Dual Surface Architectonics for Directed Self-Assembly of Ultrahigh-Resolution Electronics.

The directed self-assembly of electronic circuits using functional metallic inks has attracted intensive attention because of its high compatibility with extensive applications ranging from soft printed circuits to wearable devices. However, the typical resolution of conventional self-assembly technologies is not sufficient for practical applications in the rapidly evolving additively manufactured electronics (AMEs) market. Herein, an ultrahigh-resolution self-assembly strategy is reported based on a dual-surface-architectonics (DSA) process. Inspired by the Tokay gecko, the approach is to endow submicrometer-scale surface regions with strong adhesion force toward metallic inks via a series of photoirradiation and chemical polarization treatments. The prepared DSA surface enables the directed self-assembly of electronic circuits with unprecedented 600 nm resolution, suppresses the coffee-ring effect, and results in a reliable conductivity of 14.1 ± 0.6 µΩ cm. Furthermore, the DSA process enables the layer-by-layer fabrication of fully printed organic thin-film transistors with a short channel length of 1 µm, which results in a large on-off ratio of 106 and a high field-effect mobility of 0.5 cm2  V-1  s-1 .

Large-Scale and Galvanic Replacement Free Synthesis of Cu@Ag Core-Shell Nanowires for Flexible Electronics.

Copper nanowires (CuNWs) are considered a promising alternative to indium tin oxide due to their cost-effectiveness as well as high conductivity and transparency. However, the practical applications of copper-based conductors are greatly limited due to their rapid oxidation in atmosphere. Herein, a facile adsorption and decomposition process is developed for galvanic replacement free and large-scale synthesis of highly stable Cu@Ag core-shell nanowires. First, Ag-amine complex ([Ag(NH2R)2]+) as silver source adsorbs on CuNWs surface, and Cu@Ag-amine complex core-shell structure is formed. After that, Ag-amine complex is easily decomposed to pure Ag shell through a simple thermal annealing under air. By adjusting the concentration of Ag-aminein CuNWs solution, Cu@Ag core-shell nanowires with different thickness of silver shell can be easily obtained. The obtained core-shell nanowires exhibit high stability for at least 500 h at high temperature (140 °C) and high humidity (85 °C, 85% RH) due to the protection of Ag shell. More importantly, the conductivity and transparency of Cu@Ag nanowires-based conductors is similar to that of pure CuNWs. The large-scale and facile synthesis of Cu@Ag core-shell nanowires provides a new method to prepare stable metallic core-shell nanowires.

Three-Dimensional Stretchable and Transparent Conductors with Controllable Strain-Distribution Based on Template-Assisted Transfer Printing.

Although stretchable transparent conductors, stemmed from the strategies of both conductive composite and structural design of nonstretchable conductors, have been extensively studied, these conductors either suffer from low stretchability or require a complex fabrication process, which drastically limits their practical applications. Here, we propose a novel strategy combining the design of substrates and a simple template-assisted transfer printing process to fabricate three-dimensional (3D) transparent conductors. The strategy not only eliminates the complex and costly fabrication processes but it also endows conductors with high stretchability and long-term stability, thanks to the controllable strain distribution as well as the seamless connection between the conductor layer and the substrate. These newly designed 3D conductors achieve a low sheet resistance of 1.0 Ω/sq with a high transmittance of above 85% and remain stable without obvious resistance change during 1000 stretching-relaxation cycles until 60% strain, which are superior to most reported conductors. A large-area stretchable heater based on the 3D conductor realizes the temperature fluctuation below 10% even under a large strain, thus showing huge application prospects in the field of wearable healthcare electronics. The simple solution-processed fabrication method and high performance such as stretchability and low resistance change over a large strain range promote the practical applications of these newly designed 3D conductors.

Highly Conductive Ag Paste for Recoverable Wiring and Reliable Bonding Used in Stretchable Electronics.

