Typical Nanowires Research Articles

Nontrivial Topological Properties and Synthesis of Sn2CoS with L21 Structure.

We synthesize Sn2CoS in experiment and study its topological properties in theory. By first-principles calculations, we study the band structure and surface state of Sn2CoS with L21 structure. It is found that the material has type-II nodal line in the Brillouin zone and clear drumhead-like surface state when the spin-orbit coupling is not considered. In the case of spin-orbit coupling, the nodal line will open gap, leaving the Dirac points. To check the stability of the material in nature, we synthesize Sn2CoS nanowires with L21 structure in an anodic aluminum oxide (AAO) template directly by the electrochemical deposition (ECD) method with direct current (DC). Additionally, the diameter of the typical Sn2CoS nanowires is about 70 nm, with a length of about 70 μm. The Sn2CoS nanowires are single crystals with an axis direction of [100], and the lattice constant determined by XRD and TEM is 6.0 Å. Overall, our work provides realistic material to study the nodal line and Dirac fermions.

The optical absorption of nanowires with hexagonal cross-sections

We study the optical absorption of semiconductor nanowires with hexagonal cross-sections, such as most wurtzite nanowires. The typical semiconductor nanowires have obvious quantum confinement in the cross-sections. We focus on the extent to which this kind of nanowires could be approximated as cylindrical ones. A numerical method is developed to calculate the optical absorption in such system. It is found that whether or not the approximation of the hexagonal cross-section as a circular one is valid depends on which energy levels are mainly considered. The analysis of the linear absorptions concludes that only those subbands corresponding to the energy levels in the cross sections having hexagonal symmetry can be optically excited to. The results are helpful for the optical applications of nanowires based on their fine energy structures.

Nanomodified Rapid Hardening Concretes

One of the priority areas of modern construction production is the introduction of high-performance high-speed concrete with improved technological and construction-technical properties for the design, erection and repair of engineering structures. Modern studies have made it possible to view concrete from the nanoscale point of view as a material characterized by a complex hetero-scale structure of hydrated cement phases and mineral additives. The properties of concrete are determined by the type, size and nature of the interaction of the components of each structural level, which creates the possibility of nanotechnological regulation of the processes of structure formation and control of operational characteristics. The main task of nanomodification is to provide managed structure with more nanoscale hydration products. Typical nanowires in Portland cement systems are micro- and nanosilica SiO2, nanoglosses, TiO2, Al2O3, Fe2O3, CaCO3, carbon nanomaterials (carbon nanotubes and nanofibers). The introduction of C-S-H nanoparticles is the most effective way of accelerating the hydration of cement compared to other nanowires, by ensuring the growth of hydration products without an energy barrier in the pore space between the cement grains. The presented studies are devoted to the study of the nanomodification efficiency of concrete with a complex additive containing a polycarboxylate superplasticizer and synthesized nanoscale C-S-H particles. Based on the analysis of the results of early and design strength of nanomodified concrete, it is established that it is characterized by rapid strength gain and high durability after 28 days. The construction and technical properties of nanomodified concretes were investigated after 1 and 28 days of curing.

Frequency behavior of ultrasmall sensors using vibrating SMA nanowire-reinforced sheets under a non-uniform biaxial preload

Initial stresses can change the resonant frequency behaviour of ultrasmall structures such as small-scale beams and sheets. As a result, initial stresses can affect the size-dependent performance of ultrasmall devices such as small-scale mechanical mass sensors. In the present article, the effects of a biaxial initial stress on the frequency behavior of a small-scale mechanical sensor employing a vibrating shape memory alloy (SMA) nanowire-reinforced sheet are analyzed. The hybrid mechanical sensor consists of three various reinforced sheets. Nanowires (NWs) with SMA effects are utilized for reinforcing the mid-sheet whereas the upper and lower sheets are strengthened by typical NWs. As practical situations often include an elastic substrate, the hybrid sensor is embedded in an elastic foundation. It is assumed that the initial stress is non-uniform across the thickness of the hybrid sheet. Using the nonlocal theory of plates, a set of coupled motion equations is derived for the frequency behaviour of the mechanical sensor. Employing Galerkin’s technique, the frequency shifts are extracted. The results would be helpful in designing ultrasmall sensors employing SMA nanowires.

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The influence of elastic stresses on the formation of axial heterojunctions in ternary III-V nanowires was theoretically investigated. The composition profiles of InAs/GaAs axial heterojunction in self-catalyzed GaxIn1-xAs nanowires were obtained. It was shown that the InAs/GaAs heterojunction width amounts to tens of monolayers and increases with the increase of the nanowire radius due to elastic stresses. The elastic stress relaxation on nanowire sidewalls does not lead to the presence of miscibility gap in GaxIn1-xAs system at typical growth temperature (about 450ºC) and nanowire radius above 5 nm.

