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- 저자 : ( Stephen C Pak ) (검색결과 10 건)
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Pak, Sangyeon,Lee, Juwon,Lee, Young-Woo,Jang, A-Rang,Ahn, Seongjoon,Ma, Kyung Yeol,Cho, Yuljae,Hong, John,Lee, Sanghyo,Jeong, Hu Young,Im, Hyunsik,Shin, Hyeon Suk,Morris, Stephen M.,Cha, SeungNam,Sohn American Chemical Society 2017 NANO LETTERS Vol.17 No.9
<P/><P>van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS<SUB>2</SUB>/WS<SUB>2</SUB> vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS<SUB>2</SUB> to MoS<SUB>2</SUB> in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices.</P>
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Pak, Sangyeon,Cho, Yuljae,Hong, John,Lee, Juwon,Lee, Sanghyo,Hou, Bo,An, Geon-Hyoung,Lee, Young-Woo,Jang, Jae Eun,Im, Hyunsik,Morris, Stephen M.,Sohn, Jung Inn,Cha, SeungNam,Kim, Jong Min American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.44
<P>Phototransistors that are based on a hybrid vertical heterojunction structure of two-dimensional (2D)/quantum dots (QDs) have recently attracted attention as a promising device architecture for enhancing the quantum efficiency of photodetectors. However, to optimize the device structure to allow for more efficient charge separation and transfer to the electrodes, a better understanding of the photophysical mechanisms that take place in these architectures is required. Here, we employ a novel concept involving the modulation of the built-in potential within the QD layers for creating a new hybrid MoS<SUB>2</SUB>/PbS QDs phototransistor with consecutive type II junctions. The effects of the built-in potential across the depletion region near the type II junction interface in the QD layers are found to improve the photoresponse as well as decrease the response times to 950 μs, which is the faster response time (by orders of magnitude) than that recorded for previously reported 2D/QD phototransistors. Also, by implementing an electric-field modulation of the MoS<SUB>2</SUB> channel, our experimental results reveal that the detectivity can be as large as 1 × 10<SUP>11</SUP> jones. This work demonstrates an important pathway toward designing hybrid phototransistors and mixed-dimensional van der Waals heterostructures.</P> [FIG OMISSION]</BR>
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Pak, Sangyeon,Jang, A-Rang,Lee, Juwon,Hong, John,Giraud, Paul,Lee, Sanghyo,Cho, Yuljae,An, Geon-Hyoung,Lee, Young-Woo,Shin, Hyeon Suk,Morris, Stephen M.,Cha, SeungNam,Sohn, Jung Inn,Kim, Jong Min The Royal Society of Chemistry 2019 Nanoscale Vol.11 No.11
<P>Monolayered, semiconducting molybdenum disulfide (MoS2) is of considerable interest for its potential applications in next-generation flexible, wearable, and transparent photodetectors because it has outstanding physical properties coupled with unique atomically thin dimensions. However, there is still a lack of understanding in terms of the underlying mechanisms responsible for the photoresponse dynamics, which makes it difficult to identify the appropriate device design strategy for achieving a fast photoresponse time in MoS2 photodetectors. In this study, we investigate the importance of surface functionalization on controlling the charge carrier densities in a MoS2 monolayer and in turn the corresponding behavior of the photoresponse in relation to the position of the Fermi-level and the energy band structure. We find that the p-doping and n-doping, which is achieved through the surface functionalization of the MoS2 monolayer, leads to devices with different photoresponse behavior. Specifically, the MoS2 devices with surface functional groups contributing to p-doping exhibited a faster response time as well as higher sensitivity compared to that observed for the MoS2 devices with surface functional groups contributing to n-doping. We attribute this difference to the degree of bending in the energy bands at the metal-semiconductor junction as a result of shifting in the Fermi-level position, which influences the optoelectronic transport properties as well as the recombination dynamics leading to a low dark and thus high detectivity and fast decay time. Based upon these findings, we have also demonstrated the broad applicability of surface functionalization by fabricating a flexible MoS2 photodetector that shows an outstanding decay time of 0.7 s, which is the fastest response time observed in flexible MoS2 detectors ever reported.</P>
