Nanowire Bundles Research Articles

Spatial and temporal variation of surface-enhanced Raman scattering at Ag nanowires in aqueous solution

The spatial and temporal variation of local field enhanced Raman scattering (SERS) at Ag nanowires (NWs) in aqueous solution is presented for an improved understanding of the NW structure-SERS enhancement capability relationship. Crossed Ag NWs and Ag NW bundles are found to have SERS enhancement factors much higher than single Ag NWs because of the higher density of interstitials formed by strong surface plasmon coupling when the wires are close to each other. The role of the interstitials of Ag NWs is enhanced by using unpurified Ag NWs containing Ag nanoparticles or decorating the Ag NWs surface with gold nanoparticles using galvanic replacement reaction and electroless deposition methods. This leads to an improved SERS enhancement capability as compared to purified single Ag NWs. Raman imaging reveals a different temporal response of the SERS signal in aqueous solution in comparison to the photoluminescence background of Ag NWs in the absence of Raman-active molecules. Such a different temporal response can be potentially used to separate the SERS signal from the fluorescence background. The Discrete Dipole Approximation (DDA) method is used for the first time to calculate the local field intensity of two crossed and parallel Ag NWs. Heterogeneities in the SERS spatial distribution of the interstitials and their incident-light polarization dependence are illustrated by comparing the SEM image of a selected unpurified Ag NW bundle with its Raman image.

Syntheses and structural characterizations of CrSi2 nanostructures using Si substrates under CrCl2 vapor

Chromium disilicide (CrSi2) nanostructures were grown by the exposure of Si substrates to CrCl2 vapor at atmospheric pressure, and the growth condition dependence on the morphological and structural properties of CrSi2 was systemically investigated. The various structures of CrSi2, such as microrods, nanowire bundles along with microrods and dendritic nanowire structures, have been grown. It was found that the Si substrate temperature and the quantity of the CrCl2 source materials significantly affect the morphological and structural properties of the CrSi2 structures. The structural evolution of CrSi2 has been discussed on the basis of thermodynamic reactions, vapor deposition growth and dendritic growth phenomena. This growth process provides a simple and controllable method to grow novel nano-scaled structures of transition metal silicides for technological use.

Structure and growth of quasi-one-dimensional YSi2 nanophases on Si(100)

Quasi-one-dimensional YSi2 nanostructures are formed via self-assembly on the Si(100) surface. These epitaxial nanowires are metastable and their formation strongly depends on the growth parameters. Here, we explore the various stages of yttrium silicide formation over a range of metal coverages and growth temperatures, and establish a rudimentary phase diagram for these novel and often coexisting nanophases. We identify, in addition to previously identified stoichiometric wires, several new nanowire systems. These nanowires exhibit a variety of surface reconstructions, which sometimes coexist on a single wire. From a comparison of scanning tunneling microscopy images, tunneling spectra, and first-principles density functional theory calculations, we determine that these surface reconstructions arise from local orderings of yttrium vacancies. Nanowires often agglomerate into nanowire bundles, the thinnest of which are formed from single wire pairs. The calculations show that such bundles are energetically favored compared to well-separated single wires. Thicker bundles are formed at slightly higher temperature. They extend over several microns, forming a robust network of conducting wires that could possibly be employed in nanodevice applications.

Synthesis and characterization of WO3 nanowires and metal nanoparticle-WO3 nanowire composites

Tungsten-trioxide nanowire bundles were prepared using a simple hydrothermal method. Sodium-tungstate was used as precursor and sodium-sulfate as structure directing agent. All the reflections of the X-ray diffractogram of the synthesized wires belong to the hexagonal phase of the tungsten trioxide. The nanowires were successfully decorated with metal nanoparticles by wet impregnation. The TEM investigation showed that using different metal precursors resulted in different particle sizes and coverage on the surface.

