| The preparation and unique optical properties of metallic nanostructures, especially highly-ordered metallic nanostructures, have become a reaseach hotspot this years. When irradiated by the incident light, free electrons in the metal creats surface plasmon resonances (SPR) driven by the electromagnetic field. The SPR has been widely studied in the field of surface-enhanced Raman scattering (SERS), negative-index materials, bio-chemical sensing, solar cells and phonic devices, owing to the locally enhanced electromagnetic field. The preparation of metallic nanosturctures mainly relies on the high-cost micro-facrication processes in semiconductor indusitries, lacking flexibility on controlling the sizes of metallic structures. So it is badly in need of a simple and effective solution to fabricate metallic nanostructures. In order to solve these problems, we investigated a fast fabrication of ordered periodic metallic nanodot array using ultrathin porous anodic aluminum oxide (AAO) as the templates in this theis. Our results provide a new route to prepare period metallic nanostructures with tunable sizes and improve their optical performances. The detailed reseach contents and results are as follows:1. AAO with large intervals and the formation mechanisms. The preparation of AAO with large intervals in oxalic acid and phosphoric/citric acid based electrolytes is studied. The dependence of pore intervals on anodizing voltages is systematically investigated. It is found that the intervals increase with the voltages and the interval-to-volatge ratios in oxalic acid and phosphoric/citric acid are2.0nm/V and2.5nm/V respectively. AAO with ultra-large interval (1000run) is obtained under480V. It is also observed that the surface morphology of aluminum foils plays an important role on the formation nanopores. Both porous and barrier-type AAO can be obtained under the same conditions, depending on whether the Al foils are polished or not. Based on this experimental discovery and the simulated electric field distribution, the mechanism of defect-guided initiation of porous AAO is proposed.2. Preparation of ordered ultrathin AAO templates. Highly-ordered AAO templates with pore intervals～480nm are prepared guided by the nanostructures (period～480nm) created by pre-nanoimprinting on aluminum foils. A serial of processes, including anodization of ultrathin AAO, thin film transfer and the removal of barrier layer are investigated. Finally ultra-thin AAO templates with thickness～1μm, aspect ratio～2:1and pore diameter～250nm are obtained, in order to deposite ordered metallic nanodots.3. Preparation of ordered metallic nanodots and their optical properties. Template by the ultrathin AAO film, flexible and highly-ordered metallic nanodot arrays with period～480nm, thickness～100nm and gaps～10-30nm are fabricated through thermal evaporation, displaying unique optical properties. When varying the incident angles, the reflected light of as-prepared metallic nanodots displays different colours. The transmission can also be tuned from～50%to～60%by stretching the flexible substrate. In addition, the performances of flexible metallic nanodots in SERS with different structures, sizes and stretching ratios are investigated through finite-difference time-domain (FDTD) simulations and exmerimantal measurements. The enhancement factor reaches as high as over6000folds. |
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