Fabrication of integrated arrays of ultrahigh density magnetic nanowires on glass by anodization and electrodeposition

Integrated nanowire arrays of Fe-Pt, Co-Pt, and Ni-Pt alloys were successfully fabricated on glass substrates by successive anodization and electrodeposition. Porous alumina films, which were formed from an aluminum layers sputter-deposited on glass substrates covered with transparent oxide conductive films, were used as template-electrodes to deposit various magnetic alloys (Fe-Pt, Co-Pt, and Ni-Pt) in the nanopores by a cathodic electrodeposition, thus leading to integrated nanowire arrays with ultrahigh densities of (0.6-2.1) × 10¹⁵ wire m⁻². The as-deposited nanowires of Fe-Pt, Co-Pt, and Ni-Pt alloys are polycrystalline and composed of fine crystals (4-7 nm across) of chemically ordered tetragonal FePt, CoPt, or NiPt phase. The integrated nanowire arrays may be the promising candidate materials for ultrahigh density perpendicular magnetic recording media in terabits per square inch regime, due to the predictable enhanced perpendicular magnetic performance after appropriate annealing.     

Effect of temperature on the growth of α-PbO2 nanostructures

Ordered arrays of α-PbO₂ nanostructures were grown by galvanostatic anodic deposition into the channels of alumina templates. Electrodepositions were performed in an aqueous solution containing lead acetate and sodium acetate at pH 5.4. Bath temperature and electrodeposition time were varied to check their effect on the growth of nanostructures. It has been found that filling of alumina pores is independent of the time and electrodeposition temperature, whilst height and growth kinetics of nanostructures vary with both parameters. Temperature greatly influences morphology: wires grown at room temperature consisted of clusters of particles, leading to poorly compact structures, whilst at 40 °C the formation of compact wires was observed, some of them being partially hollow. Finally, nanowires with a very regular and dense structure were obtained at 60 °C. Also crystalline structure changes with temperature: polycrystalline nanostructures, with a preferential orientation and an average grain size of 21 nm, were obtained at room temperature, whilst at higher temperatures, wires grew almost exclusively along the plane (2 0 0) with an average grain size of approximately 28 nm. Real surface area of nanowires was evaluated from the FEG-SEM images, and it was found two orders of magnitude greater than the geometrical one.     

Magnetic properties of electrodeposited CoFeB thin films and nanowire arrays

A pulse electrodeposition method for preparing stress-free CoFeB thin films is described. The method was optimized to produce amorphous films with soft magnetic properties and the best composition was found to be Co₉₄Fe₅B₁. The optimized conditions were used to obtain arrays of ordered nanowires, using nanoporous alumina membranes as templates. While magnetization measurements demonstrate that the microstructure of the array influences the detailed characteristics of the hysteresis loops, the general pattern of the magnetization curves, characterized by high saturation fields and low squareness, is the same as for crystalline Co arrays obtained in similar conditions. This observation, as well as data obtained from variable temperature measurements, shows that the overall magnetic behavior is determined mainly by the competition of the shape anisotropy and magnetostatic interactions. Whether the wire axis is easy or hard for the array depends upon the array geometry: wire diameter, length and the packing density of the wires. In order to explain these effects, a micromagnetic model was used to calculate the saturation fields for in-plane and out-of-plane magnetized array, as a function of the geometrical parameters. These agree well with the experimental results.     

Ordered arrays of copper nanowires enveloped in polyaniline nanotubes

Copper nanowires enveloped in polyaniline (PANI) nanotubes were obtained by ‘second order’ electrodeposition into the pores of anodic porous alumina. The templated synthesis of copper nanowires was performed by both potentiostatic and galvanostatic methods. The morphology of the polyaniline nanotubes, copper nanowires as well as the copper-filled polyaniline nanotubes was investigated by means of scanning electron microscopy. The copper nanowires were protected from corrosion and oxidation by the PANI nanotubes. Energy-dispersive X-ray spectroscopy was performed for the microanalysis of the copper deposition into the polyaniline nanotubes. Cyclic voltammetry was employed to assess the electrochemical properties of the obtained nanostructures as well as the influence of the copper nanowires synthesis method on the properties of filled polyaniline nanotubes.     

