Nanoporous Alumina Templates Research Articles

Creation of High-Density Nanostructures for Use in Energy Conversion Applications

Nanostructures have garnered much interest in recent years for their potential in increasing performance of alternative energy devices, such as solar cells, thermoelectrics, batteries, and piezoelectrics. Many methods exist for the fabrication of nanostructures, but one of the most promising methods is the use of nanoporous aluminum oxide templates due to its extremely high density and self-ordering through a hexagonal, close-packed structure. However, the initial aluminum growth substrate is generally not suitable for use in energy conversion devices so a major challenge becomes the practical use of aluminum oxide templates. In this presentation, we demonstrate how to effectively utilize the unique advantages of aluminum oxide templates for a variety of applications. We introduce a method to transfer aluminum oxide templates to a substrate of interest. Using this process, we are able to pattern highly-ordered nanostructured materials with dimensions below 10 nm that may be suitable for use in thermoelectric devices. We also use this pattern transfer process to fabricate porous templates on complex multilayered substrates. These multilayered nanostructures can be used in many applications. Here, we present the growth of highly-ordered, catalyzed carbon nanotubes. These methods highlight an effective approach for creating the highly-dense, highly-ordered nanostructures required for many energy conversion applications.

Ordered nanohole arrays fabricated on a nanoporous alumina template by pulsed laser ablation

Highly ordered nanohole arrays of manganite have been synthesized using pulsed laser deposition on nanoporous alumina template. Their structure and phase formation were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive x-ray spectroscopy (EDX) and x-ray diffraction (XRD). The magnetic measurements were performed with respect to temperature and field and exhibit a ferromagnetic to paramagnetic transition at 284 K. In addition, the temperature dependence of electrical resistance was measured at different magnetic fields and an insulating phase throughout all the temperatures was observed. The low temperature ferromagnetic insulating state is discussed by the presence of a canted ferromagnetic state induced by the nanoholes. The present work shows the feasibility of combining both the nanoporous alumina template and pulsed laser ablation for the fabrication of perovskite manganite nanohole arrays which can also be extended to fabricate other multicomponent oxide nanohole materials.

Self-assembly of aligned CuO nanorod arrays using nanoporous anodic alumina template by electrodeposition on Si substrate for IR photodetectors

Hexagonally highly-ordered thin nanoporous anodic alumina (AAO) template of pre-deposited Al thin film onto Si substrate was fabricated under controllable conditions through a two-step anodization for the growth of nanostructured materials. The obtained AAO template has approximately 60nm of pore diameter and 1200nm length with 110nm interpore distances within an area of 0.75cm2. A bottom barrier layer of the AAO template was removed by wet etching using phosphoric acid (5wt%) for several minutes at room temperature. As an application, Cu nanorod-arrays embedded in AAO template was prepared by dc electrodeposition. The electrodeposited Cu nanorod-arrays onto Si based AAO template were oxidized at optimized conditions inside a conventional furnace to produce high-quality CuO nanorod arrays. The free-standing CuO nanorod arrays were formed after subsequent removal of AAO template. Fast with high-performance CuO nanorod-arrays IR photodetector based on MSM were fabricated with Al contact electrodes and its optoelectronic properties were examined. The electrical performance and photoelectric response performance were studied and the results showed that IR photodetector exhibited a high sensitivity to 808nm IR laser light. Both the response and recovery time were found to be fast; 0.19 and 0.17s, respectively, which are much shortest time compared to other IR photodetectors reported in the literature.

Cobalt electrodeposition in nanoporous anodic aluminium oxide for application as catalyst for methane combustion

The present work is focused on the cobalt electrodeposition in anodic aluminium oxide and the application of the obtained nanocomposite layer as a monolithic catalyst for complete methane oxidation. For the investigation of cobalt nanowires growing in the pores of nanoporous alumina template, cyclic voltametry and galvanostatic method were used. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were applied in order to determine the morphology and the content of the composite material in some stages of its preparation. It was found that the deposition of cobalt from a neutral borate electrolyte is accompanied by release of a small amount of hydrogen, which decreases with the process of nanowires growing.The emergence of the individual flake cobalt clusters on the alumina template is considered as the most suitable structure of the monolithic catalysts. The final resulting laminar composite structure with outer layer of Co- and Pd-oxides exhibits higher active catalytic properties for methane combustion than the catalysts obtained by conventional procedure of preparation by impregnation.

