Anodic Aluminum Oxide Structure Research Articles

Large-Scale Fabrication of 2-D Nanoporous Graphene Using a Thin Anodic Aluminum Oxide Etching Mask

A large-scale nanoporous graphene (NPG) fabrication method via a thin anodic aluminum oxide (AAO) etching mask is presented in this paper. A thin AAO film is successfully transferred onto a hydrophobic graphene surface under no external force. The AAO film is completely stacked on the graphene due to the van der Waals force. The neck width of the NPG can be controlled ranging from 10 nm to 30 nm with different AAO pore widening times. Extension of the NPG structure is demonstrated on a centimeter scale up to 2 cm2. AAO and NPG structures are characterized using optical microscopy (OM), Raman spectroscopy and field-emission scanning electron microscopy (FE-SEM). A field effect transistor (FET) is realized by using NPG. Its electrical characteristics turn out to be different from that of pristine graphene, which is due to the periodic nanostructures. The proposed fabrication method could be adapted to a future graphene-based nano device.

Enhancement of pore size distribution in one-step hybrid pulse anodization of aluminum thin films sputtered on Si substrates

The anodic aluminum oxide (AAO) has attracted more and more researches since its porous structure was discovered for many nano-enabled applications. In this article, the AAO was fabricated by one-step hybrid pulse anodizing the sputtered aluminum (Al) thin films on Si substrates for nanoporous template due to limited thickness. Surface morphology with the varied electrical field can influence the AAO characteristics during anodization process. The various Al target power and substrate bias were performed to form different surface morphology of the sputtered Al thin films which were further investigated for the AAO synthesis quality. The more non-uniform AAO structure was found on the sputtered Al thin films with rougher surface or larger mean grain size which was deposited at high target power or no bias. In contrast, the Al films with smaller mean grain size and smoother morphology deposited at relatively low target power with bias were beneficial for more uniform pore size distribution. On the constant anodizing condition, the pore size distribution can be enhanced with the reduced standard deviation from 13.01 to 6.34nm by improving surface morphology of the as-deposited Al film.

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.

Impurity-defect structure of anodic aluminum oxide produced by two-sided anodizing in tartaric acid

Porous aluminum oxide is prepared in a 0.4 M aqueous solution of tartaric acid by two-sided anodizing. Fourier Transform IR spectroscopy (FTIR) data reveal the presence, in the alumina, of unoxidized tartarate ions, as well as products of their partial (radical organic products and CO) and complete (CO2) oxidation. Carboxylate ions and elemental carbon contained in the anodic oxide impart a gray color to the films.

Photonic stop bands in quasi-random nanoporous anodic alumina structures

The existence of photonic stop bands in the self-assembled arrangement of pores in porous anodic alumina structures is investigated by means of rigorous 2D finite-difference time-domain calculations. Self-assembled porous anodic alumina shows a random distribution of domains, each of them with a very definite triangular pattern, constituting a quasi-random structure. The observed stop bands are similar to those of photonic quasicrystals or random structures. As the pores of nanoporous anodic alumina can be infiltrated with noble metals, nonlinear or active media, it makes this material very attractive and cost-effective for applications including inhibition of spontaneous emission, random lasing, LEDs and biosensors.

Tailoring of the Internal Pore Structure of Anodic Aluminum Oxide (AAO) by Pulsed Anodization of Aluminum

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Effect of anodizing voltage on the sorption of water molecules on porous alumina

The amount of water adsorbed on different centers on the surface of oxalic acid alumina films is a function of the anodizing voltage. It is decreased with increasing the anodizing voltage from 20 up to 50V, came up to maximum value at 20–30V and slightly increased at voltages above 50V. Water adsorption by oxide films formed at voltages below 50V can be due to the negative surface charge that is present on the alumina surface. The negative surface charge disappears in the films formed at voltages higher than 50V, and thus, the water is adsorbed on aluminum ions in a tetrahedral and octahedral environment. The correlation between anodizing conditions of aluminum in oxalic acid and the structure and composition of anodic alumina was established by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), thermogravimetric and differential thermal analyses (TG/DTA).

