The Simplest Plasmonic Molecules: Metal Nanoparticle Dimers and Trimers

Gilad Haran, Chemical Physics Department, Weizmann Institute of Science, Rehovot, Israel
Nir Zohar, Chemical Physics Department, Weizmann Institute of Science, Rehovot, Israel
Lev Chuntonov, Faculty of Chemistry, Technion, Haifa, Israel

Plasmonic molecules (PMs) are nanostructures in which individual plasmon modes strongly interact and show distinct collective behavior [1]. These interactions, as well as external perturbations, may lead to phenomena analogous to those found in molecular systems. Recognition of this analogy paves the way to the application of concepts developed in chemical physics to PMs.

In this lecture I will discuss our work on individual PMs consisting of two and three metal nanoparticles. First, I will show how the distance between particles in dimeric PMs affects the enhancement of Raman scattering from individual molecules situated within the inter-particle gap. We developed a method to vary this gap systematically, using ultrathin silica shells (down to 1 nm). The Raman enhancement increases as the gap size decreases, but then drops again at the shortest distance due to quantum effects.

I will then show how concepts from molecular group theory can be adopted to understand the spectroscopy of trimeric PMs self-assembled from silver nanoparticles. The most symmetric of these has an equilateral triangle form, described by the point group D_3h. Breaking this symmetry lifts the degeneracy of the plasmon modes, which can be observed in single-cluster dark-field spectra and analyzed using hybridization theory. The symmetry of PMs also affects their electromagnetic interaction with molecules. This enables a surprising modulation of the polarization of Raman scattering from these molecules

 

References:

1.            Zohar, N., L. Chuntonov, and G. Haran, Journal of Photochemistry and Photobiology C-Photochemistry Reviews, 2014. 21: p. 26-39.