Estudo teórico de nanotubos de carbono, germânio e silício

Abstract

Carbon nanotubes have attracted interest due to its versatility and can be used, for example, for electronic devices, gas sensors and as reinforcement in the formation of composites. It can be classified as single-walled nanotubes, which have only one sheet of atoms wrapped to form the cylinder or multi-walled nanotubes that are formed by more than one sheet. There is also a classification according to the way the sheet is wrapped, allowing classification of these structures in chiral and achiral (armchair and zigzag). Since the discovery of the single wall carbon nanotubes, their structural, electronic and mechanical properties have been studied, and other inorganic nanotubes have been identified, such as silicon. This aim of this work is to study carbon, silicon and germanium nanotubes, where the models were obtained through an algorithm using Linux Shell scripts. From the generated coordinates with this algorithm, the MNDO semi-empirical method was used to optimize all the structures. We unconstrained, alternately, the bonding distance, the bond angle and torsion angle, making a total of ten steps to optimize the structures. The optimized structures show that the distances of border atoms were smaller than those in the middle, because these regions are free valences, and are offset by the calculation method that reduces the bonding distance. There was also a greater change in the average charge of the nanotube borders. The dipole and the |HOMO-LUMO| energy difference was also calculated, these properties show differences between the two studied configurations, and also an increase in the diameter and length of the nanotubes. The coordinates optimized with the MNDO method were used to perform the ab initio Hartree-Fock and Density Functional theory, using the 6-31G basis function. The results were analyzed according to the average distance and charge, and the |HOMO-LUMO| difference. The energy stabilization occurs when the increase of nanotube length.