Growth of TiO2 nanowire bundle arrays and their application in dye-sensitized solar cells

Highly oriented single-crystalline TiO2 nanowire bundle arrays on transparent conductive fluorine-doped tin oxide substrates are prepared by hydrothermal method using the precursors of titanium butoxide, deionized water and hydrochloric acid. The structure and morphology characteristics of all the samples have been analyzed by X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy. Results show that the diameter, length, and density of the nanowire bundle arrays can be varied by changing the growth parameters, such as growth time, initial reactant concentration and acidity. The enhanced (002) peak in XRD patterns indicate that the nanowire is well crystallized and grow perpendicular to the substrate. The high resolution transmission electron microscope images and selected-area electron diffraction patterns confirm that there are approximately 10–30 nanowires in each bundle. The nanowire is single crystalline. Dye-sensitized solar cells assembled from oriented TiO2 nanowire bundle arrays as the photoanode are studied. The light-to-electricity conversion efficiency is about 2.17 %.

Features of Transport in Ultrathin Gold Nanowire Structures

The origin of the interface formation appearing due to the realization of contacts to ultrathin gold nanowire devices is revealed. Such interfaces play an important role in transport mechanisms in nanowire structures and can determine the electrical and operating parameters of a nanodevice. Based on experimental results, the specific electrical properties of bundles of ultrathin gold nanowires fabricated by wet chemical synthesis and subsequently assembled and contacted with gold electrodes are reported. It is demonstrated that these properties are strongly affected by the monolayers of organic molecules inevitably present on the surface of the nanowires due to synthetic conditions. In particular, such layers form a potential barrier to tunneling of the electrons from contacts to the nanowires. The electric transport behavior of the investigated nanowire structures in the temperature range from 500 mK to 300 K obeys the model of thermal fluctuation-induced tunneling conduction through the nanowire-metal electrode molecular junction. Application of this model allows calculation of the parameters of the molecular potential barrier. The formation of such a molecular barrier is verified by scanning tunneling microscope (STM) and transmission electron microscope (TEM) measurements performed using a supporting graphene layer. These findings are important for designing novel nanodevices for molecular electronics on the basis of ultrathin nanowires.

Transition of water adhesion on superhydrophobic surface during aging of polypropylene modified by oxygen capacitively coupled radio frequency plasma

Polypropylene (PP) was modified by an oxygen capacitively coupled radio frequency plasma (CCP) under a radio frequency (RF) power of 200W for an exposure time of 20min. The aging process of the plasma-modified PP was studied at temperatures of 20°C, 60°C, and 90°C for 5h. The post-aged PP surface with bundles of nanowires showed the as-modified surface morphology with a gradual decrease of the polar oxygen-containing groups. A hydrophobic over-recovery from the superhydrophilic state to the superhydrophobic state was observed on the post-aged PP surface. The as-modified superhydrophilic PP surface turned into the hydrophilic state at 20°C and then the superhydrophobic state with the high water adhesion at 60°C. The superhydrophobic state with the low water adhesion was achieved at 90°C. Under the imposed pressure of 202Pa to water droplet, the superhydrophobic surface with the high water adhesion shifted to the hydrophilic state and the superhydrophobic surface with the low water adhesion showed a good stability. The conversion from the superhydrophilic state to the superhydrophobic state on the post-aged PP surface by the plasma nanotexturing was ascribed to the change of the surface compositions under the acceleration of increased aging temperature. The change of contact geometry and the decrease of adhesive force of the liquid–solid interface in the TCL due to more intense surface restructuring under the stimulus of higher aging temperature aroused the transition of the water adhesion on the post-aged superhydrophobic PP surface by the plasma nanotexturing.

