Cylindrical Nanowires Research Articles

Pseudoelasticity and shape memory effects in cylindrical FCC metal nanowires

Molecular dynamics simulations are used to show strong pseudoelasticity and shape memory effects in a wide range of face-centered cubic metal nanowires with cylindrical shape, while similar effects have only been previously reported in face-centered metal nanowires with a unique geometry, i.e., by crystal reorientation from 〈0 0 1〉/{1 0 0} with a square cross section to 〈1 1 0〉/{1 1 1} with a rhombic cross section. The more generalized pseudoelasticity and shape memory effects reported in this work are enabled via a simple yet experimentally practical approach by tilting the nanowire axis away from the perfect 〈0 0 1〉 or 〈1 1 0〉 orientation such that the symmetry is broken in those nanowires and only one slip system is activated during the uniaxial loading. It is shown that while no pseudoelasticity and shape memory effects are found in 〈1 1 0〉 or 〈0 0 1〉 oriented cylindrical nanowires, full recovery up to ∼50% tensile (or ∼ 30% compressive) strain can be achieved in cylindrical nanowires whose axis are tilted as small as 2° (or 4°) away from 〈1 1 0〉 (or 〈0 0 1〉). This finding could open up new opportunities for synthesizing shape-memory metal nanowires for vibration damping and mechanical energy storage applications at low cost.

A Bilayer Model for Incorporating the Coupled Effects of Surface Energy and Microstructure on the Electromechanical Stability of NEMS

Herein, the static and dynamic pull-in instabilities of nanotweezers and nanoswitches fabricated from conductive cylindrical nanowires are studied. A bilayer model based on the Gurtin–Murdoch surface elasticity and the consistent couple stress theory is developed to incorporate the coupled effects of surface energy and microstructure phenomenon. The differential quadrature method is employed to solve the nonlinear governing equation of the system. The effects of various parameters on the static and dynamic pull-in parameters and stability threshold of the nanostructures are demonstrated.

Fourier-like conduction and finite one-dimensional thermal conductivity in long silicon nanowires by approach-to-equilibrium molecular dynamics

The thermal conductivity of cylindrical and smooth silicon nanowires is systematically studied as a function of diameter and length by fully atomistic simulations. A transient thermal regime is created and monitored by molecular dynamics. This approach-to-equilibrium methodology, already proven to be very efficient for bulk systems, is here applied to nanowires of length up to 1.2 micron, and diameter in the range 1 to 14 nm. It is shown that the temperature profile along the nanowire axis and its temporal evolution comply with the heat equation. A one-dimensional thermal conductivity is extracted from the characteristic decay time according to the time-dependent solution of the heat equation. Like for the bulk using the same method, the thermal conductivity exhibits a length dependence due to the slowly growing cumulative distribution of the phonon mean free paths. Unlike the bulk, the infinite-length saturation of the conductivity value can be observed in our simulations, thanks to a reduction of the phonon mean free path in the nanowires, estimated to be a few hundred nanometers. A finite thermal conductivity can be extracted for each diameter, and no divergence at long length is observed.

Electron states in a sinusoidally deformed cylindrical quantum wire

Electron states in a sinusoidally deformed cylindrical nanowire are probed within the effective mass approximation. The nanowire is immersed in a homogeneous magnetic field applied parallel to the axis of the wire. The sinusoidal deformity affects quantum properties of electrons considerably. The deformity introduces energy dependence on the axial position, enhancing (decreasing) confinement energies in regions where the radius is narrower (wider). Analysis shows that the magnetic field attenuates these sinusoidal deformity-induced distortions on electron quantum properties

Subwavelength Resonant Gratings for Micrometric Strain Sensors

A subwavelength resonant grating based on plasmonic silver nanowires is proposed and theoretically analyzed. The structure is composed of several Ag nanowires distributed in a parallel configuration with subwavelength grating constant. Despite the use of TM mode, we have discovered that for certain subwavelength distances, the combination with the plasmonic resonances of the nanowires produces a strong reflection at the resonant peaks. This structure could be used to measure deformation variation down to the nanometer range and use it as a micrometric all optical gauge. The effects of the size of the cylindrical metal nanowires on the performance of the sensing have been studied in detail. Two operation modes are observed depending on the structural parameters. Either a phase or amplitude modulation in the visible range could be obtained. In the first case, a sensitivity of 7.5 pm/µϵ has been obtained. In the second case, an amplitude modulation of –256·10–6 dB/µϵ has been obtained. This kind of sensor would be cheap and easy to measure by means of a fiber optic link. The structure is simple and easily scalable.

