Jaesung Park

Roll-to-roll production of 30-inch graphene films for transparent electrodes

The outstanding electrical, mechanical and chemical properties of graphene make it attractive for applications in flexible electronics. However, efforts to make transparent conducting films from graphene have been hampered by the lack of efficient methods for the synthesis, transfer and doping of graphene at the scale and quality required for applications. Here, we report the roll-to-roll production and wet-chemical doping of predominantly monolayer 30-inch graphene films grown by chemical vapour deposition onto flexible copper substrates. The films have sheet resistances as low as approximately 125 ohms square(-1) with 97.4% optical transmittance, and exhibit the half-integer quantum Hall effect, indicating their high quality. We further use layer-by-layer stacking to fabricate a doped four-layer film and measure its sheet resistance at values as low as approximately 30 ohms square(-1) at approximately 90% transparency, which is superior to commercial transparent electrodes such as indium tin oxides. Graphene electrodes were incorporated into a fully functional touch-screen panel device capable of withstanding high strain.1 SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano Technology (HINT), 2 Department of Chemistry, 3 Department of Mechanical Engineering, 4 School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea. 5 NanoCore & Department of Physics, National University of Singapore, Singapore 117576 & 117542, 6 Digital & IT Solution Division, Samsung Techwin, Seongnam 462-807, Korea, 7 Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565 & Faculty of Science and Engineering, Meijo University, Nagoya 468-8502, Japan.

Water-Dispersible Magnetite-Reduced Graphene Oxide Composites for Arsenic Removal

Magnetite-graphene hybrids have been synthesized via a chemical reaction with a magnetite particle size of approximately 10 nm. The composites are superparamagnetic at room temperature and can be separated by an external magnetic field. As compared to bare magnetite particles, the hybrids show a high binding capacity for As(III) and As(V), whose presence in the drinking water in wide areas of South Asia has been a huge problem. Their high binding capacity is due to the increased adsorption sites in the M-RGO composite which occurs by reducing the aggregation of bare magnetite. Since the composites show near complete (over 99.9%) arsenic removal within 1 ppb, they are practically usable for arsenic separation from water.

Enhanced Differentiation of Human Neural Stem Cells into Neurons on Graphene

However, most previous studies report that hNSCs, without biochemical motifs or co-culturing, differentiated more towards glial cells than neurons. [ 6–8 ] On the other hand, although graphene has attracted much interest for biological applications due to its exotic properties such as biocompatibility, electric conductivity, and transparency, [ 9–12 ] it has not been explored for neural stem cell behavior, yet. Herein, we report a graphene substrate that enhanced the differentiation of hNSCs into neurons. Microarray studies were performed to explore a plausible explanation for this effect. Furthermore, we demonstrated an electrical stimulation on the cells differentiated from hNSCs using graphene as a transparent electrode. Our fi ndings suggest that graphene has a unique surface property that can promote the differentiation of hNSCs toward neurons rather than glia, which should open up tremendous opportunities in stem cell research, neuroscience, and regenerative medicine. Our experimental procedure is summarized in Figure 1 . Graphene was synthesized on a large scale and transferred onto a glass substrate, following a previously reported method (see also Figure S1 and S2 in the Supporting Information). [ 10,11 ] The graphene fi lm on glass was then placed into a laminin solution (20 μ g mL − 1 in culture media for 4 h) so that laminin molecules adhered to both the graphene and the glass and helped hNSC attachment. The hNSCs were seeded on the substrate

Surface-directed molecular assembly of pentacene on monolayer graphene for high-performance organic transistors.

Organic electronic devices that use graphene electrodes have received considerable attention because graphene is regarded as an ideal candidate electrode material. Transfer and lithographic processes during fabrication of patterned graphene electrodes typically leave polymer residues on the graphene surfaces. However, the impact of these residues on the organic semiconductor growth mechanism on graphene surface has not been reported yet. Here, we demonstrate that polymer residues remaining on graphene surfaces induce a stand-up orientation of pentacene, thereby controlling pentacene growth such that the molecular assembly is optimal for charge transport. Thus, pentacene field-effect transistors (FETs) using source/drain monolayer graphene electrodes with polymer residues show a high field-effect mobility of 1.2 cm(2)/V s. In contrast, epitaxial growth of pentacene having molecular assembly of lying-down structure is facilitated by π-π interaction between pentacene and the clean graphene electrode without polymer residues, which adversely affects lateral charge transport at the interface between electrode and channel. Our studies provide that the obtained high field-effect mobility in pentacene FETs using monolayer graphene electrodes arises from the extrinsic effects of polymer residues as well as the intrinsic characteristics of the highly conductive, ultrathin two-dimensional monolayer graphene electrodes.

