Anodic Alumina Thin Films Research Articles

Optical properties and color generation mechanism of porous anodic alumina films

Porous anodic alumina thin films with iridescent colors were fabricated electrochemically in phosphoric acid electrolyte. Compared to the color saturation of alumina films fabricated in oxalic acid electrolyte, the saturation obtained using phosphoric acid was enhanced dramatically. The mechanisms behind this observation are discussed, and the microstructure and optical characteristics of the films are characterized. Multicolor patterns were obtained by an organics-assisted process for which details are given.

A colorimetric sensor based on anodized aluminum oxide (AAO) substrate for the detection of nitroaromatics

Simple and low cost colorimetric sensors for explosives detection were explored and developed. Anodized aluminum oxide (AAO) with large surface area through its porous structure and light background color was utilized as the substrate for colorimetric sensors. Fabricated thin AAO films with thickness less than ∼500 nm allowed us to observe interference colors which were used as the background color for colorimetric detection. AAO thin films with various thickness and pore-to-pore distance were prepared through anodizing aluminum foils at different voltages and times in dilute sulfuric acid. Various interference colors were observed on these samples due to their difference in structures. Accordingly, suitable anodization conditions that produce AAO samples with desired light background colors for optical applications were obtained. Thin film interference model was applied to analyze the UV–vis reflectance spectra and to estimate the thickness of the AAO membranes. We found that the thickness of produced AAO films increased linearly with anodization time in sulfuric acid. In addition, the growth rate was higher for AAO anodized using higher voltages. The thin film interference formulism was further validated with a well established layer by layer deposition technique. Coating poly(styrene sulfonate) sodium salt (PSS) and poly(allylamine hydrochloride) (PAH) layer by layer on AAO thin film consistently shifted its surface color toward red due to the increase in thickness. The red shift of UV–vis reflectance was correlated quantitatively to the number of layers been assembled. This sensitive red shift due to molecular attachment (increase in thickness) on AAO substrate was applied toward nitroaromatics detection. Aminopropyltrimethoxysilane (APTS) which can be attached onto AAO nanowells covalently through silanization and attract TNT molecules was coated and applied for TNT detection. UV–vis spectra of AAO with APTS shifted to the longer wavelength side due to TNT attachment. This red shift implied AAO thickness increased and positive detection of TNT molecules. It was also observed that both APTS and polyethyleneimine (PEI) were electron rich polymers which formed Meisenheimer complexes with TNT in solution and changed its color abruptly. This strong color change due to chemical reaction was applied as another approach for direct TNT detection. Commercial AAO films with long pores (60 μm) and white background color were coated with APTS or PEI and then exposed to TNT in solution. These membranes turned to pink rapidly and eventually became visibly orange after a few hours with a strong absorption around 500 nm that was consistent with the formation of Meisenheimer complexes. The visible color change can be observed by unaided eyes and is suitable for nitroaromatics detection at higher concentration while interference color red shift in AAO thin film is designed for nitroaromatics detection at monolayer (nm) level.

Vertical Alignment of Liquid Crystal on ITO Glass with Anodic Aluminum Oxide Thin Film

Anodic Aluminum Oxide (AAO) thin films have been demonstrated as a vertical alignment layer for nematic liquid crystal in recent years. To control the liquid crystal orientation by applying the electric field, the AAO alignment layers need to be fabricated on the transparent electrode. In this work, the liquid crystal cell with the AAO alignment layer on Indium Tin Oxide (ITO) transparent electrode has been demonstrated. The performance of the cell has been studied. The transmittance of AAO-ITO substrate is around 55%. The polar anchoring strength of the AAO-ITO cell with good vertical alignment is 1.32 · 10−4 J/m2, and the response time is around 90 ms.

Alignment Properties of Liquid Crystal on Etched Anodic Aluminum Oxide Film

Anodic aluminum oxide (AAO) is a widely studied material with self-assembled nanochannel which is perpendicular to the substrate with nanometer-scale diameters and high aspect ratios. It is used for protecting the aluminum surface or as dielectric material. In this work, the alignment properties of liquid crystal on the etched AAO thin films were studied. By controlling the etching time, not only the pore sizes but also the thickness of the etched AAO thin films can be modified. Both homeotropic and homogenous alignment properties have been demonstrated on the etched AAO thin films with controlling etching time.

Synthesis and Optical Properties of Iridescent Porous Anodic Alumina Thin Films

Highly ordered porous anodic alumina thin films with tunable structural colors covering the entire visible region have been fabricated using chemical etching. The relationship of the structural color to the anodization time and etching time in a phosphoric acid solution, as well as the angle of incidence of illuminating light are discussed. Multicolor patterns were obtained by an organics-assisted process. A theoretical study of the changes in structural color due to various factors is consistent with the experimental results. These films show promise in many areas including decoration, display and anti-counterfeiting applications.

