Showing papers by "National Tsing Hua University published in 2013"

Fullerene Derivative‐Doped Zinc Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low‐Bandgap Polymer (PTB7‐Th) for High Performance

Abstract: Modification of a ZnO cathode by doping it with a hydroxyl-containing derivative - giving a ZnO-C60 cathode - provides a fullerene-derivative-rich surface and enhanced electron conduction. Inverted polymer solar cells with the ZnO-C60 cathode display markedly improved power conversion efficiency compared to those with a pristine ZnO cathode, especially when the active layer includes the low-bandgap polymer PTB7-Th.

Alloy Design Strategies and Future Trends in High-Entropy Alloys

Abstract: High-entropy alloys (HEAs) are newly emerging advanced materials. In contrast to conventional alloys, HEAs contain multiple principal elements, often five or more in equimolar or near-equimolar ratios. The basic principle behind HEAs is that solid-solution phases are relatively stabilized by their significantly high entropy of mixing compared to intermetallic compounds, especially at high temperatures. This makes them feasibly synthesized, processed, analyzed, and manipulated, and as well provides many opportunities for us. There are huge numbers of possible compositions and combinations of properties in the HEA field. Wise alloy design strategies for suitable compositions and processes to fit the requirements for either academic studies or industrial applications thus become especially important. In this article, four core effects were emphasized, several misconceptions on HEAs were clarified, and several routes for future HEA research and development were suggested.

Structural Biological Materials: Critical Mechanics-Materials Connections

Abstract: Spider silk is extraordinarily strong, mollusk shells and bone are tough, and porcupine quills and feathers resist buckling How are these notable properties achieved? The building blocks of the materials listed above are primarily minerals and biopolymers, mostly in combination; the first weak in tension and the second weak in compression The intricate and ingenious hierarchical structures are responsible for the outstanding performance of each material Toughness is conferred by the presence of controlled interfacial features (friction, hydrogen bonds, chain straightening and stretching); buckling resistance can be achieved by filling a slender column with a lightweight foam Here, we present and interpret selected examples of these and other biological materials Structural bio-inspired materials design makes use of the biological structures by inserting synthetic materials and processes that augment the structures' capability while retaining their essential features In this Review, we explain this idea through some unusual concepts

Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light

Abstract: Nearly a century after Einstein first predicted the existence of gravitational waves, a global network of Earth-based gravitational wave observatories1, 2, 3, 4 is seeking to directly detect this faint radiation using precision laser interferometry. Photon shot noise, due to the quantum nature of light, imposes a fundamental limit on the attometre-level sensitivity of the kilometre-scale Michelson interferometers deployed for this task. Here, we inject squeezed states to improve the performance of one of the detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) beyond the quantum noise limit, most notably in the frequency region down to 150 Hz, critically important for several astrophysical sources, with no deterioration of performance observed at any frequency. With the injection of squeezed states, this LIGO detector demonstrated the best broadband sensitivity to gravitational waves ever achieved, with important implications for observing the gravitational-wave Universe with unprecedented sensitivity.

Financial Development and Innovation: Cross-Country Evidence

Abstract: We examine how financial market development affects technological innovation. Using a large data set that includes 32 developed and emerging countries and a fixed effects identification strategy, we identify economic mechanisms through which the development of equity markets and credit markets affects technological innovation. We show that industries that are more dependent on external finance and that are more high-tech intensive exhibit a disproportionally higher innovation level in countries with better developed equity markets. However, the development of credit markets appears to discourage innovation in industries with these characteristics. Our paper provides new insights into the real effects of financial market development on the economy.

Proton Structure from the Measurement of 2S-2P Transition Frequencies of Muonic Hydrogen

Abstract: Accurate knowledge of the charge and Zemach radii of the proton is essential, not only for understanding its structure but also as input for tests of bound-state quantum electrodynamics and its predictions for the energy levels of hydrogen. These radii may be extracted from the laser spectroscopy of muonic hydrogen (μp, that is, a proton orbited by a muon). We measured the 2 S 1 / 2 F = 0 - 2 P 3 / 2 F = 1 transition frequency in μp to be 54611.16(1.05) gigahertz (numbers in parentheses indicate one standard deviation of uncertainty) and reevaluated the 2 S 1 / 2 F = 1 - 2 P 3 / 2 F = 2 transition frequency, yielding 49881.35(65) gigahertz. From the measurements, we determined the Zemach radius, rZ = 1.082(37) femtometers, and the magnetic radius, rM = 0.87(6) femtometer, of the proton. We also extracted the charge radius, rE = 0.84087(39) femtometer, with an order of magnitude more precision than the 2010-CODATA value and at 7σ variance with respect to it, thus reinforcing the proton radius puzzle.

