Computationally derived rules for persistence of C60 nanowires on recumbent pentacene bilayers

The tendency for C(60) nanowires to persist on two monolayers of recumbent pentacene is studied using molecular dynamics (MD) simulations. A review of existing experimental literature for the tilt angle adopted by pentacene on noble metal surfaces shows that studies cover a limited range from 55° to 90°, motivating simulation studies of essentially the entire range of tilt angles (10°-90°) to predict the optimum surface tilt angle for C(60) nanowire formation. The persistence of a 1D nanowire depends sensitively on this tilt angle, the amount of initial tensile strain, and the presence of surface step edges. At room temperature, C(60) nanowires oriented along the pentacene short axes persist for several nanoseconds and are more likely to occur if they reside between, or within, pentacene rows for ? ≤ ～60°. The likelihood of this persistence increases the smaller the tilt angle. Nanowires oriented along the long axes of pentacene molecules are unlikely to form. The limit of stability of nanowires was tested by raising the temperature to 400 K. Nanowires located between pentacene rows survived this temperature rise, but those located initially within pentacene rows are only stable in the range ?(1) = 30°-50°. Flatter pentacene surfaces, that is, tilt angles above about 60°, are subject to disorder caused by C(60) molecules "burrowing" into the pentacene surface. An initial strain of 5% applied to the C(60) nanowires significantly decreases the likelihood of nanowire persistence. In contrast, any appreciable surface roughness, even by half a monolayer in height of a third pentacene monolayer, strongly enhances the likelihood of nanowire formation due to the strong binding energy of C(60) molecules to step edges.     

Metallic single-crystal CoSi nanowires via chemical vapor deposition of single-source precursor

We report the synthesis, structural characterization, and electrical transport properties of free-standing single-crystal CoSi nanowires synthesized via a single-source precursor route. Nanowires with diameters of 10-150 nm and lengths of greater than 10 mum were synthesized through the chemical vapor deposition of Co(SiCl(3))(CO)(4) onto silicon substrates that were covered with 1-2 nm thick SiO(2). Transmission electron microscopy confirms the single-crystal structure of the cubic CoSi. X-ray absorption and emission spectroscopy confirm the chemical identity and show the expected metallic nature of CoSi, which is further verified by room-temperature and low-temperature electrical transport measurements of nanowire devices. The average resistivity of CoSi nanowires is found to be about 510 muOmega cm. Our general and rational nanowire synthesis approach will lead to a broad class of silicide nanowires, including those metallic materials that serve as high-quality building blocks for nanoelectronics and magnetic semiconducting Fe(1-x)Co(x)Si suitable for silicon-based spintronics.     

Monte Carlo simulation of nanowires of different metals and two-metal alloys

Nanowires of different metals and two-metal alloys have been studied by means of canonical Monte Carlo simulations and the embedded atom method for the interatomic potentials. For nanowires of gold, a relatively stable three-atom-wide chain was observed. The presence of one-atom-wide linear atomic chains is not stable in any case. For two-metal alloy nanowires, the metal with a higher surface energy tends to locate in the inner region of the nanowire.     

Dielectrophoretic alignment of metal and metal oxide nanowires and nanotubes: a universal set of parameters for bridging prepatterned microelectrodes

Nanowires and nanotubes were synthesized from metals and metal oxides using templated cathodic electrodeposition. With templated electrodeposition, small structures are electrodeposited using a template that is the inverse of the final desired shape. Dielectrophoresis was used for the alignment of the as-formed nanowires and nanotubes between prepatterned electrodes. For reproducible nanowire alignment, a universal set of dielectrophoresis parameters to align any arbitrary nanowire material was determined. The parameters include peak-to-peak potential and frequency, thickness of the silicon oxide layer, grounding of the silicon substrate, and nature of the solvent medium used. It involves applying a field with a frequency >10(5) Hz, an insulating silicon oxide layer with a thickness of 2.5 μm or more, grounding of the underlying silicon substrate, and the use of a solvent medium with a low dielectric constant. In our experiments, we obtained good results by using a peak-to-peak potential of 2.1 V at a frequency of 1.2 × 10(5) Hz. Furthermore, an indirect alignment technique is proposed that prevents short circuiting of nanowires after contacting both electrodes. After alignment, a considerably lower resistivity was found for ZnO nanowires made by templated electrodeposition (2.2-3.4 × 10(-3) Ωm) compared to ZnO nanorods synthesized by electrodeposition (10 Ωm) or molecular beam epitaxy (MBE) (500 Ωm).     

