Youngseok Oh

Highly conductive, printable and stretchable composite films of carbon nanotubes and silver

Conductive films that are both stretchable and flexible could have applications in electronic devices, sensors, actuators and speakers. A substantial amount of research has been carried out on conductive polymer composites, metal electrode-integrated rubber substrates and materials based on carbon nanotubes and graphene. Here we present highly conductive, printable and stretchable hybrid composites composed of micrometre-sized silver flakes and multiwalled carbon nanotubes decorated with self-assembled silver nanoparticles. The nanotubes were used as one-dimensional, flexible and conductive scaffolds to construct effective electrical networks among the silver flakes. The nanocomposites, which included polyvinylidenefluoride copolymer, were created with a hot-rolling technique, and the maximum conductivities of the hybrid silver-nanotube composites were 5,710 S cm⁻¹ at 0% strain and 20 S cm⁻¹ at 140% strain, at which point the film ruptured. Three-dimensional percolation theory reveals that Poissons ratio for the composite is a key parameter in determining how the conductivity changes upon stretching.

Graphene‐Based Fibers: A Review

Motivated by their unique structure and excellent properties, significant progress has been made in recent years in the development of graphene-based fibers (GBFs). Potential applications of GBFs can be found, for instance, in conducting wires, energy storage and conversion devices, actuators, field emitters, solid-phase microextraction, springs, and catalysis. In contrast to graphene-based aerogels (GBAs) and membranes (GBMs), GBFs demonstrate remarkable mechanical and electrical properties and can be bent, knotted, or woven into flexible electronic textiles. In this review, the state-of-the-art of GBFs is summarized, focusing on their synthesis, performance, and applications. Future directions of GBF research are also proposed.

Omnidirectionally Stretchable High-Performance Supercapacitor Based on Isotropic Buckled Carbon Nanotube Films.

The emergence of stretchable electronic devices has attracted intensive attention. However, most of the existing stretchable electronic devices can generally be stretched only in one specific direction and show limited specific capacitance and energy density. Here, we report a stretchable isotropic buckled carbon nanotube (CNT) film, which is used as electrodes for supercapacitors with low sheet resistance, high omnidirectional stretchability, and electro-mechanical stability under repeated stretching. After acid treatment of the CNT film followed by electrochemical deposition of polyaniline (PANI), the resulting isotropic buckled acid treated CNT@PANI electrode exhibits high specific capacitance of 1147.12 mF cm(-2) at 10 mV s(-1). The supercapacitor possesses high energy density from 31.56 to 50.98 μWh cm(-2) and corresponding power density changing from 2.294 to 28.404 mW cm(-2) at the scan rate from 10 to 200 mV s(-1). Also, the supercapacitor can sustain an omnidirectional strain of 200%, which is twice the maximum strain of biaxially stretchable supercapacitors based on CNT assemblies reported in the literature. Moreover, the capacitive performance is even enhanced to 1160.43-1230.61 mF cm(-2) during uniaxial, biaxial, and omnidirectional elongations.

Silver-plated carbon nanotubes for silver/conducting polymer composites

Carbon nanotubes (CNTs) have advantages as conductive fillers due to their large aspect ratio and excellent conductivity. In this study, a novel silver/conducting polymer composite was developed by the incorporation of silver-plated CNTs. It is important to achieve a homogeneous dispersion of nanotubes and to improve the interfacial bonding to utilize the excellent properties of reinforcements in the matrix material. The homogeneous dispersion of nanotubes was achieved by an acid treatment process, and the interfacial contact was improved by electroless silver plating around nanotubes. The resistivity of the silver/conducting polymer composite was decreased by 83% by the addition of silver-plated single-walled carbon nanotubes. Conductive bumps were also screen-printed to demonstrate the capability of the composite as electrical interconnects for multi-layer printed circuit boards.

Functionalized nano-silver particles assembled on one-dimensional nanotube scaffolds for ultra-highly conductive silver/polymer composites

A novel silver/polymer composite with electrical conductivity (2.5 × 105 S cm−1) higher than that of bulk tungsten was developed through the addition of a small amount of multi-walled carbon nanotubes decorated with glutaric acid functionalized nano-silver particles (1.5 wt%). The nanotubes were used as one-dimensional conductive scaffolds constructing an effective electrical network among micron-sized silver powders.

Thermal and electrical transport in ultralow density single-walled carbon nanotube networks

The thermal, electrical, and thermoelectric properties of aerogels of single-walled carbon nanotubes are characterized. Their ultralow density enables the transport properties of the junctions to be distinguished from those of the nanotubes themselves. Junction thermal and electrical conductances are found to be orders of magnitude larger than those found in typical dense SWCNT networks. In particular, the average junction thermal conductance is close to the theoretical maximum for a van der Waals bonded SWCNT junction.

