Length Of Nanowires Research Articles

Carbon-Supported Pt and Pt–Ir Nanowires for Methanol Electro-Oxidation in Acidic Media

Direct methanol fuel cells are promising electrochemical energy conversion devices. But, more efficient and stable and less expensive catalysts are still required. Here, we successfully synthesized Pt/C and Pt0.5–Ir0.5/C, Pt0.6–Ir0.4/C, Pt0.7–Ir0.3/C, and Pt0.8–Ir0.2/C nanowires by the chemical reduction of the metallic precursors by formic acid and tested them towards methanol electro-oxidation in acidic media. Neither surfactants nor templates were used during the syntheses. The nanowires catalysts were compared with a commercial state-of-art catalyst aiming the observation of the properties improvements derived from both alloying Pt with Ir and morphology change from nanoparticles to nanowires. Well-defined and slightly agglomerated over the carbon nanowires (diameters and lengths of approximately 5 and 20 nm, respectively) were obtained, the fact that is ascribed to the 40 wt% metal loading. In addition, accelerated degradation tests showed that Pt0.6–Ir0.4/C, Pt0.7–Ir0.3/C and Pt0.8–Ir0.2/C catalysts are more stable than commercial Pt/C. All synthesized nanowires catalysts were more active towards methanol electro-oxidation than the commercial Pt/C. The Pt0.5–Ir0.5/C sample shows Pt mass activities 7 times that of commercial Pt/C. However, the Pt0.8–Ir0.2/C catalyst presented the best specific activity (6 times that of commercial Pt/C), have the highest currents in the derivative voltammetry and the oxidation potential shifts negatively 100 mV in comparison with the commercial Pt/C catalyst. Hence, the nanowires developed in this study are indicated as potential promising catalysts and can be applied successfully as direct methanol fuel cell anodes.

Impact of the Shadowing Effect on the Crystal Structure of Patterned Self-Catalyzed GaAs Nanowires.

The growth of regular arrays of uniform III-V semiconductor nanowires is a crucial step on the route toward their application-relevant large-scale integration onto the Si platform. To this end, not only does optimal vertical yield, length, and diameter uniformity have to be engineered, but also, control over the nanowire crystal structure has to be achieved. Depending on the particular application, nanowire arrays with varying area density are required for optimal device efficiency. However, the nanowire area density substantially influences the nanowire growth and presents an additional challenge for nanowire device engineering. We report on the simultaneous in situ X-ray investigation of regular GaAs nanowire arrays with different area density during self-catalyzed vapor-liquid-solid growth on Si by molecular-beam epitaxy. Our results give novel insight into selective-area growth and demonstrate that shadowing of the Ga flux, occurring in dense nanowire arrays, has a crucial impact on the evolution of nanowire crystal structure. We observe that the onset of Ga flux shadowing, dependent on array pitch and nanowire length, is accompanied by an increase of the wurtzite formation rate. Our results moreover reveal the paramount role of the secondary reflected Ga flux for VLS NW growth (specifically, that flux that is reflected directly into the liquid Ga droplet).

Degeneracy lifting of Majorana bound states due to electron-phonon interactions

We study theoretically how electron-phonon interaction affects the energies and level broadening (inverse lifetime) of Majorana bound states (MBSs) in a clean topological nanowire at low temperatures. At zero temperature, the energy splitting between the right and left MBSs remains exponentially small with increasing nanowire length $L$. At finite temperatures, however, the absorption of thermal phonons leads to the broadening of energy levels of the MBSs that does not decay with system length, and the coherent absorption/emission of phonons at opposite ends of the nanowire results in MBSs energy splitting that decays only as an inverse power-law in $L$. Both effects remain exponential in temperature. In the case of quantized transverse motion of phonons, the presence of Van Hove singularities in the phonon density of states causes additional resonant enhancement of both the energy splitting and the level broadening of the MBSs. This is the most favorable case to observe the phonon-induced energy splitting of MBSs as it becomes much larger than the broadening even if the topological nanowire is much longer than the coherence length. We also calculate the charge and spin associated with the energy splitting of the MBSs induced by phonons. We consider both a spinless low-energy continuum model, which we evaluate analytically, as well as a spinful lattice model for a Rashba nanowire, which we evaluate numerically.

