protective effects against corrosives (e.g., O₂, H₂S). In particular, the decoration of 2-mercaptobenzimidazole (MBI) offered the best resistance to H₂S and excellent stability under a high-temperature and high-humidity environment (85 °C and 85 RH %) for 4 months. Moreover, different SAMs exhibited a stabilizing or destabilizing effect on Plateau-Rayleigh instability of AgNW, which realized the tunability of degradation temperature of AgNWs, for example, increasing by ≥100 °C with MBI SAM or decreasing by ∼50 °C with octadecanethiol SAM compared with pristine AgNWs. Notably, the MBI-decorated AgNWs could survive at 400 °C which is by far the highest bearing temperature for solution-processed AgNW film. As a result, a transparent heater made of the MBI-AgNWs exhibited superior heating characteristics (higher working temperature and durability), as compared with the pristine AgNW-based heater. Our findings on the organothiols decoration not only provide a new paradigm in overall stability improvement of NW of noble metals but also show the potential in morphology controllability of metal NW.

Stability Enhancement of Silver Nanowire Networks with Conformal ZnO Coatings Deposited by Atmospheric Pressure Spatial Atomic Layer Deposition

Silver nanowire (AgNW) networks offer excellent electrical and optical properties and have emerged as one of the most attractive alternatives to transparent conductive oxides to be used in flexible optoelectronic applications. However, AgNW networks still suffer from chemical, thermal, and electrical instabilities, which in some cases can hinder their efficient integration as transparent electrodes in devices such as solar cells, transparent heaters, touch screens, and organic light emitting diodes. We have used atmospheric pressure spatial atomic layer deposition (AP-SALD) to fabricate hybrid transparent electrode materials in which the AgNW network is protected by a conformal thin layer of zinc oxide. The choice of AP-SALD allows us to maintain the low-cost and scalable processing of AgNW-based transparent electrodes. The effects of the ZnO coating thickness on the physical properties of AgNW networks are presented. The composite electrodes show a drastic enhancement of both thermal and electrical stabilities. We found that bare AgNWs were stable only up to 300 °C when subjected to thermal ramps, whereas the ZnO coating improved the stability up to 500 °C. Similarly, ZnO-coated AgNWs exhibited an increase of 100% in electrical stability with respect to bare networks, withstanding up to 18 V. A simple physical model shows that the origin of the stability improvement is the result of hindered silver atomic diffusion thanks to the presence of the thin oxide layer and the quality of the interfaces of hybrid electrodes. The effects of ZnO coating on both the network adhesion and optical transparency are also discussed. Finally, we show that the AP-SALD ZnO-coated AgNW networks can be effectively used as very stable transparent heaters.

Electrical Mapping of Silver Nanowire Networks: A Versatile Tool for Imaging Network Homogeneity and Degradation Dynamics during Failure

Electrical stability and homogeneity of silver nanowire (AgNW) networks are critical assets for increasing their robustness and reliability when integrated as transparent electrodes in devices. Our ability to distinguish defects, inhomogeneities, or inactive areas at the scale of the entire network is therefore a critical issue. We propose one-probe electrical mapping (1P-mapping) as a specific simple tool to study the electrical distribution in these discrete structures. 1P-mapping has allowed us to show that the tortuosity of the voltage equipotential lines of AgNW networks under bias decreases with increasing network density, leading to a better electrical homogeneity. The impact of the network fabrication technique on the electrical homogeneity of the resulting electrode has also been investigated. Then, by combining 1P-mapping with electrical resistance measurements and IR thermography, we propose a comprehensive analysis of the evolution of the electrical distribution in AgNW networks when subjected to increasing voltage stresses. We show that AgNW networks experience three distinctive stages: optimization, degradation, and breakdown. We also demonstrate that the failure dynamics of AgNW networks at high voltages occurs through a highly correlated and spatially localized mechanism. In particular the in situ formation of cracks could be clearly visualized. It consists of two steps: creation of a crack followed by propagation nearly parallel to the equipotential lines. Finally, we show that current can dynamically redistribute during failure, by following partially damaged secondary pathways through the crack.

Emerging Novel Metal Electrodes for Photovoltaic Applications

Emerging novel metal electrodes not only serve as the collector of free charge carriers, but also function as light trapping designs in photovoltaics. As a potential alternative to commercial indium tin oxide, transparent electrodes composed of metal nanowire, metal mesh, and ultrathin metal film are intensively investigated and developed for achieving high optical transmittance and electrical conductivity. Moreover, light trapping designs via patterning of the back thick metal electrode into different nanostructures, which can deliver a considerable efficiency improvement of photovoltaic devices, contribute by the plasmon-enhanced light-mattering interactions. Therefore, here the recent works of metal-based transparent electrodes and patterned back electrodes in photovoltaics are reviewed, which may push the future development of this exciting field.

