Avaliação teórica de propriedades estruturais e ópticas não-lineares de nanotubos de carbono

Abstract

In the present work, theoretical methodologies were applied to describe structure, thermodynamics, vibrationals and nonlinear optical properties (NLO) of carbon nanotubes. In a first part, different methods were compared for description of equilibrium geometries for single-wall carbon nanotubes (SWNT). It was observed that the PM3 semiempirical method leads to good results, when compared to the methods based on the Density Functional Theory (DFT). Using substituted stilbene (STB) bound to the tube surface it was evaluated, using a preliminary chemical model, the first hyperpolarizability (β) of the modified SWNT-STB and of the isolated STB species. The DFT results showed an intensification of the NLO property, when the STB is inserted to the wall of the nanotubes. In the second part of the project, the structure, thermodynamics and vibrational (Raman) properties of chemical models for an oxidation process of SWNTs, using molecular oxygen as oxidizing agent, were calculated using DFT level of theory. From the results, a thermodynamic viability could be observed for the formation of oxygenated functional groups, such as hydroxyl (OH), carbonyl (C=O) and carboxyl (COOH) on the walls of the tubes. Soon after, through the SWNT-COOH species, the β property was calculated for the set of molecules represented by SWNT-COO-STB and for the isolated species, as were as found in the first part of this work, it was observed a synergic behavior for the NLO property evaluated.