Estudo de sistemas nanoestruturados com propriedades eletrônicas ajustáveis

Abstract

In this thesis we show the results of first-principle calculations for two nanostructured materials with adjustable electronic properties. We studied two diferente systems: in the first system, BCN layers and nanotubes, we consider the positional disorder of the B, C, and N atoms, using a combination of first principles and simulated annealing calculations. During the annealing process, we find that the atoms segregate into isolated, irregularly shaped graphene islandsimmersed in BN. We also find that the formation of the carbon islands strongly affects the electronic properties of the materials. For instance, in the case of layers and nanotubes with the same number of B and N atoms, we find that the band gap increases during the simulatedannealing. We also find that the excess of B or N atoms results in large variations in the band gap; in the second system, a molecular-doped periodic assemblies of ligand-stabilized Au nanoparticles, we found that the most stable dopant positions are near the nanoparticle surfaces, away from the center of interstitial positions. The dopants provide an screening mechanism, reducing the nanoparticles charging energies. We also found a linear dependence of the Fermi level with dopant concentration, consistent with recent experiments, up to a criticalconcentration. For larger concentrations, a new regime is predicted. These features are well reproduced by a simple, analytical model for the material.