Paper-based silver-nanowire electronic circuits with outstanding electrical conductivity and extreme bending stability

Paper-Based Humidity Sensors as Promising Flexible Devices, State of the Art, Part 2: Humidity-Sensor Performances

This review article covers all types of paper-based humidity sensor, such as capacitive, resistive, impedance, fiber-optic, mass-sensitive, microwave, and RFID (radio-frequency identification) humidity sensors. The parameters of these sensors and the materials involved in their research and development, such as carbon nanotubes, graphene, semiconductors, and polymers, are comprehensively detailed, with a special focus on the advantages/disadvantages from an application perspective. Numerous technological/design approaches to the optimization of the performances of the sensors are considered, along with some non-conventional approaches. The review ends with a detailed analysis of the current problems encountered in the development of paper-based humidity sensors, supported by some solutions.

Paper-Based Humidity Sensors as Promising Flexible Devices: State of the Art: Part 1. General Consideration

In the first part of the review article "General considerations" we give information about conventional flexible platforms and consider the advantages and disadvantages of paper when used in humidity sensors, both as a substrate and as a humidity-sensitive material. This consideration shows that paper, especially nanopaper, is a very promising material for the development of low-cost flexible humidity sensors suitable for a wide range of applications. Various humidity-sensitive materials suitable for use in paper-based sensors are analyzed and the humidity-sensitive characteristics of paper and other humidity-sensitive materials are compared. Various configurations of humidity sensors that can be developed on the basis of paper are considered, and a description of the mechanisms of their operation is given. Next, we discuss the manufacturing features of paper-based humidity sensors. The main attention is paid to the consideration of such problems as patterning and electrode formation. It is shown that printing technologies are the most suitable for mass production of paper-based flexible humidity sensors. At the same time, these technologies are effective both in the formation of a humidity-sensitive layer and in the manufacture of electrodes.

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.

Flexible Cotton Fiber-Based Composite Films with Excellent Bending Stability and Conductivity at Cryogenic Temperature

The commonly used metal thin film or resin-based flexible composites cannot meet the requirement of cryogenic flexible conductive functional devices, which may be used in space exploration, biomedicine, and other science and technology fields facing a very low temperature environment, because of their poor fatigue and anti-bending properties at cryogenic temperature. In this work, a composite based on functionalized cotton fibers is proposed to achieve the application requirement of flexible electrical systems at cryogenic temperature. A conductive composite film with optimized strength and flexibility was obtained by controlling the size distribution of cotton fibers and adjusting the interaction force among the cotton fibers. The obtained composite film could endure over 10,000 times of bending at 77 K (-196 °C), with the resistance changing less than ±5%, indicating its excellent mechanical flexibility and electrical stability at cryogenic temperature. Finally, a demonstration was successfully conducted by applying the composite film as a flexible electrical connection to a robot arm, which worked at 77 K. This work might be a reference significance for the application of flexible conductors from room temperature to cryogenic temperature.

Oligonucleotide-Based Reusable Electrochemical Silver(I) Sensor and Its Optimization via Probe Packing Density

We report herein a selective, sensitive, and reusable electrochemical sensor for the detection of silver(I) ions. This sensor detects Ag⁺ through a structure-switching electrode-bound DNA by measuring the changes in the electron-transfer efficiency. A single-stranded DNA, featuring a methylene blue (MB)-tagged DNA hairpin structure, strategically provides selective binding for the silver-mediated coordination of cytosine-Ag⁺-cytosine complexes. The DNA-modified electrode produces a change in the electrochemical signal due to the redox current of the surface-confined MB tag. The "turn-on" signaling upon silver(I) ion binding could be attributed to a conformational change in the MB-tagged DNA from an open structure to a target-induced folding structure. Differential pulse voltammetry of the DNA-modified electrode showed that the MB reduction signal increased linearly with an increase in Ag⁺ concentrations in a range of 10-200 nM, with a detection limit of 10 nM. The structure-switching silver(I) ion sensor was amenable to regeneration by simply unfolding the electrode-bound MB-tagged DNA in 100 mM ethylenediaminetetraacetic acid, and it could be regenerated with no loss in signal gain upon subsequent silver(I) ion binding. We also demonstrated that by controlling the probe packing density on the electrode surface, the fabrication parameters can be varied to achieve optimal sensor performance.

