PE/PEO blends compatibilized by PE brush immobilized on MWNTs: improved interfacial and structural properties

Polyolefin based blends have tremendous commercial importance in view of their exceptional properties. In this study the interface of a biphasic polymer blend of PE (polyethylene) and PEO (polyethylene oxide) has been tailored to reduce the interfacial tension between the phases and to render finer morphology. This was accomplished by employing various strategies like addition of maleated PE (PE grafted maleic anhydride), immobilizing PE chains, ex situ, onto MWNTs by covalent grafting, and in situ grafting of PE chains onto MWNTs during melt processing. Multiwalled nanotubes (MWNTs) with different surface functional groups have been synthesized either a priori or were facilitated during melt mixing at higher temperature. NH2 terminated MWNTs were synthesized by grafting ethylene diamine (EDA) onto carboxyl functionalized carbon nanotubes (COOH(MWNTs) and further, was used to reactively couple with maleated PE to immobilize PE chains on the surface of MWNTs. The covalent coupling of maleated PE with NH2 terminated MWNTs was also realized in situ in the melt extruder at high temperature. Both NH2 terminated MWNTs and the in situ formed PE brush on MWNTs during melt mixing, revealed a significant improvement in the mechanical properties of the blend besides remarkably improving the dispersion of the minor phase (PEO) in the blends. Structural properties of the composites were evaluated and the tensile fractured morphology was assessed using scanning electron microscopy.

Broadband wavelength-selective reflectance and selective polarization by a tip-bent vertically aligned multi-walled carbon nanotube forest

The tunable optical properties of the bulk structure of carbon nanotubes (CNT) were recently revealed as a perfect black body material, optically reflective mirror and solar absorber. The present study demonstrates an enhanced optical reflectance of up to similar to 15% over a broad wavelength range in the near infrared region followed by a mechanical modification of the surface of a bulk CNT structure, which can be accounted for due to the grating-like surface abnormalities. In response to the specific arrangement of the so-formed bent tips of the CNT, a selective reflectance is achieved and results in reflecting only a dominant component of the polarized ight, which has not been realized so far. Modulation of this selective-optical reflectance can be achieved by ontrolling the degree of tip bending of the nanotubes, thus opening up avenues for the construction of novel dynamic light polarizers and absorbers.

Defect-Induced Enhancement and Quenching Control of Photocurrent in Few-Layer Graphene Photodetectors

A novel approach is presented for achieving an enhanced photo-response in a few layer graphene (FLG) based photodetector that is realized by introducing defect sites in the FLG. Fabrication induced wrinkle formation in graphene presented a four-fold enhancement in the photocurrent when compared to unfold PLC. Interestingly, it was observed that the addition of few multiwalled carbon nanotubes to an FLG improves the photocurrent by two-fold along with a highly stable response as compared to FLG alone.

Design and analysis of integrated optic quadrature phase shift keying modulator

Quadrature phase shift keying (QPSK) is one of the most popular modulation schemes in coherent optical communication systems for data rates in excess of 40 Gbps because of its high spectral efficiency. This paper proposes a simple method of implementing a QPSK modulator in integrated optic (IO) domain. The QPSK modulator is realized using standard IO components, such as Y-branches and electro-optic modulators (EOMs). Design optimization of EOM is carried out considering the fabrication constraints, miniaturization aspects, and simplicity. Also, the interdependency between electrode length, operating voltage, and electrode gap of an EOM has been captured in the form of a family of curves. These plots enable designing of EOMs for custom requirements. An innovative approach has been adopted in demonstrating the operation of IO QPSK modulator in terms of phase data extracted from beam propagation model. The results obtained by this approach have been verified using the conventional interferometric approach. The operation of the proposed IO QPSK modulator is experimentally demonstrated. The design of IO QPSK modulator is taken up as a part of a broader scheme that aims at generation of QPSK modulated microwave signal based on optical heterodyning. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

From (Au5Sn + AuSn) physical mixture to phase pure AuSn and Au5Sn intermetallic nanocrystals with tailored morphology: digestive ripening assisted approach

