Electronic and optical properties of organic crystals, polymers, and semiconductors

The characterization of electronic and optical properties of crystalline materials is an important step in understanding the physics of such materials, and often leads to innovative technological applications. More recently, organic materials have been studied and proposed as basis for novel electronic and electro-optical devices with properties beyond the capabilities of current semiconductor devices. Nanotechnology is now a fast expanding field, and new challenges are being posed both in developing new devices based on organic materials and in understanding the basic properties of such materials when placed in different environments. In this Dissertation, we use state-of-the-art theoretical methods in order to investigate the electronic and optical properties of materials ranging from conjugated polymers to organic molecules both in crystalline phase and as isolated molecules. We also revisit the problem of electronic band structure of covalent semiconductors and semiconductor alloys.; Our theoretical approach is based on Density Functional Theory (DFT) and many-body Green's function methods. The quasiparticle band structure is obtained within the GW Method and neutral, optical excitations of the electronic system are studied using the Bethe-Salpeter Equation (BSE) approach or alternatively Time Dependent Density Functional Theory (TDDFT). The first chapter has a summary of the theory involved in this work, complemented by an appendix on sum rules. The subsequent chapters present detailed analyses of the following studied systems: (1) Polyacetylene (C2H2), a prototypical conjugated polymer, is studied both in crystalline form and as an isolated chain. The optical absorption spectrum is calculated, and bound excitons are found to dominate the optical spectrum. (2) Pentacene (C22H 14) and other compounds in the family of linear polyacenes are studied in crystalline form. Intermolecular interactions are found to be more important than what earlier semiempirical models have predicted in determining the electronic band structure and linear optical response. (3) Photoisomerization of azobenzene (C12H10N2) is investigated using two methods: direct minimization of the potential energy of an isolated molecule in the first excited electronic state; and mapping of the potential energy surface in configuration space for the ground state and the two lowest excited states. (4) The electronic band structure of silicon, germanium and gallium arsenide is revisited, and effects on the electronic energy gap due to interactions between valence and core electrons are investigate. (Abstract shortened by UMI.)...