Sang Ouk Kim

Epitaxial self-assembly of block copolymers on lithographically defined nanopatterned substrates

Parallel processes for patterning densely packed nanometre-scale structures are critical for many diverse areas of nanotechnology. Thin films of diblock copolymers can self-assemble into ordered periodic structures at the molecular scale (∼5 to 50 nm), and have been used as templates to fabricate quantum dots, nanowires, magnetic storage media, nanopores and silicon capacitors. Unfortunately, perfect periodic domain ordering can only be achieved over micrometre-scale areas at best and defects exist at the edges of grain boundaries. These limitations preclude the use of block-copolymer lithography for many advanced applications. Graphoepitaxy, in-plane electric fields, temperature gradients, and directional solidification have also been demonstrated to induce orientation or long-range order with varying degrees of success. Here we demonstrate the integration of thin films of block copolymer with advanced lithographic techniques to induce epitaxial self-assembly of domains. The resulting patterns are defect-free, are oriented and registered with the underlying substrate and can be created over arbitrarily large areas. These structures are determined by the size and quality of the lithographically defined surface pattern rather than by the inherent limitations of the self-assembly process. Our results illustrate how hybrid strategies to nanofabrication allow for molecular level control in existing manufacturing processes.

Three‐Dimensional Self‐Assembly of Graphene Oxide Platelets into Mechanically Flexible Macroporous Carbon Films

Graphene is an atom-thick, two-dimensional material comprised of a monolayer hexagonal sp-hybridized carbons. It is flexible, has a large specific surface area, and exhibits excellent electrical and thermal conductivities and also good mechanical properties. Moreover, given the low cost of natural graphite, the potential for obtaining large quantities of graphene by a low-cost production process is high. As such, graphene and its chemically modified forms are promising building blocks for accessing highly ordered assemblies that are suitable for nanoelectronics, energy storage/conversion, catalysis, composites, and other applications. Although previous efforts have demonstrated that graphene-based platelets may be assembled into papers, thin films, or other two-dimensional constructs, the ability to control the assembly such platelets into three-dimensional (3D) structures could result in the carbon materials that exhibit very large surface areas, unusual or novel physical and electronic properties, unsurpassed chemical functionality, and other attractive features that are necessary for the aforementioned applications. Herein we demonstrate the self-assembly of graphene oxide (GO) platelets into mechanically flexible, macroporous 3D carbon films with tunable porous morphologies. Selfassembly is the spontaneous bottom-up organization of preexisting components into patterned structures. The intrinsic parallelism and scalability inherent to self-assembly can, in principle, enable low-cost, large-scale syntheses of highly ordered nanostructures. Indeed, as will be described below, the self-assembly of chemically modified graphene platelets into a complex 3D morphology was achieved by the “breath-figure” method, which is a straightforward procedure for synthesizing large-area porous polymer films. The breath-figure method as employed herein is illustrated in Figure 1A. Briefly, polymer-grafted GO platelets were synthesized and dispersed in an organic solvent. The dispersion was then cast onto a suitable substrate and exposed to a stream of humid air. Endothermic evaporation of the volatile organic solvent resulted in the spontaneous conden-

Graphene Oxide Thin Films for Flexible Nonvolatile Memory Applications

There has been strong demand for novel nonvolatile memory technology for low-cost, large-area, and low-power flexible electronics applications. Resistive memories based on metal oxide thin films have been extensively studied for application as next-generation nonvolatile memory devices. However, although the metal oxide based resistive memories have several advantages, such as good scalability, low-power consumption, and fast switching speed, their application to large-area flexible substrates has been limited due to their material characteristics and necessity of a high-temperature fabrication process. As a promising nonvolatile memory technology for large-area flexible applications, we present a graphene oxide based memory that can be easily fabricated using a room temperature spin-casting method on flexible substrates and has reliable memory performance in terms of retention and endurance. The microscopic origin of the bipolar resistive switching behavior was elucidated and is attributed to rupture and formation of conducting filaments at the top amorphous interface layer formed between the graphene oxide film and the top Al metal electrode, via high-resolution transmission electron microscopy and in situ X-ray photoemission spectroscopy. This work provides an important step for developing understanding of the fundamental physics of bipolar resistive switching in graphene oxide films, for the application to future flexible electronics.

Noncovalent Functionalization of Graphene with End-Functional Polymers

Stable dispersion of reduced graphene in various organic solvents was achieved via noncovalent functionalization with amine-terminated polymers. An aqueous dispersion of reduced graphene was prepared by chemical reduction of graphene oxide in aqueous media and was vacuum filtered to generate reduced graphene sheets. Good solvents and nonsolvents for the dried reduced graphene were evaluated using a solubility test. To achieve stable dispersion in the evaluated nonsolvents, amine-terminated polystyrene was noncovalently functionalized to the graphene, while graphene sheets were phase transferred via sonication from aqueous phase to the organic nonsolvent phase, including the amine-terminated polymers. Thorough FTIR and Raman spectroscopy investigation verified that the protonated amine terminal group of polystyrene underwent noncovalent functionalization to the carboxylate groups at the graphene surface, providing the high dispersibility in various organic media.

Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction

Cost effective hydrogen evolution reaction (HER) catalyst without using precious metallic elements is a crucial demand for environment-benign energy production. Molybdenum sulfide is one of the promising candidates for such purpose, particularly in acidic condition, but its catalytic performance is inherently limited by the sparse catalytic edge sites and poor electrical conductivity. We report synthesis and HER catalysis of hybrid catalysts composed of amorphous molybdenum sulfide (MoSx) layer directly bound at vertical N-doped carbon nanotube (NCNT) forest surface. Owing to the high wettability of N-doped graphitic surface and electrostatic attraction between thiomolybdate precursor anion and N-doped sites, ∼2 nm scale thick amorphous MoSx layers are specifically deposited at NCNT surface under low-temperature wet chemical process. The synergistic effect from the dense catalytic sites at amorphous MoSx surface and fluent charge transport along NCNT forest attains the excellent HER catalysis with onset overpotential as low as ∼75 mV and small potential of 110 mV for 10 mA/cm(2) current density, which is the highest HER activity of molybdenum sulfide-based catalyst ever reported thus far.

Flexible Nanocomposite Generator Made of BaTiO 3 Nanoparticles and Graphitic Carbons

Outdoor renewable energy sources such as solar energy (15 000 μ W/cm 3 ), [ 3 , 4 ] wind energy (380 μ W/cm 3 ), [ 5 ] and wave energy (1 000 W/cm of wave crest length) [ 6 , 7 ] can provide largescale needs of power. However, for driving small electronics in indoor or concealed environments [ 3 , 8 ] (such as in tunnels, clothes, and artifi cial skin) and implantable biomedical devices, innovative approaches have to be developed. One way of energy harvesting without such restraints is to utilize piezoelectric materials that can convert vibrational and mechanical energy sources from human activities such as pressure, bending, and stretching motions into electrical energy. [ 9–11 ]

25th Anniversary Article: Chemically Modified/Doped Carbon Nanotubes & Graphene for Optimized Nanostructures & Nanodevices

Outstanding pristine properties of carbon nanotubes and graphene have limited the scope for real-life applications without precise controllability of the material structures and properties. This invited article to celebrate the 25th anniversary of Advanced Materials reviews the current research status in the chemical modification/doping of carbon nanotubes and graphene and their relevant applications with optimized structures and properties. A broad aspect of specific correlations between chemical modification/doping schemes of the graphitic carbons with their novel tunable material properties is summarized. An overview of the practical benefits from chemical modification/doping, including the controllability of electronic energy level, charge carrier density, surface energy and surface reactivity for diverse advanced applications is presented, namely flexible electronics/optoelectronics, energy conversion/storage, nanocomposites, and environmental remediation, with a particular emphasis on their optimized interfacial structures and properties. Future research direction is also proposed to surpass existing technological bottlenecks and realize idealized graphitic carbon applications.

Polymer Brushes via Controlled, Surface‐Initiated Atom Transfer Radical Polymerization (ATRP) from Graphene Oxide

A method for growing polymers directly from the surface of graphene oxide is demonstrated. The technique involves the covalent attachment of an initiator followed by the polymerization of styrene, methyl methacrylate, or butyl acrylate using atom transfer radical polymerization (ATRP). The resulting materials were characterized using a range of techniques and were found to significantly improve the solubility properties of graphene oxide. The surface-grown polymers were saponified from the surface and also characterized. Based on these results, the ATRP reactions were determined to proceed in a controlled manner and were found to leave the structure of the graphene oxide largely intact.

Three‐Dimensional Shape Engineered, Interfacial Gelation of Reduced Graphene Oxide for High Rate, Large Capacity Supercapacitors

DOI: 10.1002/adma.201303503 Assembly of graphene into functional macroscopic objects, such as fi lms, [ 1 ] sheets, [ 2 ] fi bers, [ 3 ] foams, [ 4,5 ] and other complex architectures, [ 6 ] is of enormous research interest. How to attain desired structures in a cost effective and manufacturable manner is crucial for energy harvest/storage, catalysis, sensors and so on. Unlike fullerene or carbon nanotubes, whose assembly generally relies on weak van der Walls force or chemical modifi cation, two-dimensional graphene may straightforwardly exploit strong interlayer π – π stacking. Unfortunately, such a strong and directional interaction frequently results in graphitic stacking with minimal surface area. [ 7,8 ]

Versatile Carbon Hybrid Films Composed of Vertical Carbon Nanotubes Grown on Mechanically Compliant Graphene Films

[*] Prof. S. O. Kim, D. H. Lee, J. E. Kim, T. H. Han, J. W. Hwang, Prof. S. W. Jeon, Prof. S. H. Hong, Prof. W. J. Lee Department of Materials Science and Engineering, KAIST Daejeon 305-701 (Republic of Korea) E-mail: sangouk.kim@kaist.ac.kr Dr. S. Y. Choi Convergence Components and Materials Laboratory Electronics and Telecommunication Research Institute (ETRI) Daejoen 305-700 (Republic of Korea)