Manipulation of optical properties of Ag/Cu alloy nanowire arrays embedded in anodic alumina membranes

Arrays of Ag/Cu alloy nanowires embedded in anodic alumina membranes (AAMs) were synthesized by directly electrodepositing from a mixing electrolyte solution containing Ag⁺ and Cu²⁺ ions. Manipulations of optical properties of the resulting samples were successfully achieved by tuning the molar ratio of Ag⁺ and Cu²⁺ ions in the starting materials. When the ratio is less than 2:20, two surface plasma resonance (SPR) peaks corresponding to Ag and Cu appear, respectively. After annealing treatment, the SPR peak corresponding to Cu disappears, and that of Ag presents a red shift. Furthermore, this red shift can be up to 85 nm when the molar ratio of Ag⁺ and Cu²⁺ reduce to 1:20, which is attributed to the transferable electrons from Cu atoms.     

Crystallographically-oriented single-crystalline copper nanowire arrays electrochemically grown into nanoporous anodic alumina templates

Highly crystallographically-oriented single-crystalline copper nanowire arrays were electrochemically deposited into nanoporous commercial alumina templates. A gold/copper backward contact was needed in the template, while the nanowires were grown from a 0.5 M CuSO₄·5H₂O solution adjusted to pH=1. The kinetics of the growing process is studied by means of current vs. time curves. The pore filling is between 80 to 90%. The structure and morphology of the wires are studied by XRD, SEM and TEM. The wires have an average diameter of 150 nm corresponding to the pore diameter of the template, with the cubic face-centered copper structure. This structure is highly oriented along the [100] direction parallel to the wire axis. The preferential growing along this direction was not previously found in the literature, which may have interesting applications when such direction is required. PACS 81.05.Bx; 82.45.Qr; 81.07.-b     

Photoluminescence properties of anodic alumina membranes with ordered nanopore arrays

Photoluminescence (PL) of anodic alumina membranes (AAMs) with ordered nanopore arrays fabricated in oxalic acid has been investigated under different annealing temperatures. The PL intensity firstly increases, and at 500 °C reaches a maximum value, then decreases. The structural transition from amorphous to γ-Al₂O₃ in AAMs has been confirmed by X-ray diffraction. Thermogravimetric analysis results and electron paramagnetic resonance measurements revealed that the PL band of alumina membranes could be attributed to the oxygen-related defect centers (F⁺ centers) rather than the luminescent centers transformed from oxalic impurities.     

Fabrication and photoluminescence properties of highly ordered ZnS nanowire arrays embedded in anodic alumina membrane

ZnS nanowire arrays embedded in anodic alumina membranes (AAM) were fabricated from an electrolyte containing ZnCl₂ and elemental S in dimethylsulfoxide. Photoluminescence (PL) measurements show a broadband with three peaks centered at about 353, 425, and 520 nm that are attributed to vacancies or interstitial, sulfur vacancy, and point defects respectively.     

Magnetic characterization of FeCo nanowire arrays by first-order reversal curves

FeCo nanowire arrays have been obtained by current pulse electrodeposition into nanoporous alumina templates. First-order reversal curve (FORC) diagrams have been used to investigate magnetostatic interaction and average coercivity of individual FeCo nanowires embedded in porous alumina templates. The FeCo nanowires with a wires length up to 3 μm and wires diameter ranging from 25 to 50 nm showed interacting single-domain behavior. Using FORC diagrams, the spread of coercivity distribution was seen to be almost independent of the wires diameter, but with increase in diameter the inter-wire magnetostatic interaction was increased. It was found that for arrays with higher diameter, the coercivity of the arrays is lower than the average coercivity of the individual wires. It was detected that an increase in wire diameter results in a considerable increase in the spread of the distribution in the H_u direction of FORC distribution. Curve fitting on the experimental data proved a relatively linear relation between interaction field and square diameter of the nanowires.     

Magnetic properties and Verwey transition of quasi-one-dimensional magnetite nanowire arrays assembled in alumina templates

Magnetite (Fe₃O₄) has been successfully assembled into anodic alumina templates by an electrochemical method followed by a heat-treating process. Here, we report on the magnetic properties of these so formed nanowires and the Verwey transition measured by vibrating sample magnetometer and SQUID. A Mössbauer spectrum was collected to verify the magnetic orientation of the wires, and a tilt of the moment of 45° with respect to the wire axis was found. These wires show perpendicular magnetic anisotropy mainly due to the average easy axis of the grains pointing along the wire axis. The temperature dependence of the coercity, remanence, and the magnetization undergo a major change at 50 K, induced by the Verwey transition, which occurs at a temperature much lower than for bulk materials (120 K). The behavior of the magnetization in the vicinity of 50 K as well as its field-dependent properties was interpreted using the magneto-electronic model.     

