Semiconductor-like Sensitivity in Metallic Ultrathin Gold Nanowire-Based Sensors

Due to the ease of modification of electronic structure upon analyte adsorption, semiconductors have been the preferred materials as chemical sensors. At reduced dimension, however, the sensitivity of semiconductor-based sensors deteriorates significantly due to passivation, and often by increased band gap caused by quantum confinement. Using first-principles density functional theory combined with Boltzmann transport calculations, we demonstrate semiconductor-like sensitivity toward chemical species in ultrathin gold nanowires (AuNWs). The sensing mechanism is governed by the modification of the electronic structure of the AuNW as well as scattering of the charge carriers by analyte adsorption. Most importantly, the sensitivity exhibits a linear relationship with the electron affinities of the respective analytes. Based on this relationship, we propose an empirical parameter, which can predict an analyte-specific sensitivity of a AuNW, rendering them as effective sensors for a wide range of chemical an alytes.

Impact dynamics of high Weber number drops on chemically modified metallic surfaces

Hydrophobic/superhydrophobic metallic surfaces prepared via chemical treatment are encountered in many industrial scenarios involving the impingement of spray droplets. The effectiveness of such surfaces is understood through the analysis of droplet impact experiments. In the present study, three target surfaces with aluminum (Al-6061) as base material-acid-etched, Octadecyl Trichloro Silane (OTS) coated, and acid-etched plus OTS-coated-were prepared. Experiments on the impact of inertia dominated water drops on these chemically modified aluminum surfaces were carried out with the objective to highlight the effect of chemical treatment on the target surfaces on key sub-processes occurring in drop impact phenomenon. High speed videos of the entire drop impact dynamics were captured at three Weber number (We) conditions representative of high We (We > 200) regime. During the early stages of drop spreading, the drop impact resulted in ejection of secondary droplets from spreading drop front on the etched surfaces resembling prompt splash on rough surfaces whereas no such splashing was observable on untreated aluminum surface. Prominent development of undulations (fingers) were observed at the rim of drop spreading on the etched surfaces; between the etched surfaces the OTS-coated surface showed a subdued development of fingers than the uncoated surface. The impacted drops showed intense receding on OTS-coated surfaces whereas on the etched surface a highly irregular receding, with drop liquid sticking to the surface, was observed. Quantitative analyses were performed to reveal the effect of target surface characteristics on drop impact parameters such as temporal variation of spread factor of drop lamella, temporal variation of average finger length during spreading phase, maximum drop spreading, time taken to attain maximum spreading, sensitivity of maximum spreading to We, number of fingers at maximum spreading, and average receding velocity of drop lamella. Existing models for maximum drop spreading showed reasonably good agreement with the experimental measurements on the target surfaces except the acid-etched surface. (C) 2014 Elsevier B.V. All rights reserved.

Effect of hydrogen on the yielding behavior and shear transformation zone volume in metallic glass ribbons

The effect of hydrogen (H) charging on the shear yield strength (tau(max)) and shear transformation zone volume (Omega) of Ni-Nb-Zr metallic glass ribbons, with varying Zr content, were studied through the first pop-in loads during nanoindentation. Weight gain measurements after H charging and desorption studies were utilized to identify how the total H absorbed during charging is partitioned into mobile and immobile (or trapped) parts. These, in turn, indicate the significant role of H mobility in the amorphous structure on the yielding behavior. In high-Zr alloys, tau(max) increases significantly whereas Omega decreases. In low-Zr alloys, a slight decrease in tau(max) and increase in Omega were noted. These experimental observations are rationalized in terms of the mobility of the absorbed H in the amorphous structure and the possible role of it in the shear transformation zone dynamics during deformation of the metallic glass. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Wallner lines, crack velocity and mechanisms of crack nucleation and growth in a brittle bulk metallic glass

