• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.229.2-229.2
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• 2014

Graphene, an allotrope of carbon, is a two-dimensional material having a unique electro-mechanical property that shows significant change of the electrical conductance under the applied strain. In addition of the extraordinary mechanical strength [1], graphene becomes a prospective candidate for pressure sensor technology [2]. However, very few investigations have been carried out to demonstrate characteristics of graphene sensor as a device form. In this study, we demonstrate a pressure sensor using graphene double layer as an active channel to generate electrical signal as the response of the applied vertical pressure. For formation of the active channel in the pressure sensor, two single graphene layers which are grown on Cu foil (25 um thickness) by the plasma enhanced chemical vapor deposition (PECVD) are sequentially transformed to the poly-di-methyl-siloxane (PDMS) substrate. Dry and wet transfer methods are individually employed for formation of the double layer graphene. This sensor geometry results a switching characteristic which shows ~900% conductivity change in response to the application of pulsed pressure of 5 kPa whose on and off duration is 3 sec. Additionally, the functional reliability of the sensor confirms consistent behavior with a 200-cycle test.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.287.2-287.2
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• 2013

We synthesized a new composite of poly sodium 4-styrenesulfonate intercalated graphene oxide for energy storage devices by controlling oxidation time in the synthesis of graphite oxide. Specific capacitance was improved from 20 F/g of the previous composites to 88 F/g of the new composite at the current density of 0.3 A/g. The capacitance retention was 94% after 3000 cycles, indicating that the new composites of high cyclic stability, prominent performance as electric double layer capacitor, and even low resistance could be an excellent carbon based electrode for further energy storage devices.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.31 no.2
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• pp.112-116
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• 2018

Electric double layer capacitors (EDLCs) are promising candidates for energy storage devices in electronic applications. An EDLC yields high power density but has low specific capacitance. Carbon material is used in EDLCs owing to its large specific surface area, large pore volume, and good mechanical stability. Consequently, the use of carbon materials for EDLC electrodes has attracted considerable research interest. In this paper, in order to evaluate the electrochemical performance, graphene is used as an EDLC electrode with flake sizes of 3, 12, and 60 nm. The surface characteristic and electrochemical properties of graphene were investigated using SEM, BET, and cyclic voltammetry. The specific capacitance of the graphene based EDLC was measured in a 1 M $TEABF_4/ACN$ electrolyte at the scan rates of 2, 10, and 50 mV/s. The 3 nm graphene electrode had the highest specific capacitance (68.9 F/g) compared to other samples. This result was attributed to graphene's large surface area and meso-pore volume. Therefore, large surface area and meso-pore volume effectively enhances the specific capacitance of EDLCs.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.228.2-228.2
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• 2014

Characteristics of high Fermi velocity, high mechanical strength, and transparency offer tremendous advantages for using graphene as a promising transparent conducting material [1] in electronic devices. Although graphene is a prospective candidate for touch sensor with strong mechanical properties [2] and flexibility, only few investigations have been carried out in the field of sensor as a device form. In this study, we suggest ultra-highly sensitive and transparent graphene touch sensor fabricated by single layer graphenes. One of the graphene layers is formed in the top panel as a disconnected graphene beam transferred on PDMS, and the other of the graphene layer is formed with line-patterning on the bottom panel of triple structure PET/PI/SiO2. The touch sensor shows characteristics of flexible. Its transmittance is approximately 75% where transmittance of the top panel and the bottom panel are 86.3% and 87%, respectively, at 550 nm wavelength. Sheet resistance of each graphene layer is estimated as low as $971{\Omega}/sq$. The results show that the conductance change rate (${\Delta}C/C0$) is $8{\times}105$ which depicts ultra-high sensitivity. Moreover, reliability characteristic confirms consistent behavior up to a 100-cycle test.

• Current Optics and Photonics
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• v.8 no.4
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• pp.399-405
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• 2024

In this paper, an optical modulator based on monolayer graphene and triple-layer black phosphorus (BP) heterojunction in the optical communication band range is designed. The influences of geometric parameters, chemical potential, BP orientation and dispersion on the fundamental mode of this modulator were determined in detail by the finite-difference time-domain (FDTD) method. Using appropriate geometric parameter settings, the extinction ratio of this proposed modulator is 0.166 dB, while the modulator with a working length of 3 ㎛ can realize a 0.498 dB modulation depth. The 3-dB bandwidth of this modulator could achieve up to 2.65 GHz with 27.23 fJ/bit energy consumption. The extinction ratio and bandwidth of the proposed modulator increased by 66% and 120.83%, respectively, compared to the monolayer graphene-based ridge-type waveguide modulator. Energy consumption was reduced by 97.28%, compared to a double-layer graphene-based modulator.

