Optical properties of nanoclusters from time-dependent density-functional theory

Abstract

The ground state properties of a quantum-mechanical many-electron system can be effectively modeled by its total electron density only, which is the key idea of the density-functional theory (DFT) methods. However, electronic excitations to higher energy states are not adequately described by the standard DFT formalism. To model the optical properties, for example, absorption and emission and response to time-dependent fields such as laser fields, the extension to time-dependent DFT (TDDFT) has become a popular method. In this Thesis, the TDDFT methods are utilized to calculate the optical properties of various nanostructures including fullerenes and fullerene derivatives, silicon nanocrystals and metal-polymer hybrid structures. The main focus is in the determination of their photoabsorption spectra using a real-space implementation of TDDFT. By these calculations we study how different structural variations and changes in the chemical environment affect the electronic and optical properties of the materials. For carbon and boron nitride fullerenes, variations in their size, geometry and doping are found to have a clear impact on their photoabsorption spectra. The results strengthen the view that optical absorption can be effectively used in the experimental characterization of such structures, for example in distinguishing between different isomers. The photoabsorption is observed to be strongly affected by the chemical environment for both silicon nanocrystals and small silver nanoclusters. When silicon nanoclusters are embedded in silica, the size dependence of their absorption edge is found to change due to major changes in the electronic structure. For the silver clusters, the presence of a polymer is found to bring the absorption edge down to the visible range in some of the studied cases. These calculations shed light to the experimental observations of unexpected absorption from such structures in the visible range.