Number Density Of Nanowires Research Articles

Cu Nanowire Networks with Well-Defined Geometrical Parameters for Catalytic Electrochemical CO2 Reduction

Three-dimensional, highly interconnected copper nanowire networks are designed and fabricated by electrodeposition in etched ion track polymer templates with interconnected nanochannels and subsequently applied as a catalyst for the electrochemical CO2 reduction toward hydrocarbons and alcohols in an aqueous electrolyte. The specific surface area expressed by the nanowire networks can be adjusted by tailoring the wire length, wire diameter, and nanowire number density to values between 70 cm2 and 300 cm2 per cm2 geometrical sample area. The conversion efficiency and selectivity of CO2 reduction toward liquid- and gas-phase products are studied as a function of the applied potential. Before and after the CO2 reduction reaction, the nanowire networks are characterized by scanning and transmission electron microscopy and by X-ray diffraction, evidencing their stability during CO2 reduction in a potential region between −0.5 V and −0.93 V versus RHE.

Electrical conductivity of random metallic nanowire networks: an analytical consideration along with computer simulation.

The current interest in the study of the 2D systems of randomly deposited metallic nanowires is inspired by a combination of their high electrical conductivity with excellent optical transparency. Metallic nanowire networks show great potential for use in numerous technological applications. Although there are models that describe the electrical conductivity of the random nanowire networks through wire resistance, junction resistance, and number density of nanowires, they are either not rigorously justified or contain fitting parameters. We have proposed a model for the electrical conductivity in random metallic nanowire networks. We have mimicked such random nanowire networks as random resistor networks (RRN) produced by the homogeneous, isotropic, and random deposition of conductive zero-width sticks onto an insulating substrate. We studied the electrical conductivity of these RRNs using a mean-field approximation. An analytical dependency of the electrical conductivity on the main physical parameters (the number density and electrical resistances of these wires and of the junctions between them) has been derived. Computer simulations have been performed to validate our theoretical predictions. We computed the electrical conductivity of the RRNs against the number density of the conductive fillers for the junction-resistance-dominated case and for the case where the wire resistance and the junction resistance were equal. The results of the computations were compared with this mean-field approximation. Our computations demonstrated that our analytical expression correctly predicts the electrical conductivity across a wide range of number densities.

Analysis of incubation time preceding the Ga-assisted nucleation and growth of GaAs nanowires on Si(111)

The incubation time preceding nucleation and growth of surface nanostructures is interesting from a fundamental viewpoint but also of practical relevance as it determines statistical properties of nanostructure ensembles such as size homogeneity. Using in situ reflection high-energy electron diffraction, we accurately deduce the incubation times for Ga-assisted GaAs nanowires grown on unpatterned Si(111) substrates by molecular beam epitaxy under different conditions. We develop a nucleation model that explains and fits very well the data. We find that, for a given temperature and Ga flux, the incubation time always increases with decreasing As flux and becomes infinite at a certain minimum flux, which is larger for higher temperature. For given As and Ga fluxes, the incubation time always increases with temperature and rapidly tends to infinity above 640 {\deg}C under typical conditions. The strong temperature dependence of the incubation time is reflected in a similar variation of the nanowire number density with temperature. Our analysis provides understanding and guidance for choosing appropriate growth conditions that avoid unnecessary material consumption, long nucleation delays, and highly inhomogeneous ensembles of nanowires. On a more general ground, the existence of a minimum flux and maximum temperature for growing surface nanostructures should be a general phenomenon pertaining for a wide range of material-substrate combinations.

Study of CuO Nanowire Growth on Different Copper Surfaces

Cupric oxide (CuO) nanowires were produced by thermal oxidation of copper surfaces at temperatures up to 450 °C. Three different surfaces, namely a copper foil as well as evaporation deposited copper and an application relevant sputtered copper film on Si(100) substrates were characterized ex-situ before and after the experiment. The development of oxide layers and nanowires were monitored in-situ using grazing incidence small angle X-ray scattering. The number density of nanowires is highest for the sputtered surface and lowest for the surface prepared by evaporation deposition. This can be linked to different oxide grain sizes and copper grain boundary diffusions on the different surfaces. Small grains of the copper substrate and high surface roughness thereby lead to promoted growth of the nanowires.