Dye-sensitized solar cells based on anatase TiO2 nanoparticle/nanowire composites

Dye-sensitized solar cells were fabricated based on the composites of anatase TiO2 nanoparticles and single crystalline anatase TiO2 nanowires. Nanoparticle/nanowire composites can possess the advantages of both building blocks, i.e., the high surface area of nanoparticle aggregates and the rapid electron transport rate and the light scattering effect of single-crystalline nanowires. Three different composites were prepared with 5 wt %, 20 wt %, and 77 wt % nanowires, respectively. The performances of composite solar cells were compared with pure nanoparticle cells at a series of film thickness. With low nanowire concentrations (5 wt % and 20 wt %), the composite films maintain similar specific surface area as the pure nanoparticle films, while the composite cells show higher short-circuit current density and open-circuit voltage. An enhancement of power efficiency from 6.7% for pure nanoparticle cells to 8.6% for the composite cell with 20 wt % nanowires has been achieved under 1 Sun AM1.5 illumination (100 mW/cm2). For the composite film with 77 wt % nanowires, the nanowires became the major phase. Their less compact packing resulted in significant decrease of the specific surface area, and thus the current density. However, with the increase of film thickness, the current density showed a continuous increase in the whole thickness range up to 17 microm, indicating the improved electron diffusion length due to the formed nanowire network. The nanowires also helped to preserve crack-free thick films. These results show that employing nanoparticle/nanowire composites represents a promising approach for further improving the efficiencies of sensitized solar cells.     

Enzyme-functionalized gold nanowires for the fabrication of biosensors

Gold nanowires were prepared by an electrodeposition strategy using nanopore polycarbonate (PC) membrane, with the average diameter of the nanowires about 250 nm and length about 10 microm. The nanowires prepared were dispersed into chitosan (CHIT) solution and stably immobilized onto glassy carbon electrode (GCE) surface. The electrochemical behavior of gold nanowire modified electrode and its application to the electrocatalytic reduction of hydrogen peroxide (H(2)O(2)) were investigated. The modified electrode allows low potential detection of hydrogen peroxide with high sensitivity and fast response time. Moreover, the good biocompatibility of nanometer-sized gold, the vast surface area of the nanowire-structure make it ideal for adsorption of enzymes for the fabrication of biosensors. Glucose oxidase was adsorbed onto the nanowire surface to fabricate glucose biosensor as an application example. The detection of glucose was performed in phosphate buffer (pH 6.98) at -0.2 V. The resulting glucose biosensor exhibited sensitive response, with a short response time (<8 s), a linear range of 10(-5)-2 x 10(-2) M and detection limit of 5 x 10(-6) M.     

Temperature dependence of the field effect mobility of solution-grown germanium nanowires

The temperature dependence of the field effect mobility was measured for solution-grown single-crystal Ge nanowires. The nanowires were synthesized in hexane from diphenylgermane by the supercritical fluid-liquid-solid process using gold nanocrystals as seeds. The nanowires were chemically treated with isoprene to passivate their surfaces. The electrical properties of individual nanowires were then measured by depositing them on a Si substrate, followed by electrical connection with Pt wires using focused ion beam assisted chemical vapor deposition. The nanowires were positioned over TaN or Au electrodes covered with ZrO2 dielectric that were used as gates to apply external potentials to modulate the conductance. Negative gate potentials increased the Ge nanowire conductance, characteristic of a p-type semiconductor. The temperature-dependent source/drain current-voltage measurements under applied gate potential revealed that the field effect mobility increased with increasing temperature, indicating that the carrier mobility through the nanowire is probably dominated either by a hopping mechanism or by trapped charges in fast surface states.     

Conducting Polyaniline Nanowire Arrays Modified Electrode for High Performance Supercapacitor and Enhanced Catalysis of Nitrite Reduction

Vertically aligned conducting polymer nanowire arrays had great potential applications in supercapacitor electrode material and exhibited enhanced electrocatalytic behavior towards the reduction of nitrite. In this paper, a facial template‐free approach to synthesize large arrays of vertically aligned polyaniline (PANI) nanowires on electrochemically pretreated glassy carbon electrodes was reported by using a galvanostatic current method. The as‐prepared large arrays of PANI nanowires had very narrow diameters and were oriented perpendicular to the substrate, which was a benefit to the ion diffusion when being used as the supercapacitor electrode. The highest specific capacitance of PANI nanowire arrays was measured and kept high at a large charge‐discharge current density. Furthermore, it also can detect nitrite with ultrahigh sensitivity of 62.99 µA mM^?1 cm^?2 and a remarkable fast response time of less 1 s. The results indicated that the vertically aligned PANI nanowires could dramatically enhance the electrochemical performance.     