Confinement in Oriented Mesopores Induces Piezoelectric Behavior of Polymeric Nanowires

We report on the preparation and the piezoelectric properties of ultrathin polymeric nanowires in the oriented pores of mesoporous silica, which are embedded in the channels of a supporting anodic alumina membrane. Poly(vinylidene difluoride) [PVDF] and its copolymer, poly(vinylidene difluoride trifluoroethylene) [PVTF], were both confined to two types of columnar silica mesopores of ∼5 and 10 nm in diameter. The extreme spatial confinement induces a preferential orientation of the crystalline domains of the polymer into a ferroelectric phase, leading to ultrahigh-aspect-ratio nanowires distributed throughout the templating host, and having up to 60 μm in length, comparable to the thickness of the hosting alumina. The resulting distributed array of piezoelectric nanowires are isolated from each other by a dielectric matrix, facilitating the handling and electrical contacting. We show, for the first time, that a remarkable piezo-response, in the absence of any poling or stretching, is obtained upon nanoconfinement on the PVDF polymer, which, in contrast, does not show any polarization when in bulk or film form without poling. The piezoelectric behavior was assessed by a piezo evaluation system (PES) and we visualized polar nanowire bundles via piezoresponse force microscopy (PFM). This “nano-structuration” represents a powerful approach, holding promise for applications for nanoactuators or bioinspired ciliated sensors with high sensitivity and resolution.

Seed Concentration Control of Metal Nanowire Diameter

Gold/silver nanowires (NWs) of controlled diameters were synthesized from catalytic metal seed particles at the substrate/solution interface. Small seed nanoparticles of three different sizes: ~1 nm (11 gold atoms), ~1.4 nm (~55 gold atoms), and homemade nanoparticles of ~2 nm were used. By varying a single type of seed particle concentration in the growth solution, the NW diameters and morphology could be controlled, between bundles of ultrathin NWs of ~2-3 nm diameter to thicker isolated single NWs with a mean diameter of ~16 nm. In addition, the catalytic reduction rate leading to NW growth was found to be seed size dependent at small seed sizes (<2 nm). The two types of metallic NW films were tested for their performance as transparent electrodes after additional metal deposition for their stabilization and conductivity enhancement. The thin NW bundles exhibit superior transparent conductor properties.

Morphology control of Cu(OH)2 nanowire bundles via a simple template-free route

Orthorhombic Cu(OH)2 nanowire bundles were prepared at low cost by simply pouring NaOH into a CuSO4 and H2O2 mixing solution at room temperature. The Cu(OH)2 nanowire bundles were assembled by the directional arrangement of polycrystalline nanowires, consisting of orthorhombic Cu(OH)2 nanocrystallites. The morphology of Cu(OH)2 is significantly influenced by the H2O2 concentration. Higher H2O2 concentrations result in longer Cu(OH)2 bundles. The results of time-dependent experiments indicate that H2O2 promotes the Cu(OH)2 recrystallization and directional growth. CuO nanostructure is obtained by thermal dehydration of the as-prepared Cu(OH)2 nanowire bundles in the solid state, which preserves the morphology of the original Cu(OH)2 bundles.

Oriented assemblies of tantalum boride nanowire bundles and their luminescent property

Oriented assemblies of tantalum boride (TaB) nanowire bundles have been synthesised via a controlled thermal process. The products were characterised with scanning electron microscopy, X-ray diffraction, transmission electron microscopy and spectrofluorometry. The results show that the as-prepared products are composed of nanowires with a diameter of 60–100 nm. According to the analysis on the reaction mechanism between Ta and BN, a possible formation mechanism of TaB nanowire bundles was proposed. The TaB nanowire bundles displayed an intensive violet photoluminescence emission at around 417 nm. The special structures of the oriented assemblies of TaB nanowires may have potential application in nanoelectronics and photonics.

Porous Pr(OH)3 Nanostructures as High-Efficiency Adsorbents for Dye Removal

Herein we report the electrochemical synthesis of porous Pr(OH)(3) nanobelt arrays (NBAs), nanowire arrays (NWAs), nanowire bundles (NWBs), and nanowires (NWs) and their applications as dye absorbents in water treatment. These Pr(OH)(3) nanostructures exhibit high efficient and selective adsorption of the dyes with amine (-NH(2)) functional groups such as Congo red, reactive yellow, and reactive blue. The high efficiency and selectivity is attributed to the large effective surface area of the porous structure, plentiful hydroxyl groups, and basic sites on the Pr(OH)(3) surface. Furthermore, the toxicity studies of these porous Pr(OH)(3) nanostructure show a negligible effect on seed germination, indicating that they hold great potential as environmentally friendly absorbents in water treatment.

