Nanowire Axis Research Articles

Microwave fields driven domain wall motions in antiferromagnetic nanowires

In this work, we study the microwave field driven domain wall (DW) motion in an antiferromagnetic nanowire, using the numerical calculations based on a classical Heisenberg spin model with the biaxial magnetic anisotropy. We show that a proper combination of a static magnetic field plus an oscillating field perpendicular to the nanowire axis is sufficient to drive the DW propagation along the nanowire. More importantly, the drift velocity at the resonance frequency is comparable to that induced by temperature gradients, suggesting that microwave field can be a very promising tool to control DW motions in antiferromagnetic nanostructures. The dependences of resonance frequency and drift velocity on the static and oscillating fields, the axial anisotropy, and the damping constant are discussed in details. Furthermore, the optimal orientations of the field are also numerically determined and explained. This work provides useful information for the spin dynamics in antiferromagnetic nanostructures for spintronics applications.

Seesaw-like polarized transmission behavior of silver nanowire arrays aligned by off-center spin-coating

Straight silver nanowires were synthesized by accelerated oxidization and then aligned into ordered arrays by off-center spin-coating. Seesaw-like behavior was observed in the polarized transmission spectra of the arrays. With the increment of polarization angle (θP, defined as the angle between axis of nanowires and direction of electric field of light), transmission changed repeatedly with a period of 180°, but it moved to opposite directions between the two regions separated by supporting points locating at 494 nm. The behavior is ascribed to the competition between the extinction behaviors of the two modes of surface plasma polaritons on silver nanowires. One is the longitudinal mode which is excited by long wavelengths and tuned by function of cos2(θp) and the other is the transverse mode that is excited by short wavelengths and tuned by function of sin2(θp). Simulation was performed based on the finite-difference time domain method. The effect of the nanowire diameter and length (aspect ratio) on the position of the supporting point was studied. As nanowire width increased from 20 nm to 350 nm, the supporting point moved from 400 to 500 nm. While it changed slightly when the nanowire length increased from 3 μm to infinitely long (width fixed at 260 nm). In current study, the position of the supporting point is mainly determined by the nanowire width.

Coherent X-ray diffraction imaging meets ptychography to study core-shell-shell nanowires

AbstractWe report on the results of coherent X-ray diffraction imaging (CXDI) and ptychography measurements of two individual core-shell-shell GaAs/(In,Ga)As/GaAs nanowires (NWs) grown by molecular beam epitaxy (MBE) on patterned Si(111) substrate. CXDI at the axial GaAs 111 Bragg reflection was applied at different positions along the NW axis in order to characterize the NWs in terms of structural homogeneity along the radial directions. At each positon 3D reciprocal space maps have been recoded and inverted using phase retrieval algorithms. The CXDI were complemented by 2D ptychography measurements at GaAs 111 Bragg reflection probing the same NWs with respect to their structural homogeneity. Both methods provide structural homogeneity for NW1 and NW2 except at the bottom part of the NWs. In case of NW2 CXDI and ptychography show changes in the structure of the top part of the NW indicated by 60° rotation of the indicated three-fold rotational symmetry in the observed diffraction patterns and changes in the strain field reconstructed from ptychography.

Spin-Hall effect in the scattering of structured light from plasmonic nanowire.

Spin-orbit interactions are subwavelength phenomena that can potentially lead to numerous device-related applications in nanophotonics. Here, we report the spin-Hall effect in the forward scattering of Hermite-Gaussian (HG) and Gaussian beams from a plasmonic nanowire. Asymmetric scattered radiation distribution was observed for circularly polarized beams. Asymmetry in the scattered radiation distribution changes the sign when the polarization handedness inverts. We found a significant enhancement in the spin-Hall effect for a HG beam compared to a Gaussian beam for constant input power. The difference between scattered powers perpendicular to the long axis of the plasmonic nanowire was used to quantify the enhancement. In addition, the nodal line of the HG beam acts as the marker for the spin-Hall shift. Numerical calculations corroborate experimental observations and suggest that the spin flow component of the Poynting vector associated with the circular polarization is responsible for the spin-Hall effect and its enhancement.

Quantum interferometer based on GaAs/InAs core/shell nanowires connected to superconducting contacts

An interferometer structure was realized based on a GaAs/InAs core/shell nanowire and Nb superconducting electrodes. Two pairs of Nb contacts are attached to the side facets of the nanowire allowing for carrier transport in three different orientations. Owing to the core/shell geometry, the current flows in the tubular conductive InAs shell. In transport measurements with superconducting electrodes directly facing each other, indications of a Josephson supercurrent are found. In contrast for junctions in diagonal and longitudinal configuration a deficiency current is observed, owing to the weaker coupling on longer distances. By applying a magnetic field along the nanowires axis pronounced h/2e flux-periodic oscillations are measured in all three contact configurations. The appearance of these oscillations is explained in terms of interference effects in the Josephson supercurrent and long-range phase-coherent Andreev reflection.