Effect of electrolyte temperature on the formation of highly ordered nanoporous alumina template

In this work, we present a systematic influence of electrolyte temperature along with anodizing potential on the pore parameters during two-step anodization of Al in H2SO4 electrolyte. Top surface morphology of the nanoporous templates was examined with the help of field emission scanning electron microscope and atomic force microscope. Three-dimensional (3D) Fast Fourier Transform (FFT) image analysis was then employed to quantify pore regularity and pore periodicity as a function of both the bath temperature (1–15 °C) and the anodic potential (15–25 V). A highest pore regularity ratio of 5 × 108 was obtained at 3 °C and 25 V with a pore diameter of 32 ± 3 nm and inter-pore distance of 65 nm. With further increase in temperature, the pore regularity ratio was found to decrease drastically. It was found that higher temperature favored the dissolution of compact aluminum oxide layer isotropically along the pore length. This process in effect enhanced the pore size, growth rate, and template top surface roughness without affecting much inter-pore distance. Self-ordering of the pores was found to improve with increasing anodizing potential with a critical influence of the current density along with inter-pore distance. The mechanism of pore growth was discussed in terms of temperature-dependent activation energy controlled dissolution of aluminum. The typical activation energy evaluated at 25 V was 72.8 kJ/mol at 3 °C.

Template-assisted extrusion of biopolymer nanofibers under physiological conditions.

Biomedical applications ranging from tissue engineering to drug delivery systems require versatile biomaterials based on the scalable and tunable production of biopolymer nanofibers under physiological conditions. These requirements can be successfully met by a novel extrusion process through nanoporous aluminum oxide templates, which is presented in this study. With this simple method we are able to control the nanofiber diameter by chosing the size of the nanopores and the concentration of the biopolymer feed solution. Nanofiber assembly into different hierarchical fiber arrangements can be achieved with a wide variety of different proteins ranging from the intracellular proteins actin, α-actinin and myosin to the extracellular matrix components collagen, fibronectin, fibrinogen, elastin and laminin. The extrusion of nanofibers can even be applied to the polysaccharides hyaluronan, chitosan and chondroitin sulphate. Moreover, blends of different proteins or proteins and polysaccharides can be extruded into composite nanofibers. With these features our template-assisted extrusion process will lead to new avenues in the development of nanofibrous biomaterials.

Aluminum alloys anodisation for nanotemplates application

The aim of this investigation was to reveal the processing differences in achieving nanoporous anodized aluminium from aluminium alloys and their application for cobalt nanowires electrodeposition. The following types of aluminium were tested: pure Al (99.99%), and commercial alloys: AA1050, 6082 and 6060. Because of the differences in the surface temperature and high voltages during the anodizing steps, some stresses can be built up in the material. Therefore a strict temperature control should be done to limit thermal stresses in materials. Alloying elements (Si, Mg) cause precipitates that are observed on the surface, especially for 6060 alloy. Nevertheless, a nanoporous structure can be obtained at the end of second anodization step on all aluminium alloys investigated. It was shown that the number density of pores on the surface is practically independent on the aluminium alloys used. However, the degree of hexagonal distribution of the pores depends on the type of anodized aluminium alloy. Also, a successful fabrication of Co nanowire arrays using nanoporous anodic alumina template produced on Al alloy has been demonstrated, and the uniform filling of the template by cobalt nanowires arrays is discussed.

Magnetoelectric Coupling in Well-Ordered Epitaxial BiFeO3/CoFe2O4/SrRuO3 Heterostructured Nanodot Array.

Multiferroic magnetoelectric (ME) composites exhibit sizable ME coupling at room temperature, promising applications in a wide range of novel devices. For high density integrated devices, it is indispensable to achieve a well-ordered nanostructured array with reasonable ME coupling. For this purpose, we explored the well-ordered array of isolated epitaxial BiFeO3/CoFe2O4/SrRuO3 heterostructured nanodots fabricated by nanoporous anodic alumina (AAO) template method. The arrayed heterostructured nanodots demonstrate well-established epitaxial structures and coexistence of piezoelectric and ferromagnetic properties, as revealed by transmission electron microscopy (TEM) and peizoeresponse/magnetic force microscopy (PFM/MFM). It was found that the heterostructured nanodots yield apparent ME coupling, likely due to the effective transfer of interface couplings along with the substantial release of substrate clamping. A noticeable change in piezoelectric response of the nanodots can be triggered by magnetic field, indicating a substantial enhancement of ME coupling. Moreover, an electric field induced magnetization switching in these nanodots can be observed, showing a large reverse ME effect. These results offer good opportunities of the nanodots for applications in high-density ME devices, e.g., high density recording (>100 Gbit/in.(2)) or logic devices.