In situ real-time annealing of ultrathin vertical Fe nanowires grown by focused electron beam induced deposition

Focused Electron Beam Induced Deposition is a consolidated technique for the growth of three-dimensional (3D) nanostructures. However, this single-step nanofabrication method requires further efforts to optimize simultaneously dimensional and compositional properties, in particular for deposits with a high aspect ratio. More specifically, ferromagnetic 3D nanowires (NWs) with diameters in the sub-50 nm regime and high metallic contents up to 95 at. % attract great interest to improve the final performance of magnetic nanodevices such as magnetic tips for scanning probe microscopy. In this work, we report on real-time monitoring during chemical purification and structural crystallization processes of ultra-narrow 3D Fe NWs (<50 nm in diameter achieved) by post-growth in situ annealing in a transmission electron microscope. NW heating up to 700 °C in very high vacuum reveals the local increase of the metallic content along the entire NW length concomitant with the growth of large Fe single crystals from initially amorphous compounds. A metallic purity of 95 at. % is observed in several regions, dramatically boosting the initial Fe content of 40 at. %. The real-time in situ tracking of 3D nanostructures during thermal annealing is a key element to design and optimize novel purification processes for the fabrication of customized components to be integrated in spintronic, logic and sensing devices.

One-Dimensional Cuprous Selenide Nanostructures with Switchable Plasmonic and Super-ionic Phase Attributes.

Cuprous selenide nanocrystals have hallmark attributes, especially tunable localized surface plasmon resonances (LSPRs) and super-ionic behavior. These attributes of cuprous selenide are now integrated with a one-dimensional morphology. Essentially, Cu2 Se nanowires (NWs) of micrometer-scale lengths and about 10 nm diameter are prepared. The NWs exhibit a super-ionic phase that is stable at temperatures lower than in the bulk, owing to compressive lattice strain along the radial dimension of the NWs. The NWs can be switched between oxidized and reduced forms, which have contrasting phase transition and LSPR characteristics. This work thus makes available switchable, one-dimensional waveguides and ion-conducting channels.

High Mobility Stemless InSb Nanowires.

High aspect-ratio InSb nanowires (NWs) of high chemical purity are sought for implementing advanced quantum devices. The growth of InSb NWs is challenging, generally requiring a stem of a foreign material for nucleation. Such a stem tends to limit the length of InSb NWs and its material becomes incorporated in the InSb segment. Here, we report on the growth of chemically pure InSb NWs tens of microns long. Using a selective-area mask in combination with gold as a catalyst allows complete omission of the stem, thus demonstrating that InSb NWs can grow directly from the substrate. The introduction of the selective-area mask gives rise to novel growth kinetics, demonstrating high growth rates and complete suppression of layer deposition on the mask for Sb-rich conditions. The crystal quality and chemical purity of these NWs is reflected in the significant enhancement of low-temperature electron mobility, yielding an average of 4.4 × 104 cm2/(V s), compared to previously studied InSb NWs grown on stems.

High velocity domain wall propagation using voltage controlled magnetic anisotropy

The use of voltage-controlled magnetic anisotropy (VCMA) via the creation of a sloped electric field has been hailed as an energy-efficient approach for domain wall (DW) propagation. However, this method suffers from a limitation of the nanowire length which the DW can propagate on. Here, we propose the use of multiplexed gate electrodes to propagate DWs on magnetic nanowires without having any length constraints. The multi-gate electrode configuration is demonstrated using micromagnetic simulations. This allows controllable voltages to be applied to neighboring gate electrodes, generating large strength of magnetic anisotropy gradients along the nanowire, and the results show that DW velocities higher than 300 m/s can be achieved. Analysis of the DW dynamics during propagation reveals that the tilt of the DW and the direction of slanted gate electrode greatly alters the steady state DW propagation. Our results show that chevron-shaped gate electrodes is an effective optimisation that leads to multi-DW propagation with high velocity. Moreover, a repeating series of high-medium-low magnetic anisotropy regions enables a deterministic VCMA-controlled high velocity DW propagation.

Evolution of the Length and Radius of Catalyst-Free III-V Nanowires Grown by Selective Area Epitaxy.

We present a new model for the length and radius evolution of catalyst-free III–V nanowires grown by selective area epitaxy. We consider simultaneous axial and radial growth of nanowires, which is more typical for this technique compared to the vapor−liquid−solid growth of nanowires. Analytic expressions for the time evolution of the nanowire length and radius are derived, showing the following properties. As long as the nanowire length is shorter than the collection length of group III atoms on the sidewalls, the length evolves superlinearly and the radius evolves linearly with time. For longer nanowires, both the length and radius increase sublinearly with time. The scaling growth laws are controlled by a single parameter that depends on group V flux. The model fits well the data on the selective area growth of InAs and GaAs nanowires by different techniques. Overall, these results can be used for controlling the catalyst-free growth of III–V nanowires and their morphology, including ternary III–V material systems.