Tuning the morphology of self-assisted GaP nanowires

Patterned arrays of self-assisted GaP nanowires (NWs) were grown on a Si substrate by gas source molecular beam epitaxy using various V/III flux ratios from 1–6, and various pitches from 360–1000 nm. As the V/III flux ratio was increased from 1–6, the NWs showed a change in morphology from outward tapering to straight, and eventually to inward tapering. The morphologies of the self-assisted GaP NWs are well described by a simple kinetic equation for the NW radius versus the position along the NW axis. The most important growth parameter that governs the NW morphology is the V/III flux ratio. Sharpened NWs with a stable radius equal to only 12 nm at a V/III flux of 6 were achieved, demonstrating their suitability for the insertion of quantum dots.

Magnetic field induced mixing of light hole excitonic states in (Cd, Mn)Te/(Cd, Mg)Te core/shell nanowires

A detailed magneto-photoluminescence study of individual (Cd, Mn)Te/(Cd, Mg)Te core/shell nanowires grown by molecular beam epitaxy is performed. First of all, an enhancement of the Zeeman splitting due to sp-d exchange interaction between band carriers and Mn-spins is evidenced in these nanostructures. Then, it is found that the value of this splitting depends strongly on the magnetic field direction with respect to the nanowire axis. The largest splitting is observed when the magnetic field is applied perpendicular and the smallest when it is applied parallel to the nanowire axis. This effect is explained in terms of magnetic field induced valence band mixing and evidences the light hole character of the excitonic emission. The values of the light and heavy hole splitting are determined for several individual nanowires based on the comparison of experimental results to theoretical calculations.

Magnetization Reversal in Radially Distributed Nanowire Arrays

The magnetic properties of radially oriented Co, Ni, and CoNi alloy nanowires synthesized by pulsed electrodeposition into porous alumina structures are measured and compared with those of similar nanowires grown in a planar geometry. The alloy composition affects the anisotropy axis direction, which is determined by the balance between the magnetocrystalline and shape anisotropies, lying transverse to the nanowires for Co samples and along the nanowire axis for Ni. Monte Carlo simulations were performed to model the magnetic hysteresis of the radially oriented and planar geometry nanowires using an approach based on the conical distribution of anisotropies. The model provides an excellent fit compared with experimental hysteresis loops.

Determination of indium content of GaAs/(In,Ga)As/(GaAs) core-shell(-shell) nanowires by x-ray diffraction and nano x-ray fluorescence

We present two complementary approaches to investigate the In content in GaAs/(In,Ga)As/(GaAs) core-shell-(shell) nanowire (NW) heterostructures using synchrotron radiation. The key advantage of our methodology is that NWs are characterized in their as-grown configuration, i.e., perpendicularly standing on a substrate. First, we determine the mean In content of the (In,Ga)As shell by high-resolution x-ray diffraction (XRD) from NW ensembles. In particular, we disentangle the influence of In content and shell thickness on XRD by measuring and analyzing two reflections with diffraction vector parallel and perpendicular to the growth axis, respectively. Second, we study the In distribution within individual NWs by nano x-ray fluorescence. Both the NW (111) basal plane, that is parallel to the surface of the substrate, and the {10-1} sidewall plane were scanned with an incident nanobeam of 50 nm width. We investigate three samples with different nominal In content of the (In,Ga)As shell. In all samples, the average In content of the shell determined by XRD is in good agreement with the nominal value. For a nominal In content of 15%, the In distribution is fairly uniform between all six sidewall facets. In contrast, in NWs with nominally 25% In content, different sidewall facets of the same NW exhibit different In contents. This effect is attributed to shadowing during growth by molecular beam epitaxy. At the same time, along the NW axis the In distribution is still fairly homogeneous. In NWs with 60% nominal In content and no outer GaAs shell, the In content varies significantly both between different sidewall facets and along the NW axis. This fluctuation is explained by the formation of (In,Ga)As mounds that grow simultaneously with a thinner (In,Ga)As shell. The methodology presented here may be applied also to other core-shell NWs with a ternary shell and paves the way to correlating NW structure with functional properties that depend on the as-grown configuration of the NWs.

