Low-Temperature, Universal Synthetic Route for Mesoporous Metal Oxides by Exploiting Synergistic Effect of Thermal Activation and Plasma.

Mesoporous metal oxides exhibit excellent physicochemical properties and are widely used in various fields, including energy storage/conversion, catalysis, and sensors. Although several soft-template approaches are reported, high-temperature calcination for both metal oxide formation and template removal is necessary, which limits direct synthesis on a plastic substrate for flexible devices. Here, a universal synthetic approach that combines thermal activation and oxygen plasma to synthesize diverse mesoporous metal oxides (V2O5, V6O13, TiO2, Nb2O5, WO3, and MoO3) at low temperatures (150-200 °C), which can be applicable to a flexible polymeric substrate is introduced. As a demonstration, a flexible micro-supercapacitor is fabricated by directly synthesizing mesoporous V2O5 on an indium-tin oxide-coated colorless polyimide film. The energy storage performance is well maintained under severe bending conditions.

Selective Laser-Assisted Direct Synthesis of MoS2 for Graphene/MoS2 Schottky Junction.

Implementing a heterostructure by vertically stacking two-dimensional semiconductors is necessary for responding to various requirements in the future of semiconductor technology. However, the chemical-vapor deposition method, which is an existing two-dimensional (2D) material-processing method, inevitably causes heat damage to surrounding materials essential for functionality because of its high synthesis temperature. Therefore, the heterojunction of a 2D material that directly synthesized MoS2 on graphene using a laser-based photothermal reaction at room temperature was studied. The key to the photothermal-reaction mechanism is the difference in the photothermal absorption coefficients of the materials. The device in which graphene and MoS2 were vertically stacked using a laser-based photothermal reaction demonstrated its potential application as a photodetector that responds to light and its stability against cycling. The laser-based photothermal-reaction method for 2D materials will be further applied to various fields, such as transparent display electrodes, photodetectors, and solar cells, in the future.

Effect of Measurement System Configuration and Operating Conditions on 2D Material-Based Gas Sensor Sensitivity.

Gas sensors applied in real-time detection of toxic gas leakage, air pollution, and respiration patterns require a reliable test platform to evaluate their characteristics, such as sensitivity and detection limits. However, securing reliable characteristics of a gas sensor is difficult, owing to the structural difference between the gas sensor measurement platform and the difference in measurement methods. This study investigates the effect of measurement conditions and system configurations on the sensitivity of two-dimensional (2D) material-based gas sensors. Herein, we developed a testbed to evaluate the response characteristics of MoS2-based gas sensors under a NO2 gas flow, which allows variations in their system configurations. Additionally, we demonstrated that the distance between the gas inlet and the sensor and gas inlet orientation influences the sensor performance. As the distance to the 2D gas sensor surface decreased from 4 to 2 mm, the sensitivity of the sensor improved to 9.20%. Furthermore, when the gas inlet orientation was perpendicular to the gas sensor surface, the sensitivity of the sensor was the maximum (4.29%). To attain the optimum operating conditions of the MoS2-based gas sensor, the effects of measurement conditions, such as gas concentration and temperature, on the sensitivity of the gas sensor were investigated.

S/Mo ratio and petal size controlled MoS2 nanoflowers with low temperature metal organic chemical vapor deposition and their application in solar cells.

Vertically aligned two-dimensional (2D) molybdenum disulfide nanoflowers (MoS2 NFs) have drawn considerable attention as a novel functional material with potential for next-generation applications owing to their inherently distinctive structure and extraordinary properties. We report a simple metal organic chemical vapor deposition (MOCVD) method that can grow high crystal quality, large-scale and highly homogeneous MoS2 NFs through precisely controlling the partial pressure ratio of H2S reaction gas, P SR, to Mo(CO)6 precursor, P MoP, at a substrate temperature of 250 °C. We investigate microscopically and spectroscopically that the S/Mo ratio, optical properties and orientation of the grown MoS2 NFs can be controlled by adjusting the partial pressure ratio, P SR/P MoP. It is also shown that the low temperature MOCVD (LT-MOCVD) growth method can regulate the petal size of MoS2 NFs through the growth time, thereby controlling photoluminescence intensity. More importantly, the MoS2 NFs/GaAs heterojunction flexible solar cell exhibiting a power conversion efficiency of ∼1.3% under air mass 1.5 G illumination demonstrates the utility of the LT-MOCVD method that enables the direct growth of MoS2 NFs on the flexible devices. Our work can pave the way for practical, easy-to-fabricate 2D materials integrated flexible devices in optical and photonic applications.

Development of in-situ particle monitor with beam homogenizing module for enhancement of nanoparticle countering efficiency.