A Water-Soluble Fluorescent Probe for the Selective Sensing of Ag+ and its Application in Imaging of Living Cells and Nematodes

In this study, an imidazole-coumarin based fluorescent probe was developed for the selective and sensitive detection of Ag⁺ in aqueous solution. Using a combination of Job plot, NMR titrations, and DFT calculations, the binding properties between Ag⁺ and the probe were deeply investigated, and the results revealed a 1:1 binding stoichiometry between the probe and Ag⁺ with a binding constant of 1.02 × 10⁶ M^-1. The detection limit was found to be 150 nM, which satisfies the requirement for the quantitative detection of Ag⁺ in real water samples. Moreover, the new probe, Ic, was successfully applied to sense Ag⁺ in HeLa and HepG2 cells as well as in C. elegans, indicating that it could be a useful tool for the environmental monitoring of Ag⁺ pollution. These results demonstrated that Ic could serve as a high-efficiency and low-cost fluorescent probe for tracking Ag⁺ in an aquatic environment and biological organisms.

Silver nanowire inks for direct-write electronic tattoo applications

Room-temperature printing of conductive traces has the potential to facilitate the direct writing of electronic tattoos and other medical devices onto biological tissue, such as human skin. However, in order to achieve sufficient electrical performance, the vast majority of conductive inks require biologically harmful post-processing techniques. In addition, most printed conductive traces will degrade with bending stresses that occur from everyday movement. In this work, water-based inks consisting of high aspect ratio silver nanowires are shown to enable the printing of conductive traces at low temperatures and without harmful post-processing. Moreover, the traces produced from these inks retain high electrical performance, even while undergoing up to 50% bending strain and cyclic bending strain over a thousand bending cycles. This ink has a rapid dry time of less than 2 minutes, which is imperative for applications requiring the direct writing of electronics on sensitive surfaces. Demonstrations of conductive traces printed onto soft, nonplanar materials, including an apple and a human finger, highlight the utility of these new silver nanowire inks. These mechanically robust films are ideally suited for printing directly on biological substrates and may find potential applications in the direct-write printing of electronic tattoos and other biomedical devices.

Highly oxidation-resistant silver nanowires by C x F y polymers using plasma treatment

We demonstrate plasma-treated Ag nanowires (NWs) as flexible transparent electrode materials with enhanced long-term stability against oxidation even in a high humidity environment (80% humidity, 20 °C). Through a simple fluorocarbon (C₄F₈ or C₄F₆) plasma treatment method, a C _x F _y protective polymer was sufficiently cross-linked and attached on the surface of the AgNWs strongly and uniformly. Even though C₄F₈ and C₄F₆ activate differently on the AgNW surface due to the different dissociated radicals formed in the plasma, it was found that the C _x F _y protective polymers obtained by both chemicals work similarly as a protective layer for transparent conductive electrodes; a nearly constant sheet resistance ratio (R _s/R _o) of 1.6 was found for AgNWs treated with C₄F₈ and C₄F₆ plasmas, while the AgNWs without the plasma treatment exhibited a ratio of 176.2 after 36 days in a harsh environment. It is believed that the fluorocarbon plasma treatment can be used as a key method for ensuring long-term oxidation stability in numerous electronic applications including flexible solar cells utilizing various types of metallic nanowires.

Binding Conductive Ink Initiatively and Strongly: Transparent and Thermally Stable Cellulose Nanopaper as a Promising Substrate for Flexible Electronics

For flexible electronics, the substrates play key roles in ensuring their performance. However, most substrates suffer from weak bonding with the conductive ink and need additional aids. Here, inspired by the Ag-S bond theory, a novel cellulose nanopaper substrate is presented to improve the bond strength with the Ag nanoparticle ink through a facile printing method. The substrate is fabricated using thiol-modified nanofibrillated cellulose and exhibits excellent optical properties (∼85%@550 nm), ultra-small surface roughness (3.47 nm), and high thermal dimensional stability (up to at least 90 °C). Most importantly, it can attract Ag nanoparticles initiatively and bind them firmly, which enable the conductive ink to be printed without using the ink binder and form a strong substrate-ink bonding and maintain a stable conductivity of 2 × 10^-4 Ω cm even after extensive peeling and bending. This work may lead to exploring new opportunities to fabricate high-performance flexible electronics using the newly developed nanopaper substrate.

The direct-writing of low cost paper based flexible electrodes and touch pad devices using silver nano-ink and ZnO nanoparticles

We report a novel and simple approach for the synthesis of silver nanoparticles capped with inositol (Ag NPs/Ino) by the reduction of silver salt with ascorbic acid under basic conditions. UV-vis, TEM, FTIR and TGA techniques were used to characterize the Ag NPs/Ino to determine the size, shape and surface modification of the NPs. Stable silver nano-ink was prepared in aqueous solution containing 1% PVP (stabilizer) and glycerol (cosolvent) and was used for the direct-writing of a paper electrode with a roller ball-point pen for electrochemical applications. The solvent, stabilizing agents, concentration of NPs (10%), paper substrate, sintering temperature (40 °C) and sintering time (15 min) were optimized to obtain a uniform coating of Ag NPs on the paper substrate. Further, the synthesis and fabrication of ZnO NPs on a paper substrate was put forward to design a touch pad device based on the piezoelectric effect. The preparation of paper based devices suggests a direction for the development of a simple, low cost and compatible approach for the direct-writing of paper based flexible electrodes and electronics for future applications.

