Bismuth catalyzed growth of silicon nanowires by electron beam evaporation

Silicon nanowires (NWs) have been grown in the vapor phase for the first time with bismuth (Bi) as a catalyst using the electron beam evaporation method at a low substrate temperature of 280 degrees C. The grown Si nanowires were randomly oriented on the substrate with an average length of 900 nm for a deposition time of 15 min. Bi faceted nanoparticles (crowned) at the end of the grown Si nanowires have been observed and attributed to the Vapor-Liquid-Solid (VLS) growth mechanism. Transmission Electron Microscopy analysis on the nanowires revealed their single crystalline nature and interestingly bismuth particles were observed in Si nanowires. The obtained results have shown a new window for Si nanowires growth with bismuth as a catalyst. (C) 2012 Elsevier B.V. All rights reserved.

Nucleation-Growth Mode of Cubic-Tetragonal Transition with Coexistence of Phases over a Wide Temperature Interval in the High Temperature Ferroelectric Systems PbTiO3-BiMeO3:Me = Sc , and Zn1/2Ti1/2

Temperature dependent X-ray powder diffraction and dielectric studies have been carried out on tetragonal compositions of (1-x) PbTiO 3(x) BiMeO 3; Me similar to Sc and Zn 1/2 Ti 1/2. The cubic and the tetragonal phases coexist over more than 100 degrees C for 0.70 PbTiO 30.3 Bi ( Zn 1/2 Ti 1/2) O 3 and 0.66 PbTiO 30.34 BiScO 3. The wide temperature range of phase coexistence is shown to be an intrinsic feature of the system, and is attributed to the increase in the degree of the covalent character of the ( Pb +Bi ) O bond with increasing concentration of Bi at the Pb -site. The d-values of the {111} planes of the coexisting phases are nearly identical, suggesting this plane to be the invariant plane for the martensitic type cubic-tetragonal transformation occurring in these systems.

Growth of Polypyrrole-Horseradish Peroxidase Microstructures for H2O2 Biosensor

Conducting polymer microstructures for enzymatic biosensors are developed by a facile electrochemical route. Horseradish peroxide (HRP)-entrapped polypyrrole (PPy) films with bowl-shaped microstructures are developed on stainless steel (SS 304) substrates by a single-step process. Potentiodynamic scanning/cyclic voltammetry is used for generation of PPy microstructures using electrogenerated oxygen bubbles stabilized by zwitterionic surfactant/buffer N-2-hydroxyethylpiperazine N-2-ethanesulfonic acid as soft templates. Scanning electron microscopic images reveal the bowl-shaped structures surrounded by cauliflower-like fractal PPy films and globular nanostructures. Raman spectroscopy reveals the oxidized nature of the film. Sensing properties of PPy-HRP films for hydrogen peroxide (H2O2) are demonstrated. Electrochemical characterization of the sensor films is done by linear sweep voltammetry (LSV) and amperometry. LSV results indicated the reduction of H2O2 and linearity in response of the sensing film. The amperometric biosensor has a performance comparable to those in the literature with advantages of hard-template free synthesis procedure and a satisfactory sensitivity value of 12.8 mu A/(cm(2) . mM) in the range of 1-10 mM H2O2.

Growth and photovoltaic performance of SnS quantum dots

Tin sulphide (SnS) quantum dots of size ranging from 2.4 to 14.4 nm are prepared by chemical precipitation method in aqueous media. Growth of the SnS particles is monitored by controlling the deposition time. Both XRD and SAED patterns confirm that the particles possess orthorhombic structure. The uncapped SnS particles showed secondary phases like Sn2S3 and SnS2 which is visible in the SAED pattern. From the electrochemical characterization. HOMO-LUMO levels of both TiO2 and SnS are determined and the band alignment is found to be favorable for electron transfer from SnS to TiO2. Moreover, the HOMO-LUMO levels varied for different particle sizes. Solar cell is fabricated by sensitizing porous TiO2 thin film with SnS QDs. Cell structure is characterized with and without buffer layer between FTO and TiO2. Without the buffer layer, cell showed an open circuit voltage (V-oc) of 504 mV and short circuit current density (J(sc)) of 2.3 mA/cm(2) under AM1.5 condition. The low fill factor of this structure (15%) is seen to be increased drastically to 51%, on the incorporation of the buffer layer. The cell characteristics are analyzed using two different size quantum dots. (C) 2012 Elsevier B.V. All rights reserved.