Chemical bath deposition of ZnO nanowires at near-neutral pH conditions without hexamethylenetetramine (HMTA): understanding the role of HMTA in ZnO nanowire growth

Chemical bath deposition (CBD) is an inexpensive and reproducible method for depositing ZnO nanowire arrays over large areas. The aqueous Zn(NO(3))(2)-hexamethylenetetramine (HMTA) chemistry is one of the most common CBD chemistries for ZnO nanowire synthesis, but some details of the reaction mechanism are still not well-understood. Here, we report the use of in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to study HMTA adsorption from aqueous solutions onto ZnO nanoparticle films and show that HMTA does not adsorb on ZnO. This result refutes earlier claims that the anisotropic morphology arises from HMTA adsorbing onto and capping the ZnO {10 1 0} faces. We conclude that the role of HMTA in the CBD of ZnO nanowires is only to control the saturation index of ZnO. Furthermore, we demonstrate the first deposition of ZnO nanowire arrays at 90 °C and near-neutral pH conditions without HMTA. Nanowires were grown using the pH buffer 2-(N-morpholino)ethanesulfonic acid (MES) and continuous titratation with KOH to maintain the same pH conditions where growth with HMTA occurs. This semi-batch synthetic method opens many new opportunities to tailor the ZnO morphology and properties by independently controlling temperature and pH.     

一维有序聚苯胺纳米阵列的制备及电化学储能性能

以导电玻璃FTO为基底电极, 在硫酸溶液中, 分别研究了苯胺单体浓度和恒定电流大小对聚苯胺(PANI)形貌的影响; 同时恒定苯胺单体的浓度和工作电流, 探究了不同类型的质子酸对PANI阵列形貌的影响. 结果表明, 采用恒电流方法可以制备出一维有序PANI纳米线阵列, 而且当苯胺的浓度为0.1 mol/L, 恒电流法的工作电流密度为0.03 mA/cm²时, 所制备的PANI纳米线阵列形貌最佳; 当用HCl, HNO₃和对甲苯磺酸(p-TSA)作为合成PANI的支持液时, 得到树桩状的PANI 纳米结构, 不能得到均一的纳米线阵列结构. 电化学性能测试结果表明, 制备的最佳形貌PANI纳米线阵列的比电容值可达560 F/g; 循环1000周后电容损失率为11%.     

Investigation of microstructures of ZnCo_2O_4 on bare Ni foam and Ni foam coated with graphene and their supercapacitors performance

The graphene coating was deposited on the surface of Ni foam using the chemical vapor deposition process. A large amount of flower-like ZnCo_2O_4 microspheres with short nanowires were formed on bare Ni foam by hydrothermal method, while large-scale ZnCo_2O_4 nanowires arrays homogeneously aligned and separated adequately on Ni foam coated with graphene. This ZnCo_2O_4 nanowire structure exhibited superior supercapacitors properties. The excellent supercapacitors were mainly attributed to the large specific surface and the porosity on the nanowires which promoted the electrons and ions transportation. In addition, graphene improved conductivity of substrate for current collecting.     

ZnO-Al2O3 and ZnO-TiO2 core-shell nanowire dye-sensitized solar cells

We describe the construction and performance of dye-sensitized solar cells (DSCs) based on arrays of ZnO nanowires coated with thin shells of amorphous Al(2)O(3) or anatase TiO(2) by atomic layer deposition. We find that alumina shells of all thicknesses act as insulating barriers that improve cell open-circuit voltage (V(OC)) only at the expense of a larger decrease in short-circuit current density (J(SC)). However, titania shells 10-25 nm in thickness cause a dramatic increase in V(OC) and fill factor with little current falloff, resulting in a substantial improvement in overall conversion efficiency, up to 2.25% under 100 mW cm(-2) AM 1.5 simulated sunlight. The superior performance of the ZnO-TiO(2) core-shell nanowire cells is a result of a radial surface field within each nanowire that decreases the rate of recombination in these devices. In a related set of experiments, we have found that TiO(2) blocking layers deposited underneath the nanowire films yield cells with reduced efficiency, in contrast to the beneficial use of blocking layers in some TiO(2) nanoparticle cells. Raising the efficiency of our nanowire DSCs above 2.5% depends on achieving higher dye loadings through an increase in nanowire array surface area.     

