Two-step synthesis of one-dimensional single crystalline GaN nanowires

Straight and smooth GaN nanowires were synthesized on quartz substrates through the direct reaction of Ga₂O₃ thin films with flowing ammonia in a horizontal oven without using a template or catalyst. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), transmission electron microscopy (TEM) and photoluminescence (PL) were used to characterize the samples. The straight and smooth cylindrical nanostructures are high quality single crystalline hexagonal wurtzite GaN nanowires with diameters ranging from 5 to 30 nm and lengths up to 20 μm. The near-band-edge emission peak located at 367 nm was observed at room temperature.     

Magnetization reversal dynamics in Co nanowires with competing magnetic anisotropies

We present the magnetization reversal dynamics of Co nanowires with competing magnetic anisotropies. The aspect ratio (R) of the nanowires is varied between 2.5 and 60, and we observe a cross-over of the directions of the magnetic easy and hard axes at R=6.8. Micromagnetic simulations qualitatively reproduce the observed cross-over and give detailed insight into the reversal mechanisms associated with the cross-over. The reversal mechanism for a field applied along the long axis of the nanowire exhibits a quasi-coherent rotation mode and a corkscrew-like mode, respectively, above and below the cross-over, with the formation of a Bloch domain near the cross-over region. For a field applied along the short axis, the reversal occurs by nucleation and propagation of reversed domains from the two ends of the nanowires for very high values of the aspect ratio down to the cross-over region, but it transforms into quasi-coherent rotation mode for smaller aspect ratios (below the cross-over region).     

Structure and magnetism of electrodeposited ZnSe–Co granular films

ZnSe–Co granular films have been electrodeposited under potentiostatic condition on polished stainless steel substrates. X-ray diffraction and transmission electron microscopy measurements indicate the formation of polycrystalline granular films with Co particles dispersed in an ZnSe matrix without evidence of intermediate compound formation. Magnetic measurements reveal low values of coercive field and remanence, indicating that the overall magnetic response of the deposited films is determined by multidomain nanosized particles.     

Magnetic properties of one-dimensional embedded nickel nanostructures in gold nanowires

Magnetic nanostructures of nickel embedded in gold were successfully fabricated by electrochemical deposition in porous alumina templates. Structural characterization of the samples confirmed the formation of pure phase, crystalline multi-segmented Au-Ni-Au nanowires. Magnetic characterization of the wires reveals that ferromagnetism arises as a result of Ni embedded in Au segments. An interesting behavior of coercivity was observed that showed a rapid decrease of coercivity for smaller Ni segments while a monotonic decrease was found for the larger segments. Finally, the saturation magnetization of the wires exhibited a slower increase for smaller Ni segments while a sharp increase was observed for larger Ni segments.     

Thermal diffusivity of electrodeposited Ni and Co nanowires using infrared thermography

One-dimensional nanostructures such as Ni and Co nanowires (NWs) show anisotropic thermal properties in a direction parallel and perpendicular to the NW axis. Thermal diffusivity of Ni and Co NWs embedded in a 100-nm pore anodic alumina (AAO) template has been measured in a direction perpendicular to the NW axis, using an infrared thermography-based non-contact approach. The measured thermal diffusivity values in the radial direction are 0.728×10⁻⁶ and 0.732×10⁻⁶ m²s⁻¹, respectively, for the Ni and Co nanocomposites. The changes in the thermal diffusivity of the synthesized NWs alone were estimated using a first-order lower bound model (FOLBM). A nearly seven- and sixfold reduction, respectively, of thermal diffusivity in a direction perpendicular to the NW axis is estimated for the synthesized Ni and Co NWs.     

Magnetic nanowires

We review recent developments in the research on magnetic nanowires electrodeposited into pores of membranes. Typical nanowires fabricated by this method have a diameter in the range 30–500 nm for a length of the order of 10 μm, and can be composed of a stack of layers of different metals with thicknesses in the nanometer range (multilayered nanowires). We describe the preparation methods and present typical examples of structural characterization. We review the magnetic properties with examples of results on both arrays of nanowires and isolated nanowires. We then describe the magnetoresistance properties of multilayered nanowires, and their interest for their understanding of the CPP–GMR and the determination of spin diffusion lengths. The last section is an overview on the perspectives of future research.     