Nonrecessive and Sustained Precessional Domain Wall Motion Driven by Perpendicular Field Pulses in Thin Magnetic Nanocylinders

In a conventional field-driven domain wall (DM) motion, the external field is applied along the magnetization of the magnetic domain which is expanded during the process. Recently, a novel approach is proposed to utilize a perpendicular field pulse rather than an in-plane one to drive DWs in magnetic nanostripes. In this paper, we apply this idea to transverse DWs in thin cylindrical nanowires. In the case of driving a single DW, this approach displays apparent advantages. Owing to the zero DW mass and inertia, a net DW displacement can be attained by applying a field pulse which is initially vertical to the DW magnetization. Therefore, no artificial pinning sites are required as in flat strips. Furthermore, we propose to rotate the external field with a particular frequency to maintain a right angle between the field and the DW magnetization. In this way, a continuous DW motion with maximum speed can be realized. The feasibility of this proposed approach is demonstrated by micromagnetic simulations and an analytical calculation.

Thermal transport contributed by the torsional phonons in cylindrical nanowires: Role of evanescent modes

Thermal transport contributed by the torsional phonons in cylindrical nanowires is investigated by using the isotropic elastic continuum theory. The numerical calculations for both the concavity-shaped and convexity-shaped cylindrical structures are made to reveal the role of the evanescent modes. Results show that the evanescent modes play an important role in influencing the thermal transport in such structures. For the concavity-shaped cylindrical nanowire, the evanescent modes can enhance the thermal conductance by about 20 percent, while for the convexity-shaped cylindrical nanowire, the evanescent modes can suppress the thermal conductance by 6 percent. It is also shown that the influence of the evanescent modes on the thermal conductance is strongly related to the attenuation length of the evanescent modes. A brief analysis of these results is given.

Depinning process of magnetic domain walls in cylindrical nanowires with a chemical constraint

In this work we have studied with micromagnetic simulations the pinning process of a magnetic domain wall (DW) travelling in a cylindrical magnetic nanowire with a section where the composition has been altered with respect to the rest of the nanowire (chemical constraint). We have studied the depinning process for a non-magnetic, a paramagnetic and a ferromagnetic constraint and we show that the pinning strength in this type of defect can be tailored by a change of composition. The ferromagnetic chemical constraint is the strongest pinning site but it can stop the DW only when there is a significant reduction of the exchange energy and the saturation magnetization with respect to the rest of the wire. Chemical constraints may constitute a promising alternative to geometrical constraints in some devices such as the race-track memory.

Plasmonic characteristics of two vertically coupled graphene-coated nanowires integrated with substrate.

In this paper, we theoretically investigate the guiding properties of two vertically coupled cylindrical graphene-coated nanowires (GNWs) integrated with a thin high-index dielectric substrate. We show that the plasmonic properties of the proposed structure can be effectively modulated by changing the thickness of the dielectric substrate, the nanowire radius, the gap distance between the substrate and GNWs, and the Fermi level of graphene. Through optimizing the geometric and structural parameters, a surface plasmon mode with high optical performance and low propagation loss can be achieved in the studied structure. Compared with the plasmon mode guided in a single GNW over substrate, a highly improved figure of merit with nearly two-fold electric field enhancement can be yielded in the double GNW over substrate counterpart.

Influence of the Shape on Magnetic Properties of Ising Nanostructures

We study magnetic behaviors of phase diagrams and hysteresis loops in a cubic, hexagonal, and cylindrical type transverse Ising nanowire within the effective-field theory with correlations. From these studies, we investigate the influence of the shape on magnetic properties of Ising nanostructures. We find that the behavior of phase diagrams is more similar for the cubic and hexagonal nanowires than the cylindrical one and the system undergoes a second-order phase transition at the lowest temperature for the hexagonal nanowire and at the highest temperature for the cylindrical one. Hysteresis loop areas are the largest for a hexagonal nanowire and smallest for a cylindrical one. Hysteresis loop areas disappear at the highest temperature for a cylindrical nanowire and at the lowest temperature for a cubic one.

Accurate thermodynamic relations of the melting temperature of nanocrystals with different shapes and pure theoretical calculation

Based on the surface pre-melting model, accurate thermodynamic relations of the melting temperature of nanocrystals with different shapes (tetrahedron, cube, octahedron, dodecahedron, icosahedron, nanowire) were derived. The theoretically calculated melting temperatures are in relative good agreements with experimental, molecular dynamic simulation and other theoretical results for nanometer Au, Ag, Al, In and Pb. It is found that the particle size and shape have notable effects on the melting temperature of nanocrystals, and the smaller the particle size, the greater the effect of shape. Furthermore, at the same equivalent radius, the more the shape deviates from sphere, the lower the melting temperature is. The value of melting temperature depression of cylindrical nanowire is just half of that of spherical nanoparticle with an identical radius. The theoretical relations enable one to quantitatively describe the influence regularities of size and shape on the melting temperature and to provide an effective way to predict and interpret the melting temperature of nanocrystals with different sizes and shapes.

