Long Axis Of Nanowires Research Articles

Evaluation of structural, magnetic and concentration dependent texture variations of electrodeposited cobalt nanowires

Anodized aluminum oxide (AAO) templates have been used to synthesize Cobalt (Co) nanowires (NWs) by employing the electrodeposition technique as a function of cobalt ions concentration. Cobalt NWs show a strong dependence on cobalt ions concentration, applied during synthesis, in with reference to their structural and magnetic properties. The Co hexagonal (HCP) phase dominates X-ray diffraction, with the (002) plane oriented perpendicular to the NW long axis with lower applied deposition potential and parallel to the NW long axis at higher Co ions concentration. The reorientation of (002) HCP plane and evolution of face-cantered cubic phase (FCC) at higher concentrations of Cobalt ions results in easy axis of magnetization is switched to the long axis of NW from parallel. Co-NWs crystalline structure and magnetic characteristics might be adjusted based on Co ion concentration during synthesis.

Strong anisotropic enhancement of photoluminescence in WS2 integrated with plasmonic nanowire array

Layered transition metal dichalcogenides (TMDCs) have shown great potential for a wide range of applications in photonics and optoelectronics. Nevertheless, valley decoherence severely randomizes its polarization which is important to a light emitter. Plasmonic metasurface with a unique way to manipulate the light-matter interaction may provide an effective and practical solution. Here by integrating TMDCs with plasmonic nanowire arrays, we demonstrate strong anisotropic enhancement of the excitonic emission at different spectral positions. For the indirect bandgap transition in bilayer WS2, multifold enhancement can be achieved with the photoluminescence (PL) polarization either perpendicular or parallel to the long axis of nanowires, which arises from the coupling of WS2 with localized or guided plasmon modes, respectively. Moreover, PL of high linearity is obtained in the direct bandgap transition benefiting from, in addition to the plasmonic enhancement, the directional diffraction scattering of nanowire arrays. Our method with enhanced PL intensity contrasts to the conventional form-birefringence based on the aspect ratio of nanowire arrays where the intensity loss is remarkable. Our results provide a prototypical plasmon-exciton hybrid system for anisotropic enhancement of the PL at the nanoscale, enabling simultaneous control of the intensity, polarization and wavelength toward practical ultrathin photonic devices based on TMDCs.

Giant and Anisotropic Nonlinear Optical Responses of 1D van der Waals Material Tellurium

AbstractRecently, tellurium (Te), composed of helical Te chains stacked via the van der Waals interaction, emerges as a versatile material with high carrier mobilities and intriguing topological properties. Here, it is revealed that Te's 1D counterpart, that is, Te nanowire (NW), has advantages over the NWs of other materials in the characteristics of giant and anisotropic nonlinear optical responses. This endows the Te NW with potentials for developing nonlinear photonic and optoelectronic nanodevices with high efficiency and polarization selectivity. Compared with the well‐studied III–V and II–VI compound NWs, such as aluminium gallium arsenide, zinc oxide, and indium phosphide NWs, second harmonic generations (SHGs) and third harmonic generations (THGs) from the Te NWs are more than two orders of magnitude stronger. Its second‐order nonlinear susceptibility is estimated as 1.25 × 104 pm V−1, which is the highest value in the reported NWs, and almost four orders of magnitude higher than ZnO NW. Further, this giant optical nonlinearity promises the realizations of strong SHGs and sum‐frequency generations from Te NWs with the pump of sub‐milliwatt continuous‐wave lasers. Determined by the helical Te chain, its SHG only responds to the pump laser polarized perpendicularly to the long axis of NW, which is opposite to the polarization dependencies in other NWs, and the THG has a distorted fourfold polarimetric pattern.

