Anodic Alumina Films Research Articles

Broadband and wide-angle antireflective characteristics of nanoporous anodic alumina films for silicon-based optoelectronic applications

Nanoporous anodic alumina (NPAA) films are fabricated on silicon (Si) surfaces by the anodization of aluminum (Al). Effect of the thickness of initial Al films on the antireflective characteristics of NPAA films consisting of random pore structures is investigated in the wavelength range of 300–1,100 nm at incident light angles (θ inc) of 3°–70°, together with theoretical analysis using a rigorous coupled-wave analysis method. The reflectance strongly depends on the thickness and porosity of NPAA films, resulting from anodization parameters such as applied voltage, anodization time, and pore widening time. For the obtained NPAA film at the initial Al thickness of 100 nm and the applied voltage of 30 V, the surface reflectance is reduced over a wide wavelength region of 300–1,100 nm at normal incidence of ~3°, exhibiting the relatively lower average reflectance (R avg) of ~19 % (i.e., R avg ~ 39 % for the bare Si substrate). The lower angle-dependent reflective properties are observed compared to the bare Si substrate at θ inc = 20°–70° for unpolarized light. The calculated reflectance results of NPAA films show similar trends to the measured data.

Effect of pore depth on tribological behavior of anodic alumina films under nano-thin film lubrication

Abstract A number of parameters such as the dimple depth, dimple area density, and dimple shape, affect the tribological performance of dimpled surfaces, and a comprehensive tribological characterization of the depth-dependent nature of their material properties would be useful. However, only surfaces with dimples with depth-to-diameter ratios lower than 1 have been investigated. In this study, porous anodic alumina (PAA) films with high pore depth (PDEP)-to-pore diameter (PDIA) ratios (approximately 2.8–30) were prepared. The as-prepared films had the same PDIA value (50 nm) but different PDEP values (140 nm, 350 nm, 700 nm, 1100 nm, and 1500 nm). Perfluoropolyether was used as the lubricant to improve the tribological properties of the as-prepared films. The effects of the PDEP value on the tribological properties of the as-prepared films were investigated systematically. It was found that the antiwear performance of the PAA films was closely related to the combined effects of flexibility, mobility, and inherent lubricity of PFPE molecules and PDEP values in nanoporous structures as a reservoir for lubricants and wear particles. For PDEP-to-PDIA ratios greater than 1, under the conditions studied, the friction performance of the PAA films improved with an increase in the PDEP value. Further, after a critical PDEP value, the applied load had little effect on the friction coefficient and antiwear life of the films. The results of this study should aid the fabrication of stable films for industrial applications and act as a bridge between science and engineering on the micro/nanoscale.

Sorption of hydrophilic dyes on anodic aluminium oxide films and application to pH sensing.

The sorption of selected hydrophilic pH-sensitive dyes (bromophenol blue, bromothymol blue, bromocresol purple, alizarin red, methyl orange, congo red, rhodamine 6G) on films of anodized aluminium oxide (AAO) was investigated in this study. Depth and pore structure of the AAO channels were adjusted by changing electrolysis time and current density during treatment of aluminium foil in oxalic acid, sulfosalycilic acid and sulfuric acid at concentration levels between 0.2 and 0.6 M. The dyes were immobilized on the AAO surface by direct saturation of the films in dye solutions. It was shown by scanning electron microscopy and X-ray spectral analysis that the dyes penetrated into the AAO channels by more than 1.5 μm, even at static saturation conditions. The anionic dyes linked to the porous AAO surface exhibited differential shifts of the UV absorption bands in their acidic/basic forms. By combining several dyes, the films have an application range between pH = 0.5-9 in aqueous media. The dye-modified AAO film was a simple, portable, inexpensive and reusable pH sensor with very fast response time and clear colour transitions.

Effect of Sealing Time of Anodic Aluminum Oxide (AAO) Film for Preventing Plasma Damage

Effect of Sealing Time of Anodic Aluminum Oxide (AAO) Film for Preventing Plasma Damage

Unique AAO films with adjustable hierarchical microstructures

Anodic aluminium oxide (AAO) films with novel nano-/submicron-/micron-porous hierarchical structures have been fabricated by using a simple anodization method.

Effect of pore diameter in nanoporous anodic alumina optical biosensors.

