• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.152-152
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• 2016

We studied the effect of the silver grid size on graphene transparent conducting films for flexible organic solar cells (OSCs). The silver grid was used an assistant layer of the graphene to reduce the sheet resistance of substrates. Silver grid with various graphene sizes for optimizing transmittance and sheet resistance of substrates were fabricated on polyethylene terephthalate (PET) substrates to form the hybrid films. The optimized grid geometry on the single layer graphene (SLG) was the grid dimension $200{\mu}m{\times}200{\mu}m{\times}50nm{\times}2{\mu}m$ (length ${\times}$ width ${\times}$ height ${\times}$ linewidth), where the sheet resistance was $55.73{\Omega}/square$ with the average transmittance of ~ 92.83 % at 550 nm. The properties of the OSCs fabricated using SLG with optimized silver grids on PET substrates show a short circuit current of $10.9mA/cm^2$, an open circuit voltage of 0.58 V, a fill factor of 60.8 %, and a power conversion efficiency (PCE) of 3.9 %. The PCE was improved about 91% than that of the OSCs using the SLG without the silver grid. These results demonstrate that the optimized grid geometry to the based on the graphene transparent electrodes contribute to improving the performance of OSCs.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.280.2-280.2
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• 2016

The graphene, a single atomic sheet of graphite, has attracted tremendous interest owing to its novel properties including high intrinsic mobility, optical transparency and flexibility. However, for more diverse application of graphene devices, it is essential to tune its transport behavior by shifting Dirac Point (DP) of graphene. So, in the following context, we suggest a method to tune structural and electronic properties of graphene using atomic layer deposition. By atomic layer deposition of zinc oxide (ZnO) on graphene using 4-mercaptophenol as linker, we can fabricate n-doped graphene. Through ${\pi}-{\pi}$ stacking between chemically inert graphene and 4-mercaptophenol, conformal deposition of ZnO on graphene was enabled. The electron mobility of graphene TFT increased more than 3 times without considerably decreasing the hole mobility, compared to the pristine graphene. Also, it has high air stability. This ZnO doping method by atomic layer deposition can be applicable to large scale array of CVD graphene TFT.

• Proceeding of EDISON Challenge
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• 2015.03a
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• pp.338-342
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• 2015

Ripples and charge impurity effect of graphene are considered as the origin of charge puddles in graphene sheet on SiO2. However, this topic is very controversial among researchers in graphene community. In this study, by using density functional theory, we calculate the band structure of the rippled graphene model and charged impurity model that is located close to the (0001) ${\alpha}$-quartz surface. We expect that this study will provide great insight on this matter.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.235-235
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• 2013

We reported on the characteristics of organic solar cells (OSCs) fabricated on $HNO_3$-treated multilayer graphene (MLG) transparent electrodes. MLG electrodes were prepared using a chemical vapor deposition and a multi-transfer process. Compared to organic solar cells (OSCs) on the ITO electrodes had a fill factor of 65.97%, and a power conversion efficiency (PCE) of 3.364%, OSCs on the MLG (three-layer graphene) electrodes with sheet resistance of $274{\pm}1{\Omega}$/square and transparency of 92.1% had a fill factor of 43.46%, and a power conversion efficiency (PCE) of 2.019%. However, OSCs on the HNO3-treated MLG electrodes with lower sheet resistance of $119{\pm}1{\Omega}$/square had a fill factor of 57.54%, and a PCE of 2.861%. The results would provide a promising method to improve the performance of large-area OSCs based on MLG electrodes.

• Structural Engineering and Mechanics
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• v.75 no.1
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• pp.109-122
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• 2020

An asymptotic local plane strain elasticity theory is reformulated for the static analysis of a simply-supported, multiple graphene sheet system (MGSS) in cylindrical bending and resting on an elastic medium. The dimension of the MGSS in the y direction is considered to be much greater than those in the x and z directions, such that all the field variables are considered to be independent of the y coordinate. Eringen's nonlocal constitutive relations are used to account for the small length scale effects in the formulation examining the static behavior of the MGSS. The interaction between the MGSS and its surrounding foundation is modelled as a Winkler foundation with the parameter k_w, and the interaction between adjacent graphene sheets (GSs) is considered using another Winkler model with the parameter c_w. A parametric study with regard to some effects on the static behavior of the MGSS resting on an elastic medium is undertaken, such as the aspect ratio, the number of the GSs, the stiffness of the medium between the adjacent layers and that of the surrounding medium of the MGSS, and the nonlocal parameter.