Theoretical and experimental research of thermal conductivity of silver(Ag) nanowires in mesoporous substrate

Thermal conductivities of Ag nanowires with different sizes and temperatures were studied theoretically. The lattice and electronic thermal conductivities of Ag nanowires include contributions from interface scattering and grain boundary scattering. While the molecular dynamic simulation was used to get the contribution of interface scattering to the lattice thermal conductivity of Ag nanowires along the axial direction, a model derived from Boltzmann transport equation and the Wiedemann-Franz relation were applied to calculate electronic thermal conductivities of Ag nanowires along the axial direction. And then coupling with the lattice and electronic thermal conductivities, the effective thermal conductivity of Ag nanowire along the axial direction was obtained. Furthermore, Kubo linear-response formalism was used to approximately predict the thermal conductivity in the plane perpendicular to the axis of the Ag nanowire. It turns out that effective thermal conductivities of Ag nanowire in the r-θ plane and along the Z direction show a great anisotropy. Finally, by measuring the thermal conductivities of Ag/SBA-15, the thermal conductivity of a single Ag nanowire can be obtained by using PWDM model, and the theoretical thermal conductivities of Ag nanowire were compared with the experimental results.

Communication—Electrodeposition, Microstructure and Magnetic Properties of Co2FeSn Heusler Alloy Nanowires

Co2FeSn Heusler alloy nanowires with the diameters of 30 nm and 60 nm were firstly prepared by direct current electrodeposition. Magnetic measurements revealed the excellent soft ferromagnetism of Co2FeSn nanowires. The easy magnetization axis is perpendicular to the long axis of nanowires and the magnetic reversal of Co2FeSn nanowires occurs via coherent rotation model. In addition, the influence of the wire diameter on the magnetic properties of Co2FeSn nanowires was discussed. Co2FeSn Heusler alloy nanowires synthesized in this work shows tunable magnetic properties and are of potential interest for magnetic device and spintronic applications.

Поляризационная спектроскопия одиночной квантовой точки и одиночной квантовой нити

AbstractPhotoluminescence spectra of an isolated GaAs quantum dot within an AlGaAs quantum wire are studied. The examination of behavior of spectra in a magnetic field provided an opportunity to estimate the exciton binding energy in the quantum dot, which turned out to be 10 times higher than the bulk exciton binding energy in GaAs. It is demonstrated that the signal of exciton photoluminescence from the quantum dot emitted along the nanowire axis is linearly polarized. At the same time, the photoluminescence signal propagating in the direction perpendicular to the nanowire axis is almost unpolarized. This may be attributed to the nonaxial dot positioning in the wire under a giant increase in the binding energy of an exciton affected by an image potential.

MAGNETIC PROPERTIES OF Fe NANOWIRES ELECTRODEPOSITED IN SELF-ORDERED ALUMINA MEMBRANE

The iron nanowire arrays (NWs) were fabricated by DC electrodeposition into hexagonally ordered alumina pores. With the use of X-ray diffraction analysis, the structure of iron wires was determined. The iron wires have the Body Centered Cubic structure. The influence of cathodic deposition potential and parameters of membrane on the magnetic properties of nanowire arrays was investigated. Magnetic properties analyzed by VSM measurements suggest that the easy axis of magnetization follows the nanowire axis, with coercivity increasing with a decrease of nanowire diameter and length. The dependence of the height of Fe wires on the electrodeposition potential was determined. The low cathodic potential and smaller pore diameter are the synthesis parameters most beneficial for large coercivity with easy axis along nanowires.

1D Wires of 2D Layered Materials: Germanium Sulfide Nanowires as Efficient Light Emitters

We considered the formation of one-dimensional (1D) nanowires from the two-dimensional (2D) layered material GeS. Growth by a low-temperature vapor–liquid–solid process over a Au catalyst gives rise to single-crystalline nanowires with typical diameters of ∼100 nm and length exceeding 10 μm that consist of a crystalline GeS core with layering along the nanowire axis, surrounded by an ultrathin, sulfur-rich GeSx shell with larger bandgap that provides chemical and electronic passivation of the nanowires. Combined variable-temperature in situ electron microscopy, chemical mapping, and single-nanowire cathodoluminescence spectroscopy are used to uncover key elements of the growth process and to identify the optoelectronic properties of the GeS/GeSx core–shell nanowires. Our results suggest that vapor–liquid–solid processes can readily be extended from conventional three-dimensional crystals to the growth of 1D wires from 2D layered crystals and may provide novel low-dimensional materials for electronic, opto...

Fabrication and characterization of a germanium nanowire light emitting diode

In this letter, we demonstrate the feasibility of a germanium nanowire light emitting diode as a reasonable approach for downscaling of CMOS compatible light sources. We show room-temperature direct bandgap electroluminescence from axial p-n junction nanowire devices. The electron population in the Γ valley, necessary for direct bandgap emission, is achieved by high injection current densities. Carrier temperature is consistently found to be higher than the lattice temperature, indicating inhibited carrier cooling in small diameter wires. Strong polarization of the emission parallel to the nanowire axis is observed and attributed to dielectric contrast phenomena.

