• Journal of the Korean institute of surface engineering
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• v.49 no.2
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• pp.186-190
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• 2016

In this study, we fabricated the transparent photovoltaic device using 2-dimensional transition-metal dichalcogenides and investigated the transparency and photovoltaic characteristics. P-n heterojunction was formed by mechanical exfoliation and aligned transfer method on the transparent sheet using n-type $MoS_2$ and p-type $WSe_2$. Our transparent photovoltaic device exhibited the open-circuit voltage of ~ 0.15 V and the short-circuit current of 0.48 nA under illumination of white light.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.101.1-101.1
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• 2016

Van der Waals (vdW) heterostructures built from two-dimensional layered materials provide an unprecedented opportunity in designing new material systems because the lack of dangling bonds on the vdW surfaces enables the creation of high-quality heterointerfaces without the constraint of atomically precise commensurability. In particular, the ability to build artificial heterostructures, combined with the recent advent of transition metal dichalcogenides, allows the fabrication of unique semiconductor heterostructures in an ultimate thickness limit for fundamental studies as well as novel device applications. In this talk, we will present the characterization of the electronic and optoelectronic properties of atomically thin p-n junctions consisting of vertically stacked WSe2 and MoS2 monolayers. We observed gate-tunable diode-like current rectification and a photovoltaic response across the p-n interface. Unlike conventional bulk p-n junctions, the tunneling-assisted interlayer recombination of the majority carriers is responsible for the tenability of the charge transport and the photovoltaic response. Furthermore, we will discuss the enhanced optoelectronic characteristics in graphene-sandwiched vdW p-n junctions.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.100-100
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• 2016

Among various two-dimensional (2D) materials, 2D semiconductors and insulators have attracted a great deal of interest from nanoscience community beyond graphene, due to their attractive and unique properties. Such excellent characteristics have triggered highly active researches on 2D materials, such as hexagonal boron nitride (hBN), molybdenum disulfide (MoS2), and tungsten diselenide (WSe2). New physics observed in 2D semiconductors allow for development of new-concept devices. Especially, these emerging 2D materials are promising candidates for flexible and transparent electronics. Recently, van der Waals heterostructures (vdWH) have been achieved by putting these 2D materials onto another, in the similar way to build Lego blocks. This enables us to investigate intrinsic physical properties of atomically-sharp heterostructure interfaces and fabricate high performance optoelectronic devices for advanced applications. In this talk, fundamental properties of various 2D materials will be introduced, including growth technique and influence of defects on properties of 2D materials. We also fabricate high performance electronic/optoelectronic devices of vdWH, such as transistors, memories, and solar cells. The device platform based on van der Waals heterostructures show huge improvement of devices performance, high stability and transparency/flexibility due to unique properties of 2D materials and ultra-sharp heterointerfaces. Our work paves a new way toward future advanced electronics based on 2D materials.

• Applied Microscopy
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• v.45 no.3
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• pp.119-125
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• 2015

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted significant attention due to their unique and exotic properties attributed to their low dimensionality. In particular, semiconducting 2D TMDCs such as $MoS_2$, $WS_2$, $MoSe_2$, and $WSe_2$ have been demonstrated to be feasible for various advanced electronic and optical applications. In these regards, process to synthesize high quality 2D TMDCs layers with high reliability, wafer-scale uniformity, controllable layer number and excellent electronic properties is essential in order to use 2D TMDCs in practical applications. Vapor deposition techniques, such as physical vapor deposition, chemical vapor deposition and atomic layer deposition, could be promising processes to produce high quality 2D TMDCs due to high purity, thickness controllability and thickness uniformity. In this article, we briefly review recent research trend on vapor deposition techniques to synthesize 2D TMDCs.

• Proceedings of the Korean Vacuum Society Conference
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• 2015.08a
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• pp.213.1-213.1
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• 2015