Intrinsic Jitter in Photon Detection by Straight Superconducting Nanowires

Timing jitter inherent in photon detection by superconducting nanowire single-photon detectors has different values and behaves differently for detection events originating in bends and in straights of nanowires. Generally, jitter is larger for events in bends. Although, for typical meandering nanowire, contribution of bends to the integral jitter is almost negligible due to small geometric weight of bends in the meander, it reduces the accuracy of extracting the value of local jitter in straights. Here we report on the intrinsic jitter, which was measured in a straight nanowire without bends. Standard deviation in the intrinsic jitter for detection events in the straight nanowire is smaller than 6.5 ps for photons with the wavelength 794 nm and 7.7 ps for 1560 nm. This value is less than jitter magnitudes in straights, which were extracted from the jitter measured previously for the meandering nanowire. Coupling of photons to the nanowire through sufficiently long optical fiber increases integral jitter and causes asymmetry in the jitter histogram. However, this optical asymmetry differs qualitatively from the asymmetry caused by the detection process itself at small photon energies.

Synthesis and characterization of superconducting FeSe nanowires

Iron selenide (FeSe) nanowires have been synthesized on the large-scale by a facile surface-assisted vapor-solid reaction. The typical nanowires prepared with Fe foil/Se molar ratio of 1: 0.1 at 550 °C for 12 h have a width of ∼32 nm, a thickness of ∼5 nm, and a length up to 30 μm. The nanowires are indexed as β-FeSe (tetragonal) with trace amounts of α-FeSe (hexagonal) by X-ray diffraction (XRD). High resolution transmission electron microscope of a single nanowire shows intergrowth between β- and α-FeSe. The electric transportation measurements shows that the nanowires are superconductors of Tc0 = 9.2 K (critical temperature of zero resistance). Suitable growth conditions of the nanowires are discussed.

A nonlocal continuum model for the biaxial buckling analysis of composite nanoplates with shape memory alloy nanowires

In this study, the buckling behavior of a three-layered composite nanoplate reinforced with shape memory alloy (SMA) nanowires is examined. Whereas the upper and lower layers are reinforced with typical nanowires, SMA nanoscale wires are used to strengthen the middle layer of the system. The composite nanoplate is assumed to be under the action of biaxial compressive loading. A scale-dependent mathematical model is presented with the consideration of size effects within the context of the Eringen’s nonlocal continuum mechanics. Using the one-dimensional Brinson’s theory and the Kirchhoff theory of plates, the governing partial differential equations of SMA nanowire-reinforced hybrid nanoplates are derived. Both lateral and longitudinal deflections are taken into consideration in the theoretical formulation and method of solution. In order to reduce the governing differential equations to their corresponding algebraic equations, a discretization approach based on the differential quadrature method is employed. The critical buckling loads of the hybrid nanosystem with various boundary conditions are obtained with the use of a standard eigenvalue solver. It is found that the stability response of SMA composite nanoplates is strongly sensitive to the small scale effect.

Free and forced vibrations of nanowires on elastic substrates

This work aims to study the free and forced transverse vibrations of a nanowire on elastic substrate, in a systematic way. To this end, the governing equations are obtained from an updated mechanical model integrating the effects of surface and elastic substrate. The Winkler, Pasternak and Generalized substrate models are taken into consideration to characterize various substrates. The characteristic equations, mode shapes and effective Young’s moduli are determined for nanowires with three typical boundary conditions: simply supported, clamped-clamped and clamped-free. The Laplace transform is employed to derive the exact solutions for forced vibrations of the three typical nanowires. Numerical calculations are performed to validate the proposed solutions and to analyze the effects of surface and elastic substrates on the free and forced vibrations.

Ultra-long life nickel nanowires@nickel-cobalt hydroxide nanoarrays composite pseudocapacitive electrode: Construction and activation mechanism

A three dimensional hierarchical composite electrode composed of nickel nanowires@nickel-cobalt double hydroxide aligned on nickel foam is successfully fabricated by an in-situ growth route. In this composite electrode, the nickel nanowires are used as core materials to support high-pseudocapacitance nickel-cobalt double hydroxide and construct the typical Ni nanowires core@Ni-Co double hydroxide shell hierarchical structure on Ni foam. Compared with Ni-Co double hydroxide/Ni foam electrode (without nickel nanowires), the electrochemical performance of the obtained composite electrode is apparently improved, which shows a high areal specific capacitance (2.25 F cm−2 at 5 mA cm−2), excellent rate capability (1.65 F cm−2 at 100 mA cm−2) and superior cycle stability (151.2% retained in the 20000th cycle). And the activation mechanism of ultra-long cycle life of the obtained composite electrode is studied in this work, which indicates that the transforms of surface atoms on Ni nanowires into high-pseudocapacitance Ni(OH)2 and NiOOH during cycles make major contributions for the performance improvement.

