Computational modeling of quantum aspects in plasmonic nanostructures

Metallic nanoparticles have fascinating optical properties due to light-triggered oscillations in their free-electron plasma. These confined plasma oscillations, known as localized surface plasmons, exhibit strong resonances at specific frequencies, determined by the size, shape, and composition of the particle as well as by its surroundings. Such properties have rendered plasmons useful for nanotechnological applications in harnessing the energy of sunlight at a subwavelength size scale, in spectroscopic sensing, and in light-activated medicine, for instance. A general trend in nanotechnology is miniaturization towards smaller and smaller nanostructures. Plasma oscillations can be modeled with classical electrodynamics down to structural details of around ten nanometers, but below that, the plasmonic response starts to divert from the classical behaviour due to the quantum-mechanical nature of electrons and atomic-level fine structure. This thesis focuses on plasmons in this nanoscopic scale.In this thesis, the collective electron dynamics producing the plasma oscillations is described within the quantum-mechanical time-dependent density-functional theory (TDDFT), and the nanostructures are modeled down to their atomic structure. The thesis contains method development, which is done within the local-basis-set representation of the electron wave functions. This approach enables computational modeling from single atoms and molecules up to large nanostructures approaching the classical size regime. In this thesis, methods for controlling the accuracy of the basis sets and analyzing the character of electronic excitations are developed. The developed methods have been applied for modeling and analyzing plasmons in noble metal nanoparticles as well as in MoS2 nanostructures. The results provide insight into the plasmon formation in metal nanoparticles and into the effects of one-dimensional edge plasmon modes on the optical properties of MoS2 nanotriangles. The thesis also contains results on intrinsic quantum-mechanical phenomena revealed in plasmonic responses. The quantization of electron states is observed to manifest itself in the optical response of single small metal clusters as well as in terms of quantized electron transport between atomically-contacted nanostructures, leading to distinct quantum features in the plasmonic response. In summary, this thesis presents methods for first-principles nanoplasmonics modeling and analysis, their application for various nanostructures, and exploration of quantum phenomena therein.