Date of Award

5-2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemistry

Advisor

Stuart, Steven J

Abstract

Molecular dynamics (MD) simulation of reactive condensed-phase hydrocarbon systems is a challenging research area. The AIREBO potential is particularly useful in this area because it can simulate bond breaking and bond forming during chemical reactions. It also includes non-bonded interactions for systems with significant intermolecular interactions.

The first part of this dissertation describes a method to adaptively incorporate van der Waals interaction of carbon atoms into the AIREBO force field. In bond-order potentials, the covalent bonding interactions adapt to the local chemical environment, allowing bonded interactions to change in response to local chemical changes.
Non-bonded interactions should adjust to their
chemical environment as well, although this adaptivity has been
somewhat limited
in previous implementations.
Here the AIREBO potential is extended to include an adaptive Lennard-Jones
potential, allowing the van der Waals radius and well depth to vary adaptively
in response to chemical reactions.
The resulting potential energy surface and its gradient remain continuous,
allowing it to be used for dynamics simulations.
This new potential is parameterized for hydrocarbons, and is fit to the
energetics and densities of a variety of condensed phase molecular
hydrocarbons.
The resulting model is more accurate
for modeling aromatic and other unsaturated hydrocarbon species, for which
the original AIREBO potential had some deficiencies.
Testing on compounds not used in the fitting procedure shows that the
new model performs substantially better in predicting heats of vaporization
and pressures (or densities) of condensed-phase molecular hydrocarbons.

The second part of this dissertation describes the investigation of nanotube welding by ion bombardment. Nanotube technology has found many applications in recent years. Junctions between heterostructured nanotubes are of particular interest because of their possible application in nanoscale devices. Simulation is performed on the formation of junctions by Ar ion irradiation of nanotubes using the AIREBO force field. Two groups of nanotubes are used in this research. One includes larger nanotubes of diameter 13 \AA, another includes smaller nanotubes of diameter about 7-8 \AA. Nanotube junction formation under different bombardment ion energies is explored. In both large and small nanotubes, junctions can be formed by ion irradiation. It is found that for large nanotubes, high energy impacts are needed in order to form junctions. Smaller nanotubes are badly damaged by high energy ions. The different types of defects created by ion impact are also investigated. Cross-links in the nanotube intersection are used as a primary index of junction formation. A comparison of longer relaxation and cooling times is also performed. The evaluation of conductivity of nanotube junctions during simulation is explored. For (10,0) nanotubes, conductance across the junction becomes non-zero after the first impact for 330 eV energy impact.

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