• Journal of the Korean Crystal Growth and Crystal Technology
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• v.32 no.6
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• pp.239-245
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• 2022

Reduced graphene oxide (rGO) has attracted many attention and applications due to its excellent electrochemical ability. Therefore, standardization of rGO through structural and thermal analysis facilitates quality improvement and management, enabling users to increase efficiency and reduce relevant costs. For rGO and graphene-related materials, it is very important to determine the number of layers and define the resulting difference in physical properties. In this study, 3~4 layers of rGO-1 and 9~10 layers of rGO-2 were obtained from graphene oxide (GO) through a hydrazine reduction process. For the prepared rGOs, X-ray diffraction (XRD) pattern obtained a diffraction peak at 2θ≈25° related to (002) reflection was used to calculate the layer numbers by determining interlayer distance and FWHM value. To reduce the angular uncertainty, XRD data analysis was performed with angle correction using standard reference materials for X-ray powder diffraction analysis. Precise interlayer distance and number of layers were determined using OriginLab and open-source XRD diffraction analysis programs using the angle-corrected diffraction data. TG-DSC thermal analysis was performed to further standardize the physical properties of rGO samples.

• Proceedings of the Materials Research Society of Korea Conference
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• 2012.05a
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• pp.68.1-68.1
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• 2012

The best part of graphene is - charge-carriers in it are mass less particles which move in near relativistic speeds. Comparing to other materials, electrons in graphene travel much faster - at speeds of $10^8cm/s$. A graphene sheet is pure enough to ensure that electrons can travel a fair distance before colliding. Electronic devices few nanometers long that would be able to transmit charge at breath taking speeds for a fraction of power compared to present day CMOS transistors. Many researches try to check a possibility to make it a perfect replacement for silicon based devices. Graphene has shown high potential to be used as interconnects in the field of high frequency electrical devices. With all those advantages of graphene, we demonstrate characteristics of electrical and optical properties of graphene such as the effect of graphene geometry on the microwave properties using the measurements of S-parameter in range of 500 MHz - 40 GHz at room temperature condition. We confirm that impedance and resistance decrease with increasing the number of graphene layer and w/L ratio. This result shows proper geometry of graphene to be used as high frequency interconnects. This study also presents the optical properties of graphene oxide (GO), which were deposited in different substrate, or influenced by oxygen plasma, were confirmed using different characterization techniques. 4-6 layers of the polycrystalline GO layers, which were confirmed by High resolution transmission electron microscopy (HRTEM) and electron diffraction analysis, were shown short range order of crystallization by the substrate as well as interlayer effect with an increase in interplanar spacing, which can be attributed to the presence of oxygen functional groups on its layers. X-ray photoelectron Spectroscopy (XPS) and Raman spectroscopy confirms the presence of the $sp^2$ and $sp^3$ hybridization due to the disordered crystal structures of the carbon atoms results from oxidation, and Fourier Transform Infrared spectroscopy (FTIR) and XPS analysis shows the changes in oxygen functional groups with nature of substrate. Moreover, the photoluminescent (PL) peak emission wavelength varies with substrate and the broad energy level distribution produces excitation dependent PL emission in a broad wavelength ranging from 400 to 650 nm. The structural and optical properties of oxygen plasma treated GO films for possible optoelectronic applications were also investigated using various characterization techniques. HRTEM and electron diffraction analysis confirmed that the oxygen plasma treatment results short range order crystallization in GO films with an increase in interplanar spacing, which can be attributed to the presence of oxygen functional groups. In addition, Electron energy loss spectroscopy (EELS) and Raman spectroscopy confirms the presence of the $sp^2$ and $sp^3$ hybridization due to the disordered crystal structures of the carbon atoms results from oxidation and XPS analysis shows that epoxy pairs convert to more stable C=O and O-C=O groups with oxygen plasma treatment. The broad energy level distribution resulting from the broad size distribution of the $sp^2$ clusters produces excitation dependent PL emission in a broad wavelength range from 400 to 650 nm. Our results suggest that substrate influenced, or oxygen treatment GO has higher potential for future optoelectronic devices by its various optical properties and visible PL emission.

• Journal of Electrochemical Science and Technology
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• v.8 no.3
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• pp.250-256
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• 2017

A graphene electrode with a novel in-plane structure is proposed and successfully adopted for use in supercapacitor applications. The in-plane structure allows electrolyte ions to interact with all the graphene layers in the electrode, thereby maximizing the utilization of the electrochemical surface area. This novel structure contrasts with the conventional out-of-plane stacked structure of such supercapacitors. We herein compare the volumetric capacitances of in-plane- and out-of-plane-structured devices with reduced multi-layer graphene oxide films as electrodes. The in-plane-structured device exhibits a capacitance 2.5 times higher (i.e., $327F\;cm^{-3}$) than that of the out-of-plane-structured device, in addition to an energy density of $11.4mWh\;cm^{-3}$, which is higher than that of lithium-ion thin-film batteries and is the highest among in-plane-structured ultra-small graphene-based supercapacitors reported to date. Therefore, this study demonstrates the potential of in-plane-structured supercapacitors with high volumetric performances as ultra-small energy storage devices.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.422.1-422.1
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• 2016

