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Now showing items 1 - 16 of 34

  • 3D multilayered plasmonic nanostructures with high areal density for SERS

    Lee, MinKyoung   Jeon, Tae Yoon   Mun, ChaeWon   Kwon, Jung-Dae   Yun, Jungheum   Kim, Shin-Hyun   Kim, Dong-Ho   Chang, Seung-Cheol   Park, Sung-Gyu  

    Enhancing light-matter interactions is essential to improving nanophotonic and optoelectronic device performance. In the present work, we developed a new design for 3D plasmonic nanostructures with enhanced near-field interactions among the plasmonic nanomaterials. The 3D plasmonic nanostructures consisted of multilayered bottom Ag/polydimethylsiloxane (PDMS) nanostructures, an alumina middle layer, and top Ag nanoparticles (NPs). High areal density PDMS nanoprotrusions were self-organized by a simple maskless plasma etching process. The conformal deposition of alumina using atomic layer deposition and Ag deposition produced 3D plasmonic nanostructures. These structures induced multiple near-field interactions between the ultrahigh-areal-density (1400 mu m(-2)) top Ag NPs and the underlying Ag nanostructures, and among the top Ag NPs themselves. The high density of hot spots across the 3D space yielded highly efficient and widely tunable plasmonic responses across the entire visible range. The SERS signal enhancement measured at the 3D plasmonic nanostructures was 3.9 times the signal measured at the 2D multilayered structures and 48.0 times the signal measured at a Ag NP layer deposited onto a Si substrate. Finally, the 3D plasmonic nanostructures exhibited excellent uniformity with a variation of 6.8%, based on a microscale Raman mapping analysis. The excellent Raman signal uniformity can be attributed to the ultra high areal density of the Ag NPs and the uniform thickness of the alumina spacing layer.
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  • Fluorescence Modulation of Graphene Quantum Dots Near Structured Silver Nanofilms

    Chae, Weon-Sik   Yun, Jungheum   Nam, Sang-Hyeon   Lee, Sang-Geul   Yang, Won-Geun   Yoon, Hyewon   Park, Minsu   Jeon, Seokwoo  

    Here, we study the plasmonic metal-enhanced fluorescence properties of blue-emitting graphene quantum dots (GQDs) and green-emitting graphene oxide quantum dots (GOQDs) using fluorescence lifetime imaging microscopy. Reactive ion sputtered silver (Ag) on zinc oxide (ZnO) thin films deposited on silicon (Si) wafers are used as the substrates. The morphology of the sputtered Ag gradually changes from nanoislands, via and elongated network and a continuous film with nanoholes, to a continuous film with increasing sputtering time. The fluorescence properties of GQD and GOQD on the Ag are modulated in terms of the intensities and lifetimes as the morphology of the Ag layers changes. Although both GQD and GOQD show similar fluorescence modulation on the Ag nanofilms, the fluorescence of GQD is enhanced, whereas that of GOQD is quenched due to the charge transfer process from GOQD to ZnO. Moreover, the GQD and GOQD exhibit different fluorescence lifetimes due to the effect of their electronic configurations. The theoretical calculation explains that the fluorescence amplification on the Ag nanofilms can largely be attributed to the enhanced absorption mechanism arising from accumulated optical fields around nanogaps and nanovoids in the Ag nanofilms.
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  • Light trapping in bendable organic solar cells using silica nanoparticle arrays

    Yun, Jungheum   Wang, Wei   Kim, Soo Min   Bae, Tae-Sung   Lee, Sunghun   Kim, Donghwan   Lee, Gun-Hwan   Lee, Hae-Seok   Song, Myungkwan  

    A highly efficient light-scattering layer, composed of quasi-periodic discrete silica nanoparticles directly deposited onto polymer substrates to produce bendable organic solar cells (OSCs) with enhanced light absorption, is reported. A silica nanoparticle layer (SNL) underwent self-assembly on a highly flexible and heat-sensitive polymer at room temperature during fabrication, which employed a unique plasma-enhanced chemical vapour deposition technique. Such efficient light-scattering SNLs have not been realizable by conventional solution-based coating techniques. SNLs were optimized by precisely controlling dimensional parameters, specifically, the nanoparticle layer thickness and interparticle distance. The optimized SNL exhibited an improved transmission haze of 16.8% in the spectral range of 350-700 nm, where reduction of the total transmission was suppressed to 2%. Coating light-scattering SNLs onto polymer substrates is a promising method for improving the light harvesting abilities of OSCs by enhancing the light absorption of photoactive polymer layers. This SNL-based flexible OSC exhibited a record power conversion efficiency (PCE) of 7.4%, representing a 13% improvement, while reducing the thickness of the photoactive polymer layer by 30%.
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  • Light trapping in bendable organic solar cells using silica nanoparticle arrays

