Ki Woong Kim

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

Improved homogeneity and surface coverage of graphene oxide layers fabricated by horizontal-dip-coating for solution-processable organic semiconducting devices

We herein report an investigation of graphene oxide (GO) thin layers fabricated by simple horizontal-dip (H-dip) coating on an indium-tin-oxide (ITO) anode as used in solution-processable organic semiconducting devices. Homogeneous and smooth GO thin films were successfully deposited via an aqueous dispersion of GO on an ITO electrode, with a high surface coverage and low surface roughness, and a thickness controlled by H-dip coating. The use of an H-dip-coated GO film as a hole-injecting interfacial layer (IFL) in organic light-emitting diodes (OLEDs) resulted in a remarkable improvement in device performance (17 cd A−1), better than that (12 cd A−1) of a reference OLED with a spin-coated GO IFL. Stacked bi-IFLs of GO/poly(ethylenedioxythiophene):poly(styrene sulfonate) (GO/PEDOT:PSS) fabricated by H-dip coating were also investigated as hole-injecting IFLs in OLEDs, and these showed an even better device performance (23 cd A−1). Furthermore, it was also shown that polymer solar cells with H-dip-coated GO/PEDOT:PSS hole-collecting bi-IFLs exhibited a remarkable improvement in power conversion efficiency (6.9%), which was also higher than that (4.8%) obtained with spin-coated bi-IFLs. These results clearly indicate that the H-dip-coating of GO(/PEDOT:PSS) can effectively modify the ITO interface to yield efficient hole-selective IFLs, representing considerable promise for use as an alternative to spin-coated IFLs in the mass production of solution-processable organic semiconducting devices.

Polyethylenimine-Mediated Electrostatic Assembly of MnO2 Nanorods on Graphene Oxides for Use as Anodes in Lithium-Ion Batteries.

In recent years, the development of electrochemically active materials with excellent lithium storage capacity has attracted tremendous attention for application in high-performance lithium-ion batteries. MnO2-based composite materials have been recognized as one of promising candidates owing to their high theoretical capacity and cost-effectiveness. In this study, a previously unrecognized chemical method is proposed to induce intra-stacked assembly from MnO2 nanorods and graphene oxide (GO), which is incorporated as an electrically conductive medium and a structural template, through polyethylenimine (PEI)-derived electrostatic modulation between both constituent materials. It is revealed that PEI, a cationic polyelectrolyte, is capable of effectively forming hierarchical, two-dimensional MnO2-RGO composites, enabling highly reversible capacities of 880, 770, 630, and 460 mA·h/g at current densities of 0.1, 1, 3, and 5 A/g, respectively. The role of PEI in electrostatically assembled composite materials is clarified through electrochemical impedance spectroscopy-based comparative analysis.

Superior Lithium Storage Performance using Sequentially Stacked MnO2/Reduced Graphene Oxide Composite Electrodes

Hybrid nanostructures based on graphene and metal oxides hold great potential for use in high-performance electrode materials for next-generation lithium-ion batteries. Herein, a new strategy to fabricate sequentially stacked α-MnO2 /reduced graphene oxide composites driven by surface-charge-induced mutual electrostatic interactions is proposed. The resultant composite anode exhibits an excellent reversible charge/discharge capacity as high as 1100 mA h g(-1) without any traceable capacity fading, even after 100 cycles, which leads to a high rate capability electrode performance for lithium ion batteries. Thus, the proposed synthetic procedures guarantee a synergistic effect of multidimensional nanoscale media between one (metal oxide nanowire) and two dimensions (graphene sheet) for superior energy-storage electrodes.

3D-stacked carbon composites employing networked electrical intra-pathways for direct-printable, extremely stretchable conductors.

The newly designed materials for stretchable conductors meeting the demands for both electrical and mechanical stability upon morphological elongation have recently been of paramount interest in the applications of stretchable, wearable electronics. To date, carbon nanotube-elastomeric polymer mixtures have been mainly developed; however, the method of preparing such CNT-polymer mixtures as stretchable conductors has been limited to an ionic liquid-mediated approach. In this study, we suggest a simple wet-chemical method for producing newly designed, three-dimensionally stacked carbon composite materials that facilitate the stable morphological elongation up to a strain of 300% with normalized conductivity variation of only 0.34 under a strain of 300%. Through a comparative study with other control samples, it is demonstrated that the intraconnected electrical pathways in hierarchically structured composite materials enable the generation of highly stretchable conductors. Their direct patternability is also evaluated by printing on demand using a programmable disperser without the use of prepatterned masks.

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.

Threshold voltage manipulation of ZnO-graphene oxide hybrid thin film transistors via Au nanoparticles doping

In order to fabricate a complementary inverter, precise control of the threshold voltages for n-type semiconductor based thin film transistors (TFTs) is highly required. Here we provided a facile methodology for controlling the threshold voltage of ZnO-based TFTs. Chemically-derived graphene oxide (GO) and Au-decorated GO (Au-GO) flakes were hybridized with solution-processed ZnO thin films to control electron injection determined by the workfunction difference between ZnO and GO or Au-GO. As a result, the threshold voltages for the ZnO, GO/ZnO, and Au-GO/ZnO TFTs were 24 ± 3 V, −11 ± 4 V, and 63 ± 5 V, respectively, which determine depletion or enhancement mode TFTs without any significant change in the field effect mobility and on/off ratio.

New approach for the reduction of graphene oxide with triphenylphosphine dihalide

We developed a one-flask method for the thermal reduction of graphene oxide (GO) with triphenylphosphine dihalide (Ph3PX2) at 180 °C. Our approach offers a potential to cost-effective mass-production of graphene nanosheets under mild and environmentally friendly conditions and to avoid the use of strong acids or reducing agents. Significantly, this reduced graphene oxide (rGO) by utilizing a Ph3PX2 reductant has a C/O ratio higher than 15 and an electrical conductivity of 400 S cm−1, which indicate that this synthetic method allows us to achieve graphene nanosheets with high quality when comparing with previous reduction methods.

Morphology Effect on Enhanced Li

The electrochemical performance of Li(Mn, M)PO4 (M = Co2+/3+, Ni2+/3+) was investigated with regard to the particle morphology. Within a controlled chemical composition, Li(Mn0.92Co0.04Ni0.04)PO4, the resultant cathode exhibited somewhat spherical-shaped nanocrystalline particles and enhanced Li+-ion storage, which was even better than the undoped LiMnPO4, up to 16% in discharge capacity at 0.05 C. The outstanding electrochemical performance is attributed to the well-dispersed spherical-shaped particle morphology, which allows the fast Li+-ion migration during the electrochemical lithiation/delithiation process, especially at high current density.