Optical properties of porous anodic aluminum oxide thin films on quartz substrates

Porous anodic aluminum oxide (AAO) thin films on quartz substrates were fabricated via evaporation of a 100-nm thick Al, followed by anodization with different durations and pore widening and Al removal by chemical etching. The transmittance and reflectance of AAO films on quartz substrates were measured by optical spectrophotometry. The microstructure and morphology were examined by scanning electron microscopy. The pore diameter of AAO films after pore widening and Al removal is 60 ± 4 nm and the interpore distance is 88 ± 5 nm. It is found that the reflectance decreases and the transmittance increases with the increase of the anodization time and pore widening. Compared to a bare substrate, the transmittance of AAO films after pore widening and Al removal is about 3.0% higher, while the reflectance is about 3.0% lower over a wide wavelength range. Additionally, after pore widening and Al removal, when AAO films are prepared on both sides of the quartz substrate, the highest transmittance is about 99.0% in the wavelength range 570–680 nm. The optical constants and thickness of AAO films after pore widening and Al removal were retrieved from normal incidence transmittance data. Results show that the refractive index is lower than 1.25 in the visible optical region and that the porosity is about 0.70.

Mounted Nanoporous Anodic Alumina Thin Films as Planar Optical Waveguides

Solid-supported thin films of self-organized nanoporous anodic aluminum oxide (AAO) have been widely employed for the template preparation of nanostructured functional materials. Recently, the use of nanoporous AAO thin films in optical waveguide spectroscopy (OWS) has been explored for high sensitivity, in situ monitoring of processes occurring within these nanoporous templates. In this contribution, we demonstrate a strategy for mounting bulk anodized AAO thin films on heterogeneous solid-supports suitable for waveguide sensing experiments. Unlike conventional preparations of AAO thin films by anodization of vacuum- or electrochemically deposited Al thin films, the full range of techniques available to anodize bulk Al may potentially be applied with the present method. Moreover, we show that AAO thin films mounted on glass substrates can have superior waveguide coupling properties compared to conventionally prepared samples. The nanostructure of the AAO can be well characterized by an EMT-OWS analysis, demonstrated by comparing scanning electron microscopy images of the AAO and the pore dimensions calculated from an effective medium theory (EMT) analysis of the film refractive index measured by OWS. Finally, using a curved metallic substrate as an example, we show that our mounting technique can be used as a general strategy to functionalize objects with nanoporous AAO films.

Linear Coefficient of Thermal Expansion of Porous Anodic Alumina Thin Films from Atomic Force Microscopy

In this article, a precise and convenient technique based on the atomic force microscope (AFM) is developed to measure the linear coefficient of thermal expansion of a porous anodic alumina thin film. A stage was used to heat the sample from room temperature up to 450 K. Thermal effects on AFM probes and different operation modes at elevated temperatures were also studied, and a silicon AFM probe in the tapping mode was chosen for the subsequent measurements due to its temperature insensitivity. The topography of the porous anodic alumina sample was obtained, and the pore sizes and the surface roughness were analyzed. The thermal expansion of the sample was measured within the temperature range of 293 to 443 K. The results show that the linear coefficient of thermal expansion of the porous anodic alumina thin film is approximately two times larger than that of bulk alumina.

The selective fabrication of large-area highly orderedTiO2 nanorod and nanotube arrays on conductive transparent substrates via sol–gelelectrophoresis

Large-area free-standing arrays ofTiO2 nanorods and nanotubes were selectively synthesized on transparent conductingindium tin oxide substrates using sol–gel electrophoresis and anodic alumina(AAO) thin film templates. The effect of sol–gel ageing on the growth ofTiO2 was explained, providing a tailored ability to produce nanotubes and nanorods. An annulartungsten base electrode, stemming from the anodization of the AAO template, was foundto be crucial to the growth of nanotubes. This was supported by a study of substrateannealing in a reducing atmosphere. The work can be readily adapted for the fabrication offree-standing arrays of other metal, metal oxide, and complex oxide nanorod and nanotubearrays on conducting substrates.

