Aluminum Oxide Template Research Articles

SEM Image Analysis on the Formation of Porous Anodic Alumina Templates

SEM Image Analysis on the Formation of Porous Anodic Alumina Templates

Enhancement of Exchange Bias and Perpendicular Magnetic Anisotropy in CoO/Co Multilayer Thin Films by Tuning the Alumina Template Nanohole Size.

The interest in magnetic nanostructures exhibiting perpendicular magnetic anisotropy and exchange bias (EB) effect has increased in recent years owing to their applications in a new generation of spintronic devices that combine several functionalities. We present a nanofabrication process used to induce a significant out-of-plane component of the magnetic easy axis and EB. In this study, 30 nm thick CoO/Co multilayers were deposited on nanostructured alumina templates with a broad range of pore diameters, 34 nm ≤ Dp ≤ 96 nm, maintaining the hexagonal lattice parameter at 107 nm. Increase of the exchange bias field (HEB) and the coercivity (HC) (12 times and 27 times, respectively) was observed in the nanostructured films compared to the non-patterned film. The marked dependence of HEB and HC with antidot hole diameters pinpoints an in-plane to out-of-plane changeover of the magnetic anisotropy at a nanohole diameter of ∼75 nm. Micromagnetic simulation shows the existence of antiferromagnetic layers that generate an exceptional magnetic configuration around the holes, named as antivortex-state. This configuration induces extra high-energy superdomain walls for edge-to-edge distance >27 nm and high-energy stripe magnetic domains below 27 nm, which could play an important role in the change of the magnetic easy axis towards the perpendicular direction.

Fe-Ni-Co Electrodeposited Nanowires Decorated with Au

Alloy nanowires containing Fe, Ni and Co are of interest as electrode materials in miniaturized devices due to their ability to align them under a magnetic field. In order to improve their resistivity while retaining their magnetic behavior, Fe-Ni-Co nanowires were electrodeposited and decorated with Au; single nanowires were then further characterized (Fig. 1). The nanowires were deposition under a constant applied potential within a nanoscale alumina template, followed by dissolution of the membrane, release of the nanowires, and subsequent treatment in a gold acid electrolyte. Au clusters formed on the Fe-Ni-Co surface through simultaneous displacement and corrosion reactions. The morphology, composition and structure was examined before and after modification, revealing a change in crystallinity and composition that impacted the nanowire’s electrical and magnetic properties. Transferring a single nanowire to a lithographically prepared two-point probe enabled the electrical and magnetic characterization of the Au decorated nanowire. Averaging results of quadruplicate replicates, a low coverage of discontinuous Au clusters on Fe-Ni-Co nanowires significantly decreased the resistivity, not attributed entirely to the lower resistivity of Au, but as a consequence of changes of the Fe-Ni-Co crystallinity. A high coverage of the Fe-Ni-Co by Au had no further benefit, and even increased the resistivity. Since bulk gold is diamagnetic and decoration of Au onto the Fe-Ni-Co nanowires may compromise the overall magnetic property, the magnetoresistance and electron mobility were determined. Both decreased, as expected, with coverage of Au clusters on the nanowires, suggesting that there may be an optimal cluster density for modifying the Fe-Ni-Co nanowires. Figure 1. Electrodeposited nanowires initially amorphous then decorated with Au showing a change in crystallinity and subsequent characterization with a two-point probe. Figure 1

Magnetization reversal properties and magnetostatic interactions of disk to rod-shaped FeNi layers separated by ultra-thin Cu layers

From fast magnetic memories with low-power consumption to recording media with high densities, realizing the magnetization reversal and interaction of magnetic layers would allow for manipulating the ultimate properties. Here, we use a pulsed electrochemical deposition technique in porous alumina templates (50 nm in pore diameter) to fabricate arrays of nanowires, consisting of FeNi layers (26−227 nm in thickness) with disk to rod-shaped morphologies separated by ultra-thin (3 nm) Cu layers. By acquiring hysteresis curves and first-order reversal curves (FORCs) of the multilayer nanowire arrays, we comprehensively investigate magnetization reversal properties and magnetostatic interactions of the layers at different field angles (0° ≤ θ ≤ 90°). These involve the extraction of several parameters, including hysteresis curve coercivity (H c Hyst ), FORC coercivity (H c FORC ), interaction field distribution width (ΔH u ), and irreversible fraction of magnetization (IF m ) as a function of θ. We find relatively constant and continuously decreasing trends of H c Hyst when 0° ≤ θ ≤ 45°, and 45° < θ ≤ 90°, respectively. Meanwhile, angular dependence of H c FORC and IF m shows continuously increasing and decreasing trends, irrespective of the FeNi layer morphology. Our FORC results indicate the magnetization reversal properties of the FeNi/Cu nanowires are accompanied with vortex domain wall and single vortex modes, especially at high field angles. The rod-shaped layers also induce maximum ΔH u during the reversal process, owing to enhancements in both magnetizing and demagnetizing-type magnetostatic interactions.