Entrapment of Protein in Nanotubes Formed by a Nanochannel and Ion‐Channel Hybrid Structure of Anodic Alumina

The nanochannel (in a porous layer) and ion-channel (in a barrier layer) hybrid structure of anodic alumina is used as a protein-trapping device. The transmembrane potential drives the electromigration of the charged proteins (FITC-labeled) into the nanochannels, but electromigration across the barrier layer is impossible due to the size-exclusion effect. As a result, the proteins can be continuously trapped in the nanochannels.

Nano-AAO-Based Microsensor for Monitoring Process Carbon Monoxide

This study developed a porous nano anodic aluminum oxide (AAO) based gas microsensor for detecting process carbon monoxide (CO) at room temperature. The small sensing microdevice has stable response with high sensitivity which includes porous AAO structures, gas sensing membrane, interdigitated sensing electrode, heater and temperature sensor. The tungsten oxide (WO3) sensing membrane covers AAO to enlarge total gas sensing surface area to enhance gas sensing response. The Pt interdigitated sensing electrode can also improve the sensitivity. The experimental results showed that the CO gas concentration sensitivity is proportional to temperature. As compared with the sensor without AAO, the concentration range of CO sensed by this microsensor is 100~1000 ppm, the gas sensing resistance change rate can be increased by 87.4 %.

Distributed Bragg reflector based on porous anodic alumina fabricated by pulse anodization

In this paper, we demonstrate a distributed Bragg reflector (DBR) based on nanoporous anodic aluminum oxide (AAO) formed by pulse anodization. The AAO structure with alternating mild anodized (MA) and hard anodized (HA) layers having different porosities and thereby different refractive indices was fabricated in 0.3 M H2SO4 using potential pulses of 25 and 35 V. The effective refractive index of the HA layers can be tailored by changing the porosity of the HA layers. The porosity of the HA layers can be significantly increased by selective chemical etching of HA segments in 0.52 M H3PO4. Before etching, the porous AAO structure was supported by a polymer nanorod frame. On the selected surface area pores were infiltrated with polymers (polystyrene and PMMA). The designed AAO structure consists of alternating high and low refractive index layers and behaves as a distributed Bragg mirror reflecting light in two different ranges of wavelength. This behavior is extremely important in optical communication lines where two separate spectral bands of high reflectivity in the infrared region are desired.

Size-Dependent Surface Properties of Low-Reflectivity Nanoporous Alumina Thin-Film on Glass Substrate

The size-dependent surface properties of low-reflectivity porous alumina thin film on glass substrate produced by two-step self-organized anodization of aluminum are observed. By electro-polishing the aluminum coating, a minimum roughened surface with an Rrms of 3.91 nm is obtained. After adjusting the applied potential, an optimum regular structure of anodic aluminum oxide (AAO) is obtained, corresponding to the pore diameter and density of 23 nm and 8.67×1010 cm−2. In particular, nanoporous AAO with an air/solid ratio of 58% exhibits a maximum water contact angle of 73.4° corresponding to surface energy of 40.1×10−5 N cm−1. Furthermore, the TM-mode reflection analysis shows a diminishing Brewster angle shifted from 60°-54° with an increasing air/solid ratio from 37%-58% at 532 nm. The greatly reduced small-angle reflectance and surface energy reveals a nonlinear trend with an enlarging pore size and air/solid ratio, leading to a minimum surface reflectance and maximum water contact angle at the nanoporous AAO prepared with 60V.

Facile fabrication of nanofluidic diode membranes using anodic aluminium oxide

Active control of ion transport plays important roles in chemical and biological analytical processes. Nanofluidic systems hold the promise for such control through electrostatic interaction between ions and channel surfaces. Most existing experiments rely on planar geometry where the nanochannels are generally very long and shallow with large aspect ratios. Based on this configuration the concepts of nanofluidic gating and rectification have been successfully demonstrated. However, device minimization and throughput scaling remain significant challenges. We report here an innovative and facile realization of hetero-structured Al(2)O(3)/SiO(2) (Si) nanopore array membranes by using pattern transfer of self-organized nanopore structures of anodic aluminum oxide (AAO). Thanks to the opposite surface charge states of Al(2)O(3) (positive) and SiO(2) (negative), the membrane exhibits clear rectification of ion current in electrolyte solutions with very low aspect ratios compared to previous approaches. Our hetero-structured nanopore arrays provide a valuable platform for high throughput applications such as molecular separation, chemical processors and energy conversion.

