Nanoporous Alumina Templates Research Articles

Magnetic Behavior of Single Ni Nanowires and its Arrays Embedded in Highly Ordered Nanoporous Alumina Templates

We report on magnetization reversal and geometry dependent magnetic anisotropy of Ni nanowire arrays electrodeposited in nanoporous alumina templates. Using micromagnetic simulation we have found that magnetization reversal mechanism in arrays with different nanowire diameters is curling. This magnetic behavior appears with propagation of the domain wall along a nanowire. The calculations have been proven by the analysis of hysteresis curves. To explain magnetic properties of closely-spaced nanowire arrays we have taken into consideration the magnetostatic interaction between adjacent nanowires and their structural defects, like as boundary grains. The investigated magnetic domain pattern of individual bended nanowires confirms rather complicated magnetization reversal mechanism than either coherent rotation of magnetization or its curling. Competition between the shape and magnetoelastic anisotropies can induce an unusual zigzag-like domain pattern in a single nanowire.

3D-nanoarchitectured Pd/Ni catalysts prepared by atomic layer deposition for the electrooxidation of formic acid

Three-dimensionally (3D) nanoarchitectured palladium/nickel (Pd/Ni) catalysts, which were prepared by atomic layer deposition (ALD) on high-aspect-ratio nanoporous alumina templates are investigated with regard to the electrooxidation of formic acid in an acidic medium (0.5 M H2SO4). Both deposition processes, Ni and Pd, with various mass content ratios have been continuously monitored by using a quartz crystal microbalance. The morphology of the Pd/Ni systems has been studied by electron microscopy and shows a homogeneous deposition of granularly structured Pd onto the Ni substrate. X-ray diffraction analysis performed on Ni and NiO substrates revealed an amorphous structure, while the Pd coating crystallized into a fcc lattice with a preferential orientation along the [220]-direction. Surface chemistry analysis by X-ray photoelectron spectroscopy showed both metallic and oxide contributions for the Ni and Pd deposits. Cyclic voltammetry of the Pd/Ni nanocatalysts revealed that the electrooxidation of HCOOH proceeds through the direct dehydrogenation mechanism with the formation of active intermediates. High catalytic activities are measured for low masses of Pd coatings that were generated by a low number of ALD cycles, probably because of the cluster size effect, electronic interactions between Pd and Ni, or diffusion effects.

Synthesis of Copper Nanowires in Highly Ordered Nanoporous Anodic Alumina Template

In this paper, the fabrication of nanoporous anodic alumina template from aluminum foil has been demonstrated using hard anodization process. Copper nanowires arrays were fabricated using continuous and pulse electrodeposition technique into the porous alumina template. Copper nanowires are grown su ccessfully in anodic alumina nano template. The pulse deposition technique using 50 ms off -time showed better quality samples than those madewith continuous deposition technique due to cooling of barrier layer during the off -time.

Biosensing Surfaces Based on Quantum Dots Array

The fabrication of self-ordered semiconductor (TiO2) and noble metal (Au) QDs arrays was successfully achieved by advanced nonlithographic template based method, namely using nanoporous alumina template. The emphasis was placed on the successful preparation of QDs arrays with the desired size, homogeneous distribution and optical (especially fluorescence) properties. Titania and gold QDs characterization by SEM, EDX and fluorescence spectroscopy was performed in order to verify their surface topography, chemical composition and emission properties in UV/VIS range of spectra, respectively. The surface biofunctionalization of QDs was realized via simple physical adsorption of glutathione tripeptide, which makes these arrays suitable for potential biosensing application, mainly in optical and electrochemical detection of biomoleculesin vitro.