Design of Contact Electrodes for Semiconductor Nanowire Solar Energy Harvesting Devices

Transparent, low-resistive contacts are critical for efficient solar energy harvesting devices. It is important to reconsider the material choices and electrode design as devices move from 2D films to 1D nanostructures. In this paper, we study the effectiveness of indium tin oxide (ITO) and metals, such as Ag and Cu, as contacts in 2D and 1D systems. Although ITO has been studied extensively and developed into an effective transparent contact for 2D devices, our results show that effectiveness does not translate to 1D systems. Particularly with consideration of resistance requirement, nanowires with metal shells as contacts enable better absorption within the semiconductor as compared to ITO. Furthermore, there is a strong dependence of contact performance on the semiconductor band gap and diameter of nanowires. We found that metal contacts outperform ITO for nanowire devices, regardless of the sheet resistance constraint, in the regime of diameters less than 100 nm and band-gaps greater than 1 eV. These metal shells optimized for best absorption are significantly thinner than ITO, which enables for the design of devices with high nanowire number density and consequently higher device efficiencies.

Multiscale Study of Plasmonic Scattering and Light Trapping Effect in Silicon Nanowire Array Solar Cells.

Nanometallic structures that support surface plasmons provide new ways to confine light at deep-subwavelength scales. The effect of light scattering in nanowire array solar cells is studied by a multiscale approach combining classical electromagnetic (EM) and quantum mechanical simulations. A photovoltaic device is constructed by integrating a silicon nanowire array with a plasmonic silver nanosphere. The light scatterings by plasmonic element and nanowire array are obtained via classical EM simulations, while current-voltage characteristics and optical properties of the nanowire cells are evaluated quantum mechanically. We found that the power conversion efficiency (PCE) of photovoltaic device is substantially improved due to the local field enhancement of the plasmonic effect and light trapping by the nanowire array. In addition, we showed that there exists an optimal nanowire number density in terms of optical confinement and solar cell PCE.

Catalyst-free growth of ZnO nanowires on various-oriented sapphire substrates by pulsed-laser deposition

Catalyst-free growth of one-dimensional zinc oxide (ZnO) nanowires is reported. ZnO nanowires were synthesized on ZnO buffer layers deposited on various-oriented sapphire substrates. Syntheses of ZnO buffer layers and nanowires were performed by ultraviolet pulsed-laser deposition. ZnO nanowire's number density was the lowest in the case of using m-cut sapphire substrates. ZnO nanowires grown on a-cut sapphire substrates had vertical alignment with distances of tens to hundreds of nanometers. On the other hand, ZnO nanowires grown on c-cut sapphire substrates had the biggest nucleation rate. The dependence of crystalline orientation of ZnO buffer layers on the orientation of sapphire substrates were investigated by electron back scatter diffraction measurement. From their results, the growth models of ZnO buffer layers were suggested and the variations in morphological properties of ZnO nanowires were discussed.

The effect of nanowire length and diameter on the properties of transparent, conducting nanowire films

This article describes how the dimensions of nanowires affect the transmittance and sheet resistance of a random nanowire network. Silver nanowires with independently controlled lengths and diameters were synthesized with a gram-scale polyol synthesis by controlling the reaction temperature and time. Characterization of films composed of nanowires of different lengths but the same diameter enabled the quantification of the effect of length on the conductance and transmittance of silver nanowire films. Finite-difference time-domain calculations were used to determine the effect of nanowire diameter, overlap, and hole size on the transmittance of a nanowire network. For individual nanowires with diameters greater than 50 nm, increasing diameter increases the electrical conductance to optical extinction ratio, but the opposite is true for nanowires with diameters less than this size. Calculations and experimental data show that for a random network of nanowires, decreasing nanowire diameter increases the number density of nanowires at a given transmittance, leading to improved connectivity and conductivity at high transmittance (>90%). This information will facilitate the design of transparent, conducting nanowire films for flexible displays, organic light emitting diodes and thin-film solar cells.

