AC Electrodeposition of Uniform High Aspect-Ratio Metal Nanowires in Porous Aluminum Oxide Templates
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AC Electrodeposition of Uniform High Aspect-Ratio Metal Nanowires in Porous Aluminum Oxide Templates Nathan J. Gerein, Shazma S. Mithani, and Joel A. Haber Department of Chemistry, University of Alberta Edmonton, AB, T6G 2G2, Canada ABSTRACT The use of alternating current (ac) electrodeposition permits deposition through the resistive Al2O3 barrier layer, enabling the use of the as-grown porous aluminum oxide (PAO) template. This results in a process that is cost effective, simple, and scalable. However, achieving uniform filling using this technique is challenging. We have carried out a systematic study of the effect of multiple variables on the ac electrodeposition of copper nanowires using a fractional factorial design of experiment (FFDOE). This experiment led to the identification of template damage that occurs when continuous wave ac deposition conditions are employed, as well as to effective pulsed wave ac electrodeposition conditions. Subsequent examination of the effect of wave shape has identified the impact of this variable on electrodeposition, producing a further optimized set of ac electrodeposition conditions. The utility of these electrodeposition conditions has also been extended to the deposition of iron nanowires with similar results. INTRODUCTION Template-directed syntheses utilizing PAO templates have become a popular pathway to diverse nano-structured materials [1]. PAO templates are easily fabricated and offer the benefit of high pore densities in an hexagonal close packed array and the ability to tailor pore diameters and depths [2]. Without removal of the barrier layer or implementation of complex processing strategies, electrodeposition into PAO templates is only possible under ac conditions, as a result of the aluminum oxide barrier layer at the base of the pores formed during the anodization process. This barrier layer blocks direct current with a high resistance; however, under alternating current the anodic aluminum oxide is found to conduct preferentially in the cathodic direction [3]. This inherent rectifying property of the barrier layer allows the reduction of ions in the pores during cathodic half-cycles, without re-oxidation during the anodic half-cycles [3]. In general, ac electrodeposition through the barrier layer is a complicated process, as evidenced by the reported variation in the quality of pore filling (the percentage of pores with material deposited and the variation in the amount of material deposited in each pore) as a function of deposition conditions, including: electrolyte concentration, composition, and temperature [4]; and upon deposition voltage, frequency, and wave form (sine, square, and triangle) [4, 5]. Moreover, the optimal deposition conditions appear to depend upon the metal or compound deposited [4]. In most cases the quality of pore filling, in terms of length uniformity of the resultant wires and the height to which the pores were filled, is not reported. In cases in which complete pore filling has been reported, pore depth has ranged from 600
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