Magister Scientiae - MSc / TiO2 has been broadly used as a standard photocatalyst due to its high stability, low cost, relatively low toxicity, and excellent photocatalytic performance in comparison to other semiconductor materials. However, the large band gap of TiO2 limits its use as a photocalyst due to the high energy required for excitation of the electrons in the UV region. Research on the reduction of TiO2 band gap to the visible region of the spectrum has been explored with little success. Therefore, this study focusses on shifting the band gap of TiO2 catalyst from the UV region to the visible region by doping with graphene and nitrogen-doped graphene to form TiO2-G and TiO2-NG nanocomposites, respectively. The N-doped graphene support was prepared by doping the graphene oxide with nitrogen through Hydrothermal process, followed by the reduction of the materials. Fourier Tranform Infrared (FTIR) spectroscopy confirmed the successful doping of graphene to N-graphene by the appearance of C-N and N-H vibrational modes on the spectra. The XRD results show the fingerprint patterns of TiO2 and N-graphene, which confirms the successful preparation of the nanocomposites. Morphological studies of the nanocomposites using transmission electron microscopy (TEM) show the TiO2 nanowires dispersed on graphene related supports. The optical band gap of TiO2 from UV-Vis spectroscopy was found to be 3.2eV, which decreased to 2.7eV and 2.5eV upon incorporation of grapheme and N-graphene, respectively. These results prove the success in the achievement of the aim in this study. When electrochemical studies were further conducted on the materials, TiO2-NG was found to possess better electrochemical properties with fast electron kinetics observed on the impendance spectroscopy results. The results obtained justified use of TiO2-NG photocatalyst the optimal material for organic mineralisation in Advanced Oxidation Processes (AOPs).

This dissertation focuses on investigating the morphologies, optical and photoluminescence properties of porphyrin nanostructures prepared by the self-assembly method. The study is divided into three main parts. In the first part, a large variety of porphyrin nanostructures, including nanoplates, nanofibers, nanoparticles and nanowires, were obtained through direct acidification of tetra(p-carboxyphenyl)porphyrin (TCPP) in aqueous solution. Protonation of the carboxylate groups of TCPP resulted in the formation of nanoplates through the J-aggregation of the porphyrin. Further protonating the core nitrogens of TCPP formed the porphyrin diacids which organized into well-defined structures through their interactions with counter-anions in the solution. The structures of the resulting assemblies were found to be counterion dependent. In the second part of this work, we explored the optical memory effect of the porphyrin thin film. We found that the morphology and the emission of the porpyrin thin film on Si can be changed by varying the pH of its surrounding solution. The changing in morphology and light emission of the thin film resulted from the protonation or deprotonation of TCPP'S core nitrogens. By selectively deprotonating the TCPP dications in a confined region utilizing the water meniscus between an AFM tip and the surface, Fluorescence patterns can be generated on the thin film. The fluorescence patterns can be easily erased by re-protonating the porphyrin. In the third part of this study, porphynoid nanoparticles were deposited on a surface energy gradient, and then characterized by AFM in order to investigate how the surface energy influences thier morphologies. The surface energy gradient was prepared by selectively oxidizing a self-assembly monolayer of octadecyltrichlorosilane (OTS) by UV-ozone. The nanoparticles disassemble into smaller nanoparticles with narrower size distribution on the surface with higher surface energy. Lastly, we engaged in characterizing the morphologies of polymer nanocomposites prepared by layer-by-layer assembly for wettability control. The surface roughness of the nanocopmosite in air and in salt solutions was also measured to study the correlation between the wettability of the polymer surface and its surface roughness.