The influence of pore diameter over the optical response of nanoporous anodic alumina (NAA) films is analyzed by reflectance interference spectroscopy. NAA films manufactured by a two-step anodization procedure in oxalic acid exhibiting three well-defined pore diameter distributions with pores of 32 ± 4, 50 ± 3, and 73 ± 2 nm are studied. The optical detection of biomolecules is investigated by serially dosing protein A, human IgG and anti-human IgG into a nanoporous matrix using a custom-made flow cell. The results demonstrate that the transduction signal, the variation of effective optical thickness upon IgG binding to protein A (ΔEOT), depends on the nanopore diameter: for small pore diameter (32 nm) no significant differences in signals are observed for different protein concentrations whereas for larger pore diameters (50 nm and 73 nm) the signals increase for increasing concentrations from 10 to 100 μg mL(-1). Our experiments also show that this signal can be further enhanced by amplification with anti-human IgG due to the multiple binding events between the antigen and the antibody. These results will enable the development of more sensitive interferometric biosensors based on NAA.

The Electrical and Mechanical Properties of Porous Anodic 6061‐T6 Aluminum Alloy Oxide Film

The properties of the growth of the 6061‐T6 aluminum alloy oxide were studied using sulfuric acid anodization. The parameters for the manufacturing process include electrolyte categories, electrolyte concentration, and operating voltages. The results showed that the aluminum oxides obtained by anodization process are mainly amorphous structure and the anodic current density is an important factor affecting the rate of response for oxygen and aluminum ions in barrier. In this experiment, polish process is very important to stable the anodic aluminum oxide film and then it will get the better properties of anodic film. Besides, when using sulfuric acid as the electrolyte, the increase of anodic voltage also increases the rate of reaction which increases the mechanical and electrical properties of anodic oxide film, but too large applied anodic voltage will reduce the mechanical and electrical properties of film because of the crack of the anodic oxide film.

A Facile Method for Controllable Fabrication of Porous Anodic Aluminum Oxide Templates with Multiple-Scale Structures

Porous anodic aluminum oxide (AAO) templates with diverse nano/submicron/micron multiple-scale structures have been fabricated by a facile and controllable two-step anodization process. The first step anodization has been achieved by using a simple competitive growth process with high anodization voltage (Ua) in ethanol-oxalic acid mixture electrolytes. The effects of Ua (300–600 V), anodization current density (Ja, 100–200 A m−2), and the proportion of ethanol adding (0.3 M oxalic acid : ethanol = 1 : 1, 3 : 2, V/V) on the submicron/micron structures of AAO templates have been investigated in detail. The second step anodization has been conducted under relatively lower Ua of 30 V and 40 V, respectively. The effects of Ua, etching process, and anodization time on the nanostructures of as-fabricated porous AAO templates have been studied carefully. In addition, related intrinsic formation mechanisms have also been investigated. Based on these quantitative experimental results, the fabrication of multiple-scale porous AAO films with diverse nano/submicron/micron structures can be more controllable and designable, facilitating their practical applications in simple template synthesis of complex multiple-scale functional materials.

COMPARISON AND CONSTRUCTION OF "MILD" AND "HARD" ANODISATION REACTORS FOR THE SYNTHESIS OF POROUS ALUMINA

The mild anodization (MA) reactor is exemplified for its operational simplicity and its excellent control over the experimental parameters that are involved in the anodization process. This method provides porous anodic alumina films with a regular cell-arrangement structure. This offers a better cost-benefit ratio than the other equipment configurations that are used to fabricate nanoporous structures (i.e., ion beam lithography). Conversely, the hard anodization (HA) reactor produces oxides at a rate that is 25 to 35 times faster than the MA reactor. The produced oxides also have greater layer thicknesses and interpore distance, and with a uniform nanopore spatial order (> 1000). In contrast to MA reactors, the construction of an HA reactor requires special components to maintain anodisation at a high potential regime. Herein, we describe and compare both reactors from a technical viewpoint.

Interferometric nanoporous anodic alumina photonic coatings for optical sensing.

Herein, we present a systematic study on the development, optical optimization and sensing applicability of colored photonic coatings based on nanoporous anodic alumina films grown on aluminum substrates. These optical nanostructures, so-called distributed Bragg reflectors (NAA-DBRs), are fabricated by galvanostatic pulse anodization process, in which the current density is altered in a periodic manner in order to engineer the effective medium of the resulting photonic coatings. As-prepared NAA-DBR photonic coatings present brilliant interference colors on the surface of aluminum, which can be tuned at will within the UV-visible spectrum by means of the anodization profile. A broad library of NAA-DBR colors is produced by means of different anodization profiles. Then, the effective medium of these NAA-DBR photonic coatings is systematically assessed in terms of optical sensitivity, low limit of detection and linearity by reflectometric interference spectroscopy (RIfS) in order to optimize their nanoporous structure toward optical sensors with enhanced sensing performance. Finally, we demonstrate the applicability of these photonic nanostructures as optical platforms by selectively detecting gold(iii) ions in aqueous solutions. The obtained results reveal that optimized NAA-DBR photonic coatings can achieve an outstanding sensing performance for gold(iii) ions, with a sensitivity of 22.16 nm μM(-1), a low limit of detection of 0.156 μM (i.e. 30.7 ppb) and excellent linearity within the working range (0.9983).

