Hyojin Bong

High-mobility low-temperature ZnO transistors with low-voltage operation

Low voltage high mobility n-type thin film transistors (TFTs) based on sol-gel processed zinc oxide (ZnO) were fabricated using a high capacitance ion gel gate dielectric. The ion gel gated solution-processed ZnO TFTs were found to exhibit excellent electrical properties. TFT carrier mobilities were 13 cm2/V s, ON/OFF current ratios were 105, regardless of the sintering temperature used for the preparation of the ZnO thin films. Ion gel gated ZnO TFTs are successfully demonstrated on plastic substrates for the large area flexible electronics.

Transparent Superhydrophobic/Translucent Superamphiphobic Coatings Based on Silica−Fluoropolymer Hybrid Nanoparticles

This paper describes a simple approach to prepare a transparent superhydrophobic coating and a translucent superamphiphobic coating via spraying silica-fluoropolymer hybrid nanoparticles (SFNs) without any pre- or post-treatment of substrates; these nanoparticles create both microscale and nanoscale roughness, and fluoropolymer acts as a low surface energy binder. We also demonstrate the effects of varying the concentration of the SFN sol on the water and hexadecane repellency and on the transparency of the coated glass substrates. An increase in the concentration of the sol facilitates the transition between the superhydrophobic/transparent and superamphiphobic/translucent states. This transition results from an increase in the discontinuities in the three-phase (solid-liquid-gas) contact line and in the light scattering properties due to micropapillae tuned by varying the concentration of the sol. This versatile and controllable approach can be applied to a variety of substrates over large areas and may provide a wide range of applications for self-cleaning coatings of optoelectronics, liquid-repellent coatings, and microfluidic systems.

Evaporation‐Induced Self‐Alignment and Transfer of Semiconductor Nanowires by Wrinkled Elastomeric Templates

The evaporation-induced self-alignment of semiconductor nanowires is achieved using wrinkled elastomeric templates. The wrinkled templates, which have a surface topography that can be tuned via changes in the mechanical strain, are used as both a template to align the nanowires and as a stamp to transfer the aligned nanowires to target substrates.

Substrate-induced solvent intercalation for stable graphene doping.

Here, we report a substrate-induced intercalation phenomenon of an organic solvent at the interface between monolayer graphene and a target substrate. A simple dipping of the transferred chemical vapor deposition (CVD)-grown graphene on the SiO₂ substrate into chloroform (CHCl₃, CF), a common organic solvent, induces a spontaneous formation of CF clusters beneath the basal plane of the graphene as well as inside the wrinkles. The microscopic and spectroscopic observations showed the doping behavior of monolayer graphene, which indicates the adsorption of CF to monolayer graphene. Interestingly, the intercalated organic solvent showed remarkable stability for over 40 days under ambient conditions. To reveal the underlying mechanism of the stable solvent intercalation, desorption energy of CF molecules at the graphene/substrate interface was measured using Arrhenius plots of the conductance change upon time and temperature. Two stages of solvent intercalations with high desorption energies (70 and 370 meV) were observed along with the consecutive shrinkage of the solvent clusters at the basal plane and the wrinkles, respectively. Moreover, the theoretical calculation based on density functional theory (DFT) also shows the strong intercalation energy of CF between monolayer graphene and the SiO₂ substrate, which results from the stabilization of the graphene-SiO₂ interactions. Furthermore, the thermal response of the conductance could be utilized to maintain a certain degree of p-doping of monolayer graphene, which provides the facile, sustainable, and controllable large-area doping method of graphene for future generation of printed flexible electronics.

Inverse Transfer Method Using Polymers with Various Functional Groups for Controllable Graphene Doping

The polymer-supported transfer of chemical vapor deposition (CVD)-grown graphene provides large-area and high-quality graphene on a target substrate; however, the polymer and organic solvent residues left by the transfer process hinder the application of CVD-grown graphene in electronic and photonic devices. Here, we describe an inverse transfer method (ITM) that permits the simultaneous transfer and doping of graphene without generating undesirable residues by using polymers with different functional groups. Unlike conventional wet transfer methods, the polymer supporting layer used in the ITM serves as a graphene doping layer placed at the interface between the graphene and the substrate. Polymers bearing functional groups can induce n-doping or p-doping into the graphene depending on the electron-donating or -withdrawing characteristics of functional groups. Theoretical models of dipole layer-induced graphene doping offered insights into the experimentally measured change in the work function and the Dirac point of the graphene. Finally, the electrical properties of pentacene field effect transistors prepared using graphene electrodes could be enhanced by employing the ITM to introduce a polymer layer that tuned the work function of graphene. The versatility of polymer functional groups suggests that the method developed here will provide valuable routes to the development of applications of CVD-grown graphene in organic electronic devices.

Copper-Vapor-Assisted Growth and Defect-Healing of Graphene on Copper Surfaces

Although there is significant progress in the chemical vapor deposition (CVD) of graphene on Cu surfaces, the industrial application of graphene is not realized yet. One of the most critical obstacles that limit the commercialization of graphene is that CVD graphene contains too many vacancies or sp3 -type defects. Therefore, further investigation of the growth mechanism is still required to control the defects of graphene. During the growth of graphene, sublimation of the Cu catalyst to produce Cu vapor occurs inevitably because the process temperature is close to the melting point of Cu. However, to date few studies have investigated the effects of Cu vapor on graphene growth. In this study, how the Cu vapor produced by sublimation affects the chemical vapor deposition of graphene on Cu surfaces is investigated. It is found that the presence of Cu vapor enlarges the graphene grains and enhances the efficiency of the defect-healing of graphene by CH4 . It is elucidated that these effects are due to the removal by Cu vapor of carbon adatoms from the Cu surface and oxygen-functionalized carbons from graphene. Finally, these insights are used to develop a method for the synthesis of uniform and high-quality graphene.