Theoretical studies of growth processes and electronic properties of nanostructures on surfaces

Low dimensional nanostructures have been of particular interest because of their potential applications in both theoretical studies and industrial use. Although great efforts have been put into obtaining better understanding of the formation and properties of these materials, many questions still remain unanswered. This thesis work has focused on theoretical studies of (1) the growth processes of magnetic nanowires on transition-metal surfaces, (2) the dynamics of pentacene thin-film growth and island structures on inert surfaces, and (3) our proposal of a new type of semiconducting nanotube. In the first study, we elucidated a novel and intriguing kinetic pathway for the formation of Fe nanowires on the upper edge of a monatomic-layer-high step on Cu(111) using first-principles calculations. The identification of a hidden fundamental Fe basal line within the Cu steps prior to the formation of the apparent upper step edge Fe wire produces a totally different view of step-decorating wire structures and offers new possibilities for the study of the properties of these wires. Subsequent experiments with scanning tunneling microscopy unambiguously established the essential role of embedded Fe atoms as precursors to monatomic wire growth. A more general study of adatom behavior near transition-metal step edges illustrated a systematic trend in the adatom energetics and kinetics, resulted from the electronic interactions between the adatom and the surfaces. This work opens the possibility of controlled manufacturing of one-dimensional nanowires. In the second study, we investigated pentacene thin-films on H-diamond, H-silica and OH-silica surfaces via force field molecular dynamics simulations. Pentacene island structures on these surfaces were identified and found to have a 90-degree rotation relative to the structure proposed by some experimental groups. Our work may facilitate the design and control of experimental pentacene thin-film growth, and thus the development of organic thin-film transistors. Finally, in our third study, we proposed a new type of structurally simple and energetically stable cyanide transition metal nanotube, based on the planar structure of M(CN)2, (M = Ni, Pd, Pt). These nanotubes have semiconducting character with large band gaps (2-3 eV), which are insensitive to the chirality and diameter. We have investigated the energetic, electronic, and mechanical properties of these materials in both planar and tubular forms through first-principles density functional calculations. These calculations reveal interesting multi-center bonding features that should lead to preferential growth of tubes of a particular chirality. The unique features of these nanotubes should make them capable of being mass-produced, which is one of the most significant shortcomings of semiconducting carbon nanotubes.