Preparation and analysis of anodic aluminum oxide films with continuously tunable interpore distances

Nanoporous anodic aluminum oxides are often used as templates for preparation of nanostructures such as nanodot, nanowire and nanotube arrays. The interpore distance of anodic aluminum oxide is the most important parameter in controlling the periodicity of these nanostructures. Herein we demonstrate a simple and yet powerful method to fabricate ordered anodic aluminum oxides with continuously tunable interpore distances. By using mixed solution of citric and oxalic acids with different molar ratio, the range of anodizing voltages within which self-ordered films can be formed were extended to between 40 and 300V, resulting in the interpore distances change from 100 to 750nm. Our work realized very broad range of interpore distances in a continuously tunable fashion and the experiment processes are easily controllable and reproducible. The dependence of the interpore distances on acid ratios in mixed solutions was discussed through analysis of anodizing current and it was found that the effective dissociation constant of the mixed acids is of great importance. The interpore distances achieved are comparable to wavelengths ranging from UV to near IR, and may have potential applications in optical meta-materials for photovoltaics and optical sensing.

A Facile Method for Improving the Process of Porous Anodic Aluminum Oxide Film Preparation

A new and facile method for improving the procedure of anodic aluminum oxide (AAO) film preparation was presented, which was based on an electrochemical detachment procedure. The detachment and pore-opening were performed just through one step in a solution of HClO4and C2H5OH at a voltage of 40V. The as-obtained AAO film was characterized by scanning electron microscopy (SEM) and scanning probing microscopy (SPM). The process of detachment and pore-opening was also discussed.

Controllable Fabrication of Networked Three-Dimensional Nanoporous Anodic Alumina Films on Low-Purity Al Materials

Networked three-dimensional (3D) nanoporous alumina nanostructures—consisting of vertical cylindrical stem pore arrays (170–310 nm in diameter) and periodical transverse branched pores (20–80 nm in diameter) interspaced regularly by 190–220 nm across the vertical pore walls—were controllably fabricated from low purity Al materials (99.0%, 99.3%, and 99.56%) by anodization in a phosphoric acidic solution at 110–190 V. The formation of transverse pores depended predominantly on the purity of the Al base materials and the corrosion resistance of the anodic porous alumina films, which can be mainly attributed to anodic and/or chemical dissolution of impurities such as Fe, Cu, Zn, Mg, and Mn incorporated in the Al base materials. Moreover, the effects of the purity of Al materials on the growth and morphology of porous alumina films in oxalic and sulfuric acid solutions were also investigated by a two-step anodization of Al sheets with different purities, namely 99.0%, 99.3%, 99.56%, and 99.999% Al. The alumina films grew more slowly in all electrolytes with decreasing Al purity, which can be attributed to the lower corrosion resistance of the PAA films containing small quantities of Fe oxides and/or hydroxides produced during anodization.

Research on the Morphologies and Corrosion Resistance of Anodic Alumina Films Influenced by Additives in Sulfuric Electrolyte

The surface morphologies, chemical composition, phase composition, compactness, thickness and the corrosion resistance of porous anodic films prepared in the sulfuric electrolyte with different additives at 35°C were investigated by XRD, FESEM, EDS, drop method in this paper. The additives are including organic acids, polyhydric alcohols and rare earth (REE) salts. It is indicated that a porously anodic film with about 20 nm holes in diameter can be obtained by additives into the sulfuric electrolyte. The main chemical compositions of the anodic films are Al, O elements, and a small amount of S. The film after boiling water sealing is composed of boehmite phase (Al2O3·H2O) and alumina (Al2O3). The corrosion resistance of the anodic film can be improved by the coupling effect of the additives. The mechanism is discussed.

