Palladium Phthalocyanine Nanowire-Based Highly Sensitive Sensors for NO2(g) Detection.

Palladium phthalocyanine (PdPc) nanowires (NWs) were developed to achieve the gas sensing of NO2 in the sub-parts-per-million (ppm) range. Non-substituted metal phthalocyanine are well known for their p-type semiconducting behavior, which is responsible for its gas-sensing capabilities. Nanofabrication of the PdPc NWs was performed by physical vapor deposition (PVD) on an interdigitated gold electrode (IDE). The coordination of palladium in the structure was confirmed with UV-Vis spectroscopy. Gas-sensing experiments for NO2 detection were undertaken at different sensed gas concentrations from 4 ppm to 0.5 ppm at room temperature. In this work, the responses at different gas concentrations are reported. In addition, structural studies of the PdPc NWs with scanning electron microscopy (SEM) and electron-dispersive X-ray diffraction (EDS) are shown.

Thermoelectric properties of antimony selenide hexagonal nanotubes.

Antimony selenide (Sb2Se3) is a material widely used in photodetectors and relatively new as a possible material for thermoelectric applications. Taking advantage of the new properties after nanoscale fabrication, this material shows great potential for the development of efficient low temperature thermoelectric devices. Here we study the synthesis, the crystal properties and the thermal and thermoelectric transport response of Sb2Se3 hexagonal nanotubes (HNT) in the temperature range between 120 and 370 K. HNT have a moderate electrical conductivity ∼102 S m-1 while maintaining a reasonable Seebeck coefficient ∼430 µV K-1 at 370 K. The electrical conductivity in Sb2Se3 HNT is about 5 orders of magnitude larger and its thermal conductivity one half of what is found in bulk. Moreover, the calculated figure of merit (ZT) at room temperature is the largest value reported in antimony selenide 1D structures.

Single nanowire measurements of room temperature ferromagnetism in FeSi nanowires and the effects of Mn-doping.

Semiconductors with magnetic response at room temperature are sought for spintronics in solid-state devices. Among possible materials for this applications, the magnetic response of FeSi and doped FeSi have produced contradictory results at the nanoscale and more precise measurements and deeper studies are needed to clarify its potential capabilities. For that reason, in this work, single nanowire measurements of ferromagnetic semiconducting FeSi and Mn-doped FeSi nanostructures have been performed using magnetic force microscopy and electron holography. Results obtained confirm the presence of magnetic domains at room temperature with a magnetic moment per Fe atom of [Formula: see text] Spin polarized density functional calculations confirm a net magnetic moment between [Formula: see text] in Fe surface atoms with an estimated Curie temperature of 417 K by means of the molecular field approximation. The nanowires present a crystalline B20 cubic structure as confirmed by x-ray diffraction and high-resolution electron microscopy. Their electrical transport measurements confirm p-type nature and thermal activation. A remanent magnetization of 1.5 × 10-5 emu and 0.5 × 10-5 emu was measured at room temperature for FeSi and Mn-doped FeSi respectively, with spin freezing behavior around 30 K for the Mn-doped nanowires.

Thermoelectric properties and thermal tolerance of indium tin oxide nanowires.

Highly crystalline indium tin oxide (ITO) nanowires were grown via a vapor-liquid-solid method, with thermal tolerance up to ∼1300 °C. We report the electric and thermoelectric properties of the ITO nanowires before and after heat treatments and draw conclusions about their applicability as thermoelectric building blocks in nanodevices that can operate in high temperature conditions. The Seebeck coefficient and the thermal and electrical conductivities were measured in each individual nanowire by means of specialized micro-bridge thermometry devices. Measured data was analyzed and explained in terms of changes in charge carrier density, impurities and vacancies due to the thermal treatments.

Wet-chemical approaches to porous nanowires with linear, spiral, and meshy topologies.

We report universal approaches for porous nanowires (NWs), and porous NWs with spiral and meshy topologies that have been developed via anodic aluminum oxide (AAO) confined wet-chemical synthesis. Materials such as CuOx, Pd, and Cu NWs are taken as examples for porous NWs and porous NWs with spiral and meshy topologies. Immediate benefits are demonstrated in hydrogen sensors as examples. We observed that hydrogen concentrations as low as 0.2% (v/v) were detected, that critical temperatures of the reverse sensing behavior as low as 239.9 K were measured and that better baseline-stability was confirmed compared with those fabricated with pure Pd NWs. Our approaches are anticipated to work on the synthesis of the porous NWs of other materials that could be obtained via wet-chemistry with potential as candidates for the next generation nanodevices (e.g., gas sensors) and other applications (e.g., catalysts).

Temperature-activated reverse sensing behavior of Pd nanowire hydrogen sensors.

