Wooseok Song

Low-temperature synthesis of graphene on nickel foil by microwave plasma chemical vapor deposition

Microwave plasma chemical vapor deposition (MPCVD) was employed to synthesize high quality centimeter scale graphene film at low temperatures. Monolayer graphene was obtained by varying the gas mixing ratio of hydrogen and methane to 80:1. Using advantages of MPCVD, the synthesis temperature was decreased from 750 °C down to 450 °C. Optical microscopy and Raman mapping images exhibited that a large area monolayer graphene was synthesized regardless of the temperatures. Since the overall transparency of 89% and low sheet resistances ranging from 590 to 1855 Ω∕sq of graphene films were achieved at considerably low synthesis temperatures, MPCVD can be adopted in manufacturing future large-area electronic devices based on graphene film.

Carbon Nanotube and Graphene Hybrid Thin Film for Transparent Electrodes and Field Effect Transistors

S. H. Kim, [+] Dr. W. Song, [+] M. W. Jung, M.-A. Kang, K. Kim, Dr. S. S. Lee, Dr. J. Lim, Dr. S. Myung, Dr. K.-S. An Thin Film Materials Research Group Korea Research Institute of Chemical Technology (KRICT) Yuseong Post Offi ce Box 107 Daejeon 305-600 , Republic of Korea E-mail: msung@krict.re.kr Dr. S.-J. Chang Department of Chemistry Chung-Ang University 84 Heukseok-ro , Dongjak-gu, Seoul 156-756 , Korea Prof. J. Hwang Department of Materials Science and Engineering Hongik University Seoul 121-791 , Republic of Korea

Synthesis of Bandgap-Controlled Semiconducting Single-Walled Carbon Nanotubes

Bandgap-controlled semiconducting single-walled carbon nanotubes (s-SWNTs) were synthesized using a uniquely designed catalytic layer (Al(2)O(3)/Fe/Al(2)O(3)) and conventional thermal chemical vapor deposition. Homogeneously sized Fe catalytic nanoparticles were prepared on the Al(2)O(3) layer and their sizes were controlled by simply modulating the annealing time via heat-driven diffusion and subsequent evaporation of Fe at 800 degrees C. Transmission electron microscopy and Raman spectroscopy revealed that the synthesized SWNTs diameter was manipulated from 1.4 to 0.8 nm with an extremely narrow diameter distribution below 0.1 nm as the annealing time is increased. As a result, the bandgap of semiconducting SWNTs was successfully controlled, ranging from 0.53 to 0.83 eV, with a sufficiently narrow energy distribution, which can be applied to field-effect transistors based on SWNTs.

Wafer-Scale, Homogeneous MoS2 Layers on Plastic Substrates for Flexible Visible-Light Photodetectors

An appropriate solution is suggested for synthesizing wafer-scale, continuous, and stoichiometric MoS2 layers with spatial homogeneity at the low temperature of 450 °C. It is also demonstrated that the MoS2 -based visible-light photodetector arrays are both fabricated on 4 inch SiO2 /Si wafer and polyimide films, revealing 100% active devices with a narrow photocurrent distribution and excellent mechanical durability.

High-mobility ambipolar ZnO-graphene hybrid thin film transistors

In order to combine advantages of ZnO thin film transistors (TFTs) with a high on-off ratio and graphene TFTs with extremely high carrier mobility, we present a facile methodology for fabricating ZnO thin film/graphene hybrid two-dimensional TFTs. Hybrid TFTs exhibited ambipolar behavior, an outstanding electron mobility of 329.7 ± 16.9 cm2/V·s, and a high on-off ratio of 105. The ambipolar behavior of the ZnO/graphene hybrid TFT with high electron mobility could be due to the superimposed density of states involving the donor states in the bandgap of ZnO thin films and the linear dispersion of monolayer graphene. We further established an applicable circuit model for understanding the improvement in carrier mobility of ZnO/graphene hybrid TFTs.

Novel Fabrication of Flexible Graphene-Based Chemical Sensors with Heaters using Soft Lithographic Patterning Method

We have fabricated graphene-based chemical sensors with flexible heaters for the highly sensitive detection of specific gases. We believe that increasing the temperature of the graphene surface significantly enhanced the electrical signal change of the graphene-based channel, and reduced the recovery time needed to obtain a normal state of equilibrium. In addition, a simple and efficient soft lithographic patterning process was developed via surface energy modification for advanced, graphene-based flexible devices, such as gas sensors. As a proof of concept, we demonstrated the high sensitivity of NO2 gas sensors based on graphene nanosheets. These devices were fabricated using a simple soft-lithographic patterning method, where flexible graphene heaters adjacent to the channel of sensing graphene were utilized to control graphene temperature.

Toward Arbitrary-Direction Energy Harvesting through Flexible Piezoelectric Nanogenerators Using Perovskite PbTiO3 Nanotube Arrays

New fiber-type piezoelectric nanogenerator devices consisting of radially aligned perovskite PbTiO3 nanotubes are designed for energy harvesting from arbitrary mechanical motion. The free-standing fiber-type nanogenerators generate constant amount of electric power by bending or wind motion regardless of direction, thus, extending the possibility of their practical applications.

Homogeneous and stable p-type doping of graphene by MeV electron beam-stimulated hybridization with ZnO thin films

In this work, we demonstrate a unique and facile methodology for the homogenous and stable p-type doping of graphene by hybridization with ZnO thin films fabricated by MeV electron beam irradiation (MEBI) under ambient conditions. The formation of the ZnO/graphene hybrid nanostructure was attributed to MEBI-stimulated dissociation of zinc acetate dihydrate and a subsequent oxidation process. A ZnO thin film with an ultra-flat surface and uniform thickness was formed on graphene. We found that homogeneous and stable p-type doping was achieved by charge transfer from the graphene to the ZnO film.

Growth of Millimeter-Scale Vertically Aligned Carbon Nanotubes by Microwave Plasma Chemical Vapor Deposition

Millimeter-scale, vertically aligned carbon nanotubes (CNTs) were grown at a relatively low temperature of 700 °C by the microwave plasma chemical vapor deposition. Oxygen and water were used to investigate their role in the growth rate and crystallinity of CNTs. Scanning electron microscopy and transmission electron microscopy were adopted to observe the height and number of walls of the CNTs, respectively. The addition of oxygen significantly increased the growth rate of CNTs (~23 µm/min) for 70 min, which is three and two times higher than that of normal CNTs and water-assisted CNTs, respectively. Furthermore, the high growth rate was maintained for 220 min and the crystallinity of CNTs, which was evaluated by the G to D band ratio in Raman spectra, was also greatly improved only by introducing oxygen.

Electrical Double Layer Capacitance in a Graphene-embedded Al2O3 Gate Dielectric.

Graphene heterostructures are of considerable interest as a new class of electronic devices with exceptional performance in a broad range of applications has been realized. Here, we propose a graphene-embedded Al2O3 gate dielectric with a relatively high dielectric constant of 15.5, which is about 2 times that of Al2O3, having a low leakage current with insertion of tri-layer graphene. In this system, the enhanced capacitance of the hybrid structure can be understood by the formation of a space charge layer at the graphene/Al2O3 interface. The electrical properties of the interface can be further explained by the electrical double layer (EDL) model dominated by the diffuse layer.