Stretchable wiring and stretchable bonding between a rigid chip/component and a stretchable substrate are two key factors for stretchable electronics. In this study, a highly conductive stretchable paste has been developed with commercial Ag microflakes and poly(dimethylsiloxane), which can be used to fabricate stretchable wirings and bondings under a low curing temperature of 100 °C with printing method. Herein, recoverabilities as to recovery time and recovery resistance of the wirings are defined and discussed. The effect of Ag composition and the tensile strain rate on the recoverability of the wirings are also examined. The wiring with a low resistivity of 8.7 × 10-5 Ω cm shows much better recoverability than nanowire-based wirings due to the flake nature of the Ag particles. When stretched to 50 and 100% of strain, the resistance of the patterned wiring increases by only 10 and 110%, respectively. Moreover, the resistance of the wiring during 20% tensile cyclic test remains within 1.1 times even after 1000 cycles, thus demonstrating significant durability. The paste was utilized to fabricate conductive tracks and stretchable bondings to assemble a rigid chip to fabricate a stretchable demo. When stretched to 50% of strain, resistance of the wiring was increased by 90%. It is anticipated that the newly developed paste will be used to fabricate various stretchable wirings, bondings, and packaging structures by a simple printing process, thus enabling mass production of stretchable electronic devices.

The Surface Coating of Commercial LiFePO4 by Utilizing ZIF-8 for High Electrochemical Performance Lithium Ion Battery.

The requirement of energy-storage equipment needs to develop the lithium ion battery (LIB) with high electrochemical performance. The surface modification of commercial LiFePO4 (LFP) by utilizing zeolitic imidazolate frameworks-8 (ZIF-8) offers new possibilities for commercial LFP with high electrochemical performances. In this work, the carbonized ZIF-8 (CZIF-8) was coated on the surface of LFP particles by the in situ growth and carbonization of ZIF-8. Transmission electron microscopy indicates that there is an approximate 10 nm coating layer with metal zinc and graphite-like carbon on the surface of LFP/CZIF-8 sample. The N2 adsorption and desorption isotherm suggests that the coating layer has uniform and simple connecting mesopores. As cathode material, LFP/CZIF-8 cathode-active material delivers a discharge specific capacity of 159.3 mAh g-1 at 0.1C and a discharge specific energy of 141.7 mWh g-1 after 200 cycles at 5.0C (the retention rate is approximate 99%). These results are attributed to the synergy improvement of the conductivity, the lithium ion diffusion coefficient, and the degree of freedom for volume change of LFP/CZIF-8 cathode. This work will contribute to the improvement of the cathode materials of commercial LIB.

Highly Stable Transparent Conductive Electrodes Based on Silver-Platinum Alloy-Walled Hollow Nanowires for Optoelectronic Devices.

In industrial manufacturing, alloying can contribute to the passivation of active metals and markedly improve their corrosion resistance. This inspires us to solve the current critical problem of Ag nanowires (Ag NWs) that have poor stability against chemical and electrochemical corrosion. These problems have seriously limited the applications of Ag NWs in optoelectronic devices where they are used for transparent conductive electrodes. Here, a kind of transparent conductive electrode based on Ag@Pt alloy-walled hollow nanowires (Ag@Pt AHNWs) is successfully fabricated by introducing 12 mol % Pt into long Ag NWs to form Ag@Pt alloy. The as-synthesized electrodes exhibit better optical transmittance (82% at the wavelength of 550 nm) under high electrical conductivity (28.73 Ω/sq-1), high thermal stability up to 400 °C for 11 h, and remarkable mechanical flexibility (remaining stable after 5000 cycles bending), as well as high resistance against chemical and electrochemical corrosion. The Ag@Pt AHNWs electrodes are further applied in a primary bifunctional polyaniline electrochemical device, and the device shows promising flexibility, noticeable multicolor performances, and high specific capacitance because of the remarkable mechanical flexibility and electrochemical stability of Ag@Pt AHNWs. This work will provide an optional approach for the preparation of other metal nanomaterial electrodes with high stability.