Influence of Anodic Conditions on Self-ordered Growth of Highly Aligned Titanium Oxide Nanopores

Self-aligned nanoporous TiO₂ templates synthesized via dc current electrochemical anodization have been carefully analyzed. The influence of environmental temperature during the anodization, ranging from 2 °C to ambient, on the structure and morphology of the nanoporous oxide formation has been investigated, as well as that of the HF electrolyte chemical composition, its concentration and their mixtures with other acids employed for the anodization. Arrays of self-assembled titania nanopores with inner pores diameter ranging between 50 and 100 nm, wall thickness around 20–60 nm and 300 nm in length, are grown in amorphous phase, vertical to the Ti substrate, parallel aligned to each other and uniformly disordering distributed over all the sample surface. Additional remarks about the photoluminiscence properties of the titania nanoporous templates and the magnetic behavior of the Ni filled nanoporous semiconductor Ti oxide template are also included.     

Electrochemical Surface Modification of Aluminium Sheets for Application to Nano-electronic Devices: Anodization Aluminium and Electrodeposition of Cobalt-Copper

A nano-porous anodized aluminium oxide layer was synthesized on the surface of bulk aluminium at a wide range of anodization voltages. The barrier layer at the pore bottom of anodized aluminium oxide layer was chemically etched to make good electrical contact for nanowires electrodeposited in the pores thus formed on metallic aluminium substrates. Cathodic polarization was examined at a wide range of cathode potentials to investigate the electrodeposition behaviour of Cu and Co into the pores. Co₈₁Cu₁₉/Cu multilayered nanowires were fabricated using a pulse-plating technique into the templates. Co-alloy layer and Cu layer thicknesses were adjusted to 10 nm, by controlling the deposition times. The temperature dependence of the resistance of Co₈₁Cu₁₉/Cu multilayered nanowires grown on the template presented clean metallic characteristics and a giant magnetoresistance (GMR) of 23% was reached at 4 K.     

Synthesis of magneto-dielectric coatings in electron-beam produced plasma in medium vacuum

Using sequential electron-beam evaporation of high-temperature dielectric (alumina ceramic) and magnetic (iron) targets in various gas atmospheres (helium, air, and oxygen) in medium vacuum (5–8 Pa), magneto-dielectric coatings with thickness of around 2 μm were deposited from a multicomponent beam plasma at a deposition rate of 0.2–0.3 μm/min. The coating magnetic properties were explored by the ferromagnetic resonance technique, revealing that their effective magnetization depends on the type of operating gas and varied from 4.2 to 6.8 kGs (for deposition in helium) to 0.3 kGs (in oxygen), which is characteristic of oxide ferromagnetic materials and is considerably lower than the corresponding value (∼22 kGs) for thin iron films formed by vacuum arc deposition in high vacuum. X-ray structural analysis of coatings deposited in medium vacuum in helium showed that the magnetic layer has a magnetite (Fe₃O₄) structure. The alumina ceramic layer provides the dielectric properties of the magneto-dielectric coating; a relative dielectric constant of 6.0 and a conductivity of 8.4 mS/m were achieved.     

Ordered domain engineering and physical property modification of Ba(Co1/3Nb2/3)O3 complex perovskite ceramics

In the present work, the physical properties modification by ordered domain engineering has been systematically investigated in Ba(Co_1/3Nb_2/3)O₃ ceramics. The 1:2 ordered structure with an ordering degree up to 0.92 with uniform ordered domain size distribution is achieved in Ba(Co_1/3Nb_2/3)O₃ ceramics after post-densification annealing, where the significantly enhanced electric resistivity, thermal conductivity, and dielectric strength, and subsequently the superior energy storage characteristics are achieved. The dielectric strength increases from 618 kV cm^–1 to 972 kV cm^–1 in the present ceramics by such means of ordered domain engineering, while the energy storage density increases from 0.65 J cm^–3 to 1.42 J cm^–3. There are three primary parameters in ordered domain engineering: ordering degree, ordered domain size and its distribution, and ordered domain boundary type. The final physical properties are determined by these three parameters.     