    Yun, Jungheum   Wang, Wei   Kim, Soo Min   Bae, Tae-Sung   Lee, Sunghun   Kim, Donghwan   Lee, Gun-Hwan   Lee, Hae-Seok   Song, Myungkwan  

    A highly efficient light-scattering layer, composed of quasi-periodic discrete silica nanoparticles directly deposited onto polymer substrates to produce bendable organic solar cells (OSCs) with enhanced light absorption, is reported. A silica nanoparticle layer (SNL) underwent self-assembly on a highly flexible and heat-sensitive polymer at room temperature during fabrication, which employed a unique plasma-enhanced chemical vapour deposition technique. Such efficient light-scattering SNLs have not been realizable by conventional solution-based coating techniques. SNLs were optimized by precisely controlling dimensional parameters, specifically, the nanoparticle layer thickness and interparticle distance. The optimized SNL exhibited an improved transmission haze of 16.8% in the spectral range of 350-700 nm, where reduction of the total transmission was suppressed to 2%. Coating light-scattering SNLs onto polymer substrates is a promising method for improving the light harvesting abilities of OSCs by enhancing the light absorption of photoactive polymer layers. This SNL-based flexible OSC exhibited a record power conversion efficiency (PCE) of 7.4%, representing a 13% improvement, while reducing the thickness of the photoactive polymer layer by 30%.
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  • Ultrathin Metal films for Transparent Electrodes of Flexible Optoelectronic Devices

    Yun, Jungheum  

    The need for the development of transparent conductive electrodes (TCEs) supported on flexible polymer substrates has explosively increased in response to flexible polymer-based photovoltaic and display technologies; these TCEs replace conventional indium tin oxide (ITO) that exhibits poor performance on heat-sensitive polymers. An efficient, flexible TCE is required to exhibit high electrical conductance and high optical transmittance, as well as excellent mechanical flexibility and long-term stability, simultaneously. Recent advances in technologies utilizing an ultrathin noble-metal film in a dielectric/metal/dielectric structure, or its derivatives, have attracted attention as promising alternatives that can satisfy the requirements of flexible TCEs. This review will survey the background knowledge and recent updates of synthetic strategies and design rules toward highly efficient, flexible TCEs based on ultrathin metal films, with a special focus on the principal features and available methodologies involved in the fabrication of highly transparent, conductive, ultrathin noble-metal films. This survey will also cover the practical applications of TCEs to flexible organic solar cells and light-emitting diodes.
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  • Adhesive and Structural Failures of Oxide Coatings on Plasma-Treated Polymers

    Yun, Jungheum   Lee, Sunghun   Bae, Tae-Sung   Yun, Youngmok   Lee, Seunghoon   Kwon, Jung-Dae   Lee, Gun-Hwan  

    This study discloses (i) the chemical and morphological modifications in acrylate hard-coat and bare polyethylene terephthalate polymers occurring in the course of Ar plasma treatments and (ii) the effects of these modifications on the adhesion, barrier performance, and cohesion of silicon oxide coatings deposited on the polymers. It is concluded that the deterioration in these coating properties is dominated by the formation of nanoscopic globular polymer protrusions on the polymer surface as a result of plasma treatment. The protrusions evolve even under very mild plasma conditions in which an ion fluence of less than 1 x 10(16) ions . cm(-2) is applied with low-energy ion irradiation of 6 eV. The polymer protrusions dictate the nucleation and subsequent growth of a coating by promoting the development of a three-dimensional granular morphology in the coating. At the initial oxide nucleation stage, the wetting behavior of silicon oxide on the polymer surface in the presence of nanoscopic protrusions is directly limited by the area number density and size of the protrusions. Incomplete wetting of the protrusions with a silicon oxide coating hinders adhesion between the oxide and the polymer surface. The reduction in the contact area between the oxide and the protrusions is identified as the reason that a weak boundary layer forms at the oxide-polymer interface. Furthermore, the formation of nanoscopic defects, predominantly pinholes, is inevitable in the granular coating morphology on the polymer protrusions and weakens the oxide coating's barrier performance and cohesion strength. Variations in the polar surface free energy and chemical composition of the plasma-treated polymer surface are irrelevant to the wetting dynamics whenever the protrusions develop on the polymer surface. The effects of the polar surface free energy and chemical composition are valid only to the extent that the plasma treatment improves the wettability of a polymer surface without protrusion formation.
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  • Strategy for improving Ag wetting on oxides:Coalescence dynamics versus nucleation density

