Youngchan Kim

Direct exfoliation and dispersion of two-dimensional materials in pure water via temperature control

The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials.

Analysis of laminated beams with a layer-wise constant shear theory

Abstract Based on generalized laminate plate theory, the formulation of a one-dimensional beam finite element with layer-wise constant shear (BLCS) is presented. The linear layer-wise representation of in-plane displacements permit accurate computation of normal stresses and transverse shear stresses on each layer for laminated beams with dissimilar ply stiffnesses. The BLCS formulation is equivalent to a first-order shear deformation beam theory (Timoshenko beam theory) on each layer. For the accurate computation of interlaminar shear stresses, the layer-wise constant shear stresses obtained from constitutive relations are transformed into parabolic shear stress distributions in a post-processing operation described in detail. The accuracy of the BLCS element is demonstrated by solving several numerical examples reported in the literature. While retaining the simplicity of a laminated beam theory, the element predicts results as accurate as much more complex elasticity analyses, and it is suitable to model frame-type structures.

Progressive failure analysis of laminated composite beams

A progressive failure model for laminated composite beams is formulated using a beam finite element with layer-wise constant shear (BLCS), which permits accurate computation of stresses on each layer. This is the first study to incorporate the stress-prediction accuracy of a layer-wise element for failure prediction of laminates under bending loads. In the present formulation, based on material degradation factors and existing failure criteria, a linear elastic behavior is assumed, and a damaged layer in an element is substituted by a degraded homogeneous layer. Maximum Stress and Tsai-Wu failure criteria are used to assess failure at the Gauss points. The effect of damage accumulation is accounted for by degrading the stiffness properties of failed element-layers in the equilibrium iterations. After equilibrium is satisfied, the load is increased by a constant percentage of first-play-failure load in a load-controlled failure prediction. A displacement-controlled scheme is also implemented. The predictions of the model correlate well with experimental results for two distinct laminated composite beams: graphite-epoxy and glulam reinforced with GFRP. The study provides guidelines, through parametric studies, for the appropriate selection of material-degradation factors, load increments, and finite element mesh.

Delamination buckling of FRP layer in laminated wood beams

An analytical solution for predicting delamination buckling and growth of a thin fiber reinforced-plastic (FRP) layer in laminated wood beams under bending is presented. Based on a strength-of-materials approach, displacement functions for a delaminated beam under four-point bending are derived. Using force and displacement compatibility conditions, an explicit form relating the applied transverse load with the delamination buckling load is established. An explicit form of the strain-energy release rate is presented to study the delamination growth in beams under bending. The analytical solution is evaluated using experimental data for glued-laminated timber (glulam) beams reinforced with a thin fiber-reinforced plastic composite on the compression face. The delamination growth in bending is shown to behave differently to that of the in-plane loading case.

ATP binding to neighbouring subunits and intersubunit allosteric coupling underlie proteasomal ATPase function

The primary functions of the proteasome are driven by a highly allosteric ATPase complex. ATP binding to only two subunits in this hexameric complex triggers substrate binding, ATPase–20S association and 20S gate opening. However, it is unclear how ATP binding and hydrolysis spatially and temporally coordinates these allosteric effects to drive substrate translocation into the 20S. Here, we use FRET to show that the proteasomal ATPases from eukaryotes (RPTs) and archaea (PAN) bind ATP with high affinity at neighbouring subunits, which complements the well-established spiral-staircase topology of the 26S ATPases. We further show that two conserved arginine fingers in PAN located at the subunit interface work together as a single allosteric unit to mediate the allosteric effects of ATP binding, without altering the nucleotide-binding pattern. Rapid kinetics analysis also shows that ring resetting of a sequential hydrolysis mechanism can be explained by thermodynamic equilibrium binding of ATP. These data support a model whereby these two functionally distinct allosteric networks cooperate to translocate polypeptides into the 20S for degradation.

Wafer-scale monolayer MoS2 grown by chemical vapor deposition using a reaction of MoO3 and H2S.