Field effect transistors (FETs)를 기반으로 한 바이오센서는 빠른 응답속도, 저비용, label-free 등을 이유로 각광받고 있다. 그러나 3D 구조를 기반으로 한 FETs 바이오센서의 낮은 sensitivity의 한계점을 지니며, 이를 극복하기 위해 1D 구조의 나노튜브 등을 활용하였으나 여전히 높은 sensitivity의 확보는 힘들다. 최근에는 이러한 문제점을 극복하기 위해 이차원 반도체 물질 중 하나인 Transition metal dichalcogenide (TMD)를 이용하여, 700 이상의 sensitivity를 지니는 pH센서 및 100 이상의 sensitivity를 지니는 바이오센서가 보고되었다. 하지만 이보다 더 높은 정확성 및 반응성을 높이기 위한 연구는 부족한 실정이다. 우리는 DNA 템플릿을 이용하여, TMD FET 기반 pH 및 바이오센서의 반응성을 극대화시키는 연구를 선보인다. DNA는 7~8정도의 유전상수 (K)를 가지는 물질로 기존 $SiO_2$(K=3.9)보다 높은 유전상수를 가지며 두께를 0.7 nm로 매우 얇게 형성할 수 있는 장점이 있다. 이는 FET 기반 바이오센서의 표면 캐패시턴스를 높여 sensitivity를 극대화할 수 있으며, 기존에 사용된 high-k 기반 바이오센서와 비교하여도 약 10배 이상의 sensitivity 향상을 노릴 수 있다. 또한, TMD 물질로 우리는 $WSe_2$를 선택하였으며, pH 용액의 receptor로써 우리는 3-Aminopropyltriethoxysilane (APTES)를 활용하였고, 템플릿으로 사용된 DNA는 DX tile 및 Ring type의 두 가지를 사용하였다. 추가로, DNA의 phosphate backbone을 중성화시키고 DNA의 base pairing의 charge 안정화를 위해 구리 이온($Cu^{2+}$) 및 란타넘족($Tb^{3+}$)을 추가하였다. 완성된 바이오센서의 pH 센싱을 위해 우리는 pH 6,7,8의 표준 용액을 사용하였으며, 재현성 및 반복성의 확인하였다.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.291.1-291.1
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• 2016

Graphene by one of the two-dimensional (2D) materials has been focused on electronic applications due to its ultrahigh carrier mobility, outstanding thermal conductivity and superior optical properties. Although graphene has many remarkable properties, graphene devices have low on/off current ratio due to its zero bandgap. Despite considerable efforts to open its bandgap, it's hard to obtain appropriate improvements. To solve this problem, heterojunction barristor was proposed based on graphene. Mostly, this heterojunction barristor is made by transition metal dichalcogenides (TMDs), such as molybdenum disulfide ($MoS_2$) and tungsten diselenide ($WSe_2$), which have extremely thickness scalability of TMDs. The heterojunction barristor has the advantage of controlling graphene's Fermi level by applying gate bias, resulting in barrier height modulation between graphene interface and semiconductor. However, charged impurities between graphene and $SiO_2$ cause unexpected p-type doping of graphene. The graphene's Fermi level modulation is expected to be reduced due to this p-doping effect. Charged impurities make carrier mobility in graphene reduced and modulation of graphene's Fermi level limited. In this paper, we investigated theoretically and experimentally a relevance between graphene's Fermi level and p-type doping. Theoretically, when Fermi level is placed at the Dirac point, larger graphene's Fermi level modulation was calculated between -20 V and +20 V of $V_{GS}$. On the contrary, graphene's Fermi level modulation was 0.11 eV when Fermi level is far away from the Dirac point in the same range. Then, we produced two types heterojunction barristors which made by p-type doped graphene and graphene treated 2.4% APTES, respectively. On/off current ratio (32-fold) of graphene treated 2.4% APTES was improved in comparison with p-type doped graphene.

• Korean Journal of Cognitive Science
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• v.11 no.2
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• pp.69-78
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• 2000

Originally the Interactive Activation Model(IAM) was developed to explain World Superiority Effect(WSE) in the English words. It is known that there is a similar phenomena in Korean characters. In other words people perceive a grapheme better when it is presented as a component of a character than when it is presented alone. We modified the original IAM to explain the Character Superiority Effect(CSE) for Korean characters. However it is also reported that the degree of CSE for Korean characters varies depending on the type of the character. Especially a component between components was reported to be hard to perceive even though it is in a context. It was supposed that this special phenomenon exists for CSE of Korean characters because Korean character is a two-dimensional composition of components(graphemes). And we could explain this phenomenon by introducing weights for the input stimulus which are calculated by taking into account the two-dimensional shape of the character.

• Journal of Korea Water Resources Association
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• v.48 no.12
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• pp.1077-1087
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• 2015

The levee protect lifes, houses, and properties by blocking overflow of river. The revetment is forced to be covered on the slope of levee in order to prevent erosion. The stability of revetment is very important enough to directly connected to the stability of levee. In this study, the weak points of revetment on meandering channel were found by movable revetment experiment and the velocity and the water surface elevation (WSE) were measured at main points. The 3-D numerical simulations were performed under same conditions with experiment. And unclear flow characteristics by the limit of measuring instruments were analyzed through numerical simulation. Consequently, the section of large wall shear stress and the failure section are almost the same. Despite of small wall shear stress, the revetments located at right bank were carried away because of circulation zone due to secondary flow by meandering. With existing riprap design formula, the sizes of riprap determined using maximum local velocity were 1.5~4.7 times greater than them using mean velocity. As a result of this study, it is necessary to calculate the size of riprap in other ways for meandering and straight channel. At a later study, if the weighted value considered the radius of curvature and shape of hydraulic structure is applied to riprap design formula, it is expected that the size of revetment was evaluated rationally and accurately.