Chul Lee

Controlled selective growth of ZnO nanorod and microrod arrays on Si substrates by a wet chemical method

The use of a wet chemical method to selectively grow ZnO microrod and nanorod arrays on Si substrates is described. To control the size and position of the ZnO microrods and nanorods, polymethylmethacrylate (PMMA) submicron patterns were prepared on the Si substrates with an intermediate ZnO layer using e-beam lithography. Selective growth of the ZnO structures was achieved by the absence of ZnO nucleation sites on the PMMA mask, resulting in position-controlled growth of ZnO structures only on patterned holes where the ZnO layer was exposed. In addition, the diameters of the ZnO microrods were determined by the patterned hole size, and the diameters as small as 250nm were obtained when a hole diameter of 250nm was employed. The structural and optical characteristics of the ZnO microrods were further investigated using x-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy.

Power-Constrained Contrast Enhancement for Emissive Displays Based on Histogram Equalization

A power-constrained contrast-enhancement algorithm for emissive displays based on histogram equalization (HE) is proposed in this paper. We first propose a log-based histogram modification scheme to reduce overstretching artifacts of the conventional HE technique. Then, we develop a power-consumption model for emissive displays and formulate an objective function that consists of the histogram-equalizing term and the power term. By minimizing the objective function based on the convex optimization theory, the proposed algorithm achieves contrast enhancement and power saving simultaneously. Moreover, we extend the proposed algorithm to enhance video sequences, as well as still images. Simulation results demonstrate that the proposed algorithm can reduce power consumption significantly while improving image contrast and perceptual quality.

Wafer-scale transferable molybdenum disulfide thin-film catalysts for photoelectrochemical hydrogen production

We demonstrate that wafer-scale, transferable, and transparent thin-film catalysts based on MoS2, which consists of cheap and earth abundant elements, can provide a low onset potential of 1 mA cm−2 at 0.17 V versus a reversible hydrogen electrode and the high photocurrent density of 24.6 mA cm−2 at 0 V for a p-type Si photocathode. c-Domains with vertically stacked (100) planes in the transferable 2H-MoS2 thin films, which are grown via a thermolysis method, act as active sites for the hydrogen evolution reaction, and photogenerated electrons are efficiently transported through the n-MoS2/p-Si heterojunction.

Contrast Enhancement Based on Layered Difference Representation of 2D Histograms

A novel contrast enhancement algorithm based on the layered difference representation of 2D histograms is proposed in this paper. We attempt to enhance image contrast by amplifying the gray-level differences between adjacent pixels. To this end, we obtain the 2D histogram h(k, k+l) from an input image, which counts the pairs of adjacent pixels with gray-levels k and k+l, and represent the gray-level differences in a tree-like layered structure. Then, we formulate a constrained optimization problem based on the observation that the gray-level differences, occurring more frequently in the input image, should be more emphasized in the output image. We first solve the optimization problem to derive the transformation function at each layer. We then combine the transformation functions at all layers into the unified transformation function, which is used to map input gray-levels to output gray-levels. Experimental results demonstrate that the proposed algorithm enhances images efficiently in terms of both objective quality and subjective quality.

Hepatocyte Growth Factor Family Negatively Regulates Hepatic Gluconeogenesis via Induction of Orphan Nuclear Receptor Small Heterodimer Partner in Primary Hepatocytes

Hepatic gluconeogenesis is tightly balanced by opposing stimulatory (glucagon) and inhibitory (insulin) signaling pathways. Hepatocyte growth factor (HGF) is a pleiotropic growth factor that mediates diverse biological processes. In this study, we investigated the effect of HGF and its family member, macrophage-stimulating factor (MSP), on hepatic gluconeogenesis in primary hepatocytes. HGF and MSP significantly repressed expression of the key hepatic gluconeogenic enzyme genes, phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase (Glc-6-Pase) and reduced glucose production. HGF and MSP activated small heterodimer partner (SHP) gene promoter and induced SHP mRNA and protein levels, and the effect of HGF and MSP on SHP gene expression was demonstrated to be mediated via activation of the AMP-activated protein kinase (AMPK) signaling pathway. We demonstrated that upstream stimulatory factor-1 (USF-1) specifically mediated HGF effect on SHP gene expression, and inhibition of USF-1 by dominant negative USF-1 significantly abrogated HGF-mediated activation of the SHP promoter. Elucidation of the mechanism showed that USF-1 bound to E-box-1 in the SHP promoter, and HGF increased USF-1 DNA binding on the SHP promoter via AMPK and DNA-dependent protein kinase-mediated pathways. Adenoviral overexpression of USF-1 significantly repressed PEPCK and Glc-6-Pase gene expression and reduced glucose production. Knockdown of endogenous SHP expression significantly reversed this effect. Finally, knockdown of SHP or inhibition of AMPK signaling reversed the ability of HGF to suppress hepatocyte nuclear factor 4α-mediated up-regulation of PEPCK and Glc-6-Pase gene expression along with the HGF- and MSP-mediated suppression of gluconeogenesis. Overall, our results suggest a novel signaling pathway through HGF/AMPK/USF-1/SHP to inhibit hepatic gluconeogenesis.

