High-Resolution and Large-Area Patterning of Highly Conductive Silver Nanowire Electrodes by Reverse Offset Printing and Intense Pulsed Light Irradiation

Stretchable Electrodes with Interfacial Percolation Network

Stretchable electrodes are an essential component that determines the functionality and reliability of stretchable electronics, but face the challenge of balancing conductivity and stretchability. This work proposes a new conducting concept called the interfacial percolation network (PN) that results in stretchable electrodes with high conductivity, large stretchability, and high stability. The interfacial PN is composed of a 2D silver nanowires (AgNWs) PN and a protruding 3D AgNWs PN embedded on the surface and in the near-surface region of an elastic polymer matrix, respectively. The protruded PN is obtained by changing the arrangements of AgNWs from horizontal to quasi-vertical through introducing foreign polymer domains in the near-surface region of the polymer matrix. The resulting electrode achieves a conductivity of 13 500 S cm-1 and a stretchability of 660%. Its resistance changes under stretched conditions are orders of magnitude lower than those of conventional 2D PN and 2D + 3D PN. An interfacial PN electrode made from liquid metal remained its conductivity at 46 750 S cm-1 after the electrode underwent multiple stretch-release cycles with a deformation of >600%. The concept of interfacial PN provides fruitful implications for the design of stretchable electronics.

A Guide to Printed Stretchable Conductors

Printing of stretchable conductors enables the fabrication and rapid prototyping of stretchable electronic devices. For such applications, there are often specific process and material requirements such as print resolution, maximum strain, and electrical/ionic conductivity. This review highlights common printing methods and compatible inks that produce stretchable conductors. The review compares the capabilities, benefits, and limitations of each approach to help guide the selection of a suitable process and ink for an intended application. We also discuss methods to design and fabricate ink composites with the desired material properties (e.g., electrical conductance, viscosity, printability). This guide should help inform ongoing and future efforts to create soft, stretchable electronic devices for wearables, soft robots, e-skins, and sensors.

Ultra-robust, Highly Stretchable, and Conductive Nanocomposites with Self-healable Asymmetric Structures Prepared by a Simple Green Method

Flexible conductive polymer nanocomposites based on silver nanowires (AgNWs) have been extensively studied to develop the next generation of flexible electronic devices. Fiber materials with high strength and large stretching are an important part of high-performance wearable electronics. However, manufacturing conductive composites with both high mechanical strength and good stability remains challenging. In addition, the process of effectively dispersing conductive fillers into substrates is relatively complex, which greatly hampers its widespread application. Here, a simple green self-assembly preparation method in water is reported. The AgNW is evenly dispersed in aqueous, i.e., water-borne polyurethane (WPU) with water as the solvent, and a AgNW/WPU conductive nanocomposite film with an asymmetric structure is formed by self-assembly in one step. The film has high strength (≈49.2 MPa) and high strain (≈910%), low initial resistance (99.9 mΩ/sq), high conductivity (9968.1 S/cm), and excellent self-healing (93%) and adhesion. With good self-healing performance, fibers with a conductive filler spiral structure are formed. At the same time, the application of the conductive composite material with an asymmetric structure in intelligent wearability is demonstrated.

Facile Transfer of a Transparent Silver Nanowire Pattern to a Soft Substrate Using Graphene Oxide as a Double-Sided Adhesion-Tuning Layer

Silver nanowires (AgNWs) have been employed in various optoelectronic devices as transparent electrodes. However, it remains a great challenge to facilely pattern silver nanowires to realize desirable soft skin devices. Here, we develop an intact transfer method via a double-layered adhesion regulator of graphene oxide (GO) enabling complete transfer of a silver nanowire pattern from a tough substrate onto soft polydimethylsiloxane (PDMS) and flexible polyethylene (PE). We achieve positive and negative patterns simultaneously when selectively transferring silver nanowire patterns. The resulting patterned AgNW electrodes have uniform conductivity and long-term stability. The underlying mechanism of the clean transfer is thoroughly investigated via transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). GO plays a role in reducing the adhesion of AgNW to the donor tough substrate and enhancing adhesion of AgNW to the target soft substrate simultaneously. Finally, we demonstrate the utility of the patterned electrodes as transparent sensors detecting body motion. This work offers an effective solution to the challenging patterning problem of silver nanowires on a hydrophobic soft substrate, which is compatible with the soft component in emerging smart skin or wearable electronics.

