Templating and Pattern Transfer Using Anodized Nanoporous Alumina/Titania
This chapter provides an overview of the non-lithographic nanofabrication process known as “hard templating”. Nanoporous alumina fabricated by electrochemical anodization continues to be the most widely used hard template although anodically formed nanotu
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Karthik Shankar
Abstract
This chapter provides an overview of the non-lithographic nanofabrication process known as “hard templating”. Nanoporous alumina fabricated by electrochemical anodization continues to be the most widely used hard template although anodically formed nanotubular titania is increasing in importance for templating applications. Hard templates sustain almost no deformation to minor mechanical loads and are unchanged under the action of organic solvents and neutral salt solutions. Their mechanical robustness and relative chemical inertness allows hard templates to be compatible with a variety of chemical, electrochemical and mechanical processes typically used in nanofabrication, several of which are covered in other chapters of this book. Hard templates typically consist of a self-organized array of nanochannels of similar or identical size oriented orthogonally to a substrate. The last 15 years have seen immense progress in the construction of thinner, more versatile hard templates of greater pattern order prepared on ever more diverse substrates. Hard templating has been at the forefront of nanotechnology research as a method to direct the creation of a wide variety of metallic, semiconducting and organic nanostructures. This chapter is organized as follows: The processes used to form hard templates and to improve their pattern order are presented in Sect. 13.2, the use of the template nanochannels to grow ordered functional one-dimensional nanomaterials in Sects. 13.3 and 13.4, and the use of hard templates to affect nanoscale pattern transfer is presented in Sect. 13.5.
K. Shankar (*) Department of Electrical & Computer Engineering, University of Alberta, Edmonton, AB, Canada e-mail: [email protected] M. Stepanova and S. Dew (eds.), Nanofabrication, DOI 10.1007/978-3-7091-0424-8_13, # Springer-Verlag/Wien 2012
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13.1
K. Shankar
Introduction
Self-organized nanopore arrays of valve metal oxides can be formed nonlithographically by the electrochemical anodization of valve metals such as Al, Ti, Ta, Hf, Zr, etc. The anodization process is simple and economical and the resulting structures are mechanically robust and chemically resistant even at elevated temperatures. Therefore, anodically formed nanoporous valve metal oxides are excellent architectures for templating and pattern transfer and a wide variety of functional nanostructures have been formed using nanoporous alumina and titania. The anodic formation of porous alumina has been known since 1956 [1, 2] but has been extended to the other valve metals only in the last decade [3–7]. We shall restrict our discussion to anodic aluminum oxide (AAO) and nanotubular TiO2. In AAO, the thickness of the nanoporous film, the size of the nanopores and their spacing are the morphological parameters of interest and these can be controlled by tuning the anodization potential, the duration of the anodization and by choosing the appropriate electrolyte to perform the anodization. In TiO2 nanotube (TNT) arrays, the tubular architectur
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