Kung-Hwa Wei

Highly Efficient Electrocatalytic Hydrogen Production by MoSx Grown on Graphene‐Protected 3D Ni Foams

A three-dimensional Ni foam deposited with graphene layers on surfaces is used as a conducting solid support to load MoS(x) catalysts for electrocatalytic hydrogen evolution. The graphene sheets grown on Ni foams provide robust protection and efficiently increase the stability in acid. The superior performance of hydrogen evolution is attributed to the relatively high catalyst loading weight as well as its relatively low resistance.

Synthesis and characterization of novel segmented polyurethane/clay nanocomposites

A novel segmented polyurethane(PU)/clay nanocomposite has been synthesized by polyurethane and organoclay. 12 aminolauric acid (12COOH) and benzidine (BZD) were used as swelling agents to treat Na 1 -montmorillonite and for forming organoclay (12COOH-mont and BZD-mont) through ion exchange. The nanometer-scale silicate layers of organoclay were completely exfoliated in PU in the cases of 1, 3 and 5% 12COOH-mont/PU and in the cases of 1 and 3% BZD-mont/PU nanocomposites as confirmed by X-ray diffraction pattern and transmission electron microscopy studies. The segmented structures of PU were not interfered by the presence of the silicate layers in these nanocomposites as evidenced by their glass transition and degree of phase separation from differential scanning calorimetry and Fourier transform infrared measurements. A two-fold increase in the tensile strength and a three-fold increase in the elongation were found for 1% BZD-mont/PU as compared to that of pure PU. Additionally, both 1% 12COOH-mont/PU and 1% BZD-mont/PU exhibited lower water absorption properties than that of pure PU. q 1999 Elsevier Science Ltd. All rights reserved.

Improving device efficiency of polymer/fullerene bulk heterojunction solar cells through enhanced crystallinity and reduced grain boundaries induced by solvent additives.

Polymer solar cells based on bulk heterojunction (BHJ) structures, featuring conjugated polymers as donors and fullerene derivatives as acceptors, [ 1 ] are being developed for their potential application in the low-cost fabrication of large-area devices. In recent reports, BHJ solar cells incorporating crystalline or low-bandgap conjugated polymers [ 2 ] and fullerene derivatives have exhibited maximum power conversion effi ciencies (PCEs) of up to 8%. [ 3 ] The morphology [ 4 ] of the active layer in a BHJ solar cell incorporating a polymer/fullerene thin fi lm plays a critical role affecting the device performance; phase-separated domains in the active layer provide not only interfaces for charge separation of photogenerated excitons but also percolation pathways for charge carrier transport to the respective electrodes, critically affecting the device’s PCE. The nanoscale morphology of a polymer/fullerene thin fi lm is greatly affected by (i) the fi lm processing conditions, [ 5 ] (ii) the molar ratio (composition) of the polymer and the fullerene, [ 6 ] and (iii) the nature of the solvent additive (if any). [ 7 ] In particular, BHJ polymer solar cells can exhibit improved device performance after undergoing thermal or solvent annealing or the incorporation of solvent additives, all of which alter the fi lm morphology to a more favorable state relative to that of the as-cast fi lm or the fi lm in the absence of the additive, presumably resulting from (i) self-organization of the polymer units into ordered structures and (ii) appropriate aggregation of fullerene domains to provide percolation networks for charge carrier transport. [ 6 , 8 ] Among these approaches, the addition of solvent additive during the processing of the active layer is the simplest and most effective means of optimizing a BHJ device’s morphology; it infl uences the size of the fullerene domains and enhances the crystallinity of the self-organized polymers by improving the solubility of

Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface

Electronic junctions on edge Two-dimensional materials such as graphene are attractive materials for making smaller transistors because they are inherently nanoscale and can carry high currents. However, graphene has no band gap and the transistors are “leaky”; that is, they are hard to turn off. Related transition metal dichalcogenides (TMDCs) such as molybdenum sulfide have band gaps. Transistors based on these materials can have high ratios of “on” to “off” currents. However, it is often difficult to make a good voltage-biased (p-n) junction between different TMDC materials. Li et al. succeeded in making p-n heterojunctions between two of these materials, molybdenum sulfide and tungsten selenide. They did this not by stacking the layers, which make a weak junction, but by growing molybdenum sulfide on the edge of a triangle of tungsten selenide with an atomically sharp boundary Science, this issue p. 524 The regrowth of the second transition metal dichalcogenide monolayer by edge epitaxy creates a lateral p-n heterojunction. Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.

Synthesis and characterization of novel segmented polyurethane/clay nanocomposite via poly(ε-caprolactone)/clay

A novel segmented polyurethane/clay (PU/clay) nanocomposite based on poly(caprolactone), diphenylmethane diisocyanate, butanediol, and poly(caprolactone)/clay prepolymer was synthesized as evidenced by FTIR and X-ray diffraction studies. Poly(caprolactone)/clay (PCL/clay) prepolymer was first synthesized in a nanocomposite form as confirmed by X-ray diffraction. X-ray diffraction study showed that PU/clay contained crystalline structure due to the presence of PCL/clay. In mechanical properties, about 1.4% PCL/clay in PU/clay resulted in a large increase in the elongation of PU/clay. However, when the amount of PCL/clay was 4.2%, the elongation of PU/clay was reduced drastically. This behavior indicated that PU/clay can be transformed from an elastomer to a thermoplastic material as the amount of PCL/clay in PU/clay increased. Additionally, the lap shear stress of PU/clay was at least three times that of neat PU as a result of the PCL/clay component.