High-Quality Microprintable and Stretchable Conductors for High-Performance 5G Wireless Communication

With the advent of 5G wireless and Internet of Things technologies, flexible and stretchable printed circuit boards (PCBs) should be designed to address all the specifications necessary to receive signal transmissions, maintaining the signal integrity, and providing electrical connections. Here, we propose a silver nanoparticle (AgNP)/silver nanowire (AgNW) hybrid conductor and high-quality microprinting technology for fabricating flexible and stretchable PCBs in high-performance 5G wireless communication. A simple and low-cost reverse offset printing technique using a commercial adhesive hand-roller was adapted to ensure high-resolution and excellent pattern quality. The AgNP/AgNW micropatterns were fabricated in various line widths, from 5 μm to 5 mm. They exhibited excellent pattern qualities, such as fine line spacing, clear edge definition and outstanding pattern uniformity. After annealing via intense pulsed light irradiation, they showed outstanding electrical resistivity (15.7 μΩ cm). Moreover, they could withstand stretching up to a strain of 90% with a small change in resistance. As a demonstration of their practical application, the AgNP/AgNW micropatterns were used to fabricate 5G communication antennas that exhibited excellent wireless signal processing at operating frequencies in the C-band (4-8 GHz). Finally, a wearable sensor fabricated with these AgNP/AgNW micropatterns could successfully detected fine finger movements in real time with excellent sensitivity.

Self-assembly, alignment, and patterning of metal nanowires

Armed with the merits of one-dimensional nanostructures (flexibility, high aspect ratio, and anisotropy) and metals (high conductivity, plasmonic properties, and catalytic activity), metal nanowires (MNWs) have stood out as a new class of nanomaterials in the last two decades. They are envisaged to expedite significantly and even revolutionize a broad spectrum of applications related to display, sensing, energy, plasmonics, photonics, and catalysis. Compared with disordered MNWs, well-organized MNWs would not only enhance the intrinsic physical and chemical properties, but also create new functions and sophisticated architectures of optoelectronic devices. This paper presents a comprehensive review of assembly strategies of MNWs, including self-assembly for specific structures, alignment for anisotropic constructions, and patterning for precise configurations. The technical processes, underlying mechanisms, performance indicators, and representative applications of these strategies are described and discussed to inspire further innovation in assembly techniques and guide the fabrication of optoelectrical devices. Finally, a perspective on the critical challenges and future opportunities of MNW assembly is provided.

A review on intense pulsed light process as post-treatment for metal oxide thin films and nanostructures for device application

The intense pulsed light (IPL) post-treatment process has attracted great attention in the device fabrication due to its versatility and rapidity particularly for solution process functional structures in devices, flexible/printed electronics, and continuous manufacturing process. The metal oxide materials inherently have multi-functionality and have been widely used in form of thin films or nanostructures in device application such as thin film transistors, light emitting diodes, solar cells, supercapacitors, etc. The IPL treatment enhances the physical and/or chemical properties of the functional metal oxide through photothermal effects. However, most metal oxides are transparent to most range of visible light and require more energy for post-treatment. In this review, we have summarized the IPL post-treatment processes for metal oxide thin films and nanostructures in device applications. The sintering and annealing of metal oxides using IPL improved the device performances by employing additional light absorbing layer or back-reflector. The IPL process becomes an innovative versatile post-treatment process in conjunction with multi-functional metal oxides in near-future device applications.

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.

Preparation of Double-Layer Crossed Silver Nanowire Film and Its Application to OLED

Ordered silver nanowire (AgNW) film can effectively reduce the density of nodes, reduce the roughness of the film, and increase its conductivity and transmittance. In this paper, a double-layer crossed AgNW grid film was prepared by the auxiliary stirring method. The average transmittance of the double-layer crossed AgNW grid film was found to be 80% in the 400–1000 nm band, with a square resistance of 35 Ω/sq. As a transparent conductive anode material, the ordered AgNW film was applied to fabricate a flexible green organic light-emitting diode (OLED). The experimental results showed that the threshold voltage of the OLED was only 5 V and the maximum luminance was 1500 cd/m2.

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.