Power-constrained contrast enhancement for OLED displays based on histogram equalization

A novel power-constrained contrast enhancement algorithm for organic light-emitting diode (OLED) displays is proposed in this work. We first develop the log-modified histogram equalization (LMHE) scheme, which reduces overstretching artifacts of the conventional histogram equalization technique. Then, we model the power consumption in OLED displays, and incorporate it into LMHE to achieve the optimal tradeoff between contrast enhancement and power saving. Simulation results demonstrate that the proposed algorithm can reduce the power consumption significantly, while preserving image qualities.

Motion-Compensated Frame Interpolation Based on Multihypothesis Motion Estimation and Texture Optimization

A novel motion-compensated frame interpolation (MCFI) algorithm to increase video temporal resolutions based on multihypothesis motion estimation and texture optimization is proposed in this paper. Initially, we form multiple motion hypotheses for each pixel by employing different motion estimation parameters, i.e., different block sizes and directions. Then, we determine the best motion hypothesis for each pixel by solving a labeling problem and optimizing the parameters. In the labeling problem, the cost function is composed of color, shape, and smoothness terms. Finally, we refine the motion hypothesis field based on the texture optimization technique and blend multiple source pixels to interpolate each pixel in the intermediate frame. Simulation results demonstrate that the proposed algorithm provides significantly better MCFI performance than conventional algorithms.

Heterostructures based on inorganic and organic van der Waals systems

The two-dimensional limit of layered materials has recently been realized through the use of van der Waals (vdW) heterostructures composed of weakly interacting layers. In this paper, we describe two different classes of vdW heterostructures: inorganic vdW heterostructures prepared by co-lamination and restacking; and organic-inorganic hetero-epitaxy created by physical vapor deposition of organic molecule crystals on an inorganic vdW substrate. Both types of heterostructures exhibit atomically clean vdW interfaces. Employing such vdW heterostructures, we have demonstrated various novel devices, including graphene/hexagonal boron nitride (hBN) and MoS2 heterostructures for memory devices; graphene/MoS2/WSe2/graphene vertical p-n junctions for photovoltaic devices, and organic crystals on hBN with graphene electrodes for high-performance transistors.

Ghost-Free High Dynamic Range Imaging via Rank Minimization

We propose a ghost-free high dynamic range (HDR) image synthesis algorithm using a low-rank matrix completion framework, which we call RM-HDR. Based on the assumption that irradiance maps are linearly related to low dynamic range (LDR) image exposures, we formulate ghost region detection as a rank minimization problem. We incorporate constraints on moving objects, i.e., sparsity, connectivity, and priors on under- and over-exposed regions into the framework. Experiments on real image collections show that the RM-HDR can often provide significant gains in synthesized HDR image quality over state-of-the-art approaches. Additionally, a complexity analysis is performed which reveals computational merits of RM-HDR over recent advances in deghosting for HDR.

Single-image deraining using an adaptive nonlocal means filter

An adaptive rain streak removal algorithm for a single image is proposed in this work. We observe that a typical rain streak has an elongated elliptical shape with a vertical orientation. Thus, we first detect rain streak regions by analyzing the rotation angle and the aspect ratio of the elliptical kernel at each pixel location. We then perform the nonlocal means filtering on the detected rain streak regions by selecting nonlocal neighbor pixels and their weights adaptively. Experimental results demonstrate that the proposed algorithm removes rain streaks more efficiently and provides higher restored image qualities than conventional algorithms.