• Journal of Power System Engineering
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• v.17 no.5
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• pp.23-29
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• 2013

The activation process is the key to graphene's practical application. In this study, the effect of grinding speed in planetary ball mill on structural integrity of graphene has been studied at various grinding speed such as 100 rpm, 200 rpm, 300 rpm, 400 rpm and 500 rpm. The morphology and structure of pristine graphene and ground graphenes were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy respectively. According to these results, structural properties of graphene were improved when grinding speed was increased.

• Korean Journal of Materials Research
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• v.21 no.2
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• pp.78-82
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• 2011

Mono- and few-layer graphenes were grown on Ni thin films by rapid-thermal pulse chemical vapor deposition technique. In the growth steps, the exposure step for 60 s in $H_2$ (a flow rate of 10 sccm (standard cubic centimeters per minute)) atmosphere after graphene growth was specially established to improve the quality of the graphenes. The graphene films grown by exposure alone without $H_2$ showed an intensity ratio of $I_G/I_{2D}$ = 0.47, compared with a value of 0.38 in the films grown by exposure in H2 ambient. The quality of the graphenes can be improved by exposure for 60 s in $H_2$ ambient after the growth of the graphene films. The physical properties of the graphene films were investigated for the graphene films grown on various Ni film thicknesses and on 260-nm thick Ni films annealed at 500 and $700^{\circ}C$. The graphene films grown on 260-nm thick Ni films at $900^{\circ}C$ showed the lowest $I_G/I_{2D}$ ratio, resulting in the fewest layers. The graphene films grown on Ni films annealed at $700^{\circ}C$ for 2 h showed a decrease of the number of layers. The graphene films were dependent on the thickness and the grain size of the Ni films.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.159-159
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• 2013

Atomically thin graphene is the ideal model system for studying nanoscale friction due to its intrinsic two-dimensional anisotropy. Furthermore, modulating its tribological properties could be an important milestone for graphene-based micro and nano-mechanical devices. Here, we report that the tribological properties can be easily altered via simple chemical modifications of the graphene surface. Friction force microscopy measurements show that hydrogenated, fluorinated, and oxidized graphenes exhibit, 2-, 6-, and 7-fold enhanced nanoscale friction on their surfaces, respectively, compared to pristine graphene. The measured nanoscale friction should be associated with the adhesive and elastic properties of the chemically modified graphenes. Density functional theory calculations suggest that, while the adhesive properties of chemically modified graphenes are marginally reduced down to ~30%, the out-of-plane elastic properties are drastically increased up to 800%. Based on these findings, we propose that nanoscale friction on graphene surfaces is characteristically different from that on conventional solid surfaces; stiffer graphene exhibits higher friction, whereas a stiffer three-dimensional solid generally exhibits lower friction. The unusual friction mechanics of graphene is attributed to the intrinsic mechanical anisotropy of graphene, which is inherently stiff in plane, but remarkably flexible out of plane. The out-of-plane flexibility can be modulated up to an order of magnitude by chemical treatmentof the graphene surface. The correlation between the measured nanoscale friction and the calculated out-of-plane flexibility suggests that the frictional energy in graphene is mainly dissipated through the out-of-plane vibrations, or the flexural phonons of graphene.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.27 no.1
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• pp.56-60
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• 2014

The polyimide composite membranes were prepared with polyimide composite solutions including graphenes by using the phase inversion method. The morphologies of these membranes were significantly changed according to the graphene loadings in composite solutions and the solvent systems of the composite solutions. The finger-like macro-voids were formed in the hollow fiber membranes which were prepared in the NMP solvent system with a small amount of ethanol. As increasing the content of the viscous alcohols such as glycerol or 1,3-propanediol in the composite solution, however, the morphologies of the hollow fiber membranes were changed to sponge-like types. In case of flat membranes, the increase of graphene content in polyimide composites causes that their membranes change from the finger-like macro-porous to sponge-like morphologies.

• Journal of Electrochemical Science and Technology
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• v.9 no.4
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• pp.251-259
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• 2018

In general, the charge storage characteristics and overall performance of electrochemical energy devices (such as lithiumion batteries and supercapacitors) significantly depends on the structural and geometrical factors of the electrodes' active materials. The most widely used active materials of electrochemical energy storage devices are based on carbons of various forms. Each carbon type has drawbacks and advantages when used as the electrode material. Studies have been recently carried out to combine different types of carbons, in particular nanostructured carbons, in order to overcome the structure-originated limitations and thus enhance the overall electrochemical performances. In this feature article, we report the recent progress on the development of this novel class of materials (multidimensional nanocarbons), and their applications for supercapacitors. Multidimensional nanocarbons include graphenes/carbon nanotubes (CNTs), CNTs/carbon films, CNTs/fullerenes, and ternary carbon nanostructures. Various applications using these multidimensional nanocarbons have been proposed and demonstrated in the literature. Owing to the recent extensive studies on electrochemical energy storage devices and considering that carbons are their most fundamental electrode materials, the number of reports on nanocarbons employed as electrodes of the electrochemical energy storage devices is rapidly increasing. Recently, numerous multidimensional nanocarbons have been designed, prepared, and utilized as electrodes of electrochemical capacitors or supercapacitors, which are considered next-generation energy devices owing to their unique merits compared to the conventional structures. In this review, we summarize the basic motivations, preparation methods, and resultant supercapacitor performances of each class of multidimensional nanocarbons published in the literature, focusing on recent reports.