Highly Conductive Graphene/Ag Hybrid Fibers for Flexible Fiber-Type Transistors

Mechanically robust, flexible, and electrically conductive textiles are highly suitable for use in wearable electronic applications. In this study, highly conductive and flexible graphene/Ag hybrid fibers were prepared and used as electrodes for planar and fiber-type transistors. The graphene/Ag hybrid fibers were fabricated by the wet-spinning/drawing of giant graphene oxide and subsequent functionalization with Ag nanoparticles. The graphene/Ag hybrid fibers exhibited record-high electrical conductivity of up to 15,800 S cm−1. As the graphene/Ag hybrid fibers can be easily cut and placed onto flexible substrates by simply gluing or stitching, ion gel-gated planar transistors were fabricated by using the hybrid fibers as source, drain, and gate electrodes. Finally, fiber-type transistors were constructed by embedding the graphene/Ag hybrid fiber electrodes onto conventional polyurethane monofilaments, which exhibited excellent flexibility (highly bendable and rollable properties), high electrical performance (μh = 15.6 cm2 V−1 s−1, Ion/Ioff > 104), and outstanding device performance stability (stable after 1,000 cycles of bending tests and being exposed for 30 days to ambient conditions). We believe that our simple methods for the fabrication of graphene/Ag hybrid fiber electrodes for use in fiber-type transistors can potentially be applied to the development all-organic wearable devices.

Highly Sensitive Wearable Textile-Based Humidity Sensor Made of High-Strength, Single-Walled Carbon Nanotube/Poly(vinyl alcohol) Filaments

Textile-based humidity sensors can be an important component of smart wearable electronic-textiles and have potential applications in the management of wounds, bed-wetting, and skin pathologies or for microclimate control in clothing. Here, we report a wearable textile-based humidity sensor for the first time using high strength (∼750 MPa) and ultratough (energy-to-break, 4300 J g-1) SWCNT/PVA filaments via a wet-spinning process. The conductive SWCNT networks in the filaments can be modulated by adjusting the intertube distance by swelling the PVA molecular chains via the absorption of water molecules. The diameter of a SWCNT/PVA filament under wet conditions can be as much as 2 times that under dry conditions. The electrical resistance of a fiber sensor stitched onto a hydrophobic textile increases significantly (by more than 220 times) after water sprayed. Textile-based humidity sensors using a 1:5 weight ratio of SWCNT/PVA filaments showed high sensitivity in high relative humidity. The electrical resistance increases by more than 24 times in a short response time of 40 s. We also demonstrated that our sensor can be used to monitor water leakage on a high hydrophobic textile (contact angle of 115.5°). These smart textiles will pave a new way for the design of novel wearable sensors for monitoring blood leakage, sweat, and underwear wetting.

Preformed Nanoporous Carbon Nanotube Scaffold-Based Multifunctional Polymer Composites

Multifunctional polymer nanocomposites that simultaneously possess high modulus and strength, high thermal stability, novel optical responses, and high electrical and thermal conductivity have been actively researched. Carbon nanotubes are considered an ideal additive for composites because of their superlative physical, electronic and optical properties. While nanotubes have successfully added electrical conductivity, thermal stability, and novel optical responses to polymers, mechanical reinforcements, although substantial, have been well below any theoretical estimations. Here, we integrated preformed hydrogels and aerogels of individually dispersed nanotubes with polymer to increase elastic modulus of composites according to Halpin-Tsai model up to at least 25 vol % of nanotubes. Our solution-based fabrication method allowed us to create bulk composites with tunable form-factors, and with polymers that were incompatible with nanotubes. Further, in this approach, nanotubes were not covalently linked among themselves and to the polymer, so intrinsic optical, electrical, and thermal properties of nanotubes could be exploited. The optically active nanotubes, for example, added a strain-dependent, spatially resolved fluorescence to these composites. Finally, the nanoporous nanotube networks suppressed the polymer glass transition and extended the mechanical integrity of polymer well above its polymer melting point, and both the nanotubes and polymer remained thermally stable above their decomposition temperatures.

High-Strength Single-Walled Carbon Nanotube/Permalloy Nanoparticle/Poly(vinyl alcohol) Multifunctional Nanocomposite Fiber.

Magnetic nanocomposite fibers are a topic of intense research due to their potential breakthrough applications such as smart magnetic-field-response devices and electromagnetic interference (EMI) shielding. However, clustering of nanoparticles in a polymer matrix is a recognized challenge for obtaining a property-controllable nanocomposite fiber. Another challenge is that the strength and ductility of the nanocomposite fiber decrease significantly with increased weight loading of magnetic nanoparticles in the fiber. Here, we report high-strength single-walled carbon nanotube (SWNT)/permalloy nanoparticle (PNP)/poly(vinyl alcohol) multifunctional nanocomposite fibers fabricated by wet spinning. The weight loadings of SWNTs and PNPs in the fiber were as high as 12.0 and 38.0%, respectively. The tensile strength of the fiber was as high as 700 MPa, and electrical conductivity reached 96.7 S m(-1). The saturation magnetization (Ms) was as high as 24.8 emu g(-1). The EMI attenuation of a fabric woven from the prepared fiber approached 100% when tested with electromagnetic waves with a frequency higher than 6 GHz. The present study demonstrates that a magnetic-field-response device can be designed using the fabricated multifunctional nanocomposite fiber.