An in situ particle monitor (ISPM) was developed to measure the concentration of several hundred nanosized contaminant particles generated from the semiconductor process. It is difficult to measure particles below 300 nm owing to low sensitivity and reliability. To improve the sensitivity and reduce the uncertainty caused by the Gaussian distribution of laser, a beam homogenizing module was applied to transform the Gaussian beam into a flat-top beam by total internal reflection. The performance of the beam-homogenizing ISPM was evaluated by measuring standard polystyrene latex particles in vacuum. We analyzed the measurement efficiency by a comparative evaluation with a scanning mobility particle sizer. Following this, the count of particles generated from the exhaust line of a plasma-enhanced chemical vapor deposition process was measured for real-time process diagnosis.

Unsupervised Learning for Depth, Ego-Motion, and Optical Flow Estimation Using Coupled Consistency Conditions.

Herein, we propose an unsupervised learning architecture under coupled consistency conditions to estimate the depth, ego-motion, and optical flow. Previously invented learning techniques in computer vision adopted a large amount of the ground truth dataset for network training. A ground truth dataset, including depth and optical flow collected from the real world, requires tremendous effort in pre-processing due to the exposure to noise artifacts. In this paper, we propose a framework that trains networks while using a different type of data with combined losses that are derived from a coupled consistency structure. The core concept is composed of two parts. First, we compare the optical flows, which are estimated from both the depth plus ego-motion and flow estimation network. Subsequently, to prevent the effects of the artifacts of the occluded regions in the estimated optical flow, we compute flow local consistency along the forward-backward directions. Second, synthesis consistency enables the exploration of the geometric correlation between the spatial and temporal domains in a stereo video. We perform extensive experiments on the depth, ego-motion, and optical flow estimation on the Karlsruhe Institute of Technology and Toyota Technological Institute (KITTI) dataset. We verify that the flow local consistency loss improves the optical flow accuracy in terms of the occluded regions. Furthermore, we also show that the view-synthesis-based photometric loss enhances the depth and ego-motion accuracy via scene projection. The experimental results exhibit the competitive performance of the estimated depth and the optical flow; moreover, the induced ego-motion is comparable to that obtained from other unsupervised methods.

Highly sensitive two-dimensional MoS2 gas sensor decorated with Pt nanoparticles.

A two-dimensional molybdenum disulfide (MoS2)-based gas sensor was decorated with Pt nanoparticles (NPs) for high sensitivity and low limit of detection (LOD) for specific gases (NH3 and H2S). The two-dimensional MoS2 film was grown at 400°C using metal organic gas vapour deposition. To fabricate the MoS2 gas sensor, an interdigitated Au/Ti electrode was deposited using the electron beam (e-beam) evaporation method with a stencil mask. The MoS2 gas sensor without metal decoration sensitively detects NH3 and H2S gas down to 2.5 and 30 ppm, respectively, at room temperature (RT). However, for improved detection of NH3 and H2S gas, we investigated the functionalization strategy using metal decoration. Pt NP decoration modulated the electronic properties of MoS2, significantly improving the sensitivity of NH3 and H2S gas by 5.58× and 4.25×, respectively, compared with the undecorated MoS2 gas sensor under concentrations of 70 ppm. Furthermore, the Pt NP-decorated MoS2 sensor had lower LODs for NH3 and H2S gas of 130 ppb and 5 ppm, respectively, at RT.

Observation of photoluminescence from large-scale layer-controlled 2D ß-Cu2S synthesized by the vapor-phase sulfurization of copper thin films.

Two-dimensional (2D) copper chalcogenides (Cu2-x X where X = S, Se, Te) have had much attention regarding various applications due to their remarkable optical and electrical properties, abundance, and environmentally friendly natures. This work indicates that highly uniform Cu2-x S (where 0 < x < 1) nanosheets can be obtained by the two-step method of Cu deposition by sputtering with precisely controlled and extremely low growth rate followed by vapor-phase sulfurization. The phase transformations of thin Cu2-x S films upon the Cu seed layer thickness are investigated. A unique thickness-constrained synthesis process using vapor-phase sulfurization is employed here, which evolves from a vertical to lateral growth mechanism based on the optimization of the Cu seed layer thickness. Atomically thin 2D ß-Cu2S film was successfully synthesized using the thinnest Cu seed film. We have systematically investigated the phase- and thickness-dependent optical properties of Cu2-x S films at room temperature. Micro-photoluminescence (PL) spectroscopy reveals that the 2D ß-Cu2S film possesses a direct band gap with an energy of 1.1 eV while the PL intensities are greatly suppressed in the multilayer Cu2-x S (where 0 ≤ x < 1).

Wafer-scale production of highly uniform two-dimensional MoS2 by metal-organic chemical vapor deposition.

