Growth of silicon nanowires by electron beam evaporation using indium catalyst

For the first time silicon nanowires have been grown on indium (In) coated Si (100) substrates using e-beam evaporation at a low substrate temperature of 300 degrees C. Standard spectroscopic and microscopic techniques have been employed for the structural, morphological and compositional properties of as grown Si nanowires. The as grown Si nanowires have randomly oriented with an average length of 600 nm for a deposition time of 15 min. As grown Si nanowires have shown indium nanoparticle (capped) on top of it confirming the Vapor Liquid Solid (VLS) growth mechanism. Transmission Electron Microscope (TEM) measurements have revealed pure and single crystalline nature of Si nanowires. The obtained results have indicated good progress towards finding alternative catalyst to gold for the synthesis of Si nanowires. (C) 2011 Elsevier B.V. All rights reserved.

Investigation of biodegradable and biocompatible castor oil poly(mannitol-citric-sebacate) polyester as a drug carrier

Castor oil-based poly(mannitol-citric sebacate) was synthesized by simple, catalyst-free melt condensation process using monomers having potential to be metabolized in vivo. The polymer was characterized using various techniques and the tensile and hydration properties of the polymers were also determined. The biocompatibility of the polymer was tested using human foreskin fibroblasts cells. The in vitro degradation studies show that the time for complete degradation of the polymer was more than 21 days. The usage of castor oil polyester as a drug carrier was analysed by doping the polymer with 5-fluorouracil model drug and the release rate was studied by varying the percentage loading of drugs and the pH of the PBS solution medium. The cumulative drug-release profiles exhibited a biphasic release with an initial burst release and cumulative 100% release within 42 h. To understand the role of the polymer as a drug carrier in the release behaviour, drug-release studies were conducted with another drug, isoniazid. The release behaviour of isoniazid drug from the same polymer matrix followed an nth order kinetic model and 100% cumulative release was achieved after 12 days. The variation in the release behaviour for two model drugs from the same polymer matrix suggests a strong interaction between the polymer and the drug molecule.

Polymer blend based photovoltaic devices.

Solar cells on thin conformable substrates require conventional plastics such asPS and PMMA that provide better mechanical and environmental stability with cost reduction. We can also tune charge transfer between PPV derivatives and fullerene derivatives via morphology control of the plastics in the solar cells. Our group has conducted morphology evolution studies in nano- and microscale light emitting domains in poly (2-methoxy, 5-(2'-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) and poly (methyl methacrylate) (PMMA) blends. Our current research has been focused on tricomponent-photoactive solar cells which comprise MEH-PPV, PMMA, and [6,6]-phenyl C61-butyric acid methyl ester (PCBM, Figure 1) in the photoactive layer. Morphology control of the photoactive materials and fine tuning of photovoltaic properties for the solar cells are our primary interest. Similar work has been done by the Sariciftci research group. Additionally, a study on inter- and intramolecular photoinduced charge transfer using MEH-PPV derivatives that have different conjugation lengths (Figure 1, n=1 and 0.85) has been performed.

Polymer based photo detectors

Recent developments in our laboratory related to polymer-based light sensors are reviewed. The inherent processibility of the active polymer medium is utilized in the implementation of different designs for the opto-electronic applications. The utility of these devices as sensitive photodetectors, image sensors and position sensitive detectors is demonstrated. The schottky-type layer formation at interfaces of polymers such as polyalkylthiophenes and aluminum accompanied by the enhanced photo-induced charge separation due to high local electric field is tapped for some of these device structures. The sensitivity of polymer-based field effect transistors to light also provides a convenient lateral geometry for efficient optical-coupling and control of the transistor state. ne range of these polymer-detectors available with the option of operating in the diode and transistor modes should be an attractive feature for many potential applications.