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Hong, John,Lee, Young-Woo,Ahn, Docheon,Pak, Sangyeon,Lee, Juwon,Jang, A-Rang,Lee, Sanghyo,Hou, Bo,Cho, Yuljae,Morris, Stephen M.,Shin, Hyeon Suk,Cha, SeungNam,Sohn, Jung Inn,Kim, Jong Min Elsevier 2017 Nano energy Vol.39 No.-
<P><B>Abstract</B></P> <P>Designing and tailoring the assembly of complex ternary transition metal oxide (TTMO) structures are a key step in the pursuit of high performance pseudo-capacitive materials for the development of next-generation energy storage devices. Here, we present uniquely assembled 3D porous heterostructures with hierarchically-coordinated TTMOs, comprising the multiply interconnected primary nanoporous frameworks of ZnCo<SUB>2</SUB>O<SUB>4</SUB>/NiMoO<SUB>4</SUB> core-shell structures and the secondary protruding structures of NiMoO<SUB>4</SUB> layered nanosheets. By benefiting from the combination of hierarchically cooperative two TTMOs, the developed 3D ZnCo<SUB>2</SUB>O<SUB>4</SUB>/NiMoO<SUB>4</SUB> heterostructures with their stable, porous, and conductive features exhibit robust pseudo-capacitive performance with high capacitances of 6.07Fcm<SUP>–2</SUP> and 1480.48Fg<SUP>–1</SUP> at 2mAcm<SUP>–2</SUP> as well as an excellent cycling stability of 90.6% over 15,000 cycles. Moreover, an asymmetric supercapacitor device can deliver a high energy density of 48.6Whkg<SUP>–1</SUP> and a power density of 2820Wkg<SUP>–1</SUP>. The superior pseudo-capacitive energy storage characteristics are strongly attributed to the interconnected 3D nanoporous network architectures of the TTMOs along with the secondary layered nanosheets that provide 1) the enlarged surface area with the high conductivity, 2) the facile and multi-access ion paths, and 3) the favorable structural stability. Combined, these results highlight the importance of novel nanostructure design in maximizing the pseudo-capacitive performance and provide a viable way to develop new electrode materials.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A hierarchically-coordinated two TTMOs based heterostructures were proposed and synthesized. </LI> <LI> The TTMOs showed multiple interconnected nanoporous architecture and protruding nanosheets. </LI> <LI> The resultant electrode exhibited high capacitance with ultra-high cyclability. </LI> <LI> The full cell demonstrated superior energy and power densities. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
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Hong, John,Hou, Bo,Lim, Jongchul,Pak, Sangyeon,Kim, Byung-Sung,Cho, Yuljae,Lee, Juwon,Lee, Young-Woo,Giraud, Paul,Lee, Sanghyo,Park, Jong Bae,Morris, Stephen M.,Snaith, Henry J.,Sohn, Jung Inn,Cha, Se Royal Society of Chemistry 2016 Journal of materials chemistry. A, Materials for e Vol.4 No.48
<▼1><P>We report a PbS QD hybrid passivation structure to eliminate trap sites while increasing charge extraction in QD solar cells.</P></▼1><▼2><P>Colloidal quantum dots (CQDs) are extremely promising as photovoltaic materials. In particular, the tunability of their electronic band gap and cost effective synthetic procedures allow for the versatile fabrication of solar energy harvesting cells, resulting in optimal device performance. However, one of the main challenges in developing high performance quantum dot solar cells (QDSCs) is the improvement of the photo-generated charge transport and collection, which is mainly hindered by imperfect surface functionalization, such as the presence of surface electronic trap sites and the initial bulky surface ligands. Therefore, for these reasons, finding effective methods to efficiently decorate the surface of the as-prepared CQDs with new short molecular length chemical structures so as to enhance the performance of QDSCs is highly desirable. Here, we suggest employing hybrid halide ions along with the shortest heterocyclic molecule as a robust passivation structure to eliminate surface trap sites while decreasing the charge trapping dynamics and increasing the charge extraction efficiency in CQD active layers. This hybrid ligand treatment shows a better coordination with Pb atoms within the crystal, resulting in low trap sites and a near perfect removal of the pristine initial bulky ligands, thereby achieving better conductivity and film structure. Compared to halide ion-only treated cells, solar cells fabricated through this hybrid passivation method show an increase in the power conversion efficiency from 5.3% for the halide ion-treated cells to 6.8% for the hybrid-treated solar cells.</P></▼2>