A model for growth and engulfment of gas microporosity during aluminum alloy solidification process

A new coupled approach is presented for modeling the hydrogen bubble evolution and engulfment during an aluminum alloy solidification process in a micro-scale domain. An explicit enthalpy scheme is used to model the solidification process which is coupled with a level-set method for tracking the hydrogen bubble evolution. The volume averaging techniques are used to model mass, momentum, energy and species conservation equations in the chosen micro-scale domain. The interaction between the solid, liquid and gas interfaces in the system have been studied. Using an order-of-magnitude study on growth rates of bubble and solid interfaces, a criterion is developed to predict bubble elongation which can occur during the engulfment phase. Using this model, we provide further evidence in support of a conceptual thought experiment reported in literature, with regard to estimation of final pore shape as a function of typical casting cooling rates. The results from the proposed model are qualitatively compared with in situ experimental observations reported in literature. The ability of the model to predict growth and movement of a hydrogen bubble and its subsequent engulfment by a solidifying front has been demonstrated for varying average cooling rates encountered in typical sand, permanent mold, and various casting processes. (C) 2012 Elsevier B.V. All rights reserved.

Morphology Controlled Growth of Meso-Porous Co3O4 Nanostructures and Study of Their Electrochemical Capacitive Behavior

The morphology of nanocrystalline Co3O4 synthesized through microwave irradiation of a solution of a cobalt complex is found to depend reproducibly on the conditions of synthesis and, in particular, on the composition of the solvent used. Despite the rapidity of the process, oriented aggregation occurs under certain conditions, depending on solvent composition. Annealing the oriented samples leads to microstructures with significant porosity, rendering the material suitable as electrodes for electrochemical capacitors. Electrochemical analysis of the oxide samples was carried out in 0.1M Na2SO4 electrolyte vs. Ag/AgCl electrode. A stable specific capacitance of 221 F/g was measured for a meso-porous sample displaying oriented aggregation. Stability of these oxide materials were checked for longer charge-discharge cycling. (C) 2012 The Electrochemical Society. DOI: 10.1149/2.002210jes] All rights reserved.

Diffusion and growth mechanism of phases in the Pd-Sn system

Growth mechanism of phases and atomic mechanism of diffusion are discussed in the Pd-Sn system. The Kirkendall marker plane location indicates that the PdSn4 phase grows because of diffusion of Sn. Atomic arrangement in the crystal indicates that Sn can diffuse through its own sublattice but Pd cannot diffuse unless antisites are present. The negligible diffusion of Pd indicates the absence of Pd antisites. The activation energy value indicates that the contribution from grain boundary diffusion cannot be neglected although experiments were conducted in the homologous temperature range of 0.7-0.79.

Influence of growth ambience and doping on the structural properties of multiferroic DyMnO3

We report on the single crystal growth of 50% Sr and Y doped multiferroic DyMnO3 using optical floating zone technique. A comparison of the effect of growth ambience and of chemical substitution on the crystal structure of DyMnO3 is attempted. It is observed that DyMnO3 adopts Pm3m cubic structure with 50% Sr doping whereas with 50% Y doping, the crystal structure is hexagonal P6(3)cm. Orthorhombic Pnma structure is adopted by DyMnO3 when grown in air, whereas hexagonal P6(3)cm structure is obtained when grown under the ambience of argon. The structural polymorphism is discussed in terms of difference in ionic sizes of Sr, Y and Dy, comparable Gibbs free energies and coordination schemes of surrounding oxygens for hexagonal and orthorhombic structures of DyMnO3. (C) 2012 Elsevier B.V. All rights reserved.

Subdiffusion in hair bundle dynamics: The role of protein conformational fluctuations

The detection of sound signals in vertebrates involves a complex network of different mechano-sensory elements in the inner ear. An especially important element in this network is the hair bundle, an antenna-like array of stereocilia containing gated ion channels that operate under the control of one or more adaptation motors. Deflections of the hair bundle by sound vibrations or thermal fluctuations transiently open the ion channels, allowing the flow of ions through them, and producing an electrical signal in the process, eventually causing the sensation of hearing. Recent high frequency (0.1-10 kHz) measurements by Kozlov et al. Proc. Natl. Acad. Sci. U. S. A. 109, 2896 (2012)] of the power spectrum and the mean square displacement of the thermal fluctuations of the hair bundle suggest that in this regime the dynamics of the hair bundle are subdiffusive. This finding has been explained in terms of the simple Brownian motion of a filament connecting neighboring stereocilia (the tip link), which is modeled as a viscoelastic spring. In the present paper, the diffusive anomalies of the hair bundle are ascribed to tip link fluctuations that evolve by fractional Brownian motion, which originates in fractional Gaussian noise and is characterized by a power law memory. The predictions of this model for the power spectrum of the hair bundle and its mean square displacement are consistent with the experimental data and the known properties of the tip link. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4768902]

Directional grain growth from anisotropic kinetic roughening of grain boundaries in sheared colloidal crystals