Interfacial optimization of SiC nanocomposites reinforced by SiC nanowires with high volume fraction

AbstractHigh volume fraction SiC nanowires‐reinforced SiC composites (SiCNWs/SiC) were prepared by hybrid process of chemical vapor infiltration and polymer impregnation/pyrolysis in this research. SiCNWs networks are first to be made promising a high volume fraction (20 vol%), and the pyrolytic carbon (PyC) interphase with 5 nm is designed on SiCNWs surface to optimize the bonding condition between SiCNWs and SiC matrix. Nanoindentation shows a modulus of 494 ± 14 GPa of SiCNWs/SiC composites without interphase comparing to the one with PyC interphase of 452 ± 13 GPa. However, the 3‐point bending test shows a higher strength of the composite with PyC interphase (273 ± 32 MPa) comparing with the one without interphase (240 ± 38 MPa). The fracture surface is observed under SEM, which shows a longer SiCNWs pullout of the composite with PyC interphase. The energy dissipation during the 3‐point bending test is calculated by the length of nanowire pull‐out, it demonstrates that the SiCNWs with PyC interphase possess better performance for toughening composite. Further characterization proves that the PyC interphase can give SiCNWs/SiC composites higher fracture toughness (4.49 ± 0.44 MPa·m1/2) than the composites without interphase (3.66 ± 0.28 MPa·m1/2).

Light Management in Organic Photovoltaics Processed in Ambient Conditions Using ZnO Nanowire and Antireflection Layer with Nanocone Array.

Low carrier mobility and lifetime in semiconductor polymers are some of the main challenges facing the field of organic photovoltaics (OPV) in the quest for efficient devices with high current density. Finding novel strategies such as device structure engineering is a key pathway toward addressing this issue. In this work, the light absorption and carrier collection of OPV devices are improved by employment of ZnO nanowire (NW) arrays with an optimum NW length (50 nm) and antireflection (AR) film with nanocone structure. The optical characterization results show that ZnO NW increases the transmittance of the electron transporting layer as well as the absorption of the polymer blend. Moreover, the as-deposited polymer blend on the ZnO NW array shows better charge transfer as compared to the planar sample. By employing PC70BM:PV2000 as a promising air-stable active-layer, power conversion efficiencies of 9.8% and 10.1% are achieved for NW devices without and with an AR film, indicating 22.5% and 26.2% enhancement in PCE as compared to that of planar device. Moreover, it is shown that the AR film enhances the water-repellent ability of the OPV device.

Fixed vortex domain wall propagation in FeNi/Cu multilayered nanowire arrays driven by reversible magnetization evolution

While studying domain wall (DW) propagation in magnetic nanowires (NWs) may pave the way for future research and technological applications in recording heads and novel sensors, no attention has been paid to the investigation of magnetic reversal modes in multilayered NWs using angular first-order reversal curve (AFORC) analysis. Here, the magnetization reversal process of uniform FeNi/Cu NW arrays with a diameter of 45 nm electrodeposited in the anodic aluminum oxide template is systematically studied by AFORC analysis for the field angle θ (0° ≤ θ ≤ 90°) and compared with the average magnetic behavior of reversal modes based on conventional hysteresis loop measurements. The FeNi segment aspect ratio is kept constant at about 5, whereas the Cu segment length (LCu) increases from 2.5 to 25 nm. AFORC coercivity increases continuously with increasing θ, indicating that the NWs reverse their magnetization by nucleation and propagation of vortex DW (VDW). At θ = 0°, the respective hysteresis loop coercivity and magnetostatic coupling between FeNi segments along the NW length are reduced by increasing LCu from 2.5 to 25 nm, resulting in an enhancement in the reversible fraction of NWs from 10% to 48%. However, the VDW reversal mode is not influenced by the increase in NW reversibility as a function of θ for the different LCu, which arises from constant properties of the FeNi segments. The AFORC analysis of the reversal mechanism is also found to be in agreement with recent angle-dependent anisotropic magnetoresistance measurements in single multilayered NWs.

