Dense Nanowires Research Articles

Atomic Force Microscopic and Raman Investigation of Boron-Doped Diamond Nanowire Electrodes and Their Activity toward Oxygen Reduction

Reactive ion etching of diamond interfaces using oxygen plasma is a widely used approach for the formation of diamond nanowires. In this paper, we highlight the influence of the doping level of the etched diamond substrate on the density of the resulting nanowires. Heavily boron-doped diamond interfaces result in very dense diamond nanowires, while etching of low boron-doped diamond substrates results in sparsely formed nanostructures, as boron dopant atoms in the diamond act as masks during the etching process. In pursuit of a better understanding of doping and plasma etching effects, we demonstrated by performing Raman imaging on single diamond nanowires that the etching process leads to a dedoping of the wire tip and a partial transformation of diamond to sp2 carbon. The etching process does not, however, alter the initial diamond feature of the rest of the nanowire. Finally, the activity of the different diamond nanowires toward oxygen reduction in alkaline solution was investigated. Interestingly, hi...

Bending fatigue behavior of silver nanowire networks with different densities

This study examines the deformation behavior of silver nanowire networks with different densities under bending fatigue testing conditions. Up to 400,000 bending cycles are applied to the various silver nanowire networks while monitoring the resistance in situ. The least dense silver nanowire network shows a moderate decrease in resistance during the early bending cycles due to mechanical welding, followed by a rapid increase as the bending test progresses. As the nanowire density increases, however, this initial decrease in the resistance becomes smaller, and the resistance of the densest silver nanowire network increases sharply from the very start of the cyclic bending. A microstructural analysis reveals that the junction formation by mechanical welding in the early stages of bending is pronounced for the less dense silver nanowire networks, while the densest network showed severe deformation, including broken or disintegrated nanowires due to their confined geometry, which cannot easily accommodate the bending strain by stretching the network. Our findings underscore that different criteria should be considered to evaluate the mechanical reliability of various flexible electronic devices using silver nanowire electrodes with different densities.

97 percent light absorption in an ultrabroadband frequency range utilizing an ultrathin metal layer: randomly oriented, densely packed dielectric nanowires as an excellent light trapping scaffold.

In this paper, we propose a facile and large scale compatible design to obtain perfect ultrabroadband light absorption using metal-dielectric core-shell nanowires. The design consists of atomic layer deposited (ALD) Pt metal uniformly wrapped around hydrothermally grown titanium dioxide (TiO2) nanowires. It is found that the randomly oriented dense TiO2 nanowires can impose excellent light trapping properties where the existence of an ultrathin Pt layer (with a thickness of 10 nm) can absorb the light in an ultrabroadband frequency range with an amount near unity. Throughout this study, we first investigate the formation of resonant modes in the metallic nanowires. Our findings prove that a nanowire structure can support multiple longitudinal localized surface plasmons (LSPs) along its axis together with transverse resonance modes. Our investigations showed that the spectral position of these resonance peaks can be tuned with the length, radius, and orientation of the nanowire. Therefore, TiO2 random nanowires can contain all of these features simultaneously in which the superposition of responses for these different geometries leads to a flat perfect light absorption. The obtained results demonstrate that taking unique advantages of the ALD method, together with excellent light trapping of chemically synthesized nanowires, a perfect, bifacial, wide angle, and large scale compatible absorber can be made where an excellent performance is achieved while using less materials.

Spectroscopic ellipsometry of columnar porous Si thin films and Si nanowires

Columnar mesoporous Si thin films and dense nanowire (SiNW) carpets were investigated by spectroscopic ellipsometry in the visible-near-infrared wavelength range. Porous Si layers were formed by electrochemical etching while structural anisotropy was controlled by the applied current. Layers of highly oriented SiNWs, with length up to 4.1μm were synthesized by metal-assisted chemical etching. Ellipsometric spectra were fitted with different multi-layered, effective medium approximation-based (EMA) models. Isotropic, in-depth graded, anisotropic and hybrid EMA models were investigated with the help of the root mean square errors obtained from the fits. Ellipsometric-fitted layer thicknesses were also cross-checked by scanning electron microscopy showing an excellent agreement. Furthermore, in the case of mesoporous silicon, characterization also revealed that, at low current densities (<100mA/cm2), in-depth inhomogeneity shows a more important feature in the ellipsometric spectra than anisotropy. On the other hand, at high current densities (>100mA/cm2) this behavior turns around, and anisotropy becomes the dominant feature describing the spectra. Characterization of SiNW layers showed a very high geometrical anisotropy. However, the highest fitted geometrical anisotropy was obtained for the layer composed of ∼1μm long SiNWs indicating that for thicker layers, collapse of the nanowires occurs.