Majorana states in prismatic core-shell nanowires

We consider core-shell nanowires with conductive shell and insulating core and with polygonal cross section. We investigate the implications of this geometry on Majorana states expected in the presence of proximity-induced superconductivity and an external magnetic field. A typical prismatic nanowire has a hexagonal profile, but square and triangular shapes can also be obtained. The low-energy states are localized at the corners of the cross section, i.e., along the prism edges, and are separated by a gap from higher energy states localized on the sides. The corner localization depends on the details of the shell geometry, i.e., thickness, diameter, and sharpness of the corners. We study systematically the low-energy spectrum of prismatic shells using numerical methods and derive the topological phase diagram as a function of magnetic field and chemical potential for triangular, square, and hexagonal geometries. A strong corner localization enhances the stability of Majorana modes to various perturbations, including the orbital effect of the magnetic field, whereas a weaker localization favorizes orbital effects and reduces the critical magnetic field. The prismatic geometry allows the Majorana zero-energy modes to be accompanied by low-energy states, which we call pseudo Majorana, and which converge to real Majoranas in the limit of small shell thickness. We include the Rashba spin-orbit coupling in a phenomenological manner, assuming a radial electric field across the shell.

Functionalized porous Si nanowires for selective and simultaneous electrochemical detection of Cd(II) and Pb(II) ions

Thiol and amino-functionalized porous Si nanowires have been demonstrated to selectively detect Cd(II) and Pb(II) with high sensitivity using square wave anodic stripping voltammetry (SWASV) analysis. Through a typical Ag-assisted chemical etching approach, uniform and porous Si nanowires have been fabricated on heavily doped Si wafer. Continuously functionalized by (3-mercaptopropyl)trimethoxysilane (MPTMS) and (3-aminopropyl)triethoxysilane (APTES), the internal and external surfaces of porous Si nanowires have been decorated with thiol and amino groups, respectively. Inspired by the intrinsic properties of porous nanostructures, their electrochemical performances toward heavy metal ions have been further evaluated. The results indicate that Cd(II) and Pb(II) can be detected with high sensitivity owing to the strong complexing capacity of thiol and amino groups. Additionally, porous Si nanowires decorated with thiol groups present different stripping behaviors from those with amino groups toward the investigated heavy metal ions, realizing the selective and simultaneous detection of Cd(II) and Pb(II). Furthermore, simultaneous detection of Cd(II) and Pb(II) has also been demonstrated without any interference and their individual high sensitivity has been fundamentally preserved. Expectedly, porous Si nanowires as an effective modifier can be further extended to selectively detect other heavy metal ions through modifying with other functional organic groups.

Single-electron transport in InAs nanowire quantum dots formed by crystal phase engineering

We report electrical characterization of quantum dots formed by introducing pairs of thin wurtzite (WZ) segments in zinc blende (ZB) InAs nanowires. Regular Coulomb oscillations are observed over a wide gate voltage span, indicating that WZ segments create significant barriers for electron transport. We find a direct correlation of transport properties with quantum dot length and corresponding growth time of the enclosed ZB segment. The correlation is made possible by using a method to extract lengths of nanowire crystal phase segments directly from scanning electron microscopy images, and with support from transmission electron microscope images of typical nanowires. From experiments on controlled filling of nearly empty dots with electrons, up to the point where Coulomb oscillations can no longer be resolved, we estimate a lower bound for the ZB-WZ conduction-band offset of 95 meV.

