Teppei Araki

Strongly adhesive and flexible transparent silver nanowire conductive films fabricated with a high-intensity pulsed light technique

Large-scale silver nanowire (AgNW) mesh films have received increasing attention as new transparent conductive films used in various printed devices. However, there are two crucial issues in implementing AgNWs that need to be addressed: (1) strong adhesion between AgNW film and substrate and (2) high conductivity with short treatment time for low-cost printed technology. Here, a high-intensity pulsed light (HIPL) sintering technique, which provides extreme heating locally in the AgNW film and at the interface between the film and polymer substrate, sinters the AgNW film to produce high conductivity with strong adhesion on the substrate. Importantly, light intensity, exposure time, and AgNW amount can be adjusted simply to form films that meet specific device needs. A flexible AgNW film with sheet resistance of 19 Ω sq−1 and transmittance of 83% at 550 nm is obtained with only one-step on a polyethylene terephthalate substrate with a light intensity of 1.14 J cm−2 under an exposure time of only 50 μs. The film can endure multiple peeling tests, which will play an important role in printed electronics.

Facile synthesis of very-long silver nanowires for transparent electrodes

Silver nanowires >60 μm and even 100 μm in length have been synthesized using a polyol process by adjusting the stirring speed at 130 °C. The length is over three times longer than that of normal AgNWs. These wires have a uniform ∼60 nm diameter, independent of the stirring speed. At 91% transmittance at 550 nm, AgNW films fabricated at room temperature achieved 25 Ω per square sheet resistance, which is superior to that of expensive ITO films.

Low haze transparent electrodes and highly conducting air dried films with ultra-long silver nanowires synthesized by one-step polyol method

Transparent electrodes made of silver nanowires (AgNWs) exhibit higher flexibility when compared to those made of tin doped indium oxide (ITO) and are expected to be applied in plastic electronics. However, these transparent electrodes composed of AgNWs show high haze because the wires cause strong light scattering in the visible range. Reduction of the wire diameter has been proposed as a way to weaken light scattering, although there have seldom been any studies focusing on the haze because of the difficulty involved in controlling the wire diameter. In this report, we show that the haze can be easily reduced by increasing the length of AgNWs with a large diameter. Ultra-long (u-long) AgNWs with lengths in the range of 20–100 μm and a maximum length of 230 μm have been successfully synthesized by adjusting the reaction temperature and the stirring speed of a one-step polyol process. Compared to typical AgNWs (with diameter and length of 70 nm and 10 μm, respectively) and ITO, a transparent electrode consisting of u-long AgNWs 91 nm in diameter demonstrated a low haze of 3.4%-1.6% and a low sheet resistance of 24–109 Ω/sq. at a transmittance of 94%–97%. Even when fabricated at room temperature without any post-treatment, the electrodes composed of u-long AgNWs achieved a sheet resistance of 19 Ω/sq. at a transmittance of 80%, which is six orders of magnitude lower than that of typical AgNWs.

Printable and Stretchable Conductive Wirings Comprising Silver Flakes and Elastomers

Smart textiles such as wearable computers and health-care sensors on everyday clothing will be realized likely in the near future. Here, we report that highly electrical conductors were fabricated by filling half of the volume of a polyurethane elastomer with microsized silver flakes under a low temperature of 70 °C. Given the high affinity of polyurethane with silver flakes and substrates, high conductivity was maintained on polyurethane substrates stretched up to a strain of 600% and on folded cellulosic substrates. Illumination of a light-emitting diode was also successful with the stretched wirings.

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.