Monolayer MoS2 nanosheets are potentially useful in optoelectronics, photoelectronics, and nanoelectronics due to their flexibility, mechanical strength, and direct band gap of 1.89 eV. Experimentalists have studied the synthesis of MoS2 using chemical vapor deposition (CVD) methods in an effort to fabricate wafer-scale nanofilms with a high uniformity and continuity for practical electronic applications. In this work, we applied the CVD method to a reaction of MoO3 powder and H2S gas to grow high-quality polycrystalline monolayer MoS2 sheets with unprecedented uniformity over an area of several centimeters. The monolayer MoS2 was characterized using Raman spectroscopy, photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), x-ray photoemission spectroscopy (XPS), and transmission electron microscopy (TEM). The top-gate field-effect transistor prepared with a 30 nm HfO2 capping layer displayed an electrical mobility of 1 cm(2) v(-1) s(-1) and an I on/off of ~10(5). This method paves the way for the development of practical devices with MoS2 monolayers and advances fundamental research.

Ultraclean and Direct Transfer of a Wafer‐Scale MoS2 Thin Film onto a Plastic Substrate

An ultraclean method to directly transfer a large-area MoS2 film from the original growth substrate to a flexible substrate by using epoxy glue is developed. The transferred film is observed to be free of wrinkles and cracks and to be as smooth as the film synthesized on the original substrate.

Large-Area CVD-Grown Sub-2 V ReS2 Transistors and Logic Gates

We demonstrated the fabrication of large-area ReS2 transistors and logic gates composed of a chemical vapor deposition (CVD)-grown multilayer ReS2 semiconductor channel and graphene electrodes. Single-layer graphene was used as the source/drain and coplanar gate electrodes. An ion gel with an ultrahigh capacitance effectively gated the ReS2 channel at a low voltage, below 2 V, through a coplanar gate. The contact resistance of the ion gel-gated ReS2 transistors with graphene electrodes decreased dramatically compared with the SiO2-devices prepared with Cr electrodes. The resulting transistors exhibited good device performances, including a maximum electron mobility of 0.9 cm2/(V s) and an on/off current ratio exceeding 104. NMOS logic devices, such as NOT, NAND, and NOR gates, were assembled using the resulting transistors as a proof of concept demonstration of the applicability of the devices to complex logic circuits. The large-area synthesis of ReS2 semiconductors and graphene electrodes and their applications in logic devices open up new opportunities for realizing future flexible electronics based on 2D nanomaterials.

A layerwise beam element for analysis of frames with laminated sections and flexible joints

Abstract Fiber-reinforced plastic (FRP) composites are being used currently as reinforcement for beams of conventional materials, such as concrete and glued-laminated timber (glulam). The common assumption of plane cross-sections through the laminate for laminated beams with dissimilar ply stiffnesses can lead to inaccurate results, articularly for rectangular sandwich beams with soft cores. In this paper, we extend the formulation of a 3-node beam finite element with layerwise constant shear (BLCS) to the analysis of plane frames with rectangular laminated sections and flexible joints. Experimental results for sandwich beams and glulam-FRP beams are used to illustrate the accuracy of BLCS. An A-frame with two distinct laminated cross-sections and a rigid or flexible apex joint is analyzed with BLCS and also plane-stress and layered-shell elements of ANSYS. The BLCS element predicts the response of the A-frame accurately and does not exhibit shear-locking.

Line-defect mediated formation of hole and Mo clusters in monolayer molybdenum disulfide

The production of hole and Mo cluster by electron beam irradiation in molybdenum disulfide (MoS2), which consists of S-Mo-S layers, is monitored over time using atomic resolution transmission electron microscopy. S vacancies are firstly formed due to knocking off of S atoms and then line defects are induced due to accumulation of S vacancies in MoS2 sheet instead of forming a hole. The line defects tend to be merged at a point and a hole is formed subsequently at the point. Mo atoms tend to be clustered discretely as a nano sheet along the edge of the hole due to difference in displacement threshold energy between Mo and S atoms under electron irradiation. After Mo clusters are nearly separated from MoS2 sheet, the clusters are transformed into body-centered cubic nanocrystal of Mo during prolonged electron beam irradiation. The line defect mediated formation of hole and Mo cluster only occurs within a single grain of monolayer MoS2 sheet.