Agglomeration in Porous Silicon Prepared from Si-Nanowires Structures

Silicon nanowires (SiNW’s) arrays prepared by electroless etching method are attractive due to their significant light absorption properties across the visible spectrum [1]. Typical silicon nanowires grown by electroless etching is shown in Figure 1a. Following this preparation of silicon nanowires, these samples are etched in an electrolytic HF and ethanol solution to produce porous silicon quantum confined structures. Mechanical behavior of materials at nanoscale is different due to their quantum confinement effect. When porous silicon nano-structures are prepared from SiNWs obtained through electroless process, they may have a tendency to become thin. Ultimately these nano-structures may collapse and form bunches on the surface [2], as shown in Figure 1b. However, bunched silicon nanowires structures may be desirable in some applications but may not be for some others. In order to avoid bunching of SiNWs, it may be necessary to reduce the surface tension during the growth of nanowires [3]. (a) (b) Fig 1. Cross-section images of silicon nanowires (a) grown by electroless etching, (b) Porous silicon structures prepared from SiNWs. SiNW’s were grown by eletroless etching in a solution of hydrofluoric acid (HF) and silver nitrate (AgNO3) at room temperature [4]. The prepared nanowires were then etched in an electrolytic solution containing HF and ethanol to make porous Si quantum confined structures. Reflectance and photoluminescence studies are performed on this structures Figure 1

In-plane trapping and manipulation of ZnO nanowires by a hybrid plasmonic field.

In general, when a semiconductor nanowire is trapped by conventional laser beam tweezers, it tends to be aligned with the trapping beam axis rather than confined in the horizontal plane, and this limits the application of these nanowires in many in-plane nanoscale optoelectronic devices. In this work, we achieve the in-plane trapping and manipulation of a single ZnO nanowire by a hybrid plasmonic tweezer system on a flat metal surface. The gap between the nanowire and the metallic substrate leads to an enhanced gradient force caused by deep subwavelength optical energy confinement. As a result, the nanowire can be securely trapped in-plane at the center of the excited surface plasmon polariton field, and can also be dynamically moved and rotated by varying the position and polarization direction of the incident laser beam, which cannot be performed using conventional optical tweezers. The theoretical results show that the focused plasmonic field induces a strong in-plane trapping force and a high rotational torque on the nanowire, while the focused optical field produces a vertical trapping force to produce the upright alignment of the nanowire; this is in good agreement with the experimental results. Finally, some typical ZnO nanowire structures are built based on this technique, which thus further confirms the potential of this method for precise manipulation of components during the production of nanoelectronic and nanophotonic devices.

Electronic and thermal transport study of sinusoidally corrugated nanowires aiming to improve thermoelectric efficiency

Improvement of thermoelectric efficiency has been very challenging in the solid-state industry due to the interplay among transport coefficients which measure the efficiency. In this work, we modulate the geometry of nanowires to interrupt thermal transport with causing only a minimal impact on electronic transport properties, thereby maximizing the thermoelectric power generation. As it is essential to scrutinize comprehensively both electronic and thermal transport behaviors for nano-scale thermoelectric devices, we investigate the Seebeck coefficient, the electrical conductance, and the thermal conductivity of sinusoidally corrugated silicon nanowires and eventually look into an enhancement of the thermoelectric figure-of-merit from the modulated nanowires over typical straight nanowires. A loss in the electronic transport coefficient is calculated with the recursive Green function along with the Landauer formalism, and the thermal transport is simulated with the molecular dynamics. In contrast to a small influence on the thermopower and the electrical conductance of the geometry-modulated nanowires, a large reduction of the thermal conductivity yields an enhancement of the efficiency by 10% to 35% from the typical nanowires. We find that this approach can be easily extended to various structures and materials as we consider the geometrical modulation as a sole source of perturbation to the system.

Ballistic-diffusive heat conduction in multiply-constrained nanostructures

Ballistic-diffusive heat conduction in multiply-constrained nanostructures is theoretically studied based on the phonon Boltzmann transport equation. The results show that different constraints influence the thermal transport in different ways. In the direction parallel to the heat flow, the phonon ballistic transport can cause temperature jumps at the boundaries in contact with the phonon baths. In contrast, for lateral constraint, the heat flux is reduced near the boundaries due to phonon-boundary scattering. A thermal conductivity model for multiply-constrained nanostructures is then derived from the phonon Boltzmann transport equation. The influences of different constraints are combined on the basis of Matthiessen's rule. The model accurately characterizes the thermal conductivities of various typical nanostructures, including nanofilms (in-plane and cross-plane) and finite length nanowires of various cross-sectional shapes (e.g. circular and rectangular). The model predictions also agree well with Monte Carlo simulations and experimental data for silicon nanofilms and nanowires.