• Polymer(Korea)
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• v.36 no.3
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• pp.251-261
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• 2012

The nanocomposites containing graphene oxide flakes were prepared in order to improve the mechanical properties of artificial marbles based on poly(methyl methacrylate)(PMMA) matrix. Graphene oxide flakes were prepared from graphite by oxidation with Hummers method followed by exfoliation with thermal treatment. Surface of graphene oxide flakes were modified with oxyfluorination in various oxygene:fluorine compositions to improve the interfacial compatibility. The nanocomposites containing graphenes modified with oxyfluorination in the oxygen content of 50% and higher showed the significant increase in flexural strength, flexural modulus, Rockwell hardness, Barcol hardness, and Izod impact strength. The morphology of fractured surface showed the improved interfacial adhesion between PMMA matrix and the graphenes which were properly treated with oxyfluorination. The mechanical properties of nanocomposite were deteriorated by increasing the content of graphene above 0.07 phr due to the nonuniform dispersion of graphenes.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.491.1-491.1
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• 2014

Heteroatom phosphorous-doped graphene aerogel (PGA) with high surface area is successfully synthesized via hydrothermal method for high power and energy supercapacitors, including the advantage of three dimensional internetwork and constitutive graphene skeletons. The morphology of PGA was investigated by the scanning electron microscope, transmission electron microscope. The chemical structure and circumstances were confirmed by Raman and X-ray photoelectron spectroscopy, the phosphorus is successfully incorporated with the graphene sheets. As evidenced by electrochemical measurements, cyclic voltammetry and galvanostatic charge discharge, the hierarchically PGA has an unprecedented high capacitance, which contributes to the excellent high-rate performance of this material for supercapacitor application.

• Korean Journal of Materials Research
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• v.22 no.3
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• pp.130-135
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• 2012

We present a method of graphene synthesis with high thickness uniformity using the thermal chemical vapor deposition (TCVD) technique; we demonstrate its application to a grid supporting membrane using transmission electron microscope (TEM) observation, particularly for nanomaterials that have smaller dimensions than the pitch of commercial grid mesh. Graphene was synthesized on electron-beam-evaporated Ni catalytic thin films. Methane and hydrogen gases were used as carbon feedstock and dilution gas, respectively. The effects of synthesis temperature and flow rate of feedstock on graphene structures have been investigated. The most effective condition for large area growth synthesis and high thickness uniformity was found to be $1000^{\circ}C$ and 5 sccm of methane. Among the various applications of the synthesized graphenes, their use as a supporting membrane of a TEM grid has been demonstrated; such a grid is useful for high resolution TEM imaging of nanoscale materials because it preserves the same focal plane over the whole grid mesh. After the graphene synthesis, we were able successfully to transfer the graphenes from the Ni substrates to the TEM grid without a polymeric mediator, so that we were able to preserve the clean surface of the as-synthesized graphene. Then, a drop of carbon nanotube (CNT) suspension was deposited onto the graphene-covered TEM grid. Finally, we performed high resolution TEM observation and obtained clear image of the carbon nanotubes, which were deposited on the graphene supporting membrane.

• Proceedings of the Korean Vacuum Society Conference
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• 2015.08a
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• pp.239.2-239.2
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• 2015

Graphene is a most interesting material due to its unique and outstanding properties. However, semi-metallic properties of graphene along with zero bandgap energy structure limit further application to optoelectronic devices. Recently, many researchers have shown that band gap can be induced in the Bernal stacked bilayer graphene. Several methods have been used for the controlled growth of the Bernal staked bilayer graphene, but it is still challenging to control the growth process. In this paper, we synthesize the large area Bernal stacked bilayer graphene using multi heating zone low pressure chemical vapor deposition (LPCVD). The synthesized bilayer graphenes are characterized by Raman spectroscopy, optical microscope (OM), scanning electron microscopy (SEM). High resolution transmission electron microscopy (HRTEM) is used for the observation of atomic resolution image of the graphene layers.

• Carbon letters
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• v.14 no.2
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• pp.63-75
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• 2013

Graphene, a carbon crystal sheet of molecular thickness, shows diverse and exceptional properties ranging from electrical and thermal conductivities, to optical and mechanical qualities. Thus, its potential applications include not only physicochemical materials but also extends to biological uses. Here, we review recent experimental studies about graphene for such bioapplications. As a prerequisite to the search to determine the potential of graphene for bioapplications, the essential qualities of graphene that support biocompatibility, were briefly summarized. Then, direct examples of tissue regeneration and tissue engineering utilizing graphenes, were discussed, including uses for cell scaffolds, cell modulating interfaces, drug delivery, and neural interfaces.