Hierarchical Branched Vanadium Oxide Nanorod@Si Nanowire Architecture for High Performance Supercapacitors

Vanadium oxide (VOx ) nanorods are uniformly synthesized on dense Si nanowire arrays. This 3D hierarchical nanoarchitecture offers a novel high-performance supercapacitor electrode design with significantly improved specific capacitance and high-rate capability.

Correction to Highly Dense Cu Nanowires for Low-Overpotential CO2 Reduction.

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to Highly Dense Cu Nanowires for Low-Overpotential CO2 ReductionDavid Raciti, Kenneth J. Livi, and Chao Wang*Cite this: Nano Lett. 2016, 16, 10, 6716Publication Date (Web):September 26, 2016Publication History Published online26 September 2016Published inissue 12 October 2016https://doi.org/10.1021/acs.nanolett.6b03897Copyright © 2016 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1749Altmetric-Citations4LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (99 KB) Get e-Alerts Get e-Alerts

High Density Crossbar Arrays with Sub- 15 nm Single Cells via Liftoff Process Only.

Emerging nano-scale technologies are pushing the fabrication boundaries at their limits, for leveraging an even higher density of nano-devices towards reaching 4F2/cell footprint in 3D arrays. Here, we study the liftoff process limits to achieve extreme dense nanowires while ensuring preservation of thin film quality. The proposed method is optimized for attaining a multiple layer fabrication to reliably achieve 3D nano-device stacks of 32 × 32 nanowire arrays across 6-inch wafer, using electron beam lithography at 100 kV and polymethyl methacrylate (PMMA) resist at different thicknesses. The resist thickness and its geometric profile after development were identified to be the major limiting factors, and suggestions for addressing these issues are provided. Multiple layers were successfully achieved to fabricate arrays of 1 Ki cells that have sub- 15 nm nanowires distant by 28 nm across 6-inch wafer.

Highly Dense Cu Nanowires for CO2 and CO Reduction

Electrochemical reduction of CO2, an artificial way of carbon recycling, represents one promising solution for energy and environmental sustainability. Despite the many advantages, electrochemical reduction of CO2 is challenged by the absence of efficient catalysts for this reaction. Copper (Cu) is the most studied material capable of catalyzing CO2 reduction at significant rates, but it still requires large overpotentials; e.g., to reach a current density of 1 mA/cm2 on polycrystalline Cu electrode, it typically requires an overpotential of >0.5 V for producing CO and HCOOH (two-electron processes) and >0.8 V for further reduced products such as CH4 and C2H4. Moreover, hydrogen evolution competes with CO2 reduction, reducing the Faradaic efficiency (FE) towards carbon-containing compounds. Here we report the synthesis of highly dense Cu nanowires and tailoring of their surface structures to achieve superior performance for electrocatalytic CO2 and CO reduction, with particular attention to low-overpotential conditions. CuO nanowires were first grown by oxidizing Cu mesh in air which were then subjected to electrochemical reduction or annealing in a reducing atmosphere to form Cu nanowires. By tailoring the conditions for the reduction, high activity and selectivity were achieved for the production of CO from CO2, and ethanol/acetate from CO, at E < 0.4 V. It was found that the catalytic performances of the Cu nanowires are strongly correlated to the crystalline and surface structures in the nanoscale, based on which the active sites capable of selective reduction of CO2 and CO were identified. Our work indicates the great potential of electrochemical conversion of CO2 for synthesis of fuel molecules and production of chemical feedstocks.