UV/Ozone-Oxidized Large-Scale Graphene Platform with Large Chemical Enhancement in Surface-Enhanced Raman Scattering

We fabricated a highly oxidized large-scale graphene platform using chemical vapor deposition (CVD) and UV/ozone-based oxidation methods. This platform offers a large-scale surface-enhanced Raman scattering (SERS) substrate with large chemical enhancement in SERS and reproducible SERS signals over a centimeter-scale graphene surface. After UV-induced ozone generation, ozone molecules were reacted with graphene to produce oxygen-containing groups on graphene and induced the p-type doping of the graphene. These modifications introduced the structural disorder and defects on the graphene surface and resulted in a large chemical mechanism-based signal enhancement from Raman dye molecules [rhodamine B (RhB), rhodamine 6G (R6G), and crystal violet (CV) in this case] on graphene. Importantly, the enhancement factors were increased from ∼10(3) before ozone treatment to ∼10(4), which is the largest chemical enhancement factor ever on graphene, after 5 min ozone treatment due to both high oxidation and p-doping effects on graphene surface. Over a centimeter-scale area of this UV/ozone-oxidized graphene substrate, strong SERS signals were repeatedly and reproducibly detected. In a UV/ozone-based micropattern, UV/ozone-treated areas were highly Raman-active while nontreated areas displayed very weak Raman signals.

Work-Function Engineering of Graphene Electrodes by Self-Assembled Monolayers for High-Performance Organic Field-Effect Transistors

We have devised a method to optimize the performance of organic field-effect transistors (OFETs) by controlling the work functions of graphene electrodes by functionalizing the surface of SiO2 substrates with self-assembled monolayers (SAMs). The electron-donating NH2-terminated SAMs induce strong n-doping in graphene, whereas the CH3-terminated SAMs neutralize the p-doping induced by SiO2 substrates, resulting in considerable changes in the work functions of graphene electrodes. This approach was successfully utilized to optimize electrical properties of graphene field-effect transistors and organic electronic devices using graphene electrodes. Considering the patternability and robustness of SAMs, this method would find numerous applications in graphene-based organic electronics and optoelectronic devices such as organic light-emitting diodes and organic photovoltaic devices.

Single‐Gate Bandgap Opening of Bilayer Graphene by Dual Molecular Doping

Dual doping-driven perpendicular electric field with opposite directions remarkably increase the on/off current ratio of bilayer graphene field-effect transistors. This unambiguously proves that it is possible to open a bandgap with two molecular dopants (F4-TCNQ and NH2 -functionalized self-assembled monolayers (SAMs)) even in a single-gate device structure.

Transparent flexible organic transistors based on monolayer graphene electrodes on plastic.

There has been much interest in graphene-based electronic devices because graphene provides excellent electrical, optical, and mechanical properties. [ 1 ] In this sense, organic electronic devices using graphene electrodes have attracted considerable attention, and several reports have described the use of graphene source/drain electrodes in organic fi eld-effect transistors (OFETs). [ 2 ] One of the ultimate goals in the fabrication of OFETs using graphene electrodes lies in the fabrication of fl exible and transparent organic transistors, assembled on plastics substrates, that maintain their high performance under ambient conditions. However, no reports have described the fabrication of organic transistors assembled on plastic substrates because the synthesis of either graphene or reduced graphene oxide requires high-temperature fabrication processes. Another important goal in the context of fabricating organic electronic devices with graphene electrodes lies in the fabrication of highly transparent graphene electrodes that cover large areas. Graphene transmittance decreases linearly as the number of layers increases in n-layer graphene. [ 3 ] Thus, the use of monolayer graphene is necessary to achieve high transparency in graphene electrodes, provided that the conductivity of the graphene is suffi cient for device electrode applications. Another merit of monolayer graphene is its extremely low thickness (3–4 Å). Source/drain electrodes in staggered bottomcontact FET structures should be thin to ensure step coverage of the active layer during sequential transistor fabrication. [ 4 ] For this reason, one-atom-thick monolayer graphene provides ideal source/drain electrodes for effi cient charge injection. Recently, several groups succeeded in fabricating high-quality/largearea graphene with preferential monolayer thickness using a

Ambipolar Memory Devices Based on Reduced Graphene Oxide and Nanoparticles

[*] Prof. S. Hong, S. Myung, H. Lee Department of Physics and Astronomy, Seoul National University Seoul 151-747 (Korea) Department of Biophysics and Chemical Biology, Seoul National University Seoul 151-747 (Korea) E-mail: shong@phya.snu.ac.kr Prof. K. S. Kim, J. Park Center for Superfunctional Materials, Department of Chemistry Pohang University of Science and Technology Pohang 790-784 (Korea) E-mail: kim@postech.ac.kr

Control of Graphene Field‐Effect Transistors by Interfacial Hydrophobic Self‐Assembled Monolayers

Graphene has received considerable attention as a potential alternative material for use in next-generation semiconductor technology. [ 1 , 2 ] Since its discovery, graphene-based electronic devices have been fabricated on SiO 2 /Si substrates to take advantage of the optical contrast between the monolayer graphene and SiO 2 and because of the ease of fabrication of devices with metal-oxide-semiconductor (MOS) structures. [ 3 ]