Graphene and Graphene Nanomesh Spintronics

Spintronics, which manipulate spins but not electron charge, are highly valued as energy and thermal dissipationless systems. A variety of materials are challenging the realization of spintronic devices. Among those, graphene, a carbon mono-atomic layer, is very promising for efficient spin manipulation and the creation of a full spectrum of beyond-CMOS spin-based nano-devices. In the present article, the recent advancements in graphene spintronics are reviewed, introducing the observation of spin coherence and the spin Hall effect. Some research has reported the strong spin coherence of graphene. Avoiding undesirable influences from the substrate are crucial. Magnetism and spintronics arising from graphene edges are reviewed based on my previous results. In spite of carbon-based material with only sp2 bonds, the zigzag-type atomic structure of graphene edges theoretically produces spontaneous spin polarization of electrons due to mutual Coulomb interaction of extremely high electron density of states (edge states) localizing at the flat energy band. We fabricate honeycomb-like arrays of low-defect hexagonal nanopores (graphene nanomeshes; GNMs) on graphenes, which produce a large amount of zigzag pore edges, by using a nonlithographic method (nanoporous alumina templates) and critical temperature annealing under high vacuum and hydrogen atmosphere. We observe large-magnitude ferromagnetism, which arises from polarized spins localizing at the hydrogen-terminated zigzag-nanopore edges of the GNMs, even at room temperature. Moreover, spin pumping effects are found for magnetic fields applied in parallel with the few-layer GNM planes. Strong spin coherence and spontaneously polarized edge spins of graphene can be expected to lead to novel spintronics with invisible, flexible, and ultra-light (wearable) features.

Contact line behavior on nanoporous alumina surfaces

The aim of this study is to describe the behavior and the thickness of a three-phase system contact line on nanoporous alumina surfaces based on wetting hysteresis measurements. The nanoporous alumina templates were fabricated by anodization of aluminum foil in an oxalic or sulfuric acid solution. The oxide film consists of a well-ordered hexagonal array of pores with uniform diameter. The morphology was controlled by varying the pore diameter, depth and spacing. The wetting behavior of the nanoporous alumina templates was investigated using advancing and receding contact angle measurements carried out by varying the volume of the water droplets used. The surfaces showed a large hysteresis, with a receding contact angle always equal to zero making them strongly adhesive despite their hydrophobicity. Comparing linear and areal models indicates that the contact line thickness is negligible compared with the size of the nanopores. Contact line pinning highlights the role of the surface shape around the pores in the marked adhesive behavior of our templates.

An approach for in situ measurement of anode temperature during the growth of self-ordered nanoporous anodic alumina thin films: Influence of Joule heating on pore microstructure

Anodic aluminium oxide (AAO) thin films, containing hexagonally ordered nanopores, are widely used as templates for fabricating a large number of zero and one- dimensional functional nanostructures such as: nanodots, nanowires and nanotubes. The localized pore temperature developed due to the heat generation phenomenon during anodization plays a key role in the growth morphology of the nanoporous structure. The influence of temperature on the electrochemical process of anodization is generally evaluated by considering the electrolyte temperature, while little importance is given to the localized electrode temperature. In the present work, nanoporous alumina templates were prepared using a two step anodization process with oxalic acid as electrolyte under different bath temperatures and anodizing potentials. We report a novel approach to measure the in situ anode temperature during the anodization process that uses a Pt thin film Resistance Temperature Detector sensor which is deposited on partially fabricated alumina through anodization. The in situ anode temperature has been recorded at different anodization conditions and their contribution to the rise in the anode temperature is presented. Our results indicate that Joule heating is a major source of heat developed during the anodization

Effect of Barrier Layer on Fabrication of FePt Nanowires by Electrodeposition into Nanoporous Alumina Templates

It is well known that the nanoporous aluminum oxide film is an electrical insulator. In addition the homogeneity of electrodeposited nanowire arrays in nanoporous alumina templates increases drastically with the usage of AC electrodeposition in comparison to DC electrodeposition. This is revealing of the effect of dielectric properties of alumina that is used as template for electrodeposition. In this work integrated nanowire arrays of FePt alloys were successfully fabricated by alternating current electrodeposition on nanoporous alumina; also we characterize the nanoporous alumina membrane that is used as a template for FePt nanowire electrodeposition using EIS allowing the electrochemical impedance contributions from the barrier layer, nanoporous structure, and electrolyte solution resistance to be calculated through the use of an equivalent circuit model. The impedance spectra of the porous film prepared under different anodization conditions were measured. The data is attained at open circuit potential over a frequency range between 1 Hz to 100 KHz with an AC potential amplitude of 10 mV. As our result demonstrates the resistance of the bariyer layer decreases with the anodization temperature then the electrochemical process of FePt nanowires at template which fabricated on low anodization temperature will be difficult due to the higher resistance.

