Due to their extraordinary electronic properties, Quantum Dots (QDs) are potentially very useful
nanoscale devices and research tools. As their electrons are confined in all three dimensions, the
energy spectra of QDs is descrete, similar to atoms and molecules. Because the gaps between
these energy levels is inversely related to the size of the QD, very small QDs are desirable.
Carbon nanotubes have long been touted as fundamental units of nanotechnology, due to
their structural, optical and electronic properties, many of which are a result of the confinement
of electrons in the trans-axial plane of the nanotube. It is known that their band gap structure
is altered under deformation of their cross section.
It is proposed that one way to fabricate a very small quantum dot is by confining electrons
in the nanotube so that they may not freely move along its length. A structure to produce this
confinement has been described elsewhere, namely the carbon nanotube cross, consisting of two
carbon nanotubes, with the the one draped over the other at ninety degrees. It is thought that
this structure will induce local physical deformations in the nanotube, resulting in local changes
in electronic structure of the top nanotube at the junction of the cross. These band gap shifts
may cause metal-semiconductor transitions, resulting in tunnel barriers that axially the confine
electrons in the nanotube. This thesis investigates the possibility that the carbon nanotube cross
may exhibit QD behavior at the junction of the cross, due to these local band gap shifts.
A device for carbon nanotube growth, using Chemical Vapor Deposition, has been designed,
and may be built using microfabrication techniques. This device consists of electrodes (for electrical
measurements of the nanotubes) and catalyst regions (to initiate nanotube growth), lithographically
patterned in a configuration that promotes carbon nanotube formation. Unfortunately,
due to fabrication issues, this effort is a work in progress, and these devices have not yet
been constructed. However, an experimental methodolgy has been developed, which provides a
framework for eventually building a carbon nanotube cross, and investigating the possibility of
QD behavior at the junction of the cross.
This structure has also been investigated computationally. Molecular dynamics simulations
were used to obtain equilibrium geometries of the carbon nanotube cross, and it was found
that their are many different meta stable states, corresponding to different types of nanotube,
and different physical arrangements of these nanotubes. The electronic structure of the carbon
nanotube cross was calculated using the density functional theory. Band gap energies similar to
experimental values were obtained. A one-to-one spatial correlation between deformation and
band gap and conduction band shifts were observed in the top carbon nanotube of the nanotube
cross. Small tunnel barriers, inferred from both the calculated band gap and LUMO energies, are
observed, and could well be sufficient to confine electrons along the axis of the nanotube.
The results described in this thesis, while not definitive, certainly indicate that a QD probably
would form at the junction of a carbon nanotube cross, and that further investigation, both
experimental and computational, is warranted.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/841 |
| Date | 05 1900 |
| Creators | Woodsworth, Daniel James |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | text |
| Rights | Woodsworth, Daniel James |
| Relation | Undergraduate Honours Theses |
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