Min Wook Jung

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

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.

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.

Direct growth of graphene nanopatches on graphene sheets for highly conductive thin film applications

Graphene nanopatches (GNs) on graphene films grown by chemical vapor deposition (CVD) were synthesized by Ni nanoparticle assembly and subsequent CVD growth to enhance their electrical conductivity. As a result, the sheet resistance of the hexagonally shaped GN-assembled graphene films decreased from 681.7 ± 11.2 to 527.2 ± 47.0 Ω sq−1 with 97.9% transparency. This improvement in electrical conductivity was the result of p-type doping of the GNs on graphene films and the generation of additional charge carrier conducting paths to diminish defect scattering, which was a result of the enhanced extracted-hole mobility of the GN-assembled graphene films.

Fabrication of graphene-based flexible devices utilizing a soft lithographic patterning method

There has been considerable interest in soft lithographic patterning processing of large scale graphene sheets due to the low cost and simplicity of the patterning process along with the exceptional electrical or physical properties of graphene. These properties include an extremely high carrier mobility and excellent mechanical strength. Recently, a study has reported that single layer graphene grown via chemical vapor deposition (CVD) was patterned and transferred to a target surface by controlling the surface energy of the polydimethylsiloxane (PDMS) stamp. However, applications are limited because of the challenge of CVD-graphene functionalization for devices such as chemical or bio-sensors. In addition, graphene-based layers patterned with a micron scale width on the surface of biocompatible silk fibroin thin films, which are not suitable for conventional CMOS processes such as the patterning or etching of substrates, have yet to be reported. Herein, we developed a soft lithographic patterning process via surface energy modification for advanced graphene-based flexible devices such as transistors or chemical sensors. Using this approach, the surface of a relief-patterned elastomeric stamp was functionalized with hydrophilic dimethylsulfoxide molecules to enhance the surface energy of the stamp and to remove the graphene-based layer from the initial substrate and transfer it to a target surface. As a proof of concept using this soft lithographic patterning technique, we demonstrated a simple and efficient chemical sensor consisting of reduced graphene oxide and a metallic nanoparticle composite. A flexible graphene-based device on a biocompatible silk fibroin substrate, which is attachable to an arbitrary target surface, was also successfully fabricated. Briefly, a soft lithographic patterning process via surface energy modification was developed for advanced graphene-based flexible devices such as transistors or chemical sensors and attachable devices on a biocompatible silk fibroin substrate. Significantly, this soft lithographic patterning technique enables us to demonstrate a simple and efficient chemical sensor based on reduced graphene oxide (rGO), a metallic nanoparticle composite, and an attachable graphene-based device on a silk fibroin thin film.

Effect of nitrogen plasma on the surface of indium oxide nanowires

The change in the atomic nitrogen concentration on a semiconducting nanowires surface and the consequent changes in the electrical characteristics of a nanowire transistor were investigated by exposing In(2)O(3) nanowires to nitrogen (N(2)) plasma. After plasma was applied at N(2) flow rates of 20, 40, and 70 sccm with a fixed source power of 50 W, the In(2)O(3) nanowire transistor exhibited changes in the threshold voltage (V(th)), subthreshold slope (SS), and on-current (I(on)). In particular, after treatment at an N(2) flow rate of 40 sccm, V(th) shifted positively by ~2.3 V, the SS improved by ~0.24 V/dec, and I(on) increased by ~0.8 μA on average. The changes are attributed to the combination of nitrogen ions produced by the plasma with oxygen vacancies or indium interstitials on the nanowires. Optimization of the plasma treatment conditions is expected to yield desirable device characteristics by a simple, nondestructive process.

Heat-driven size manipulation of Fe catalytic nanoparticles for precise control of single-walled carbon nanotube diameter

Size-tailored Fe catalytic nanoparticles (NPs) were formed by a heat-driven evaporation process, and precise control of single-walled carbon nanotube (SWCNT) diameter was achieved. The size and surface concentration of Fe NPs significantly decreased with increasing evaporation temperature, which was investigated by transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy. TEM and Raman spectroscopy revealed that the synthesis of SWCNTs with an extremely narrow diameter distribution was achieved and their diameter can be manipulated by evaporation temperature. This diameter-controlled growth of SWCNTs is a step towards SWCNT-based applications.

Long-term air-stable n-type doped graphene by multiple lamination with polyethyleneimine

We demonstrate homogeneous and air-stable n-type doping of graphene grown by thermal chemical vapor deposition. Laminated Polyethyleneimine (PEI) layers fabricated by a simple polymer solution coating were employed. The doping stability of multiple laminated structures (graphene/PEI, PEI/graphene, and graphene/PEI/graphene) was systematically investigated. The resulting graphene/PEI/graphene laminated structure was highly stable in air. Moreover, the work function and carrier concentration of doped graphene can be tuned by altering the laminated structure.

P-Type Doping of Graphene Films by Hybridization with Nickel Nanoparticles

Here, we demonstrate the decoration of Ni nanoparticles (NPs) on graphene films by simple annealing for p-type doping of graphene. Scanning electron microscopy and atomic force microscopy revealed that high-density, uniformly sized Ni NPs were formed on the graphene films. The density of the Ni NPs increased gradually, whereas the size of the Ni NPs decreased with increasing NiCl26H2O solution concentration. The formation of Ni NPs on graphene films was explained by heat-driven dechlorination and subsequent nano-particlization, as investigated by X-ray photoelectron spectroscopy. The doping effect of Ni NPs onto graphene films was verified by Raman spectroscopy and electrical transport measurements. This method may provide a facile and universal way to obtain metal NPs on graphene if the metal forms a compound with Cl.