Effect of temperature and time on morphology of CuO nanoparticle during synthesis

In the past decades, many methods have been developed to synthesize zero- dimensional nanoparticles, quasi–one–dimensional (1D) CuO nanostructures, such as metal organic deposition technique, microwave irradiation, sol-gel-like dip technique, reverse micelleassisted route, chemical method, and simple template free solution route. Among those synthesis methods, hydrothermal and chemical reaction methods are of particular interest because they are safe and environmental-friendly synthesis. By use of nonionic surfactant polyethylene glycol (PEG), the clew CuO nanostructure with the diameter of about 4 µm was prepared at 150˚C for 11 hr. As the temperature increased to 160C for 11 hr, the gear wheel CuO nanostructure with the diameter of about 70 nm was made. Furthermore, two other temperatures (180 and 200) are utilized for 20 hr to produce nanowire. The product was characterized by powder X-Ray diffraction, and scanning electron microscopy SEM showed the various shapes of CuO nanostructures, including clew, gearwheel, and nanowire bundles.

Ta4FeTe4: a new material for metallic single molecular wires

Bulk Ta4FeTe4 is a highly anisotropic metallic compound consisting of single molecular wires. In the crystalline bulk phase, sub-nanometer wires self-assemble to form a hexagonal packing of rods resulting in a fibrous crystal morphology. In this work we report the synthesis and crystal growth of single crystalline Ta4FeTe4 nanowire bundles with a length of several millimeters and an aspect ratio exceeding > 105. The nanowire bundle diameter could further be decreased by intercalation-exfoliation. Our results show that depending on the unbundling method nanowires of different length and diameter are accessible. A complete unbundling to single molecular wires of 7.9 A is feasible with this methodology. As the Ta4FeTe4 nanowires are held together only by van der Waals interactions the properties of single molecular wires may be expected similar to those of the metallic bulk material.

Fabrication and properties of ultraviolet photo-detectors based on SiC nanowires

A new type of ultraviolet photo-detectors (UVPDs) based on a bundle of highly aligned SiC nanowires was fabricated and the photo-electric properties of the UVPDs including I–V characteristics and time response were studied in this work. SiC nanowires were prepared by pyrolysis of a polymer precursor with ferrocene as the catalyst by a CVD route. The diameters of SiC nanowires varied from 100 to 200 nm while they were some centimeters long and the SiC nanowires were with zinc blended cubic form (β-SiC) tested by X-ray diffraction. A bundle of nanowires was fixed onto two legs’ base by conductive silver paste to form the UVPDs. The electrical measurement of the device showed a significant increase of current when the device was exposed to 254 nm UV light, and the rising time of the device is very short, but the falling time is relatively long. Our results show that the UVPDs based on SiC nanowires have excellent electrical and optical properties which can be potentially applied.

Formation of mesa-type vertically aligned silicon nanowire bundle arrays by selective-area chemical oxidation and etching processes

A novel top-down scheme for post patterning of vertically aligned silicon nanowires (VA-SiNWs) is proposed and demonstrated by means of chemical oxidation, photolithographic patterning, and chemical etching processes. The authors discovered that only ∼1.4 nm chemical oxide layer is sufficient to protect SiNWs from the attack of KOH etchant. VA-SiNWs can simultaneously serve as an optical antireflective layer between photoresist and silicon substrate for eliminating the backreflection during photolithographic patterning due to the strong optical absorption and light trapping behavior at ultraviolet spectral region. After selective-area SiNW removal with chemical etching procedures, SiNWs at unmasked areas are drastically shortened with a decreased wire number density while oxide masked SiNWs are still vertically aligned with high wire number density and good uniformity. The patterned SiNWs preserves the superior antireflective performance of original SiNWs which makes every SiNW bundle to have dark appearance. Contrary to prepatterning scheme which creates embedded VA-SiNW patterns, this post patterning scheme enables the synthesis of patterned mesa-type SiNWs on silicon surface. The patterned VA-SiNW bundles can be employed for applications in arrayed sensors, arrayed photodiodes, or arrayed field emitters.