Durable Transition-Metal-Carbide-Supported Pt-Ru Anodes for Direct Methanol Fuel Cells

Molybdenum carbide (MoC) and tungsten carbide (WC) are synthesized by direct carbonization method. PtRu catalysts supported on MoC, WC, and Vulcan XC-72R are prepared, and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy in conjunction with electrochemistry. Electrochemical activities for the catalysts towards methanol electro-oxidation are studied by cyclic voltammetry. All the electro-catalysts are subjected to accelerated durability test (ADT). The electrochemical activity of carbide-supported electro-catalysts towards methanol electro-oxidation is found to be higher than carbon-supported catalysts before and after ADT. The study suggests that PtRu/MoC and PtRu/WC catalysts are more durable than PtRu/C. Direct methanol fuel cells (DMFCs) with PtRu/MoC and PtRu/WC anodes also exhibit higher performance than the DMFC with PtRu/C anode.

Catalyst-Packed Non-Thermal Plasma Reactor for Removal of Nitrogen Oxides

A single-stage plasma-catalytic reactor in which catalytic materials were packed was used to remove nitrogen oxides. The packing material was scoria being made of various metal oxides including Al2O3, MgO, TiO2, etc. Scoria was able to act not only as dielectric pellets but also as a catalyst in the presence of reducing agent such as ethylene and ammonia. Without plasma discharge, scoria did not work well as a catalyst in the temperature range of 100 °C to 200 °C, showing less than 10% of NOx removal efficiency. When plasma is produced inside the reactor, the NOx removal efficiency could be increased to 60% in this temperature range.

DeNOx Study in Diesel Engine Exhaust Using Barrier Discharge Corona Assisted by V2O5/TiO2 Catalyst

A plasma-assisted catalytic reactor was used to remove nitrogen oxides (NOx) from diesel engine exhaust operated under different load conditions. Initial studies were focused on plasma reactor (a dielectric barrier discharge reactor) treatment of diesel exhaust at various temperatures. The nitric oxide (NO) removal efficiency was lowered when high temperature exhaust was treated using plasma reactor. Also, NO removal efficiency decreased when 45% load exhaust was treated. Studies were then made with plasma reactor combined with a catalytic reactor consisting of a selective catalytic reduction (SCR) catalyst, V2O5/TiO2. Ammonia was used as a reducing agent for SCR process in a ratio of 1:1 to NOx. The studies were focused on temperatures of the SCR catalytic reactor below 200°C. The plasma-assisted catalytic reactor was operated well to remove NOx under no-load and load conditions. For an energy input of 96 J/l, the NOx removal efficiencies obtained under no-load and load conditions were 90% and 72% respectively at an exhaust temperature of 100°C.

High emission currents and low threshold fields in multi-wall carbon nanotube-polymer composites in the vertical configuration

In this work, we present field emission characteristics of multi-wall carbon nanotube (MWCNT)-polystyrene composites at various weight fractions along the cross-section of sample. Scanning electron microscope images in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with weight fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High current density of 100 mA/cm(2) was achieved at a field of 2.2 V/lm for 0.15 weight fraction. The field emission is observed to follow the Fowler-Nordheim tunneling mechanism, however, electrostatic screening is observed to play a role in limiting the current density at higher weight fractions. (C) 2012 American Institute of Physics. [doi:10.1063/1.3685754]

Structural Transformations, Composition Anomalies and a Dramatic Collapse of Linear Polymer Chains in Dilute Ethanol-Water Mixtures

Water-ethanol mixtures exhibit many interesting anomalies, such as negative excess partial molar volume of ethanol, excess sound absorption coefficient at low concentrations, and positive deviation from Raoult's law for vapor pressure, to mention a few. These anomalies have been attributed to different, often contradictory origins, but a quantitative understanding is still lacking. We show by computer simulation and theoretical analyses that these anomalies arise from the sudden emergence of a bicontinuous phase that occurs at a relatively low ethanol concentration of x(eth) approximate to 0.06-0.10 (that amounts to a volume fraction of 0.17-0.26, which is a significant range!). The bicontinuous phase is formed by aggregation of ethanol molecules, resulting in a weak phase transition whose nature is elucidated. We find that the microheterogeneous structure of the mixture gives rise to a pronounced nonmonotonic composition dependence of local compressibility and nonmonotonic dependence in the peak value of the radial distribution function of ethyl groups. A multidimensional free energy surface of pair association is shown to provide a molecular explanation of the known negative excess partial volume of ethanol in terms of parallel orientation and hence better packing of the ethyl groups in the mixture due to hydrophobic interactions. The energy distribution of the ethanol molecules indicates additional energy decay channels that explain the excess sound attenuation coefficient in aqueous alcohol mixtures. We studied the dependence of the solvation of a linear polymer chain on the composition of the water-ethanol solvent. We find that there is a sudden collapse of the polymer at x(eth) approximate to 0.05-a phenomenon which we attribute to the formation of the microheterogeneous structures in the binary mixture at low ethanol concentrations. Together with recent single molecule pulling experiments, these results provide new insight into the behavior of polymer chain and foreign solutes, such as enzymes, in aqueous binary mixtures.

