Ankara : The Department of Physics and the Graduate School of Engineering and Science of Bilkent University, 2014.Thesis (Ph. D.) -- Bilkent University, 2014.Includes bibliographical references leaves 94-112.While the hierarchical assembling as well as the dramatic miniaturization of Si
nanowires (NWs) are on-going, an understanding of the underlying physics is of
great importance to enable custom design of nanostructures tailored to specific
functionalities. This work presents a large-scale atomistic insight into the electronic
properties of NW-based complex structures, starting from the subsystem
level up to the full assembly, within the framework of pseudopotential-based linear
combination of bulk bands method. Laying the groundwork by grasping single
Si NWs, we get into a large extent an unexplored territory of NW networks and
kinked NWs. As one end product, a versatile estimator is introduced for the band
gap and band-edge lineups of multiply-crossing Si NWs that is valid for various
diameters, number of crossings, and NW alignments. Aiming for an exploration
of the low-lying energy landscape, real space wave function analysis is undertaken
for tens of states around band edges which reveal underlying features for a variety
of crossings. Predominantly, the valence states spread throughout the network,
in contrast the conduction minima are largely localized at the crossings. Given
the fact that substantial portion of the band edge shift drives from the confined
conduction states, branched Si NWs and nanocrystals have quite close band gap
values as the networks of similar wire diameters. Further support to wave function
analysis is provided via quantum ballistic transport calculations employing
the Kubo-Greenwood formalism. The intriguing localization behaviors are identified,
springing mainly at the crossings and kinks of NWs. The ballistic transport
edge set apart the conducting extended states from the localized-band gap determining
ones. Our findings put forward useful information to realize functionality
encoded synthesis of NW-based complex structures, both in the bottom-up and
top-down fabrication paradigms.Keleş, ÜmitPh.D