Flexible Electronics Based on Micro/Nanostructured Paper

Over the past several years, a new surge of interest in paper electronics has arisen due to the numerous merits of simple micro/nanostructured substrates. Herein, the latest advances and principal issues in the design and fabrication of paper-based flexible electronics are highlighted. Following an introduction of the fascinating properties of paper matrixes, the construction of paper substrates from diverse functional materials for flexible electronics and their underlying principles are described. Then, notable progress related to the development of versatile electronic devices is discussed. Finally, future opportunities and the remaining challenges are examined. It is envisioned that more design concepts, working principles, and advanced papermaking techniques will be developed in the near future for the advanced functionalization of paper, paving the way for the mass production and commercial applications of flexible paper-based electronic devices.

Printed Thin-Film Transistors: Research from China

Thin-film transistors (TFTs) have experienced tremendous development during the past decades and show great promising applications in flat displays, sensors, radio frequency identification tags, logic circuit, and so on. The printed TFTs are the key components for rapid development and commercialization of printed electronics. The researchers in China play important roles to accelerate the development and commercialization of printed TFTs. In this review, we comprehensively summarize the research progress of printed TFTs on rigid and flexible substrates from China. The review will focus on printing techniques of TFTs, printed TFT components including semiconductors, dielectrics and electrodes, as well as fully printed TFTs and printed flexible TFTs. Furthermore, perspectives on the remaining challenges and future developments are proposed.

A reversible fluorescent probe for monitoring Ag(I) ions

Silver-containing nanomaterials are of interest for their antibiotic properties, for a wide range of applications from medicine to consumer products. However, much remains to be learnt about the degradation of such materials and their effects on human health. While most analyses involve measurement of total silver levels, it is important also to be able to measure concentrations of active free Ag(I) ions. We report here the preparation of a coumarin-based probe, thiocoumarin silver sensor 1 (TcAg1), that responds reversibly to the addition of silver ions through the appearance of a new fluorescence emission peak at 565 nm. Importantly, this peak is not observed in the presence of Hg(II), a common interferent in Ag(I) sensing. To establish the utility of this sensor, we prepared silver-doped phosphate glasses with demonstrated bactericidal properties, and observed the Ag(I) release from these glasses in solutions of different ionic strength. TcAg1 is therefore a useful tool for the study of the environmental and medical effects of silver-containing materials.

Conductivity enhancement of silver nanowire networks via simple electrolyte solution treatment and solvent washing

As a promising replacement material for indium tin oxide in flexible electronics, silver nanowires (AgNWs) usually need complicated post-treatment to reduce the high contact resistance across the intersections when used as transparent conductive films. In this work, a widely applicable nano-joining method for improving the overall conductivity of AgNW networks with different kinds of electrolyte solutions is presented. By treatment with an electrolyte solution with appropriate ionic strengths, the insulating surfactant layer (polyvinylpyrrolidone, PVP) on the AgNWs could be desorbed, and the AgNW network could be densified. The sheet resistance of the AgNW film on a glass slide is reduced by 60.9% (from 67.5 to 26.4 Ohm sq^-1) with a transmittance of 92.5%. High-resolution transmission electron microscopy analysis indicates that atomic diffusion occurs at the intersection of two AgNWs. Thus, metallurgical bonding on the nanometer scale is achieved across the junctions of the AgNWs, leading to a significant enhancement in the conductivity of the AgNW network.

In Situ and Real-Time Studies, via Synchrotron X-ray Scattering, of the Orientational Order of Cellulose Nanocrystals during Solution Shearing

In this manuscript, we report on the ordering of the cellulose nanocrystals (CNCs) as they experience shear forces during the casting process. To achieve these measurements, in situ and in real time, we used synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAX). We believe that the GIWAX technique, although not commonly used to probe these types of phenomena, can open new avenues to gain deeper insights into film formation processes and surface-driven phenomena. In particular, we investigated the influence of solution concentration, shear-cast velocity, and drying temperature on the ordering of cellulose nanocrystals (CNCs) using GIWAXS. The films were prepared from aqueous suspensions of cellulose nanocrystals at two concentration values (7 and 9 wt %). As the films were cast, the X-ray beam was focused on a fixed position and GIWAXS patterns were recorded at regular time intervals. Structural characterization of the dry films was carried out via polarized optical microscopy and scanning electron microscopy. In addition, a rheological study of the CNC suspensions was performed. Our results show that the morphology of the CNC films was significantly influenced by shear velocity, concentration of the precursor suspension, and evaporation temperature. In contrast, we observed that the orientation parameter of the films was not significantly affected. The scattering intensity of the peak (200) was analyzed as a function of time, following a sigmoidal profile, hence indicating short- and long-range interactions within the anisotropic domains as they reached their final orientation state. A model capable of describing the resulting film morphologies is also proposed. The results and analysis presented in this manuscript provide new insights into the controlled alignment of cellulose nanocrystals under shear. This controlled alignment has significant implications in the development of advanced coatings and films currently used in a myriad of applications, such as catalysis, optics, electronics, and biomedicine.

