Investigating mechanical and physical properties of nanostructures.

摘要

In the first part of the dissertation, mechanical properties of bundles of carbon nanotubes are investigated. Earlier experiments showed that these tubes are tightly packed in the bundles, proving that they are strongly interacting. Furthermore, extensive applications of bundles of CNTs in microelectronics such as electrical interconnect have been reported which makes determining their mechanical properties very important. Having used beams with partial interlayer resistance and Euler-Bernouli beams, it was found that the stiffness of the system of tubes is more than the summation of the tubes individual stiffness. In the second part, the growth of single end cap carbon nanotubes is investigated using Molecular dynamics and Monte Carlo simulations. Simulations show that adding dimers to cap lead to the tube growth with preserving the hexagonal structure. Results from both methods were consistent providing a proof that 5-7 dislocations cause the structure to anneal and remove defects. Furtheremore, a rotation theory for growth of chiral tubes was introduced. In the third part, edge evolution of finite graphene structures is investigated. Graphene edges could be an arrangement of Armchair (AC) or Zigzag (ZZ). It has been always a chellenging problem to find which edge (AC or ZZ) is dominant during Graphene evaporation. Having investigated the problem through different geometries and edge types, it was found that irrespective of initial geometry and shape, all structures tend to reach a common equilibrium introducing AC edge as a dominant edge. In the last part, we study novel boron structures and their mechanical and electronic properties, using ab initio calculations. The alpha-sheet has been proven to be the most stable structure energetically out of the two dimensional boron sheets. The properties of achrial (AC and ZZ) tubes obtianed from alpha-sheets were reported earlier. In the same line, the properties of chiral nanotubes obtained from a-sheet is investigated. The computations confirm their high stability and mechanical stiffness parameters within 50% of CNTs. Relaxation results reveals the curvature-induced buckling of certain atoms off the original plane. Finally, the electronic structure of the chiral tubes were found to be consistent with the earlier work results which confirmed that for small tubes (diameter 1.7nm) the tubes are expected to be metallic, while semi-conducting for larger tubes.