Novel curved and strained carbon-based nanostructures: a theoretical study of their electronic properties and their response to laser excitations.

Abstract

"In this thesis we review and discuss, from a theoretical point of view, the physical properties of a wide variety of carbon-based nano-materials. This work is focused on systems in which the atomic structures exhibit one - or a combination - of the following features: i) negative curvature, ii) highly strained C-C bonds, or iii) localized bond strain due to mechanical defects. The electronic and structural properties of these systems have been calculated using rst principles and tight-binding approaches. The mechanical stability of negatively curved graphitic crystals (the Triply Periodic Graphitic Surfaces, also known as Schwarzites) is reviewed for a representative variety of avors. It is shown that, for small crystals, identi cation could be achieved using vibrational spectroscopy; whereas larger systems would be practically indistinguishable from amorphous phases of graphite-like materials. The electronic properties of such systems and their response to optical excitations are related to the local atomic environment and di erent families could be characterized using Electron Energy Loss Spectroscopy (EELS). In contrast to other curved carbons, such as single walled carbon nanotubes, metalicity is only possible for structures containing seven membered rings. Atomic doping also could be achieved by substitutions of Boron or Nitrogen atoms, or by the introduction of pyridine-like groups. One and two dimensional networks built from the cubic cubane cage have been also investigated. These networks are shown to be mechanically stable. Their electronic properties are intrinsically related to those of the cubane molecule and, large electronic gaps have been observed. The introduction of electronically rich linkers could be used to reduce the gap without a ecting signi cantly their mechanical stability. In addition, the interaction of laser pulses with graphitic carbon nanostructures is studied in detail. Novel paths for defect healing are shown to be available using the contribution of the electronic entropy to the system's free energy. These studies suggest that it may be possible to anneal selectively defects (e.g. pentagon-heptagon pairs) using femtosecond laser pulses, in order to modify the electronic and mechanical properties of carbon nano-systems. We have extended the calculation framework for laser-matter interactions, in order to allow for the study of systems with more than one atomic species (e.g. binary semiconductors). These studies, originally designed for systems containing a single atomic species, has been extended so that further investigations, binary semiconductors would be possible."