Anodic Alumina Films Research Articles

Fabrication of a meniscus microlens array made of anodic alumina by laser irradiation and electrochemical techniques

An anodic alumina microlens array was fabricated by laser irradiation and electrochemical techniques. An aluminum specimen covered with a porous oxide film was irradiated with a pulsed Nd-YAG laser, and then electropolished to dissolve the aluminum substrate. A well-defined semi-elliptical micropore was formed on the aluminum by electropolishing. The immersion of the electropolished specimen in a CrO3/H3PO4 solution resulted in the dissolution of the remaining anodic oxide film. Subsequent re-anodizing enabled the formation of a characteristic meniscus-shaped oxide film on the micropore. A microlens array made of the thin anodic alumina film, which showed flexibility and heat resistance, was successfully fabricated by the lift-off of the anodic oxide.

Re-Anodizing Technique as a Method of Investigation of Thermally Activated Defects in Anodic Alumina Films

A new approach of determination of energy of shallow thermally activated traps in the anodic alumina films is suggested. The approach consists of the heat-treatment of the anodic Al2O3|Al structure at 100–350°C (this means thermal release of electrons from the traps) and then re-anodizing of the anodic films in the barrier type electrolytes in potentiodynamic mode (refilling of electron traps during transient processes). In comparison with existing methods (thermoactivated and surface soft-X-ray absorption and photoemission spectroscopy), this method is relatively simple and can be applied to investigate electron trap levels both in semiconductor and insulator films. The algorithm of data-processing of experimental results was also proposed to obtain the parameters of thermally activated electron traps. It was applied to calculate the activation energy of electron traps that is equal to 0.147 and 0.060 eV for sulfuric and oxalic acid alumina films, respectively. The concentration of the traps was determined to be on the order of 1019 cm−3. The electron traps are believed to be due to the presence of coordinatively unsaturated aluminum ions, their presence was confirmed by FTIR spectroscopy. On the basis of data obtained a model is suggested for the transitions that take place in the barrier layer of the anodic alumina during heating and re-anodizing.

Experimental validation of the novel theory explaining self-organization in porous anodic alumina films

The lack of a reliable method for theoretical prediction of nanoporous anodic alumina films obtained from non-familiar electrolytes prompted the search of a viable solution to this problem. The theory explaining the self-assembly mechanism was described in our preceding work. Here, the results of an extensive validation test are presented.

Preparation and Breakdown Property of Aluminum Oxide Thin Films Deposited onto Anodized Aluminum Substrate

Aluminum oxide thin films prepared from wet chemistry were deposited onto anodized aluminum substrates, where aluminum iso-propylate was used as the precursor to prepare the aluminum oxide films. Barrier-type anodized aluminum oxide film with different thickness was deposited in an aqueous solution of ammonium pentaborate under selected anodizing voltages. The electro-transport characteristics (I–V) are used to identify their dielectric behaviors. The FESEM micrographs and metallographic photographs were used to analyze the physical variation after the breakdown. The nonlinear electrical transport behavior and the breakdown phenomenon of the aluminum oxide thin films are studied.

Fabricating 20 cm × 20 cm Porous Template Using Anodic Aluminum Oxide

This paper proposes anodic aluminum oxide (AAO) on a 20 cm × 20 cm tin-doped indium oxide (ITO)/glass substrate by using an anodization process with a specially designed carrier. The proposed AAO can be used at a high applied bias (>110 V) to improve the uniformity of current density. The enhancement of light transmittance is observed by modulating the porous diameter. Therefore, the increased transmittance of AAO/ITO/glass with increased time of pore-widening treatment is attributed to the decreased refractive index of AAO film. By using pore-widening and post-annealing treatments, the morphologies and optical transmission spectra of AAO/ ITO/glass were also examined.

Effects of Electrolyte Species and Their Combination on Film Structures and Dielectric Properties of Crystalline Anodic Alumina Films Formed by Two-Step Anodization

The effects of electrolyte species and their combination during the two-step anodization of anodic oxide films formed in various ammonium salts solutions on aluminum in terms of the crystallinity and dielectric properties for use as an electrolytic capacitor were investigated. The aluminum substrates were pretreated by dipping in boiling water to form a hydrated layer. Anodic films were annealed at 500°C after the first anodization. The CV (capacitance C multiplied by withstand voltage V) of the films formed with a second anodization in ammonium borate was higher than that of the films formed using the same electrolyte for the first and second anodizations. The increase in CV due to the improved crystallinity when using borate for the second anodization and the following electrolytes for the first anodization increased in the order salicylate < succinate < tartrate < phosphate < citrate < adipate < borate. When phosphate or organic electrolytes were used in the second anodization, considerable spike noise was observed. However, the spike noise was significantly suppressed when borate was used in the second anodization. [doi:10.2320/matertrans.L-M2013826]

