Quantum theory of plasmons in nanostructures

In this thesis, ab initio quantum-mechanical calculations are used to study the properties of plasmons in nanostructures that involve atomic length-scales. The plasmon is an electronic excitation that corresponds to oscillations in the electron charge density in metals, often visualized as water ripples in a pond where the water represents a sea of free electrons.

Plasmons on metal surfaces and in nanostructured materials, such as metal nanoparticles and atomically thin two-dimensional materials, have several technological applications due to their ability to confine light on nanoscale. For a theoretical description of plasmon in such materials, where the electrons are heavily confined in one or more directions, a quantum mechanical description of the electrons in the material is necessary.

In this thesis, the ab initio methods Density functional theory (DFT) and linear response time-dependent DFT are applied to calculate the properties of plasmons in nanostructures in different dimensions. In order to identify and visualize localized plasmon modes, a method for calculating plasmon eigenmodes within the ab initio framework has been developed.

In the studied materials, quantum mechanical effects such as coupling to single-electronic transitions, electron spill-out from the surface, tunneling, and spatial non-locality, are shown to alter the plasmon excitations. The studied systems include two-dimensional materials, such as thin metal films, monolayer transition metal dichalcogenides, and graphene.

Here, also van der Waals heterostructures (vdWh), which are stacks of different twodimensional materials, are considered. A new multi-scale approach for calculating the dielectric-function of vdWh, which extends ab initio accuracy to the description of hundreds of atomic layers, is presented. Also, one-dimensional plasmons are studied in the case of atomically thin nanowires and edge-states of MoS2.