Laser-Printed In-Plane Micro-Supercapacitors: From Symmetric to Asymmetric Structure

Here, we propose and demonstrate a complete solution for efficiently fabricating in-plane micro-supercapacitors (MSCs) from a symmetric to asymmetric structure. By using an original laser printing process, symmetric MSC with reduced graphene oxide (rGO)/silver nanowire (Ag-NW) hybrid electrodes was facilely fabricated and a high areal capacitance of 5.5 mF cm^-2 was achieved, which reaches the best reports on graphene-based MSCs. More importantly, a "print-and-fold" method has been creatively proposed that enabled the rapid manufacturing of asymmetric in-plane MSCs beyond the traditional cumbersome technologies. α-Ni(OH)₂ particles with high tapping density were successfully synthesized and employed as the pseudocapacitive material. Consequently, an improved supply voltage of 1.5 V was obtained and an areal capacitance as high as 8.6 mF cm^-2 has been realized. Moreover, a demonstration of a miniaturized MSC pack was performed by multiply-folding the serial Ag-NW-connected MSC units. As a result, a compact MSC pack with a high supply voltage of 3 V was obtained, which can be utilized to power a light-emitting diode light. These presented technologies may pave the way for the efficiently producing high performance in-plane MSCs, meanwhile offering a solution for the achievement of practical power supply packs integrated in limited spaces.

Fused silver nanowires with silica sol nanoparticles for smooth, flexible, electrically conductive and highly stable transparent electrodes

AgNWs-silica nanoparticles composite TCE with smooth surface and superior opto-electrical properties has been manufactured via AgNW-silica sol composite ink coating on PET through Mayer rod method, which is a promising alternative to ITO films.

The Preparation of Ag Nanoparticle and Ink Used for Inkjet Printing of Paper Based Conductive Patterns

Ag nanoparticles were successfully prepared using a liquid reduction method with a suitable mixture reductant of polyethylene glycol (PEG) and ethylene glycol (EG). OP-10 as a dispersing agent, was used to prepare the conductive Ag ink. Ag nanoparticles with an average particle size of 40 nm were prepared while the ratio of PEG to EG was 1:2. Meanwhile, the Ag particles had a narrow size distribution and great dispersion performance. The effects of paper substrates, sintering temperature, and sintering time on the conductivity of the printed Ag ink pattern were also studied. It was found that Lucky porous high glossy photo paper was a good candidate as the printing substrate. The resistivity of the printed pattern could reach 5.1 × 10⁻³ Ω·cm after heated at 100 °C for 2 h. Hence, the printed pattern showed good conductivity which led to the LED light being on. Furthermore, the Ag nanoparticle ink could be printed to form any pattern as required that still showed good electrical conductivity after being sintered under low-temperature. This could provide new possibilities for the preparation of flexible electrodes.

A paper-based touch sensor with an embedded micro-probe array fabricated by double-sided laser printing

Touch sensor is one of the key components for human interfacing devices. However, although various touch sensors have been demonstrated, their sophisticated fabrication processes and complicated structures make them expensive and delicate, and thus they are not considered to be practical for wide application in daily life. Herein, we present a low-cost and scalable paper-based touch sensor suitable for practical applications. The sensor is based on the novel structure of embedded silver nanowire micro-probe arrays in a paper substrate, which exhibits high sensitivity to multiple touch inputs and compact structure with a total thickness of ca. 100 μm. Silver nanowire electrodes on two sides are manufactured at the same time via an original double-sided laser printing technique. Since this technique is mask-free, solvent-free and highly efficient, it is very suitable for paper substrates that cannot endure solvent processing. The sensing properties of the sensor in various extreme situations are examined and the spatial distributions of touch pressure are detected by arranging the sensing units in arrays. Demonstration examples of the touch sensor and pressure mapping are presented, and finally, the successful application of the sensor array in an electronic lock system is shown to further illustrate its applicability.