Finite temperature magnetic phase transition features of the quenched disordered binary alloy cylindrical nanowire

Thermal and magnetic phase transition properties of a quenched disordered binary alloy cylindrical nanowire of the type ApB1−p have been elucidated by making use of Monte Carlo simulation based on a local spin update Metropolis algorithm. Magnetic components A with spin-½ and B with spin-1 are distributed randomly throughout the cylindrical nanowire with a probability of p and 1−p, respectively. The phase boundary lines, magnetization and also magnetic susceptibility profiles of the system in both presence and absence of a single-ion anisotropy term are investigated. Our Monte Carlo simulation results indicate that the critical behavior of the system sensitively depends on the concentration value of the type-A atoms, the spin-spin exchange interaction strength between unlike magnetic components as well as on the single-ion anisotropy interaction. Finally, a comparison of our observations with those of recently published studies for the clean cylindrical nanowire systems corresponding to p = 0.0 and p = 1.0 cases is presented and it is found that there exist satisfactory agreements.

A Generalization of Mean Field Theory in a Cluster with Many Sites on the Ising Model from the Bogoliubov Inequality: Hexagonal Nanowire and Nanotube

A generalization of mean field theory in a cluster with many sites was obtained for the spin-1/2 Ising model from the Gibbs-Bogoliubov inequality. The expressions for the free energy and the magnetization were obtained. The generalization was applied in a structure of the nanowire and nanotube hexagonal lattices, for clusters of seven sites and six sites, respectively. The results for the magnetization, the free energy, the internal energy, the entropy, the specific heat, and the critical frontiers were obtained. The critical temperature and the compensation temperature in a cylindrical Ising nanowire are investigated, in order to clarify the distinction between the ferromagnetic and ferrimagnetic behaviors when the core-shell exchange coupling takes a different sign. The results were compared with other works.

Pseudo-monocrystalline properties of cylindrical nanowires confinedly grown by electrodeposition in nanoporous alumina templates

Four different cylindrical nanowires systems with single-crystal-like properties have been characterized by transmission electron microscopy and selected area electron diffraction (SAED) under different tilting angles.

Surface Morphology and Magnetic Properties of Isolated Cylindrical Nickel Nanowires

In this article, the surface morphology and magnetic properties of isolated cylindrical nickel (Ni) nanowires fabricated by electrodeposition have been thoroughly investigated using scanning electron microscopy and Magneto-Optical Kerr Effect (MOKE) magnetometry, respectively. The surfaces of most nanowires were found to be homogenous, uniform, and cylindrical in shape. Some others show different diameters and surface features, including; protrusions and branches along their length. The diameter distribution of a wide range of nanowires was found to differ from their template pore diameters. These all variations are more likely due to defects exist in the internal surfaces of the pores within the template itself, or may be associated with the trapped air pockets within the pores during nanowires growth or due to the oxide formation or residual contaminants which may cover these wires. The nanowires lengths were found to differ from their actual lengths estimated during deposition growth. This was attributed to the breakage of nanowires into small sections during releasing process. The hysteresis loops obtained by applying a magnetic field at different angles with respect to the nanowires long axis showed square hysteresis loops with a sharp jump of Kerr signal during switching behaviour, as well as a high squareness ratio, indicating the dominance of shape anisotropy. These results are quite different from the measurements of high density templated nanowires reported in the literature, due to the small number of nearest neighbour nanowires, and hence no magneto-static interaction. The magnetisation reversal of such wires is well described by the non-uniform rotation of the curling model of domain reversal.

Superplastic Pd50Pt50 monocrystalline bimetallic alloy nanowire: a molecular dynamics simulation study

We report uniaxial tensile studies of monocrystalline cylindrical Pd50 Pt50 bimetallic alloy nanowire (NW) (11.67 A diameter × 101.14 A length) subjected to strain rates of 1%, 3%, 5%, 8% and 10% ps−1 and temperatures of 100 K, 300 K, 500 K and 700 K using molecular dynamics simulations. The tensile test is carried out along y -axis [0 1 0] with periodic boundary conditions and non-periodic along x -axis and z -axis. Dissipative mechanisms such as dislocations, partial dislocations and stair rod dislocations are also identified during deformation. It is found that NW is sensitive to strain rates and temperatures and exhibits superplastic behavior with strain >100% at higher strain rates. Young's modulus is found to be sensitive to strain rate and temperature and is about 25% less that of bulk value at strain rates >5% ps−1 and at 300 K. Yield strength increases linearly with strain rate and decreases with increase in temperature as expected. Crystalline to amorphous-phase transformations take place at strain rates >5% ps−1 and could be the reason behind large plastic strain. The calculated mechanical properties of NWs will be useful in devices applicable to sensing and catalysis.

