Pore-opening Process in Nanoporous AAO Membranes for Electrodeposition of Semiconducting Nanowires
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Pore-opening Process in Nanoporous AAO Membranes for Electrodeposition of Semiconducting Nanowires Sovannary Phok1, Suresh S. Rajaputra2, Vijay P. Singh2, Hamda A. Al-Thani1 and Falah Hasoon3. 1 National Energy and Water Research Center, Abu Dhabi Water and Electricity Authority, Po. Box 54111, Abu Dhabi, U.A.E. 2 Center for Nanoscale, Electrical and Computer Engineering, University of Kentucky, 453 F. Paul Anderson Tower, Lexington, KY 40506-0046, U.S.A. 3 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, U.S.A.
ABSTRACT The fabrication of nanoporous aluminum oxide (Al2O3) membranes for large scale production of nanowires is performed at room temperature by a two-step anodization of commercially available aluminum foil tapes. During the anodization process, an oxide barrier layer is formed at the interface with aluminum. In the present work, the removal of the barrier is performed by (i) ramping down the voltage with a rate in the range of 0.5 V per 60s to 2 V per 60s and (ii) immersing the substrate in 50% phosphoric acid for up to 5 minutes. Depending on the removal conditions, several morphologies at the oxide-aluminum interface are observed by Scanning Electron Microscopy (SEM). Ramping down the voltage at less than 0.3 V per 15s combined by immersion in 50% phosphoric acid for less than 3 minutes is found to open the barrier layer of the nanopores. The pores have root-like structure with branches as small as few nanometers due to the slow voltage ramping. Several amorphous anodized Al2O3 (AAO) templates with pore diameter ranging from 30 to 40 nm and with length up to 25 μm were prepared by two-step anodization for the cathodic electrodeposition of photoactive nanowire semiconductors such as copper indium diselenide and cadmium sulfide. INTRODUCTION Nanoporous membranes have shown great potential in areas of inclusion chemistry, guest-host synthesis, molecular manipulations and reaction in the nanoscale for making nanoparticles, nanowires [1-6] and nanotubes [7, 8] with their unusual physical properties. High aspect-ratio (HAR) nanostructured materials have attracted great interests for nanoscale device applications in microelectronics, non-linear optics and optoelectronics [9]. In particular, solar devices based on photoactive HAR semiconductors are predicted to exhibit higher efficiency when compared to planar thin-film solar cells due to enhanced optoelectronic properties by quantum confinement effects and larger effective surface area. In order to develop a large-scale production approach of HAR nanostructure solar devices, the HAR semiconductors are fabricated by a well-established bottom-up strategy integrating lost cost and non-vacuum electrochemical techniques. In general, such nanofabrication requires a conducting substrate and a chemically inert template with well-ordered nano-channels in which the semiconductor material is directly deposited. In our strategy, the HAR nanostructures are fabricated by electrodepositing the materials inside the nano-channels of anodized aluminum
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