On the Characteristics and Corrosion Resistance of Anodic Alumina Films Prepared in Sulfuric Electrolyte with Different Additives

The characteristics including surface morphology, phase constitution, thickness and the corrosion resistance of the porously anodic films prepared in the sulfuric electrolyte with different additives at 35°C were investigated by SEM, EDS, XRD and electrochemical polarization method in this paper. The additives are organic acids, polyhydric alcohols and rare earth (REE) salts. A porous anodic film with about 20 nm holes in diameter can be obtained by additives into the sulfuric electrolyte. The main chemical compositions of the anodic films are Al, O elements, and a small amount of S. The film after boiling water sealing is composed of boehmite (Al2O3·H2O) and alumina (Al2O3) phases. The compactness and the thickness of the anodic films can be improved by the coupling effect of the additives, which increases the corrosion resistance of the film. The effect of additives on morphologies and corrosion resistance is discussed.

Templated deposition of multiscale periodic metallic nanodot arrays with sub-10 nm gaps on rigid and flexible substrates

Multiscale metallic nanostrucutures, which support hybrid coupling of plasmon resonances, are essential for the engineering of plasmonic devices. The fabrication of large area periodic multiscale structures still remains a challenge, considering the cost and efficiency. In this work, highly ordered multiscale Ag nanoarrays with lateral dimensions of up to 6 mm × 6 mm have been successfully fabricated on both rigid silicon and flexible polydimethylsiloxane (PDMS) substrate by thermal evaporation using ultrathin anodic aluminum oxide films as masks. Owing to the peculiarities of thermal evaporation and the variance of substrate surface energy, the unit cell of the periodic arrays consist of a core-satellite structure on silicon and randomly distributed child particles on PDMS, with gaps as small as 10 nm. The flexible Ag nanoarrays on PDMS demonstrate a broadband extraordinary optical transmission with an enhancement up to 2.7 times when normalized to the exposed area. Moreover, the transmission and diffraction properties can readily be controlled by stretching the PDMS. These tunable optical properties support the multiscale Ag nanoarrays to be applied in some optical and optoelectronic devices.

Anodic oxide film growth on thin magnetron sputter-deposited titanium layer

A ~100nm thick sputter-deposited titanium layer on electropolished aluminium is used for the investigation of anodic film growth in 1M H3PO4. It is found that the thin titanium layer could not provide sufficient current efficiency for titanium anodic film growth when anodized to the voltage over 80V due to the occurrence of oxygen evolution. The ruptures of titanium anodic film and the sputtering layer are induced by the development of oxygen bubbles. The penetrated phosphoric acid electrolyte in the ruptured regions of sputtering titanium layer contacts with the aluminium substrate, which is leading to the anodic oxide growth of aluminium. The thickness of the titanium anodic film increases from 10 to 100V, however, it is reduced from 100 to 150V due to the thinning of titanium layer. Over 80V, the sputtering layer at some regions where it is completely ruptured, the anodic film growth of titanium could not be created. A thicker aluminium anodic film is formed on such regions due to the direct connection with the electrolyte.

Growth behavior of anodic oxide formed by aluminum anodizing in glutaric and its derivative acid electrolytes

The growth behavior of anodic oxide films formed via anodizing in glutaric and its derivative acid solutions was investigated based on the acid dissociation constants of electrolytes. High-purity aluminum foils were anodized in glutaric, ketoglutaric, and acetonedicarboxylic acid solutions under various electrochemical conditions. A thin barrier anodic oxide film grew uniformly on the aluminum substrate by glutaric acid anodizing, and further anodizing caused the film to breakdown due to a high electric field. In contrast, an anodic porous alumina film with a submicrometer-scale cell diameter was successfully formed by ketoglutaric acid anodizing at 293K. However, the increase and decrease in the temperature of the ketoglutaric acid resulted in non-uniform oxide growth and localized pitting corrosion of the aluminum substrate. An anodic porous alumina film could also be fabricated by acetonedicarboxylic acid anodizing due to the relatively low dissociation constants associated with the acid. Acid dissociation constants are an important factor for the fabrication of anodic porous alumina films.

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 stochastic route to simulate the growth of porous anodic alumina

Porous anodic alumina (PAA) film is composed of highly ordered and controllable structures, and their extensive application requires the understanding of their growing mechanism. Herein, we present a localized oxidation model to unravel the phenomena of PAA growth, showing that random processes converge into an ordered formation due to the unique characteristics of ion transport confinement in alumina. The anodizing voltage shows a quadratic relationship with barrier layer depth. In addition, we predict the furcate conditions of PAA and the voltage threshold to produce a PAA by our model.