Estudo de propriedades Eletrônicas, energéticas e estruturais de Nanotubos BxCyNz de camadas simples e dupla via cálculos de primeiros princípios

Abstract

In this dissertation we have done theoretical studies, via rst-principles calculations, of the structural, energetic and electronic properties of boron (B) and nitrogen (N) doped carbon (C) nanotubes (NTs), with single and double layers. We have opted for a con guration in which the carbon atoms are distributed in two diametrically opposed stripes separated by two BN stripes. To perform the calculations, we have used the SIESTA computational code, which is based on the Density Functional Theory (DFT), with the exchangecorrelation term treated through the Generalized Gradient Approximation (GGA), using the norm-conserving pseudopotentials and a basis set obtained by a linear combination of atomic orbitals. Regarding the structural analysis, we have noticed an atomic reorganization that caused a deformation. This distortion was more visible in the zigzag structures. Through the energetic analysis, we have observed that the diameter, the chirality, the quantity and the types of bonds, are factors that in uence the nanotubes stability. Considering the diameter, when we keep the chirality and stoichiometry xed, the stability follows one simple rule: the larger the diameter, the more stable the structures are. Regarding the chemical bonds, we have concluded that the higher the obtained value for the ratio between the number of total bonds (T L ) and the number of unfavorable bonds (L ) in the unit cell, the more stable a given system will be (considering the same stoichiometry). However, when we compare structures with di erent chiralities, we note that the zigzag nanotubes are more stable, even if they present lower diameter magnitudes and lower values for the ratio T L =L D . We have also observed that the BC N stoichiometry is more stable than the others studied in this work. For the double layer structures, an energetic stability rule (n,0)@(n+9,0), which is independent of the stoichiometry, was observed. And the distance between the stripes for these systems are more elevated than the ones for CNTs and pure BNNTs. Finally, an electronic analysis was performed with the use of the electronic band structure of the nanotubes, which determines the materials electronic behavior, and where we can obtain the data related to the systems energy gaps. The studied nanotubes behaved as semiconductors with energy gaps ranging between 0.75 eV and 0.06 eV, with energy gaps both direct and indirect, and also as metallic systems. We have also observed that the nanotubes with double layers are metallic for smaller systems and direct gap semiconductors for the larger systems, showing interesting variations around the Fermi level. For the double walled nanotubes, we have observed that the greater contribution for the valence and conduction bands, in states near the Fermi level, comes from the inner tube.