Kyoung Soon Choi

Bioactive effects of graphene oxide cell culture substratum on structure and function of human adipose‐derived stem cells

Nanoscale topography of artificial substrates can greatly influence the fate of stem cells including adhesion, proliferation, and differentiation. Thus the design and manipulation of nanoscale stem cell culture platforms or scaffolds are of great importance as a strategy in stem cell and tissue engineering applications. In this report, we propose that a graphene oxide (GO) film is an efficient platform for modulating structure and function of human adipose-derived stem cells (hASCs). Using a self-assembly method, we successfully coated GO on glass for fabricating GO films. The hASCs grown on the GO films showed increased adhesion, indicated by a large number of focal adhesions, and higher correlation between the orientations of actin filaments and vinculin bands compared to hASCs grown on the glass (uncoated GO substrate). It was also found that the GO films showed the stronger affinity for hASCs than the glass. In addition, the GO film proved to be a suitable environment for the time-dependent viability of hASCs. The enhanced differentiation of hASCs included osteogenesis, adipogenesis, and epithelial genesis, while chondrogenic differentiation of hASCs was decreased, compared to tissue culture polystyrene as a control substrate. The data obtained here collectively demonstrates that the GO film is an efficient substratum for the adhesion, proliferation, and differentiation of hASCs.

Microlitre scale solution processing for controlled, rapid fabrication of chemically derived graphene thin films

We report a new class of rapid solution processes for fabricating highly uniform chemically derived graphene thin films with control over the thickness on the subnanometre scale. The film deposition directly on various substrates is driven by dragging a meniscus of microlitre graphene oxide (GO) suspension trapped between two plates. The fine tuning of the optoelectronic properties of the graphene thin films is achieved by simply varying the number of depositions, GO concentration, dragging speed, and angle between two plates. This coating technique is simple, inexpensive, and easy to scale for large-area graphene films used as transparent electrodes with a significant reduction of the material consumption.

Reduced Graphite Oxide-Indium Tin Oxide Hybrid Materials for use as a Transparent Electrode

for use as a Transparent Electrode Kyoung Soon Choi, Yensil Park, Ki-Chang Kwon, Jooheon Kim, Chang Keun Kim, Soo Young Kim, Kihyon Hong and Jong-Lam Lee b,z School of Chemical Engineering and Materials Science, Chung-Ang University, Dongjak-gu, Seoul, 156-756, Korea Division of Advanced Materials Science and Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk, 790-784, Korea

Drastically enhanced hydrogen evolution activity by 2D to 3D structural transition in anion-engineered molybdenum disulfide thin films for efficient Si-based water splitting photocathodes

We synthesized transferrable and transparent anion-engineered molybdenum disulfide thin-film catalysts through a simple thermolysis method by using [(NH4)2MoS4] solution and powder precursors with different sulphur/phosphorus weight ratios. The synthesized sulphur-doped molybdenum phosphide (S:MoP) thin film changed from a two-dimensional van der Waals structure to a three-dimensional hexagonal structure by introduction of phosphorus atoms in the MoS2 thin film. The S:MoP thin film catalyst, which is composed of cheap and earth abundant elements, could provide the lowest onset potential and the highest photocurrent density for planar p-type Si photocathodes. The density functional theory calculations indicate that the surface of S:MoP thin films absorb hydrogen better than that of MoS2 thin films. The structurally engineered thin film catalyst facilitates the easy transfer of photogenerated electrons from the p-Si light absorber to the electrolyte. Anion-engineering of the MoS2 thin film catalyst would be an efficient way to enhance the catalytic activity for photoelectrochemical water splitting.

Enhanced Photocatalytic Performance Depending on Morphology of Bismuth Vanadate Thin Film Synthesized by Pulsed Laser Deposition

We have fabricated high quality bismuth vanadate (BiVO4) polycrystalline thin films as photoanodes by pulsed laser deposition (PLD) without a postannealing process. The structure of the grown films is the photocatalytically active phase of scheelite-monoclinic BiVO4 which was obtained by X-ray diffraction (XRD) analysis. The change of surface morphology for the BIVO4 thin films depending on growth temperature during synthesis has been observed by scanning electron microscopy (SEM), and its influence on water splitting performance was investigated. The current density of the BiVO4 film grown on a glass substrate covered with fluorine-doped tin oxide (FTO) at 230 °C was as high as 3.0 mA/cm2 at 1.23 V versus the potential of the reversible hydrogen electrode (VRHE) under AM 1.5G illumination, which is the highest value so far in previously reported BiVO4 films grown by physical vapor deposition (PVD) methods. We expect that doping of transition metal or decoration of oxygen evolution catalyst (OEC) in our BiVO4 film might further enhance the performance.

Engineering structures and functions of mesenchymal stem cells by suspended large-area graphene nanopatterns

Inspired by the hierarchical nanofibrous and highly oriented structures of natural extracellular matrices, we report a rational design of chemical vapor deposition graphene-anchored scaffolds that provide both physical and chemical cues in a multilayered organization to control the adhesion and functions of cells for regenerative medicine. These hierarchical platforms are fabricated by transferring large graphene film onto nanogroove patterns. The top graphene layer exhibits planar morphology with slight roughness (~20 nm between peaks) due to the underlying topography, which results in a suspended structure between the nanoridges. We demonstrate that the adhesion and differentiation of human mesenchymal stem cells were sensitively controlled and enhanced by the both the nanotopography and graphene cues in our scaffolds. Our results indicate that the layered physical and chemical cues can affect the apparent cell behaviors, and can synergistically enhance cell functionality. Therefore, these suspended graphene platforms may be used to advance regenerative medicine.