Influence of Reducer on the Nanopore Structure of Porous Anodic Alumina

In recent years, attention has been focused on adjustment and control of the nanostructures of porous anodic alumina (PAA) and porous anodic TiO2 nanotubes (PATNT). Because the formation mechanism of PAA and PATNT is still unclear, it is difficult to adjust the nanostructures of PAA and PATNT. To validate the novel viewpoint of the nanopore resulting from an oxygen bubble mold, an innovative chemical approach was used to adjust the PAA nanostructures. One successful approach is to use a reducer to absorb the oxygen bubbles in the nanopores. A novel anodic alumina film was obtained in a mixed solution of the reducer and oxalic acid. The influence of the reducer on the PAA nanostructures which formed in H3PO4 solution was investigated in detail. The experimental results showed that the regularity and the diameters of the nanopores in the PAA decreased as the reducer content increased. The differences in the voltage-time curves between electrolytes with and without the reducer were analyzed quantitatively. The results showed that the conductivity of the anodic oxide film that formed in the electrolyte with the reducer was better than that in the electrolyte without the reducer. When aluminum anodizes in a sealed case, oxygen bubbles are easily absorbed by the reducer, the oxygen bubble mold effect disappears, and a compact alumina film is obtained. Overall, these results clearly demonstrate that nanopores result from the oxygen bubble mold effect.

The use of PEEK nanorod arrays for the fabrication of nanoporous surfaces under high temperature: SiNx example

Large area silicon nitride (SiN(x)) nanoporous surfaces are fabricated using poly(ether-ether-ketone) (PEEK) nanorod arrays as a template. The procedure involves manipulation of nanoporous anodic aluminum oxide (AAO) templates in order to form an ordered array of PEEK nanopillars with high temperature resistant characteristics. In this context, self-ordered AAO templates are infiltrated with PEEK melts via the "precursor film" method. Once the melts have been crystallized in the porous structure of AAO, the basis alumina layer is removed, yielding an ordered array of PEEK nanopillars. The resulting structure is a high temperature and chemical resistant polymeric nanomold, which can be utilized in the synthesis of nanoporous materials under aggressive conditions. Such conditions are high temperatures (up to 320 °C), vacuum, or extreme pH. For example, SiN(x) nanopore arrays have been grown by plasma enhanced chemical vapor deposition at 300 °C, which can be of interest as mold for nanoimprint lithography, due to its hardness and low surface energy. The SiN(x) nanopore array portrays the same characteristics as the original AAO template: 120 nm diameter pores and an interpore distance of 430 nm. Furthermore, the aspect ratio of the SiN(x) nanopores can be tuned by selecting an AAO template with appropriate conditions. The use of PEEK as a nanotemplate extends the applicability of polymeric nanopatterns into a temperature regime up to now not accessible and opens up the simple fabrication of novel nanoporous inorganic surfaces.

Effects of pore-channel ordering on the mechanical properties of anodic aluminum oxide nano-honeycombs

The mechanical properties of anodic aluminum oxide (AAO) nano-honeycombs with different spatial ordering of pore-channels are investigated by nanoindentation. The pore-channel ordering is systematically varied by carefully adjusting the orientation of the aluminum used for anodization. The results indicate that the strength of AAO structures increases significantly with the regularity of their pore-channel arrangement, whereas the elastic modulus is less sensitive to pore-channel regularity.

