Showing papers by "National Tsing Hua University published in 2011"

Surface plasmonic effects of metallic nanoparticles on the performance of polymer bulk heterojunction solar cells

Abstract: We have systematically explored how plasmonic effects influence the characteristics of polymer photovoltaic devices (OPVs) incorporating a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We blended gold nanoparticles (Au NPs) into the anodic buffer layer to trigger localized surface plasmon resonance (LSPR), which enhanced the performance of the OPVs without dramatically sacrificing their electrical properties. Steady state photoluminescence (PL) measurements revealed a significant increase in fluorescence intensity, which we attribute to the increased light absorption in P3HT induced by the LSPR. As a result, the rate of generation of excitons was enhanced significantly. Furthermore, dynamic PL measurements revealed that the LSPR notably reduced the lifetime of photogenerated excitons in the active blend, suggesting that interplay between the surface plasmons and excitons facilitated the charge transfer process. This phenomenon reduced the recombination level...

High-Quality Thin Graphene Films from Fast Electrochemical Exfoliation

Abstract: Flexible and ultratransparent conductors based on graphene sheets have been considered as one promising candidate for replacing currently used indium tin oxide films that are unlikely to satisfy future needs due to their increasing cost and losses in conductivity on bending. Here we demonstrate a simple and fast electrochemical method to exfoliate graphite into thin graphene sheets, mainly AB-stacked bilayered graphene with a large lateral size (several to several tens of micrometers). The electrical properties of these exfoliated sheets are readily superior to commonly used reduced graphene oxide, which preparation typically requires many steps including oxidation of graphite and high temperature reduction. These graphene sheets dissolve in dimethyl formamide (DMF), and they can self-aggregate at air-DMF interfaces after adding water as an antisolvent due to their strong surface hydrophobicity. Interestingly, the continuous films obtained exhibit ultratransparency (∼96% transmittance), and their sheet resistance is <1k Ω/sq after a simple HNO3 treatment, superior to those based on reduced graphene oxide or graphene sheets by other exfoliation methods. Raman and STM characterizations corroborate that the graphene sheets exfoliated by our electrochemical method preserve the intrinsic structure of graphene.

Physical layer security in wireless networks: a tutorial

Abstract: Wireless networking plays an extremely important role in civil and military applications. However, security of information transfer via wireless networks remains a challenging issue. It is critical to ensure that confidential data are accessible only to the intended users rather than intruders. Jamming and eavesdropping are two primary attacks at the physical layer of a wireless network. This article offers a tutorial on several prevalent methods to enhance security at the physical layer in wireless networks. We classify these methods based on their characteristic features into five categories, each of which is discussed in terms of two metrics. First, we compare their secret channel capacities, and then we show their computational complexities in exhaustive key search. Finally, we illustrate their security requirements via some examples with respect to these two metrics.

Bifunctional Au−Fe3O4 Heterostructures for Magnetically Recyclable Catalysis of Nitrophenol Reduction

Abstract: The dumbbell- and flower-like Au−Fe3O4 heterostructures by thermal decomposition of the iron−oleate complex in the presence of Au nanoparticles (NPs) have been successfully fabricated using different sizes of Au NPs as the seeds for magnetically recyclable catalysis of p-nitrophenol and 2,4-dinitrophenol reduction. The heterostructures exhibit bifunctional properties with high magnetization and excellent catalytic activity toward nitrophenol reduction. The epitaxial linkages in dumbbell- and flower-like heterostructures are different, leading to the change in magnetic and catalytic properties of the heterostructured nanocatalysts. The pseudo-first-order rate constants for nitrophenol reduction are 0.63−0.72 min−1 and 0.38−0.46 min−1 for dumbbell- and flower-like Au−Fe3O4 heterostructures, respectively. In addition, the heterostructured nanocatalysts show good separation ability and reusability which can be repeatedly applied for nearly complete reduction of nitrophenols for at least six successive cycles....

Equation of state for dark energy in $f(T)$ gravity

Abstract: We study the cosmological evolutions of the equation of state for dark energy wDE in the exponential and logarithmic as well as their combination f(T) theories. We show that the crossing of the phantom divide line of wDE = −1 can be realized in the combined f(T) theory even though it cannot be in the exponential or logarithmic f(T) theory. In particular, the crossing is from wDE > −1 to wDE < −1, in the opposite manner from f(R) gravity models. We also demonstrate that this feature is favored by the recent observational data.