Engineering Silver Nanowire Networks: From Transparent Electrodes to Resistive Switching Devices

Metal nanowires (NWs) networks with high conductance have shown potential applications in modern electronic components, especially the transparent electrodes over the past decade. In metal NW networks, the electrical connectivity of nanoscale NW junction can be modulated for various applications. In this work, silver nanowire (Ag NW) networks were selected to achieve the desired functions. The Ag NWs were first synthesized by a classic polyol process, and spin-coated on glass to fabricate transparent electrodes. The as-fabricated electrode showed a sheet resistance of 7.158 Ω □^-1 with an optical transmittance of 79.19% at 550 nm, indicating a comparable figure of merit (FOM, or Φ_TC) (13.55 × 10^-3 Ω^-1). Then, two different post-treatments were designed to tune the Ag NWs for not only transparent electrode but also for threshold resistive switching (RS) application. On the one hand, the Ag NW film was mechanically pressed to significantly improve the conductance by reducing the junction resistance. On the other hand, an Ag@AgO_x core-shell structure was deliberately designed by partial oxidation of Ag NWs through simple ultraviolet (UV)-ozone treatment. The Ag core can act as metallic interconnect and the insulating AgO_x shell acts as a switching medium to provide a conductive pathway for Ag filament migration. By fabricating Ag/Ag@AgO_x/Ag planar structure, a volatile threshold switching characteristic was observed and an on/off ratio of ∼100 was achieved. This work showed that through different post-treatments, Ag NW network can be engineered for diverse functions, transforming from transparent electrodes to RS devices.

Growth of a Large-Area, Free-Standing, Highly Conductive and Fully Foldable Silver Film with Inverted Pyramids for Wearable Electronics Applications

A promising new concept is the application of flexible and foldable conductive film or paper for wearable electronics, in which silver nanowires, carbon nanotubes, and graphene are primarily used as conductive materials. However, their insufficient nanostructure contacts lead to poor electrical conductivity and mechanical fracture. Here, we demonstrate a simple and innovative strategy for fabricating a free-standing silver film with inverted pyramids by replicating pyramids on a textured silicon wafer under a hydrothermal reaction. In this unique structure, the inverted pyramids on the film surface can provide sufficient buffer space for a mechanically foldable and unfoldable cushion, and the continuous film ensures an uninterrupted electron transport pathway. As a result, the silver film with inverted pyramids can exhibit extremely high conductivity, with a sheet resistance as low as 2.55 × 10^-3 Ω/sq, corresponding to an electrical conductivity of 4.2 × 10⁵ S cm^-1 for a 9.2-μm-thick film (67.7% of bulk silver's conductivity). Surprisingly, this film has outstanding mechanical folding stability, with less than a 0.5% deviation from the initial resistance after 35,000 repetitive folding and unfolding cycles when tested at the folding site. The film is free-standing, thin, flexible, foldable, and suitable for cutting and patterned growth, which makes it suitable for wearable electronics, showing a much wider range of applications than substrate-based ones.

One-Step Fabrication of 3D Nanohierarchical Nickel Nanomace Array To Sinter with Silver NPs and the Interfacial Analysis

Three-dimensional (3D) nanohierarchical Ni nanomace (Ni NM) array was fabricated on copper substrate by only one step with electroplating method, the unique structure was covered with Au film (Ni/Au NM) without changing its morphology, and in the following step, it was sintered with silver nanoparticle (Ag NP) paste. The structure of the Ni NM array and its surface morphology were characterized by X-ray diffraction, scanning electron microscope (SEM), and atomic force microscope. The sintered interface was investigated by SEM, transmission electron microscopy, and energy-dispersive X-ray spectroscopy to analyze the sintering mechanism. The results showed that a metallurgical bond was successfully achieved at 250 °C without any gas or vacuum shield and extra pressure. The Cu substrate with Ni/Au NM array was able to join with the Ag NP paste without obvious voids. Due to the compatible chemical potential between Ag NPs and Ni/Au NM array, the Au element was able to diffuse into the Ag layer with about 800 nm distance. Based on the excellent 3D nanohierarchical structure, the shear strength of Ni/Au NM array was 6 times stronger than the flat Ni/Au coated substrate. It turned out that the substrate surface played a crucial role in improving the shear strength and sintering efficiency. The 3D Ni NM array had achieved an excellent bonding interface and had great potential application in the microelectronics packaging field.

Paper microchip with a graphene-modified silver nano-composite electrode for electrical sensing of microbial pathogens

Rapid and sensitive point-of-care diagnostics are of paramount importance for early detection of infectious diseases and timely initiation of treatment. Here, we present cellulose paper and flexible plastic chips with printed graphene-modified silver electrodes as universal point-of-care diagnostic tools for the rapid and sensitive detection of microbial pathogens or nucleic acids through utilizing electrical sensing modality and loop-mediated isothermal amplification (LAMP). We evaluated the ability of the developed paper-based assay to detect (i) viruses on cellulose-based paper microchips without implementing amplification in samples with viral loads between 106 and 108 copies per ml, and (ii) amplified HIV-1 nucleic acids in samples with viral loads between 10 fg μl-1 and 108 fg μl-1. The target HIV-1 nucleic acid was amplified using the RT-LAMP technique and detected through the electrical sensing of LAMP amplicons for a broad range of RNA concentrations between 10 fg μl-1 and 108 fg μl-1 after 40 min of amplification time. Our assay may be used for antiretroviral therapy monitoring where it meets the sensitivity requirement of the World Health Organization guidelines. Such a paper microchip assay without the amplification step may also be considered as a simple and inexpensive approach for acute HIV detection where maximum viral replication occurs.