Smart polymer mediated purification and recovery of active proteins from inclusion bodies

Obtaining correctly folded proteins from inclusion bodies of recombinant proteins expressed in bacterial hosts requires solubilization with denaturants and a refolding step. Aggregation competes with the second step. Refolding of eight different proteins was carried out by precipitation with smart polymers. These proteins have different molecular weights, different number of disulfide bridges and some of these are known to be highly prone to aggregation. A high throughput refolding screen based upon fluorescence emission maximum around 340 nm (for correctly folded proteins) was developed to identify the suitable smart polymer. The proteins could be dissociated and recovered after the refolding step. The refolding could be scaled up and high refolding yields in the range of 8 mg L-1 (for CD4D12, the first two domains of human CD4) to 58 mg L-1 (for malETrx, thioredoxin fused with signal peptide of maltose binding protein) were obtained. Dynamic light scattering (DLS) showed that polymer if chosen correctly acted as a pseuclochaperonin and bound to the proteins. It also showed that the time for maximum binding was about 50 min which coincided with the time required for incubation (with the polymer) before precipitation for maximum recovery of folded proteins. The refolded proteins were characterized by fluorescence emission spectra, circular dichroism (CD) spectroscopy, melting temperature (T-m), and surface hydrophobicity measurement by ANS (8-anilinol-naphthalene sulfonic acid) fluorescence. Biological activity assay for thioredoxin and fluorescence based assay in case of maltose binding protein (MBP) were also carried out to confirm correct refolding. (C) 2012 Elsevier B.V. All rights reserved.

Nonlinear viscoelasticity of freestanding and polymer-anchored vertically aligned carbon nanotube foams

Vertical arrays of carbon nanotubes (VACNTs) show unique mechanical behavior in compression, with a highly nonlinear response similar to that of open cell foams and the ability to recover large deformations. Here, we study the viscoelastic response of both freestanding VACNT arrays and sandwich structures composed of a VACNT array partially embedded between two layers of poly(dimethylsiloxane) (PDMS) and bucky paper. The VACNTs tested are similar to 2 mm thick foams grown via an injection chemical vapor deposition method. Both freestanding and sandwich structures exhibit a time-dependent behavior under compression. A power-law function of time is used to describe the main features observed in creep and stress-relaxation tests. The power-law exponents show nonlinear viscoelastic behavior in which the rate of creep is dependent upon the stress level and the rate of stress relaxation is dependent upon the strain level. The results show a marginal effect of the thin PDMS/bucky paper layers on the viscoelastic responses. At high strain levels (epsilon - 0.8), the peak stress for the anchored CNTs reaches similar to 45 MPa, whereas it is only similar to 15MPa for freestanding CNTs, suggesting a large effect of PDMS on the structural response of the sandwich structures. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.3699184]

Low temperature charge transport and microwave absorption of carbon coated iron nanoparticles-polymer composite films

In this paper, the low temperature electrical conductivity and microwave absorption properties of carbon coated iron nanoparticles-polyvinyl chloride composite films are investigated for different filler fractions. The filler particles are prepared by the pyrolysis of ferrocene at 980 degrees C and embedded in polyvinyl chloride matrix. The high resolution transmission electron micrographs of the filler material have shown a 5 nm thin layer graphitic carbon covering over iron particles. The room temperature electrical conductivity of the composite film changes by 10 orders of magnitude with the increase of filler concentration. A percolation threshold of 2.2 and an electromagnetic interference shielding efficiency (EMI SE) of similar to 18.6 dB in 26.5-40 GHz range are observed for 50 wt% loading. The charge transport follows three dimensional variable range hopping conduction. (C) 2012 Elsevier Ltd. All rights reserved.