Microrod Patterned Lithium Metal Surface for High-performance Solid-state Lithium Batteries

Lithium metal is considered as the “holy grail” of energy density, however, there is a dendrite growth issue, which inhibits its practical applications. Herein, a microrod patterned lithium metal surface was fabricated by a hot-press process using a porous alumina template. As an anode for solid-state lithium metal batteries, it shows remarkable cyclability and improved rate capability. The specific capacity reaches 60 mAh g−1 at 7 C with excellent cycling performance. The improved performance is mainly attributed to the dispersed current on the lithium metal surface compared with that of the pristine lithium foil. This work provides an economically viable surface treatment method for high performance metal batteries.

Photomechanical Structures Based on Porous Alumina Templates Filled with 9-Methylanthracene Nanowires

9-Methylanthracene (9MA) undergoes a concerted [4 + 4] photodimerization in its crystal form that can be harnessed in order to generate photomechanical motions such as bending, twisting, and expansion. As described in this paper, 9MA nanowires were grown in anodic aluminum oxide (AAO) templates with the goal of using the crystal expansion to generate a net increase in the height of the composite disk. The growth conditions were optimized in order to raise the filling amount from 28% to 77% of the available volume in the porous AAO. A new experimental method for detecting motion, based on the analysis of data from a dynamically misaligned Michelson interferometer, was developed. Template bending was observed, showing that the photodimerization of the confined nanowires generated mechanical work, but no conclusive evidence for surface disruption or vertical translation was observed. Optical measurements, as well as atomic force and scanning electron microscopy, showed that incomplete filling, crystal orientation, and debris from template polishing likely prevented the observation of vertical actuation in these nanocrystal composites. This work highlights some of the practical challenges that are involved in creating photomechanical actuators using the organic–inorganic composite approach, with the two most significant being (1) the uniform filling of the porous template with the organic active material and (2) the removal of excess organic material from the template’s surface.

Hierarchical sparse Co nanowire/carbon nanofiber arrays as 3D Li+ gateways for high rate, long-life lithium-sulfur batteries

Lithium‑sulfur batteries (LSBs) have been investigated as promising electrochemical energy storage owing to their high theoretical energy density. However, practical applications of LSBs are still hindered by rapid capacity decay and short cycle life, especially at high rates. Here, highly hierarchical sparse Co-nanowire/carbon nanofiber/sulfur arrays (CoNws/CNFs/S) were fabricated inside porous alumina template using a combination of electrodeposition, polymer pyrolysis, and infiltration methods. As a consequence, the sulfur host structure made with CoNWs/CNFs can deliver high rate capability and cycle stability with a high capacity of 1010 mAh g-1 and a low capacity fading rate of 0.3% per cycle (after 200 cycles under a rate of 1C). Our result suggests that designing hybrid coaxial architecture could represent an attractive path to improve high power density LSBs.

Cicada-Wing-Inspired Nanopillar Hydrogels Consisting of Poly(vinyl alcohol) and Poly(methacrylic acid) for Capturing Bacteria through Their Flexibility and Wide Range of Motion.

The human race has learned about countering bacterial infection from other living organisms. In this study, cicada-wing-inspired hydrophilic nanopillars were prepared using commercially available porous alumina templates without the use of specialized equipment and additional materials. Hydrophilic polymers of poly(vinyl alcohol) (PVA) and poly(methacrylic acid) (poly(MAAc)) were used to construct these nanopillars. The water-insoluble nanopillars formed by thermal cross-linking between the hydroxy and carboxy groups were highly flexible, with no significant structural disruption being observed on tilting them. The nanopillars captured Gram-negative and Gram-positive bacteria, despite their negatively charged surface leading an electrostatic repulsion with bacteria. The wing of the cicada has evolved to remain dry and maintain its lightweight. The water-containing cicada-wing structure may be an unknown encounter for bacteria and, thus, may become a countermeasure against the ever-changing bacteria.

Strongly Quantum-Confined Perovskite Nanowire Arrays for Color-Tunable Blue-Light-Emitting Diodes.