Electro-reduction of oxygen and electro-oxidation of methanol at Pd monolayer-modified macroporous Pt electrode

With polystyrene latex spheres self-assembled on indium tin oxide-coated glass electrode as templates, highly ordered macroporous Pt was prepared by electrochemical deposition. Then, the macroporous Pt was modified by Pd monolayer involving the galvanic displacement of Cu monolayer formed by under-potential deposition on macroporous Pt. Electrocatalytic properties of the Pd-modified macroporous Pt electrode for oxygen reduction were investigated by cyclic voltammetry and chronoamperometry in O₂-saturated solution containing 0.1 M HClO₄. Methanol electro-oxidation on the Pd-modified macroporous Pt surfaces in 0.5 M H₂SO₄ containing 1 M CH₃OH was studied by cyclic voltammetry. The corresponding results showed that Pd-modified macroporous Pt electrode had negative catalytic activity for methanol oxidation in compared with macroporous Pt. However, Pd-modified macroporous Pt electrode had positive electrocatalytic activity to O₂ reduction.     

Electrodeposition of hard magnetic films and microstructures

Electrochemical deposition of materials with hard magnetic properties in the as-deposited state is essential for the efficient integration of micro-magnetic components into MEMS devices. Here we discuss the growth process and the microstructure-magnetic properties correlation for two Co-rich alloys, Co-Ni-P and Co-Pt. Under suitable synthesis conditions, these materials exhibit perpendicular anisotropy and hard magnetic properties in the as-deposited state; in addition, such properties are maintained up to several micrometer film thickness through close control of the film microstructure. In the case of Co-Ni-P films we achieved a saturation magnetization of 1.21 T (963 emu/cm³), perpendicular coercivity up to 188 kA/m (2.36 kOe) at a thickness of 10 μm, and energy products up to 4.2 kJ/m³. Co-rich Co-Pt films were grown on several substrates - Cr, Cu(0 0 1), Cu(1 1 1), and Ru(0 0 0 1) - in order to control magnetic anisotropy and achieve optimum hard magnetic properties. Cu(1 1 1) contributes to stabilize the hexagonal hcp phase at high current density yielding excellent hard magnetic properties, although only in films thicker than 100 nm; saturation magnetization in these films was about 1.04 T (828 emu/cm³). Perpendicular coercivities up to 485.6 kA/m (6.1 kOe) were obtained in 1 μm thick film deposited at 50 mA/cm². Ru(0 0 0 1) seed layers provide an appropriate interface structure to further facilitate the epitaxial growth of hcp films, yielding hard magnetic properties and perpendicular coercivity with a squareness ∼1 in films as thin as 10 nm. The hard magnetic properties were only marginally compromised at large film thickness. Deposition at higher current density (50 mA/cm²) favored markedly improved hard magnetic properties. The Co-Pt films on Ru exhibited perpendicular anisotropy with anisotropy constant up to 1.2 MJ/m³. The electrodeposition process was further extended to fill lithographically patterned hole arrays (850 nm diameter, center-to-center distance 2550 nm and about 700 nm thick resist), yielding arrays of micron-sized hard magnetic cylinders with perpendicular coercivity of 361 kA/m (4.54 kOe) and high squareness.     

Effects of anodization and electrodeposition conditions on the growth of copper and cobalt nanostructures in aluminum oxide films

Anodic aluminum oxide (AAO) films were prepared by alternative current (ac) oxidation in sulfuric acid and phosphoric acid solution. The porous structure of the AAO templates was probed by ac electrodeposition of copper. AAO templates grown using an applied square waveform signal in cold sulfuric acid solution exhibit a greater pore density and a more homogeneous barrier layer. UV–vis–NIR reflectance spectra of the Cu/AAO assemblies exhibit a plasmon absorption peak centered at 580 nm, consistent with the formation of Cu nanostructures slightly larger than 10 nm in diameter. Spectroscopic data also indicate that there is little or no oxide layer surrounding the Cu nanostructures grown by ac electrodeposition. The effect of pH of the cobalt plating solution on the magnetic properties of the Co/AAO assemblies was also investigated. Co nanowire arrays electrodeposited at pH 5.5 in H₂SO₄-grown AAO templates exhibit a fair coercivity of 1325 Oe, a magnetization squarness of about 72%, and a significant effective anisotropy. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Tubular Sn-filled carbon nanostructures on ITO: Nanocomposite material for multiple applications

