AAO Nanowells: Synthesis, in-situ Growth Study, and Applications in Ultra-sensitive Chemical Detection
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0951-E09-10
AAO Nanowells: Synthesis, in-situ Growth Study, and Applications in Ultra-Sensitive Chemical Detection H. Hau Wang1, Kyung-In Son1, Byeongdu Lee2, Jianjiang Lu1, and Catherine Han1 1 Materials Science Division, Argonne National Laboratory, Argonne, IL, 60439 2 X-Ray Science Division, Argonne National Laboratory, Argonne, IL, 60439
ABSTRACT Anodized aluminum oxide (AAO) membranes consist of locally highly ordered nanopores. The pore diameter (20-200 nm) and pore-to-pore distance are controlled through the anodizing voltage and the choice of etching solution. High aspect ratio over 1,000 can easily be achieved which makes AAO membranes the ideal templates for making nanowires and nanotubes. In this work, the early stage of nanopore formation was studied in-situ with the use of small angle x-ray scattering (SAXS) and glazing incidence GISAXS techniques. The nanopores were found to grow as a function of the square root of growth time. The resulting short nanopores or nanowells with length below 200 nm were further characterized with use of SEM and AFM. These AAO nanowells showed interference color. Their reflectance UV-Vis spectra indicated strong angular dependence and can be understood with a simple single layer (nanoporous alumina over aluminum) model. These spectra are influenced by the nanowells thickness and diameter. Therefore, large array of interferometric AAO nanowell sensors are possible. When these nanowells are coated with Ag or Au thin films, in addition to enhanced interference color, highly sensitive surface enhanced Raman scattering (SERS) effect has been observed. INTRODUCTION Anodized aluminum oxide (AAO) membrane is a unique material due to the presence of highly ordered nanopores. These nanopores are straight, perpendicular to the membrane, and high aspect ratio (length/diameter) over 1,000 can easily be obtained. The pore diameter (20 to 200 nm), pore-to-pore distance, and pore length (up to ~100 µm), are controllable through the anodizing potential, etching solution, and etching time. These nanopores are highly ordered within a domain, typically 20 times of the pore-to-pore distance, but generally lack of long range order. These AAO membranes can be prepared in a standard wet chemistry lab and many exciting fundamental research and applications have been developed based on the use of these materials. For examples, when AAO membranes were used as templates, multiwalled carbon nanotube superconductor was reported with Tc at 12 K,1a giant magnetoresistance and spin-valve effect was observed in Co/Cu multilayered nanowires,1b on-wire lithography was developed that allowed sub-100 nm gap control,1c and metal nanohole arrays were fabricated that indicated photonic bandgap behavior.1d Moreover, chemical and biochemical applications were also reported such as submicrometer bar codes for fluorescence detection,2a DNA separation (with polycarbonate membranes),2b AAO membrane catalysis with high product selectivity,2c polymer nanowires and nanotube synthesis,2d and biomimetic super-hydrophobic
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