The comprehensive effects of visible light irradiation on silver nanowire transparent electrode.

The silver nanowire (AgNW) transparent electrode is one of the promising components for flexible electronics due to its high electrical and thermal conductivity, optical transparency and flexibility. However, the application of the AgNW electrode with an improved performance is generally limited by its poor long-term stability. As the name suggests, the transparent electrode is usually exposed to visible light in various applications. Unlike other electrode materials, AgNWs show unique and complicated behavior under long-term visible light illumination. In this study, the comprehensive effect of visible light irradiation on the AgNW transparent electrode is initially investigated in detail. Light irradiation induces the migration of silver to enhance the nanowire contacts while also leading to the generation and growth of particles and diameter loss in the nanowire. Light irradiation accelerates the sulfidation and oxidation of the AgNWs as well, resulting in the emergence of degradation products on the nanowire surface. All these effects influence the sheet resistance of the AgNW electrode during light illumination. The light-induced change of sheet resistance also relates to the nanowire concentration due to the sensitivity of the wire-wire contact resistance near the percolation threshold. It is believed that this work will be a valuable reference for the design, processing and application of transparent electrodes used in numerous optoelectronic devices.

Self-catalyzed copper-silver complex inks for low-cost fabrication of highly oxidation-resistant and conductive copper-silver hybrid tracks at a low temperature below 100 °C.

Cu-Ag complex inks are developed for printing conductive tracks of low cost, high stability, and high conductivity on heat-sensitive substrates such as polyethylene terephthalate (PET) substrates. The inks show an obvious self-catalyzed characteristic due to the in situ formation of fresh metal nanoparticles which promote rapid decomposition and sintering of the inks at a low temperature below 100 °C. The temperature is 40-60 °C lower than those of general Cu complex inks and 100-120 °C lower than those of general Cu/Ag particle inks. Highly conductive Cu-Ag tracks of 2.80 × 10-5 Ω cm and 6.40 × 10-5 Ω cm have been easily realized at 100 °C and 80 °C, respectively. In addition, the printed Cu-based tracks not only show high oxidation resistance at high temperatures of up to 140 °C (the maximum tolerable temperature of current PET substrate) but also show excellent stability at high humidity of 85% because of the very uniform Cu-Ag hybrid structure. The printable tracks exhibit great potential application in various wearable devices fabricated on textiles, papers, and other heat-sensitive substrates.

Highly conductive and transparent copper nanowire electrodes on surface coated flexible and heat-sensitive substrates.

Copper nanowire (CuNW) based flexible transparent electrodes have been extensively investigated due to their outstanding performances and low price. However, commonly used methods for processing CuNW transparent electrodes such as thermal annealing and photonic sintering inevitably damage the flexible substrates leading to low transmittance. Herein, a surface coating layer was demonstrated to protect the heat-sensitive polyethylene terephthalate (PET) polymer from being destroyed by the instantaneous high temperature during the photonic sintering process. The stable ceramic surface coating layer avoided the direct exposure of PET to intense light, further reduced the heat releasing to the bottom part of the PET, protecting the flexible PET base from destruction and ensuring high transparency for the CuNW transparent electrodes. A CuNW transparent electrode on surface coated PET (C-PET) substrates with a sheet resistance of 33 Ohm sq-1 and high transmittance of 82% has been successfully fabricated by the photonic sintering method using light intensity of 557 mJ cm-2 within several seconds in ambient conditions. The surface coating layers open a novel method to optimize the rapid photonic sintering technique for processing metal nanomaterials on heat-sensitive substrates.

Nanoridge patterns on polymeric film by a photodegradation copying method for metallic nanowire networks.