Here we present digestive ripening facilitated interatomic diffusion for the phase controlled synthesis of homogeneous intermetallic nanocrystals of Au-Sn system. Au and Sn metal nanoparticles synthesized by a solvated metal atom dispersion (SMAD) method are employed as precursors for the fabrication of AuSn and Au5Sn which are Au-rich Au-Sn intermetallic nanocrystals. By optimizing the stoichiometry of Au and Sn in the reaction mixture, and by employing growth directing agents, the formation of phase pure intermetallic AuSn and Au5Sn nanocrystals could be realized. The as-prepared Au and Sn colloidal nanoparticles and the resulting intermetallic nanocrystals are thoroughly characterized by powder X-ray diffraction, transmission electron microscopy (TEM and STEM-EDS), and optical spectroscopy. The results obtained here demonstrate the potential of solution chemistry which allows synthesizing phase pure Au-Sn intermetallics with tailored morphology.

CHLORAL HYDRATE AS A WATER CARRIER FOR THE EFFICIENT DEPROTECTION OF ACETALS, DITHIOACETALS, AND TETRAHYDROPYRANYL ETHERS IN ORGANIC SOLVENTS

The efficient deprotection of several acetals, dithioacetals, and tetrahydropyranyl (THP) ethers under ambient conditions, using chloral hydrate in hexane, is described. Excellent yields were realized for a wide range of both aliphatic and aromatic substrates. The method is characterized by mild conditions (room temperatures or below), simple workup, and the ready availability of chloral hydrate. High chemoselectivity was also observed in the deprotection, acetonides, esters, and amides being unaffected under the reaction conditions. Products were generally purified chromatographically and identified spectrally. These results constitute a novel addition to current methodology involving a widely employed deprotection tactic in organic synthesis. It seems likely that the mechanism of the reaction involves adsorption of the substrate on the surface of the sparingly soluble chloral hydrate.

Magnetic field induced tailoring of mechanical behavior of fluid filled micro porous carbon nanotube foam

Compressive loading of the carbon nanotube (CNT) has attracted much attention due to its entangled cellular like structure (CNT foam). This report investigates the mechanical behavior of magnetorheological fluid impregnated micro porous CNT foam that has not been realized before at this scale. Compressive behavior of CNT foam is found to greatly depend on the variation in both fluid viscosity as well as magnetic field intensity. Moreover, maximum achieved stress and energy absorption in CNT foam followed a power law behavior with the magnetic field intensity. Magnetic field induced movement of both CNT and iron oxide particles along the field direction is shown to dominate compressive behavior of CNT foam over highly attractive van der Waals forces between individual CNT. Therefore, this study demonstrates a method for tailoring the mechanical behavior of the fluid impregnated CNT foam. (C) 2014 AIP Publishing LLC.

Nonlinear optical absorption in a graphene infrared photodetector

The photoresponse of the graphene photodetector elucidated strong dependence on several optical parameters, such as the angle of incidence and the incident power of infrared exposure at room temperature. The sinusoidal dependence of the photoresponse on incidence angle, which had not been realized before, has now been revealed. The combined effect of the photo excited charge carrier and the photon drag effect explain this nonlinear optical absorption in graphene at lower incident power. The nonlinear dependence of the charge carrier generation on the incident power revealed that this process contributed to the nonlinear photoresponse. However, a deviation is observed at a higher incident power due to the induction of thermal effects in the graphene lattice. This work demonstrates the tunability of the graphene photodetector under a systematic variation that involves both parameters.

A framework for post-silicon realization of arbitrary instruction extensions on reconfigurable data-paths

In this paper we present a framework for realizing arbitrary instruction set extensions (IE) that are identified post-silicon. The proposed framework has two components viz., an IE synthesis methodology and the architecture of a reconfigurable data-path for realization of the such IEs. The IE synthesis methodology ensures maximal utilization of resources on the reconfigurable data-path. In this context we present the techniques used to realize IEs for applications that demand high throughput or those that must process data streams. The reconfigurable hardware called HyperCell comprises a reconfigurable execution fabric. The fabric is a collection of interconnected compute units. A typical use case of HyperCell is where it acts as a co-processor with a host and accelerates execution of IEs that are defined post-silicon. We demonstrate the effectiveness of our approach by evaluating the performance of some well-known integer kernels that are realized as IEs on HyperCell. Our methodology for realizing IEs through HyperCells permits overlapping of potentially all memory transactions with computations. We show significant improvement in performance for streaming applications over general purpose processor based solutions, by fully pipelining the data-path. (C) 2014 Elsevier B.V. All rights reserved.