Catalyst-free growth of single-crystalline alumina nanowire arrays

Alumina nanowires were synthesized on a large-area silicon substrate via simple thermal evaporation of a mixture of aluminum and alumina powders. The microstructure of the as-grown alumina nanowires was characterized using X-ray diffraction and transmission electron microscopy techniques. These nanowires usually have a straight morphology and are single crystalline with the wire axis parallel to the (001) direction. Arrays of the alumina nanowires were also observed grown on the surface of many large particles. The catalyst-free growth of the alumina nanowires was explained in the framework of vapor–solid growth. PACS 81.07.-b; 81.16.-c; 81.07.Bc     

Magnetic crossover effect in Nickel nanowire arrays

A crossover effect in the magnetic reversal mechanism within arrays of Nickel nanowires whose diameter varies from 15 to 100 nm is observed around 50 nm. Hysteresis loops and FMR measurements confirm that nanowire diameter controls effectively the nanowire easy axis as well as the magnetization reversal mechanism. This might be very interesting for spintronic devices based on current-induced domain motion such as non-volatile magnetic memory elements (MRAM) and low Ohmic loss devices.     

Photoluminescence of anodic alumina membranes: pore size dependence

The photoluminescence (PL) of anodic alumina membranes with different pore sizes is investigated in this paper. We observe that a small pore size leads to a decrease of photoluminescence intensity, an increase of the recombination lifetime, and a broadening of the emission band. The origin of the PL characteristics are discussed together with the decay measurement of the emission band. PACS 78.47.+p; 78.55.Mb     

Magnetic domain structure in small diameter magnetic nanowire arrays

Fe_0.3Co_0.7 alloy nanowire arrays were prepared by ac electrodepositing Fe²⁺ and Co²⁺ into a porous anodic aluminum oxide (PAO) template with diameter about 50 nm. The surface of the samples were polished by 100 nm diamond particle then chemical polishing to give a very smooth surface (below ±10 nm/μm²). The morphology properties were characterized by SEM and AFM. The bulk magnetic properties and domain structure of nanowire arrays were investigated by VSM and MFM respectively. We found that such alloy arrays showed strong perpendicular magnetic anisotropy with easy axis parallel to nanowire arrays. Each nanowire was in single domain structure with several opposite single domains surrounding it. Additionally, we investigated the domain structure with a variable external magnetic field applied parallel to the nanowire arrays. The MFM results showed a good agreement with our magnetic hysteresis loop.     

Magnetic properties of amorphous β-FeOOH nanowire arrays

β-FeOOH nanowire arrays with diameters of 50–200 nm have been fabricated by electrochemical deposition using two-step anodic porous alumina templates. The as-prepared nanowires are homogeneous and have large aspect ratios. The selected area electron diffraction photo performed on a single wire was used to confirm the amorphous crystal structure further. The magnetic properties of these nanowire arrays were firstly investigated by using a SQUID magnetometry. The ZFC and FC studies show that these nanowire arrays exhibit spin-freezing phenomena at low temperature. The temperature-dependent magnetization curves show that the Neel transition temperatures are much lower than that of bulk material. Moreover, hysteresis was found at 5 K and the coercivities up to about 1500 Oe. The size-dependent magnetic properties were also investigated. These abnormal magnetic behaviours can be interpreted in terms of the amorphous crystal structure and the low dimensionality of the nanowire arrays.     

Profiling measurements of metal ion distribution in thin polymer inclusion membranes by Rutherford backscattering spectrometry

Polymer inclusion membranes (PIMs) composed of a homogeneous mixture of cellulose triacetate matrix, 2-nitro-phenyl-octyl-ether as plasticizer and tri-octyl-phosphine-oxyde as carrier were synthesized by the spin coating method. Synthesized membranes were doped with molybdenum metal ions and then characterized by four experimental techniques: thermo gravimetric and differential analyses, scanning electron microscopy (SEM), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and Rutherford backscattering (RBS) spectrometry using a 3.2 MeV He⁺ ion beam. The RBS analysis has established both the elemental composition as well as the Mo⁺ metal profiling of the studied PIMs. The experimental irradiation conditions were optimized in order to determine the ion fluence thresholds resulting in measurable changes in elemental composition of membranes. Changes in physico-chemical properties of the irradiated PIMs vs He⁺ ion fluence were observed with the ATR-FTIR analysis. Also, the SEM analysis of PIMs surfaces has revealed a porous texture, while the thermal analysis of annealed PIMs at 105°C has showed no significant changes of mass (～1%) of the studied samples.     