• Advances in nano research
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• v.10 no.2
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• pp.101-114
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• 2021

An analytical investigation has been performed on the mechanical performance of waves propagated in a Single-Layered Graphene Sheet (SLGS) when an In-plane Varying Bending (IVB) load is interacted. It has been supposed that the Graphene Sheet (GS) is located on an elastic medium. Employing a two-parameter elastic foundation, the effects of elastic substrate on the GS behavior are modeled. Besides, the kinematic equations are derived by the means of a trigonometric two-variable refined plate theory. Moreover, in order to indicate the size-dependency of the SLGS, a Nonlocal Strain Gradient Theory (NSGT) was considered. The nonlocal governing differential equations are achieved in the framework of Hamilton's Principle (HP). Also, an analytical approach was used to detect the unknowns of the final eigenvalue equation. Finally, the effects of each parameters using some dispersion charts were determined.

• Korean Chemical Engineering Research
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• v.55 no.2
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• pp.253-257
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• 2017

Graphene is a monolayer carbon material which consists of $sp^2$ bonding between carbon atoms. Its excellent intrinsic properties allow graphene to be used in various research fields. Many researchers believe that graphene is suitable for electronic device materials due to its high electrical conductivity and carrier mobility. Through chemical doping, n- or p-type graphene can be obtained, and consequently graphene-based devices which have more comparable structure to common semiconductor-based devices can be fabricated. In our research, we introduced the dielectrophoresis process to the chemical doping step in order to improve the effect of chemical doping of graphene selectively. Under 10 kHz and $5V_{pp}$ (peak-to-peak voltage), doping was conducted and the Au nanoparticles were effectively formed, as well as aligned along the edges of graphene. Effects of the selective chemical doping on graphene were investigated through Raman spectroscopy and the change of its electrical properties were explored. We proposed the method to enhance the doping effect in local region of a graphene layer.

• Carbon letters
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• v.13 no.1
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• pp.44-47
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• 2012

We analyzed the effect of etchants for metal catalysts in terms of the characteristics of resulting graphene films, such as sheet resistance, hall mobility, transmittance, and carrier concentration. We found the residue of $FeCl_3$ etchant degraded the sheet resistance and mobility of graphene films. The residue was identified as an iron oxide containing a small amount of Cl through elemental analysis using X-ray photoelectron spectroscopy. To remove this residue, we provide an alternative etching solution by introducing acidic etching solutions and their combinations ($HNO_3$, HCl, $FeCl_3$ + HCl, and $FeCl_3+HNO_3$). The combination of $FeCl_3$ and acidic solutions (HCl and $HNO_3$) resulted in more enhanced electrical properties than pure etchants, which is attributed to the elimination of left over etching residue, and a small amount of amorphous carbon debris after the etching process.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.67-67
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• 2012

Graphene, two-dimensional one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice, exhibits fascinating electrical properties, such as a linear energy dispersion relation and high mobility in addition to a wide-range optical absorption and high thermal conductivity. Graphene's outstanding tensile strength allows graphene-based electronic and photonic devices to be flexible, bendable, or even stretchable. Recently many groups have reported high performance electronic and optoelectronic devices based on graphene materials, i.e. field-effect transistors, gas sensors, nonvolatile memory devices, and plasmonic waveguides, in which versatile properties of graphene materials have been incorporated into a flexible electronic or optoelectronic platform. However, there are several fundamental or technological hurdles to be overcome in real applications of graphene in electronics and optoelectronics. In this tutorial we will present a short introduction to the basic material properties and recent progresses in applications of graphene to electronics and optoelectronics and discuss future outlook of graphene-based devices.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.13-13
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• 2011

Graphene based nano-electronic and nano-electromechanical devices will be introduced in this presentation. The first part of the presentation will be covered by our recent results on the fabrication and physical properties of artificially twisted bilayer graphene. Thanks to the recently developed contact transfer printing method, a single layer graphene sheet is stacked on various substrates/nano-structures in a controlled manner for fabricating e.g. a suspended graphene device, and single-bilayer hybrid junction. The Raman and electrical transport results of the artificially twisted bilayer indicates the decoupling of the two graphene sheets. The graphene based electromechanical devices will be presented in the second part of the presentation. Carbon nanotube based nanorelay and A new concept of non-volatile memory based on the carbon nanotube field effect transistor together with microelectromechanical switch will be briefly introduced at first. Recent progress on the graphene based nano structures of our group will be presented. The array of graphene resonators was fabricated and their mechanical resonance properties are discussed. A novel device structures using carbon nanotube field effect transistor combined with suspended graphene gate will be introduced in the end of this presentation.