Analytical Model of Mid-Infrared Surface Plasmon Modes in a Cylindrical Long-Range Waveguide With Double-Layer Graphene

An optical waveguide for working in mid-infrared range is highly desirable. However, such a waveguide is required to yield a long propagation distance and deep subwavelength confinement. Although the graphene-based surface plasmon polariton (SPP) waveguides have exhibited some potential in this regard, their propagation lengths are currently insufficient (~100 μm). In this paper, we propose a graphene-based cylindrical long-range SPP (LRSPP) waveguide, which is composed of a cylindrical silicon nanowire core surrounded by an inner graphene layer, a silica layer, and an outer graphene layer from inside to out. First, the electromagnetic field equation and the dispersion equation of this waveguide are derived. Then, combining these derived equations with the numerical simulation, the propagation properties of the LRSPP and the short-range SPP modes are analyzed. The results show that the proposed waveguide with the LRSPP fundamental mode has a notable advantage for simultaneously achieving a larger propagation length (~10 μm) and a deep subwavelength confinement (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , where A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the diffraction-limited mode area). This waveguide has potential for application in high-density photonic integrated circuits in the mid-infrared range.

The electrical conductivity and longitudinal magnetoresistance of metallic nanotubes

Proceeding from exact solutions of the Boltzmann transport equation in the relaxation time approximation, we present formulas for the electrical conductivity and longitudinal magnetoresistance of single-crystalline cylindrical nanotubes. The effects of surface scattering are taken into account by introducing different specularity parameters at the inner and outer surfaces. For small values of the inner diameter, these formulas reduce to the respective expressions for cylindrical nanowires. It is found that the existing measurements of the resistivity of nanotubes (Venkata Kamalakar and Raychaudhuri, New J. Phys. 14, 043032 (2012)) can be accurately described by this formalism.

Hybrid plasmonic waveguide consisting of two identical semiconductor nanowires and metal film with semi-cylinder ridges

In this article, a hybrid plasmonic waveguide (HPW) consisting of a thin metal film sandwiched between two identical cylindrical semiconductor nanowires is proposed and investigated numerically. With two air grooves carved symmetrically on the upper and lower surfaces of the metal film and two nanoscale semi-cylindrical ridges formed, the structured metal film and the semiconductor nanowires are embedded in a low-index silicon-dioxide medium. Based on the finite element method, our simulation results show that the proposed HPW can achieve a propagation length longer than 1000 μm in all circumstances, as well as a mode area as small as 5.21 × 10−4λ2, and an excellent figure of merit. The high-performance of the novel HPW may provide theoretical guidance for further research of HPW and related applications in photonic integrated circuits.

Transfer matrix method solving interface optical phonons in wurtzite core-multishell nanowires of III-nitrides

Within the framework of dielectric continuum and Loudon’s uniaxial crystal models, the transfer matrix method (TMM) is developed to investigate interface optical phonons (IOPs) in cylindrical wurtzite core-multishell nanowires (CMSNWs) consisting of ternary mixed crystals (TMCs). The IOPs in GaN/InxGa1-xN/InyGa1-yN and GaN/InxGa1-xN/InyGa1-yN/InzGa1-zN CMSNWs are calculated as examples. The results show that there may be several types of IOPs existing in certain frequency regions in CMSNWs for a given component due to the phonon dispersion anisotropy in wurtzite nitrides. The IOPs are classified by possible combinations of the interfaces in CMSNWs. Furthermore, the dispersion relations and electro-static potentials of each kind of IOPs are discussed in detail. The dispersion relations of IOPs in CMSNWs is found to be the combination of that in each nearest two layer CSNW. It can explain the fact that the total branch number of IOPs obey the 2n rule. It is also found that the peak positions of electro-static potentials are decided by the layer component order from the inner layer to outside in CMSNWs. The results indicate that TMM for IOPs is available and can be commodiously extended to other cylindrical wurtzite III-nitride CMSNWs. Based on this method, one can further discuss the IOPs related photoelectric properties in nitride CMSNWs consisting of TMCs.