Self-organized GaN nanostructures have been accomplished with lattice-(mis)matched using MOCVD. A lattice mismatched system (i.e. GaN nanostructure/ AlN) was utilized with S-K mode mechanism, whereas, metallic droplet method (i.e. Vapor-Liquid-Solid method) was employed in the lattice matched system (i.e. GaN nanostructure / AlGaN). The nanostructure size is adjustable by changing growth parameters (height: 2 ~ 15nm and diameter: 10 ~ 100nm). It has been found that the photon emission energy is tunable relative to the nanostructure size, and smaller nanostructures have larger photon energy. However, a numerical modeling was performed to investigate the relationship between quantum confinement (and/or piezoelectric polarization) and the dot size. For dot height < 4.1nm, the confinement effect is larger than the piezoelectric effect, otherwise the piezoelectric effect is more dominant. In addition, GaN nanostructures grown on Al0.15Ga0.85N have smaller lattice mismatch (less than 0.5%) than the GaN nanostructures grown on AlN. Therefore, the quantum confinement in a GaN/Al0.15Ga0.85N system is more dominant in determining photon emission energy than in a GaN/AlN system. The nanostructure advantages of quantum confinement and high thermal stability have been studied for the achievement of room temperature ferromagnetism using TM (transition metal; Mn or Fe). The transition metal (Mn or Fe) enhances nucleation of islands, resulting in size and density improvements. The magnetization measurements revealed magnetic properties of ferromagnetic nanostructure. Especially, room temperature ferromagnetism was observed in GaFeN nanostructures, which can contribute to ferromagnetic semiconductors operating above room temperature.

Radiative properties of a material is the core of thermal science and optics, which play critical roles in modern technologies, including microelectronics, energy conversion, and nanotechnology. The key to modify or enhance radiative properties is employing one-, two-, and three-dimensional (1, 2, and 3D) periodic micro/nanostructures. Since their applications are not fully uncovered and very few comprehensive studies are available, the objective of this dissertation is to explore applications of periodic micro/nanostructures with modified radiative properties in modern technologies through both numerically and experimentally investigations. Theses representative applications include the thermal control in rapid thermal processing, the design of a wavelength-selective radiator for thermophotovoltaic systems, and the nanothermal manufacturing. The theoretical foundation of the study is built on the rigorous coupled-wave analysis (RCWA) for numerical calculation of the far-field radiative properties and the electromagnetic field distribution in the near-field regime. Measurements of diffraction efficiencies are conducted on fabricated 1D and 2D periodic silicon microstructures with a laser scatterometer/diffractometer with high angular resolution. The diffraction efficiency can be employed for non-contact surface profile inspection tool because it strongly depends on structure patterns. For better temperature control during rapid thermal processing, the dissertation performs a parametric study on radiation absorption of a generic CMOS device together with its simplified nanoscale structures. The applicability of approximation models, which homogenize micro/nanostructures into a film, is also evaluated. Next, a new concept of complex gratings is proposed for actively tailoring the radiative properties and serving as a thermophotovoltaic (TPV) radiator. The radiator exhibits a wide-band and angle-independent high transverse magnetic wave emittance matching the bandgap of TPV cells so that the energy conversion efficiency can be improved. Furthermore, the nanoscale metallic slit arrays show polarization-dependant enhanced transmission and highly localized electromagnetic energy density, which hold promising potentials in nanothermal manufacturing. Three submicrometer metallic slit arrays are fabricated on top of a silicon substrate. Their spectral transmittance is measured with a Fourier-transform infrared spectrometer and largely agrees with RCWA modeling results. In short, the dissertation clearly demonstrates that precise control and tuning of radiative properties using micro/nanofabrication are not only feasible but also may have numerous technological impacts.

Thesis (Ph. D.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 66-71). Also available on the World Wide Web.

Thèse de doctorat : Sciences chimiques. Polymères : Bordeaux 1 : 2008. / Titre provenant de l'écran-titre.

Thèse doctorat : Physique. Physique de la Matière Condensée : Strasbourg 1 : 2006. / Titre provenant de l'écran-titre. Bibliogr. 5 p.

Thèse de doctorat : Sciences Odontologiques : Strasbourg : 2009. / Thèse soutenue sur un ensemble de travaux. Titre provenant de l'écran-titre. Bibliogr. 9 p.

Reproduction de : Thèse doctorat : Physique : Lyon 1 : 2003. / Titre provenant de l'écran titre. 289 Réf. bibliogr.

Thesis (Ph.D.)--University of Texas at Arlington, 2008.