Study of the magnetization behavior of ferromagnetic nanowire array: Existence of growth defects revealed by micromagnetic simulations

High aspect ratio nanowires were electrodeposited in nanoporous anodic alumina template by a potentiostatic method. The angular dependence of the coercive field and remanence magnetization extracted from magnetometry measurements are compared with micromagnetic simulations. Inclusion of magnetostatic interactions between Ni nanowires in simulations is required to explain some of the properties of the magnetization reversal. However, it is not sufficient to reproduce fully the angular dependence of the coercive field. Due to the polycrystalline nature of nanowires and thus to the presence of grain boundaries, defects are included in simulations. A good agreement between theory and experiment is then clearly highlighted, in particular in the nanowire easy axis direction. The achieved results allow a description of several experimental data published in the literature and consequently to get a better understanding of reversal mechanisms that operate in such nanowire arrays. A complementary study of composite nanowire array is successfully performed to prove the adequacy of the simulations method to describe the magnetic properties of nanowire array.

On the anodic aluminium oxide refractive index of nanoporous templates

In the present study, we have determined the intrinsic refractive index of anodic aluminium oxide, which is originated by the formation of nanoporous alumina templates. Different templates have been fabricated by the conventional two-step anodization procedure in oxalic acid. Their porosities were modified by chemical wet etching allowing the tuning of their effective refractive indexes (air-filled nanopores + anodic aluminium oxide). By standard spectroscopic light transmission measurements, the effective refractive index for each different template was extracted in the VIS–NIR region. The determination of the intrinsic anodic aluminium oxide refractive index was performed by using the Maxwell–Garnett homogenization theory. The results are coincident for all the fabricated samples. The obtained refractive index (~1.55) is quite lower (~22%) than the commonly used Al2O3 handbook value (~1.75), showing that the amorphous nature of the anodic oxide structure strongly conditions its optical properties. This difference is critical for the correct design and modeling of optical plasmonic metamaterials based on anodic aluminium oxide nanoporous templates.

Laser nanostructured Co nanocylinders-Al2O3 cermets for enhanced & flexible solar selective absorbers applications

We report on the structural and optical properties of laser surface structured Co nanocylinders-Al2O3 cermets on flexible Aluminium substrate for enhanced solar selective absorbers applications. This new family of solar selective absorbers coating consisting of Co nanocylinders embedded into nanoporous alumina template which were produced by standard electrodeposition and thereafter submitted to femtosecond laser surface structuring. While their structural and chemical properties were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry and atomic force microscopy, their optical characteristics were investigated by specular & diffuse reflectance. The optimized samples exhibit an elevated optical absorptance α(λ) above 98% and an emittance ɛ(λ) ∼0.03 in the spectral range of 200–1100nm. This set of values was suggested to be related to several surface and volume phenomena such as light trapping, plasmon surface effect as well as angular dependence of light reflection induced by the ultrafast laser multi-scale structuring.

(Invited) Polymer Nanopatterned Layers for Organic Solar Cell Applications

Recent advances in polymer-based organic solar cells have been possible due to different approaches such as design of new devices and synthesis of new materials with low band-gaps, control of the nanoscale morphology with additives, variation of the ratio of the donor/acceptor in the bulk heterojunction, application of thermal or solvent annealing process, the use of polymer nanostructures (nanogratings, nanopillars, nanorods and nanodots), etc. [1-5]. Nanoporous anodic alumina templates (NAAT) have been used to obtain polymer nanopatterned layers [6]. Template synthesis has been demonstrated to be an excellent technique to transfer the nanoporous shape to polymer and it is a cost-effective technique. In an organic bulk heterojunction solar cell is very important to improve the exciton dissociation, charge transport and to reduce the recombination. Polymer nanopatterned structures allow that the excitons can reach until the donor/acceptor interface to be dissociated into electrons and holes and the free charge carriers can travel along the uninterrupted pathway until they reach their respective electrodes. Interdigitated polymer nanostructures allow the domains in the p-type donor material (polymer) and the n-type acceptor material (fullerene) to be aligned normal to the electrode surfaces, thus increasing their crystallinity and charge carrier mobility and reducing recombination rates inside the device [7]. In this scenario, herein, we present the manufacture of the NAAT and interdigitated morphology to obtain the nanopatterned organic solar cell. Figure 1a and 1b show the environment scanning electron microscopy (ESEM) images of NAAT and P3HT-nanopatterned structure. Fig. 1c shows the nanopatterned device with the stack ITO / PEDOT:PSS / poly (3-hexylthiophene) P3HT-nanopatterned / [6,6]-phenyl-C71-butyric acid methyl (PC70BM) / Ca / Ag. Furthermore, we characterized and analysed the current-voltage characteristic of this solar cell measured under light and dark conditions. Finally, we discuss the future and potential applications of the NAAT to be used in nanopatterned layers manufactured with other semiconductor materials.

