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Herein we present a new modeling approach to nanostar plasmonics that treats nanostar spikes as dual cavity systems where coupled bulk and surface polaritons propagate and form standing waves. Finite element simulations of the optical behavior of gold nanostars in water reveal a new view of collective electron cloud oscillations, where localized surface plasmon resonances coexist with coherent delocalized interface waves, i.e. propagating surface plasmons. Gold nanostar spikes long enough to allow propagating polaritons, and short enough to resonate with the spherical core, serve as the substrate for the observed overlap between propagating modes and localized ones. Transverse plane plots reveal bulk polaritons coupled to surface polaritons. In light of these, we explore the mechanisms that drive the plasmonic coupling in nanostars from the single spike level to multi-spiked systems and to complex interparticle coupling ensembles. Our successful predictions in experimentally synthesized systems of increasing complexity allow us to test our method in various regimes.
First, we explore changes in gold nanostar spike resonances when SiO2 shells are progressively grown onto the spikes. As the SiO2 layer thickens, the plasmonic enhancement dampens reaching a minimum due to the disrupted polaritonic coupling
on the spikes. We determine a strong correlation between the nanostar morphology and its silica coating layer, the enhanced electric field, and the surface enhanced Raman scattering (SERS) signal enhancements. The modelled behavior is expressed in terms of power losses maxima and it is compared to the experimentally measured SERS signal enhancement, as both values depend on the absolute value of the electric field. A successful prediction of the trend secures the applicability of our modelling approach to systems with spatially varying and frequency dependent dielectric functions.
We then calculate the shape dependent extinction coefficient, the volume, and the surface area of that real particle by introducing a detailed nanostar tomogram into our computational method and calculating its electric field under illumination with 8 different polarization orientations. In comparison to a semi-empirical, simplified model, used to calculate the same fundamental physical and optical parameters, which assumes a perfectly spherical core and identical protruding spikes, and other methods from bibliography, and based on the close agreement among the values obtained with the various approaches, we are confident that our method could be generalized for nanostars of any dimensions and arbitrary shape synthesized in solution using seed-mediated protocols.
Having successfully applied our approach to structurally anisotropic particles, we propose a method for the rational design of plasmonic particles. Using the conclusions from our computational study and working in parallel with a synthetic team we establish a causal relationship between structural and plasmonic properties. By way of comparison between the observed shifts and the spectral positions of the various resonances in the experiment and the model, we use this relationship to fine
tune the synthesis. Having optimized the synthesis, we focus on the resonances from the isolated single particle level studied via Ultra-Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy (Ultra-STEM EELS), to the highly coupled ensemble level studied via Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (FTIR-ATR). A new resonant window is covered, stretching to wavelengths in the short- wave infrared (SWIR). We have also showcased a method to determine the level of monodispersity and thus the applicability of our emerging method.
In brief, we have developed a numerical approach for the detailed study of gold nanostars. Tested in anisotropic, asymmetric, and arbitrary shaped systems it proved to be a useful tool for accurate predictions of the optical and the physical properties of plasmonic particles. We utilized this approach for the development of an emerging form of a plasmonic material that covers new grounds in how far, how strong, and how narrow these particles can resonate and enhance the impinging electric field.

ETD_9418

Ph.D.

Includes bibliographical references

by Theodoros V. Tsoulos

doi:10.7282/t3-ka18-kz74

ETD doctoral

The author owns the copyright to this work.