Processes, parameters and mechanisms controlling the normal and abnormal growth of porous anodic alumina films

Aluminium was anodised in H2SO4 solutions 0.25–1.53 M, temperatures 0–35 °C, times up to 90 min and voltage 25 V. Anodising was followed chronoamperometrically. The passing charge, consumed Al, formed oxide, transport numbers of oxygen anions in barrier layer and film thicknesses were determined. Films were examined to detect abnormal film growth, called burning. It was found that the current in Al anode is closely solely ionic, and the only processes occurring are those related with ionic charge transport and heat release and its abduction for which suitable equations were formulated relating many parameters. At each concentration, the temperatures, current densities and times up to which normal film growth occurs and above which stain-like mild, island-like intermediate, strong, strong-destructive and mixed burning appears were found. Burning emerges at lower concentrations, temperatures and times for thinner Al. Low concentrations and temperatures among thresholds where mild and next-kind burning first appear define windows of conditions where thick enough dark or black films grow. Peculiar chronoamperometric plot characterises each film growth type. Though other burning kinds can appear at each surface region or in the whole surface, strong and strong-destructive emerge only at the lower Al side around which the efficiency of solution stirring is highest. The mechanisms of normal and abnormal film growth were formulated showing that different processes and interacting variable parameters, some of which are noted for first time, regulate each film growth type. These can predict methods to avoid abnormal film growth.

WS2 nanoparticles–encapsulated amorphous carbon tubes: A novel electrode material for supercapacitors with a high rate capability

The present communication reports on the preparation of WS2 nanoparticles–encapsulated amorphous carbon tubes (WS2NPs/CTs) via two-step processes. Firstly, CTs were grown in anodic aluminum oxide (AAO) film by hydrothermal carbonization of glucose. Secondly, the as-prepared CTs/AAO film was impregnated with (NH4)2WS4 followed by high temperature treatment in Ar atmosphere. We further demonstrate the application of the resulting WS2NP/CT composites as a novel electrode material for supercapacitor. Such composites were found to deliver a specific capacitance of 337F/g at a high current density of 10A/g with good cycle stability.

Tribological Behaviors of Lubricants Modified Nanoporous Anodic Alumina Film

A uniform closely packed hexagonal array of porous anodic alumina (PAA) film with an average pore diameter of ~100 nm and a pore height of ~330 nm was obtained by anodizing in oxalic acid solution. Low surface energy molecules of perfluoropolyether (PFPE) and octadecyltrichlorosilane (OTS) were used to enhance the hydrophobicity of the PAA and aluminum (Al) surface. The tribological performances were investigated in detail. The results showed that the static friction coefficient of PAA was much lower than that of smooth Al, which could be due to the higher hardness of the PAA than that of Al. PAA modified with PFPE was characterized by lower friction coefficient and longer wear life as compared with bare Al, PAA, and PAA modified with OTS. This is caused by the combined effects of flexibility, mobility, and inherent lubricity of PFPE molecules and the nanoporous structure of PAA as a reservoir for lubricants and wear particles.

Effect of porosity on the laser induced damage threshold of porous anodic alumina films

Thermal stress of porous alumina films has been simulated by finite element method based on thermal transfer equation and thermal stress formulas. The influence of equivalent thermal conductivity and elastic modulus on laser induced damage threshold (LIDT) has been studied. It was found that the biggest circumferential tensile stress will be small with the porosity from 15% to 35%, and it effectively improves the LIDT. The equivalent thermal conductivity and LIDT decreases with the increase of porosity. The equivalent elastic modulus decreases and LIDT increase with the increase of porosity.

Blind reconstruction of atomic force microscopy tip morphology by using porous anodic alumina membrane

An atomic force microscopy (AFM) image of a surface is a convolution of the tip geometry and sample features. It is important to develop tip characterisers and estimate the tip shape for a more accurate AFM image. With the traditional characterisers with special microstructures it is difficult to accurately determine tip shape because of their dimensional uncertainty. Combined with tip blind reconstruction algorithms, some nanomaterials with arrayed nanostructures are often used to estimate the AFM tip morphology. However, the blind reconstruction algorithms are sensitive to image noise. To solve such problems, the porous anodic alumina (PAA) film with well-ordered porous nanostructures was fabricated and used as a new tip characteriser. By setting the appropriate scanning routine and scanning mode, the two-dimensional and three-dimensional tip morphology was accurately calculated. PAA film as the AFM tip characteriser can effectively reduce the influence of AFM image noise and sample-dimensional uncertainty of tip blind estimation results, especially avoiding tip wear and damage.