• Proceedings of the Materials Research Society of Korea Conference
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• 2012.05a
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• pp.69.3-69.3
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• 2012

A high-performance low-voltage graphene field-effect transistor (FED array was fabricated on a flexible polymer substrate using solution-processable, high-capacitance ion gel gate dielectrics. The high capacitance of the ion gel, which originated from the formation of an electric double layer under the application of a gate voltage, yielded a high on-current and low voltage operation below 3 V. The graphene FETs fabricated on the plastic substrates showed a hole and electron mobility of 203 and 91 $cm^2/Vs$, respectively, at a drain bias of - I V. Moreover, ion gel gated graphene FETs on the plastic substrates exhibited remarkably good mechanical flexibility. This method represents a significant step in the application of graphene to flexible and stretchable electronics.

• The Journal of the Korea institute of electronic communication sciences
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• v.9 no.2
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• pp.167-171
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• 2014

Using chemical vapor deposition(CVD), the synthesis of graphene was performed on poly and single crystalline Cu substrates. The growth behavior of graphene and its characterization were shown utilizing the optical microscopic image and its image analysis. As a result in the analysis of graphene growth, it was found out the graphene is growing always in particular direction in relation to the crystalline direction of a single grain in polycrystalline Cu substrate. With the image analysis it was possible to show the characterization of graphene, such as the growth direction and the number of layers showing single, double and triple layers, within the neighboring single grains in polycrystalline Cu. In addition, the relatively large area of graphene with about $3mm^2$ on Cu(111) having high quality, single layer, and single crystalline was shown along with its characterization.

• Journal of Sensor Science and Technology
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• v.26 no.2
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• pp.122-127
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• 2017

Graphene grown on copper-foil substrates by chemical vapor deposition (CVD) has been attracting interest for sensor applications due to an extraordinary high surface-to-volume ratio and capability of large-scale device fabrication. However, CVD graphene has a polycrystalline structure and a high density of grain boundaries degrading its electrical properties. Recently, processes such as electropolishing for flattening copper substrate has been applied before growth in order to increase the grain size of graphene. In this study, we systemically analyzed the effects of the process condition of electropolishing copper foil on the quality of CVD graphene. We observed that electropolishing process can reduce surface roughness of copper foil, increase the grain size of CVD graphene, and minimize the density of double-layered graphene regions. However, excessive process time can rather increase the copper foil surface roughness and degrade the quality of CVD graphene layers. This work shows that an optimized electropolishing process on copper substrates is critical to obtain high-quality and uniformity CVD graphene which is essential for practical sensor applications.

• Advances in nano research
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• v.9 no.4
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• pp.221-235
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• 2020

The following composition establishes a nonlocal strain gradient plate model that is essentially related to mass sensors laying on Winkler-Pasternak medium for the vibrational analysis from graphene sheets. To achieve a seemingly accurate study of graphene sheets, the posited theorem actually accommodates two parameters of scale in relation to the gradient of the strain as well as non-local results. Model graphene sheets are known to have double variant shear deformation plate theory without factors from shear correction. By using the principle of Hamilton, to acquire the governing equations of a non-local strain gradient graphene layer on an elastic substrate, Galerkin's method is therefore used to explicate the equations that govern various partition conditions. The influence of diverse factors like the magnetic field as well as the elastic foundation on graphene sheet's vibration characteristics, the number of nanoparticles, nonlocal parameter, nanoparticle mass as well as the length scale parameter had been evaluated.

• Applied Chemistry for Engineering
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• v.18 no.5
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• pp.522-526
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• 2007

Structural features and electrochemical performances of cokes pyrolized from oxidized cokes were examined, and compared with KOH-activated coke. Needle cokes ($d_{002}=3.5{\AA} $), having a graphene layer structure, were changed to a single phase of graphite oxide after oxidation treatment with an acidic solution having an $NaCLO_3$/needle coke composition ratio of above 7.5, and the inter-layer distance of the oxidized coke was expanded to $6.9{\AA} $ with increasing oxygen content. After heating at $200^{\circ}C$, the oxidized coke was pyrolized to the graphene layer structure with inter-layer distance of $3.6{\AA} $. However, the change of the inter-layer distance of the needle coke was not observed in the KOH activation process. On the other hand, an intercalation of electrolyte ions into the pyrolized coke, observed at first charge, occurred at 1.0 V, in which the value was lower than that of KOH-activation coke. The cell capacitor using pyrolized coke exhibited a lower internal resistance of $0.57{\Omega}$ in 1 kHz, and a larger capacitance per weight and volume of 30.3 F/g and 26.9 F/ml at the two-electrode system in the potential range 0~2.5 V than those of the cell capacitor using KOH-activation of coke. This better electrochemical performance may be associated with structure defects in the graphene layer derived from the process of the inter-layer expansion and shrinkage.