Yeongjun Lee

Rapid Fabrication of Designable Large-Scale Aligned Graphene Nanoribbons by Electro-hydrodynamic Nanowire Lithography

A new technique, electro-hydrodynamic nanowire (e-NW) lithography , is demonstrated for the rapid, inexpensive, and efficient fabrication of graphene nanorib bons (GNRs) on a large scale while simultaneously controlling the location and alignment of the GNRs. A series of interesting GNR architectures, including parallel lines, grids, ladders, and stars are produced. A sub-10-nm-wide GNR is obtained to fabricate field-effect transistors that show a room-temperature on/off current ratio of ca. 70.

Individually Position‐Addressable Metal‐Nanofiber Electrodes for Large‐Area Electronics

A individually position-addressable large-scale-aligned Cu nanofiber (NF) array is fabricated using electro-hydrodynamic nanowire printing. The printed single-stranded Cu NF has a diameter of about 710 nm and resistivity of 14.1 μΩ cm and is effectively used as source/drain nanoelectrode in pentacene transistors, which show a 25-fold increased hole mobility than that of a device with Cu thin-film electrodes.

Versatile Metal Nanowiring Platform for Large-Scale Nano- and Opto-Electronic Devices.

A versatile metal nanowiring platform enables the fabrication of Ag nanowires (AgNW) at a desired position and orientation in an individually controlled manner. A printed, flexible AgNW has a diameter of 695 nm, a resistivity of 5.7 μΩ cm, and good thermal stability in air. Based on an Ag nanowiring platform, an all-NW transistors array, as well as various optoelectronic applications, are successfully demonstrated.

Flexible transparent electrodes for organic light-emitting diodes

The use of flexible organic light-emitting diodes (OLEDs) for the next-generation displays and solid-state lightings has been considered, but the widely used transparent conducting electrode (TCE), indium–tin-oxide (ITO), should be replaced by flexible electrodes due to its brittleness and increasing cost. Therefore, many kinds of alternative TCEs have been increasingly studied. In this paper, the properties and applications of the candidate transparent flexible electrodes classified into four categories (conducting polymer, silver nanowire, carbon nanotube and graphene) are described. This paper finally suggests how to develop alternative TCEs for replacing the conventional ITO electrode.

Room-Temperature-Processable Wire-Templated Nanoelectrodes for Flexible and Transparent All-Wire Electronics

Sophisticated preparation of arbitrarily long conducting nanowire electrodes on a large area is a significant requirement for development of transparent nanoelectronics. We report a position-customizable and room-temperature-processable metallic nanowire (NW) electrode array using aligned NW templates and a demonstration of transparent all-NW-based electronic applications by simple direct-printing. Well-controlled electroless-plating chemistry on a polymer NW template provided a highly conducting Au NW array with a very low resistivity of 7.5 μΩ cm (only 3.4 times higher than that of bulk Au), high optical transmittance (>90%), and mechanical bending stability. This method enables fabrication of all-NW-based electronic devices on various nonplanar surfaces and flexible plastic substrates. Our approach facilitates realization of advanced future electronics.

Tough and Water‐Insensitive Self‐Healing Elastomer for Robust Electronic Skin

An electronic (e-) skin is expected to experience significant wear and tear over time. Therefore, self-healing stretchable materials that are simultaneously soft and with high fracture energy, that is high tolerance of damage or small cracks without propagating, are essential requirements for the realization of robust e-skin. However, previously reported elastomers and especially self-healing polymers are mostly viscoelastic and lack high mechanical toughness. Here, a new class of polymeric material crosslinked through rationally designed multistrength hydrogen bonding interactions is reported. The resultant supramolecular network in polymer film realizes exceptional mechanical properties such as notch-insensitive high stretchability (1200%), high toughness of 12 000 J m-2 , and autonomous self-healing even in artificial sweat. The tough self-healing materials enable the wafer-scale fabrication of robust and stretchable self-healing e-skin devices, which will provide new directions for future soft robotics and skin prosthetics.

Simple, Inexpensive, and Rapid Approach to Fabricate Cross‐Shaped Memristors Using an Inorganic‐Nanowire‐Digital‐Alignment Technique and a One‐Step Reduction Process

A rapid, scalable, and designable approach to produce a cross-shaped memristor array is demonstrated using an inorganic-nanowire digital-alignment technique and a one-step reduction process. Two-dimensional arrays of perpendicularly aligned, individually conductive Cu-nanowires with a nanometer-scale Cux O layer sandwiched at each cross point are produced.

Flexible organic light-emitting diodes for solid-state lighting

Abstract. Flexible organic light-emitting diodes (OLEDs) are candidates for next-generation solid-state lighting because they have merits such as low driving voltage, various color tuning, designable form, and large-area light emission. Although OLEDs’ efficiency, luminance, and lifetime have been improved enough to be commercialized, they are still inflexible despite being based on organic materials. To achieve efficient and reliable flexible OLEDs for solid-state lighting, flexible substrates for OLEDs should be developed. For this purpose, progress must be made in developing good flexible substrates, electrode materials, and encapsulation techniques compatible with these flexible substrates. Here, we review and discuss progress made in these three technologies for solid-state lighting using flexible OLEDs. Addressing the technical challenges associated with the development of high performing flexible substrates, electrode materials compatible with these substrates and good encapsulation techniques would lead to efficient and reliable flexible OLEDs and make flexible solid-state lighting commercially feasible.

Deformable Organic Nanowire Field‐Effect Transistors

Deformable electronic devices that are impervious to mechanical influence when mounted on surfaces of dynamically changing soft matters have great potential for next-generation implantable bioelectronic devices. Here, deformable field-effect transistors (FETs) composed of single organic nanowires (NWs) as the semiconductor are presented. The NWs are composed of fused thiophene diketopyrrolopyrrole based polymer semiconductor and high-molecular-weight polyethylene oxide as both the molecular binder and deformability enhancer. The obtained transistors show high field-effect mobility >8 cm2 V-1 s-1 with poly(vinylidenefluoride-co-trifluoroethylene) polymer dielectric and can easily be deformed by applied strains (both 100% tensile and compressive strains). The electrical reliability and mechanical durability of the NWs can be significantly enhanced by forming serpentine-like structures of the NWs. Remarkably, the fully deformable NW FETs withstand 3D volume changes (>1700% and reverting back to original state) of a rubber balloon with constant current output, on the surface of which it is attached. The deformable transistors can robustly operate without noticeable degradation on a mechanically dynamic soft matter surface, e.g., a pulsating balloon (pulse rate: 40 min-1 (0.67 Hz) and 40% volume expansion) that mimics a beating heart, which underscores its potential for future biomedical applications.

One-dimensional conjugated polymer nanomaterials for flexible and stretchable electronics

Stretchable electronics will be essential components of future wearable electronics, biomedical applications, and robotics. Conjugated polymers (CPs) have good mechanical compliance and can be processed via facile solution-based methods; thus, they can be effectively adapted to flexible and stretchable electronics. In addition, the electrical and mechanical properties of CPs can be tuned to satisfy the requirements of various flexible and stretchable next-generation applications. Typically, one effective and simple approach to apply CPs to stretchable electronics is to form one-dimensional (1D) CP nanostructures. This article reviews the recent work on the development of flexible and stretchable 1D CP nanomaterials, including nanofibril networks and printed single nanowires, and their flexible and stretchable applications and then presents some perspectives for future research on 1D CP nanomaterials for flexible and stretchable electronics.