Hydrogen sensors built with individual palladium nanowires (Pd NWs) have been achieved by integrating Pd NWs across microelectromechanical system (MEMS) electrodes, followed by assembling and bonding them to a chip carrier platform. The sensing measurements reveal that the sensors with individual Pd NWs show reverse sensing behaviors between the temperature zones of (370-263 K) and (263-120 K).

Colorimetric sensors: temperature-activated reverse sensing behavior of pd nanowire hydrogen sensors (small 2/2013).

The image shows an artistic version of a Pd nanowire surrounded by hydrogen molecules. The nanowire is electrically connected to the Pt electrodes of a sensor device by L. F. Fonseca and co-workers to study the effects of reduced temperature on its electrical response during hydrogen gas exposure. The TEM image shows the ordered crystal structure of the material. As described on page 188, when temperature is reduced, a crossover from a bulk- to a percolationcontrolled response is observed. This effect was confirmed in nanowires arrays on interdigitated electrodes and single nanowires integrated to MEMS devices.

Mechanical characterization of pristine and hydrogen-exposed palladium nanowires by in situ TEM.

Changes in the crystal lattice of palladium nanowires (Pd NWs) upon hydrogen exposure by absorption and interstitial introduction of hydrogen atoms within the matrix can induce swelling of the nanostructure and generate dislocations through the solid that may alter the overall mechanical performance of the material. Understanding the mechanical behavior of Pd NW-based hydrogen sensors may provide crucial information regarding material changes where the integrity of the sensing device can be compromised. The plastic behavior of hydrogen sensing Pd NWs was studied prior to-and subsequently to-hydrogen exposure via in situ transmission electron microscope-atomic force microscope (TEM-AFM) experiments to understand the role of hydrogenation in the NWs mechanical performance simultaneous to real-time observation. Quantitative and qualitative analysis was performed for deformed NWs upon compression and tension. Large plastic deformation was observed for pristine Pd NWs whereas little plastic deformation was observed for hydrogen-exposed Pd NWs. Tested pristine NWs behaved in a ductile manner, and necking events were observed for all tested specimens upon tension. Lowered ductility was observed for the hydrogen-exposed specimen, in accordance with hydrogen embrittlement observed in bulk palladium.

Growth and characterization of branched carbon nanostructures arrays in nano-patterned surfaces from porous silicon substrates.

A method to grow branched carbon nanostructures arrays is presented. We employ the electron-beam-induced deposition method using a transmission electron microscope in poor vacuum conditions where hydrocarbons are present in the chamber. The hydrocarbons are attracted to the substrates by the local electric fields. Saw-tooth nano-patterns were made with a focused ion beam in porous silicon substrates with high porosity in order to create sites with high-local electric fields. We found that the adequate ion dose to create well-defined saw-tooth nano-patterns was between 8 and 10 nC/microm(2). Raman and electron energy-loss spectroscopy on the branched carbon nanostructures show a high concentration of sp(2) sites suggesting that they are made of graphite-like hydrogenated amorphous carbon. Selected area electron diffraction, high-resolution images and energy dispersive X-ray analysis (EDS) are also presented.

Palladium/cobalt nanowires with improved hydrogen sensing stability at ultra-low temperatures.

The metallic dopants in palladium (Pd) sensing materials enable modification of the d-band center of Pd, which is expected to tune the α-ß phase transitions of the PdHx intermediate, thus improve the sensing stability to hydrogen. Here, the boosted hydrogen-sensing stability at ultra-low temperatures has been achieved with palladium/cobalt nanowires (PdCo NWs) as the sensing material. The various Co contents in PdCo NWs were modulated via AAO-template-confined electrodeposition. The temperature-dependent sensing evaluations were performed in 0.1-3 v/v% hydrogen. Such sensors integrated with PdCo NWs are able to stably detect hydrogen as low as 0.1 v/v%, even when the temperature is lowered to 273 K. In addition, the critical temperatures of "reverse sensing behavior" of the PdCo NWs (Pd82Co18: Tc = 194 K; Pd63Co37: Tc = 180 K; Pd33Co67: Tc = 184 K) are observed much lower than that of pristine Pd NWs (Tc = 287 K). Specifically, the Pd63Co37 NWs (∼37 at% Co content) sensor shows outstanding stability of sensing hydrogen against α-ß phase transitions within the wide temperature range of 180-388 K, which is attributed to both the electronic interactions between Pd and Co and the lattice compression strain caused by Co dopants. Moreover, the "reverse sensing behavior" of the PdCo NWs is explicitly interpreted using the α-ß phase transition model.

Elaboration and Biocompatibility of an Eggshell-Derived Hydroxyapatite Material Modified with Si/PLGA for Bone Regeneration in Dentistry.