Mode I fracture experiments were conducted on brittle bulk metallic glass (BMG) samples and the fracture surface features were analyzed in detail to understand the underlying physical processes. Wollner lines, which result from the interaction between the propagating crack front and shear waves emanating from a secondary source, were observed on the fracture surface and geometric analysis of them indicates that the maximum crack velocity is similar to 800 m s(-1), which corresponds to similar to 0.32 times the shear wave speed. Fractography reveals that the sharp crack nucleation at the notch tip occurs at the mid-section of the specimens with the observation of flat and half-penny-shaped cracks. On this basis, we conclude that the crack initiation in brittle BMGs is stress-controlled and occurs through hydrostatic stress-assisted cavity nucleation ahead of the notch tip. High magnification scanning electron and atomic force microscopies of the dynamic crack growth regions reveal highly organized, nanoscale periodic patterns with a spacing of similar to 79 nm. Juxtaposition of the crack velocity with this spacing suggests that the crack takes similar to 10(-10) s for peak-to-peak propagation. This, and the estimated adiabatic temperature rise ahead of the propagating crack tip that suggests local softening, is utilized to critically discuss possible causes for the nanocorrugation formation. Taylor's fluid meniscus instability is unequivocally ruled out. Then, two other possible mechanisms, viz. (a) crack tip blunting and resharpening through nanovoid nucleation and growth ahead of the crack tip and eventual coalescence, and (b) dynamic oscillation of the crack in a thin slab of softened zone ahead of the crack-tip, are critically discussed. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Impact of Stone-Wales and lattice vacancy defects on the electro-thermal transport of the free standing structure of metallic ZGNR

We report the effect of topological as well as lattice vacancy defects on the electro-thermal transport properties of the metallic zigzag graphene nano ribbons at their ballistic limit. We employ the density function theory-Non equilibrium green's function combination to calculate the transmission details. We then present an elaborated study considering the variation in the electrical current and the heat current transport with the change in temperature as well as the voltage gradient across the nano ribbons. The comparative analysis shows, that in the case of topological defects, such as the Stone-Wales defect, the electrical current transport is minimum. Besides, for the voltage gradient of 0.5 Volt and the temperature gradient of 300 K, the heat current transport reduces by similar to 62 % and similar to 50% for the cases of Stones-Wales defect and lattice vacancy defect respectively, compared to that of the perfect one.

High quality oxide-free metallic nanoparticles: a strategy for synthesis through laser ablation in aqueous medium

The paper explores the synthesis of oxide-free nanoparticles of Ag and Cu through laser ablation of pure targets under aqueous medium and tuning the quality and size through addition of Polyvinylpyrrolidone (PVP) in the medium. The size distribution of nanoparticles reduces from 37 +/- 30 nm and 13 +/- 5 nm to 32 +/- 12 nm and 4 +/- 1 nm for Ag and Cu with changes in PVP concentration from 0.00 to 0.02 M, respectively. Irregular shaped particles of Ag with Ag2O phase and a Cu-Cu2O core-shell particles form without the addition of PVP, while oxide layer is absent with 0.02 M of PVP. The recent understanding of the mechanism of particle formation during laser ablation under liquid medium allows us to rationalize our observation.

Direct correlation between non-random distribution of cations and ion transport mechanism in soda-lime silicate glasses

We report a direct correlation between dissimilar ion pair formation and alkali ion transport in soda-lime silicate glasses established via broad band conductivity spectroscopy and local structural probe techniques. The combined Raman and Nuclear Magnetic Resonance (NMR) spectroscopy techniques on these glasses reveal the coexistence of different anionic species and the prevalence of Na+-Ca2+ dissimilar pairs as well as their distributions. The spectroscopic results further confirm the formation of dissimilar pairs atomistically, where it increases with increasing alkaline-earth oxide content These results, are the manifestation of local structural changes in the silicate network with composition which give rise to different environments into which the alkali ions hop. The Na+ ion mobility varies inversely with dissimilar pair formation, i.e. it decreases with increase of non-random formation of dissimilar pairs. Remarkably, we found that increased degree of non-randomness leads to temperature dependent variation in number density of sodium ions. Furthermore, the present study provides the strong link between the dynamics of the alkali ions and different sites associated with it in soda-lime silicate glasses. (C) 2014 Elsevier B.V. All rights reserved.