Utilizing Dog-Boning to Build up High-Aspect-Ratio Nanofences

The material built-up at sharp corners of a workpiece during electroplating, so-called dog-boning, was utilized for bottom-up creation of nanofences – free standing lines of high aspect ratio nanowires – on silicon oxide substrates. This was realized by electroplating gold into nanoporous aluminum oxide templates with underlying micropatterned gold pads, serving as working electrodes. Using the right parameter set, initial preferred nucleation at the gold pad rim site and the unique nanomatrix geometry led to the formation of lines of nanowires at the rim site separated by an adjacent depletion area from an extended bulk nanorod array region. It was found that the underlying mechanisms of this growth process are the increased electric field densities at the electrode edges and a field screening effect due to already grown neighboring wires, both identified by finite element electric field simulations, as well as the restricted diffusion dynamics of gold ions in the presence of nanopores. The obtained high aspect ratio nanofences might be applicable for enhanced catalysis, dielectric index sensing or in optical filters.

Electrochemical Anodizing, Structural and Mechanical Characterization of Nanoporous Alumina Templates

Highly ordered Anodic Aluminum Oxide (AAO) structures produced from aluminum by using an electrochemical anodizing method were developed towards its application for the next generation of micro/nanomedical and energy devices. In addition of analyzing the anodizing current profile, the surface morphology was characterized by using Scanning Electron Microscopy (SEM), the crystalline structure by X-Ray Diffraction (XRD) and the mechanical properties by nanoindentation experiments. The anodizing time and applied potential determines the nanopores regularity and their size, although the effect of the potential is more pronounced than the effect of temperature in the transformation from crystalline alumina to amorphous alumina. Optimum pore growth was achieved with an applied potential of 17 V which led to a pore fraction - P(f) - of about 17.5%. The experimental Berkovich nanoindentation method was used to determine the AAO hardness as a function of the indenter depth, during the loading stage, using mechanical response and deformation behaviour of the nanopores structure. From the experimental data of the load-displacement curves, this method allows the calculation of the indenter contact depth at each reloading point, thus leading to the estimation of the materials hardness. The results reveal that the hardness depends on the processing conditions used for the production of the AAO samples that also strongly influences the organization and pore size uniformity.

Fabrication of Biomolecules Self-Assembled on Au Nanodot Array for Bioelectronic Device

In the present study, an nano-platform composed of Au nanodot arrays on which biomolecules could be self-assembled was developed and investigated for a stable bioelectronic device platform. Au nanodot pattern was fabricated using a nanoporous alumina template. Two different biomolecules, a cytochrome c and a single strand DNA (ssDNA), were immobilized on the Au nanodot arrays. Cytochorme c and single stranded DNA could be immobilized on the Au nanodot using the chemical linker 11-MUA and thiol-modification by covalent bonding, respectively. The atomic structure of the fabricated nano-platform device was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electrical conductivity of biomolecules immobilized on the Au nanodot arrays was confirmed by scanning tunneling spectroscopy (STS). To investigate the activity of biomolecule-immobilized Au-nano dot array, the cyclic voltammetry was carried out. This proposed nano-platform device, which is composed of biomolecules, can be used for the construction of a novel bioelectronic device.

Crystal anisotropy induced temperature dependent magnetization in cobalt nanowires electrodeposited within alumina template

Cobalt nanowires were deposited within nanoporous alumina template by direct current electrodeposition. The effect of pH on temperature dependent hysteresis behavior and magnetic properties of Co nanowires has been analyzed and discussed in detail. The easy axis of magnetization was observed to change uniformly from parallel to perpendicular direction to the wire axis with decrease of temperature from 400K to 5K. The effect could be attributed to increased magnetocrystalline anisotropy at lower temperatures (e.g. 5K). Strong pH dependence of cross-over temperature that changes from 300K to 50K was observed and the change was delayed with increase of pH.

The effect of anode surface area on nanoporous oxide formation during anodizing of low purity aluminum (AA1050 alloy)

Porous anodic alumina layers were obtained by a simple two-step anodization of low purity aluminum (99.5 % Al, AA1050 alloy) in a 0.3 M oxalic acid electrolyte at 45 V and 20 °C. The effect of anode surface area on structural features of nanoporous oxide and process of oxide formation was investigated. An ordered structure composed of nanostripes or nanopores was formed on the Al surface during electrochemical polishing in a mixture of perchloric acid and ethanol. This nanopattern is then replicated during the anodic oxide formation. It was found that the pore diameter, interpore distance, and porosity increase slightly with increasing surface area of the aluminum sample exposed to the anodizing electrolyte. On the other hand, a slight decrease in pore density and cell wall thickness was observed with increasing surface area of the sample. The detailed inspection of current density vs. time curves was also performed. The obtained results revealed that the higher surface area of the anode, the local current density minimum, was reached faster during first step of anodization and the increase in current density corresponding to the pore rearrangement process was observed earlier. Finally, a dense array of Pd nanowires (∼90 nm in diameter) was synthesized by simple electrodeposition of metal inside the channels of through-hole nanoporous anodic alumina templates with relatively large surface areas (4 cm2).