Design and tailoring of a hierarchical graphene-carbon nanotube architecture for supercapacitors

Abstract: Stacking of individual graphene sheets (GS) is effectively inhibited by introducing one-dimensional carbon nanotubes (CNTs) to form a 3-D hierarchical structure which significantly enhances the electrochemical capacitive performances of GS-based composites. From SEM images, inserting proper quantity of CNTs as nanospacers can effectively impede the stacking of GS and enlarge the space between GS sheets, leading to obtain a highly porous nanostructure. The specific capacitance of GS-CNTs-9-1 (∼326.5 F g−1 at 20 mV s−1) is much higher than that of GS material (∼83 F g−1). Furthermore, the energy and power densities of GS-CNTs-9-1 are respectively as high as 21.74 Wh kg−1 and 78.29 kW kg−1, revealing that the hierarchical graphene-CNT architecture provides remarkable effects on enhancing the capacitive performance of GS-based composites. Therefore, the GS-CNT composites are promising carbon materials for supercapacitors.

Lead-free NaNbO3 nanowires for a high output piezoelectric nanogenerator

Abstract: Perovskite ferroelectric nanowires have rarely been used for the conversion of tiny mechanical vibrations into electricity, in spite of their large piezoelectricity. Here we present a lead-free NaNbO3 nanowire-based piezoelectric device as a high output and cost-effective flexible nanogenerator. The device consists of a NaNbO3 nanowire–poly(dimethylsiloxane) (PDMS) polymer composite and Au/Cr-coated polymer films. High-quality NaNbO3 nanowires can be grown by hydrothermal method at low temperature and can be poled by an electric field at room temperature. The NaNbO3 nanowire–PDMS polymer composite device shows an output voltage of 3.2 V and output current of 72 nA (current density of 16 nA/cm2) under a compressive strain of 0.23%. These results imply that NaNbO3 nanowires should be quite useful for large-scale lead-free piezoelectric nanogenerator applications.

QoS-Based Transmit Beamforming in the Presence of Eavesdroppers: An Optimized Artificial-Noise-Aided Approach

Abstract: Secure transmission techniques have been receiving growing attention in recent years, as a viable, powerful alternative to blocking eavesdropping attempts in an open wireless medium. This paper proposes a secret transmit beamforming approach using a quality-of-service (QoS)-based perspective. Specifically, we establish design formulations that: i) constrain the maximum allowable signal-to-interference-and-noise ratios (SINRs) of the eavesdroppers, and that ii) provide the intended receiver with a satisfactory SINR through either a guaranteed SINR constraint or SINR maximization. The proposed designs incorporate a relatively new idea called artificial noise (AN), where a suitable amount of AN is added in the transmitted signal to confuse the eavesdroppers. Our designs advocate joint optimization of the transmit weights and AN spatial distribution in accordance with the channel state information (CSI) of the intended receiver and eavesdroppers. Our formulated design problems are shown to be NP-hard in general. We deal with this difficulty by using semidefinite relaxation (SDR), an approximation technique based on convex optimization. Interestingly, we prove that SDR can exactly solve the design problems for a practically representative class of problem instances; e.g., when the intended receiver's instantaneous CSI is known. Extensions to the colluding-eavesdropper scenario and the multi-intended-receiver scenario are also examined. Extensive simulation results illustrate that the proposed AN-aided designs can yield significant power savings or SINR enhancement compared to some other methods.

Using multi-objective genetic algorithm for partner selection in green supply chain problems

Abstract: Partner selection is an important issue in the supply chain management. Since environment protection has been of concern to public in recent years, and the traditional supplier selection did not consider about this factor; therefore, this paper introduced green criteria into the framework of supplier selection criteria. The aim of this research was to develop an optimum mathematical planning model for green partner selection, which involved four objectives such as cost, time, product quality and green appraisal score. In order to solve these conflicting objectives, we adopted two multi-objective genetic algorithms to find the set of Pareto-optimal solutions, which utilized the weighted sum approach that can generate more number of solutions. In experimental analysis, we introduced a {4,4,4,4} supply chain network structure, and compared average number Pareto-optimal solutions and CPU times of two algorithms.

Fusing generic objectness and visual saliency for salient object detection

Abstract: We present a novel computational model to explore the relatedness of objectness and saliency, each of which plays an important role in the study of visual attention. The proposed framework conceptually integrates these two concepts via constructing a graphical model to account for their relationships, and concurrently improves their estimation by iteratively optimizing a novel energy function realizing the model. Specifically, the energy function comprises the objectness, the saliency, and the interaction energy, respectively corresponding to explain their individual regularities and the mutual effects. Minimizing the energy by fixing one or the other would elegantly transform the model into solving the problem of objectness or saliency estimation, while the useful information from the other concept can be utilized through the interaction term. Experimental results on two benchmark datasets demonstrate that the proposed model can simultaneously yield a saliency map of better quality and a more meaningful objectness output for salient object detection.