Hollow carbon nanostructures filled by metallic Sn were fabricated by means of chemical vapor deposition on transparent Indium Tin Oxide (ITO). We found no need for catalytic particles, and the growth happens in the temperature range 820–940 K. Upon annealing in an oxygen atmosphere, the carbon skin could be burned out, leaving SnO_x pillars on the ITO substrate. The electrical and optical properties of the grown Sn/C and SnO_x nanopillars were characterized.This growth strategy is versatile and can suitably be adapted to different substrate materials, provided that ITO can be deposited and annealed at the temperature required for the formation of the nanostructures. The rational control of this simple growth process and the lack of deposited external catalysts allow the fabrication of ordered, possibly, vertically aligned nanopillars over large areas, with tunable morphological, electrical and optical characteristics. This approach is envisaged as a promising path to develop energy generation and storage electrodes or chemical sensors with improved efficiency.     

Electrochemical energy storage in ordered porous carbon materials

Highly ordered porous carbon materials obtained by a replica technique have been used for supercapacitor application and electrochemical hydrogen storage. For the preparation of the well-tailored carbons, MCM-48, SBA-15 and MSU-1 molecular sieves served as templates, whereas a sucrose solution, propylene and pitch were the carbon source. A careful physico-chemical characterization (CO₂ and N₂ adsorption, X-ray diffraction, electron microscopy observations) allowed to estimate the total surface area, the pore size distribution, the micro/mesopore volume as well as the structure and the microtexture of the investigated carbons. The specific capacitance (F/g) and the hydrogen adsorption capacity in the carbon nanopores were correlated with the microtextural properties. Especially, a linear dependence has been found between the capacitance or the amount of electrochemically stored hydrogen and the ultramicropores (pores smaller than 0.7 nm) volume. It clearly indicates that in these carbons: (a) the major part of the electrical double layer is charged with non-solvated ions; (b) ultramicropores play a determinant role for hydrogen storage.     

Filling of mesoporous silicon with copper by electrodeposition from an aqueous solution

Copper filling into mesopores formed in highly doped p-type silicon was investigated. When the copper electrodeposition was carried out at a very small constant current density (−6.4 μA cm⁻²), the mesopores with 4 μm depth were filled with copper continuously from the bottom to the opening. When the electrodeposition current was set at an absolute value twice as large as in the above condition, the isolated particles were electrodeposited in the mesopores. The depth also affected the filling behavior. The pores with 8 μm in depth were not continuously filled with copper even in the condition at which the pores of 4 μm in length were completely filled. Electrodeposition behavior in mesopores was also simulated using a simple model. The numerical simulation suggested that the diffusion-limited electrodeposition could be achieved in mesopores at a very small current, at which the diffusion-limited condition had never been realized on a planar electrode.     

Preparation of hydroxyapatite coating on smooth implant surface by electrodeposition

A novel process for the deposition of a hydroxyapatite (HA) coating on a smooth implant surface has been developed. Specimens were firstly subjected to electrodeposition at −1.8 V (versus Ag/AgCl) in a mixed solution of 0.042 M Ca(NO₃)₂·4H₂O and 0.025 M NH₄H₂PO₄ at 85 °C for 5 s, and then post-treated in 1 M NaOH solution for 30 min. The experimental results showed the specimens prepared by the designed process to have better adhesion properties than those prepared by the traditional electrodeposition process.     

Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>–Au and Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–Au Hybrid Nanorods: Layer-by-Layer Assembly Synthesis and Their Magnetic and Optical Properties

A layer-by-layer technique has been developed to synthesize FeOOH–Au hybrid nanorods that can be transformed into Fe₂O₃–Au and Fe₃O₄–Au hybrid nanorods via controllable annealing process. The homogenous deposition of Au nanoparticles onto the surface of FeOOH nanorods can be attributed to the strong electrostatic attraction between metal ions and polyelectrolyte-modified FeOOH nanorods. The annealing atmosphere controls the phase transformation from FeOOH–Au to Fe₃O₄–Au and α-Fe₂O₃–Au. Moreover, the magnetic and optical properties of as-synthesized Fe₂O₃–Au and Fe₃O₄–Au hybrid nanorods have been investigated.     