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

Abstract: 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.

Graphene-based photothermal agent for rapid and effective killing of bacteria.

Abstract: Conventional antibiotic therapies are becoming less efficient due to the emergence of antibiotic-resistant bacterial strains. Development of novel antibacterial material to effectively inhibit or kill bacteria is crucial. A graphene-based photothermal agent, magnetic reduced graphene oxide functionalized with glutaraldehyde (MRGOGA), was synthesized for efficient capture and effective killing of both gram-positive Staphylococcus aureus (S. aureus) and gram-negative Escherichia coli (E. coli) bacteria upon near-infrared (NIR) laser irradiation. In the present work, we took advantage of the excellent photothermal properties of reduced graphene oxide upon NIR laser irradiation and glutaraldehyde as an efficient capturing agent toward both bacteria. Its magnetic characteristic allows bacteria to be readily trapped in a small volume by the external magnet. The synergetic effects increase the heating extent by MRGOGA upon NIR laser irradiation and the killing of the captured bacteria. The survival rate and memb...

A Generalized Low-Rank Appearance Model for Spatio-temporally Correlated Rain Streaks

Abstract: In this paper, we propose a novel low-rank appearance model for removing rain streaks. Different from previous work, our method needs neither rain pixel detection nor time-consuming dictionary learning stage. Instead, as rain streaks usually reveal similar and repeated patterns on imaging scene, we propose and generalize a low-rank model from matrix to tensor structure in order to capture the spatio-temporally correlated rain streaks. With the appearance model, we thus remove rain streaks from image/video (and also other high-order image structure) in a unified way. Our experimental results demonstrate competitive (or even better) visual quality and efficient run-time in comparison with state of the art.

Selective decoration of Au nanoparticles on monolayer MoS2 single crystals.

Abstract: We report a controllable wet method for effective decoration of 2-dimensional (2D) molybdenum disulfide (MoS2) layers with Au nanoparticles (NPs). Au NPs can be selectively formed on the edge sites or defective sites of MoS2 layers. The Au-MoS2 nano-composites are formed by non-covalent bond. The size distribution, morphology and density of the metal nanoparticles can be tuned by changing the defect density in MoS2 layers. Field effect transistors were directly fabricated by placing ion gel gate dielectrics on Au-decorated MoS2 layers without the need to transfer these MoS2 layers to SiO2/Si substrates for bottom gate devices. The ion gel method allows probing the intrinsic electrical properties of the as-grown and Au-decorated MoS2 layers. This study shows that Au NPs impose remarkable p-doping effects to the MoS2 transistors without degrading their electrical characteristics.

Gated silicene as a tunable source of nearly 100% spin-polarized electrons

Abstract: Silicene is a one-atom-thick two-dimensional crystal of silicon with a hexagonal lattice structure that is related to that of graphene but with atomic bonds that are buckled rather than flat. This buckling confers advantages on silicene over graphene, because it should, in principle, generate both a band gap and polarized spin-states that can be controlled with a perpendicular electric field. Here we use first-principles calculations to show that field-gated silicene possesses two gapped Dirac cones exhibiting nearly 100% spin-polarization, situated at the corners of the Brillouin zone. Using this fact, we propose a design for a silicene-based spin-filter that should enable the spin-polarization of an output current to be switched electrically, without switching external magnetic fields. Our quantum transport calculations indicate that the proposed designs will be highly efficient (nearly 100% spin-polarization) and robust against weak disorder and edge imperfections. We also propose a Y-shaped spin/valley separator that produces spin-polarized current at two output terminals with opposite spins.