Forecasting stock markets using wavelet transforms and recurrent neural networks: An integrated system based on artificial bee colony algorithm

Abstract: This study presents an integrated system where wavelet transforms and recurrent neural network (RNN) based on artificial bee colony (abc) algorithm (called ABC-RNN) are combined for stock price forecasting. The system comprises three stages. First, the wavelet transform using the Haar wavelet is applied to decompose the stock price time series and thus eliminate noise. Second, the RNN, which has a simple architecture and uses numerous fundamental and technical indicators, is applied to construct the input features chosen via Stepwise Regression-Correlation Selection (SRCS). Third, the Artificial Bee Colony algorithm (ABC) is utilized to optimize the RNN weights and biases under a parameter space design. For illustration and evaluation purposes, this study refers to the simulation results of several international stock markets, including the Dow Jones Industrial Average Index (DJIA), London FTSE-100 Index (FTSE), Tokyo Nikkei-225 Index (Nikkei), and Taiwan Stock Exchange Capitalization Weighted Stock Index (TAIEX). As these simulation results demonstrate, the proposed system is highly promising and can be implemented in a real-time trading system for forecasting stock prices and maximizing profits.

Direct formation of wafer scale graphene thin layers on insulating substrates by chemical vapor deposition.

Abstract: Direct formation of high-quality and wafer scale graphenethinlayersoninsulatinggatedielectricssuchasSiO2is emergent for graphene electronics using Si-wafer compatible fabrication. Here, we report that in a chemical vapor deposition process the carbon species dissociated on Cu surfaces not only resultingraphenelayersontopofthecatalyticCuthin filmsbut also diffuse through Cu grain boundaries to the interface between Cu and underlying dielectrics. Optimization of the process parameters leads to a continuous and large-area gra- phene thin layers directly formed on top of the dielectrics. The bottom-gated transistor characteristics for the graphene films have shown quite comparable carrier mobility compared to the top-layer graphene. The proposed method allows us to achieve wafer-sized graphene on versatile insulating substrates without the need of graphene transfer.

A Secured Authentication Protocol for Wireless Sensor Networks Using Elliptic Curves Cryptography

Abstract: User authentication is a crucial service in wireless sensor networks (WSNs) that is becoming increasingly common in WSNs because wireless sensor nodes are typically deployed in an unattended environment, leaving them open to possible hostile network attack. Because wireless sensor nodes are limited in computing power, data storage and communication capabilities, any user authentication protocol must be designed to operate efficiently in a resource constrained environment. In this paper, we review several proposed WSN user authentication protocols, with a detailed review of the M.L Das protocol and a cryptanalysis of Das’ protocol that shows several security weaknesses. Furthermore, this paper proposes an ECC-based user authentication protocol that resolves these weaknesses. According to our analysis of security of the ECC-based protocol, it is suitable for applications with higher security requirements. Finally, we present a comparison of security, computation, and communication costs and performances for the proposed protocols. The ECC-based protocol is shown to be suitable for higher security WSNs.

Defect Engineering in Cubic Cerium Oxide Nanostructures for Catalytic Oxidation

Abstract: Traditional nanostructured design of cerium oxide catalysts typically focuses on their shape, size, and elemental composition We report a different approach to enhance the catalytic activity of cerium oxide nanostructures through engineering high density of oxygen vacancy defects in these catalysts without dopants The defect engineering was accomplished by a low pressure thermal activation process that exploits the nanosize effect of decreased oxygen storage capacity in nanostructured cerium oxides

Clean transfer of graphene for isolation and suspension.

Abstract: Fabrication of large-area clean graphene sheets is the first step toward the development of high-performance applications in surface chemistry and biotechnology as well as in high-mobility electronics. Here we demonstrate the clean transfer of graphene grown by chemical vapor deposition on Cu foil, with surface cleanness defined by transmission electron microscopy (TEM) in combination with Raman scattering on the same position of suspended graphene sheets. For clean graphene, the Raman spectra exhibit distinctive features that can explicitly discriminate from that of graphene covered with a thin layer of amorphous carbon such as residual poly(methyl methacrylate) (PMMA). By applying this technique to graphene sheets with various degrees of surface cleanness, we show that the quantitative characterization of the thickness of surface contaminants is possible based on multiple reflections and interference of light in samples.

