Nov 25, 2009
Anti-bonding modes visible in silver nanorings
Noble metal nanostructures exhibit a rich resonant extinction spectrum at visible frequencies. For complex structures, the resonances can be described as bonding and anti-bonding arrangements of the parent plasmon resonances of the individual constituents. The anti-bonding plasmonic resonance mode in individual nanoparticles or nanostructures, however, is difficult to observe due to its energetic overlap with the interband transitions in metals and the interference by high-ordered multipolar modes. Now, researchers at Interuniversity Microelectronics Center (IMEC) of Belgium have reported on a clear experimental observation of the plasmonic dipolar anti-bonding mode in silver nanorings. The team's result highlights the role that plasmon hybridization theory can play as an effective tool for understanding, designing and predicting optical properties of metallic nanostructures.
Silver nanoring structures were chosen for three reasons. First, the plasmon resonances in nanorings are prototype examples of hybridized resonances where the plasmons at the inner and outer surfaces couple strongly. Second, only the dipolar resonances are present and high-ordered multipolar modes remain dark for incident light normal to the nanorings, which can minimize the influence of multipolar modes on the dipolar anti-bonding mode. Finally, silver is a suitable metal. Its interband transitions start at a relatively higher energy region (~3.8 eV), which decreases the damping of anti-bonding resonance and makes it observable within our experimentally accessible UV-vis-NIR spectral range.
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In the framework of the plasmon hybridization model, the nanorings can be considered as a composite of nanodiscs and nanoholes. The interaction of nanodiscs and nanoholes leads to an energy splitting of resonance modes, resulting in a low-energy bonding mode and a high-energy anti-bonding mode for nanorings. The researchers have experimentally detected the bonding and anti-bonding resonances in the extinction spectrum of silver nanorings. The numerical simulations of the electric field and surface charge profiles confirm the presence of the bonding and anti-bonding modes as well.
Silver nanorings are fabricated using a nanosphere-lithography technique by sputtering a 30 nm thick metal layer on the top of 100 nm sized polystyrene sulphate latex beads, followed by an ion milling process. Gold nanorings are constructed for comparison, and in this case only the dipolar bonding modes appear. The dipolar anti-bonding modes are absent due to the strong damping by the interband transitions in gold.
The researchers presented their work in Nanotechnology.
About the author
Jian Ye is a PhD candidate at KULeuven and Dr Pol Van Dorpe is a postdoctoral researcher at IMEC/KULeuven. They are exploring new physics and device applications based on plasmonic nanostructures at IMEC, in the Functional Nano-Systems (FNS) Group of Prof. Liesbet Lagae. The FNS group covers a broad range of topics in nano-bioelectronics, including magnetic and plasmonic biosensors, nanoparticles and nanopores. Prof. Guido Maes is the head of the Quantum Chemistry and Physical Chemistry Group in the Department of Chemistry of KULeuven. Prof. Gustaaf Borghs is a senior fellow and scientific director at IMEC, responsible for long-term research in novel materials, devices and systems.