Simple Composite Dipole Model for the Optical Modes of Strongly-Coupled Plasmonic Nanoparticle Aggregates

Abstract

Self-assembled strongly coupled plasmonic aggregates exhibit optical spectra which show complex plasmonic resonances. To understand the optics of such systems, we introduce an effective composite dipole model extending previous effective models of aggregates into the plasmonic domain. The ingredients in this model are found by comparing the time-resolved extinction of self-assembling growing aggregates of gold nanoparticles spaced by rigid sub-nm gaps to recent rigorous electromagnetic simulations of this geometry. The highly reproducible spectral signatures from experiments match our simulations, confirming that the electromagnetic response of such fractal plasmonic clusters can be well-understood in terms of embedded straight chains of plasmonically coupled nanoparticles surrounded by an optically decoupled halo of dimers. We show how to derive simple analytical formulas that lead to rapid extraction of key parameters from such experimental spectra and which properly account for the long-wavelength lineshapes. In particular, we find these effective parameters describe the extent of plasmon delocalization along such chains, the eccentricity of these optically dominant cores, and the fraction of nanoparticles active within them. This underpins applications which depend on spectral selectivity and field enhancements in such tightly coupled plasmonic systems.

Optical Simulations of Self-assembly Relevant Gold Aggregates: A Comparative Study

“…These simulation results are in agreement with the ones obtained by Taylor et al [19], who investigated the optical properties of quasi-fractal aggregates experimentally and theoretically as well. They compared time-resolved spectroscopic results with simulations for gold nanoparticles with diameters of 20 and 60 nm; the aggregates featured a sub-nanometer (0.9 nm) interparticle gap.…”

“…The reason to investigate only this mode (polarization direction) is that for coupled systems the strength of the coupled mode exceeds the single particle mode by orders of magnitude i.e. it becomes the optically dominant mode [19]. The structure itself is highly anisometric, hence besides the coupled (longitudinal) mode the transversal excitation contributes the overall optical response to a smaller extent as well, analogously to gold nanorods.…”

“…The clustering of the particles and the evolution of the different assemblies can be followed by UV-Vis spectroscopy. There are several literature examples, where assignment of the evolved nanoparticle cluster structure has been carried out solely based on spectroscopic characterization (or by naked eye due to the color change) and the identification of dimers, trimers [13], chains [18,19] or larger clusters [15] based on the extinction spectra has been claimed. Two-dimensional nanoparticle arrays have been also reported with potentially useful optical properties due to the collective plasmonic oscillations [20,21].…”

“…Consequently, a multitude of coupled modes can exist simultaneously that can be excited by the external source. However, the overall optical response of the system is determined by the optically dominant centers [19], which is responsible e.g. for the presence of a 'distinct' coupled mode in the extinction spectrum during the aggregation of gold…”

Aggregation kinetics and cluster structure of amino-PEG covered gold nanoparticles

“…Thirdly, their aggregation can be monitored using simple visible light spectroscopy, due to pronounced spectral changes upon plasmon coupling between the NPs. [9][10][11] Finally, the surface of gold nanoparticles can be easily modified by a number of molecules, e.g. thiol containing polymers, [12][13] opening up a wide range of possibilities for post synthesis functionalization.…”