Controlling the cross-section of waveguides inscribed on bulk chalcogenide glasses using fast laser

Waveguides have been fabricated on melt-quenched, bulk chalcogenide glasses using the femto-second laser inscription technique at low repetition rates in the single scan regime. The inscribed waveguides have been characterized by butt-coupling method and the diameter of the waveguide calculated using the mode-field image of the waveguide. The waveguide cross-section symmetry is analyzed using the heat diffusion model by relating the energy and translation speed of the laser. The net-fluence and symmetry of the waveguides are correlated based on the theoretical values and experimental results of guiding cross-section.

Morphological Evolution in Air-Stable Metallic Iron Nanostructures and Their Magnetic Study

Iron nanostructures with morphology ranging from discrete nanoparticles to nearly monodisperse hierarchical nanostructures have been successfully synthesized using solvated metal atom dispersion (SMAD) method. Such a morphological evolution was realized by tuning the molar ratio of ligand to metal. Surface energy minimization in confluence with strong magnetic interactions and ligand-based stabilization results in the formation of nanospheres of iron. The as-prepared amorphous iron nanostructures exhibit remarkably high coercivity in comparison to the discrete nanoparticles and bulk counterpart. Annealing the as-prepared amorphous Fe nanostructures under anaerobic conditions affords air-stable carbon-encapsulated Fe(0) and Fe3C nanostructures with retention of the morphology. The resulting nanostructures were thoroughly analyzed by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and Raman spectroscopy. TGA brought out that Fe3C nanostructures are more robust toward oxidation than those of a-Fe. Finally, detailed magnetic studies were carried out by superconducting quantum interference device (SQUID) magnetometer and it was found that the magnetic properties remain conserved even upon exposure of the annealed samples to ambient conditions for months.

Uninterrupted galvanic reaction for scalable and rapid synthesis of metallic and bimetallic sponges/dendrites as efficient catalysts for 4-nitrophenol reduction

Here, we demonstrate an uninterrupted galvanic replacement reaction (GRR) for the synthesis of metallic (Ag, Cu and Sn) and bimetallic (Cu M, M=Ag, Au, Pt and Pd) sponges/dendrites by sacrificing the low reduction potential metals (Mg in our case) in acidic medium. The acidic medium prevents the oxide formation on Mg surface and facilitates the uninterrupted reaction. The morphology of dendritic/spongy structures is controlled by the volume of acid used for this reaction. The growth mechanism of the spongy/dendritic microstructures is explained by diffusion-limited aggregate model (DLA), which is also largely affected by the volume of acid. The significance of this method is that the yield can be easily predicted, which is a major challenge for the commercialization of the products. Furthermore, the synthesis is complete in 1-2 minutes at room temperature. We show that the sponges/dendrites efficiently act as catalysts to reduce 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using NaBH4-a widely studied conversion process.

Enhancing the mechanical and biological performance of a metallic biomaterial for orthopedic applications through changes in the surface oxide layer by nanocrystalline surface modification