    Zhao, Guoqing   Jeong, Eunwook   Choi, Eun-Ae   Yu, Seung Min   Bae, Jong-Seong   Lee, Sang-Geul   Han, Seung Zeon   Lee, Gun-Hwan   Yun, Jungheum  

    Wetting of noble metals on oxide structures becomes problematic when attempting to grow two-dimensional, atomically smooth and continuous thin film structures because of a strong tendency for three-dimensional growth. This study redefines the crucial factor that controls the wetting of vacuum deposited Ag on SiO2 substrates and refutes the currently accepted assertion that increasing the nucleation density of Ag is responsible for promoting wetting. Ultrahigh-resolution electron microscopy, supported by X-ray spectroscopic techniques, was used to observe how fast Ag nanoparticles evolve to a continuous film by manipulating the contribution of the crucial factor on Ag wetting, and the results were numerically interpreted using first-principles density functional theory calculations to reveal the relevant dynamics. The experimental and numerical results confirmed that the earliest transition of the coalescence dynamics of Ag nanoparticles from full to partial coalescence is the dominating factor in improving Ag wetting. Surprisingly, Ag wetting is independent of the initial nucleation density of Ag nanoparticles. Favorable partial coalescence with more irregularities in the shapes was readily attainable by artificially reducing the thermodynamic surface and interfacial free energies of the evolving Ag nanoparticles through the addition of a small concentration of Al or O as a wetting agent.
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  • Fabrication and near-field visualization of a wafer-scale dense plasmonic nanostructured array

    Yun, Jungheum   Lee, Haemi   Mun, ChaeWon   Jahng, Junghoon   Morrison, William A.   Nowak, Derek B.   Song, Jung-Hwan   Lim, Dong-Kwon   Bae, Tae-Sung   Kim, Hyung Min   Kim, Nam Hoon   Nam, Sang Hwan   Kim, Jongwoo   Seo, Min-Kyo   Kim, Dong-Ho   Park, Sung-Gyu   Suh, Yung Doug  

    Developing a sensor that identifies and quantifies trace amounts of analyte molecules is crucially important for widespread applications, especially in the areas of chemical and biological detection. By non-invasively identifying the vibrational signatures of the target molecules, surface-enhanced Raman scattering (SERS) has been widely employed as a tool for molecular detection. Here, we report on the reproducible fabrication of wafer-scale dense SERS arrays and single-nanogap level near-field imaging of these dense arrays under ambient conditions. Plasmonic nanogaps densely populated the spaces among globular Ag nanoparticles with an areal density of 120 particles per mu m(2) upon application of a nanolithography-free simple process consisting of the Ar plasma treatment of a polyethylene terephthalate substrate and subsequent Ag sputter deposition. The compact nanogaps produced a high SERS enhancement factor of 3.3 x 10(7) and homogeneous (coefficient of variation of 8.1%) SERS response. The local near fields at these nanogaps were visualized using photo-induced force microscopy that simultaneously enabled near-field excitation and near-field force detection under ambient conditions. A high spatial resolution of 3.1 nm was achieved. Taken together, the generation of a large-area SERS array with dense plasmonic nanogaps and the subsequent single-nanogap level characterization of the local near field have profound implications in the nanoplasmonic imaging and sensing applications.
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  • Fabrication and near-field visualization of a wafer-scale dense plasmonic nanostructured array

    Yun, Jungheum   Lee, Haemi   Mun, ChaeWon   Jahng, Junghoon   Morrison, William A.   Nowak, Derek B.   Song, Jung-Hwan   Lim, Dong-Kwon   Bae, Tae-Sung   Kim, Hyung Min   Kim, Nam Hoon   Nam, Sang Hwan   Kim, Jongwoo   Seo, Min-Kyo   Kim, Dong-Ho   Park, Sung-Gyu   Suh, Yung Doug  

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  • Analysis of carbon nitride growth in pedestal reactors by chemical vapor deposition

    Yun, Jungheum   Dandy, David S.  