Facile fabrication of stretchable Ag nanowire/polyurethane electrodes using high intensity pulsed light

Silver nanowires (AgNWs) have emerged as a promising nanomaterial for next generation stretchable electronics. However, until now, the fabrication of AgNWbased components has been hampered by complex and time-consuming steps. Here, we introduce a facile, fast, and one-step methodology for the fabrication of highly conductive and stretchable AgNW/polyurethane (PU) composite electrodes based on a high-intensity pulsed light (HIPL) technique. HIPL simultaneously improved wire–wire junction conductivity and wire–substrate adhesion at room temperature and in air within 50 μs, omitting the complex transfer–curing–implanting process. Owing to the localized deformation of PU at interfaces with AgNWs, embedding of the nanowires was rapidly carried out without substantial substrate damage. The resulting electrode retained a low sheet resistance (high electrical conductivity) of <10 Ω/sq even under 100% strain, or after 1,000 continuous stretching–relaxation cycles, with a peak strain of 60%. The fabricated electrode has found immediate application as a sensor for motion detection. Furthermore, based on our electrode, a light emitting diode (LED) driven by integrated stretchable AgNW conductors has been fabricated. In conclusion, our present fabrication approach is fast, simple, scalable, and costefficient, making it a good candidate for a future roll-to-roll process.

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.

Ultraflexible and ultrathin polymeric gate insulator for 2 V organic transistor circuits

We have developed a high-yield process for fabricating organic transistors with ultraflexible and ultrathin polymeric (parylene) insulators. In a top-contact bottom-gate configuration, an oxygen plasma treatment for a Au gate surface before parylene deposition significantly improved the yield of transistors, enabling the parylene thickness to be reduced to 18 nm. Taking full advantage of the ultraflexible and ultrathin insulator, we have demonstrated 2 V ring oscillator circuits, where the yield was 97% for 360 transistors inside the area of 7 × 7 cm2. The highly reliable ultrathin insulator is useful for large-area circuits with low-voltage organic transistors.

Laser-induced forward transfer of high-viscosity silver precursor ink for non-contact printed electronics

Non-contact printing techniques are receiving increasing interest in the field of printed electronics, because they can be used to pattern various inks on arbitrary substrates without applying mechanical pressure or damaging pre-patterned components. The ink-jet process is frequently used for non-contact printing of various conductive inks. However, the ink-jet printing process is restricted by the viscosity of the ink, because the nozzle can become clogged by high-viscosity inks. Here we successfully demonstrate a non-contact printing technique for high-viscosity and high-concentration silver precursor inks using the laser-induced forward transfer (LIFT) process. The process conditions for LIFT printing, including the triazene polymer sacrificial layer thickness, the laser fluence, and the donor-acceptor distance, have been investigated in detail. LIFT printing of a hexylamine-based 70 wt% silver precursor ink with viscosity of 60 mPa s was achieved, and produced fine conductive lines with widths of 141 μm, thicknesses of 490 nm, and volume resistivity of 11.6 μΩ cm. It is envisaged that this non-contact printing method can pave the way towards non-contact and maskless printing of high viscosity inks in the manufacture of printed electronics. ©2015 The Royal Society of Chemistry.

Rapid self-assembly of ultrathin graphene oxide film and application to silver nanowire flexible transparent electrodes

Featuring outstanding electrical and optical properties, silver nanowires (AgNWs) have been regarded as one of the most promising candidates for ITO to manufacture transparent conductive electrodes. However, the poor long-term stability of bare AgNWs, due to sulfidation/oxidation corrosion, is an unavoidable and urgent problem in practical applications. In the present work, a large-area ultrathin and uniform graphene oxide (GO) film was freely self-assembled at the interface of pentane–water by a rapid process within only 3 minutes, and subsequently transferred onto the surface of AgNW film by a simple dip coating process, resulting in an impressive improvement in the conductive performance and stability of the AgNWs. The ultrathin GO film was formed by the evaporation driven instability effect of acetone to induce the self assembly of GO nanosheets and an assistant thermal treatment to accelerate the formation rate. The thickness of GO film could be effectively controlled by changing the amount of acetone and the self-assembly time. The sheet resistance of the GO/AgNW electrode has been decreased approximately 3–4 times, with only a 2% loss in transmittance, compared to the original AgNW electrode. A GO/AgNW electrode with a sheet resistance of 21.5 Ω sq−1 at 90% transmittance has been achieved. The stability of the AgNW electrodes at room temperature and high temperature (120 °C) environments has been improved using GO as a protective film. The uniform and large-scale GO film can be transferred onto various substrates by a simple dip coating method with an arbitrary shape, which will open a new window for the protection of various metal nanowires.