This thesis describes a study of polar semiconductor nanorods using first-principles
quantum-mechanical computer simulations.
Semiconductor nanostructures in solution are a very exciting class of material due
to our growing ability to manipulate their shapes, sizes and the superstructures they
assemble in,to produce a wide range of technologically useful properties.
Nanocrystals of binary semiconductor, such as those of ZnO, have been observed
to exhibit very large dipole moments which affect their internal electronic structure
(and therefore their optical properties, for example) as well as their interactions with
the surrounding environment, which can affect the kinetics of self-assembly and the
stability of the structures formed.
A detailed understanding of the factors contributing to this large polarity in nanocrystals
has proven elusive for two main reasons: (1) the multitude of factors that are
involved, ranging from the effects of surface chemistry, to the non-centrosymmetric
nature of the underlying crystal, to quantum confinement, to long-range electrostatics,
to interactions with the solvent and considerations of thermodynamic stability;
and (2) the limitations of current experimental technique, which don’t allow us the
level of control over, or knowledge of the state of our system, that is necessary to
be able to disaggregate these factors.
The main advantage of computer simulation is the level of control over and knowledge
of our ‘experimental’ conditions that it allows, thus making it an ideal tool for
addressing this problem. Recent developments in linear-scaling density-functional
theory, combined with improvements in computational power, have for the first time
brought accurate quantum-mechanical methods in to the realm of applicability to
nanocrystals of realistic size.
This thesis focuses on how a nanorod’s polarity and electronic structure are affected
by changes in the surface terminating species, by surface relaxations, nanorod
size, semiconductor type, applied electric fields, and interactions with neighbouring
nanorods.