Direct synthesis of vertical α-Fe2O3 nanowires from sputtered Fe thin film

α -Fe 2 O 3 (hematite) nanowires were synthesized from Fe thin films. The bladelike nanowires were synthesized by the simple annealing of a sputtered Fe thin film in a furnace with the annealing time varied between 0 and 100 min. The length of the nanowires was proportional to the logarithm of the annealing time. The number density of nanowires was saturated within an annealing time of 0–1 min. The Fe thin film may have been oxidized during annealing and the iron oxide molecules may have diffused to the roots of nanowires in accordance with the solid-phase growth model or they may have vaporized and then have been deposited on the surface of nanowires in accordance with the vapor-liquid-solid model. The authors characterized the nanowires by Raman spectroscopy and transmission electron microscopy (TEM). The Raman shift and the results of TEM indicated that the nanowires consisted of trigonal α-Fe2O3.

Transport Limited Growth of Zinc Oxide Nanowires

Vertical arrays of crystalline zinc oxide (ZnO) nanowires grown on various substrates find applications in dye-sensitized and hybrid organic/inorganic bulk-heterojunction solar cells. The ability to grow dense nanowires at high rates and the fundamental understanding of the growth process are important for these applications. Herein, we show that heterogeneous growth of ZnO nanowires on substrates seeded with ZnO nanoparticles in an aqueous solution of methenamine and zinc nitrate is mass transport limited. Mass transport limited growth leads to an inverse relationship between the nanowire dimensions (height and diameter) and the nanowire number density. This mass transport limitation also leads to nonuniform growth near the boundaries between seeded and unseeded regions. Stirring the reaction solution increases the nanowire growth rate. Experimental results were interpreted within the framework of two simple but nontrivial models of the solution phase species transport and the nanowire growth. Additional...

Fabrication of UV detectors based on ZnO nanowires using silicon microchannel

Aligned ZnO nanowires were grown by metal organic chemical vapor deposition on patterned silicon substrate. The shape of nanostructures was greatly influenced by the micropatterned surface. The aspect ratio, packing fraction and the number density of nanowires on top surface are around 10, 0.8 and 107 per mm2, respectively, whereas the values are 20, 0.3 and 5×107 per mm2, respectively, towards the bottom of the cavity. XRD patterns suggest that the nanostructures have good crystallinity. High-resolution transmission electron microscopy confirmed the single-crystalline growth of the ZnO nanowires along the [0001] direction. Photosensitivity of the nanowires, grown on both top and bottom surface of the microchannel, was observed. However, the nanowires grown on bottom surface have shown better UV response with base line recovery at dark condition.

Formation of Nanowires Based on π-Conjugated Polymers by High-Energy Ion Beam Irradiation

High-energy ion beam irradiation of cross-linking type polymers gives nanowires formed by cross-linking reactions along the ion track trajectories. A fairly simple technique has been developed to control the size and number density of nanowires based on polystyrene (PS) by selecting appropriate ion beam characteristics and molecular sizes of the target polymers. This technique has also been extended to the formation of nanowires based on polyaniline (PANi) and poly(3-hexylthiophene) (P3HT). Characteristic optical properties (absorption and photoluminescence) observed for the various sizes of nanowires based on P3HT on a quartz substrate are different than those from solid state P3HT, suggesting that the nanowires are not just the finely patterned analogs of the conjugated polymers.

Fabrication of Large Number Density Platinum Nanowire Arrays by Size Reduction Lithography and Nanoimprint Lithography

Large number density Pt nanowires with typical dimensions of 12 microm x 20 nm x 5 nm (length x width x height) are fabricated on planar oxide supports. First sub-20 nm single crystalline silicon nanowires are fabricated by size reduction lithography, and then the Si nanowire pattern is replicated to produce a large number of Pt nanowires by nanoimprint lithography. The width and height of the Pt nanowires are uniform and are controlled with nanometer precision. The nanowire number density is 4 x 10(4) cm(-1), resulting in a Pt surface area larger than 2 cm(2) on a 5 x 5 cm(2) oxide substrate. Bimodal nanowires with different width have been generated by using a Pt shadow deposition technique. Using this technique, alternating 10 and 19 nm wide nanowires are produced.