• 한국진공학회:학술대회논문집
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• 한국진공학회 2014년도 제46회 동계 정기학술대회 초록집
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• pp.185.2-185.2
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• 2014

We have studied the atomic and electronic structure of graphene nanoribbons (GNRs) on a hexagonal boron nitride (h-BN) sheet with intercalated atoms using first-principles calculations. The h-BN sheet is an insulator with the band gap about 6 eV and then it may a good candidate as a supporting dielectric substrate for graphene-based nanodevices. Especially, the h-BN sheet has the similar bond structure as graphene with a slightly longer lattice constant. For the computation, we use the Vienna ab initio simulation package (VASP). The generalized gradient approximation (GGA) in the form of the PBE-type parameterization is employed. The ions are described via the projector augmented wave potentials, and the cutoff energy for the plane-wave basis is set to 400 eV. To include weak van der Waals (vdW) interactions, we adopt the Grimme's DFT-D2 vdW correction based on a semi-empirical GGA-type theory. Our calculations reveal that the localized states appear at the zigzag edge of the GNR on the h-BN sheet due to the flat band of the zigzag edge at the Fermi level and the localized states rapidly decay into the bulk. The open-edged graphene with a large corrugation allows some space between graphene and h-BN sheet. Therefore, atoms or molecules can be intercalated between them. We have considered various types of atoms for intercalation. The atoms are initially placed at the edge of the GNR or inserted in between GNR and h-BN sheet to find the effect of intercalated atoms on the atomic and electronic structure of graphene. We find that the impurity atoms at the edge of GNR are more stable than in between GNR and h-BN sheet for all cases considered. The nickel atom has the lowest energy difference of ~0.2 eV, which means that it is relatively easy to intercalate the Ni atom in this structure. Finally, the magnetic properties of intercalated atoms between GNR and h-BN sheet are investigated.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2014년도 제46회 동계 정기학술대회 초록집
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• pp.433.1-433.1
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• 2014

The research of graphene, a monolayer of carbon atoms with honeycomb lattice structure, has explosively increased after appeared in 2004. As a result, its high transmittance, mobility, thermal conductivity, and outstanding mechanical and chemical stability have been proved. Especially, many researches were executed about the field of transparent electrode highlighting material of substituting the indium tin oxide (ITO). In addition, qualitative and quantitative improvements have been achieved due to many synthesis methods were discovered. Among them, mostly used method is chemical vapour deposition of graphene grown on copper or nickel. The transmittance, mobility, sheet resistance, and other many properties are completely changed according to these two types of synthesis method of graphene. In this research, considering the difference of characteristics as the synthesis method of graphene, what types of graphene should be used and how to use it were studied. The stacked graphene harvested on copper and multi-layer graphene harvested on nickel were compared and analyzed, as a result, the transmittance of 90% and the sheet resistance of $70{\Omega}{\square}$ was showed even though stacked graphene layers were 4 layers. The reason that could bring these results is lowered sheet resistance due to stacked monolayer graphenes. Moreover, light output power of the three stacked graphene spreading layer shows the highest value, but light-emitting diode with multi-layer graphene died out from 12mA due to also its high sheet resistance. Therefore, we need to clarify about what types of graphene and how to use the graphene in use.

• 센서학회지
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• 제23권4호
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• pp.234-237
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• 2014

In this paper, a copper foil-thick grapheme (thin graphite sheet)-copper foil structure is reported to achieve mechanically strong and high thermal conductive layer suitable for heat spreading components. Since graphene provides much higher thermal conductivity than copper, thick graphene embedded copper layer can achieve higher effective thermal conductivity which is proportional to graphene/copper thickness ratio. Since copper is nonreactive with carbon material which is graphene, chromium is used as adhesion layer to achieve copper-thick graphene-copper bonding for graphene embedded copper layer. Both sides of thick graphene were coated with chromium as an adhesion layer followed by copper by sputtering. The copper foil was bonded to sputtered copper layer on thick graphene. Angstrom's method was used to measure the thermal conductivity of fabricated copper-thick graphene-copper structure. The thermal conductivity of the copper-thick graphene-copper structures is measured as $686W/m{\cdot}K$ which is 1.6 times higher than thermal conductivity of pure copper.

• Applied Science and Convergence Technology
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• 제26권4호
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• pp.55-61
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• 2017

Graphene, with a carrier mobility achieving up to $140,000cm^2/Vs$ at room temperature, makes it an ideal material for application in semiconductor devices. However, when the metal comes in contact with the graphene sheet, an energy barrier forms at the metal-graphene interface, resulting in a drastic reduction of the carrier mobility of graphene. In this review, the various methods of forming metal-graphene covalent contacts to lower the contact resistance are discussed. Furthermore, the graphene sheet in the area of metal contact can be cut in certain patterns, also discussed in this review, which provides a more efficient approach to forming covalent contacts, ultimately reducing the contact resistance for the realization of high-performance graphene devices.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2015년도 제49회 하계 정기학술대회 초록집
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• pp.221.2-221.2
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• 2015

Graphene is very interesting 2 dimensional material providing unique properties. Especially, graphene has been investigated as a stretchable and transparent conductor due to its high mobility, high optical transmittance, and outstanding mechanical properties. On the contrary, high sheet resistance of extremely thin monolayer graphene limits its application. Artificially stacked multilayer graphene is used to decrease its sheet resistance and has shown improved results. However, stacked multilayer graphene requires repetitive and unnecessary transfer processes. Recently, growth of multilayer graphene has been investigated using a chemical vapor deposition (CVD) method but the layer controlled synthesis of multilayer graphene has shown challenges. In this paper, we demonstrate controlled growth of multilayer graphene using a two-step process with multi heating zone low pressure CVD. The produced graphene samples are characterized by optical microscope (OM) and scanning electron microscopy (SEM). Raman spectroscopy is used to distinguish a number of layers in the multilayer graphene. Its optical and electrical properties are also analyzed by UV-Vis spectrophotometer and probe station, respectively. Atomic resolution images of graphene layers are observed by high resolution transmission electron microscopy (HRTEM).

