Fabrication of nano-scale porous template

In recent years, there has been increasing interest in the fabrication of nanometer-sized fine structures because of their potential utilization in electronic, optical, and micromechanical devices. Although, Several techniques have been proposed to synthesis high-density and regular nano-pore arrays, such as e-beam and x-ray lithography, proton beam writing (PBW) and AAO [59-65]. The AAO is one approach to the fabrication of nanometer-sized structures, and the AAO has been considered as a naturally occurring structure as a host for the fabrication [64-71].

This approach is promising, especially for the preparation of large-area, nanometer-sized structures with high aspect ratios, which are difficult to form by a conventional lithographic process. Over all techniques, AAO is the better choice by most material researchers, because of its simple and rapid fabrication and cost effectiveness. The technique of porous formed in electrolytes under anodic bias has been studied and reported in 1953. [72] In 1970s, O’Sullivan and Wood [73]

presented a model based on the electric field distribution to explain why pores grow at

all and why their size distribution is quite narrow. The further refined models [74-76]

can give microscopic explanations for the dependence of, e.g., pore diameters and pore distances on applied voltage or electrolyte composition. In recent decade, the attractive issue about low-dimensional science is giving rise to a lot of research on growing porous anodic aluminum oxide (AAO) [77-85]. Anodic porous alumina, which is prepared by the anodic oxidation of aluminum in an acidic electrolyte, is one of the typical self-organized fine structures with a nanopore array [86, 87]. Anodic porous alumina has a packed array of columnar hexagonal cells with central, cylindrical, uniformly sized holes ranging from 7 to 400 nm in diameter. Such nanopore arrays of alumina (Al2O3) are known to exhibit hexagonally ordered pores and considerable structural strength on the nanoscale. Many types of nano-composites have been fabricated with anodic porous alumina used as a host material; when used for the preparation of magnetic recording media [88, 89], optical devices [90], functional electrodes [91, 92], and electrochromic [93] and electroluminescence display devices [94], the holes in these materials are filled with metals or semiconductors.

Self-organized formation of hexagonal pore arrays in anodic alumina provides a conventional tool to fabricate the low-dimension materials. Porous oxide growth on aluminum under anodic bias in various electrolytes has been studied for several decades [86]. But highly regular polycrystalline pore structures occur only for a quite small processing window. Recently reported self-organized pore growth, leading to a densely packed hexagonal pore structure for certain sets of parameters.

Especially, within an oxalic acid, the pore size is a linear relationship to anodic voltage [95]. Figure 4.1 shows this published result revealing this dependence.

According to these parameters and follow the process, it is easy to select the parameter for getting suitable size of pores.

Figure 4.1 Relationship between pore diameter and growth rate of anodic alumina membrane and anodic voltage.

Experiment

The chemical reactions of aluminum oxidation are [96]

Anodic reaction: 2 Al → 2 Al³⁺ + 6 e

-Oxide-electrolyte interface: 2 Al³⁺ + 3H₂O → Al₂O₃ + 6 H⁺ Cathodic reaction: 6 H⁺ + 6 e^- → 3H₂

Overall reaction: 2 Al + 3 H₂O → Al₂O₃ + 3 H₂

This reaction occurs spontaneously until the compact barrier layer is formed.

Chemical reactions in aqueous are always complex, so that the diameter of pores in

AAO template well depend on temperature, electrolyte composition, and electrical potential. In order to obtain the high regular, uniform pore diameter, and thick alumina foil, a low temperature experimental set-up is designed to provide all components at low temperature. Figure 4.2 shows the experimental set-up.

Temperature of the electrolyte and components are maintained at low temperature around 0 °C using a commercial refrigerator. A resistance temperature detector (RTD) was used for the temperature sensing, and a 400 mL beaker was used to contain the electrolyte.

Temperature controller and Stirrer

Re fr ig era to r

Aluminum foil Sourcemeter

Figure 4.2 The setup scheme for the fabrication of nano-porous template. sThe materials of anode and cathode electrode are copper and platinum, respectively.

This whole set were placing on a temperature and stirrer controller in a refrigerator.

A pure Aluminum foil (purity ~ 99.9999%) is degreased by ultrasonic cleaner with acetone and ethanol in sequence. The pure and cleaned aluminum foil was mounted on a copper plate which was served as the anode electrode, and subjected to electro-polishing in a H3PO4: H2SO4: H2O solution with weight ratio 4:4:2 for obtaining smooth surface. The applied potential is about 20 Volt to flatten the surface of foil. Then, a two-step anodic oxidization process is performed to fabricate AAO template. The flattened foil is anodized in an acid aqueous at a suggested applied voltage about 0 ~ 3 °C. Pretreated foil then goes through the second anodizing with the same condition of first step. To this step, sample formed three layers, they are pore-layer, barrier-layer, and aluminum-layer. The barrier-layer is a thin alumina layer, which is covering and stoking nanochannels to stop the passing of electrolyte. For the goal to remove the barrier layer, CuCl2 and 2 wt% NaOH are used to etch the aluminum and barrier layer in sequence. Figure 4.3 shows some SEM images of AAO templates by the above condition. This detail processes will show in Figure 4.4.

Figure 4.3 Three top-views and one side-view of anodic alumina membrane with pore size ~60, 20, and 10 nm.

Figure 4.4 The scheme of a whole procedure to deposit nanowires in AAO template.

Aluminum

Aqueous electrolyte

Aluminum

Electropolish

Anodize

Soaking of CuCl2, NaOH

Depositing of electrode

Electro-deposition