Color tunability of perovskite light-emitting diodes (PeLEDs) by mixed halide compositional engineering is one of the primary intriguing characteristics of PeLEDs. However, mixed halide PeLEDs are often susceptible to color red-shifting caused by halide ion segregation. In this work, strongly quantum-confined perovskite nanowires (QPNWs) made of CsPbBr3 are grown in nanoporous anodic alumina templates using a closed space sublimation process. By tuning the pore size with atomic layer deposition, QPNWs with a diameter of 6.6 to 2.8 nm have been successfully obtained, with continuous tunable photoluminescence emission color from green (512 nm) to pure blue (467 nm). To better understand the photophysics of QPNWs, carrier dynamics and the benefit of alumina passivation are studied and discussed in detail. Eventually, PeLEDs using various diameters of CsPbBr3 QPNWs are successfully fabricated with cyan color (492 nm) PeLEDs, achieving a record high 7.1% external quantum efficiency (EQE) for all CsPbBr3-based cyan color PeLEDs. Sky blue (481 nm) and pure blue (467 nm) PeLEDs have also been successfully demonstrated, respectively. The work here demonstrates a different approach to achieve quantum-confined one-dimensional perovskite structures and color-tunable PeLEDs, particularly blue PeLEDs.

Fabrication of FePt nanowires through pulsed electrodeposition into nanoporous alumina templates

Fabrication of FePt nanowires through pulsed electrodeposition into nanoporous alumina templates

Synthesis of CoZn nanowire in anodic aluminum templets and their characterization by Rutherford backscattering spectrometry

In this research, the CoZn alloy nanowire arrays were deposited in a porous aluminum oxide template by alternative current pulse electrodeposition. The aluminum oxide template was fabricated through two-step soft anodization of pure aluminum foils and its morphology was determined using a scanning electron microscope.In order to systematically study the constructed sample, non-destructive ion beam analysis techniques were used. Among these methods, Rutherford backscattering spectrum analysis was performed by interaction of proton ions with energy of 2000 keV and 2500 keV and helium ions with energy of 2500 keV with the sample. Analysis of the RBS spectrum of the sample along with simulations performed by SIMNRA code made it possible to determine the concentration, depth profile of the elements in the sample, and determination of stoichiometry of the alumina template at different depths. The best results are related to a proton with energy 2500 keV, which reported the depth of 10 to 14 μm for CoZn alloy nanowire. Also, for validation of the results, FE-SEM images were prepared from the sample, which confirms the agreement with the results obtained from the RBS method.

Effects of annealing temperature on morphological and optical properties of Ag nanoparticle array and its application in SERS

Ordered metal nanoparticle (NP) arrays have attracted tremendous attention for potential plasmonic applications due to their unique surface plasmon resonance. Herein, we fabricated highly ordered Ag NP arrays employing anodized aluminum (Al) templates and the vacuum thermal evaporation technique. The morphological evolution and corresponding optical property changes of the Ag NP arrays during annealing treatment has been investigated. It is found that there is a transition point approximately at annealing temperature of 290°C, where the shape of Ag NPs is most spherical and the peak position of quadrupole mode has a largest wavelength about 500 nm. Then, the surface-enhanced Raman spectra of Rhodamine 6G on Ag NP arrays annealed at different temperatures has been demonstrated at an excitation wavelength of 532 nm. The maximum SERS enhancement is also observed at 290°C. Our findings provide a way to further improve the performance of the plasmonic structures.

Possible routes for the synthesis of nanowires of intermetallic compounds: The case of CeIn3

In this work, we investigated the role of different parameters in the synthesis of intermetallic nanowires of CeIn3 by the metallic-flux nanonucleation (MFNN) method such as template pore diameter, crystallization temperature, heat treatment temperature, and synthesis time. Depending on the growing parameters, we obtained CeIn3 nanowires (d ∼ 350 nm) or CeAlO3 nanotubes. For the nanowires, we observed a suppression of the CeIn3 antiferromagnetic transition from the bulk TN ∼ 10 K to the nanowire system TN ∼ 3 K, which may be associated with the dimensionality affecting the interplay between magnetic exchange interactions, crystalline electrical field, and Kondo effects. We assume that the CeAlO3 nanotubes may result from a reaction with the alumina template and consequent rare-earth oxidation. Our work shows that even it is a great challenge to find the correct growth path of a particular intermetallic compound, the MFNN method can be a promising route to obtain rare-earth based nanowires.

Spatial magnetic imaging of non-axially symmetric vortex domains in cylindrical nanowire by transmission X-ray microscopy.