    The chemical vapor deposition of polycrystalline carbon nitride in stagnation flow reactors is simulated. A model is used to predict the gas phase chemistry, temperature and velocity profiles, potential gaseous film growth precursors, and to evaluate the likelihood of bond rearrangement occurring in the bulk phase or on the deposition surface once the gaseous precursors are adsorbed. Numerical studies are carried out to predict the effects of inlet and substrate temperatures, reactor pressure, and inlet gas composition on the gas phase chemistry. Potential gaseous film growth precursors of carbon nitride are determined by quantitatively comparing the calculated results against existing experimental data. Results of the model indicate that the gas phase chemistry, including the gas composition at the deposition surface, is strongly affected by reactor pressure and inlet gas composition. However, the gas composition at the deposition surface does not depend strongly on the inlet temperature, while it is found to be strongly dependent on the substrate temperature. Since no correlation is found between the predicted near-surface concentrations of potential film growth precursors and experimentally measured bond types in the carbon nitride films, the experimentally measured bond types in the films must therefore result from chemical bond rearrangement occurring on the deposition surface or in the bulk phase once the gaseous precursors are adsorbed. Comparison between the calculated film growth rate using potential growth precursors and experimental data indicates that the CHx (x = 0,2,3), C2H2, NHx (x = 0,1,2), and HxCN (x = 1,2) species are the most probable crystalline carbon nitride growth species. Among these, C and CH3 dominate the carbon contribution to the film growth, and N is the primary nitrogen bearing species responsible for the film growth. The sum of predicted film growth rates for carbon bearing species is comparable to the experimentally determined film growth rate.
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  • Analysis of carbon nitride growth in pedestal reactors by chemical vapor deposition

    Yun, Jungheum   Dandy, David S.  

    The chemical vapor deposition of polycrystalline carbon nitride in stagnation flow reactors is simulated. A model is used to predict the gas phase chemistry, temperature and velocity profiles, potential gaseous film growth precursors, and to evaluate the likelihood of bond rearrangement occurring in the bulk phase or on the deposition surface once the gaseous precursors are adsorbed. Numerical studies are carried out to predict the effects of inlet and substrate temperatures, reactor pressure, and inlet gas composition on the gas phase chemistry. Potential gaseous film growth precursors of carbon nitride are determined by quantitatively comparing the calculated results against existing experimental data. Results of the model indicate that the gas phase chemistry, including the gas composition at the deposition surface, is strongly affected by reactor pressure and inlet gas composition. However, the gas composition at the deposition surface does not depend strongly on the inlet temperature, while it is found to be strongly dependent on the substrate temperature. Since no correlation is found between the predicted near-surface concentrations of potential film growth precursors and experimentally measured bond types in the carbon nitride films, the experimentally measured bond types in the films must therefore result from chemical bond rearrangement occurring on the deposition surface or in the bulk phase once the gaseous precursors are adsorbed. Comparison between the calculated film growth rate using potential growth precursors and experimental data indicates that the CHx (x = 0,2,3), C2H2, NHx (x = 0,1,2), and HxCN (x = 1,2) species are the most probable crystalline carbon nitride growth species. Among these, C and CH3 dominate the carbon contribution to the film growth, and N is the primary nitrogen bearing species responsible for the film growth. The sum of predicted film growth rates for carbon bearing species is comparable to the experimentally determined film growth rate.
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  • Fabrication of a Completely Transparent and Highly Flexible ITO Nanoparticle Electrode at Room Temperature