Novel Flexible Transparent Conductive Films with Enhanced Chemical and Electromechanical Sustainability: TiO2 Nanosheet-Ag Nanowire Hybrid

Flexible transparent conductive films (TCFs) of TiO₂ nanosheet (TiO₂ NS) and silver nanowire (Ag NW) network hybrid were prepared through a simple and scalable solution-based process. The as-formed TiO₂ NS-Ag NW hybrid TCF shows a high optical transmittance (TT: 97% (90.2% including plastic substrate)) and low sheet resistance (R_s: 40 Ω/sq). In addition, the TiO₂ NS-Ag NW hybrid TCF exhibits a long-time chemical/aging and electromechanical stability. As for the chemical/aging stability, the hybrid TCF of Ag NW and TiO₂ NS reveals a retained initial conductivity (ΔR_s/R_s < 1%) under ambient oxidant gas over a month, superior to that of bare Ag NW (ΔR_s/R_s > 4000%) or RuO₂ NS-Ag NW hybrid (ΔR_s/R_s > 200%). As corroborated by the density functional theory simulation, the superb chemical stability of TiO₂ NS-Ag NW hybrid is attributable to the unique role of TiO₂ NS as a barrier, which prevents Ag NW's chemical corrosion via the attenuated adsorption of sulfidation molecules (H₂S) on TiO₂ NS. With respect to the electromechanical stability, in contrast to Ag NWs (ΔR/R₀ ∼ 152.9%), our hybrid TCF shows a limited increment of fractional resistivity (ΔR/R₀ ∼ 14.4%) after 200 000 cycles of the 1R bending test (strain: 6.7%) owing to mechanically welded Ag NW networks by TiO₂ NS. Overall, our unique hybrid of TiO₂ NS and Ag NW exhibits excellent electrical/optical properties and reliable chemical/electromechanical stabilities.

Healable and flexible transparent heaters

Challenges associated with the mechanical fracture of electrical conductors have hindered the realization of truly flexible high performance wearable electronics. Here, transparent healable electrodes have been developed and examined to alleviate these problems. The composite electrode features a layer of an interconnecting AgNW network on a polyurethane film modified with Diels-Alder adducts (PU-DA). Surface modification using hydrophilic molecules improved adhesion of the AgNW network and resulted in mechanically robust flexible electrodes with a figure of merit sheet resistance of 13.3 Ω □^-1 and 77% transmittance at 550 nm. Transparent and flexible healable heaters (TFHH) with good mechanical and thermal stability were fabricated using these electrodes for potential applications in thermochromics, electrically driven displays and defrosters. The PU-DA TFHHs exhibited high Joule heating temperatures of 102 °C with a low operation voltage (6 V), fast thermal response (150 s) and enhanced robustness to endure large repeated mechanical strain for over 500 bending cycles with a small variance in resistance (<10%). After deliberate damage by a knife cut, the electrodes were healed and recovered to their original conductivity via a simple heat treatment at 120 °C. Uniquely, the healing process can also be triggered by utilising electrical power.

All-Polymeric Flexible Transparent Heaters

All-polymeric flexible transparent heaters (THs) are demonstrated for the first time. Thin films of four poly(3,4-ethylenedioxythiophene) (PEDOT)-based materials embedding different dopants exhibit low sheet resistances, down to 57 Ω sq^-1 associated with good transparencies (>87%) and a haze lower than 1%. These transparent thin films show excellent heating properties, with high heating rates (up to 1.6 °C s^-1) and steady-state temperatures exceeding 100 °C when subjected to 12 V bias. Very high areal power densities were also measured, reaching almost 10 000 W m^-2. The temperature increase is finely fitted to a thermal model. It is further demonstrated that these new THs can be efficiently integrated for applications in thermochromic displays and visor deicers.

Transparent Electrodes Based on Silver Nanowire Networks: From Physical Considerations towards Device Integration

The past few years have seen a considerable amount of research devoted to nanostructured transparent conducting materials (TCM), which play a pivotal role in many modern devices such as solar cells, flexible light-emitting devices, touch screens, electromagnetic devices, and flexible transparent thin film heaters. Currently, the most commonly used TCM for such applications (ITO: Indium Tin oxide) suffers from two major drawbacks: brittleness and indium scarcity. Among emerging transparent electrodes, silver nanowire (AgNW) networks appear to be a promising substitute to ITO since such electrically percolating networks exhibit excellent properties with sheet resistance lower than 10 Ω/sq and optical transparency of 90%, fulfilling the requirements of most applications. In addition, AgNW networks also exhibit very good mechanical flexibility. The fabrication of these electrodes involves low-temperature processing steps and scalable methods, thus making them appropriate for future use as low-cost transparent electrodes in flexible electronic devices. This contribution aims to briefly present the main properties of AgNW based transparent electrodes as well as some considerations relating to their efficient integration in devices. The influence of network density, nanowire sizes, and post treatments on the properties of AgNW networks will also be evaluated. In addition to a general overview of AgNW networks, we focus on two important aspects: (i) network instabilities as well as an efficient Atomic Layer Deposition (ALD) coating which clearly enhances AgNW network stability and (ii) modelling to better understand the physical properties of these networks.

The dependence of silver nanowire stability on network composition and processing parameters

Variables such as nanowire diameter and density are found to have a significant effect on the degradation of silver nanowires.