Topographical patterns are widely applied in many manufacturing areas due to the unique role in modifying performance related to physical, chemical and biological fundamentals. The patterns are usually realized by buckling or wrinkling, self-assembly or epitaxy, and lithography techniques. However, the combination of satisfactory controllability, ridge robustness, cost and dimensional precision is still difficult to achieve by any of the strategies above. A novel, simple and low-cost nanopatterning technique named "photodegradation copying method" with high technological flexibility has been initially proposed in this study. As a perfect example, a nanoridge-patterned surface has been successfully realized on a polymeric film thanks to the selective photodegradation of polymer and the shielding effect of silver nanowire (AgNW) networks. Roughness, wettability and transmittance of the polymeric film became simply and effectively controllable by adjusting the photodegradation time or the size and distribution of AgNWs. In addition, the ridge-patterned film could also be employed as a substrate in transfer printing for more flexible devices. Various topographical nanopatterns are expected to be simply realized by the photocopying method, just replacing nanowires with other masks like nanodisks, nanocubes, nanotriangles, and so on. This promising photocopying technique is believed to play an important role in the development of topographical nanopatterns, and enable more intriguing applications simply, flexibly and inexpensively.

Printable and Flexible Copper-Silver Alloy Electrodes with High Conductivity and Ultrahigh Oxidation Resistance.

Printable and flexible Cu-Ag alloy electrodes with high conductivity and ultrahigh oxidation resistance have been successfully fabricated by using a newly developed Cu-Ag hybrid ink and a simple fabrication process consisting of low-temperature precuring followed by rapid photonic sintering (LTRS). A special Ag nanoparticle shell on a Cu core structure is first created in situ by low-temperature precuring. An instantaneous photonic sintering can induce rapid mutual dissolution between the Cu core and the Ag nanoparticle shell so that core-shell structures consisting of a Cu-rich phase in the core and a Ag-rich phase in the shell (Cu-Ag alloy) can be obtained on flexible substrates. The resulting Cu-Ag alloy electrode has high conductivity (3.4 µΩ·cm) and ultrahigh oxidation resistance even up to 180 °C in an air atmosphere; this approach shows huge potential and is a tempting prospect for the fabrication of highly reliable and cost-effective printed electronic devices.

Biaxially stretchable silver nanowire conductive film embedded in a taro leaf-templated PDMS surface.

A biaxially wave-shaped polydimethylsiloxane (PDMS) surface was developed simply by using a taro leaf as the template. The resulting leaf-templated PDMS (L-PDMS) possesses a micro-sized curved interface structure, which is greatly beneficial for the exact embedding of a silver nanowire (AgNW) network conductive film covering the L-PDMS surface. The intrinsically curved AgNW/L-PDMS film surface, without any dangling nanowire, could prevent the fracture of AgNWs due to stretching stress even after cyclic stretching. More importantly, it also exhibited a biaxial stretchability, which showed ultra-stable resistance after continuous stretching for 100 cycles each in X- and Y-directions. This biaxially stretchable AgNW/L-PDMS film could extend the application fields in stretchable electronics.

Stretchable and transparent electrodes based on patterned silver nanowires by laser-induced forward transfer for non-contacted printing techniques.

Silver nanowires (AgNWs) are excellent candidate electrode materials in next-generation wearable devices due to their high flexibility and high conductivity. In particular, patterning techniques for AgNWs electrode manufacture are very important in the roll-to-roll printing process to achieve high throughput and special performance production. It is also essential to realize a non-contact mode patterning for devices in order to keep the pre-patterned components away from mechanical damages. Here, we report a successful non-contact patterning of AgNWs-based stretchable and transparent electrodes by laser-induced forward transfer (LIFT) technique. The technique was used to fabricate a 100% stretchable electrode with a width of 200 µm and electrical resistivity 10-4 Ωcm. Experiments conducted integrating the stretchable electrode on rubber substrate in which LED was pre-fabricated showed design flexibility resulting from non-contact printing. Further, a patterned transparent electrode showed over 80% in optical transmittance and less than 100 Ω sq-1 in sheet resistance by the optimized LIFT technique.

One-Step Fabrication of Stretchable Copper Nanowire Conductors by a Fast Photonic Sintering Technique and Its Application in Wearable Devices.

