Minhyeok Choe

Large-scale patterned multi-layer graphene films as transparent conducting electrodes for GaN light-emitting diodes

This work demonstrates a large-scale batch fabrication of GaN light-emitting diodes (LEDs) with patterned multi-layer graphene (MLG) as transparent conducting electrodes. MLG films were synthesized using a chemical vapor deposition (CVD) technique on nickel films and showed typical CVD-synthesized MLG film properties, possessing a sheet resistance of [Formula: see text] with a transparency of more than 85% in the 400-800 nm wavelength range. The MLG was applied as the transparent conducting electrodes of GaN-based blue LEDs, and the light output performance was compared to that of conventional GaN LEDs with indium tin oxide electrodes. Our results present a potential development toward future practical application of graphene electrodes in optoelectronic devices.

The application of graphene as electrodes in electrical and optical devices

Graphene is a promising next-generation conducting material with the potential to replace traditional electrode materials such as indium tin oxide in electrical and optical devices. It combines several advantageous characteristics including low sheet resistance, high optical transparency and excellent mechanical properties. Recent research has coincided with increased interest in the application of graphene as an electrode material in transistors, light-emitting diodes, solar cells and flexible devices. However, for more practical applications, the performance of devices should be further improved by the engineering of graphene films, such as through their synthesis, transfer and doping. This article reviews several applications of graphene films as electrodes in electrical and optical devices and discusses the essential requirements for applications of graphene films as electrodes.

Three-Dimensional Integration of Organic Resistive Memory Devices

Since the discovery of conducting polymers [ 1 ] , organic-based electronics such as organic light-emitting diodes, transistors, photovoltaics, and memory devices have been spotlighted as potentially innovative devices given their easy and lowcost fabrication by spin-coating or ink-jet printing, and their fl exibility. [ 2–15 ] Among these, organic memories have been extensively investigated for data-storage application. [ 11 , 14 , 16–21 ]

Stable Switching Characteristics of Organic Nonvolatile Memory on a Bent Flexible Substrate

Organic-based electronics have received great attention due to their material variety and advantageous properties such as fl exibility, printability, and light-weightness. [ 1 , 2 ] Their low costs, based on their ease of fabrication and large-area processing capabilities, increase the merits of organic electronics even more. [ 3 , 4 ] Consequently, organic electronics, including organic solar cells, light-emitting diodes, thin-fi lm transistors, and memories, have been extensively investigated for the realization of practical device applications. [ 5–8 ] Among these, organic memories have emerged as an excellent candidate for the nextgeneration information storage media because of their potential application in fl exible memory devices. [ 8–18 ] There are different types of organic memories. They are distinguished as ferroelectric, [ 13 , 14 , 18 ] fl ash, [ 15 , 18 ] and resistive-type organic memories [ 16–18 ]

A New Approach for Molecular Electronic Junctions with a Multilayer Graphene Electrode

www.MaterialsViews.com C O M M A New Approach for Molecular Electronic Junctions with a Multilayer Graphene Electrode U N IC A Gunuk Wang , Yonghun Kim , Minhyeok Choe , Tae-Wook Kim , and Takhee Lee * IO N Interest in the fi eld of molecular electronics is grounded in the fact that devices based on molecules constitute the ultimate device miniaturization limit that both inorganicand organic-based electronics aspire to reach. [ 1–10 ] The non-linear current–voltage characteristics of molecular junctions have been extensively investigated with a variety of platforms and techniques, such as scanning probe microscope-based techniques, [ 10–13 ] break junctions, [ 5 , 14–17 ] crossed-wire tunnel junctions, [ 18–20 ] and various solid-state device-based methods. [ 4 , 6 , 21–25 ] Within these efforts, the creation of a stable solid-state molecular junction has been a long-standing challenge in terms of understanding molecular charge transport mechanisms and practical device applications. Most fabrication techniques involve evaporating a metal onto the molecules as the top electrode. [ 21–24 , 26 , 27 ] This process causes electrical short circuits and unstable and unexpected current–voltage characteristics due to fi lamentary paths and damage to the molecules. [ 22 , 23 , 26–29 ] These inevitable uncertainties in the fabrication technique lead to relatively large variations in the junction conductance, despite the use of identical molecular components, and this is an obstacle for truly understanding molecular charge transport mechanisms and device applications. New techniques and ideas have been developed to resolve this issue. [ 4 , 6 , 21 , 30 , 31 ] The fabrication of molecular junctions using a conductive polymer (PEDOT:PSS) between the top electrode and the molecules has been one of the most successful techniques in terms of high device yields and stable junctions. [ 21 ] Nevertheless, the use of a conductive polymer has some limitations and presents some uncertainties as a platform for physical-organic studies, because the properties of the interface between the polymer layer and the molecules are not well-understood. [ 21 , 30–33 ] For example, it has been reported that the resistance of the materials fabricated using this technique is signifi cantly different to those of molecular junctions that do not have the polymer interlayer [ 30–33 ] due to poor contact between PEDOT:PSS and