Glass, Gel, and Liquid Crystals: Arrested States of Graphene Oxide Aqueous Dispersions

Colloidal systems with competing interactions are known to exhibit a range of dynamically arrested states because of the systems' inability to reach its underlying equilibrium state due to intrinsic frustration. Graphene oxide (GO) aqueous dispersions constitute a class of 2D-anisotropic colloids with competing interactions long-range electrostatic repulsion, originating from ionized groups located on the rim of the sheets, and weak dispersive attractive interactions originating from the unoxidized graphitic domains. We show here that aqueous dispersions of GO exhibit a range of arrested states, encompassing fluid, glass, and gels that coexist with liquid-crystalline order with increasing volume fraction. These states can be accessed by varying the relative magnitudes of the repulsive and attractive forces. This can be realized by changing the ionic strength of the medium. We observe at low salt concentrations, where long-range electrostatic repulsion dominates, the formation of a repulsive Wigner glass, while at high salt concentrations, when attractive forces dominate, the formation of gels exhibits a nematic to columnar liquid-crystalline transition. The present work highlights how the chemical structure of GO hydrophilic ionizable groups and hydrophobic graphitic domains coexisting on a single sheet gives rise to a rich and complex array of arrested states.

Finite size scaling in crossover among different random matrix ensembles in microscopic lattice models

Using numerical diagonalization we study the crossover among different random matrix ensembles (Poissonian, Gaussian orthogonal ensemble (GOE), Gaussian unitary ensemble (GUE) and Gaussian symplectic ensemble (GSE)) realized in two different microscopic models. The specific diagnostic tool used to study the crossovers is the level spacing distribution. The first model is a one-dimensional lattice model of interacting hard-core bosons (or equivalently spin 1/2 objects) and the other a higher dimensional model of non-interacting particles with disorder and spin-orbit coupling. We find that the perturbation causing the crossover among the different ensembles scales to zero with system size as a power law with an exponent that depends on the ensembles between which the crossover takes place. This exponent is independent of microscopic details of the perturbation. We also find that the crossover from the Poissonian ensemble to the other three is dominated by the Poissonian to GOE crossover which introduces level repulsion while the crossover from GOE to GUE or GOE to GSE associated with symmetry breaking introduces a subdominant contribution. We also conjecture that the exponent is dependent on whether the system contains interactions among the elementary degrees of freedom or not and is independent of the dimensionality of the system.

Design and Modeling of an Active Five-Axis Compliant Micromanipulator

This paper presents the design and modeling of an active five-axis compliant micromanipulator whose tip orientation can be independently controlled by large angles about two axes and the tip-position can be controlled in three dimensions. These features enable precise control of the contact point of the tip and the tip-sample interaction forces with three-dimensional nanoscale objects, including those features that are conventionally inaccessible. Control of the tip-motion is realized by means of electromagnetic actuation combined with a novel kinematic and structural design of the micromanipulator, which, in addition, also ensures compatibility with existing high-resolution motion-measurement systems. The design and analysis of the manipulator structure and those of the actuation system are first presented. Quasi-static and dynamic lumped-parameter (LP) models are then derived for the five-axis compliant micromanipulator. Finite element (FE) analysis is employed to validate these models, which are subsequently used to study the effects of tip orientation on the mechanical characteristics of the five-axis micromanipulator. Finally, a prototype of the designed five-axis manipulator is fabricated by means of focused ion-beam milling (FIB).

Subexponential Algorithm for d-Cluster Edge Deletion: Exception or Rule?