电化学沉积Fe与FePd纳米线阵列的磁性

利用电化学沉积方法在氧化铝模板中制备了一维Fe和Fe_095Pd_{0 05}合金纳米线阵列.两种样品均有(110)晶向择优取向，纳米线直径为60nm.在这一直径下形状各向异性 和内禀晶体各向异性的竞争结果很适合考察Pd掺杂的磁性行为.研究发现在FePd纳米线中， 由于极少量Pd在Fe中的合金化，减弱了晶体各向异性与形状各向异性的影响，改变了磁畴结 构，增强了畴壁钉扎作用，结果在Fe_095Pd_005纳米线 中便显示出强烈的沿线方向的各向异性，方形度和矫顽力也有较大改善. 关键词： 纳米线 电化学沉积 磁性     

Optical properties of alumina membranes prepared by anodic oxidation process

The luminescence property of anodic alumina membranes (AAMs) with ordered nanopore arrays prepared by electrochemically anodizing aluminum in oxalic acid solutions have been investigated. Photoluminescence emission (PL) measurement shows that a blue PL band occurs in the wavelength ranges of 300-600 nm. The PL intensity and peak position of AAMs depend markedly on the excitation wavelength. A new peak located at 518 nm can be observed under a monitoring wavelength at 429 nm in the photoluminescence excitation (PLE) spectra. Convincing evidences have been presented that the PLE would be associated with the residual aluminum ions in the membrane. The PLE and PL of AAMs, as a function of anodizing times, have been discussed. It is found that the oxalic impurities incorporated in the AAMs would have important influences on the optical properties of AAMs in the initial stage of anodization. The PL and PLE spectra obtained show that there are three optical centers, of which the first is originated from the F⁺ centers in AAMs, the second is correlated with the oxalic impurities incorporated in the AAMs, and the third is associated with the excess aluminum ions in the membrane.     

Fabrication and optical properties of platinum nanowire arrays on anodic aluminium oxide templates

Arrays of Pt nanowires, fabricated by electrodepositing Pt metal into nanoporous anodic aluminium oxide (AAO) templates, exhibit a preferable optical absorption band in the ultraviolet-visible (UV-VIS) spectra and present a blueshift as the wire aspect ratio increases or its radius decreases. This type of optical property of Pt nanowire/porous alumina composites has been theoretically explored using Maxwell－Garnett (MG) effective medium theory. The MG theory, however, is only applicable to nanowires with an infinitesimally small radius relative to the wavelength of an incident light. The nanowire radius is controlled by the pore radius of the host alumina, which depends on anodizing conditions such as the selected electrolyte, anodizing time, temperature and voltage. The nanowire aspect ratios depend on the amount of Pt deposited into the nanopores of AAO films. The optical absorption properties of the arrays of Pt nanowires with diameters of 24, 55 and 90 nm have been investigated by the UV-VIS spectra, which show that the extinction maximum (λ_max) shifts to shorter wavelength side as the wire aspect ratio increases or its radius decreases. The results are qualitatively consistent with those calculated based on the MG theory.     

Magnetic optical bifunctional CoPt_3/Co multilayered nanowire arrays

CoPt3/Co multilayered nanowire(NW) arrays are synthesized by pulsed electrodeposition into nanoporous anodic aluminum oxide(AAO) templates. The electrochemistry deposition parameters are determined by cyclic voltammetry to realize the well control of the ratio of Co to Pt and the length of every segment. The x-ray diffraction(XRD) patterns show that both Co and CoPt3 NWs exhibit face-centered cubic( fcc) structures. In the UV-visible absorption spectra,CoPt3/Co NW arrays show a red-shift with respect to pure CoPt3 NWs. Compared with the pure Co nanowire arrays, the CoPt3/Co multilayered nanowire arrays show a weak shape anisotropy and well-modulated magnetic properties. CoPt3/Co multilayered nanowires are highly encouraging that new families of bimetallic nanosystems may be developed to meet the needs of nanomaterials in emerging multifunctional nanotechnologies.     

Magnetic and magnetization properties of electrodeposited fcc CoPt nanowire arrays

Magnetic and magnetization properties of fcc Co_1−xPt_x (x?0.3) alloy nanowires fabricated by electrodeposition into self-synthesized anodic alumina templates are investigated. Magnetization curves, measured for varying wire geometries, show a crossover of easy axis of magnetization from parallel to perpendicular to the nanowire axis as a function of the diameter and length. The measured values of coercivity (H_c) and remanent squareness (SQ) of CoPt nanowire arrays, as a function of angle (θ) between the field and wire axis, support the crossover of easy axis of magnetization. The curling mode of the magnetization reversal process is observed for CoPt nanowire arrays. At low temperatures, the easy axis for magnetization of the nanowires is observed to deviate from the room-temperature orientation.     

Self-assembled growth of C60 nanowhiskers on anodic porous alumina membranes

Unique structured claw-like C₆₀ nanowhiskers were fabricated on an anodic porous alumina membrane. Tips of the C₆₀ nanowhiskers are shaped like cusps, which directly extend into the nanopores of the alumina membrane. It is revealed that the surface stress of the alumina membrane with a highly ordered nanopore arrangement due to thermal treatment is responsible for self-assembled growth of the observed C₆₀ nanowhiskers. The ordering of the nanopore arrangement in the anodic porous alumina membrane helps to form the C₆₀ nanowhiskers along the nanopore channels. The shape and structure of the C₆₀ nanowhiskers are promising characteristics for applications in nanodevices such as field-emission tips and nanoprobes. PACS 61.46.+w; 61.48.+c; 33.20.Fb