    Yun, Jungheum   Park, Yeon Hyun   Bae, Tae-Sung   Lee, Sunghun   Lee, Gun-Hwan  

    We report the fabrication of a highly flexible indium tin oxide (ITO) electrode that is completely transparent to light in the visible spectrum. The electrode was fabricated via the formation of a novel ITO nanoarray structure, consisting of discrete globular ITO nanoparticles superimposed on an agglomerated ITO layer, on a heat-sensitive polymer substrate. The ITO nanoarray spontaneously assembled on the surface of the polymer substrate by a simple sputter coating at room temperature, without nanolithographic or solution-based assembly processes being required. The ITO nanoarray exhibited a resistivity of approximately 2.3 x 10(-3) Omega cm and a specular transmission of about 99% at 550 nm, surpassing all previously reported values of these parameters in the case of transparent porous ITO electrodes synthesized via solution-based processes at elevated temperatures. This novel nanoarray structure and its fabrication methodology can be used for coating large-area transparent electrodes on heat-sensitive polymer substrates, a goal unrealizable through currently available solution-based fabrication methods.
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  • Fabrication of a Completely Transparent and Highly Flexible ITO Nanoparticle Electrode at Room Temperature

    Yun, Jungheum   Park, Yeon Hyun   Bae, Tae-Sung   Lee, Sunghun   Lee, Gun-Hwan  

    We report the fabrication of a highly flexible indium tin oxide (ITO) electrode that is completely transparent to light in the visible spectrum. The electrode was fabricated via the formation of a novel ITO nanoarray structure, consisting of discrete globular ITO nanoparticles superimposed on an agglomerated ITO layer, on a heat-sensitive polymer substrate. The ITO nanoarray spontaneously assembled on the surface of the polymer substrate by a simple sputter coating at room temperature, without nanolithographic or solution-based assembly processes being required. The ITO nanoarray exhibited a resistivity of approximately 2.3 x 10(-3) Omega cm and a specular transmission of about 99% at 550 nm, surpassing all previously reported values of these parameters in the case of transparent porous ITO electrodes synthesized via solution-based processes at elevated temperatures. This novel nanoarray structure and its fabrication methodology can be used for coating large-area transparent electrodes on heat-sensitive polymer substrates, a goal unrealizable through currently available solution-based fabrication methods.
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  • Model of morphology evolution in the growth of polycrystalline β-SiC films

    Yun, Jungheum   Dandy, David S.  

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  • Model of morphology evolution in the growth of polycrystalline β-SiC films

    Yun, Jungheum   Dandy, David S.  

    The growth of β-SiC films via chemical vapor deposition (CVD) has been under intensive investigation because this is viewed to be an enabling material for a variety of new semiconductor devices in areas where silicon cannot effectively compete. However, the difficulty in achieving single-crystal or highly textured surface morphology in films with low bulk defect density has limited the use of β-SiC films in electronic devices. Although several researchers have reported results relating the morphology of β-SiC films to deposition parameters, including substrate temperature and gas composition, detailed knowledge of the effects of deposition parameters on film morphology and crystallographic texture is still lacking. If these relationships between deposition parameters and film morphology can be quantified, then it may be possible to obtain optimal β-SiC film morphologies via CVD for specific applications such as high-power electronic devices.
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  • Ultrathin Silver Film Electrodes with Ultralow Optical and Electrical Losses for Flexible Organic Photovoltaics

    Zhao, Guoqing   Shen, Wenfei   Jeong, Eunwook   Lee, Sang-Geul   Yu, Seung Min   Bae, Tae-Sung   Lee, Gun-Hwan   Han, Seung Zeon   Tang, Jianguo   Choi, Eun-Ae   Yun, Jungheum  

    Improving the wetting ability of Ag on chemically heterogeneous oxides is technically important to fabricate ultrathin, continuous films that would facilitate the minimization of optical and electrical losses to develop qualified transparent Ag film electrodes in the state-of-the-art optoelectronic devices. This goal has yet to be attained, however, because conventional techniques to improve wetting of Ag based on heterogeneous metallic wetting layers are restricted by serious optical losses from wetting layers. Herein, we report on a simple and effective technique based on the partial oxidation of Ag nanoclusters in the early stages of Ag growth. This promotes the rapid evolution of the subsequently deposited pure Ag into a completely continuous layer on the ZnO substrate, as verified by experimental and numerical evidence. The improvement in the Ag wetting ability allows the development of a highly transparent, ultrathin (6 nm) Ag continuous film, exhibiting an average optical transmittance of 94% in the spectral range 400-800 nm and a sheet resistance of 12.5 Omega sq(-1), which would be well-suited for application to an efficient front window electrode for flexible solar cell devices fabricated on polymer substrates.
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