Semiconducting two-dimensional (2D) materials, particularly extremely thin molybdenum disulfide (MoS2) films, are attracting considerable attention from academia and industry owing to their distinctive optical and electrical properties. Here, we present the direct growth of a MoS2 monolayer with unprecedented spatial and structural uniformity across an entire 8 inch SiO2/Si wafer. The influences of growth pressure, ambient gases (Ar, H2), and S/Mo molar flow ratio on the MoS2 layered growth were explored by considering the domain size, nucleation sites, morphology, and impurity incorporation. Monolayer MoS2-based field effect transistors achieve an electron mobility of 0.47 cm2 V-1 s-1 and on/off current ratio of 5.4 × 104. This work demonstrates the potential for reliable wafer-scale production of 2D MoS2 for practical applications in next-generation electronic and optical devices.

Hierarchical, Dual-Scale Structures of Atomically Thin MoS2 for Tunable Wetting.

Molybdenum disulfide (MoS2), a well-known solid lubricant for low friction surface coatings, has recently drawn attention as an analogue two-dimensional (2D) material beyond graphene. When patterned to produce vertically grown, nanoflower-structures, MoS2 shows promise as a functional material for hydrogen evolution catalysis systems, electrodes for alkali metal-ion batteries, and field-emission arrays. Whereas the wettability of graphene has been substantially investigated, that of MoS2 structures, especially nanoflowers, has remained relatively unexplored despite MoS2 nanoflower's potential in future applications. Here, we demonstrate that the wettability of MoS2 can be controlled by multiscale modulation of surface roughness through (1) tuning of the nanoflower structures by chemical vapor deposition synthesis and (2) tuning of microscale topography via mechanical strain. This multiscale modulation offers broadened tunability (80-155°) compared to single-scale tuning (90-130°). In addition, surface adhesion, determined from contact angle hysteresis (CAH), can also be tuned by multiscale surface roughness modulation, where the CAH is changed in range of 20-40°. Finally, the wettability of crumpled MoS2 nanoflowers can be dynamically and reversibly controlled through applied strain (∼115-150° with 0-200% strain), and remains robust over 1000 strain cycles. These studies on the tunable wettability of MoS2 will contribute to future MoS2-based applications, such as tunable wettability coatings for desalination and hydrogen evolution.

Development of particle characteristics diagnosis system for nanoparticle analysis in vacuum.

A particle characteristics diagnosis system (PCDS) was developed to measure nano-sized particle properties by a combination of particle beam mass spectrometry, scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDS). It allows us to measure the size distributions of nano-sized particles in real time, and the shape and composition can be determined by in situ SEM imaging and EDS scanning. PCDS was calibrated by measuring the size-classified nano-sized NaCl particles generated using an aqueous solution of NaCl by an atomizer. After the calibration, the characteristics of nano-sized particles sampled from the exhaust line of the plasma-enhanced chemical vapor deposition process were determined using PCDS.

Low-temperature growth of layered molybdenum disulphide with controlled clusters.

Layered molybdenum disulphide was grown at a low-temperature of 350 °C using chemical vapour deposition by elaborately controlling the cluster size. The molybdenum disulphide grown under various sulphur-reaction-gas to molybdenum-precursor partial-pressure ratios were examined. Using spectroscopy and microscopy, the effect of the cluster size on the layered growth was investigated in terms of the morphology, grain size, and impurity incorporation. Triangular single-crystal domains were grown at an optimized sulphur-reaction-gas to molybdenum-precursor partial-pressure ratio. Furthermore, it is proved that the nucleation sites on the silicon-dioxide substrate were related with the grain size. A polycrystalline monolayer with the 100-nm grain size was grown on a nucleation site confined substrate by high-vacuum annealing. In addition, a field-effect transistor was fabricated with a MoS2 monolayer and exhibited a mobility and on/off ratio of 0.15 cm(2) V(-1) s(-1) and 10(5), respectively.

Theoretical analysis for quantitative evaluation of in-situ nanoparticle measurement probability.

A probability equation based on the proper assumptions of the particle trajectory and fundamental physics has been developed by analyze beam properties such as beam width and intensity distribution for an in situ particle monitor (ISPM). The radius coordinate which has the same intensity and portion of beam area for detection voltage range were analyzed to calculate particle measurement probability. The particle measurement probability is defined at a ratio of entire beam area to specified beam area which decided by detection voltage range. A probability measurement, given as a function of the detection voltage range, was performed 5 times using 200, 300, 500, 700 nm polystyrene latex standard particles at a pressure of 100 Torr with an in-house ISPM. The theoretical calculation results show good agreement with the experimental results and the maximum error is 20% by calculating probability differences between theoretical and experimental values. A calibration method based on the proposed probability equation enables to developed and increase accuracy of ISPM.

Histopathological pictures of the initial changes of the hair bulbs in alopecia areata.

A biopsy of the seemingly normal scalp of a patient who had just begun to develop alopecia areata showed distinctive changes in bulbar morphology, in addition to peribulbar lymphocytic infiltrates. One of these changes was a loss of structural integrity of the centrally located supramatrical upper bulbar region. The other was the shrinkage of hair bulbs in the direction of club shape. Uninvolved intact anagen follicles were also present among these involved follicles.