Self-organized anodized alumina membranes

Nanostructures have attracted an enormous interest in the last decades because of the different and unusual properties that may arise when at least one dimension is reduced in the nanometer range. For the mass production of functional materials large-scale templates chemically inert are needed. In a pioneering work [1] Masuda etal showed that the anodization of aluminum under suitable acid and voltage conditions, and using a two-step anodization procedure, leads to the formation of ordered arrays of pores in an alumina matrix. Since the starting material is often a sheet of aluminum one speaks of membranes, hence the name Anodized Alumina Membranes (AAM). Since that time the fabrication of pores of diameters ranging from a few tens of nanometers to above one micron have been demonstrated. Such templates have been extensively used to grow metallic, semiconducting and oxides nanowires having potential applications in electronic, catalysis and biology.

 
We have implemented this technique and are now able to tune pore diameter and period based on the electrolyte and voltage. We developed electrochemistry procedures to fill the template with wires and tubes of functional metals, alloys, oxides and semiconductors.

The principle of the self-organization anodization process is the following. During the first anodization step the pores are created. They are initially rather disordered, and self-organize themselves progressively in depth in hexagonal arrays, as a consequence of strain in the matrix. After dissolution of the alumina matrix the ordered roughness at the bottom of the previous pores serves in the second anodization step to initiate pores parallel and ordered from the very surface, ready to be filled by functional materials in the form of wires and tubes.
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FIG 1 : Schematics of the two-steps anodization procedure.
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FIG 2 : Illustration of the control over the pore diameter in our arrays of pores, depending on the anodizing procedure (SEM plane-view for the first line - Cross-section below).

Corresponding author : L. Cagnon

[1] H. Masuda, K. Fukuda, Science 268 (1995) 1466

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