chapter  1
7 Pages

9 TEMPLATE METHODS

The growth of thin films displaying special features like aligned pores perpendicularly to the substrate surface and nanoporous structures has attracted the attention of many research groups. A lot of bottom-up techniques, particularly those in which, self-assembly processes play a relevant role in the growth mechanisms of that nanostructures have been reported. Among them, electrochemical techniques constitute one of the most used to fabricate highly ordered nanostructures for replicating other nanostructured materials and for growing functionalized material arrays. Such electrochemical nanofabrication is mainly based on ordered and nanoporous anodic aluminum oxide membranes (AAO), anodic titania membranes, colloidal polystyrene (PS) latex spheres, self-assembled monolayers (SAMs) and self-assembled block copolymers and even carbon nanotubes. Template methods offer very important strategies for complicated nanostructures. 1.9.1 Anodized Aluminum Oxide (AAO) MembranesA simple and completely nonlithographic preparation technique for free-standing nanostructured films with a close-packed hexagonal array of nanoembossments has been developed by porous anodic aluminum oxide (AAO) membrane templates with different pore diameters. They have been largely used to construct different free-standing inorganic and organic nanowires [125-127], nanotubes [128] and ordered arrays of nanoparticles [129] since invented. Aluminum anodization provides a simple and inexpensive way to obtain nanoporous templates with uniform and controllable pore diameters and periods over a wide range. The usual electrochemical method for producing the AAO film is the anodization of high-purity Al plates at constant voltage (e.g., anodized at 22 V from an Al foil and detached by the reverse-bias method) [130]. Membranes with several different pore sizes can be made, for example, in the following electrolytes: Aqueous solutions of H2SO4 at 10-20 V for

pores ~10-25 nm, H2C2O4 at 40-80 V for pores ~40-100 nm, and H3PO4 at 100-140 V for pores ~100-170 nm. The pore diameter is linearly related to the anodizing voltage (1.2 nm/V). A voltage reduction was done to the thin barrier layer that inhibits anodic current during electrodeposition [131]. Other attempts have been made to create nanoporous symmetries other than hexagonal packing [132]. Recently, a novel AAO membrane with a six-membered ring symmetry coexisting with the usual hexagonal structure has been fabricated by constant current anodization [133].The pore sizes of this structure can be tailored by changing the processing conditions. Ordered arrays of nanodots with novel structure have been fabricated by such AAO template. In the final stage, the porous alumina substrate can be removed by etching in KOH. Moreover, one of the interesting possibilities afforded by the anodization process is that the anodization can take place on arbitrary surfaces, such as curved surfaces. Unique features including cessation, bending, and branching of pore channels are observed when fabricating AAO templates on curved surfaces [134]. The new structures may open new opportunities in optical, electronic, and electrochemical applications.Many strategies have been ingeniously implemented to fabricate complicated nanostructures based on the AAO templates. For example, hexagonally ordered Ni nanocones have been fabricated using an a porous AAO template where the pores are of a cone shape [135]. The conical AAO film was found to exhibit hexagonal order with a period of 100 nm. The Ni nanocones and the surface morphology of the nanoconical film exhibit the same periodic structure of the template as shown in Fig. 1.11.