Plasmonic excitations and coupling in atomic wires
Authors: Zamin Mamiyev
Date: 2020-09-22 00:00:00
Tasks: Band Gap, Single Particle Analysis

Metal-induced atomic wires on the semiconductor surfaces are the ultimate limit of long-range ordered quasi-one-dimensional electronic systems. In this thesis, the plasmonic excitations and their coupling with the structural parameters, as well as the role of the embedding medium on the Au atomic wires, self-organized on flat and high-indexed Si surfaces were investigated. The atomic wire systems in the focus of this thesis, Si(111)-(5×2)-Au and Si(hhk)-(1×2)-Au, were generated by adsorption of submonolayers of gold onto the reconstructed surfaces. These systems feature a broad range of characteristics that can exemplarily be classified based on the number of atomic chains per terrace, inter-wire spacing, local structure of step edges, number of metallic bands and band filling. High-resolution EELS-LEED and SPA-LEED were used as the main experimental methods to address the mentioned properties and furthers. These experimental results were compared with the atomistic DFT calculations. Moreover, for the Si(553)-Au system, IR transmittance experiments were performed to investigate the optically active standing wave formation after oxidation. As a prominent property of conductive electrons, collective excitation is strongly coupled to the crystal lattice, electronic band structure, electronic and spatial confinement as well as properties of the surrounding physical media. This makes plasmonic excitation an adequate tool to probe a variety of interactions and coupling between those parameters associated with metallic structures. Taking this advantage, the present thesis investigates plasmonic excitations and their coupling to structural and environmental parameters. As a result of this investigation, a strong cross-coupling of the electronic and structural properties was revealed. For example, electronic doping to the Si(553)-Au system enhances order along the wires, which also results in band gap opening at the same time. Moreover, modification of individual structural motifs on the Si(557)-Au surface leads to a unique rearrangement of the band structure while preserving the metallicity. Also, combining the plasmon dispersion with calculated band structures, the kinetic sequence of the oxidation of the different atomic groups could be studied. For most investigated systems, an almost quantitative agreement between atomistic calculations and plasmon spectroscopy results was achieved, validating the calculated band structure and model used. In particular, the unoccupied part of the band structure was investigated for atomic wire systems. As an example, for the very first time unoccupied electronic band structure of the Si(111)-(5×2)-Au surface was investigated by comparing plasmon dispersion with the available DFT calculated bands. Moreover, doping of the latter surface with surplus Au and atomic H resulted in a metal-insulator transition. However, due to their highly robust anisotropic structure and electronic properties, the doping and oxidation of Si(hhk)-Au systems underline more specific mechanisms in the fine-tuning of metallicity. Some of these particular mechanisms include switching of bands, electronic interaction of adjacent terraces, cross-talking of dimerization and band gap opening, self-healing of defects, robust metallicity due to the site-specific oxidation, and indirect charge transfer to the metallic states, etc. have been extensively studied in this thesis.