Formulation of concentrated and stable ink of silver nanowires with applications in transparent conductive films

Concentrated and long-term stable Ag nanowire ink is formulated to coat transparent conductive films with superior comprehensive performance after simple cleaning.

Seeds triggered massive synthesis and multi-step room temperature post-processing of silver nanoink—application for paper electronics

Ink synthesis, room-temperature post-processing and applications for flexible 3D paper electronics.

Rapid Laser Printing of Paper-Based Multilayer Circuits

Laser printing has been widely used in daily life, and the fabricating process is highly efficient and mask-free. Here we propose a laser printing process for the rapid fabrication of paper-based multilayer circuits. It does not require wetting of the paper, which is more competitive in manufacturing paper-based circuits compared to conventional liquid printing process. In the laser printed circuits, silver nanowires (Ag-NWs) are used as conducting material for their excellent electrical and mechanical properties. By repeating the printing process, multilayer three-dimensional (3D) structured circuits can be obtained, which is quite significant for complex circuit applications. In particular, the performance of the printed circuits can be exactly controlled by varying the process parameters including Ag-NW content and laminating temperature, which offers a great opportunity for rapid prototyping of customized products with designed properties. A paper-based high-frequency radio frequency identification (RFID) label with optimized performance is successfully demonstrated. By adjusting the laminating temperature to 180 °C and the top-layer Ag-NW areal density to 0.3 mg cm(-2), the printed RFID antenna can be conjugately matched with the chip, and a big reading range of ∼12.3 cm with about 2.0 cm over that of the commercial etched Al antenna is achieved. This work provides a promising approach for fast and quality-controlled fabrication of multilayer circuits on common paper and may be enlightening for development of paper-based devices.

Substrateless Welding of Self-Assembled Silver Nanowires at Air/Water Interface

Integrating connected silver nanowire networks with flexible polymers has appeared as a popular way to prepare flexible electronics. To reduce the contact resistance and enhance the connectivity between silver nanowires, various welding techniques have been developed. Herein, rather than welding on solid supporting substrates, which often requires complicated transferring operations and also may pose damage to heat-sensitive substrates, we report an alternative approach to prepare easily transferrable conductive networks through welding of self-assembled silver nanowires at the air/water interface using plasmonic heating. The intriguing welding behavior of partially aligned silver nanowires was analyzed with combined experimental observation and theoretical modeling. The underlying water not only physically supports the assembled silver nanowires but also buffers potential overheating during the welding process, thereby enabling effective welding within a broad range of illumination power density and illumination duration. The welded networks could be directly integrated with PDMS substrates to prepare high-performance stable flexible heaters that are stretchable, bendable, and can be easily patterned to explore selective heating applications.

Extremely Robust and Patternable Electrodes for Copy-Paper-Based Electronics

We propose a fabrication process for extremely robust and easily patternable silver nanowire (AgNW) electrodes on paper. Using an auxiliary donor layer and a simple laminating process, AgNWs can be easily transferred to copy paper as well as various other substrates using a dry process. Intercalating a polymeric binder between the AgNWs and the substrate through a simple printing technique enhances adhesion, not only guaranteeing high foldability of the electrodes, but also facilitating selective patterning of the AgNWs. Using the proposed process, extremely crease-tolerant electronics based on copy paper can be fabricated, such as a printed circuit board for a 7-segment display, portable heater, and capacitive touch sensor, demonstrating the applicability of the AgNWs-based electrodes to paper electronics.

Characterisation of graphene fibres and graphene coated fibres using capacitively coupled contactless conductivity detector

The use of capacitively coupled contactless conductivity detection (C⁴D) for the characterisation of thin conductive graphene fibres, graphene composite fibres, and graphene coated fibrous materials is demonstrated for the first time.

Electron beam irradiated silver nanowires for a highly transparent heater

Transparent heaters have attracted increasing attention for their usefulness in vehicle windows, outdoor displays, and periscopes. We present high performance transparent heaters based on Ag nanowires with electron beam irradiation. We obtained an Ag-nanowire thin film with 48 ohm/sq of sheet resistance and 88.8% (substrate included) transmittance at 550 nm after electron beam irradiation for 120 sec. We demonstrate that the electron beam creates nano-soldering at the junctions of the Ag nanowires, which produces lower sheet resistance and improved adhesion of the Ag nanowires. We fabricated a transparent heater with Ag nanowires after electron beam irradiation, and obtained a temperature of 51 °C within 1 min at an applied voltage of 7 V. The presented technique will be useful in a wide range of applications for transparent heaters.