Light absorption modeling of ordered bulk heterojunction organic solar cells

Ordered bulk heterojunction organic solar cells are devices that combine the advantages of the planar bilayer and the bulk heterojunction architectures. They offer uninterrupted pathways to electrodes for effective charge collection and an extended Donor–Acceptor interface for efficient exciton dissociation. Additionally, this interface can also be a potential approach to increase photon absorption by light trapping. Light absorption and charge carrier generation of organic nanostructures are studied by means of finite-element modeling for a wide range of structuring widths, periods and heights for poly(3-hexylthiophene):1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (P3HT:PCBM) structures. Results show an increase in light absorption at certain wavelengths in the P3HT region with respect to an equivalent planar bilayer model. This increase can be attributed to two phenomena: for the smallest periods the structures behaves like an effective medium, while for periods of the order of magnitude of the incident light wavelength there is light trapping. The maximum increase in absorption was achieved for a 250 nm-width and 500 nm periodicity structure with a height of 40 nm. Exciton diffusion has also been studied to evaluate the effective amount of absorbed light contributing to photocurrent. In this case, best results correspond to the smallest sizes (1.25–12.5 nm-width) for all the considered heights, achieving an increment in the photocurrent up to more than a factor 6 if compared with that of the reference planar bilayer device. This study can be used to optimize our devices, which are achieved via nanoporous anodic alumina templates.

Temperature dependence of the training effect in electrodeposited Co/CoO nanotubes

High aspect ratio Co/CoO nanotubes (NTs) were obtained by potentiostatic electrodeposition of Co inside nanoporous alumina templates followed by the natural oxidation of their inner walls. Magnetic measurements performed at low temperatures after field cooling the samples from above its blocking temperature (TB∼220 K), evidenced the existence of exchange bias (EB) coupling between the Co ferromagnetic outer wall and the CoO antiferromagnetic inner wall of the NTs. A decrease in the magnitude of the EB field was measured at T<TB when cycling the Co/CoO NT arrays through consecutive hysteresis loops. This decrease is known as the training effect (TE) and is here studied in the 6 K≤T<TB temperature range. The TE was fitted using the recursive Binek formula, giving small values for the characteristic decay rate of the training behavior, and evidencing a decrease of EB with increasing antiferromagnetic layer thickness. A phenomenological theory for the temperature dependence of the TE in exchange biased systems was applied for the first time to core-shell nanotubular structures. The good agreement obtained between the experimental results and the theoretical data, provided a strong confirmation of the qualitative correctness of the spin configuration relaxation model used in these systems.

Fe-Co and Fe-Ni Nanocluster Wires by Hydrogen Reduction in Nanoporous Alumina Templates

This paper reports the fabrication, formation mechanism, nanostructure and magnetic properties of Fe50Co50, Fe65Co35 and Fe50Ni50 (at.%) nanocluster wires (NCWs) with different aspect ratios fabricated by thermal decomposition of metal nitrates and subsequent hydrogen reduction in nanoporous anodized aluminum oxide (AAO) templates. The as-synthesized NCWs have diameters in the range of 80-360 nm, and lengths in the range of 0.5-10 μm. The NCWs are composed of roughly round-shaped nanoclusters in the range of about 5-50 nm in size and a weighted average size of 14 nm with a mixture of single-crystal and poly-crystalline structures. The Fe-Co NCWs have a bcc structure while the Fe50Ni50 NCWs have an fcc structure. An intrinsic coercivity at room temperature iHc of 1.15 kOe was obtained for the Fe65Co35 NCWs. The NCWs exhibit the easy magnetization direction along the length of the nanowires as a result of the shape anisotropy.