(Invited) Facile Horizontal GaAs Nanowire Schottky Solar Cells with Record High Efficiency

Nowadays, it is still technological challenging to fabricate solar cells with the high efficiency and low cost aimed for the third generation photovoltaics as well as for powering nano-devices by smart power supply. In this study, highly crystalline, long and dense GaAs nanowires (NWs) are easily synthesized on amorphous SiO2/Si substrates in the solid source chemical vapor deposition (CVD) system. And the single GaAs NWs are fabricated into Au-Al asymmetric Schottky contact solar cells [2] by a simple ultraviolet photolithography process. The NW synthesis avoids the use of expensive single crystalline epitaxial substrates such as GaAs(111)B used in metal-organic (MO) CVD or molecular beam epitaxy (MBE) methods [1], and device fabrication gets rid of the complicated p-i-n structures complementation. Besides these low-cost advantages of the NW synthesis and device fabrication, the horizontal NW solar cell show a record high efficiency of ~17% under air mass 1.5 global (AM 1.5G) illumination as shown in Figure 1. This produces another record in addition to the vertically aligned p-i-n GaAs NW solar cells previously reported (40%), in which the superior photovoltaic performance is attributed to the light concentrating effect of NWs with diameter in the order of light wavelength [3]. Moreover, the GaAs NWs can be printed onto glass and plastics to enhance the output power as well as provide the transparent and flexible solar cells as shown in Figure 1. All these results show the promising prospect of the high quality GaAs NWs synthesis in low cost solid source CVD system, and the highly efficient photovoltaic applications in the simple Schottky contact configuration. 1 N. Han, J. J. Hou, F. Y. Wang, S. Yip, Y. T. Yen, Z. X. Yang, G. F. Dong, T. Hung, Y. L. Chueh and J. C. Ho, ACS Nano 7 (2013) 9138. 2 N. Han, Z. Yang, F. Wang, G. Dong, S. Yip, X. Liang, T. F. Hung, Y. Chen and J. C. Ho, ACS Appl. Mater. Interfaces 7 (2015) 20454. 3 P. Krogstrup, H. I. Jorgensen, M. Heiss, O. Demichel, J. V. Holm, M. Aagesen, J. Nygard and A. F. I. Morral, Nat. Photonics 7 (2013) 306. Figure 1

Self-assembled growth of GaN nanowires on amorphous AlxOy: from nucleation to the formation of dense nanowire ensembles

We present a comprehensive description of the self-assembled nucleation and growth of GaN nanowires (NWs) by plasma-assisted molecular beam epitaxy on amorphous AlxOy buffers (a-AlxOy) prepared by atomic layer deposition. The results are compared with those obtained on nitridated Si(111). Using line-of-sight quadrupole mass spectrometry, we analyze in situ the incorporation of Ga starting from the incubation and nucleation stages till the formation of the final nanowire ensemble and observe qualitatively the same time dependence for the two types of substrates. However, on a-AlxOy the incubation time is shorter and the nucleation faster than on nitridated Si. Moreover, on a-AlxOy we observe a novel effect of decrease in incorporated Ga flux for long growth durations which we explain by coalescence of NWs leading to reduction of the GaN surface area where Ga may reside. Dedicated samples are used to analyze the evolution of surface morphology. In particular, no GaN nuclei are detected when growth is interrupted during the incubation stage. Moreover, for a-AlxOy, the same shape transition from spherical cap-shaped GaN crystallites to the NW-like geometry is found as it is known for nitridated Si. However, while the critical radius for this transition is only slightly larger for a-AlxOy than for nitridated Si, the critical height is more than six times larger for a-AlxOy. Finally, we observe that in fully developed NW ensembles, the substrate no longer influences growth kinetics and the same N-limited axial growth rate is measured on both substrates. We conclude that the same nucleation and growth processes take place on a-AlxOy as on nitridated Si and that these processes are of a general nature. Quantitatively, nucleation proceeds somewhat differently, which indicates the influence of the substrate, but once shadowing limits growth processes to the upper part of the NW ensemble, they are not affected anymore by the type of substrate.

Dry-growth of silver single-crystal nanowires from porous Ag structure

A fabrication method of single crystal Ag nanowires in large scale is introduced without any chemical synthesis in wet processes, which usually generates fivefold twinned nanowires of fcc metals. Dense single-crystal nanowires grow on a mechanically polished surface of micro-porous Ag structure, which is created from Ag micro-particles. The diameter and the length of the nanowires can be controlled simply by changing the temperature and the time of the heating during the nanowire growth in air. Unique growth mechanism is described in detail, based on stress-induced migration accelerated by the micro-porous structure where the origin of Ag nanowires growth is incubated. Transmission electron microscopy analysis on the single crystal nanowires is also presented. This simple method offered an alternative preparation for metallic nanowires, especially with the single crystal structure in numerous applications.