Wearable Electronics of Silver-Nanowire/Poly(dimethylsiloxane) Nanocomposite for Smart Clothing

Wearable electronics used in smart clothing for healthcare monitoring or personalized identification is a new and fast-growing research topic. The challenge is that the electronics has to be simultaneously highly stretchable, mechanically robust and water-washable, which is unreachable for traditional electronics or previously reported stretchable electronics. Herein we report the wearable electronics of sliver nanowire (Ag-NW)/poly(dimethylsiloxane) (PDMS) nanocomposite which can meet the above multiple requirements. The electronics of Ag-NW/PDMS nanocomposite films is successfully fabricated by an original pre-straining and post-embedding (PSPE) process. The composite film shows a very high conductivity of 1.52 × 10(4) S cm(-1) and an excellent electrical stability with a small resistance fluctuation under a large stretching strain. Meanwhile, it shows a robust adhesion between the Ag-NWs and the PDMS substrate and can be directly machine-washed. These advantages make it a competitive candidate as wearable electronics for smart clothing applications.

Highly Stable and Sensitive Paper-Based Bending Sensor Using Silver Nanowires/Layered Double Hydroxides Hybrids

Highly sensitive flexible piezoresistive materials using silver nanowires (AgNWs) composites have been widely researched due to their excellent electrical, optical, and mechanical properties. Intrinsically, AgNWs tend to aggregate in polymer matrix because of the intense depletion-induced interactions, which seriously influence the percolation threshold of the composites. In this study, we report a highly stable and sensitive paper-based bending sensor using the AgNWs and layered double hydroxides (LDHs) to construct a hybrid conductive network in waterborne polyurethane that is easy to destruct and reconstruct under bending deformation. The nonconductive 2D LDH nanosheets are embedded into AgNWs network and assist dispersion of AgNWs, which depends on the hydrogen bonding between the two nanostructures. The percolation threshold of the composites decreases from 10.8 vol % (55 wt %) to 3.1 vol % (23.8 wt %), and the composites reaches a very low resistivity (10(-4) Ω·cm) with a small amount of AgNWs (8.3 vol %) due to the dispersion improvement of AgNWs with the effect of LDH nanosheets. The as-prepared conductive composites with low percolation threshold can be manufactured on paper via various methods such as rollerball pen writing, inkjet printing, or screen printing. The bending sensor prepared by manufacturing the composites on paper shows low-cost, excellent conductivity, flexibility (>3000 bending cycles), sensitivity (0.16 rad(-1)), fast response (120 ms) and relaxation time (105 ms), and nontoxicity. Therefore, a simple but efficient wearable sensor is developed to monitor the human motions (such as fingers and elbow joints movements) and presents good repeatability, stability, and responsiveness, making the bending sensor possibly able to meet the needs in numerous applications for robotic systems.

Robust Ag nanoplate ink for flexible electronics packaging

Nanoinks are currently a topic of heightened interest with respect to low temperature bonding processes and printable electronics. We have developed an innovative polyvinylpyrrolidone (PVP)-stabilized Ag nanoplate ink amenable to very strong low temperature packaging, and investigated the relationship between bonding strength and electrical conductivity post-bonding. PVP shell plastic deformations observed in failure microcracks with the formation of PVP nanofibers, revealed bonding strength at low temperatures (<250 °C) was primarily due to adhesive bonding. It is found that, utilizing photonic sintering, ∼ 70 °C reduction of transformation temperature from adhesive to metallic bonding was achieved compared to that of thermal sintering. A numerical simulation was developed to better understand the influences of the light-induced heat generation, which demonstrated near-infrared light can facilitate sintering. Bonding strengths of 27 MPa were achieved at room temperatures, and 29.4 MPa at 210 °C with photonic sintering. Moreover, the anisotropic resistivity was observed with different thermal dependences. These results demonstrate Ag nanoplate inks have potential for low temperature 3D interconnections in lead-free microcircuits, flexible electronic packaging, and diverse sensing applications.