Graphene quantum dots (GQDs), a new kind of carbon-based photo luminescent nanomaterial from chemically modified graphene oxide (CMGO) or chemically modified graphene (CMG), has attracted extensive research attention in the last few years due to its outstanding chemical, optical and electrical properties. To further extended its potential applications as optoelectronic devices, solar cells, bio and bio-sensors and so on, intensive research efforts have been devoted to the CMG. However, the CMG, a suspension of aqueous, have problematic since they are prone to agglomeration after drying a solvent. In this study, we synthesized the GQDs from graphite and deposited on silicon substrate by kinetic spray. The photo luminescent properties of deposited GQD films were analyzed and compared with initial GQDs suspension. In addition, its carbon properties were investigated with GQDs solution properties. The properties of deposited GQD films by kinetic spray were similar to that of the GQDs suspension in water. We could provide a pathway for silicon-based silicon based device applications. Finally, the well-adjusted GQD films with photo luminescence effects will show Energy-Down-Shift layer effects on silicon solar cells. The GQD layers deposited at nozzle scan speeds of 40, 30, 20, and 10 mm/s were evaluated after they were used to fabricate crystalline-silicon solar cells; the results indicate that GQDs play an important role in increasing the optical absorptivity of the cells. The short-circuit current density (Jsc) was enhanced by about 2.94 % (0.9 mA/cm2) at 30 mm/s. Compared to a reference device without a GQD energy-down-shift layer, the PCE of p-type silicon solar cells was improved by 2.7% (0.4 percentage points).

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.309-309
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• 2012

Heteroepitaxial growth remains as one of the continuously growing interests, because the heterogeneous crystallization on different substrates is a common feature in the fabrication processes of many semiconductor materials and devices, such as molecular beam epitaxy, pulsed laser deposition, sputtering, chemical bath deposition, chemical vapor deposition, hydrothermal synthesis, vapor phase transport and so on [1,2]. By using the R.F. sputtering system, ZnO thin films were deposited on graphene 4 and 6 mono layers, which is grown on 400 nm and 600 nm $SiO_2$ substrates, respectively. The ZnO thin layer was deposited at various temperatures by using a ZnO target. In this experimental, the working power and pressure were $3{\times}10^{-3}$ Torr and 50 W, respectively. The base pressure of the chamber was kept at a pressure around $10^{-6}$ Torr by using a turbo molecular pump. The oxygen and argon gas flows were controlled around 5 and 10 sccm by using a mass flow controller system, respectively. The structural properties of the samples were analyzed by XRD measurement. The film surface and carrier concentration were analyzed by an atomic force microscope and Hall measurement system. The surface morphologies were observed using field emission scanning electron microscope (FE-SEM).

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.464-464
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• 2013

그래핀 옥사이드(Graphene Oxide)는 그래핀과 마찬가지로 많은 분야로의 응용 가능성을 보이는 소자중 하나로 각광받고 있다. 그래핀 옥사이드가 가지는 유전체 특징은 전하 트랩층(charge trap layer)으로 사용을 가능하게 하고 또한 물에 녹는 수용성 특징은 스핀코터(spin coator)를 이용한 간단한 도포과정을 통하여 저비용으로 간단하게 소자를 제작 가능하게 한다. 이 연구에서 우리는 금속-절연체-반도체 구조를 가지는 메모리 소자를 제작하여 0.4 mg/ml의 농도로 DI에 용해된 그래핀 옥사이드가 플로팅게이트(floating gate)로써 사용되었을 때의 특성을 알아보기 위해 Boonton 720를 사용하여 C-V (hysteresis) 커브와 C-T(Capacitance-Time)를 측정하여 그래핀 옥사이드의 유무에 따른 메모리 윈도우 폭의 증가 및 저장된 정보가 손실되지 않고 얼마나 길게 유지 되는지를 살펴봄으로 플로팅게이트로써 그래핀 옥사이드의 특성을 살펴보았다. 먼저 터널링층으로 쓰이는 SiO2가 5 nm 증착된 P타입 Si기판위에 플로팅게이트로 쓰이는 그래핀 옥사이드층을 쉽게 쌓기 위하여 APTES 자기조립 단분자막 코팅을 한 후 그래핀 옥사이드를 3,000 rpm으로 40초간 스핀코팅을 하였다. 그 후 블로킹층으로 쓰이는 400 nm 두께의 폴리비닐페놀(PVP)를 3,000 rpm으로 40초간 스핀코팅을 하고 $130^{\circ}C$에서 열처리를 하였으며 $10^{-5}$ Torr의 압력에서 진공 열증착으로 알루미늄 게이트 전극을 증착했다.