Template-Assisted Hydrothermal Growth of Aligned Zinc Oxide Nanowires for Piezoelectric Energy Harvesting Applications.

A flexible and robust piezoelectric nanogenerator (NG) based on a polymer-ceramic nanocomposite structure has been successfully fabricated via a cost-effective and scalable template-assisted hydrothermal synthesis method. Vertically aligned arrays of dense and uniform zinc oxide (ZnO) nanowires (NWs) with high aspect ratio (diameter ∼250 nm, length ∼12 μm) were grown within nanoporous polycarbonate (PC) templates. The energy conversion efficiency was found to be ∼4.2%, which is comparable to previously reported values for ZnO NWs. The resulting NG is found to have excellent fatigue performance, being relatively immune to detrimental environmental factors and mechanical failure, as the constituent ZnO NWs remain embedded and protected inside the polymer matrix.

Template-free fabrication of Ag nanowire arrays/Al2O3 assembly with flexible collective longitudinal-mode resonance and ultrafast nonlinear optical response

We utilized a co-sputtering technique without any templates, featuring growing and etching synchronously, to delicately fabricate dense and ultrafine Ag nanowire arrays/alumina matrix composite films. Both the diameter and separation distance of the Ag nanowire arrays in the composites are not only within the scope of sub-10 nm but also tunable, which is very hard to accomplish for the conventional optical lithography- or template-based method. It is exhibited that the collective longitudinal plasmon resonance of the composite films, covering a wide range from visible to the near infrared region, is extremely sensitive to the geometrical parameters of the Ag nanowires, owing to the strong plasmonic coupling among neighboring nanowires. The experimental observations were also theoretically supported by the near-field electromagnetic numerical simulation. More interestingly, the fabricated composite films demonstrated ultrafast nonlinear optical response in the visible light region under femtosecond laser excitation, possessing a short relaxation time of 1.45 ps for the longitudinal mode (L mode) resonance. These results indicate that the proposed composite films as a building block with exotic optical properties could provide an opportunity to construct integrated nanodevices for plasmonic optical applications.

Rapid nanosheets and nanowires formation by thermal oxidation of iron in water vapour and their applications as Cr(VI) adsorbent

Thermal oxidation of iron foil was done at 400°C and 500°C in for 2h to form multilayered oxide scale with outer oxide layer of α-Fe2O3 comprising of nanowires and nanosheets respectively. Iron oxidized at 300°C formed a rather compact film with no noticeable nanostructures. The morphologies of oxide formed in different oxidation environment (water vapour or dry air) were compared; densely packed nanostructures were produced in water vapour compared to dry air. Time variation study indicated rapid growth of nanostructure whereby for 1min at 500°C dense nanowires with some noticeable nanosheets were already observed. The nanowires and nanosheets were used to adsorb Cr(VI) from aqueous solution. Adsorption of 10ppm of Cr(VI) on the nanowires and nanosheets was found to be successful with much faster removal efficiency for the nanosheets. Both samples displayed complete adsorption for less than 1h.

Laser-driven absorption/desorption of catalysts for producing nanowire arrays in solution

Highly dense CdTe nanowire arrays were synthesized in solution by laser-driven absorption and desorption of gold catalysts.

Zero-bias photocurrents in highly-disordered networks of Ge and Si nanowires

Semiconducting nanowire (NW) devices have garnered attention in self-powered electronic and optoelectronic applications. This work explores and exhibits, for the first time for visible light, clear evidence of the zero-biased optoelectronic switching in randomly dispersed Ge and Si NW networks. The test bench, on which the NWs were dispersed for optoelectronic characterization, was fabricated using a standard CMOS fabrication process, and utilized metal contacts with dissimilar work functions—Al and Ni. The randomly dispersed NWs respond to light by exhibiting substantial photocurrents and, most remarkably, demonstrate zero-bias photo-switching. The magnitude of the photocurrent is dependent on the NW material, as well as the channel length. The photocurrent in randomly dispersed GeNWs was found to be higher by orders of magnitude compared to SiNWs. In both of these material systems, when the length of the NWs was comparable to the channel length, the currents in sparse NW networks were found to be higher than those in dense NW networks, which can be explained by considering various possible arrangements of NWs in these devices.