Ternary Ag/epoxy adhesive with excellent overall performance

Excellent electrical conductivity (EC) generally conflicts with high lap shear strength (LSS) for electrically conductive adhesives (ECAs) since EC increases while LSS decreases with increasing conductive filler content. In this work, the ECAs with the excellent overall performance are developed based on the ternary hybrid of Ag microflakes (Ag-MFs), Ag nanospheres (Ag-NSs), and Ag nanowires (Ag-NWs). First, a low silver content adhesive system is determined. Then, the effects of the relative contents of Ag fillers on the EC and the LSS are studied. It is shown that a small amount of Ag-NSs or Ag-NWs can dramatically improve the EC for the Ag-MF/epoxy adhesives. The Ag-NSs and Ag-NWs with appropriate contents have a synergistic effect in improving the EC. Meanwhile, the LSS of the as-prepared adhesive with the appropriate Ag contents reaches an optimal value. Both the EC and the LSS of the as-prepared ternary hybrid ECA with a low content of 40 wt % Ag are higher than those of the commercial ECAs filled with the Ag-MF content over 60 wt %. Finally, the ternary hybrid ECA with the optimal formulation is shown to be promising for printing the radio frequency identification tag antennas as an immediate application example.

Electrochemical detection of aqueous Ag+ based on Ag+-assisted ligation reaction

In this work, a novel strategy to fabricate a highly sensitive and selective biosensor for the detection of Ag(+) is proposed. Two DNA probes are designed and modified on a gold electrode surface by gold-sulfur chemistry and hybridization. In the presence of Ag(+), cytosine-Ag(+)-cytosine composite forms and facilitates the ligation event on the electrode surface, which can block the release of electrochemical signals labeled on one of the two DNA probes during denaturation process. Ag(+) can be sensitively detected with the detection limit of 0.1 nM, which is much lower than the toxicity level defined by U.S. Environmental Protection Agency. This biosensor can easily distinguish Ag(+) from other interfering ions and the performances in real water samples are also satisfactory. Moreover, the two DNA probes are designed to contain the recognition sequences of a nicking endonuclease, and the ligated DNA can thus be cleaved at the original site. Therefore, the electrode can be regenerated, which allows the biosensor to be reused for additional tests.

Recent developments and directions in printed nanomaterials

In this review, we survey several recent developments in printing of nanomaterials for contacts, transistors, sensors of various kinds, light-emitting diodes, solar cells, memory devices, and bone and organ implants. The commonly used nanomaterials are classified according to whether they are conductive, semiconducting/insulating or biological in nature. While many printing processes are covered, special attention is paid to inkjet printing and roll-to-roll printing in light of their complexity and popularity. In conclusion, we present our view of the future development of this field.

Direct-writing of circuit interconnects on cellulose paper using ultra-long, silver nanowires based conducting ink

A highly stable conducting nanoink based on silver ultra-long nanowires (Ag ULNWs) was developed by a self-seeding polyol method with controlled doping of silver acetate for flexible electronics applications.

Direct writing on paper of foldable capacitive touch pads with silver nanowire inks

Paper-based capacitive touch pads can be fabricated utilizing high-concentration silver nanowire inks needle-printed directly onto paper substrates through a 2D programmable platform. Post deposition, silver nanowire tracks can be photonically sintered using a camera flash to reduce sheet resistance similar to thermal sintering approaches. Touch pad sensors on a variety of paper substrates can be achieved with optimized silver nanowire tracks. Rolling and folding trials, which yielded only modest changes in capacitance and no loss of function, coupled with touch pad functionality on curved surfaces, suggest sufficient flexibility and durability for paper substrate touch pads to be used in diverse applications. A simplified model to predict touch pad capacitance variation ranges with differing touch conditions was developed, with good agreement against experimental results. Such paper-based touch pads have the advantage of simple structure, easy fabrication, and fast sintering, which holds promise for numerous commercial applications including low-cost portable devices where ultrathin and lightweight features, coupled with reliable bending stability are desirable.

High performance of carbon nanotubes/silver nanowires-PET hybrid flexible transparent conductive films via facile pressing-transfer technique

To obtain low sheet resistance, high optical transmittance, small open spaces in conductive networks, and enhanced adhesion of flexible transparent conductive films, a carbon nanotube (CNT)/silver nanowire (AgNW)-PET hybrid film was fabricated by mechanical pressing-transfer process at room temperature. The morphology and structure were characterized by scanning electron microscope (SEM) and atomic force microscope (AFM), the optical transmittance and sheet resistance were tested by ultraviolet-visible spectroscopy (UV-vis) spectrophotometer and four-point probe technique, and the adhesion was also measured by 3M sticky tape. The results indicate that in this hybrid nanostructure, AgNWs form the main conductive networks and CNTs as assistant conductive networks are filled in the open spaces of AgNWs networks. The sheet resistance of the hybrid films can reach approximately 20.9 to 53.9 Ω/□ with the optical transmittance of approximately 84% to 91%. The second mechanical pressing step can greatly reduce the surface roughness of the hybrid film and enhance the adhesion force between CNTs, AgNWs, and PET substrate. This process is hopeful for large-scale production of high-end flexible transparent conductive films.