Enhancing dispersion of copper nanowires in melt‐mixed polystyrene composites

Copper nanowire (CuNWs)/polystyrene (PS) composites were prepared by melt mixing using unfunctionalized and functionalized nanowires. Alkanethiols were utilized to modify the surface of CuNWs postsynthesis and enable their dispersion in a polymer melt. Unfunctionalized nanowires decreased the electrical resistivity of PS by nine orders of magnitude with 2.0 vol % Cu, and resulted in composites with a viscoelastic behavior dominated by polymer–polymer networks indicating that electrical percolation occurred without a transition from liquid‐like to solid‐like behavior (i.e., rheological percolation). Results from transmission electron microscopy (TEM), scanning electron microscopy (SEM), and melt rheology characterization indicated that surface modification of CuNWs contributed to the dispersion of the nanofiller in the polymer matrix. CuNWs functionalized with 1‐octanethiol and 1‐butanethiol produced rheological percolation and a gradual decrease in the electrical resistivity of the PS nanocomposites with increasing concentration of nanowires. Polymer nanocomposites with low concentrations of functionalized nanowires showed lower complex viscosities than pure PS; this was attributed to a plasticizing effect introduced by the alkanethiols. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2064–2078, 2008     

Photoelectrochemical study on charge transfer properties of TiO2-B nanowires with an application as humidity sensors

One-dimensional (1-D) TiO2-B nanowires have been synthesized via a facile solvothermal route. The morphology and crystalline structures of the nanowires were characterized by using powder X-ray diffraction, low/high-resolution transmission electron microscopy, and Brunauer-Emmett-Teller methods. It is important with the calcination treatment at 350 degrees C to maintain 1-D morphologies of the material in the form of single-crystalline TiO2-B nanowires. In addition, a simple method was used to study the photogenerated charge transfer and photoelectrochemical properties of the TiO2-B nanowires in comparison with commercial TiO2 P25 nanoparticles based on the experimental data from the electric field-effected photocurrent action spectrum and Mott-Schottky measurements. It was revealed that TiO2-B nanostructures played an important role in the photoelectrochemical processes. The synthetic TiO2-B nanowire electrode exhibited unique electronic properties, e.g., favorable charge-transfer ability, negative-shifted appearing flat-band potential, existence of abundant surface states or oxygen vacancies, and high-level dopant density. Moreover, the obtained TiO2-B nanowires were found to display excellent humidity sensing abilities as functional materials in the humidity sensor application. With relative humidity increased from 5% to 95%, about one and half orders of magnitude change in resistance was observed in the TiO2-B nanowire-based surface-type humidity sensors.     

Bismuth nanowires for potential applications in nanoscale electronics technology.

Nanowires of bismuth with diameters ranging from 10 to 200 nm and lengths of 50 microm have been synthesized by a pressure injection method. Nanostructural and chemical compositional studies using environmental and high resolution transmission electron microscopy with electron stimulated energy dispersive X-ray spectroscopy have revealed essentially single crystal nanowires. The high resolution studies have shown that the nanowires contain amorphous Bi-oxide layers of a few nanometers on the surface. In situ environmental high resolution transmission electron microscopy (environmental-HRTEM) studies at the atomic level, in controlled hydrogen and other reducing gas environments at high temperatures demonstrate that gas reduction can be successfully applied to remove th oxide nanolayers and to maintain the dimensional and structural uniformity of the nanowires, which is key to attaining low electrical contact resistance.     

Titania nanowires as substrates for sensing and photocatalysis of common textile industry effluents

Sensing and photocatalysis of textile industry effluents such as dyes using mesoporous anatase titania nanowires are discussed here. Spectroscopic investigations show that the titania nanowires preferentially sense cationic (e.g. Methylene Blue, Rhodamine B) over anionic (e.g. Orange G, Remazol Brilliant Blue R) dyes. The adsorbed dye concentration on titania nanowires increased with increase in nanowire dimensions and dye solution pH. Electrochemical sensing directly corroborated spectroscopic findings. Electrochemical detection sensitivity for Methylene Blue increased by more than two times in magnitude with tripling of nanowire average length. Photodegradation of Methylene Blue using titania nanowires is also more efficient than the commercial P25-TiO₂ nanopowders. Keeping illumination protocol and observation times constant, the Methylene Blue concentration in solution decreased by only 50% in case of P25-TiO₂ nanoparticles compared to a 100% decrease for titania nanowires. Photodegradation was also found to be function of exposure times and dye solution pH. Excellent sensing ability and photocatalytic activity of the titania nanowires is attributed to increased effective reaction area of the controlled nanostructured morphology.     