Improvement in the microstructure and magnetic properties in arrays of dc pulse electrodeposited Co nanowires induced by Cu pre-plating

Co nanowire arrays were pulse electrodeposited into nanoporous alumina templates via the dendrite pores of a thinned barrier layer formed by exponentially non-equilibrium anodization, either without or with Cu pre-plating. Electrolyte acidity (pH) dependence of the microstructure and magnetic properties of the prepared Co nanowires was investigated and compared. The process of Cu pre-plating prior to electrodeposition of Co nanowires along with an adjustment of the pH value was shown to significantly improve the magnetic properties of the Co nanowires and ultimately result in a high coercivity (about 3000 Oe) and squareness up to 98% at pH ∼ 5.7. The improvement of magnetic properties may be caused by the rotation of the hexagonal close packed c-axis of more crystal grains along with the wire axis, which supplements the shape anisotropy of the nanowires. The angular dependence of the coercivity (Hc(θ)) of both types of prepared Co nanowires was also studied.

Influence of Surface Property on the Crystallization of Hentetracontane under Nanoscopic Cylindrical Confinement

The crystallization behavior and the orientation of linear alkane hentetracontane (C41) confined in cylindrical nanoporous alumina templates with different surface energies were investigated by nonisothermal crystallization and X-ray diffraction. The surface of pristine nanoporous alumina was modified to have low surface energy by grafting with polydimethylsiloxane. In the pristine nanoporous alumina, C41 crystallized at two crystallization temperature ranges, lower than bulk, and exhibited the decreased Avrami exponents. C41 in the surface-modified nanoporous alumina showed the inhibition of crystallization at higher temperature range among the two crystallization temperature ranges but the enhancement of crystallization at much lower temperature ranges than in the pristine nanoporous alumina. It was clearly shown that those variations of crystallization behavior imply the surface effect on crystallization. The crystal orientation was also affected by surface-modification of the alumina template. The a-axis of orthorhombic C41 crystals in the pristine nanoporous alumina was preferentially oriented parallel to the pore axis, while b- and c-axes were perpendicular to the pore axis. C41 crystals in the surface-modified nanoporous alumina showed two types of orientation. One was identical to that in the pristine nanoporous alumina, and the other was the orientation that the crystals were tilted with respect to the c-axis as the (110) plane parallel to the pore axis.

Enhancing the performance of free-standing TiO2 nanotube arrays based dye-sensitized solar cells via ultraprecise control of the nanotube wall thickness

Free standing TiO2 nanotube (TNT) array film was fabricated via template-assisted Atomic Layer Deposition (ALD) in nanoporous anodic alumina templates followed by alumina dissolution. Effect of TiO2 nanotube wall thickness on the photovoltaic performance was studied on the dye sensitized solar cells with the TNT wall thickness precisely controlled by ALD between 250 and 550 cycles. The results show that the photovoltaic performance can be improved by optimizing the TNT wall thickness. A thick enough tube wall is crucial for forming space charge layer to allow efficient charge separation, but too thick wall thickness will reduce dye loading relatively. A photovoltaic energy conversion efficiency of 4.65% was obtained on the device with 450 ALD cycles of TNT wall thickness, which is about 1.8 times of that obtained with 250 cycles thickness.

Magnetic interactions and reversal mechanisms in Co nanowire and nanotube arrays

Ordered hexagonal arrays of Co nanowires (NWs) and nanotubes (NTs), with diameters between 40 and 65 nm, were prepared by potentiostatic electrodeposition into suitably modified nanoporous alumina templates. The geometrical parameters of the NW/NT arrays were tuned by the pore etching process and deposition conditions. The magnetic interactions between NWs/NTs with different diameters were studied using first-order reversal curves (FORCs). From a quantitative analysis of the FORC measurements, we are able to obtain the profiles of the magnetic interactions and the coercive field distributions. In both NW and NT arrays, the magnetic interactions were found to increase with the diameter of the NWs/NTs, exhibiting higher values for NW arrays. A comparative study of the magnetization reversal processes was also performed by analyzing the angular dependence of the coercivity and correlating the experimental data with theoretical calculations based on a simple analytical model. The magnetization in the NW arrays is found to reverse by the nucleation and propagation of a transverse-like domain wall; on the other hand, for the NT arrays a non-monotonic behavior occurs above a diameter of ∼50 nm, revealing a transition between the vortex and transverse reversal modes.