Biomass burning contribution to Beijing aerosol

Abstract: . Biomass burning, the largest global source of elemental carbon (EC) and primary organic carbon (OC), is strongly associated with many subjects of great scientific concern, such as secondary organic aerosol and brown carbon which exert important effects on the environment and on climate in particular. This study investigated the relationships between levoglucosan and other biomass burning tracers (i.e., water soluble potassium and mannosan) based on both ambient samples collected in Beijing and source samples. Compared with North America and Europe, Beijing was characterized by high ambient levoglucosan concentrations and low winter to summer ratios of levoglucosan, indicating significant impact of biomass burning activities throughout the year in Beijing. Comparison of levoglucosan and water soluble potassium (K+) levels suggested that it was acceptable to use K+ as a biomass burning tracer during summer in Beijing, while the contribution of fireworks to K+ could be significant during winter. Moreover, the levoglucosan to K+ ratio was found to be lower during the typical summer period (0.21 ± 0.16) compared with the typical winter period (0.51 ± 0.15). Levoglucosan correlated strongly with mannosan (R2 = 0.97) throughout the winter and the levoglucosan to mannosan ratio averaged 9.49 ± 1.63, whereas levoglucosan and mannosan exhibited relatively weak correlation (R2 = 0.73) during the typical summer period when the levoglucosan to mannosan ratio averaged 12.65 ± 3.38. Results from positive matrix factorization (PMF) model analysis showed that about 50% of the OC and EC in Beijing were associated with biomass burning processes. In addition, a new source identification method was developed based on the comparison of the levoglucosan to K+ ratio and the levoglucosan to mannosan ratio among different types of biomass. Using this method, the major source of biomass burning aerosol in Beijing was suggested to be the combustion of crop residuals, while the contribution from softwood burning was also non-negligible, especially in winter.

A New Droop Control Method for the Autonomous Operation of Distributed Energy Resource Interface Converters

Abstract: Microgrid is widely accepted as an effective mean of integrating various distributed energy resources (DERs) through their interface converters to provide electric power of high quality and reliability. These distributed resources interface converters can operate in an autonomous fashion without any communication for enhanced system reliability and reduced complexity. Conventionally, the real power-frequency droop control and the reactive power-voltage magnitude droop are adopted as the decentralized control strategies in these DERs interface converters for the autonomous power sharing operations. However, the reactive power sharing of $Q\hbox{--}V$ droop control often deteriorates due to its dependence on the line impedances. In this paper, a $Q\hbox{--}\dot{V}$ droop control method with $\dot{V}$ restoration mechanism is proposed to improve reactive power sharing. Its operation principle and control method are explained and analyzed. Simulation and experimental results are presented to validate the effectiveness of the proposed method.

Dynamic evolution of conducting nanofilament in resistive switching memories.

Abstract: Resistive random access memory (ReRAM) has been considered the most promising next-generation nonvolatile memory. In recent years, the switching behavior has been widely reported, and understanding the switching mechanism can improve the stability and scalability of devices. We designed an innovative sample structure for in situ transmission electron microscopy (TEM) to observe the formation of conductive filaments in the Pt/ZnO/Pt structure in real time. The corresponding current-voltage measurements help us to understand the switching mechanism of ZnO film. In addition, high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) have been used to identify the atomic structure and components of the filament/disrupted region, determining that the conducting paths are caused by the conglomeration of zinc atoms. The behavior of resistive switching is due to the migration of oxygen ions, leading to transformation between Zn-dominated ZnO(1-x) and ZnO.

Surface electronic structure of the topological Kondo-insulator candidate correlated electron system SmB6.

Abstract: The Kondo insulator SmB6 has long been known to exhibit low-temperature transport anomalies whose origin is of great interest. Here we uniquely access the surface electronic structure of the anomalous transport regime by combining state-of-the-art laser and synchrotron-based angle-resolved photoemission techniques. We observe clear in-gap states (up to ~4 meV), whose temperature dependence is contingent on the Kondo gap formation. In addition, our observed in-gap Fermi surface oddness tied with the Kramers' point topology, their coexistence with the two-dimensional transport anomaly in the Kondo hybridization regime, as well as their robustness against thermal recycling, taken together, collectively provide strong evidence for protected surface metallicity with a Fermi surface whose topology is consistent with the theoretically predicted topological Fermi surface. Our observations of systematic surface electronic structure provide the fundamental electronic parameters for the anomalous Kondo ground state of correlated electron material SmB6.