Exciton-plasmon interaction in a composite metal-insulator-semiconductor nanowire system

We report about the synthesis and optical properties of a composite metal-insulator-semiconductor nanowire system which consists of a wet-chemically grown silver wire core surrounded by a SiO2 shell of controlled thickness, followed by an outer shell of highly luminescent CdSe nanocrystals. With microphotoluminescence (micro-PL) experiments, we studied the exciton-plasmon interaction in individual nanowires and analyzed the spatially resolved nanocrystal emission for different nanowire length, SiO2-shell thickness, nanocrystal shape, pump power, and emission polarization. For an SiO2 spacer thickness of approximately 15 nm, we observed an efficient excitation of surface plasmons by excitonic emission of CdSe nanocrystals. For nanowire lengths up to approximately 10 microm, the composite metal-insulator-semiconductor nanowires ((Ag)SiO2)CdSe act as a waveguide for 1D-surface plasmons at optical frequencies with efficient photon outcoupling at the nanowire tips, which is promising for efficient exciton-plasmon-photon conversion and surface plasmon guiding on a submicron scale in the visible spectral range.     

Templateless synthesis of polypyrrole nanowires by non-static solution-surface electropolymerization

Polypyrrole nanowires (PPy-NWs) are synthesized by a novel templateless approach based on non-static solution-surface (NSSS) electropolymerization. The mechanism responsible for PPy-NW formation is the simultaneous oxidation of pyrrole and water, with concomitant formation of hydroxyl radicals and dioxygen nanobubbles. In particular, a localized PPy-NW deposition at the solution-air interface is enabled by solution-surface electropolymerization due to the surface excess of the monomer at the interface favored by the large surface tension of the solvent. In the proposed approach, solution-surface electropolymerization is performed in non-static conditions (NSSS), as the solution-air interface is shifted by flowing the electrolyte solution over the electrode surface. This allows a PPy-NW homogeneous deposition on whatever large area electrode to be rapidly achieved. Parameters influencing the morphology of PPy-NWs are studied, particularly focusing on flow rate, pH of the electrolyte solution, and electropolymerization time. The growth process of PPy-NWs is examined and the way of tuning their resulting morphology is discussed. Morphological investigation by scanning electron microscopy and chemical/electrochemical characterization of PPy-NWs by X-ray photoelectron spectroscopy and cyclic voltammetry, respectively, further support the proposed nanowire formation mechanism. Nanowires with diameter in the range of 40–300 nm are obtained, and the possibility of depositing differently sized nanowires with a predetermined spatial distribution on the same substrate is also demonstrated.     

An atomistic model and key parameters for devising single molecular nanowire sensors

Based on one impurity model Hamiltonian describing a nanowire upon adsorption of a molecule, we obtain an analytical formula of the conductance which is governed clearly by modulating key parameters. The formula shows that the conductance change in nanowire upon adsorption of a molecule is mainly controlled by three factors, electron hopping between adsorbed molecule and nanowire, chemical potential, and the change of atomic configurations of the nanowires near the adsorption site. Conductance is very sensitive to the choice of these key parameters; therefore, a proper nanowire system that renders matched chemical potential as well as hopping strength between the nanowire and the adsorbed molecule should be devised for the sensor applications. Our model calculations give similar conductance features to the conductance obtained by the first principle calculations for a singe-molecule-adsorbed molecular wire. It is worthy of note that the system can be in antiresonance, which is characterized by a quick drop in conductance when a molecule is adsorbed on the nanowires.     

离子液体中电沉积制备钴纳米线阵列

Porous anodic alumina (PAA) was used as a template to prepare Co nanowires array from 1-ethyl-3-methylimidazolium chloride ionic liquid by direct current method. The surface morphology of porous anodic alumina template was observed by field emission-scanning tunneling microscopy (FE-SEM) before and after the electrodeposition of Co nanowires. The electrodeposition of Co nanowires was characterized by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). TEM results indicate that the Co nanowire surface is coarse and porous when aqueous solution was used as electrolyte， and the Co nanowire deposited from the ionic liquid is uniform and smooth. XRD results show that the electrodeposition of Co is a mixture of crystal and microcrystal phase.