Highly ordered hexagonally arranged sub‐200 nm diameter vertical cylindrical pores on p‐type Si using non‐lithographic pre‐patterning of the Si substrate

AbstractAnodically etched two‐dimensional (2‐D) arrays of highly ordered sub‐200 nm in diameter vertical cylindrical pores were fabricated on p‐type Si wafers, with a resistivity of 6–8 Ω cm, by non‐lithographic pre‐patterning of the silicon substrate through a self‐assembled porous anodic alumina (PAA) thin film, directly grown on the Si wafer. The PAA film was grown by electrochemical oxidation of a thin Al film in an oxalic acid aqueous solution electrolyte. Through the PAA pores, concave etch pits were formed on Si by chemical etching, that were then used as pore initiation sites for electrochemical macroporous silicon formation. The so formed vertical cylindrical pore arrays showed perfect hexagonal arrangement on the Si surface. A pore diameter down to 180 nm and a pore height up to ∼1 μm were achieved for the first time on p‐type Si. The developed technology is particularly interesting for photonic crystals and sensors applications. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of porous anodic alumina templates in selected micrometer-sized areas on a Si substrate. Application for growing ordered Ti nanopillars

The local formation of porous anodic alumina (PAA) thin films on confined areas measuring few μm2 through an SiO2 masking layer on the silicon substrate has been developed. The locally grown porous anodic alumina thin films showed highly symmetric vertical cylindrical pores with a pore density that was much higher than that of films on non-confined areas. Pore density as high as 6 × 1010 pores/cm2 was achieved, compared to ∼1010 pores/cm2 in corresponding large-area samples. Pore diameter was as small as 30–50 nm. Alumina thin films on confined areas on a Si substrate are very interesting for use as masking layers for local Si nanopatterning or as templates for local growth of different nanostructures, e.g. nanowires of different materials, on wafer level. In this work, we used the PAA films as sacrificial templates to grow locally on a silicon wafer using sputtering, regular arrays of Ti nanopillars on pre-selected areas on the Si wafer. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence in the blue spectral region from fluorene molecules embedded in porous anodic alumina thin films on silicon

The photoluminescence (PL) in the blue spectral region from fluorene molecules embedded in the pores of anodic alumina thin films on silicon was investigated in detail. It was found that the PL was strongly dependent on the diameter and depth of the pores, as well as on the solvent used to dissolve the fluorene molecules. A photoluminescence blue shift with a maximum value of 30 nm was observed when the fluorenes were embedded into 40 nm pores of the alumina film compared to the corresponding spectrum in solution. The results have shown that the penetration of the molecules into the pores, as well as the formation of nanosize aggregates are favored when the molecules are dissolved in an aromatic solvent.

Resistance States and Photoluminescence in Anodic Oxide Alumina Films

Reproducible resistance switching behaviors induced by the external voltage were investigated in anodic oxide alumina (AOA) thin films. Electric-transport measurements showed that a low resistance state in the Ag/AOA/A1 structures might be caused by the formation of the conductive filaments. Switching to a high resistance state occurred due to the rupture of these filaments. Change in the photoluminescence properties of the AOA films depending on the resistance states was observed.

Self-assembled hexagonal ordering of Si nanocrystals embedded inSiO2 nanodots

Highly dense hexagonally ordered two-dimensional arrays of Si nanocrystals embedded inSiO2 nanodots were fabricated on a silicon substrate by using a self-assembled porous anodic aluminathin film as a masking layer through which electrochemical oxidation of the Si substrateand ultralow energy Si implantation took place. After removal of the alumina film and hightemperature annealing of the samples, hexagonally ordered Si nanocrystals embedded withinSiO2 nanodots were obtained, having sizes in the few tens of nanometer range. The fabricatedordered structures show significant potential for applications either in basic physicsexperiments or as building blocks for nanoelectronic and nanophotonic devices.

Highly ordered hexagonally arranged nanostructures on silicon through a self-assembledsilicon-integrated porous anodic alumina masking layer

A combined process of electrochemical formation of self-assembled porous anodic aluminathin films on a Si substrate and Si etching through the pores was used to fabricate ideallyordered nanostructures on the silicon surface with a long-range, two-dimensionalarrangement in a hexagonal close-packed lattice. Pore arrangement in the alumina film wasachieved without any pre-patterning of the film surface before anodization. Perfect patterntransfer was achieved by an initial dry etching step, followed by wet or electrochemicaletching of Si at the pore bottoms. Anisotropic wet etching using tetramethyl ammoniumhydroxide (TMAH) solution resulted in pits in the form of inverted pyramids, whileelectrochemical etching using a hydrofluoric acid (HF) solution resulted in concave nanopitsin the form of semi-spheres. Nanopatterns with lateral size in the range 12–200 nm, depth inthe range 50–300 nm and periodicity in the range 30–200 nm were achieved either onlarge Si areas or on pre-selected confined areas on the Si substrate. The poresize and periodicity were tuned by changing the electrolyte for porous anodicalumina formation and the alumina pore widening time. This parallel large-areananopatterning technique shows significant potential for use in Si technology anddevices.

