Graduation Semester and Year

2008

Language

English

Document Type

Thesis

Degree Name

Master of Science in Physics

Department

Physics

First Advisor

Asok Ray

Abstract

First-principles calculations for the electronic and geometric structures of three different types of armchair and zigzag silicon carbide nanotubes from (3, 3) to (11, 11) and (3, 0) to (11, 0) have been performed using hybrid density functional theory and the finite cluster approximation. Full geometry and spin optimizations have been performed without any symmetry constraints. A detailed comparison of the structures and stabilities of the three types of nanotubes is presented. The dependence of the electronic band gaps on the respective tube diameters, energy density of states and dipole moments as well as Mulliken charge distributions have been investigated. For type 1 armchair nanotubes Si atoms moved toward the tube axis and C atoms moved in the opposite direction after relaxation, consistent with other SiC nanotubes found in literature. For type 2 and the newly proposed type 3 armchair, this displacement direction is reversed. For all types of zigzag SiC nanotubes Si atoms moved outward the tube axis making two concentric cylinders of Si and C atoms after relaxation contrary to some published results in the literature for type 1 nanotubes. The band gaps for type 1 armchair nanotubes are larger than bulk 3C-SiC gap, while type 2 and type 3 armchair nanotubes have significantly lower band gaps. The band gaps for type 2 armchair nanotubes, also type 1 and type 2 zigzag nanotubes show an oscillatory pattern as the diameter increases. Unlike the other two types, band gap for type 3 nanotubes in both chirality decreases monotonically with increasing tube diameter. None of the armchair tubes appear to be magnetic. On the other hand all the zigzag tubes studied here appear to have triplet ground states except for type 1 (3, 0). As a continuation we have also investigated the interaction of Fe atom with these SiC nanotubes. A systematic study of Fe atom encapsulation and adsorption in both armchair and zigzag SiC nanotubes has been performed using the same computational formalism. A detailed comparison of the binding energies, equilibrium positions, Mulliken charges, spin magnetic moments of Fe atoms, the electronic states, HOMO-LUMO gaps, and changes in gaps with respect to the bare nanotube gaps have been investigated. Our results show that the properties of SiC nanotubes can be modified by Fe atom encapsulation and adsorption. All the structures are found to have magnetic ground states with high magnetic moments. It is expected that both pristine and Fe doped SiC nanotubes will have interesting and important applications in the field of band gap engineering, molecular electronics and spintronics.

Disciplines

Physical Sciences and Mathematics | Physics

Comments

Degree granted by The University of Texas at Arlington

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