• 한국분말재료학회지
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• 제26권1호
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• pp.11-15
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• 2019

In this study, a method to remove residual powder on a multi-layered graphene and a new approach to transfer multi-layered graphene at once are studied. A graphene one-step transfer (GOST) method is conducted to minimize the residual powder comparison with a layer-by-layer transfer. Furthermore, a residual powder removing process is investigated to remove residual powder at the top of a multi-layered graphene. After residual powder is removed, the sheet resistance of graphene is decreased from 393 to 340 Ohm/sq in a four-layered graphene. In addition, transmittance slightly increases after residual powder is removed from the top of the multi-layered graphene. Optical and atomic-force microscopy images are used to analyze the graphene surface, and the Ra value is reduced from 5.2 to 3.7 nm following residual powder removal. Therefore, GOST and residual powder removal resolve the limited application of graphene electrodes due to residual powder.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2014년도 제46회 동계 정기학술대회 초록집
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• pp.210.1-210.1
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• 2014

The hexagonal boron nitride (h-BN) sheet, a 2D material like graphene sheet, is comprised of boron and nitrogen atoms. Similar to graphene, h-BN sheet has attractive mechanical properties while it has a wide band gap unlike graphene. Recently, many experimental groups studied the growth of single BN layer by chemical vapor deposition (CVD) method on the copper substrate. To study the initial stage of h-BN growth on the copper surface, we have performed density functional theory calculations. We investigate several adsorption sites of a boron or nitride atom on the Cu surfaces. Then, by increasing the number of adsorbed B and N atoms, we study formation behaviors of the BN flakes on the surface. Several types of BN flakes atoms such as triangular, linear, and hexagonal shapes are considered on the copper surface. We find that the formation of the BN flake in triangular shape is most favorable on the surface. On the basis of the theoretical results, we discuss the growth mechanism of h-BN layer on the copper surfaces in terms of its shapes in the initial stage of growth.

• Structural Engineering and Mechanics
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• 제66권6호
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• pp.693-701
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• 2018

This paper develops a nonlocal strain gradient plate model for vibration analysis of graphene sheets under thermal environments. For more accurate analysis of graphene sheets, the proposed theory contains two scale parameters related to the nonlocal and strain gradient effects. Graphene sheet is modeled via a two-variable shear deformation plate theory needless of shear correction factors. Governing equations of a nonlocal strain gradient graphene sheet on elastic substrate are derived via Hamilton's principle. Differential quadrature method (DQM) is implemented to solve the governing equations for different boundary conditions. Effects of different factors such as temperature rise, nonlocal parameter, length scale parameter, elastic foundation and aspect ratio on vibration characteristics a graphene sheets are studied. It is seen that vibration frequencies and critical buckling temperatures become larger and smaller with increase of strain gradient and nonlocal parameter, respectively.

• 한국광학회지
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• 제32권3호
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• pp.114-119
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• 2021

산업용으로 이용하는 대면적 그래핀 시트는 주로 Cu foil 위에서 화학증기증착법(chemical vapor deposition, CVD)을 이용하여 성장된다. 그러나 모든 면적에서 균일하게 성장되는 것은 아니므로, 품질이 불만족스러운 그래핀 필름을 제외시키는 과정이 필수적으로 요구된다. 비침습적인 반사형 광학적인 방법을 사용하면, 그래핀의 성장프로세스 도중이나 성장 후에 빠르고 편리하게 분류할 수 있다. 본 논문에서는 국소적인 그래핀 필름의 파장별 반사 대비율(reflectance contrast)이 그래핀의 품질과 밀접한 관련이 있어 품질이 불만족스러운 그래핀 필름을 제외시키는 데에 효과적인 데이터로 이용될 수 있음을 밝혔다. 파장별 반사 대비율을 계산하기 위해서, 화학적 퍼텐셜(chemical potential)과 전자간 널뛰기(hopping) 에너지, 그리고 온도 등의 요소가 성장된 그래핀의 광학적 반사 대비율에 어떠한 영향을 미치는지 조사하였다.

• Advances in nano research
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• 제6권2호
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• pp.147-162
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• 2018

A new nonlocal higher order shear deformation theory (HSDT) is developed for buckling properties of single graphene sheet. The proposed nonlocal HSDT contains a new displacement field which incorporates undetermined integral terms and contains only two variables. The length scale parameter is considered in the present formulation by employing the nonlocal differential constitutive relations of Eringen. Closed-form solutions for critical buckling forces of the graphene sheets are obtained. Nonlocal elasticity theories are used to bring out the small scale influence on the critical buckling force of graphene sheets. Influences of length scale parameter, length, thickness of the graphene sheets and shear deformation on the critical buckling force have been examined.