Nanostructured metals are a promising class of biomaterials for application in orthopedics to improve the mechanical performance and biological response for increasing the life of biomedical implants. Surface mechanical attrition treatment (SMAT) is an efficient way of engineering nanocrystalline surfaces on metal substrates. In this work, 316L stainless steel (SS), a widely used orthopedic biomaterial, was subjected to SMAT to generate a nanocrystalline surface. Surface nanocrystallization modified the nature of the oxide layer present on the surface. It increased the corrosion-fatigue strength in saline by 50%. This increase in strength is attributed to a thicker oxide layer, residual compressive stresses, high strength of the surface layer, and lower propensity for intergranular corrosion in the nanocrystalline layer. Nanocrystallization also enhanced osteoblast attachment and proliferation. Intriguingly, wettability and surface roughness, the key parameters widely acknowledged for controlling the cellular response remained unchanged after nanocrystallization. The observed cellular behavior is explained in terms of the changes in electronic properties of the semiconducting passive oxide film present on the surface of 316L SS. Nanocrystallization increased the charge carrier density of the n-type oxide film likely preventing denaturation of the adsorbed cell-adhesive proteins such as fibronectin. In addition, a net positive charge developed on the otherwise neutral oxide layer, which is known to facilitate cellular adhesion. The role of changes in the electronic properties of the oxide films on metal substrates is thus highlighted in this work. This study demonstrates the advantages of nanocrystalline surface modification by SMAT for processing metallic biomaterials used in orthopedic implants.

ESD Behavior of Metallic Carbon Nanotubes

ESD behavior of metallic carbon nanotubes (CNTs) is explored. Unique TLP I-V characteristics and failure mechanism of carbon shells are discussed. ESD failure in CNTs is attributed to shell burning. It was found that CNT interconnect changes resistance in steps of fundamental quantum resistance (h/2e(2)) after individual shell burning.

Anodization of sputtered metallic films: The microstructural connection

A simple microstructural rationale for successful anodization of metallic films into ordered oxide nanostructures has been identified. It applies to three of the most commonly studied systems, Zr, Ti and Al films and can be extended to other such oxides. A dense Zone T or II microstructure, in sputtered films, is the most critical ingredient. While T-substrate > 0.3T(melting) Ching is the simplest route, pressure and plasma heating can also be exploited. Such microstructures are also associated with a unique growth stress signature. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Thermal and spectroscopic investigations of glasses in the system PbO-PbCl2-PbBr2

Glass formation has been examined in the system 15PbO.xPbCl(2).(85-x)PbBr2 (where 0 <= x <= 25)where the PbO concentration is kept constant while PbCl2 and PbBr2 concentrations are varied. The glasses have been examined using thermal and spectroscopic techniques. T-8, Delta C-p, refractive index, optical basicity have been found to remain unaffected by the composition which is a curious feature of these glasses. It is found that there is a wide infrared window available for use in the investigated glasses. The IR window extends from 1000 to 1500 cm(-1) and in glasses where PbCl2 is less than 20 mol%, the window extends up to 2260 cm(-1). X-ray photoelectron spectra (XPS) revealed that the 4f(5/2) and 4f(7/2) peaks due to f-level transitions have a constant difference in energies, but with energy and FWHM values that varying sensitively and systematically with composition. The observations are discussed and suitable explanations are provided.

Robust, metallic Pd17Se15 and Pd7Se4 phases from a single source precursor and their use as counter electrodes in dye sensitized solar cells

Thin films of conducting palladium selenide phases (Pd17Se15 and Pd7Se4) are prepared using a single source molecular precursor by thermolysis. Varying the mole ratios of palladium and selenium precursors results in palladium organo-selenolate complexes which on thermolysis at different temperatures yield Pd17Se15 and Pd7Se4 phases that are very stable and adherent to the substrate. The organo-selenolate complexes are characterized using small angle XRD, Se-77 NMR and thermogravimetric analysis (TGA). The palladium selenide films are characterized by various techniques such as XRD, XPS, TEM and SEM. Electrical conductivities of the films are determined using the four probe method. The strong adherence of the films to glass substrates coupled with high corrosion resistant behavior towards strong acid and alkaline environments render them to be very effective as electrocatalysts. The catalytic activity towards the I-3(-)/I- redox couple, which is an important reaction in the regeneration of the dye in a dye-sensitized solar cell, is studied. Between the two phases, the Pd17Se15 film shows superior activity as the counter electrode for dye sensitized solar cells with a photocurrent conversion efficiency of 7.45%.