Nucleation and growth mechanism of Co–Pt alloy nanowires electrodeposited within alumina template

Co–Pt alloy nanowires were electrodeposited by direct current electrodeposition within nanoporous alumina templates with varying deposition potentials. The effect of deposition potential on nucleation and growth mechanisms during electrodeposition of Co–Pt alloy nanowires was investigated. The less negative deposition potential (−0.9 V) favours the instantaneous nucleation mechanism. The positive deviation from theoretical instantaneous and progressive nucleation mechanisms occurs at higher negative deposition potentials. The hysteresis behaviour and magnetic properties of electrodeposited Co–Pt alloy nanowires altered with varying deposition potential. The easy magnetization direction was in direction perpendicular to the wire axis. The deposition potential dependent change in hysteresis behaviour with increased coercivity and scattered remanence ratio was observed. This is attributed to better crystallinity with reduced defect density and hydrogen evolution causing structural changes at more negative deposition potentials.

Fabrication of TiO2 Nanotanks Embedded in a Nanoporous Alumina Template

The feasibility of surface nanopatterning with TiO2 nanotanks embedded in a nanoporous alumina template was investigated. Self‐assembled anodized aluminium oxide (AAO) template, in conjunction with sol gel process, was used to fabricate this nanocomposite object. Through hydrolysis and condensation of the titanium alkoxide, an inorganic TiO2 gel was moulded within the nanopore cavities of the alumina template. The nanocomposite object underwent two thermal treatments to stabilize and crystallize the TiO2. The morphology of the nanocomposite object was characterized by Field Emission Scanning Electron Microscopy (FESEM). The TiO2 nanotanks obtained have cylindrical shapes and are approximately 69 nm in diameter with a tank‐to‐tank distance of 26 nm. X‐ray diffraction analyses performed by Transmission Electron Microscopy (TEM) with selected area electron diffraction (SAED) were used to investigate the TiO2 structure. The optical properties were studied using UV‐Vis spectroscopy.

Substrate integrated waveguide isolator based on ferromagnetic nanowires in porous alumina template

We demonstrate the operation of a fully integrated and miniaturized waveguide based on ferromagnetic nanowires as a promising alternative to macroscopic ferrite slab-loaded metallic rectangular waveguides used as microwave isolators. Nanowires of various heights are selectively grown at dedicated areas into a low-loss nanoporous alumina template in order to create the shielding walls of a Substrate Integrated Waveguide (SIW) topology and the ferromagnetic slab supporting a circularly polarized nonreciprocal propagation. A model is proposed for the microwave response of the SIW and its non-reciprocity, which takes into account the substrate permittivity and the tensorial permeability of the ferromagnetic nanowires, and its predictions are validated by measurements. A significant isolation of 7 dB/cm is obtained experimentally at 13.5 GHz, without the need of a DC bias magnetic field.

Growth of TiO2 nanotube arrays in semiconductor porous anodic alumina templates

Abstract

Giant increase in the metal-enhanced fluorescence of organic molecules in nanoporous alumina templates and large molecule-specific red/blue-shift of the fluorescence peak.