Cr掺杂ZnO纳米线的电子结构和磁性

采用自旋极化密度泛函理论系统研究了Cr掺杂ZnO纳米线的电学、磁学以及光学属性.计算结果显示,Cr原子沿[0001]方向替代ZnO纳米线中的Zn原子时体系一般呈现铁磁耦合,沿[1010]和[0110]方向替代Zn原子时体系呈现反铁磁耦合,且磁性耦合状态在费米能级附近出现了明显的自旋劈裂现象,发生了强烈的Cr 3d和O 2p杂化效应.自旋态密度计算结果显示,磁矩主要来源于Cr原子未成对3d态电子的贡献,磁矩的大小与Cr原子的电子排布有关.光学性质计算结果显示,Cr掺杂ZnO纳米线在远紫外和近紫外都具有明显的吸收峰,吸收峰发生了明显的红移.这些结果都表明Cr掺杂ZnO纳米线也许是一种很有前途的稀磁半导体材料. 关键词： ZnO 纳米线 第一性原理 磁性     

Microstructural and magnetic properties of CoPt nanowires

In this work, the magnetic and microstructural properties of CoPt nanowires are presented as a function of the electrolyte pH and current density during electrodeposition into anodized alumina templates. CoPt nanowires of high aspect ratio have been prepared using electrolyte pH values in the range from 2 to 6. The as-made samples exhibit a face centered cubic (fcc) structure with soft magnetic properties which transform into the face centered tetragonal (fct) L10 phase after thermal treatment. Different pH values of the electrolyte during electrodeposition lead to significantly different microstructures and, therefore, different magnetic properties. The CoPt nanowires prepared at high pH value are composed of fcc nanorods of about 25 nm in length. Thermal annealing of these samples leads to a preferred (0 0 1) orientation (along the direction perpendicular to the direction of nanowires) which increases with annealing time. On the other hand, the CoPt nanowires prepared at lower pH value are composed of uniform fcc nanograins with the size ∼2−3 nm. Magnetization curves for the later sample are virtually identical in both directions indicating an isotropic behavior.     

Gas sensing properties of single crystalline ZnO nanowires grown by thermal evaporation technique

The ZnO NWs were applied as effective material for the fabrication of ethanol (C₂H₅OH) and carbon monoxide (CO) gas sensor. The ZnO NWs were grown by thermal evaporation techniques on non-catalytic Si (100) substrates. The average width and length of ZnO NWs was 60 nm and 20 μm, respectively and they were single crystalline in nature. The maximum response was 51.64 at 300 °C for 1000 ppm of CO gas, while 104.23 at 400 °C for 250 ppm of ethanol gas. The response of ZnO NWs was very high for ethanol compared to the CO, whereas the recovery time for ethanol was very poor compare to CO gas. The response of ZnO NWs was about 25 times higher for ethanol compare to CO, at 400 °C for 100 ppm of each gas. The high response for ethanol is related to electron donating effect of ethanol (10e^?) which was higher than the CO gas (2e^?). The high response of ZnO NWs was attributed to large contacting surface area for electrons, oxygen, target gas molecule, and abundant channels for gas diffusion.     

Structural and electrical studies of template synthesized copper nanowires

The template strategy combined with electrodeposition technique has been used to produce copper nanowires in the cylindrical pores of track-etched polycarbonate membranes. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy have been used to characterize as-prepared copper nanowires. XRD study shows the face centered cubic crystal structure of copper nanowires. Williamson–Hall (WH) analysis has been used to determine the crystallite size and microstrain induced due to lattice deformation. FESEM results reveal that copper nanowires are continuous, well aligned with uniform diameter and having high aspect ratio. The optical absorption spectra exhibit a strong peak at 568 nm attributed to the surface plasmon resonance. The current–voltage (I–V) characteristics show an ohmic behavior of the fabricated copper nanowires. The increase in resistivity of nanowires than that of bulk counterpart has been attributed to the surface and size effects in nanowires and explained in the light of Fuchs–Sondeimer and Mayadas–Shatzkes models.     

An optical study of cobalt nanowires dispersed in liquid phase

The Co nanowires were electrodeposited in polycarbonate membrane (PCT). SEM, TEM and XPS techniques were used to characterize the morphology, structure and size of nanowires as well as chemical composition. The influence of different mediums was studied on the optical absorption of dispersed cobalt nanowires. The absorption spectrum of cobalt nanowires in water showed a broad shoulder at 290 nm, but in ethanol solution it was not observed in the visible region of the spectra up to 200 nm. Cobalt nanowires dispersed in methanol presented a peak at 236 nm. We attribute the data to oxidation of cobalt in water and low dielectric constant of methanol relative to ethanol and water. We found rather good agreement between the experimental results and the calculation based on Gans’ theory.     