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( Robert A Kobet ),( Xiao Ping Pan ),( Bao Hong Zhang ),( Stephen C Pak ),( Adam S Asch ),( Myon Hee Lee ) 한국응용약물학회 2014 Biomolecules & Therapeutics(구 응용약물학회지) Vol.22 No.5
The nematode Caenorhabditis elegans (C. elegans) offers a unique opportunity for biological and basic medical researches due to its genetic tractability and well-defined developmental lineage. It also provides an exceptional model for genetic, molecular, and cellular analysis of human disease-related genes. Recently, C. elegans has been used as an ideal model for the identification and functional analysis of drugs (or small-molecules) in vivo. In this review, we describe conserved oncogenic signaling pathways (Wnt, Notch, and Ras) and their potential roles in the development of cancer stem cells. During C. elegans germline development, these signaling pathways regulate multiple cellular processes such as germline stem cell niche specification, germline stem cell maintenance, and germ cell fate specification. Therefore, the aberrant regulations of these signaling pathways can cause either loss of germline stem cells or overproliferation of a specific cell type, resulting in sterility. This sterility phenotype allows us to identify drugs that can modulate the oncogenic signaling pathways directly or indirectly through a high-throughput screening. Current in vivo or in vitro screening methods are largely focused on the specific core signaling components. However, this phenotype based screening will identify drugs that possibly target upstream or downstream of core signaling pathways as well as exclude toxic effects. Although phenotype-based drug screening is ideal, the identification of drug targets is a major challenge. We here introduce a new technique, called Drug Affinity Responsive Target Stability (DARTS). This innovative method is able to identify the target of the identified drug. Importantly, signaling pathways and their regulators in C. elegans are highly conserved in most vertebrates, including humans. Therefore, C. elegans will provide a great opportunity to identify therapeutic drugs and their targets, as well as to understand mechanisms underlying the formation of cancer.
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Hong, John,Lee, Juwon,Lee, Young-Woo,Park, Woon Bae,Ahn, Docheon,Park, Jong Bae,Pak, Sangyeon,Baik, Jaeyoon,Morris, Stephen M.,Cha, SeungNam,Sohn, Kee-Sun,Sohn, Jung Inn Elsevier 2019 Journal of Power Sources Vol.444 No.-
<P><B>Abstract</B></P> <P>The search for new materials that exhibit rapid Faradaic energy-storing behavior continues to be ever more important as they offer a promising alternative to battery technology because of their unrivalled ability to deliver large amounts of power along with large amounts of energy. Here, we present a reduced binary anion compound (r-BAC) as a first demonstration of redox-active materials, which are fabricated by a facile and direct activation synthetic method. The r-BAC exhibits excellent energy storage characteristics compared to non-reduced full binary anion compound (f-BAC). Based on the density functional theory (DFT) calculations and the <I>ex-situ</I> chemical study, it is found that the superior electrochemical performance is strongly attributed to not only the Ni cation sites (Ni<SUP>2+</SUP>/Ni<SUP>3+</SUP> redox couple) that are energetically more activated by oxygen vacancies, but also to the additive electrochemical activity at the unsaturated sulfur sites (S<SUP>4+</SUP>/S<SUP>6+</SUP> redox couple) in a binary anion. Thus, we expect that this study on the binary anion material and the corresponding anion-based charge transfer mechanisms may provide a new strategy for the efficient storage of charge in next-generation energy storage applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We first propose an electrochemically active binary anion compound. </LI> <LI> A novel binary anion compound has the tailored oxygen vacancy site. </LI> <LI> The electrode shows excellent electrochemical energy storage abilities. </LI> <LI> The electrochemical features are highly dependent on anion-mediated active sites. </LI> <LI> DFT calculations exhibit the possibility of anion-based Faradaic redox reactions. </LI> </UL> </P>