High-Resolution and Facile Patterning of Silver Nanowire Electrodes by Solvent-Free Photolithographic Technique Using UV-Curable Pressure Sensitive Adhesive Film

Patterning of silver nanowires (AgNWs) used in fabricating flexible and transparent electrodes (FTEs) is essential for constructing a variety of optoelectronic devices. However, patterning AgNW electrodes using a simple, inexpensive, high-resolution, designable, and scalable process remains a challenge. Therefore, herein a novel solvent-free photolithographic technique using a UV-curable pressure sensitive adhesive (PSA) film for patterning AgNWs is introduced. The UV-curable PSA film can be selectively patterned by photopolymerization under UV exposure through a photomask. The AgNWs embedded in the non-photocured adhesive areas of the film are firmly held by a crosslinked network of photocurable resin when the patterned film is attached to the AgNW-coated substrate and additionally irradiated by UV light. After peeling off the film, the positive pattern of AgNW electrodes remains on the substrate, while the negative pattern is transferred to the film. This solvent-free photolithographic technique, which does not use toxic solvents, provides superior pattern features, such as fine line widths and spacings, sharp line edges, and low roughness. Therefore, the developed technique could be successfully applied in the development of flexible and transparent optoelectronic devices, such as a self-cleaning electro-wetting-on-dielectric (EWOD) devices, transparent heaters, and FTEs.

Highly efficient patterning technique for silver nanowire electrodes by electrospray deposition and its application to self-powered triboelectric tactile sensor

A patterned transparent electrode is a crucial component of state-of-the-art wearable devices and optoelectronic devices. However, most of the patterning methods using silver nanowires (AgNWs), which is one of the outstanding candidate materials for the transparent electrode, wasted a large amount of unused AgNWs during the patterning process. Here, we report a highly efficient patterning of AgNWs using electrospray deposition with grounded electrolyte solution (EDGE). During electrospray deposition, a patterned electrolyte solution collector attracted AgNWs by strong electrostatic attraction and selectively deposited them only on the patterned collector, minimizing AgNW deposited elsewhere. The enhanced patterning efficiency was verified through a comparison between the EDGE and conventional process by numerical simulation and experimental validation. As a result, despite the same electrospray deposition conditions for both cases except for the existence of the electrolyte solution collector, the coverage ratio of AgNWs fabricated by the EDGE process was at least six times higher than that of AgNWs produced by the conventional process. Furthermore, the EDGE process provided high design flexibility in terms of not only the material of the substrate, including a polymer and a ceramic but also the shape of the substrate, including a 2D flat and 3D curved surface. As an application of the EDGE process, a self-powered touch sensor exploiting the triboelectric effect was demonstrated. Thus, the EDGE process would be utilized in further application in wearable or implantable devices in the field of biomedicine, intelligent robots, and human-machine interface.

Embedded Reverse-Offset Printing of Silver Nanowires and Its Application to Double-Stacked Transparent Electrodes with Microscale Patterns

We propose an embedded reverse-offset printing (EROP) method, which generates silver nanowire (AgNW) transparent electrodes for display applications. The proposed EROP method can solve the two critical issues of microscale pattern formation and surface planarization. The AgNW electrode had a transmittance of 82% at 550 nm, a sheet resistance of 12.2 Ω/sq, and a 3.27 nm smooth surface. We realized the roll-based pattern formation of AgNW on a plastic substrate as small as 10 μm with negligible step differences to facilitate the proposed method. The proposed EROP method also produced a double-stacked AgNW electrode, enabling the simultaneous operation of separately micropatterned devices. To verify the usefulness of EROP, we fabricated an organic light-emitting diode (OLED) device to demonstrate leakage current reduction and efficiency improvement compared with a conventional indium tin oxide (ITO)-based OLED device. The EROP-based OLED showed 38 and 25% higher current efficiencies than an insulator-patterned AgNW OLED and a conventional ITO-based OLED, respectively.

Recent advances in printed flexible heaters for portable and wearable thermal management

Flexible resistive heaters (FRHs) with high heating performance, large-area thermal homogeneity, and excellent thermal stability are very desirable in modern life, owing to their tremendous potential for portable and wearable thermal management applications, such as body thermotherapy, on-demand drug delivery, and artificial intelligence. Printed electronic (PE) technologies, as emerging methods combining conventional printing techniques with solution-processable functional ink have been proposed to be promising strategies for the cost-effective, large-scale, and high-throughput fabrication of printed FRHs. This review summarizes recent progress in the main components of FRHs, including conductive materials and flexible or stretchable substrates, focusing on the formulation of conductive ink systems for making printed FRHs by a variety of PE technologies including screen printing, inkjet printing, roll-to-roll (R2R) printing and three-dimensional (3D) printing. Various challenges facing the commercialization of printed FRHs and improved methods for portable and wearable thermal management applications have been discussed in detail to overcome these problems.