The correlation clustering problem is a fundamental problem in both theory and practice, and it involves identifying clusters of objects in a data set based on their similarity. A traditional modeling of this question as a graph theoretic problem involves associating vertices with data points and indicating similarity by adjacency. Clusters then correspond to cliques in the graph. The resulting optimization problem, Cluster Editing (and several variants) are very well-studied algorithmically. In many situations, however, translating clusters to cliques can be somewhat restrictive. A more flexible notion would be that of a structure where the vertices are mutually ``not too far apart'', without necessarily being adjacent. One such generalization is realized by structures called s-clubs, which are graphs of diameter at most s. In this work, we study the question of finding a set of at most k edges whose removal leaves us with a graph whose components are s-clubs. Recently, it has been shown that unless Exponential Time Hypothesis fail (ETH) fails Cluster Editing (whose components are 1-clubs) does not admit sub-exponential time algorithm STACS, 2013]. That is, there is no algorithm solving the problem in time 2 degrees((k))n(O(1)). However, surprisingly they show that when the number of cliques in the output graph is restricted to d, then the problem can be solved in time O(2(O(root dk)) + m + n). We show that this sub-exponential time algorithm for the fixed number of cliques is rather an exception than a rule. Our first result shows that assuming the ETH, there is no algorithm solving the s-Club Cluster Edge Deletion problem in time 2 degrees((k))n(O(1)). We show, further, that even the problem of deleting edges to obtain a graph with d s-clubs cannot be solved in time 2 degrees((k))n(O)(1) for any fixed s, d >= 2. This is a radical contrast from the situation established for cliques, where sub-exponential algorithms are known.

A Brief Overview of the Recent Bio-Medical Applications of Fiber Bragg Grating Sensors

Fiber Bragg Grating (FBG) sensors have become one of the most widely used sensors in the recent times for a variety of applications in the fields of aerospace, civil, automotive, etc. It has been recently realized that FBGs and etched FBGs can play an important role in biomedical applications. This article provides a brief overview of the recent advancements in the application of FBG sensors in bio-mechanical, bio-sensing and bio-medical fields.

Metallacyclocumulenes: A Theoretical Perspective on the Structure, Bonding, and Reactivity

CONSPECTUS: Transition metals help to stabilize highly strained organic fragments. Metallacycles, especially unsaturated ones, provide much variety in this area. We had a sustained interest in understanding new C-C bond formation reactions affected by binuclear transition metal fragments Cp2M. One such study led to the exploration of the bimetallic C-C cleavage and coupled complexes, where the acetylide ligands bridge two metal atoms. The underlying M-C interaction in these complexes inspired the synthesis of a five-membered cyclocumulene complex, which opened a new phase in organometallic chemistry. The metallacyclocumulene produces a variety of C-C cleavage and coupled products including a radialene complex. Group 4 metallocenes have thus unlocked a fascinating chemistry by stabilizing strained unsaturated C4 organic fragments in the form of five-membered metallacyclocumulenes, metallacyclopentynes, and metallacycloallenes. Over the years, we have carried out a comprehensive theoretical study to understand the unusual stability and reactivity of these metallacycles. The unique (M-C-beta) interaction of the internal carbon atoms with the metal atom is the reason for unusual stability of the metallacycles. We have also shown that there is a definite dependence of the C-C coupling and cleavage reactions on the metal of metallacyclocumulenes. It demonstrates unexpected reaction pathways for these reactions. Based on this understanding, we have predicted and unraveled the stabilization factors of an unusual four-membered metallacycloallene complex. Indeed, our prediction about a four-membered heterometallacycle has led to an interesting bonding situation, which is experimentally realized. This type of M-C bonding is intriguing from a fundamental perspective and has great relevance in synthesizing unusual structures with interesting properties. In this Account, we first give a short prologue of what led to the present study and describe the salient features of the structure and bonding of the metallacyclocumulenes. The unusual reaction pathway of this metallacycle is explored next. Similar features of the metallacyclopentynes and metallacycloallenes are briefly mentioned. Then, we discuss the exploitation of the unique M-C bonding to design some exotic molecules such as a four-membered metallacycloallene complex. Our efforts to build a conceptual framework to understand these metallacycles and to exploit their chemistry continue.