MoS2-Nanosheet/Graphene-Oxide Composite Hole Injection Layer in Organic Light-Emitting Diodes

In this work, composite layers comprising two-dimensional MoS2 and graphene oxide (GO) were employed as hole injection layers (HILs) in organic light-emitting diodes (OLEDs). MoS2 was fabricated by the butyllithium (BuLi) intercalation method, while GO was synthesized by a modified Hummers method. The X-ray diffraction patterns showed that the intensity of the MoS2 (002) peak at 14.15° decreased with increase in GO content; the GO (001) peak was observed at 10.07°. In the C 1s synchrotron radiation photoemission spectra, the contributions of the C-O, C=O, and O-C=O components increased with increase in GO content. These results indicated that GO was well mixed with MoS2. The lateral size of MoS2 spanned from a few hundreds of nanometers to 1 μm, while the size of GO was between 400 nm and a few micrometers. Thus, the coverage of the MoS2-GO composite on the ITO surface improved as the GO content increased, owing to the large particle size of GO. Notably, GO with large size could fully cover the indium tin oxide film surface, thus, lowering the roughness. The highest maximum power efficiency (PEmax) was exhibited by the OLED with MoS2-GO 6:4 composite HIL, indicating that similar contents of MoS2 and GO in MoS2-GO composites provide the best results. The OLED with GO HIL showed very high PEmax (4.94 lm W−1) because of very high surface coverage and high work function of GO. These results indicate that the MoS2-GO composites can be used to fabricate HILs in OLEDs.

Polarized Light‐Emitting Diodes Based on Patterned MoS2 Nanosheet Hole Transport Layer

Here, this study successfully fabricates few-layer MoS2 nanosheets from (NH4 )2 MoS4 and applies them as the hole transport layer as well as the template for highly polarized organic light-emitting diodes (OLEDs). The obtained material consists of polycrystalline MoS2 nanosheets with thicknesses of 2 nm. The MoS2 nanosheets are patterned by rubbing/ion-beam treatment. The Raman spectra shows that {poly(9,9-dioctylfluorene-alt-benzothiadiazole), poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]} (F8BT) on patterned MoS2 exhibits distinctive polarization behavior. It is discovered that patterned MoS2 not only improves the device efficiency but also changes the polarization behavior of the devices owing to the alignment of F8BT. This work demonstrates a highly efficient polarized OLED with a polarization ratio of 62.5:1 in the emission spectrum (166.7:1 at the peak intensity of 540 nm), which meets the manufacturing requirement. In addition, the use of patterned MoS2 nanosheets not only tunes the polarization of the OLEDs but also dramatically improves the device performance as compared with that of devices using untreated MoS2 .

Effect of ZnCdSe/ZnSe Core/Shell Quantum Dots on the Efficiency of Organic Photovoltaic Cells

Efficiency of Organic Photovoltaic Cells Kyoung Soon Choi, Yensil Park, Jae Soo Yoo,* Soo Young Kim, Heesun Yang, Gwan Ho Jung, and Jong-Lam Lee* School of Chemical Engineering and Materials Science, Chung-Ang University, Dongjak-gu, Seoul 156-756, Korea Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Korea

Hierarchically Micro- and Nanopatterned Topographical Cues for Modulation of Cellular Structure and Function

Living cells receive biochemical and physical information from the surrounding microenvironment and respond to this information. Multiscale hierarchical substrates with micro- and nanogrooves have been shown to mimic the native extracellular matrix (ECM) better than conventional nanopatterned substrates; therefore, substrates with hierarchical topographical cues are considered suitable for investigating the role of physical factors in tissue functions. In this study, precisely controllable, multiscale hierarchical substrates that could mimic the micro- and nanotopography of complex ECMs were fabricated and used to culture various cell types, including fibroblasts, endothelial cells, osteoblasts, and human mesenchymal stem cells. These substrates had both microscale wrinkles and nanoscale patterns and enhanced the alignment and elongation of all the cells tested. In particular, the nanotopography on the microscale wrinkles promoted not only the adhesion, but also the functions of the cells. These findings suggest that the hierarchical multiscale substrates effectively regulated cellular structure and functions and that they can be used as a platform for tissue engineering and regenerative medicine.Living cells receive biochemical and physical information from the surrounding microenvironment and respond to this information. Multiscale hierarchical substrates with micro- and nanogrooves have been shown to mimic the native extracellular matrix (ECM) better than conventional nanopatterned substrates; therefore, substrates with hierarchical topographical cues are considered suitable for investigating the role of physical factors in tissue functions. In this study, precisely controllable, multiscale hierarchical substrates that could mimic the micro- and nanotopography of complex ECMs were fabricated and used to culture various cell types, including fibroblasts, endothelial cells, osteoblasts, and human mesenchymal stem cells. These substrates had both microscale wrinkles and nanoscale patterns and enhanced the alignment and elongation of all the cells tested. In particular, the nanotopography on the microscale wrinkles promoted not only the adhesion, but also the functions of the cells. These findings suggest that the hierarchical multiscale substrates effectively regulated cellular structure and functions and that they can be used as a platform for tissue engineering and regenerative medicine.