Plasmonic nanostructures made by anodized aluminum oxide templates

Abstract

Plasmonic nanostructures, such as Au and Ag nanostructures, have become one of the most appealing and active research topics in nanotechnology, enabling many fundamental studies and applications in numerous scientific disciplines. By tailoring the size, shape, composition, and environment of plasmonic nanostructures, light can be controlled in many unique means. While the relatively simple, generally isotropic, and homogenous nanostructures have demonstrated some of plasmonic properties, the building structures with increased complexity and hierarchy will be necessary to realize the most useful nanoscale architectures. One of the methods for preparation of complex and intricate nanostructures is the electrochemical deposition in anodized aluminum oxide (AAO) template method with the key advantage of length and composition control.

Therefore, it is desirable to generate novel plasmonic nanosystems with precise control over dimensions by developing techniques based on AAO template method, and at the same time to explore their plasmonic properties for SERS applications. In order to achieve these aims, the following works have been carried out.Au/Ag alloy nanorings with controlled length were thus obtained. Additionally, dimers of nanorings were fabricated by this technique in combination with the on-wire lithography (OWL) method. We found that nanoring dimers have higher average enhancement than the Au nanodisk dimer which is attributed to their geometrical advantage where nanoring has less radiation damping compared to the Au nanodisk.

Second, the AAO template method has been developed and optimized to fabricate Ag-Au nanodisk heterodimers with controllable disk-thickness and gap size in high yield. The average enhancement factor (EF) of optimized Ag-Au heterodimer is observed to be in between those of Au and Ag homodimers at 633 nm and 488 nm excitation wavelengths. Interestingly, when the heterodimer illuminated by 488 nm light, the EF from Au disk (in the heterodimer) is higher than that of single Au disk, whereas the EF of Ag disk (in the heterodimer) is lower than that of single Ag disk, indicating the plasmonic energy transfer from Ag disk to Au disk in the heterodimer.

Third, layered AuNi nanodisks with controlled thickness have been prepared by the AAO method to study the plasmonic interaction between plasmonically active and non-active metals. LSPR damping was found to be induced by the Ni segment. Both SERS experiment and theoretical calculation suggest that the hot-spots can be delocalized to Ni segments in the layered AuNi nanodisks. These optical phenomena should be a consideration in designing multifunctional nanostructures that combine plasmonically active and non-active metals.