Fabrication of cobalt-nickel binary nanowires in a highly ordered alumina template via AC electrodeposition

Cobalt-nickel (Co-Ni) binary alloy nanowires of different compositions were co-deposited in the nanopores of highly ordered anodic aluminum oxide (AAO) templates from a single sulfate bath using alternating current (AC) electrodeposition. AC electrodeposition was accomplished without modifying or removing the barrier layer. Field emission scanning electron microscope was used to study the morphology of templates and alloy nanowires. Energy-dispersive X-ray analysis confirmed the deposition of Co-Ni alloy nanowires in the AAO templates. Average diameter of the alloy nanowires was approximately 40 nm which is equal to the diameter of nanopore. X-ray diffraction analysis showed that the alloy nanowires consisted of both hexagonal close-packed and face-centered cubic phases. Magnetic measurements showed that the easy x-axis of magnetization is parallel to the nanowires with coercivity of approximately 706 Oe. AC electrodeposition is very simple, fast, and is useful for the homogenous deposition of various secondary nanostuctured materials into the nanopores of AAO.     

Production of alumina templates suitable for electrodeposition of nanostructures using stepped techniques

In recent years, the possibility of using anodized aluminium as a template to obtain nanowires and nanotubes via electrodeposition has been firmly established. The main problem with this method is the high electrical resistance generated by the barrier layer that isolates the metallic base from the electrodeposition bath. An easy way to decrease this resistance is to perform an additional step before deposition: a barrier layer thinning (BLT) process . Different BLT procedures have been described and although some of them work well, the process is not yet well understood. Here, we present an optimized stepped galvanostatic BLT process and discuss how BLT is achieved. As opposed to the common belief that BLT takes place via dendritic porous growth at the bottom of the porous anodic alumina (PAA), we show that during thinning, a controllable branched-shaped porous structure is generated. Finally, we also analyze the use of stepped techniques to obtain alumina templates with narrower pores than expected in an oxalic acid bath.     

Plasma enhanced aerosol–gel deposition of Al2O3 coatings

The new plasma enhanced aerosol–gel technique has been used for alumina films preparation, in this work. This process integrates aerosol–gel deposition of films and their plasma treatment in one reactor. The alumina films deposited by aerosol–gel method on Si substrate were plasma or thermally treated. The influence of deposition and condensation conditions on properties of the films was studied. Produced coatings were characterized in terms of surface morphology (SEM, AFM) as well as crystalline and chemical structure (FTIR, XRD). Plasma discharge used for modification of the substrates prior to the deposition process improved homogeneity of produced coatings. Coatings obtained at room temperature exhibit boehmite structure which was transformed into γ-Al₂O₃ after annealing. A similar transformation was induced by low temperature oxide plasma discharge treatment for sufficiently thin coatings.     

New Trends in Nanoparticles: Syntheses and Their Applications to Fuel Cells,Health Care,and Magnetic Storage

This paper reviews important research on chemical and electrochemical synthesis and application of nanoparticles, especially our recent results in this field: (i) catalytic metal nanoparticles for micro-fuel cells, (ii) magnetic oxide nanoparticles for drug delivery systems, and (iii) magnetic metal nanoparticles for magnetic recording media. To fulfill the requirements of each application, we chose and modified those synthetic methods for obtaining suitable properties, e.g., morphology, catalytic activity, and magnetic properties. (i) For micro-fuel cells, electrodeposition is attractive because of its selective deposition onto current collectors and possible elimination of an annealing process. As a result, we have successfully synthesized Pt, PtRu alloy, and PdCo alloy, which consisted of dendritic structures macroscopically and of interconnected nanoparticles microscopically. (ii) For drug delivery systems, since magnetic nanoparticles should possess ferromagnetism, be dispersible in water, and be nontoxic, Fe₃O₄ nanoparticles synthesized by hydrolysis in aqueous media are suitable. As a result, we have successfully controlled the size (10–40 nm in diameter) and the magnetic properties of Fe₃O₄ nanoparticles by means of adjusting the molar ratio of ferrous to ferric ions in the precursor solution. (iii) For magnetic recording materials, since magnetic nanoparticles should possess high coercivity, a controlled shape, and a uniform small size, we have modified a chemical method for synthesizing FePt by adjusting the growth temperature. As a result, we have succeeded in synthesizing FePt nanoparticles with a controlled shape (cubic) and a uniform size (ca. 5.6 nm).