Strategically Controlled Flash Irradiation on Silicon Anode for Enhancing Cycling Stability and Rate Capability toward High-Performance Lithium-Ion Batteries

Si has attracted considerable interest as a promising anode material for next-generation Li-ion batteries owing to its outstanding specific capacity. However, the commercialization of Si anodes has been consistently limited by severe instabilities originating from their significant volume change (approximately 300%) during the charge-discharge process. Herein, we introduce an ultrafast processing strategy of controlled multi-pulse flash irradiation for stabilizing the Si anode by modifying its physical properties in a spatially stratified manner. We first provide a comprehensive characterization of the interactions between the anode materials and the flash irradiation, such as the condensation and carbonization of binders, sintering, and surface oxidation of the Si particles under various irradiation conditions (e.g., flash intensity and irradiation period). Then, we suggest an effective route for achieving superior physical properties for Si anodes, such as robust mechanical stability, high electrical conductivity, and fast electrolyte absorption, via precise adjustment of the flash irradiation. Finally, we demonstrate flash-irradiated Si anodes that exhibit improved cycling stability and rate capability without requiring costly synthetic functional binders or delicately designed nanomaterials. This work proposes a cost-effective technique for enhancing the performance of battery electrodes by substituting conventional long-term thermal treatment with ultrafast flash irradiation.

Recycling silver nanoparticle debris from laser ablation of silver nanowire in liquid media toward minimum material waste

As silver nanowires (Ag NWs) are usually manufactured by chemical synthesis, a patterning process is needed to use them as functional devices. Pulsed laser ablation is a promising Ag NW patterning process because it is a simple and inexpensive procedure. However, this process has a disadvantage in that target materials are wasted owing to the subtractive nature of the process involving the removal of unnecessary materials, and large quantities of raw materials are required. In this study, we report a minimum-waste laser patterning process utilizing silver nanoparticle (Ag NP) debris obtained through laser ablation of Ag NWs in liquid media. Since the generated Ag NPs can be used for several applications, wastage of Ag NWs, which is inevitable in conventional laser patterning processes, is dramatically reduced. In addition, electrophoretic deposition of the recycled Ag NPs onto non-ablated Ag NWs allows easy fabrication of junction-enhanced Ag NWs from the deposited Ag NPs. The unique advantage of this method lies in using recycled Ag NPs as building materials, eliminating the additional cost of junction welding Ag NWs. These fabricated Ag NW substrates could be utilized as transparent heaters and stretchable TCEs, thereby validating the effectiveness of the proposed process.

Optically Programmable Plateau-Rayleigh Instability for High-Resolution and Scalable Morphology Manipulation of Silver Nanowires for Flexible Optoelectronics

The ability to engineer microscale and nanoscale morphology upon metal nanowires (NWs) has been essential to achieve new electronic and photonic functions. Here, this study reports an optically programmable Plateau-Rayleigh instability (PRI) to demonstrate a facile, scalable, and high-resolution morphology engineering of silver NWs (AgNWs) at temperatures <150 °C within 10 min. This has been accomplished by conjugating a photosensitive diphenyliodonium nitrate with AgNWs to modulate surface-atom diffusion. The conjugation is UV-decomposable and able to form a cladding of molten salt-like compounds, so that the PRI of the AgNWs can be optically programmed and triggered at a much lower temperature than the melting point of AgNWs. This PRI self-assembly technique can yield both various novel nanostructures from single NW and large-area microelectrodes from the NW network on various substrates, such as a nanoscale dot-dash chain and the microelectrode down to 5 μm in line width that is the highest resolution ever fabricated for the AgNW-based electrode. Finally, the patterned AgNWs as flexible transparent electrodes were demonstrated for a wearable CdS NW photodetector. This study provides a new paradigm for engineering metal micro-/nanostructures, which holds great potential in fabrication of various sophisticated devices.