Magneto-quantum oscillations in Bi2Se3 nanowires

We report preparation of nanoribbons (crossection ∼ 250*25 nm2) by focused ion beam etching of single-crystalline Bi2Se3 and detailed measurements of their magnetoresistance at temperatures down to 4.2 K, magnetic field up to 9 T. In a magnetic field parallel to the axis of nanowire the magnetoresistance shows up oscillations. Surprisingly, the Fourier analysis shows the presence not only of oscillations with a period corresponding to the flux quantum (Φ0 = hc/e), but also oscillations with a period of 2Φ0 and 4Φ0. Possible mechanisms of the observed effect are discussed.

Transversely Divergent Second Harmonic Generation by Surface Plasmon Polaritons on Single Metallic Nanowires.

Coherently adding up signal wave from different locations are a prerequisite for realizing efficient nonlinear optical processes in traditional optical configurations. While nonlinear optical processes in plasmonic waveguides with subwavelength light confinement are in principle desirable for enhancing nonlinear effects, so far it has been difficult to improve the efficiency due to the large momentum mismatch. Here we demonstrate, using remotely excited surface plasmon polaritons (SPPs), axial collimated but transversely divergent second harmonic (SH) generation in a single silver nanowire-monolayer molybdenum disulfide hybrid system. Fourier imaging of the generated SH signal confirms the momentum conservation conditions between the incident and reflected SPPs and reveals distinct features inherent to the 1D plasmonic waveguides: (i) the SH photons are collimated perpendicular to the nanowire axis but are divergent within the perpendicular plane; (ii) the collimation (divergence) is inversely proportional to the length of the active region (lateral confinement of the SPPs); and (iii) the SH emission pattern resembles that of an aligned dipole chain on top of the substrate with an emission peak at the critical angle. Our results pave the way to generate and manipulate SH emission around subwavelength waveguides and open up new possibilities for realizing high efficiency on-chip nonlinear optics.

In Situ Visualization of Fast Surface Ion Diffusion in Vanadium Dioxide Nanowires.

We investigate in situ ion diffusion in vanadium dioxide (VO2) nanowires (NWs) by using photocurrent imaging. Alkali metal ions are injected into a NW segment via ionic liquid gating and are shown to diffuse along the NW axis. The visualization of ion diffusion is realized by spatially resolved photocurrent measurements, which detect the charge carrier density change associated with the ion incorporation. Diffusion constants are determined to be on the order of 10-10 cm2/s for both Li+ and Na+ ions at room temperature, while H+ diffuses much slower. The ion diffusion is also found to occur mainly at the surface of the NWs, as metal contacts can effectively block the ion diffusion. This novel method of visualizing ion distribution is expected to be applied to study ion diffusion in a broad range of materials, providing key insights on phase transition electronics and energy storage applications.

Photoluminescence Characterization of Cadmium Sulphide (CdS) Nanowires for Polarization Studies

In this paper, the polarizing properties for CdS nanowire arrays were explored for their potential use in the design of nanowire based polarizers and optical switches. These free standing cadmium sulphide (CdS) nanowires were grown in anodized aluminum oxide (AAO) template via dc electrodeposition. Raman and photoluminescence (PL) measurements were investigated for parallel and perpendicular polarization with two orientations of the sample having light propagating parallel to the nanowire axis in one orientation and light propagating perpendicular to the nanowire axis in other orientation. Polarization-sensitive measurements show strong polarization anisotropy in the photoluminescence (PL) intensity measurements observed in parallel and perpendicular orientation to the long axis of a nanowire. The measured PL ratio, ρ, for parallel to perpendicular orientation was around 0.80- 0.85 which shows strong polarization anisotropy for the grown CdS nanowires. Strong peaks of A1 (TO) at 235 cm-1, E2 mode at 255 cm-1 along with 1 LO (longitudinal optical) at 303 cm-1, 2 LO peak at 604 cm-1 of the CdS nanowires were seen with different polarizations for Raman spectral studies. These polarization studies show that these dc electrodeposited grown CdS nanowire arrays are well suited for uses in polarization-based nanoscale devices such as in optical switches, and high performance photodetectors.

Anisotropic phase coherence in GaAs/InAs core/shell nanowires

Low-temperature transport in nanowires is accompanied by phase-coherent effects, which are observed as modulation of the conductance in an external magnetic field. In the GaAs/InAs core/shell nanowires investigated here, these are h/e flux periodic oscillations in a magnetic field aligned parallel to the nanowire axis and aperiodic universal conductance fluctuations in a field aligned perpendicularly to the nanowire axis. Both electron interference effects are used to analyse the phase coherence of the system. Temperature-dependent measurements are carried out, in order to derive the phase coherence lengths in the cross-sectional plane as well as along the nanowire sidewalls. It is found that these values show a strong anisotropy, which can be explained by the crystal structure of the GaAs/InAs core/shell nanowire. For nanowires with a radius as low as 45 nm, flux periodic oscillations were observed up to a temperature of 55 K.