Morphology control of magnetic properties in cobalt nanowires

Cobalt nanowires with different shapes and sizes were synthesized by reduction of carboxylate salts of CoII in 1, 2-butanediol using a solvothermal chemical process. The well-crystallized Co nanowires with hexagonal close-packed (hcp) phase are observed and the (002) crystalline direction is along the long axis of nanowires. The morphology control is strongly dependent on the reaction parameters. By varying the amount of capping agent in proper ranges, the effect of reaction parameters on controlling the size and shape of Co nanowires is demonstrated. With the amount of capping agent increasing, the aspect ratio of Co nanowires increases remarkably. However, the magnetic measurement of cobalt nanowires shows that the coercivity of the Co nanocrystals does not increase with the increase in aspect ratio monotonously, which suggests that the tip shape and microstructure also play an important role in the magnetization reversal process of the Co nanocrystals, and the aspect ratio plays a much less role as the ratio value exceeds 11. To further understand the effect of size on the magnetic properties in the Co nanowires, micromagnetic simulations were performed, which confirms that the magnetic properties are barely affected by the aspect ratio larger than 10. The highest coercivity of 624 kA·m−1 is obtained for ellipsoid nanowires with a mean length of 200 nm, which also displays a strong magnetic anisotropy. As a result, the highest energy product of the wires reaches 248 kJ·m−3.

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.

Oriented arrays of iron nanowires: synthesis, structural and magnetic aspects

Iron nanowires with the diameter of ca. 40 nm and a length up to few dozens of microns are fabricated via templated electrodeposition using anodic aluminum oxide (AAO) film as porous matrix. Despite polycrystalline structure of wires the technique allows fabrication of dense deposits with micrometer-sized single crystalline grains within AAO templates and high chemical stability towards oxidation. Nanowire arrays exhibit strong magnetization anisotropy with saturation magnetization of 180 emu/g and coercive field of 815 Oe in direction parallel to the long axis of nanowires and 230 Oe in perpendicular direction. The effective hyperfine fields on iron atoms as extracted from Mossbauer and Nuclear Forward Scattering of sample in demagnetized state indicates slight deviation of magnetization vector (~ 6°) from nanowire long axis appearing probably due to curling of magnetic moments by antisymmetric exchange interactions at the surface of nanowires.

Magnetically extracted microstructural development along the length of Co nanowire arrays: The interplay between deposition frequency and magnetic coercivity

Providing practical implications for developing the design and optimizing the performance of hard magnets based on nanowires (NWs) requires an in-depth understanding of the processes in fabrication and magnetic parameters. Here, an electrochemical deposition technique with different frequencies is used to fabricate 50 nm diameter Co NW arrays into the nanopores of anodic aluminum oxide templates. The resulting NWs with dendrites at the base are subsequently exposed to a chemical etching with which to prepare cylindrical Co NWs with an aspect ratio of 200. In this way, the coercivity at room temperature increases up to 20% for different deposition frequencies, indicating the occurrence of a magnetic hardening along the NW length. Decreasing the length of the cylindrical NWs in ascending order whilst also using a successive magnetometry, the deposition frequency is found to be an important parameter in further enhancing the initial coercivity up to 65% in the length range of 10 to 3 μm. The first-order reversal curve diagrams evaluated along the NW length evidence the elimination of a soft magnetic phase and the formation of harder magnetic domains when reducing the length. Alternatively, X-ray diffraction patterns show improvements in the crystallinity along the [002] direction, pertaining to the alignment of the hexagonal close-packed c-axis of cobalt and long axis of NWs when reducing the length. These results may address the growing need for the creative design and low cost fabrication of rare-earth-free permanent magnets with high coercivity and availability.

Single-Photon Emission from GaAs Quantum Dots Embedded in Nanowires**Supported by the National Key Basic Research Program of China under Grant No 2013CB922304, and the National Natural Science Foundation of China under Grant Nos 11474275 and 11204297.

A highly efficient single-photon source based on a semiconductor quantum dot (QD) is a promising candidate in quantum information processing. We report a single-photon source based on self-assembled GaAs QDs in nanowires with an extraction efficiency of 14%. The second-order correlation function g(2) (0) at saturate excitation power is estimated to be 0.28. The measured polarization of QD emission depends on the geometric relations between the directions of PL collection and the long axis of nanowires.

Confinement Induced Preferential Orientation of Crystals and Enhancement of Properties in Ferroelectric Polymer Nanowires.

