Graduation date: 1997

Structural evolution and its effect on optical absorption/emission behavior of derivatives of PFs upon heat treatment at different temperatures were studies by means of a combination of x-ray diffraction, transmission electron microscopy and molecular simulation. The main physical characteristics from results of this study over a series of PFs with alkyl side-chains may be summarized as the following: (1) The crystal structure of poly (9,9-di-n-octyl- 2,7-fluorene, PF8) and poly(9,9-bis(2- ethylhexyl)- 2,7-fluorene, PF26) are determined via a combination of selected area electron diffraction and molecular simulation. In PF8 case, there are 8 chains in an orthorhombic unit cell with dimensions a = 2.56 nm, b = 2.34 nm, c (chain axis) = 3.32 nm, space group P212121, and calculated density of 1.041 gcm-3. On the other hand, in PF26 case, there are 3 chains in a trigonal unit cell with dimensions a = 1.67 nm, b = 1.67 nm, c (chain axis) = 4.04 nm, space group P3, and calculated density of 0.991 gcm-3. (2) All the simulation results indicate that branched side-chains in the case of PF26 tend to fill the space among backbones. In contrast, the linear side-chains in the case of PF8 appear to embrace the neighboring backbone, favoring formation of layered structure. (3) As a consequence, co-planrity of PF backbones is decreased by the attached alkyl side-chains. This in turn results in lowered conjugation length, and in favor of blue light emission.

Thesis (M. Phil.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 57-58).

Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references (leaf 99).

Gas electron diffraction (GED) is a technique that has been developed to study the molecular structure of species in the gas phase. This thesis focuses on the reconstruction of the Canterbury GED apparatus (moved from Edinburgh, UK) and the requirements for modifying the apparatus to incorporate a mass spectrometer (MS) so diffraction and MS data can be obtained within a single experiment. The combined GED-MS system has been identified in previous work in the Masters group as a necessary development for studying the structure of short-lived species generated in situ. This is particularly true for the study of ketene, which as shown in this thesis, can be generated from several precursors as part of a multiple product pyrolysis system. While GED data for ketene generated from acetic anhydride has been refined, the species formed from the pyrolysis of Meldrum’s acid were determined to be too difficult to deconvolute without additional experimental data from MS. A computational study of possible ketene derivatives that could be studied with a GED-MS apparatus is also presented. Lastly, this thesis details a structural study of the gas-phase structures of tris(chloromethyl)amine and a family of substituted disilane systems which have been determined in the gas phase for the first time. A comprehensive GED, Raman spectroscopy and ab initio study have been undertaken for tris(chloromethyl)amine [N(CH2Cl)3] which is shown to have a different structure in the solid and gas phase. Further work in the form of a molecular dynamics investigation has been identified as necessary to describe the low amplitude motion of one of the CH2Cl groups in the gas phase to allow for the GED refinement to be completed. The work on the substituted disilane systems X3SiSiXMe2 (X = F, Cl, Br, I) and X3SiSiMe3 (X = H, F, Cl, Br) demonstrates the effect of increased halogen substitution on the electronic effects of the disilanes, and the effect that the methyl groups have as larger halogens increase the steric bulk of the system.

published_or_final_version / abstract / toc / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

The various electronic phenomena of electrons in the quasi-one-dimensional semiconducotor heterostructures have been largely investigated in the past research, due to its importance both on the theoretical understanding and the design of nanodevices. In particular, most research is currently based on the GaAs-AIGaAs material system with a 2-DEG interface. From the study of Hua Wu, following the Bohm's interpretation of quantum mechanics, energetic electrons approximately approach the classical behavior. The goal of this theoretical study is to investigate how the flow of energetic electrons may be controlled by the use of a tunable reflector. When encountering hard potential walls, energetic electrons in the nanostructure nearly follow the law of reflection. In addition, if the hard potential walls function as a reflector, the bouncing ball trajectory is also predicted. In this project, the fact that energetic electrons demonstrate semi-classical periodical flow motion is conceptually verified.The quantum wire (QW) with a tab and a notch nanostructure is selected as the practical model to achieve the project's goal. The resonant properties of the QW with a tab and the QW with a notch are individually investigated. The tight-binding recursive Green's function method is the theory underlying the numerical computation of the conductance in a nanodevice. / Department of Physics and Astronomy

Low-energy electron scattering was used as a tool to study electron-stimulated processes at complex interfaces. The electron diffraction in each complex interface is theoretically treated by a multiple scattering formalism for quantitative analysis. Mathematical descriptions of electron-stimulated processes and a multiple scattering expansion extended from the single-scattering case are presented. This analysis method was applied in three research topics: These are 1) electron-stimulated desorption of Cl+ from Si surfaces, 2) characterization of epitaxial graphene on Si-terminated SiC(0001), and 3) low-energy electron induced DNA damage. Zone-specific desorption of Cl+ from Si(111)- 7X7:Cl surfaces was demonstrated. Graphene epitaxially grown on SiC(0001) surfaces was analyzed using Auger electron diffraction and Raman scattering spectroscopy. Finally, the roles of interfacial water and dissociative electron attachment resonances in low-energy electron-induced DNA damage were revealed. Electron scattering calculations using the "path approach" were applied in all of the above mentioned studies. The combination of theory and experiment has lead to insight regarding electron scattering with complex targets.