Preparing nano-hole arrays by using porous anodic aluminum oxide nano-structural masks for the enhanced emission from InGaN/GaN blue light-emitting diodes

We report on the achievement of the enhanced cathodoluminescence (CL) from InGaN/GaN light-emitting diodes (LEDs) by using roughening surface. Nanoporous anodic aluminum oxide (AAO) mask was utilized to form nano-hole arrays on the surface of InGaN/GaN LEDs. AAO membranes with ordered hexagonal structures were fabricated from aluminum foils by a two-step anodization method. The average pore densities of ∼1.0 × 1010 cm−2 and 3.0 × 1010 cm−2 were fabricated with the constant anodization voltages of 25 and 40 V, respectively. Anodic porous alumina film with a thickness of ∼600 nm has been used as a mask for the induced couple plasma etching process to fabricate nano-hole arrays on the LED surface. Diameter and depth of nano-holes can be controlled by varying the etching duration and/or the diameter of AAO membranes. Due to the reduction of total internal reflection obtained in the patterned samples, we have observed that the cathodoluminescence intensity of LEDs with nanoporous structures is increased up to eight times compared to that of samples without using nanoporous structure.

Photoluminescence emission of nanoporous anodic aluminum oxide films prepared in phosphoric acid

The photoluminescence emission of nanoporous anodic aluminum oxide films formed in phosphoric acid is studied in order to explore their defect-based subband electronic structure. Different excitation wavelengths are used to identify most of the details of the subband states. The films are produced under different anodizing conditions to optimize their emission in the visible range. Scanning electron microscopy investigations confirm pore formation in the produced layers. Gaussian analysis of the emission data indicates that subband states change with anodizing parameters, and various point defects can be formed both in the bulk and on the surface of these nanoporous layers during anodizing.

Continuous and Nanostructured TiO2 Films Grown by dc Sputtering Magnetron

The growth of Anatase nanostructured films using dc reactive magnetron sputtering and post-annealing treatment is reported. TiO2 has been deposited on Porous Anodic Alumina Films used as templates which were previously grown in phosphoric acid solution and etched to modify their pore diameters. This synthesis via results in the formation of vertically aligned and spatially ordered TiO2 nanostructures replicating the underlying template. Previously, the growth optimization of TiO2 thin films deposited by dc magnetron sputtering on flat silicon substrates was done. The crystalline structure and Ti in-depth concentration profile were determined by grazing incidence X-ray diffraction and Rutherford backscattering spectrometry, respectively. The surface morphology of the samples was explored by mean of a Field Emission Gun scanning electron microscope. Optical properties of the nanostructured samples were studied by using the reflectance spectra received in the UV-visible range. In these spectra different band gap values and complex light absorption features were observed.

Obvious improvement of light extraction obtained by anodic aluminum oxide coverage on GaN surface

Anodic aluminum oxide (AAO) films with different porosities were fabricated on the surfaces of InGaN/GaN multiple quantum wells (MQWs). We measured the photoluminescence spectra of these samples and then calculated the light extraction enhancement factors (E). It was found that the light extraction of the MQWs with the AAO films has been greatly enhanced compared to the area without AAO coverage. The maximum value of E reaches 3.5 at a certain wavelength and 2.29 in the integral intensity. The increase of E with the decrease of the porosities demonstrates that the nano-hole structure in the AAO films plays an important role in enhancing the light extraction efficiency.

Relationship between microstructure and optical properties of a novel perovskite C12PbI4 embedded in matrix of porous alumina

In this study, organic–inorganic hybrid perovskite multiple quantum wells (PbI QWs) embedded in porous anodic alumina (PAA) thin films on glass and aluminum substrates are investigated in detail. The pore height and diameter of the nanoscale structure of porous anodic alumina (PAA) film produced by the anodization technique are controllable. The synthesized films are characterized morphologically using the atomic force microscopy (AFM). Scanning electron microscopy (SEM) study showed granular surface. The structural and optical properties were investigated by X-ray diffraction (XRD), photoluminescence (PL) and UV–Vis–NIR spectrophotometer. The effect of the two different substrates on the impregnation of the PbI QW in the PAA is presented. Both PL and AFM studies show a better penetration of the PbI QW in the case of the Al substrate providing a wider pore diameter. Remarkable enhancement of quantum confinement is demonstrated.