The spatial magnetization texture of a cylindrical nanowire has been determined by Transmission X-ray Microscopy (TXM) and X-ray magnetic circular dichroism (XMCD). For this purpose, nanowires with designed geometry, consisting of CoNi/Ni periodic segments, have been grown by designed electrodeposition into alumina templates. Experimental data allow one to conclude the presence of mono- and trivortex magnetic domains in CoNi segments but, unusually, these states are characterized by an asymmetric XMCD contrast across the nanowire's section. Micromagnetic modelling shows non-trivial three-dimensional structures with ellipsoidal vortex cores and non-axially symmetric magnetization along the nanowire direction. The modelled TXM contrast of micromagnetic structures allows to correlate the experimental asymmetric XMCD contrast to the easy axis direction of the uniaxial magnetocrystalline anisotropy.

Templated electrodeposition as a scalable and surfactant-free approach to the synthesis of Au nanoparticles with tunable aspect ratios.

A high-throughput method for the fabrication of ordered arrays of Au nanoparticles is presented. It is based on pulsed electrodeposition into porous anodic alumina templates. In contrast to many synthesis routes, it is cyanide-free, prior separation of the alumina template from the aluminium substrate is not required, and the use of contaminating surfactants/capping agents often found in colloidal synthesis is avoided. The aspect ratio of the nanoparticles can also be tuned by selecting an appropriate electrodeposition time. We show how to fabricate arrays of nanoparticles, both with branched bases and with hemispherical bases. Furthermore, we compare the different morphologies produced with electron microscopies and grazing-incidence synchrotron X-ray diffraction. We find the nanoparticles are polycrystalline in nature and are compressively strained perpendicular to the direction of growth, and expansively strained along the direction of growth. We discuss how this can produce dislocations and twinning defects that could be beneficial for catalysis.

Nanoimprinted and Anodized Templates for Large-Scale and Low-Cost Nanopatterning.

Nanopatterning to fabricate advanced nanostructured materials is a widely employed technology in a broad spectrum of applications going from spintronics and nanoelectronics to nanophotonics. This work reports on an easy route for nanopatterning making use of ordered porous templates with geometries ranging from straight lines to square, triangular or rhombohedral lattices, to be employed for the designed growth of sputtered materials with engineered properties. The procedure is based on large-scale nanoimprinting using patterned low-cost commercial disks, as 1-D grating stamps, followed by a single electrochemical process that allows one to obtain 1-D ordered porous anodic templates. Multiple imprinting steps at different angles enable more complex 2-D patterned templates. Subsequently, sputtering facilitates the growth of ferromagnetic antidot thin films (e.g., from 20 to 100 nm Co thick layers) with designed symmetries. This technique constitutes a non-expensive method for massive mold production and pattern generation avoiding standard lithographical techniques. In addition, it overcomes current challenges of the two-stage electrochemical porous anodic alumina templates: (i) allowing the patterning of large areas with high ordering and/or complex antidot geometries, and (ii) being less-time consuming.

Fabrication and characterization of well-ordered PbS nanowires in aluminum oxide template by sulfurization and vacuum injection molding processes

Lead (Pb) nanowire arrays were fabricated with anodic aluminum oxide (AAO) templates of 30, 100 and 300 nm in pore diameters. Through vacuum injection molding process, Pb/AAO composite was obtained, and lead sulfide (PbS) could further be synthesized after exposing to sulfur gas. AAO templates with different pore sizes were fabricated by using pure aluminum in a two-step anodization. Three types of solutions, which are 10 vol% sulfuric acid, 3 wt% oxalic acid and 1 vol% phosphoric acid, were adopted to achieve AAO of various pore sizes. Different sulfurization temperatures and time spans were applied for studying on the formation mechanism of PbS. Finally, the morphology, composition, structure and elements distribution of the as-prepared Pb and PbS nanowires were confirmed through the use of scanning electron microscopy, energy dispersive x-ray spectroscopy, element-mapping, x-ray diffraction and transmission electron microscopy analysis. The results indicated that Pb nanowires were successfully obtained after applying vacuum injection molding process with 50 kgf cm−2 hydraulic pressure, and PbS nano arrays can be formed by sulfurization at 500 °C for 5 h. Furthermore, an optical property, ultraviolet–visible (UV–Vis) absorption, was also measured. The measurement of the PbS nanowires showed that a significant quantum confinement effect made the energy gap produce a blue shift from 0.41 eV to 1.65 eV or 1.72 eV.