Outage Constrained Robust Transmit Optimization for Multiuser MISO Downlinks: Tractable Approximations by Conic Optimization

Abstract: In this paper we consider a probabilistic signal-to-interference and-noise ratio (SINR) constrained problem for transmit beamforming design in the presence of imperfect channel state information (CSI), under a multiuser multiple-input single-output (MISO) downlink scenario. In particular, we deal with outage-based quality-of-service constraints, where the probability of each user's SINR not satisfying a service requirement must not fall below a given outage probability specification. The study of solution approaches to the probabilistic SINR constrained problem is important because CSI errors are often present in practical systems and they may cause substantial SINR outages if not handled properly. However, a major technical challenge is how to process the probabilistic SINR constraints. To tackle this, we propose a novel relaxation- restriction (RAR) approach, which consists of two key ingredients-semidefinite relaxation (SDR), and analytic tools for conservatively approximating probabilistic constraints. The underlying goal is to establish approximate probabilistic SINR constrained formulations in the form of convex conic optimization problems, so that they can be readily implemented by available solvers. Using either an intuitive worst-case argument or specialized probabilistic results, we develop various conservative approximation schemes for processing probabilistic constraints with quadratic uncertainties. Consequently, we obtain several RAR alternatives for handling the probabilistic SINR constrained problem. Our techniques apply to both complex Gaussian CSI errors and i.i.d. bounded CSI errors with unknown distribution. Moreover, results obtained from our extensive simulations show that the proposed RAR methods significantly improve upon existing ones, both in terms of solution quality and computational complexity.

Systematic Investigation of the Metal-Structure–Photophysics Relationship of Emissive d10-Complexes of Group 11 Elements: The Prospect of Application in Organic Light Emitting Devices

Abstract: A series of new emissive group 11 transition metal d10-complexes 1–8 bearing functionalized 2-pyridyl pyrrolide together with phosphine ancillary such as bis[2-(diphenylphosphino)phenyl] ether (POP) or PPh3 are reported. The titled complexes are categorized into three classes, i.e. Cu(I) complexes (1–3), Ag(I) complexes (4 and 5), and Au(I) metal complexes (6–8). Via combination of experimental and theoretical approaches, the group 11 d10-metal ions versus their structural variation, stability, and corresponding photophysical properties have been investigated in a systematic and comprehensive manner. The results conclude that, along the same family, how much a metal d-orbital is involved in the electronic transition plays a more important role than how heavy the metal atom is, i.e. the atomic number, in enhancing the spin–orbit coupling. The metal ions with and without involvement of a d orbital in the lowest lying electronic transition are thus classified into internal and external heavy atoms, respectiv...

Preparation and properties of graphene oxide/polyimide composite films with low dielectric constant and ultrahigh strength via in situpolymerization

Abstract: This study proposes an effective approach using in situpolymerization, to fabricate large-area graphene oxide (GO)/polyimide (PI) composite films with outstanding mechanical properties. The GO/PI composite films provide ultrahigh tensile strength (up to 844 MPa) and Young's modulus (20.5 GPa). The NH2-functionalized GO (ODA-GO) is a versatile starting platform for polymer grafting, promoting excellent dispersion of GO within the polymer matrix, and forming strong links with the polymer to facilitate load transfer. The Young's modulus of the integrated GO–PI composite films with 3.0 wt% ODA-GO loading is 15 times greater, and the tensile strength is 9 times greater than comparable properties of pure PI film. The dielectric constant decreases with increasing GO content and a dielectric constant (Dk) of 2.0 was achieved. This approach provides a strategy for developing ultrahigh performance GO–polymer composite materials.

On the superior hot hardness and softening resistance of AlCoCrxFeMo0.5Ni high-entropy alloys

Abstract: Effects of Cr content on microstructures and hot hardness of AlCoCrxFeMo0.5Ni high-entropy alloys (x = 0–2.0) were investigated. The cast microstructure of AlCoCrxFeMo0.5Ni consists of B2 and σ phases, both being multi-element solid solutions. Increasing Cr content increases the volume fraction of σ phase and causes the matrix phase of the dendrite to change from B2 phase to σ phase. The alloy hardness increases from Hv 601 at x = 0 to Hv 867 at x = 2.0 as a result of increasing amount of hard σ phase. A phase diagram for the AlCoCrxFeMo0.5Ni alloy system is constructed based on SEM, HTXRD and DTA analyses, providing useful information for understanding and designing high-entropy alloys. All the AlCoCrxFeMo0.5Ni alloys possess higher hot-hardness level than that of Ni-based superalloys, In 718 and In 718 H, from room temperature to 1273 K. Cr-1.5 and Cr-2.0 alloys exhibit a transition temperature higher than that of Co-based alloy T-800 by about 200 K and have respective hardness values, Hv 374 and Hv 450 at 1273 K, being much higher than those, around Hv 127, of In 718 and In 718 H. The mechanism of larger strengthening and softening resistance is related with B2 and σ phases, both having multi-principal-element effect. The AlCoCrxFeMo0.5Ni alloy system has a potential in high-temperature applications.