A Keplerian disk around a Class 0 source: ALMA observations of VLA1623A

Abstract: Context. Rotationally supported disks are critical in the star formation process. The questions of when they form and what factors influence or hinder their formation have been studied but are largely unanswered. Observations of early-stage YSOs are needed to probe disk formation. Aims: VLA1623 is a triple non-coeval protostellar system, with a weak magnetic field perpendicular to the outflow, whose Class 0 component, VLA1623A, shows a disk-like structure in continuum with signatures of rotation in line emission. We aim to determine whether this structure is in part or in whole a rotationally supported disk, i.e. a Keplerian disk, and what its characteristics are. Methods: ALMA Cycle 0 Early Science 1.3 mm continuum and C$^{18}$O (2-1) observations in the extended configuration are presented here and used to perform an analysis of the disk-like structure using position-velocity (PV) diagrams and thin disk modeling with the addition of foreground absorption. Results: The PV diagrams of the C$^{18}$O line emission suggest the presence of a rotationally supported component with a radius of at least 50 AU. Kinematical modeling of the line emission shows that the disk out to 180 AU is actually rotationally supported, with the rotation described well by Keplerian rotation out to at least 150 AU, and the central source mass is ~{}0.2 M$_{⊙}$ for an inclination of 55{deg}. Pure infall and conserved angular momentum rotation models are excluded. Conclusions: VLA1623A, a very young Class 0 source, presents a disk with an outer radius R$_{out}$ = 180 AU with a Keplerian velocity structure out to at least 150 AU. The weak magnetic fields and recent fragmentation in this region of {$ρ$} Ophiuchus may have played a leading role in the formation of the disk. Appendices are available in electronic form at http://www.aanda.org

Criterion for Sigma Phase Formation in Cr- and V-Containing High-Entropy Alloys

Abstract: Formation of the σ phase has been observed in quite a few high-entropy alloys (HEAs) recently. The σ phase significantly enhances the hardness of the alloys but reduces their ductility. Thus, controlling the formation of σ phase through proper design is critical for HEAs. However, theories to predict the σ phase formation based on HEA composition are still not available. Here, we demonstrate that the σ phase formation is directly related to the valence electron concentration (VEC) of the alloy. The σ-phase-forming VEC range was systematically studied and revealed. Such finding is of crucial importance to the future design of HEAs.

Nitrogen-Doped Graphene Sheets Grown by Chemical Vapor Deposition: Synthesis and Influence of Nitrogen Impurities on Carrier Transport

Abstract: A significant advance toward achieving practical applications of graphene as a two-dimensional material in nanoelectronics would be provided by successful synthesis of both n-type and p-type doped graphene. However, reliable doping and a thorough understanding of carrier transport in the presence of charged impurities governed by ionized donors or acceptors in the graphene lattice are still lacking. Here we report experimental realization of few-layer nitrogen-doped (N-doped) graphene sheets by chemical vapor deposition of organic molecule 1,3,5-triazine on Cu metal catalyst. When reducing the growth temperature, the atomic percentage of nitrogen doping is raised from 2.1% to 5.6%. With increasing doping concentration, N-doped graphene sheet exhibits a crossover from p-type to n-type behavior accompanied by a strong enhancement of electron-hole transport asymmetry, manifesting the influence of incorporated nitrogen impurities. In addition, by analyzing the data of X-ray photoelectron spectroscopy, Raman spectroscopy, and electrical measurements, we show that pyridinic and pyrrolic N impurities play an important role in determining the transport behavior of carriers in our N-doped graphene sheets.

Intrinsic Spin Seebeck Effect in Au/YIG

Abstract: The acute magnetic proximity effects in $\mathrm{Pt}/\mathrm{YIG}$ compromise the suitability of Pt as a spin current detector. We show that $\mathrm{Au}/\mathrm{YIG}$, with no anomalous Hall effect and a negligible magnetoresistance, allows the measurements of the intrinsic spin Seebeck effect with a magnitude much smaller than that in $\mathrm{Pt}/\mathrm{YIG}$. The experiment results are consistent with the spin polarized density functional calculations for Pt with a sizable and Au with a negligible magnetic moment near the interface with YIG.

Enhancing Physical-Layer Secrecy in Multiantenna Wireless Systems: An Overview of Signal Processing Approaches

Abstract: This article provides an overview of the signal processing techniques used to enhance secrecy in the physical layer of multiantenna wireless communication systems. Motivated by results in information theory, signal processing techniques in both the data transmission and the channel estimation phases have been explored in the literature to enlarge the signal quality difference at the destination and the eavesdropper. In the data transmission phase, secrecy beamforming and precoding schemes are used to enhance signal quality at the destination while limiting the signal strength at the eavesdropper. Artificial noise (AN) is also used on top of beamformed or precoded signals to further reduce the reception quality at the eavesdropper. In the channel estimation phase, training procedures are developed to enable better channel estimation performance at the destination than at the eavesdropper. As a result, the effective signal-to-noise ratios (SNRs) at the two terminals will be different and a more favorable secrecy channel will be made available for use in the data transmission phase. Finally, future research directions are discussed.