Hydroxyapatite (HAp) is the most commonly used biomaterial in modern bone regeneration studies because of its chemical similarity to bone, biocompatibility with different polymers, osteoconductivity, low cost, and lack of immune response. However, to overcome the disadvantages of HAp, which include fragility and low mechanical strength, current studies typically focus on property modification through the addition of other materials. Objective. To develop and evaluate the biocompatibility of a HAp material extracted from eggshells and modified with silicon (Si) and poly(lactic-co-glycolic) acid (PLGA). Materials and Methods. An in vitro experimental study in which a HAp material prepared from eggshells was synthesized by wet chemical and conventional chemical precipitation. Subsequently, this material was reinforced with Si/PLGA using the freezing/lyophilization method, and then osteoblast cells were seeded on the experimental material (HAp/Si/PLGA). To analyse the biocompatibility of this composite material, scanning electron microscopy (SEM) and fluorescence confocal microscopy (FCM) techniques were used. PLGA, bovine bone/PLGA (BB/PLGA), and HAp/PLGA were used as controls. Results. A cellular viability of 96% was observed for the experimental HAp/Si/PLGA material as well as for the PLGA. The viability for the BB/PLGA material was 90%, and the viability for the HAp/PLGA was 86%. Cell adhesion was observed on the exterior surface of all materials. However, a continuous monolayer and the presence of filopodia were observed over both external and internal surface of the experimental materials. Conclusions. The HAp/Si/PLGA material is highly biocompatible with osteoblastic cells and can be considered promising for the construction of three-dimensional scaffolds for bone regeneration in dentistry.

Thermoelectric properties of SnSe nanowires with different diameters.

Tin selenide (SnSe) has been the subject of great attention in the last years due to its highly efficient thermoelectricity and its possibilities as a green material, free of Pb and Te. Here, we report for the first time a thermoelectricity and transport study of individual SnSe micro- and nano-wires with diameters in the range between 130 nm and 1.15 µm. X-ray diffraction and transmission electron microscopy analyses confirm an orthorhombic SnSe structure with Pnma (62) symmetry group and 1:1 Sn:Se atomic ratio. Electrical and thermal conductivity and the Seebeck coefficient were measured in each individual nanowire using a specialized suspended microdevice in the 150-370 K temperature range, yielding a thermal conductivity of 0.55 Wm-1 K-1 at room temperature and ZT ~ 0.156 at 370 K for the 130 nm diameter nanowire. The measured properties were correlated with electronic information obtained by model simulations and with phonon scattering analysis. The results confirm these structures as promising building blocks to develop efficient temperature sensors, refrigerators and thermoelectric energy converters. The thermoelectric response of the nanowires is compared with recent reports on crystalline, polycrystalline and layered bulk structures.

Shape-controlled synthesis of palladium and copper superlattice nanowires for high-stability hydrogen sensors.

For hydrogen sensors built with pure Pd nanowires, the instabilities causing baseline drifting and temperature-driven sensing behavior are limiting factors when working within a wide temperature range. To enhance the material stability, we have developed superlattice-structured palladium and copper nanowires (PdCu NWs) with random-gapped, screw-threaded, and spiral shapes achieved by wet-chemical approaches. The microstructure of the PdCu NWs reveals novel superlattices composed of lattice groups structured by four-atomic layers of alternating Pd and Cu. Sensors built with these modified NWs show significantly reduced baseline drifting and lower critical temperature (259.4 K and 261 K depending on the PdCu structure) for the reverse sensing behavior than those with pure Pd NWs (287 K). Moreover, the response and recovery times of the PdCu NWs sensor were of ~9 and ~7 times faster than for Pd NWs sensors, respectively.

Single-crystal γ-MnS nanowires conformally coated with carbon.

We report for the first time the fabrication of single-crystal metastable manganese sulfide nanowires (γ-MnS NWs) conformally coated with graphitic carbon via chemical vapor deposition technique using a single-step route. Advanced spectroscopy and electron microscopy techniques were applied to elucidate the composition and structure of these NWs at the nanoscale, including Raman, XRD, SEM, HRTEM, EELS, EDS, and SAED. No evidence of α-MnS and ß-MnS allotropes was found. The γ-MnS/C NWs have hexagonal cross-section and high aspect ratio (∼1000) on a large scale. The mechanical properties of individual γ-MnS/C NWs were examined via in situ uniaxial compression tests in a TEM-AFM. The results show that γ-MnS/C NWs are brittle with a Young's modulus of 65 GPa. The growth mechanism proposed suggests that the bottom-up fabrication of γ-MnS/C NWs is governed by vapor-liquid-solid mechanism catalyzed by bimetallic Au-Ni nanoparticles. The electrochemical performance of γ-MnS/C NWs as an anode material in lithium-ion batteries indicates that they outperform the cycling stability of stable micro-sized α-MnS, with an initial capacity of 1036 mAh g(-1) and a reversible capacity exceeding 503 mAh g(-1) after 25 cycles. This research advances the integration of carbon materials and metal sulfide nanostructures, bringing forth new avenues for potential miniaturization strategies to fabricate 1D core/shell heterostructures with intriguing bifunctional properties that can be used as building blocks in nanodevices.