Length-controlled and selective growth of individual indium nitride nanowires by localized laser heating

Length-controlled indium nitride (InN) nanowires are grown by localized laser-assisted metal organic vapor phase epitaxy. Laser irradiation results in spatially confined, rapid heating that enables precise nucleation control and subsequent nanowire growth. This localization of the laser-driven growth can realize on-demand and site-selective direct synthesis of length-controlled nanowires on catalytic gold nanodots. The length of the InN nanowires is controlled by the laser irradiation time at a fixed power. Energy dispersive X-ray, Raman and photoluminescence spectroscopic analyzes respectively characterize the elemental composition, crystallinity, and emission properties of as-grown InN nanowires.

High-Yield Synthesis of Long Silver Nanowires via Chromic Chloride and a Stable Reaction Environment

The search for suitable synthesis methods and parameters capable of controlling the length, diameter, and yield of silver nanowires (AgNWs) is still an emerging strategy today. Therefore, a method for high-yield synthesis of long AgNWs via chromic chloride and a stable reaction environment was proposed. The results show that Cr3+ could restore the adsorbed atomic oxygen quickly and provide a high efficiency in the prevention of the oxidative etching, for the ion of Cr2+ oxidized to Cr3+ has a lower standard electrode potential, and a more stable reaction environment provided by the coupling method could avoid disturbing the growth of the {111} reactive sites of the wires; then, the yield and length of the AgNWs were improved. The length of the AgNWs was over 75 μm and even 160 μm; the yield of the AgNWs was over 90%, which provides the referable basis for the synthesis of ultralong AgNWs.

Level-expansion: A statistical sequential design methodology with application to nanomaterial synthesis

Nanotechnology is an era-defining breakthrough across science and engineering. For example, one-dimensional nanostructures such as nanowires, nanotubes, and nanobelts are widely regarded as critical building blocks for creating the next generation of devices in electronics, optics, energy, and biomedicine. Motivated by a practical problem of sequential synthesis of nanowires, we propose a new statistical design augmentation method, called level-expansion. For a fractional factorial design at two levels, this method creates a follow-up design by expanding some of the factors in the initial design to four elaborately chosen levels and reversing the signs of the remaining factors. The augmented design produced as such strikes a fine balance between dealiasing and entertaining nonlinear effects. Some statistical properties of the proposed method are derived. The effectiveness of the proposed method is successfully illustrated with a case study for growing a type of zinc-oxide nanowire. The use of level-expansion in the case study unveils some previously unknown nonlinear relationships between the concentration of polyethyleneimine and the length of nanowires. This finding is important for nanoscientists to invent new zinc-oxide nanowires with better macroscopic transport properties. Besides nanotechnology, the proposed method applies broadly to problems in many other scientific fields with similar traits where a follow-up design is needed for the dual purposes of investigating nonlinear effects and dealising.

Fabrication of large-area ZnO nanowire field emitter arrays by thermal oxidation for high-current application

Large-area ZnO nanowire field emitter arrays have important applications in flat-panel displays, light sources, photodetectors and X-ray sources, where it is crucial to realize a high field emission current and current density over a large-area. In this paper, large-area ZnO nanowire arrays with various array spacings were prepared by the thermal oxidation technique from patterned Zn film arrays, and their field emission properties were studied. The results show that the Zn array density is related to the nanowire length and population density. A mechanism is proposed wherein the stress-induced process and mass transport process both play key roles in the growth. Herein, ZnO nanowire arrays in a 4.7 × 4.7 cm2 area exhibited a sufficient quantity of emission sites and diminished field screening effect and a high current of 12.9 mA with a current density of 0.58 mA/cm2. This work provides useful guidance for optimizing ZnO nanowires and enhancing field emission current performance for large-area ZnO nanowire field emission devices.