Porous anodic alumina thin films on Si: interface characterization

AbstractPorous anodic alumina (PAA) thin films (thickness ∼ 50nm) were fabricated on Si by anodization of thin Al films under constant voltage of 20 V in sulphuric acid aqueous solution. The films exhibit cylindrical vertical pores of diameter ∼ 13–15 nm, arranged in hexagonal close packed structure. Electrochemical oxidation of the Si substrate through PAA, used as masking layer with openings in the pores, resulted in the formation of SiO2 dots at each pore tip. Two different kinds of films, namely with or without SiO2 dots at pore tips, were fabricated. In order to characterize the electrical quality of the interface of PAA thin films with Si, C‐V and G‐V measurements were performed on Metal‐Insulator‐Semiconductor (MIS) structures with Al metallization. The measurements were carried out in the voltage range +1.0 V to –3.0 V in steps of 0.05 V and in the frequency range 1 MHz to 100 Hz. The typical form of C‐V and G‐V curves of a MIS structure was obtained. In order to determine the interface trap density Dit, C‐f and G‐f measurements were performed as a function of the applied gate voltage in the depletion region. Dit was evaluated following the Conductance Method (E. H. Nicollian, and J. R. Brews, MOS Physics and Technology (J. Wiley & Sons, New York, 1982), p. 222 [1]). Both types of samples exhibit values of Dit in the order of 1011 eV–1cm–2. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Porous anodic alumina thin films on Si: interface characterization

AbstractThe cover picture of this issue of physica status solidi (c) has been taken from the article [1]. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Highly ordered diamond and hybrid triangle-diamond patterns in porous anodic alumina thin films

Utilizing nonequilibrium formation kinetics in porous anodic alumina (PAA) thin films, diamond and hybrid triangle-diamond pore patterns are achieved. During anodization, the self-compensation abilities of PAA allow diamond-shaped pores to form by omitting certain sites in the surface prepatterning process. The effects of tessellation on cell formation in these arrangements yield elongated, regular, and partially compressed hexagonal cell structures leading to diamond, circular, and triangular pores, respectively. The diamond-shaped porous templates provide a low-cost option for the preparation of scalable nanostructures with diamond-shaped cross sections with utility in a range of nanoscale applications, including enhanced sensing and field emission.

Modeling Ion Diffusion Current in Nanochannel Using Infinitesimal Distribution Resistor–Capacitor Circuits

A theoretical model that considers both the diffusion and electrical osmosis fluxes to estimate the diffusion current through a nanochannel is proposed. The equivalent impedance of a nanochannel is modeled as a series connection of finite resistor–capacitor (RC) circuits. The impedances of the electrical double layer and buffer solution in each infinitesimal RC circuit are represented using a capacitor and a resistor, respectively. The diffusion-induced current in the proposed model is position dependent due to the electrical double layer effect. Computer simulations and experiments using a nanoporous anodic aluminum oxide (AAO) thin film as the filter to separate electrolysis with an ion concentration gradient were conducted. Electrochemical impedance spectroscopy (EIS) was performed to measure the impedance of the porous AAO thin film in the frequency domain such that the capacitor and resistor can be estimated accordingly. A high degree of coincidence between the theoretical and experimental data is observed.

Nanostructuring Si surface and Si/SiO 2 interface using porous-alumina-on-Si template technology. Electrical characterization of Si/SiO 2 interface

In this work, anodic porous alumina thin films with pores in the nanometer range are grown on silicon by electrochemistry and are used as masking material for the nanopatterning of the silicon substrate. The pore diameter and density are controlled by the electrochemical process. Through the pores of the alumina film chemical oxidation of the silicon substrate is performed, leading to the formation of regular arrays of well-separated stoichiometric silicon dioxide nanodots on silicon, with a density following the alumina pores density and a diameter adjustable by adjusting the chemical oxidation time. The alumina film is dissolved chemically after the SiO 2 nanodots growth, revealing the arrays of silicon dioxide dots on silicon. In a next step, the nanodots are also removed, leaving a nanopatterned bare silicon surface with regular arrays of nanopits at the footprint of each nanodot. This silicon surface structuring finds interesting applications in nanoelectronics. One such application is in silicon nanocrystals memories, where the structuring of the oxidized silicon surface leads to the growth of discrete silicon nanocrystals of uniform size. In this work, we examine the electrical quality of the Si/SiO 2 interface of a nanostructured oxidized silicon surface fabricated as above and we find that it is appropriate for electronic applications (an interface trap density below 1–3×10 10 eV −1 cm −2 is obtained, indicative of the high quality of the thermal silicon oxide).