Growth And Field Emission Properties Of Vertically-aligned ZnO Nanowire Array On Biaxially Textured Ni-W Substrate By Thermal Evaporation

Vertically well aligned and highly dense ZnO nanowires (NWs) have been grown on biaxially textured Ni substrates by a simple thermal evaporation technique over a large area without using any catalyst. The grown ZnO NWs have crystallized in wurtzite hexagonal structure and have grown along [0001] direction. It is also observed that the degree of vertical alignment of NWs increases with increasing growth temperature. An intense photoluminescence peak at 383 nm with a negligible deep band emission revealed the good crystalline quality of ZnO NWs. Field emission properties of the grown NWs have been examined and a field enhancement factor of 1573 has been obtained, indicating the suitability of grown nanowires for field emission applications. Copyright © 2015 VBRI Press.

Synthesis and Characterization of ZnO Nanowires and ZnO–CuO Nanoflakes from Sputter-Deposited Brass (Cu0.65–Zn0.35) Film and Their Application in Gas Sensing

A novel method was presented for synthesis of ZnO and ZnO–CuO composites in the form of nanowires, nanorods and nanoflakes on oxidized silicon substrates. Further, the use of the synthesized nanostructures for gas sensing was demonstrated. Pure brass (Cu 0.65 –Zn 0.35 ) films were deposited on oxidized Si substrate by radio frequency (RF) diode sputtering. Subsequently, these films having thickness in the range of 100–200 nm were oxidized in different oxidizing ambient in the temperature range of 300–550 °C. The effect of temperature, time and oxidizing ambient on the growth of nanostructures was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and photoluminescence (PL) techniques. The nanostructures surface was analyzed by X-ray photoelectron spectroscopy (XPS). The synthesized nanowires had diameter in the range of 60–100 nm and length up to 50 µm. Based on these observations, the growth mechanism has been suggested. For the nanorods, the diameter was observed to be ~150 nm. Samples having dense nanowires, nanorods and nanoflakes were used as a gas sensing material. The performance of the sensor was investigated for different nanostructured materials for various volatile organic compounds (VOCs). It was observed that ZnO–CuO nanoflakes were more sensitive to VOC sensing compared to ZnO nanowires and nanorods.

Highly Dense Cu Nanowires for Low-Overpotential CO2 Reduction.

Electrochemical reduction of CO2, an artificial way of carbon recycling, represents one promising solution for energy and environmental sustainability. However, it is challenged by the lack of active and selective catalysts. Here, we report a two-step synthesis of highly dense Cu nanowires as advanced electrocatalysts for CO2 reduction. CuO nanowires were first grown by oxidation of Cu mesh in air and then reduced by either annealing in the presence of hydrogen or applying a cathodic electrochemical potential to produce Cu nanowires. The two reduction methods generated Cu nanowires with similar dimensions but distinct surface structures, which have provided an ideal platform for comparative studies of the effect of surface structure on the electrocatalytic properties. In particular, the Cu nanowires generated by electrochemical reduction were highly active and selective for CO2 reduction, requiring an overpotential of only 0.3 V to reach 1 mA/cm(2) electrode current density and achieving Faradaic efficiency toward CO as high as ∼60%. Our work has advanced the understanding of the structure-property relationship of Cu-based nanocatalysts, which could be valuable for the further development of advanced electrocatalytic materials for CO2 reduction.

Highly Dense ZnO Nanowires Grown on Graphene Foam for Ultraviolet Photodetection.

Growth of highly dense ZnO nanowires (ZnO NWs) is demonstrated on three-dimensional graphene foam (GF) using resistive thermal evaporation technique. Photoresponse of the as-grown hybrid structure of ZnO NWs on GF (ZnO NWs/GF) is evaluated for ultraviolet (UV) detection. Excellent photoresponse with fast response and recovery times of 9.5 and 38 s with external quantum efficiency of 2490.8% is demonstrated at low illumination power density of 1.3 mW/cm(2). In addition, due to excellent charge carrier transport, mobility of graphene reduces the recombination rate of photogenerated charge carriers, hence the lifetime of photogenerated free charge carriers enhances in the photodetectors.