Copper nanowire (CuNW) conductors have been considered to have a promising perspective in the area of stretchable electronics due to the low price and high conductivity. However, the fabrication of CuNW conductors suffers from harsh conditions, such as high temperature, reducing atmosphere, and time-consuming transfer step. Here, a simple and rapid one-step photonic sintering technique was developed to fabricate stretchable CuNW conductors on polyurethane (PU) at room temperature in air environment. It was observed that CuNWs were instantaneously deoxidized, welded and simultaneously embedded into the soft surface of PU through the one-step photonic sintering technique, after which highly conductive network and strong adhesion between CuNWs and PU substrates were achieved. The CuNW/PU conductor with sheet resistance of 22.1 Ohm/sq and transmittance of 78% was achieved by the one-step photonic sintering technique within only 20 µs in air. Besides, the CuNW/PU conductor could remain a low sheet resistance even after 1000 cycles of stretching/releasing under 10% strain. Two flexible electronic devices, wearable sensor and glove-shaped heater, were fabricated using the stretchable CuNW/PU conductor, demonstrating that our CuNW/PU conductor could be integrated into various wearable electronic devices for applications in food, clothes, and medical supplies fields.

Fast fabrication of copper nanowire transparent electrodes by a high intensity pulsed light sintering technique in air.

Copper nanowire transparent electrodes have received increasing interest due to the low price and nearly equal electrical conductivity compared with other TEs based on silver nanowires and indium tin oxide (ITO). However, a post-treatment at high temperature in an inert atmosphere or a vacuum environment was necessary to improve the conductivity of Cu NW TEs due to the easy oxidation of copper in air atmosphere, which greatly cancelled out the low price advantage of Cu NWs. Here, a high intensity pulsed light technique was introduced to sinter and simultaneously deoxygenate these Cu NWs into a highly conductive network at room temperature in air. The strong light absorption capacity of Cu NWs enabled the welding of the nanowires at contact spots, as well as the removal of the thin layer of residual organic compounds, oxides and hydroxide of copper even in air. The Cu NW TE with a sheet resistance of 22.9 Ohm sq(-1) and a transparency of 81.8% at 550 nm has been successfully fabricated within only 6 milliseconds exposure treatment, which is superior to other films treated at high temperature in a hydrogen atmosphere. The HIPL process was simple, convenient and fast to fabricate easily oxidized Cu NW TEs in large scale in an air atmosphere, which will largely extend the application of cheap Cu NW TEs.

Highly Reliable Silver Nanowire Transparent Electrode Employing Selectively Patterned Barrier Shaped by Self-Masked Photolithography.

The transparent electrode based on silver nanowire (AgNW) networks is one promising alternative of indium tin oxide film in particular for advanced flexible and printable electronics. However, the widespread application of AgNW electrode is hindered by its poor long-term reliability. Although the reliability can be improved by applying traditional overcoating layer or the core-shell structure, the transmittance or conductivity is inevitably undermined. In this paper, a novel patterned barrier of photoresist in situ assembled on the nanowire surface realized the reliability enhancement by simply employing AgNWs themselves as the mask in the photolithography process. The patterned barrier selectively covered the nanowires, while keeping the high transmittance and conductivity unchanged and improving the adhesion of AgNW networks on substrate. After 720 h storage in 85 °C/85% relative humidity (RH) environment, the resistance of electrode with patterned barrier only increased by 0.72 times. This study proposes a new way, i.e., the in situ patterned barrier containing light-sensitive substance, to selectively protect AgNW networks, which can be expanded to various metallic networks including nanowires, nanorods, nanocables, electrospun nanofibers, and so on.

A highly sensitive and flexible pressure sensor with electrodes and elastomeric interlayer containing silver nanowires.