Localized Growth of Silicon Oxide Nanowires by Micro‐Afterglow Oxidation

AbstractSilicon oxide nanowires are synthesised below 523 K by a non‐VLS mechanism on different substrates: Fe‐0.5 wt.‐%Si and Fe‐1.5 wt.‐%Si alloys and silicon wafers coated or not by 30‐nm thick coatings of iron. An Ar‐O2 micro‐afterglow is used to feed the substrate with oxidising species over an area of ≈2 mm2. In this area, circular bundles of nanowires appear after 15 min and are randomly distributed. Each bundle diameter increases with time and reaches 50 µm. After several hours, bundles overlap and give a continuous carpet of nanowires. TEM analyses show that no iron is found in the nanowires, whatever the substrate. A growth mechanism, based on the emergence of a surface defect is proposed. magnified image

Synthesis of ZnS Nanowire Bundles and their Photocatalytic Activity for Dye Degradation

Synthesis of ZnS Nanowire Bundles and their Photocatalytic Activity for Dye Degradation

Decamethylferrocene Redox Chemistry and Gold Nanowire Electrodeposition at Salt Crystal|Electrode|Nonpolar Organic Solvent Contacts

This report describes exploratory experimental findings for electrochemical processes in nonpolar solvents (hexane, toluene, and dichloroethane). Conventional 3 mm diameter glassy-carbon-disk electrodes are used in contact with a crystalline salt electrolyte (ammonium nitrate) immersed in nonpolar solvents. The insoluble salt is employed as a “surface thin film electrolyte”, with humidity causing electrical connection from the working electrode to the SCE counter-reference electrode. The organic solvents are employed without intentionally added electrolyte. Humidity in the nonpolar solvents is shown to be essential for the processes to work. The oxidation of decamethylferrocene is demonstrated as a test organometallic redox system. The electrochemical reduction of Au(III) in toluene (solubilized with tetraoctylammonium bromide, TOABr) is employed to demonstrate and visualize the reaction zone around salt crystal|working electrode contact points. Gold nanowire bundle formation is observed, presumably due to an ordered interfacial surfactant microphase at salt|electrode contact points. The triple phase boundary nature of these processes is discussed, and future applications are suggested.

SiC纳米线紫外光探测器光电性能的研究 Fabrication and Properties of Ultraviolet Photo-Detector Based on SiC Nanowires

基于一束高取向排列的SiC纳米线，本文制备了一种新型的紫外光探测器，并测试研究了其光电性能。采用传统化学气相沉积技术，以二茂铁为催化剂，通过聚合物热解的方法制备了长度达数厘米、直径为100~200 nm的SiC纳米线。探测器由导电银浆固定一束高取向排列的SiC纳米线到器件基座两个引脚上形成，其光电性能测试研究表明，器件暗电流很小、并且在254 nm紫外光照射时，其光电流迅速变大；器件反应时间很短而回复时间相对较长。 A new type of Ultraviolet Photodetector (UVPDs) based on a bundle of highly aligned SiC nanowires was fabricated and the photo-electric properties of the UVPDs including I-V characteristic and time response et al. were studied in this paper. SiC nanowires were prepared by pyrolysis of a polymer precursor with ferrocene as the catalyst by a CVD route. The diameter of SiC nanowires varied from 100 to 200 nm while they were some centimeters long. A bundle of nanowires was fixed onto two legs in a custom base by conductive silver paste to form the UVPDs. The electrical measurements of the device show a big increase of current when explored the device to 254 nm UV light, and the rise time of the device is very short, but the fall time is relatively long.