Oriented layered Bi2O2Se nanowire arrays for ultrasensitive photodetectors

Due to its high carrier mobility, superior air stability, and intriguing self-modulation doping effects, Bi2O2Se has shown great potential for applications in high performance field-effect transistors and infrared photodetectors. However, Bi2O2Se generally tends to form 2D micromorphology because of its native layered structure. Here, we developed a space-confined CVD method to realize the epitaxial growth of highly oriented 1D Bi2O2Se arrays. The controllable length and width of the vertical growth nanowires mainly distribute in the ranges of 30–42 μm and 120–160 nm, respectively. The orientation of Bi2O2Se nanowires was determined by the epitaxial relationship between Bi2O2Se {110} and mica {00n} planes. In addition, various morphologies of Bi2O2Se including 1D nanowires, 2D nanosheets, and rectangular nanosheets can be controllably synthesized by precisely modifying the growth temperature. The integrated detector based on the as-synthesized Bi2O2Se nanowire arrays demonstrated excellent performances in the spectrum from the UV to near-infrared regime. The responsivity, detectivity, external quantum efficiency, and response time can reach up to 722.2 A W−1, 5.64 × 1011 Jones, 189 000%, and 0.267 ms, respectively. These findings manifest that the oriented Bi2O2Se nanowire arrays have great prospects in ultrafast and near-infrared photodetection applications.

Constructing Metal Arch Nanobridges Utilizing a Photothermal‐Induced Nanobonding Technique

AbstractConstruction of multilayered arch nanobridges and nanocantilever structures consisting of silver nanowires using a photothermal‐induced nanobonding technique is demonstrated. The fabricated nanobridges are of different height (300 nm to 20 µm) and span (25–70 µm). Their current–voltage characteristic curves indicate superior electrical connection between the metal nanowires and the abutments (gold thin film). Moreover, super mechanical strength of these arch nanobridges and nanocantilevers is demonstrated by putting gold nanoplates (a few tens of nm thick, a few tens of µm in diameter, and a maximum weight of 3 pN—about 20 times heavier than the silver nanowire of same length) on top of these nanostructures. The multilayered arch nanobridges demonstrate their potential applicability as short‐circuit‐free multipronged electronic connections in nanoelectronic devices. Furthermore, by recording the change in vibration frequency due to proximity of foreign contamination, these nanobridges and the nanocantilevers can be utilized to identify them; that is, these nanostructures can be used as biological sensors to detect viruses, bacteria, and other pathogens.

Nanowire Length, Density, and Crystalline Quality Retrieved from a Single Optical Spectrum.

We propose spectral domain attenuated reflectometry (SDAR) for fast characterization of nanomaterial growth. The method is demonstrated here for zinc oxide (ZnO) nanowires (NWs) which are grown vertically in random forest fashion showing that it is not limited to well-ordered NWs. We show how SDAR can provide, on the basis of a single measured spectrum, simultaneous information on nanowire length, nanowire density (through nanowire/air filling ratio), and crystalline quality (through band gap). The robustness of the proposed method is assessed first through comparison with information obtained from SEM and XRD taken as reference. In SDAR, the process for fast extraction of NW thickness and filling ratio values makes use of the interference pattern contrast and the spectral periodicity in the reflection response which involve a best fit of the measured spectra with simple theoretical modeling based on the effective medium approach, achieved with a mean square error down to 0.1%. The results also suggest the existence of either 2 or 3 layers of different effective refractive index, hence providing insight on possible growth mechanisms.

Sophorolipid induced hydrothermal synthesis of Cu nanowires and its modulating effect on Cu nanostructures

Cu nanowires are important class of one dimensional metal nanowires with high electrical conductivity comparable to silver nanowires. Sophorolipid (SL), a well-known biosurfactant and glycolipid, plays significant role in the synthesis of nanomaterials as reducing and capping agent. Herein, we report the synthesis of Cu nanowires using sophorolipid by alkyl amine mediated hydrothermal synthesis and in this process, octadecylamine (ODA) used as alkyl amine. Cu nanowires of length ∼5−40μm and width ∼50−250 nm are produced at SL/ODA weight ratio of 0.13:1. Further structural investigations suggest that the morphology of these Cu nanostructures changes from nanowires to micron size rods and decahedron particles by modulating the weight ratio of SL and ODA.

Dielectric permittivity and microwave absorption properties of SiC nanowires with different lengths

SiC nanowires with different lengths were prepared, and the effect of nanowire length on their dielectric permittivity and microwave absorption properties were investigated. The results reveal that SiC nanowires with longer length can improve the electrical conductivity of microwave absorbing materials. This may be attributed to the fact that longer nanowires are more easily to interconnect with each other, which facilitates the construction of a more complete electrical conduction network in the matrix. Cole-Cole curves also indicate that with increasing nanowire length, the dominant loss mechanism of SiC nanowires is changed from polarization loss to conductance loss. Therefore, the conductivity-dependent dielectric permittivity and microwave absorbing ability of SiC nanowires were significantly improved with the increase in the nanowire lengths.