Yeongun Ko

Novel Synthesis, Coating, and Networking of Curved Copper Nanowires for Flexible Transparent Conductive Electrodes

In this work, a whole manufacturing process of the curved copper nanowires (CCNs) based flexible transparent conductive electrode (FTCE) is reported with all solution processes, including synthesis, coating, and networking. The CCNs with high purity and good quality are designed and synthesized by a binary polyol coreduction method. In this reaction, volume ratio and reaction time are the significant factors for the successful synthesis. These nanowires have an average 50 nm in width and 25-40 μm range in length with curved structure and high softness. Furthermore, a meniscus-dragging deposition (MDD) method is used to uniformly coat the well-dispersed CCNs on the glass or polyethylene terephthalate substrate with a simple process. The optoelectrical property of the CCNs thin films is precisely controlled by applying the MDD method. The FTCE is fabricated by networking of CCNs using solvent-dipped annealing method with vacuum-free, transfer-free, and low-temperature conditions. To remove the natural oxide layer, the CCNs thin films are reduced by glycerol or NaBH4 solution at low temperature. As a highly robust FTCE, the CCNs thin film exhibits excellent optoelectrical performance (T = 86.62%, R(s) = 99.14 Ω ◻(-1)), flexibility, and durability (R/R(0) < 1.05 at 2000 bending, 5 mm of bending radius).

Surface Energy Engineered, High‐Resolution Micropatterning of Solution‐Processed Reduced Graphene Oxide Thin Films

Graphene, an atomically thin layer of two-dimensional carbon nanostructure, has received intense attention in recent years because of its extraordinary optoelectronic properties and potential applications in microelectronics. [ 1–4 ] While high-quality graphene has been produced by chemical vapor deposition (CVD) on metallic surfaces [ 5 , 6 ] and graphitization of a single crystal SiC, [ 7 ] reduced graphene oxide (rGO) is also considered as a promising electronic nanomaterial because of its solution processability, residual chemically active sites, and high-volume production at low cost. [ 4 , 8 , 9 ] In the form of a single-layer sheet or fi lms of a few layers, rGO has been employed in various electronic devices including chemical/biological sensors, [ 10 , 11 ] fi eldeffect transistors (FETs), [ 8 , 12 ] transparent electrodes, [ 13 , 14 ] and photovoltaics. [ 15 ] However, previous studies have largely focused on a single electronic device or sensor. To fabricate practical and reproducible rGO-based microelectronics, a scalable and effective method for high-resolution rGO micropatterns on various substrates is highly desirable. Top-down lithographic techniques have been widely used to create rGO micropatterns by selectively etching parts of rGO thin fi lms. [ 12 , 16–18 ] Although a variety of well-defi ned rGO patterns can be obtained from such lithographic methods, they are time-consuming, involve complex procedures, and give rise to undesirable contamination of the patterned surface from contact with sacrifi cial masks. Alternatively, rGO patterning has been explored with nonlithographic routes such as micromolding in capillaries [ 19 , 20 ] and solvent evaporation-driven self-assembly process. [ 21 ] These methods, however, are often limited to simple patterned structures such as stripes, because the assembly of GO fl akes occurs in a restricted geometry. In addition, although various printing techniques including inkjet printing, [ 22 , 23 ] transfer printing [ 24 , 25 ] and imprinting [ 26 ] have also been applied for rGO patterning, the production of highresolution and reproducible rGO micropatterns on a large scale still remains a challenging task.

Highly Transparent and Stretchable Conductors Based on a Directional Arrangement of Silver Nanowires by a Microliter-Scale Solution Process

We report an effective method for fabricating highly transparent and stretchable large-area conducting films based on a directional arrangement of silver nanowires (AgNWs) driven by a shear force in a microliter-scale solution process. The thin conducting films with parallel AgNWs or cross-junctions of AgNWs are deposited on the coating substrate by dragging a microliter drop of the coating solution trapped between two plates. The optical and electrical properties of the AgNW thin films are finely tuned by varying the simple systematic parameters in the coating process. The transparent thin films with AgNW cross-junctions exhibit the superior electrical conductivity with a sheet resistance of 10 Ω sq(-1) at a transmittance of 85% (λ = 550 nm), which is well described by the high ratio of DC to optical conductivity of 276 and percolation theory in a two-dimensional matrix model. This simple coating method enables the deposition of AgNW thin films with high optical transparency, flexibility, and stretchability directly on plastic substrates.

Wafer-scale and environmentally-friendly deposition methodology for extremely uniform, high-performance transistor arrays with an ultra-low amount of polymer semiconductors

We report on a new class of microliter-scale solution processes for fabricating highly uniform and large-area transistor arrays with extremely low consumption of semiconducting polymers. These processes are accomplished by applying a vertical phase separation of polymers with an environmentally benign solvent, a random copolymerization strategy between two highly conductive repeating units, and a meniscus-dragging deposition technique. The successful realization of these three processes, as confirmed by the structural and morphological in-depth characterizations, has enabled the fabrication of high-performance polymeric field-effect transistors that were uniformly distributed, without a single failure, on a 4 inch wafer using only 40 μg of semiconducting polymers. The resulting transistor arrays showed an average mobility of 0.28 cm2 V−1 s−1, with a low standard deviation of 0.04, as well as ultra-uniform near-zero threshold voltages. Our simple strategy shows great promise for fabricating large-scale organic electronic devices in the future using a truly low-cost process.

Design and Fabrication of Wettability Gradients with Tunable Profiles through Degrafting Organosilane Layers from Silica Surfaces by Tetrabutylammonium Fluoride

Surface-bound wettability gradients allow for a high-throughput approach to evaluate surface interactions for many biological and chemical processes. Here we describe the fabrication of surface wettability gradients on flat surfaces by a simple, two-step procedure that permits precise tuning of the gradient profile. This process involves the deposition of homogeneous silane SAMs followed by the formation of a surface coverage gradient through the selective removal of silanes from the substrate. Removal of silanes from the surface is achieved by using tetrabutylammonium fluoride which selectively cleaves the Si-O bonds at the headgroup of the silane. The kinetics of degrafting has been modeled by using a series of first order rate equations, based on the number of attachment points broken to remove a silane from the surface. Degrafting of monofunctional silanes exhibits a single exponential decay in surface coverage; however, there is a delay in degrafting of trifunctional silanes due to the presence of multiple attachment points. The effects of degrafting temperature and time are examined in detail and demonstrate the ability to reliably and precisely control the gradient profile on the surface. We observe a relatively homogeneous coverage of silane (i.e., without the presence of islands or holes) throughout the degrafting process, providing a much more uniform surface when compared to additive approaches of gradient formation. Linear gradients were formed on the substrates to demonstrate the reproducibility and tuneability of this subtractive approach.