Competition between Fullerene Aggregation and Poly(3-hexylthiophene) Crystallization upon Annealing of Bulk Heterojunction Solar Cells

Concomitant development of [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) aggregation and poly(3-hexylthiophene) (P3HT) crystallization in bulk heterojunction (BHJ) thin-film (ca. 85 nm) solar cells has been revealed using simultaneous grazing-incidence small-/wide-angle X-ray scattering (GISAXS/GIWAXS). With enhanced time and spatial resolutions (5 s/frame; minimum q ≈ 0.004 Å(-1)), synchrotron GISAXS has captured in detail the fast growth in size of PCBM aggregates from 7 to 18 nm within 100 s of annealing at 150 °C. Simultaneously observed is the enhanced crystallization of P3HT into lamellae oriented mainly perpendicular but also parallel to the substrate. An Avrami analysis of the observed structural evolution indicates that the faster PCBM aggregation follows a diffusion-controlled growth process (confined by P3HT segmental motion), whereas the slower development of crystalline P3HT nanograins is characterized by constant nucleation rate (determined by the degree of supercooling and PCBM demixing). These two competing kinetics result in local phase separation with space-filling PCBM and P3HT nanodomains less than 20 nm in size when annealing temperature is kept below 180 °C. Accompanying the morphological development is the synchronized increase in electron and hole mobilities of the BHJ thin-film solar cells, revealing the sensitivity of the carrier transport of the device on the structural features of PCBM and P3HT nanodomains. Optimized structural parameters, including the aggregate size and mean spacing of the PCBM aggregates, are quantitatively correlated to the device performance; a comprehensive network structure of the optimized BHJ thin film is presented.

Layer-by-Layer Graphene/ TCNQ Stacked Films as Conducting Anodes for Organic Solar Cells

Large-area graphene grown by chemical vapor deposition (CVD) is a promising candidate for transparent conducting electrode applications in flexible optoelectronic devices such as light-emitting diodes or organic solar cells. However, the power conversion efficiency (PCE) of the polymer photovoltaic devices using a pristine CVD graphene anode is still not appealing due to its much lower conductivity than that of conventional indium tin oxide. We report a layer-by-layer molecular doping process on graphene for forming sandwiched graphene/tetracyanoquinodimethane (TCNQ)/graphene stacked films for polymer solar cell anodes, where the TCNQ molecules (as p-dopants) were securely embedded between two graphene layers. Poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction polymer solar cells based on these multilayered graphene/TCNQ anodes are fabricated and characterized. The P3HT/PCBM device with an anode structure composed of two TCNQ layers sandwiched by three CVD graphene layers shows optimum PCE (∼2.58%), which makes the proposed anode film quite attractive for next-generation flexible devices demanding high conductivity and transparency.

Opening an Electrical Band Gap of Bilayer Graphene with Molecular Doping

The opening of an electrical band gap in graphene is crucial for its application for logic circuits. Recent studies have shown that an energy gap in Bernal-stacked bilayer graphene can be generated by applying an electric displacement field. Molecular doping has also been proposed to open the electrical gap of bilayer graphene by breaking either in-plane symmetry or inversion symmetry; however, no direct observation of an electrical gap has been reported. Here we discover that the organic molecule triazine is able to form a uniform thin coating on the top surface of a bilayer graphene, which efficiently blocks the accessible doping sites and prevents ambient p-doping on the top layer. The charge distribution asymmetry between the top and bottom layers can then be enhanced simply by increasing the p-doping from oxygen/moisture to the bottom layer. The on/off current ratio for a bottom-gated bilayer transistor operated in ambient condition is improved by at least 1 order of magnitude. The estimated electrical band gap is up to ∼111 meV at room temperature. The observed electrical band gap dependence on the hole-carrier density increase agrees well with the recent density-functional theory calculations. This research provides a simple method to obtain a graphene bilayer transistor with a moderate on/off current ratio, which can be stably operated in air without the need to use an additional top gate.

Graphene/MoS2 Heterostructures for Ultrasensitive Detection of DNA Hybridisation

The photoluminescence signals of a graphene/MoS2 heterostructural stacking film are sensitive to environmental charges, which allows the single-base sequence-selective detection of DNA hybridization with sensitivity to the level of aM.

Mechanical properties of clay-polyimide (BTDA-ODA) nanocomposites via ODA-modified organoclay

Clay–polyimide [3,3′, 4,4′-benzophenone tetracarboxylic dianhydride–4,4′-oxydianiline (BTDA–ODA)] nanocomposites were synthesized from ODA-modified montmorillonite (organoclay) and poly(amic acid). The layered silicates of organoclay were intercalated by polyimide (BTDA–ODA), as confirmed by X-ray diffraction and by transmission electron microscopy, and the tensile mechanical properties of the nanocomposites were measured. It was found that the modulus and the maximum stress of these organoclay/BTDA–ODA nanocomposites were much higher than those of pure BTDA–ODA: a twofold increase in the modulus and a one-half-fold increase in the maximum stress in the case of 7/93 organoclay–BTDA-ODA. In addition, the elongation-for-break of organoclay/BTDA–ODA nanocomposites is even slightly higher than that of pure BTDA–ODA, which is a sharp contrast to that of conventional inorganics-filled polymer composites.