Sungkyunkwan University

About: Sungkyunkwan University is a education organization based out in Seoul, South Korea. It is known for research contribution in the topics: Thin film & Graphene. The organization has 28229 authors who have published 56428 publications receiving 1352733 citations. The organization is also known as: 성균관대학교.

Confocal absorption spectral imaging of MoS2: optical transitions depending on the atomic thickness of intrinsic and chemically doped MoS2

Abstract: We performed a nanoscale confocal absorption spectral imaging to obtain the full absorption spectra (over the range 1.5-3.2 eV) within regions having different numbers of layers and studied the variation of optical transition depending on the atomic thickness of the MoS2 film. Three distinct absorption bands corresponding to A and B excitons and a high-energy background (BG) peak at 2.84 eV displayed a gradual redshift as the MoS2 film thickness increased from the monolayer, to the bilayer, to the bulk MoS2 and this shift was attributed to the reduction of the gap energy in the Brillouin zone at the K-point as the atomic thickness increased. We also performed n-type chemical doping of MoS2 films using reduced benzyl viologen (BV) and the confocal absorption spectra modified by the doping showed a strong dependence on the atomic thickness: A and B exciton peaks were greatly quenched in the monolayer MoS2 while much less effect was shown in larger thickness and the BG peak either showed very small quenching for 1 L MoS2 or remained constant for larger thicknesses. Our results indicate that confocal absorption spectral imaging can provide comprehensive information on optical transitions of microscopic size intrinsic and doped two-dimensional layered materials.

Nanowire perovskite solar cell.

Abstract: Organolead iodide perovskite, CH3NH3PbI3, was prepared in the form of nanowire by means of a small quantity of aprotic solvent in two-step spin-coating procedure. One-dimensional nanowire perovskite with the mean diameter of 100 nm showed faster carrier separation in the presence of hole transporting layer and higher lateral conductivity than the three-dimensional nanocuboid crystal. Reduction in dimensionality resulted in the hypsochromic shift of both absorption and fluorescence spectra, indicative of more localized exciton states in nanowires. The best performing device employing nanowire CH3NH3PbI3 delivered photocurrent density of 19.12 mA/cm2, voltage of 1.052 V, and fill factor of 0.721, leading to a power conversion efficiency (PCE) of 14.71% at standard AM 1.5G solar illumination. A small I–V hysteresis was observed, where a PCE at forward scan was measured to be 85% of the PCE at reverse scan.

Diffusion Mechanism of Lithium Ion through Basal Plane of Layered Graphene

Abstract: Coexistence of both edge plane and basal plane in graphite often hinders the understanding of lithium ion diffusion mechanism. In this report, two types of graphene samples were prepared by chemical vapor deposition (CVD): (i) well-defined basal plane graphene grown on Cu foil and (ii) edge plane-enriched graphene layers grown on Ni film. Electrochemical performance of the graphene electrode can be split into two regimes depending on the number of graphene layers: (i) the corrosion-dominant regime and (ii) the lithiation-dominant regime. Li ion diffusion perpendicular to the basal plane of graphene is facilitated by defects, whereas diffusion parallel to the plane is limited by the steric hindrance that originates from aggregated Li ions adsorbed on the abundant defect sites. The critical layer thickness (lc) to effectively prohibit substrate reaction using CVD-grown graphene layers was predicted to be ∼6 layers, independent of defect population. Our density functional theory calculations demonstrate that...

Spurious Welfare Reversals in International Business Cycle Models

Abstract: Papers on international business cycles have documented spurious welfare reversals: incomplete markets produce a higher level of welfare than the complete market. This paper first demonstrates how conventional linearization, as used in King, Plosser, and Rebelo (1988), can generate approximation errors that can result in welfare reversals. Using a two-country production economy, we argue that spurious welfare reversals are not only possible but also plausible under reasonable values for model parameters including labor supply elasticity. As a constructive alternative, this paper then proposes an approximation method that modifies the conventional linearization by a bias correction - the linear approximation around a 'stochastic' steady state. We show that this method can be easily implemented and very well approximates the exact solution. The accuracy of the proposed method is by far better than that of the conventional linearization method and as good as that of a perturbation method involving a second-order expansion.

Sound‐Driven Piezoelectric Nanowire‐Based Nanogenerators

Abstract: Thus, sound power can be used for various novel applications including mobile phones that can be charged during conversations and sound-insulating walls near highways that generate electricity from the sound of passing vehicles. The latter development would have the additional benefi t of reducing noise levels near highways by absorbing the sound energy of vehicles. Here, we report the fi rst power-generating performance of sound-driven nanogenerators based on piezoelectric ZnO nanowires. Figure 1a and b show a schematic diagram of an integrated nanogenerator with piezoelectric ZnO nanowires and a crosssectional fi eld-emission scanning electron microscopy (FE-SEM) image of vertically well-aligned ZnO nanowire arrays (acting as a piezoelectric active layer), respectively. The nanowires were grown using a thermal chemical vapor deposition (CVD) system via a vapor-liquid-solid mechanism on an n-type GaN thin fi lm (acting as a bottom electrode)-deposited sapphire substrate. [ 14 ] It was found that there are no additional peaks other than (0002) and (0004) peaks of ZnO in X-ray diffraction measurements (not shown), indicating good alignment of the ZnO nanowires along the c -axis direction on GaN. A palladium gold (PdAu)coated polyethersulfone (PES) substrate were used as both a top electrode and a vibration plate and placed above the ZnO nanowire arrays. The integrated device was then sealed at the edges to prevent physical and chemical damage. The average length and diameter of the ZnO nanowires were ∼ 10 µ m and ∼ 150 nm, respectively. The integrated nanogenerator was then connected to a measurement system.