The fluorescence of organic fluorophore molecules is enhanced when they are placed in contact with certain metals (Al, Ag, Cu, Au, etc.) whose surface plasmon waves couple into the radiative modes of the molecules and increase the radiative efficiency. Here, we report a hitherto unknown size dependence of this metal-enhanced fluorescence (MEF) effect in the nanoscale. When the molecules are deposited in nanoporous anodic alumina films with exposed aluminum at the bottom of the pores, they form organic nanowires standing on aluminum nanoparticles whose plasmon waves have much larger amplitudes. This increases the MEF strongly, resulting in several orders of magnitude increase in the fluorescence intensity of the organic fluorophores. The increase in intensity shows an inverse superlinear dependence on nanowire diameter because the nanowires also act as plasmonic "waveguides" that concentrate the plasmons and increase the coupling of the plasmons with the radiative modes of the molecules. Furthermore, if the nanoporous template housing the nanowires has built-in electric fields due to space charges, a strong molecule-specific red- or blue-shift is induced in the fluorescence peak owing to a renormalization of the dipole moment of the molecule. This can be exploited to detect minute amounts of target molecules in a mixture using their optical signature (fluorescence) despite the presence of confounding background signals. It can result in a unique new technology for biosensing and chemical sensing.

A new approach to fabricating magnetic multilayer nanowires by modifying the ac pulse electrodeposition in a single bath

This work focused on the development of a new single bath technique to fabricate compositionally modulated Co/Cu, CoFe/Cu and Fe/Cu multilayer nanowires in nanoporous alumina templates prepared by the hard and mild anodization methods. The approach was based on ac pulse electrodeposition, employing successive cycles of alternating continuous and pulsed sine waves with designated off-time between pulses and reduction/oxidation voltages. The substantial control over the composition of each segment was achieved by simultaneous change in the off-time between pulses and the ac deposition voltage. The multilayered nature of the nanostructures was substantiated by transmission electron microscopy. Each layer thickness was also nearly uniform, and could be readily adjusted by the number of pulses. The proposed method facilitates the fabrication of various multilayer nanowires in a single bath, which speeds up the fabrication process and is desirable for their application in nanodevices and nanoelectronics. The effect of magnetic layer thickness on the magnetic behaviour was also studied. Decreasing the magnetic layer thickness caused the parallel coercivity and squareness values to approach those measured in the perpendicular direction. The magnetic easy axis changed from parallel to nearly perpendicular to the nanowire axis, depending on the magnetic layers' aspect ratio and shape anisotropies.

Characterization of CdS Nanowires Self-Assembled in a Nanoporous Alumina Template

CdS nanowires were self-assembled in a thin film (∼200 nm) anodic aluminum oxide template on an indium tin oxide-coated glass substrate via dc electrodeposition. Raman spectral studies were done to probe the vibrational properties of scattering CdS phonons. Strong 1 longitudinal optical (LO), 2 LO, and 3 LO peaks were observed at 302 cm−1, 603 cm−1, and 906 cm−1 having an energy separation of 37 meV, which is in accordance with the CdS bulk values. The photoluminescence spectra showed improved intensity of emission on annealing of the CdS nanowires. Field-emission scanning microscopy confirms the growth of nanowires of diameters ranging from 10 nm to 25 nm for these templates. These diameters agreed with those extracted from the luminescence emission energies.

Effects of Barrier Layer Thinning and Pore Opening on Nanowire Growth in Nanoporous Aluminum Oxide Templates

Nanowires have garnered much interest in recent years for their potential in increasing performance of devices, such as solar cells, thermoelectrics, batteries, and piezoelectrics. Many methods exist for the fabrication of nanowires, but one of the most promising methods is the use of nanoporous aluminum oxide templates due to its extremely high density and self-ordering through a hexagonal close packed structure. However, fabricating uniform arrays of nanowires in contact with a conductive substrate still remains a challenge. A vital step in the process is removal of the barrier layer between the pore and the underlying substrate. This step is needed to establish direct contact from the pores to the substrate. The objective is to remove the barrier without sacrificing the pore walls. Several methods exist for barrier layer thinning, including stepping down the anodization voltage or ramping down the applied current. However, the thinning method will influence the outcome of the pore opening, which is the second and final step in creating direct contact between the pore and substrate. Here, different barrier layer thinning techniques will be discussed and their effect on the pore opening process examined. Both step-down voltage and ramped current methods were used to understand the barrier layer thinning process. These results were correlated to the amount of pore opening achieved, measured through a combination of electrochemical impedance spectroscopy and chronoamperometry. Finally, nanowires were grown within these aluminum oxide templates to study the effects of different barrier layer thinning and pore opening techniques on nanowire growth and uniformity. This research provides a comprehensive approach towards fabricating uniform, high-density nanowire arrays.