Acunzo, Adriano Anisotropic Nanoparticles for Enhanced Plasmonic Effects. [Tesi di dottorato]

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Abstract

Chapter 1 includes a selection of topics from Plasmonics. The concept of volume plasmons as excited collective oscillations of an interacting electron gas (due to the long‒range part of the Coulomb potential) is first introduced in a classical hamiltonian formalism, and then discussed in both the first and second quantization formalisms. The possibility to excite plasmons in metal nanoparticles by optical radiation marks the transition to the classical Mie theory. The two main predictions of the quasi‒static approximation of Mie theory are then pragmatically discussed by using nanophotonic simulations. The simulation software and the two simulation workspaces used throughout this work are presented and discussed in detail. Finally, a selection of simulations from our recent works are presented to highlight the essential role played by simulations in plasmonics. Chapter 2 is devoted to nanofabrication. Both isotropic (spherical) and anisotropic (cubic) gold nanoparticles were synthesized combining seed‒mediated methods with Liz-Marzán’s dissolution reaction, embracing Mirkin’s renewed attention to seeds quality. Meticulous morphological characterizations and analysis of the nanoparticles products are presented, particularly for nanocubes. All interesting experimental results are corroborated by FDTD simulations run by Ansys Lumerical software. In this case, they were paramount for the modelling of nanocubes as “rounded” nanocubes. Finally, electrostatic self‒assembly technique was used for the fabrication of random arrays of nanospheres and nanocubes on glass substrate. Collective plasmon phenomena were observed and then deeply investigated by FDTD simulations. Chapter 3 starts with the analytical treatment of the metal‒enhanced fluorescence (MEF) developed by Khurgin and Sun (et al.). Both absorption and emission of molecules placed close to a metal nanosphere are described as two‒step processes in the presence of the plasmon modes supported by the sphere. A clear understanding of what is and what is not possible to achieve by MEF is provided. In particular, the role played by the total absorption cross section of the molecules in limiting the absorption enhancement is unveiled. The negative impact of the higher‒order modes of a sphere on the emission enhancement is highlighted. Then, the special case of fluorophores with high original quantum yield is presented through the classical works of Anger, Bharadwaj, and Novotny. Especially for these fluorophores, whose quantum yield can only diminish during MEF processes, the breaking of the spherical symmetry is advised as a route to limit the quenching. Anisotropic nanoparticles are thus introduced as a natural way to address the issue, as well as to further increase the absorption enhancement thanks to higher electric fields produced by their sharp features. Chapter 4 present a comparative study of the distance‒dependent MEF performance on the fabricated nanostructures. Generally, the chapter has been organised to guide the reader into such a complex topic as MEF, according to our understanding of it. In particular, the study was conducted from a biosensing perspective, leading to specific choices, discussed in detail. Double strand DNA (dsDNA) was employed as nanometric spacer as most appealing from a biosensing viewpoint. The experimental results were analyzed and discussed one by one with the help of the simulated electromagnetic field profiles of target nanoparticles from the arrays. Few results could not be fully explained by plasmonics (i.e., by MEF) and are possibly related to the complexity of dsDNA. In particular, we speculate about a novel effect. After a brief conclusion, the last part of the manuscript includes supplementary, but relevant, material such as: nanofabrication protocols and procedures (Appendix A and B); technical details about the morphological analysis of electron micrographs by ImageJ software (Appendix C); dsDNA sequences (Appendix D); MEF experiments procedures (Appendix E); and supplementary data, analysis, and simulations (Appendix F). The list of references finally closes this manuscript.

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