Solution and Evaporation Hybrid Approach to Enhance the Stability and Pattern Resolution Characteristics of Organic Light-Emitting Diodes

The solution process and vacuum evaporation, both fabrication methods for conventional organic light-emitting diodes (OLEDs), are intrinsically restricted with regard to their ability to enhance pattern resolutions and film stability outcomes. Here, we introduce a novel approach of the solution process followed by intense pulsed light (IPL) evaporation for producing high-resolution line patterns of OLEDs. Through control of the wettability between the banks and microchannels via a mask-free selective surface treatment, we successfully deposited phosphorescent green and red inks only into the microchannels. Then, high-resolution patterns of an emitting layer (EML) layer were uniformly evaporated onto the device substrate using IPL evaporation. Ultimately, we fabricated green and red phosphorescent OLED devices with a high pixel density of a line-patterned EML with a width of 6 μm and a pitch of 13.6 μm. In addition, we demonstrated that the IPL-evaporated films have many advantages compared to those fabricated by the conventional solution process. We also showed that the IPL evaporation process can be less sensitive to problems related to the aggregation of organic molecules during a drying or annealing process. Hence, the device performance and lifetime of the IPL-evaporated OLEDs were enhanced compared to those of the spin-coated OLEDs.

Facile Patterning of Silver Nanowires with Controlled Polarities via Inkjet-Assisted Manipulation of Interface Adhesion

Facile patterning technologies of silver nanowires (AgNWs) with low-cost, high-resolution, designable, scalable, substrate-independent, and transferable characteristics are highly desired. However, it remains a grand challenge for any material processing method to fulfil all desirable features. Herein, a new patterning method is introduced by combining inkjet printing with adhesion manipulation of substrate interfaces. Both positive and negative patterns (i.e., AgNW grid and rectangular patterns) have been simultaneously achieved, and the pattern polarity can be reversed through adhesion modification with judiciously selected supporting layers. The electrical performance of the AgNW grids depends on the AgNW interlocking structure, manifesting a strong structure-property correlation. High-resolution and complex AgNW patterns with line width and spacing as small as 10 μm have been demonstrated through selective deposition of poly(methyl methacrylate) layers. In addition, customized AgNW patterns, such as logos and words, can be fabricated onto A4-size samples and subsequently transferred to targeted substrates, including Si wafers, a curved glass vial, and a beaker. This reported inkjet-assisted process therefore offers a new effective route to manipulate AgNWs for advanced device applications.

Micro-transfer patterning of dense nanoparticle layers: roles of rheology, adhesion and fracture in transfer dynamics

Liquid inks deposited on substrates undergo spreading, coalescence, dewetting and subsequent drying kinetics, which limit the controllability of the cross-sectional shape and resolution of the printed patterns. By contrast, when the ink layers are previously semidried (highly-concentrated) and patterned on a polydimethylsiloxane sheet, single-micrometer features are resolved. Here we present the rheological, fracture and adhesive properties of semidried nanoparticle dispersion ink layers, which optimize the patterning of reverse offset printing with 5 μm spatial resolution. Under the appropriate patterning conditions, when the volume fraction φ of the particles in the semidried layers was approximately 46 v/v%, the layer elasticity was dominant in the linear viscoelastic region and a Burgers-type creeping property appeared. Under tensile strain, the semidried layers suddenly fractured at the sites of patterns with sharply defined sidewalls. In the semidried thin layers dominated by viscosity (lower φ), the pattern edges were degraded owing to local transfer instability and possible subsequent spreading. Over-drying reduced the adhesiveness of the ink layers, implying an upper limit of φ for successful patterning. The characteristics of semidried inks contribute to establishing a versatile ink-formulation scheme of various functional nanomaterials for high-resolution printed applications.

Scalable, Alternating Narrow Stripes of Polyvinyl Alcohol Support and Unmodified PEDOT:PSS with Maintained Conductivity Using a Single-Step Slot Die Coating Approach

Slot die coating has been established as an economical approach for deposition of parallel narrow stripes, a constituent pattern feature in many printed device applications. However, the minimum feature size that can be achieved using this approach is constrained by wetting and liquid bridge phenomena at the deposition region. We hypothesize that pattern resolution and process control can be improved by co-depositing a support fluid to stabilize the pattern. Electrically conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is slot die-coated in parallel stripes on flexible poly(ethylene terephthalate) substrate, without wettability-enhancing dopants or substrate pretreatment. A miscible liquid phase, polyvinyl alcohol, is used as the support material. Feature size performance and conductivity of PEDOT:PSS stripe regions are evaluated across a range of process conditions. Narrow PEDOT:PSS stripes produced using our technique range from 400 to 850 μm and exhibit conductivity approaching 1.5 S cm^-1. This electrical performance falls within the upper range expected prior to standard conductivity-enhancing post-treatments. Significantly, dewetting effects normally present with undoped PEDOT:PSS on the plastic substrate are fully mitigated with our deposition technique. These results indicate high ease of processing and good feature size performance, with few inherent drawbacks to the functional properties of the patterned films.