Yensil Park

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.

Superhydrophobic and antireflective nanograss-coated glass for high performance solar cells

We present a facile method for producing superhydrophobic nanograss-coated (SNGC) glass surfaces that possess both reduced reflectivity and self-cleaning properties at the air/glass interface. The refractive index of a CaF2 nanograss (NG) layer on a glass substrate, deposited by glancing angle vapor deposition, is 1.04 at 500 nm, which is the second-lowest value ever reported so far. The fluorinated NG layer gives rise to a high water contact angle (>150°) and very efficient cleaning out of dust with water drops. Using the dual functionalities of the SNGC glass, we demonstrate superhydrophobic and antireflective organic photovoltaic cells with excellent power conversion efficiency.

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

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

The effects of device structure on the performances of distyrylbiphenyl compounds based organic light emitting diodes

Abstract The effects of device structure and applied current density on the performances of blue organic light emitting diodes based on unsymmetrical distyrylbiphenyl compounds were investigated. According to the attached functional groups on the unsymmetrical distyrylbiphenyls, two kinds of compounds were synthesized and evaluated: ( Z )-4-(2- p -tolylvinyl)-4′-[1-phenyl-2-(4- tert -butylphenyl)vinyl]biphenyl (U-DSB_tBM) and ( Z )-4-[2-(4- tert -butylphenyl)vinyl]-4′-[1-phenyl-2-(4- tert -butylphenyl)vinyl]biphenyl (U-DSB_tBtB). As the emission layers of U-DSB_tBM or U-DSB_tBtB thickened, the operation voltage increased and the emitted light became more blue. After inserting a hole blocking layer, the values of the Commission Internationale de l’Eclairage (CIE) chromaticity coordinates were (0.14–0.21, 0.15–0.28). As the applied current increased, the y coordinate of the sample using U-DSB_tBM increased; however, that of the sample using U-DSB_tBtB decreased regardless of the device structure. The X-ray diffraction data and the atomic force microscopy imaging results suggest that the hole mobility of U-DSB_tBM is greater than that of U-DSB_tBtB, indicating that the recombination region is located in U-DSB_tBM at a low current, and that it expands to the electron transport layer at a high current. Therefore, the device structure and mobility affect device performance, and both factors should be considered to optimize the device properties.

Organic photovoltaic cells with core/shell quantum dots embedded in poly(methyl methacrylate)

In this letter, poly(methyl methacrylate) (PMMA), in which ZnCdS/ZnS core/shell QDs or ZnCdSe/ZnS core/shell QDs were embedded, was used to enhance the absorbance of light and, thereby, deliver power at greater efficiencies, as shown in Fig. 1. In order to attach QDs on the opposite side of an indium tin oxide (ITO)/glass substrate, a PMMA layer was used as a supporting material. The effects of the QDs + PMMA on the performance of the OPV cells are discussed. Furthermore, the dependence of the efficiency of the OPV cell on the photoluminescence wavelength of the core/shell QDs is discussed by comparing the two types of QDs.