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Hong, John,Kim, Byung-Sung,Yang, Seungmo,Jang, A-Rang,Lee, Young-Woo,Pak, Sangyeon,Lee, Sanghyo,Cho, Yuljae,Kang, Dongwoo,Shin, Hyeon Suk,Hong, Jin Pyo,Morris, Stephen M.,Cha, SeungNam,Sohn, Jung Inn Royal Society of Chemistry 2019 Journal of Materials Chemistry A Vol.7 No.6
<P>Traditional synthetic routes for transition metal sulfides typically involve solution and thermal-based processes to exploit their favorable pseudo-capacitive properties. However, there is a practical need to develop alternative processes to fabricate metal sulfide electrodes because of the time-consuming processes (>12 h), additional heat-treatment to active reactants, relatively high post-heat-treatment temperature (200-400 °C) and non-scalable nature of existing synthetic routes. Herein, utilizing a solution-based sulfur precursor, one-dimensional single-crystalline Cu2S nanostructures have been successfully prepared <I>via</I> a solution-based direct synthesis process within 10 min at room temperature without the need for thermal treatment steps. The fabricated electrode exhibits a capacitance of 750 mF cm<SUP>−2</SUP> at a current density of 2 mA cm<SUP>−2</SUP>. Moreover, the rate capacitance is maintained at about 82.3% as the current density is increased to 40 mA cm<SUP>−2</SUP>, and the capacity retains 90.5% of the initial value after 20 000 cycles. Importantly, as this method involves a solution-based formulation it is compatible with roll-to-roll processes, which is promising for mass and scalable production of the electrodes. The synthetic method ensures a facile and efficient approach to fabricating scalable one-dimensional single crystalline Cu2S nanostructures, highlighting the uniqueness of the solution-based sulfur activation method.</P>
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Kaon BSMB-parameters using improved staggered fermions fromNf=2+1unquenched QCD
Choi, Benjamin J.,Jang, Yong-Chull,Jung, Chulwoo,Jeong, Hwancheol,Kim, Jangho,Kim, Jongjeong,Kim, Sunghee,Lee, Weonjong,Leem, Jaehoon,Pak, Jeonghwan,Park, Sungwoo,Sharpe, Stephen R.,Yoon, Boram American Physical Society 2016 Physical review. D Vol.93 No.1
<P>We present results for the matrix elements of the additional Delta S = 2 operators that appear in models of physics beyond the Standard Model (BSM), expressed in terms of four BSM B-parameters. Combined with experimental results for Delta M-K and epsilon(K), these constrain the parameters of BSM models. We use improved staggered fermions, with valence hypercubic blocking transfromation (HYP)-smeared quarks and N-f = 2 + 1 flavors of 'asqtad' sea quarks. The configurations have been generated by the MILC Collaboration. The matching between lattice and continuum four-fermion operators and bilinears is done perturbatively at one-loop order. We use three lattice spacings for the continuum extrapolation: a approximate to 0.09, 0.06 and 0.045 fm. Valence light-quark masses range down to approximate to m(s)(phys)/13 while the light sea-quark masses range down to approximate to m(s)(phys)/20. Compared to our previous published work, we have added four additional lattice ensembles, leading to better controlled extrapolations in the lattice spacing and sea-quark masses. We report final results for two renormalization scales, mu = 2 and 3 GeV, and compare them to those obtained by other collaborations. Agreement is found for two of the four BSM B-parameters (B-2 and B-3(SUSY)). The other two (B-4 and B-5) differ significantly from those obtained using regularization independent momentum subtraction (RI-MOM) renormalization as an intermediate scheme, but are in agreement with recent preliminary results obtained by the RBC-UKQCD Collaboration using regularization independent symmetric momentum subtraction (RI-SMOM) intermediate schemes.</P>
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