Estudo ab initio de nanotubos, com ênfase na adsorção e encapsulamento de átomos e moléculas

Abstract

In this thesis, ab initio calculations are performed using the Density Functional Theory (DFT) formalism, to investigate the electronic, optical, structural and energetics of vari- ous nanotubular systems. Firstly, the single-walled nanotubular prototypes CB, CN and BN of zig-zag and armchair configurations are considered to determine the energetic sta- bility of the systems and also their electronic structural properties. The identification of the semiconductor nature of the BN nanotubes inspired us to perform adsorptions of the hydrogen atom and various diatomic (H 2 , O 2 , N 2 , CO, N O) and triatomic (CO 2 , N O 2 ) molecules in their zig-zag and armchair versions. Finally, we study the encapsulation of the above agents in the (9.0) and (5.5) versions. The DFT calculations provided data on the energetic stability, density of states (DOS), band structure and enabled calculations of transport and optical properties. We analyzed three types of nanotubes in the zig- zag and armchair configurations, consisting of carbon-boron (CBNT), carbon-nitrogen (CNNT) and boron-nitrogen (BNNT). The above study enables us to understand the impact of diameter and chirality on the binding energy or the structural stability, the electronic property of the nanotubular prototypes. The results show that all CBNTs have a metallic characteristic independent of the chirality, while the metallic aspect is predomi- nant in CNNT. The results show that BNNTs have semiconductor properties with small energy gap at the Fermi level for n ≥ 5. All large diameter nanotubes are more stable structurally and energetically. The adsorptions of the atoms and molecules as discussed above on the surface of the zigzag and the armchair boron-nitrogen nanotubes were consi- dered both at the nitrogen and boron sites of the nanotubes. The structural parameters, binding energies, intramolecular bond-lengths of the structures demonstrated stability. The band gaps are analyzed to understand the adsorption properties of the complexes. Furthermore, density of states (DOS) band structures and charge-density transfer calcu- lations are also performed. Among all adsorbates, the hydrogen atom adsorption on both versions of SBNNT demonstrates the greatest stability. The adsorptions affected the gap which could be observed in the band structure in the density of states and in the quan- tum transmittance calculations. The encapsulation of the above atoms and molecules in (5,5) and (9,0) BNNTs also demonstrated stable structures. The results show that NO 2 - SBNNT is the most stable among all the encapsulated agents. When BNNT confine the hydrogen atom and the O 2 , N O and N O 2 molecules, they undergo a transition from the semiconductor to the metallic state. This transition is exhibited in band structures, DOS and quantum transmittance, where the energy gap at the Fermi level is modified. We also verified the charge transfer between SBNNTs and the atoms/molecules in adsorptions and encapsulations. The optical properties of the systems have been addressed through dielectric function calculation.