Comparative Study of Fe‐Doped ZnO Nanowire Bundle and Their Thin Film for NO2 and CH4 Gas Sensing

SummaryThe free standing Fe‐doped ZnO nanowire bundle is synthesized by employing a typical vapor phase transport method in single step from Fe+Zn powder in O2+Ar flow. Fe‐doped ZnO nanowires are characterized by Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS) suggesting formation of hexagonal wurtzite phase of ZnO with 7 at% Fe in ZnO with typical nanowire diameter in the range of 20 nm–70 nm. These nanowire bundles are ultrasonicated and processed into thin film using spin coating technique. Both of the Fe‐ doped ZnO nanowire bundle and thin films are tested for their NO2 and CH4 (20–100 ppm) sensing properties. The thin film exhibited higher operating temperature (300°C) and response time (45 sec) as compared to nanowire bundle (150°C and 15 sec).

In Situ TEM Observations of Sn-Containing Silicon Nanowires Undergoing Reversible Pore Formation Due to Fast Lithiation/Delithiation Kinetics

In situ transmission electron microscopy (TEM) studies were carried out to observe directly in real time the lithiation and delithiation of silicon (Si) nanowires with significant amounts of tin (Sn). The incorporation of Sn significantly enhances the lithiation rate compared to typical Si nanowires: surface diffusion is enhanced by 2 orders of magnitude and the bulk lithiation rate by 1 order of magnitude, resulting in a sequential surface-then-core lithiation mechanism. Pore formation was observed in the nanowires during delithiation as a result of the fast lithiation/delithiation kinetics of the nanowires. Pore formation was found to be associated with nonlithiated crystalline domains in the nanowire, which prevent uniform structural changes of the nanowire, and the resulting pores increase in size after each cycle. When an amorphous Si shell was applied to the nanowires, pore formation was not observed during the in situ TEM experiments. Ex situ TEM analysis of Sn-containing Si nanowires cycled in coi...

(Invited) Strain and Phonon-Carrier Interactions in Ge-Si0.5Ge0.5 Core-Shell Nanowires Probed Using Tip-Enhanced Raman Spectroscopy

Group IV nanowires and nanowire heterostructures are of current interest for a broad range of applications including field-effect transistors with enhanced carrier mobility, and thermoelectric devices with engineered thermal and carrier transport properties. The energy band alignments among Si, Ge, and SixGe1-x alloys enable quantum confinement of either holes or electrons, while compositional variations and structural morphology at the nanoscale can lead to new phonon scattering and confinement effects. Because of the lattice mismatch between Si and Ge, however, the dimensions and alloy compositions in nanowire heterostructures must be carefully controlled and characterized to maintain coherent strain. Here we describe the use of tip-enhanced Raman spectroscopy to characterize nanoscale correlations between strain and nanowire dimensions in Ge-SixGe1-x core-shell nanowires grown by ultrahigh-vacuum chemical vapor deposition, and to elucidate phonon-carrier interactions that are manifested via shifts in Raman peaks originating from Ge-Ge vibrational modes within the quantum-confined hole gas formed at the Ge-SixGe1-x interface. Typical nanowire structures consisted of a Ge core 35-55nm in diameter surrounded by a 5nm thick Si0.5Ge0.5 shell, leading to compressive strain in the Ge core and formation of a quantum-confined hole gas at the Ge-Si0.5Ge0.5 interface. Tip-enhanced Raman spectroscopy (TERS) was performed under ambient atmospheric conditions on nanowires dispersed on an Au surface using an Au-coated atomic force microscope probe tip and 633nm laser excitation in a side illumination geometry. Tip-enhanced Raman spectra exhibit two distinct peaks associated with Ge-Ge vibrational modes – one arising from Ge-Ge vibrations in the nanowire Ge core region, and the other from Ge-Ge vibrations occurring within the quantum-confined hole gas at the Ge-Si0.5Ge0.5 interface. Both peaks exhibit a dependence of Raman shift on local nanowire diameter consistent with coherent strain in the nanowire, demonstrating that the TERS measurements provide the ability to probe strain at the nanoscale. The difference in Raman shift associated with these peaks is shown to be a consequence of coupling between intervalence band transitions and the Ge-Ge phonon mode that leads to Fano resonance behavior and a shift in the phonon self-energy. In addition, positioning of the Au-coated probe tip in closer proximity to the nanowire leads directly to an increase in the separation between these peaks and in the relative amplitude of the peak from the Ge-Si0.5Ge0.5 interface region, confirming that the TERS measurement is sensitive to phenomena within an extremely localized region in the vicinity of the probe tip apex. Computational electromagnetic simulations confirm that proximity to the probe tip leads to a large increase in electromagnetic field amplitude in the nanowire within ~10-20nm of the probe tip surface, and provide insight into the dependence of coupling to intervalence-band carrier transitions on proximity to the probe tip during the TERS measurement. Figure 1