The fabrication of functional materials via grain growth engineering implicitly relies on altering the mobilities of grain boundaries (GBs) by applying external fields. Although computer simulations have alluded to kinetic roughening as a potential mechanism for modifying GB mobilities, its implications for grain growth have remained largely unexplored owing to difficulties in bridging the widely separated length and time scales. Here, by imaging GB particle dynamics as well as grain network evolution under shear, we present direct evidence for kinetic roughening of GBs and unravel its connection to grain growth in driven colloidal polycrystals. The capillary fluctuation method allows us to quantitatively extract shear-dependent effective mobilities. Remarkably, our experiments reveal that for sufficiently large strains, GBs with normals parallel to shear undergo preferential kinetic roughening, resulting in anisotropic enhancement of effective mobilities and hence directional grain growth. Single-particle level analysis shows that the mobility anisotropy emerges from strain-induced directional enhancement of activated particle hops normal to the GB plane. We expect our results to influence materials fabrication strategies for atomic and block copolymeric polycrystals as well.

Antibodies against the extracellular domain of human Notch1 receptor reveal the critical role of epidermal-growth-factor-like repeats 25-26 in ligand binding and receptor activation

The Notch signalling pathway is implicated in a wide variety of cellular processes throughout metazoan development. Although the downstream mechanism of Notch signalling has been extensively studied, the details of its ligand-mediated receptor activation are not clearly understood. Although the role of Notch ELRs EGF (epidermal growth factor)-like-repeats] 11-12 in ligand binding is known, recent studies have suggested interactions within different ELRs of the Notch receptor whose significance remains to be understood. Here, we report critical inter-domain interactions between human Notch1 ELRs 21-30 and the ELRs 11-15 that are modulated by calcium. Surface plasmon resonance analysis revealed that the interaction between ELRs 21-30 and ELRs 11-15 is similar to 10-fold stronger than that between ELRs 11-15 and the ligands. Although there was no interaction between Notch 1 ELRs 21-30 and the ligands in vitro, addition of pre-clustered Jagged1Fc resulted in the dissociation of the preformed complex between ELRs 21-30 and 11-15, suggesting that inter-domain interactions compete for ligand binding. Furthermore, the antibodies against ELRs 21-30 inhibited ligand binding to the full-length Notch1 and subsequent receptor activation, with the antibodies against ELRs 25-26 being the most effective. These results suggest that the ELRs 25-26 represent a cryptic ligand-binding site which becomes exposed only upon the presence of the ligand. Thus, using specific antibodies against various domains of the Notch1 receptor, we demonstrate that, although ELRs 11-12 are the principal ligand-binding site, the ELRs 25-26 serve as a secondary binding site and play an important role in receptor activation.

Novel low-temperature growth of SnO2 nanowires and their gas-sensing properties

A simple thermal evaporation method is presented for the growth of crystalline SnO2 nanowires at a low substrate temperature of 450 degrees C via an gold-assisted vapor-liquid-solid mechanism. The as-grown nanowires were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction, and were also tested for methanol vapor sensing. Transmission electron microscopy studies revealed the single-crystalline nature of the each nanowire. The fabricated sensor shows good response to methanol vapor at an operating temperature of 450 degrees C. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Uninterrupted and reusable source for the controlled growth of nanowires

Generally, the length of the oxide nanowires grown by vapor phase transport is limited by the degradation of the source materials. Furthermore, the source material is used once for the nanowires growth. By exploiting the Si-Zn phase diagram, we have developed a simple methodology for the non-catalytic growth of ultralong ZnO nanowires in large area with controllable aspect ratio and branched structures. The insolubility of Zn in Si and the use of a Si cap on the Zn source to prevent local source oxidation of Zn (i.e. prevents the degradation of the source) are the keys to grow longer nanowires without limitations. It has been shown that the aspect ratio can be controlled by thermodynamically (temperature) and more importantly by kinetically (vapor flux). One of the interesting findings is that the same source material can be used for several depositions of oxide nanostructured materials.

Ductile fracture by multiple void growth and interaction ahead of a notch tip in polycrystalline plastic solids

The objectives of this paper are to study the effects of plastic anisotropy and evolution in crystallographic texture with deformation on the ductile fracture behaviour of polycrystalline solids. To this end, numerical simulations of multiple void growth and interaction ahead of a notch tip are performed under mode I, plane strain, small scale yielding conditions using two approaches. The first approach is based on the Hill yield theory, while the second employs crystal plasticity constitutive equations and a Taylor-type homogenization in order to represent the ductile polycrystalline solid. The initial textures pertaining to continuous cast Al-Mg AA5754 sheets in recrystallized and cold rolled conditions are considered. The former is nearly-isotropic, while the latter displays pronounced anisotropy. The results indicate distinct changes in texture in the ligaments bridging the voids ahead of the notch tip with increase in load level which gives rise to retardation in porosity evolution and increase in tearing resistance for both materials.