The next-generation application of pressure sensors is gradually being extended to include electronic artificial skin (e-skin), wearable devices, humanoid robotics and smart prosthetics. In these advanced applications, high sensing capability is an essential feature for high performance. Although surface patterning treatments and some special elastomeric interlayers have been applied to improve sensitivity, the process is complex and this inevitably raises the cost and is an obstacle to large-scale production. In the present study a simple printing process without complex patterning has been used for constructing the sensor, and an interlayer is employed comprising elastomeric composites filled with silver nanowires. By increasing the relative permittivity, εr, of the composite interlayer induced by compression at high nanowire concentration, it has been possible to achieve a maximum sensitivity of 5.54 kPa(-1). The improvement in sensitivity did not sacrifice or undermine the other features of the sensor. Thanks to the silver nanowire electrodes, the sensor is flexible and stable after 200 cycles at a bending radius of 2 mm, and exhibits outstanding reproducibility without hysteresis under similar pressure pulses. The sensor has been readily integrated onto an adhesive bandage and has been successful in detecting human movements. In addition to measuring pressure in direct contact, non-contact pressures such as air flow can also be detected.

Silver Nanowire Electrodes: Conductivity Improvement Without Post-treatment and Application in Capacitive Pressure Sensors.

Transparent electrode based on silver nanowires (AgNWs) emerges as an outstanding alternative of indium tin oxide film especially for flexible electronics. However, the conductivity of AgNWs transparent electrode is still dramatically limited by the contact resistance between nanowires at high transmittance. Polyvinylpyrrolidone (PVP) layer adsorbed on the nanowire surface acts as an electrically insulating barrier at wire-wire junctions, and some devastating post-treatment methods are proposed to reduce or eliminate PVP layer, which usually limit the application of the substrates susceptible to heat or pressure and burden the fabrication with high-cost, time-consuming, or inefficient processes. In this work, a simple and rapid pre-treatment washing method was proposed to reduce the thickness of PVP layer from 13.19 to 0.96 nm and improve the contact between wires. AgNW electrodes with sheet resistances of 15.6 and 204 Ω sq-1 have been achieved at transmittances of 90 and 97.5 %, respectively. This method avoided any post-treatments and popularized the application of high-performance AgNW transparent electrode on more substrates. The improved AgNWs were successfully employed in a capacitive pressure sensor with high transparency, sensitivity, and reproducibility.

'Chrysanthemum petal' arrangements of silver nano wires.

Highly ordered 'Chrysanthemum petal' arrangements of silver nano wires were fabricated in a biodegradable polymer of polyvinyl alcohol using a simple one-step blending method without any template. The degree of the arrangement increased with the decreasing content of polyvinyl alcohol. The mechanism for the formation of these 'Chrysanthemum petal' arrangements was discussed specifically. These 'Chrysanthemum petal' arrangements will be helpful to increase the electrical conductivity of silver nano wires films.

Thin-film copper indium gallium selenide solar cell based on low-temperature all-printing process.

In the solar cell field, development of simple, low-cost, and low-temperature fabrication processes has become an important trend for energy-saving and environmental issues. Copper indium gallium selenide (CIGS) solar cells have attracted much attention due to the high absorption coefficient, tunable band gap energy, and high efficiency. However, vacuum and high-temperature processing in fabrication of solar cells have limited the applications. There is a strong need to develop simple and scalable methods. In this work, a CIGS solar cell based on all printing steps and low-temperature annealing is developed. CIGS absorber thin film is deposited by using dodecylamine-stabilized CIGS nanoparticle ink followed by printing buffer layer. Silver nanowire (AgNW) ink and sol-gel-derived ZnO precursor solution are used to prepare a highly conductive window layer ZnO/[AgNW/ZnO] electrode with a printing method that achieves 16 Ω/sq sheet resistance and 94% transparency. A CIGS solar cell based on all printing processes exhibits efficiency of 1.6% with open circuit voltage of 0.48 V, short circuit current density of 9.7 mA/cm(2), and fill factor of 0.34 for 200 nm thick CIGS film, fabricated under ambient conditions and annealed at 250 °C.