The physical properties of polymers strongly depend on the molecular or supermolecular order and orientation. Here we demonstrate the preferential orientation of lamellar crystals and the enhancement of ferro/piezoelectric properties in individual poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) nanowires fabricated from anodic alumina oxide (AAO) templates. The crystallographic a axis of P(VDF-TrFE) was found to be aligned along the long axis of nanowires due to geometrical confinement and grapho-expitaxial crystals growth. The alignment of lamellar crystals in P(VDF-TrFE) nanowires and enhancement of crystallization translated into improved ferro/piezoelectric properties such as lower coercive field and higher piezoelectric coefficient, testified by piezoresponse force microscopy images and piezoresponse hysteresis loops.

Pt supported self-assembled nest-like-porous WO3 hierarchical microspheres as electrocatalyst for methanol oxidation

Hexagonal tungsten trioxide (hex-WO3) hierarchical microspheres with nest-like pores are synthesized by a facile hydrothermal method. The nest-like-porous (NLP) WO3 hierarchical microspheres with 5–6μm in diameters are self-assembled of single-crystal nanowires. The nanowires have lengths of several hundred nanometers and diameters of 5–30nm; the long axis of nanowire is oriented toward 〈001〉 direction. The specific surface area of hex-WO3 microspheres is 62m2g−1. 20wt.% Pt nanoparticles with ∼7nm are loaded onto the WO3 microspheres using a conventional microwave-assisted ethylene glycol (EG) method. The electrocatalytic activity for methanol oxidation of Pt/NLP-WO3 microspheres is investigated by cyclic voltammetry and chronoamperometry. Due to the large tunnels of hexagonal structure and high porous surface morphology, great enhancement of electrochemical performance is achieved. The Pt/NLP-WO3 microspheres are demonstrated to be a promising anode material for direct methanol fuel cells (DMFC).

The effect of substrate on magnetic properties of Co/Cu multilayer nanowire arrays

Ordered Co/Cu multilayer nanowire arrays have been fabricated into anodic aluminium oxide templates with Ag and Cu substrate by direct current electrodeposition. This paper studies the morphology, structure and magnetic properties by transmission electron microscopy, selective area electron diffraction, x-ray diffraction, and vibrating sample magnetometer. X-ray diffraction patterns reveal that both as-deposited nanowire arrays films exhibit face-centred cubic structure. Magnetic measurements indicate that the easy magnetization direction of Co/Cu multilayer nanowire arrays films on Ag substrate is perpendicular to the long axis of nanowire, whereas the easy magnetization direction of the sample with Cu substrate is parallel to the long axis of nanowire. The change of easy magnetization direction attributed to different substrates, and the magnetic properties of the nanowire arrays are discussed.

Effects of applied magnetic field on microstructure and magnetic properties of template-based Co nanowire arrays

Arrays of Co nanowires were fabricated by ac electrochemical deposition of metal into high order anodic alumina templates prepared through two-step oxidation method. The effects of applied magnetic field and the pH value of the electrolyte on growth of nanowires were investigated. Arrays of Co nanowires of hcp structure were deposited in the electrolyte whose pH values are 6.0 and 6.5 without magnetic field, then 0.3 T magnetic field was applied along the long axis of nanowires. The experimental results show that external magnetic field and adjustment of pH value of the electrolyte impel the easy magnetization axis of the hcp Co grain to grow along length direction of nanowires. The Co nanowire arrays show higher perpendicular anisotropy, larger coercivity and high M/Ms because the superposition of the crystal anisotropy of hcp Co grains and shape anisotropy of nanowire along length direction.

Optical and magnetic properties of 30 and 60 nm Ni nanowires

Ni nanowire arrays of high aspect ratio with the diameters of about 30 nm and 60 nm were prepared by DC applied AC electrodeposition. We observe the different preferred orientation and various magnetic behaviors of 30 and 60 nm diameter nanowires. In addition, the coercivity H c(||), squareness S(||) and the ratio H c(||)/ H c(⊥) where the applied field is parallel (||) and perpendicular (⊥) to the long axis of nanowires increase with decreasing wire diameter. This is the first time that optical results of Ni nanowires were presented.