Capacitive humidity sensor with a coplanar electrode structure based on anodised porous alumina film

A simple and new relative humidity (RH) capacitive sensor with a two-layer planar structure based on anodic aluminium oxide (AAO) film serving as a sensing layer has been fabricated. A pair of separated tantalum thin films under the AAO layer served as two coplanar electrodes. AAO film was fabricated by anodising deposited aluminium thin film in oxalic acid. The influence of the pore size and thickness of the AAO film, and the electrode spacing, on the performance of the sensor was investigated. The RH sensor with a thicker AAO film and narrower electrode spacing was more sensitive. By pore widening, the sensitivity of the sensor can be improved. The achieved average sensitivity as high as ∼3.57 pF/RH% was obtained for sensors with an AAO layer of 2 µm thickness and 80 nm pore size. These results were explained using the partial capacitance method and parallel-serial model. In addition, under the humidity range of 20–80%, sensors with 70 nm pores had the highest sensitivity, which was interpreted in terms of effective permittivity and anion concentration. This study provides an easy method to fabricate high-performance humidity sensors with very simple structures for practical applications.

Cavity-type hypersonic phononic crystals

We report on the engineering of the phonon dispersion diagram in monodomain anodic porous alumina (APA) films through the porosity and physical state of the material residing in the nanopores. Lattice symmetry and inclusion materials are theoretically identified to be the main factors which control the hypersonic acoustic wave propagation. This involves the interaction between the longitudinal and the transverse modes in the effective medium and a flat band characteristic of the material residing in the cavities. Air and filled nanopores, therefore, display markedly different dispersion relations and the inclusion materials lead to a locally resonant structural behavior uniquely determining their properties under confinement. APA films emerge as a new platform to investigate the rich acoustic phenomena of structured composite matter.

Deposition of Polyelectrolyte Multilayer Film on a Nanoporous Alumina Membrane for Stable Label-Free Optical Biosensing

The stabilization of inorganic nanostructures is a key challenge in developing label-free optical biosensors using inorganic nanoporous films. In the present study, a polyelectrolyte multilayer film (PMF) composed of poly(acrylic acid) and poly(allylamine hydrochloride) was deposited on porous anodic alumina (PAA) film (pore diameter = 40 nm) by the layer-by-layer technique. The characteristics of the PMF as a protective coating layer against dissolution of the PAA film were examined by means of in situ optical waveguide spectroscopy (OWS), ex situ scanning electron microscope/energy-dispersive spectroscopy (SEM/EDS), and infrared reflection–adsorption spectroscopy (IR-RAS). The results obtained by OWS and SEM/EDS indicated the formation of PMF at both the alumina pore surface and the PAA film surfaces. Processing the PMF-deposited PAA (PMF–PAA) film at 180 °C to promote thermal cross-linking allowed PMF to act as a strong protective coating layer in aqueous buffer solutions (pH 2.5–8.7) but also in common alumina matrix etchant. Residual amino groups in the cross-linked PMF could also be used to conjugate single-probe DNA at the PMF–PAA film for OWS-based DNA–DNA hybridization assay. From the results obtained in this study, it was concluded that the deposition of PMF followed by thermal cross-linking is potentially useful not only as a protective coating and but also for immobilizing biorecognition elements in the PAA film.

Electrochemical behavior of anodized AA6063-T6 alloys affected by matrix structures

AA 6063 alloys were cold-rolled (CR) either before or after solution treatment (S) and then different samples were artificially aged (T6) to obtain different samples (CRST6 and SCRT6). The highest dislocation density was observed in the SCRT6 sample which also showed the lowest particle count among the three samples; ST6, CRST6 and SCRT6. Subsequently, all samples were anodized in a 15wt% sulfuric acid solution for different time spans to obtain anodic aluminum oxide (AAO) films. The anodized samples were further analyzed with X-ray Photoelectron Spectroscopy (XPS) analysis. We determined that the constituent phases in the AAO film were composed of hydrated amorphous alumina, hydrated oxide (Al(OH)3) and oxyhydroxide (AlOOH) phases together with some silicon-containing particles trapped in the films on all samples. In the electrochemical test, the silicon-containing particles and hydrated Al(OH)3 oxide that existed at the electrolyte/film (e/f) interface were found to inversely influence the corrosion resistance of the anodized samples.