Electrodeposition of nano-Cu, Ni and binary Ni/Cu into nano-porous AAO layer for high-efficiency black spectrally selective coating

BackgroundAnodic aluminum oxide (AAO) template is widespread due to its diverse metal nanostructures. Various solar selective black coatings on aluminum oxide template were investigated. Spectrally selective nano-coating of nickel, copper and nickel–copper on anodized aluminum was produced.ResultsThe coatings were performed via electrodeposition and evaluated by measurement of coating thickness, hardness and optical properties. Also, these coatings were analyzed by scanning electron microscope, energy-dispersive X-ray spectroscopic and polarization studies in 3.5% NaCl solution. The anodized aluminum showed higher corrosion resistance (4.8284 KΩ) and lower corrosion rate (0.02189 mm/year). However, the electro-colored Al with Cu for 60 min showed the highest corrosion rate of 0.1942 mm/y, compared with other Al samples. The effect of anodizing time on the metal density and the optical efficiency of black copper coating was studied.ResultsThe obtained solar panels exhibit low values of solar reflectance within the visible range and high solar absorption efficiency. These coatings are highly efficient and adequate for any solar system.

Narrow Segment Driven Multistep Magnetization Reversal Process in Sharp Diameter Modulated Fe67Co33 Nanowires.

Magnetic nanomaterials are of great interest due to their potential use in data storage, biotechnology, or spintronic based devices, among others. The control of magnetism at such scale entails complexing the nanostructures by tuning their composition, shape, sizes, or even several of these properties at the same time, in order to search for new phenomena or optimize their performance. An interesting pathway to affect the dynamics of the magnetization reversal in ferromagnetic nanostructures is to introduce geometrical modulations to act as nucleation or pinning centers for the magnetic domain walls. Considering the case of 3D magnetic nanowires, the modulation of the diameter across their length can produce such effect as long as the segment diameter transition is sharp enough. In this work, diameter modulated Fe67Co33 ferromagnetic nanowires have been grown into the prepatterned diameter modulated nanopores of anodized Al2O3 membranes. Their morphological and compositional characterization was carried out by electron-based microscopy, while their magnetic behavior has been measured on both the nanowire array as well as for individual bisegmented nanowires after being released from the alumina template. The magnetic hysteresis loops, together with the evaluation of First Order Reversal Curve diagrams, point out that the magnetization reversal of the bisegmented FeCo nanowires is carried out in two steps. These two stages are interpreted by micromagnetic modeling, where a shell of the wide segment reverses its magnetization first, followed by the reversal of its core together with the narrow segment of the nanowire at once.

Micro and Nanopatterned Ion Exchange Membranes for Applications in Electrochemical Devices

Topographical patterns on ion exchange membranes’ (IEMs’) surfaces have been show to improve the performance and energy efficiency in electrochemical energy conversion and water treatment processes. Substituting flat IEMs with patterned/profiled IEMs in electrodialysis reduces spacer channel mass transfer limitations by increasing the interfacial area between the liquid solution and membrane materials. Additionally, the extended surface of the profiled cation-exchange and anion-exchange membranes augment the spacer channel conductivity without obfuscating bulk liquid flow. Topographically patterned ion exchange membranes are fabricated using several methodologies that include photo-/soft-lithography, hot pressing, solution casting and 3D printing. The minimum lateral feature size that can be obtained by these methods is practically on the order of a few microns and potentially could be as low as few hundred nanometers. Block copolymer (BCP) lithography is a nanopatterning platform that generates lateral feature sizes of 5 to 50 nm. Thin films of diblock copolymers can self-assemble on a neutral layer resulting in lamella, cylinder, or sphere morphologies when the film is thermal or solvent vapor annealed. The morphology of the film primarily depends on volume fraction (fA, fB) of each block and interaction parameter between the blocks (χ). Poly(styrene-block-methylmethacrylate) (PS-b-PMMA) is one of the most studied block copolymer for BCP lithography because it form perpendicular aligned nanostructures via thermal annealing without topcoat layers to control the free surface interfacial energy. The etch contrast between the styrene and MMA blocks also makes it easier to transfer the pattern onto substrates – especially if sequential infiltration synthesis (SIS) is used.In this work, for the first time, a method to prepare nanopatterned IEMs using BCP lithography is reported. The SIS process post PS-b-PMMA self-assembly converts MMA into alumina creating organic-inorganic hard mask. The styrene block is removed via oxygen plasma in a reactive ion-etcher leaving behind a nanoporous alumina template on a silicon wafer. Sulfonated poly(arylene ether ether ketone) (SPEEK) is dropcasted onto the nanoporous alumina and the solvent is evaporated. Once releasing the SPEEK from the substrate, a nanopatterned cation-exchange membrane is attained. The electrochemical properties of the nanopatterned membrane such as conductivity, permselectivity and transference number are measured and compared to a flat, non-patterned membrane. The efficiency of these nanopatterned membranes for simple electrochemical devices, like a hydrogen pump, will also be presented.This work is supported from NSF Award # 1703307. Figure 1