Enhanced Charge Injection in Pentacene Field‐Effect Transistors with Graphene Electrodes

S. Lee , G. Jo , S.-J. Kang , G. Wang , M. Choe , W Park , . Prof. D.-Y. Kim , H. Dr. . Y Kahng , Prof. Lee . TDepartment of Nanobio Materials and Electronics Department of Materials Science and Engineering Gwangju Institute of Science and Technology Gwangju 500–712, Korea E-mail: yhkahng@gist.ac.kr; tlee@gist.ac.kr Dr. Y. H. KahngResearch Institute for Solar and Sustainable Energies Gwangju Institute of Science and Technology Gwangju 500–712, Korea

Tuning of a graphene-electrode work function to enhance the efficiency of organic bulk heterojunction photovoltaic cells with an inverted structure

We demonstrate the fabrication of inverted-structure organic solar cells (OSCs) with graphene cathodes. The graphene film used in this work was work-function-engineered with an interfacial dipole layer to reduce the work function of graphene, which resulted in an increase in the built-in potential and enhancement of the charge extraction, thereby enhancing the overall device performance. Our demonstration of inverted-structure OSCs with work-function-engineering of graphene electrodes will foster the fabrication of more advanced structure OSCs with higher efficiency.

Tuning of the Electronic Characteristics of ZnO Nanowire Field Effect Transistors by Proton Irradiation

We demonstrated a controllable tuning of the electronic characteristics of ZnO nanowire field effect transistors (FETs) using a high-energy proton beam. After a short proton irradiation time, the threshold voltage shifted to the negative gate bias direction with an increase in the electrical conductance, whereas the threshold voltage shifted to the positive gate bias direction with a decrease in the electrical conductance after a long proton irradiation time. The electrical characteristics of two different types of ZnO nanowires FET device structures in which the ZnO nanowires are placed on the substrate or suspended above the substrate and photoluminescence (PL) studies of the ZnO nanowires provide substantial evidence that the experimental observations result from the irradiation-induced charges in the bulk SiO(2) and at the SiO(2)/ZnO nanowire interface, which can be explained by a surface-band-bending model in terms of gate electric field modulation. Our study on the proton-irradiation-mediated functionalization can be potentially interesting not only for understanding the proton irradiation effects on nanoscale devices, but also for creating the property-tailored nanoscale devices.

Enhancement in the photodetection of ZnO nanowires by introducing surface-roughness-induced traps

We investigated the enhanced photoresponse of ZnO nanowire transistors that was introduced with surface-roughness-induced traps by a simple chemical treatment with isopropyl alcohol (IPA). The enhanced photoresponse of IPA-treated ZnO nanowire devices is attributed to an increase in adsorbed oxygen on IPA-induced surface traps. The results of this study revealed that IPA-treated ZnO nanowire devices displayed higher photocurrent gains and faster photoswitching speed than transistors containing unmodified ZnO nanowires. Thus, chemical treatment with IPA can be a useful method for improving the photoresponse of ZnO nanowire devices.

Au nanoparticle-decorated graphene electrodes for GaN-based optoelectronic devices

We studied GaN-based optoelectronic devices such as light-emitting diodes (LEDs) and solar cells (SCs) with graphene electrodes. A decoration of Au nanoparticles (NPs) on multi-layer graphene films improved the electrical conductivity and modified the work function of the graphene films. The Au NP-decorated graphene film enhanced the current injection and electroluminescence of GaN-based LEDs through low contact resistance and improved the power conversion efficiency of GaN-based SCs through additional light absorption and energy band alignment. Our study will enhance the understanding of the role of Au NP-decorated graphene electrodes for GaN-based optoelectronic device applications.