• Composites Research
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• v.29 no.4
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• pp.167-172
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• 2016

$Co_9S_8/reduced$ graphene (CSRG) has been prepared by a facile two step hydrothermal method and used as a supercapacitor electrode material. It is anticipated that the $Co_9S_8$ and reduced graphene oxide (RGO) would serve as a spacer material to each other to stop the agglomeration and simultaneous contribution of electrical double layer capacitance (RGO) and pseudocapacitance ($Co_9S_8$) would provide high electrochemical properties. The chemical analysis has been done by Fourier transform infrared spectroscopy and the morphology is characterised by field emission scanning electron microscopy. CSRG shows a high electrical conductivity of $98S\;m^{-1}$. The symmetric supercapacitor shows a specific capacitance of ${\sim}728F\;g^{-1}$ with a current density of $2A\;g^{-1}$. CSRG also showed an energy density of $25.2Wh\;kg^{-1}$ with a power density of $1000W\;kg^{-1}$.

• Membrane and Water Treatment
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• v.9 no.4
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• pp.211-220
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• 2018

Incorporating nano-materials in thin-film composite (TFC) membranes has been considered to be an approach to achieve higher membrane performance in various water treatment processes. This study investigated the rejection efficiency of three target compounds, i.e., reserpine, norfloxacin and tetracycline hydrochloride, by TFC membranes with different graphene oxide proportions. Graphene oxide (GO) was incorporated into the polyamide active layer of a TFC membrane via an interfacial polymerization (IP) reaction. The TFC membranes were characterized with FTIR, FE-SEM, AFM; in addition, the water contact angle measurements as well as the permeation and separation performance were evaluated. The prepared GO-TFC membranes exhibited a much higher flux ($3.11{\pm}0.04L/m2{\cdot}h{\cdot}bar$) than the pristine TFC membranes ($2.12{\pm}0.05L/m2{\cdot}h{\cdot}bar$) without sacrificing their foulant rejection abilities. At the same time, the GO-modified membrane appeared to be less sensitive to pH changes than the pure TFC membrane. A significant improvement in the anti-fouling property of the membrane was observed, which was ascribed to the favorable change in the membrane's hydrophilicity, surface morphology and surface charge through the addition of an appropriate amount of GO. This study predominantly improved the understanding of the different PA/GO membranes and outlined improved industrial applications of such membranes in the future.

• Journal of Korean Society of Environmental Engineers
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• v.37 no.11
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• pp.649-655
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• 2015

As a novel water treatment membrane, concept of ceramic-based graphene membrane (CbGM) was suggested, and its mass transport behavior was investigated. The selectivity of CbGM was given by graphene material which is consisting of active layer, only transmitting water, but rejecting salts. Filtration-assisted assembly methods was employed as a facile method to fabricate CbGM. Surface morphology and characteristics of CbGM were analyzed by scanning electron microscopy (SEM) and contact angle. In addition, three different kinds of solutes (i.e., NaCl, $MgCl_2$, $Na_2SO_4$) were tested in batch forward osmosis system to confirm the mass transport behavior. Through surface morphology analysis and mass transport behavior, it was revealed that interlocking between graphene layers is very important, rather than thickness of laminated graphene layers, in terms of selectivity to CbGM. All the anions in each solute showed faster transport than those of cations. In addition, solutes which have high ion valence charge ratio of anion to cation ($Z^-/Z^+$) was easier to be passed through CbGM. It indirectly implied that the surface charge of CbGM appear to be positive. In addition, It showed that surface charge of CbGM has a great role on mass transport, in particular, transport of matter having charges, generally ions.

• Membrane Journal
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• v.26 no.4
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• pp.239-252
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• 2016

Two-dimensional materials offer unique characteristics for membrane applications to water technology. With its atomic thickness, availability and stackability, graphene in particular is attracting attention in the research and industrial communities. Here, we present a brief overview of the recent research activities in this rising topic with bringing two membrane architecture into focus. Pristine graphene in single- and polycrystallinity poses a unique diffusion barrier property for most of chemical species at broad ambient conditions. If well designed and controlled, physical and chemical perforation can turn this barrier layer to a thinnest feasible membrane that permits ultimate permeation at given pore sizes. For subcontinuum pores, both molecular dynamics simulations and experiments predict potential salt rejection to envisage a seawater desalination application. Another novel membrane architecture is a stack of individual layers of 2D materials. When graphene-based platelets are chemically modified and stacked, the interplanar spacing forms a narrow transport pathway capable of separation of solvated ions from pure water. Bearing unbeknownst permeance and selectivity, both membrane architecture - ultrathin porous graphene and stacked platelets - offer a promising prospect for new extraordinary membranes for water technology applications.