Magnetic field driven domain-wall propagation in magnetic nanowires

The mechanism of magnetic field induced magnetic domain-wall (DW) propagation in a nanowire is revealed: A static DW cannot exist in a homogeneous magnetic nanowire when an external magnetic field is applied. Thus, a DW must vary with time under a static magnetic field. A moving DW must dissipate energy due to the Gilbert damping. As a result, the wire has to release its Zeeman energy through the DW propagation along the field direction. The DW propagation speed is proportional to the energy dissipation rate that is determined by the DW structure. The negative differential mobility in the intermediate field is due to the transition from high energy dissipation at low field to low energy dissipation at high field. For the field larger than the so-called Walker breakdown field, DW plane precesses around the wire, leading to the propagation speed oscillation.     

Ultrafine Fe-Co nanowires: Fabrication and heat treatment influence on the structure and magnetic properties

Ultrafine nanowires of Fe-Co with a diameter around 15 nm have been fabricated by electrodeposition method using anodic porous alumina as a template. The alloy nanowires were in the form of arrays and consisting of polycrystalline structures. They showed obvious shape anisotropy parallel to the axis of nanowires and the perpendicular coercivity (Hc_⊥) was found to be 2576.8 Oe which is higher than any coercivity value reported in the literature. The effects of critical factors such as heat treatment and temperature of annealing on the structure and magnetic properties of the ultrafine nanowire arrays were studied and discussed.     

Magnetoresistance of single Co nanowires

Polycrystalline single Co nanowires are prepared by electron beam lithography on GaAs substrates at room temperature. The width of the Co nanowires is varied between 150 and 4000 nm. Magnetoresistance measurements are carried out in a temperature range between 1.5 and 45 K applying magnetic fields μ₀H up to 4.5 T parallel and perpendicular to the current direction. The in plane (longitudinal) magnetoresistance (MR) shows pronounced features at magnetic fields H_c (coercive fields) indicating the magnetization reversal process. From the MR-curves we determined H_c as a function of the angle α between current and field direction (from in plane to out of plane) and of the width w of the Co nanowires. The H_c=H_c(α,w) behavior allows to discuss the reversal process in more detail.     

C掺杂ZnO纳米线的磁性研究

运用第一性原理方法研究了C掺杂ZnO纳米线的电子性质和磁性质.研究发现C原子趋于替代纳米线表面的O原子.所有掺杂纳米线显示了半导体特性.纳米线的总磁矩主要来源于C原子2p轨道的贡献.由于杂化,相邻的Zn原子和O原子也产生了少量自旋.在超原胞内,C、Zn和O原子磁矩平行排列,表明它们之间是铁磁耦合.铁磁态和反铁磁态的能量差达到了186meV,表明C掺杂ZnO纳米线可能存在室温铁磁性,在自旋电子学领域有很大应用前景.     

Competition between surface energy and interphase energy in transition region and diameter-dependent orientation of silicon nanowires

A small sandwiched transition region between the Au catalysis droplet and silicon nanowires (SiNWs) is proposed to investigate the diameter-dependent orientation of SiNWs grown by the vapor-liquid-solid (VLS) mechanism. Atomic-scale calculation shows that for a given transition region width, there is always a critical diameter. Below the critical value, surface energy dominates and the 〈1 1 0〉 orientation is preferred, whereas at larger diameters, the interphase energy dominates and SiNWs grow along the 〈1 1 1〉 direction. The variability of the critical diameter is also included in our model by adjusting the transition region width. The theoretical results are in agreement with those from experiments.     

Influence of the electrical parameters on the fabrication of copper nanowires into anodic alumina templates

Metallic copper nanowires have been grown into the pores of alumina membranes by electrodeposition from an aqueous solution containing CuSO₄^.and H₃BO₃ at pH 3. In order to study the influence of the electrical parameters on growth and structure of nanowires, different deposition potentials (both in the region where hydrogen evolution reaction is allowed or not) and voltage perturbation modes (constant potential or unipolar pulsed depositions) were applied. In all cases, pure polycrystalline Cu nanowires were fabricated into template pores, having lengths increasing with the total deposition time. These nanowires were self-standing, because they retain their vertical orientation and parallel geometry even after total template dissolution.However, the electrical parameters influence the growth rate, length uniformity and crystal size of the nanowires. Continuous electrodeposition resulted in higher growth rates but less uniform lengths of nanowires grown inside different membrane pores, whilst a square pulse deposition produced a slower growth but quite uniform lengths. Also the grain size, of the order of 50 nm, was slightly influenced by the potential perturbation mode.     

Characterizing magnetic interactions in Ni nanowires by FORC analysis

Polycrystalline Ni nanowires with different diameters were electrodeposited in nanoporous anodized alumina membranes. First-Order Reversal Curves (FORCs) were measured and FORC distributions were calculated. They clearly showed an asymmetric behavior with a strong maximum at negative interaction fields, evidencing the dominant demagnetizing interactions which depend on the geometry of the nanowires.