Fabrication of enhanced anodic aluminum oxide performance at room temperatures using hybrid pulse anodization with effective cooling

Conventional anodic aluminum oxide (AAO) template was performed using potentiostatic method of direct-current anodization (DCA) on costly high-purity (99.997%) aluminum foils at low temperatures of 0–10°C to avoid dissolution effects which occurred frequently at room temperatures (RT) of 20–30°C. In this paper, we show the hybrid pulse anodization (HPA) method with pulsing normal-positive and small-negative potential differences at RT for enhancing performance of AAO structure for both the cheap low-purity (99%) and costly high-purity (99.997%) aluminum foils. The HPA mainly takes advantages of effective cooling that arise from the nearly zero cathodic current and high-thermal-conductivity liquid electrolyte on the foils. The HPA is different from the traditional pulse anodization with alternating both high and low positive potential differences (/currents) or both one-positive and one-zero potential differences. The HPA not only merits manufacturing convenience and cost reduction but also promotes pore distribution uniformity of AAO at severe conditions of cheap low-purity Al foils and relatively high room temperature. The pore distribution uniformity can be improved by HPA in a suitable duration compared with the DCA. Very good AAO distribution uniformity (91%) was achieved in high-purity aluminum foil by HPA because it can suppress the Joule's heat to diminish the dissolution reaction. The evolution of AAO distribution uniformity for both the HPA and DCA on Al foil purities and process durations were comparatively investigated.

Nanoporous Aluminum Oxide Thin Films on Si Substrate: Structural Changes as a Function of Interfacial Stress

This paper reports the effect of Al2O3−SiO2 interfacial stress on porous anodic alumina (PAA) structures formed by anodizing 150 nm thick aluminum thin films (ATF) deposited on silicon (Si) and glass substrates. The increase in the interfacial stress was achieved by growing thicker SiO2 layers by anodizing the substrate after complete ATF oxidation. Thicker SiO2 layers induced structural changes on the PAA. The nanopore diameter and interpore distance increased from 50 to 80 nm and 90 to 150 nm, respectively, and the nanopore density decreased from 60 to 43 nanopores μm−2. The modifications of the PAA structure are related to the expansion of the SiO2 layer, which exerts a bottom-up pressure on the PAA film to minimize the stress generated on the Al2O3−SiO2 interface.

A Facile Approach to Fabricate Alumina Fibers with Different Nano-Structures

In dilute phosphoric acid aqueous solutions (6 wt.%), alumina nano-tubes, nano-belts and nano-wires were fabricated from porous anodic alumina oxide (AAO) under different etching times of 20 min, 35 min and 45 min. These materials were characterized by techniques of scanning electron microscope (SEM) and transmission electron microscope (TEM) and high-resolution transmission electron microscopy (HRTEM), respectively. The morphological evolution is time-dependent, which is explained by the special hexagonal structure of porous anodic alumina and the composition of an individual cell wall. Besides the etching effect of phosphoric acid on the structure, a loosening effect also occurs along the boundaries of the colloid bundles. Because of the incorporation of acid anions during anodization, the composition of the individual cell wall is affected and three different layers formed. Therefore, the etching rates of different layers on the cell wall are influenced, which leads to the formation of various nano-structures.

Highly ordered porous alumina with tailor-made pore structures fabricated by pulseanodization

A new anodization method for the preparation of nanoporous anodic aluminum oxide(AAO) with pattern-addressed pore structure was developed. The approach is based onpulse anodization of aluminum employing a series of potential waves that consist of two ormore different pulses with designated periods and amplitudes, and provides uniquetailoring capability of the internal pore structure of anodic alumina. Pores of the resultingAAOs exhibit a high degree of directional coherency along the pore axes withoutbranching, and thus are suitable for fabricating novel nanowires or nanotubes, whosediameter modulation patterns are predefined by the internal pore geometry of AAO. Itis found from microscopic analysis on pulse anodized AAOs that the effectiveelectric field strength at the pore base is a key controlling parameter, governing notonly the size of pores, but also the detailed geometry of the barrier oxide layer.

Morphological modification of the surface of polymers by the replication of the structure of anodic aluminum oxide

Anodic aluminum oxide films have been used as stamps to modify polymer materials in order to create superhydrophobic surfaces. Using polyethylene terephthalate, it has been shown that a high replication degree leads to a strong increase in the water wetting contact angle (up to 174°). However, the inverse situation is observed for the case of low replication degrees: hydrophilization of the surface occurs, which can be explained by a change of the Cassie-Baxter wetting mechanism to the Wenzel mechanism.