In this thesis, I report on the investigation of the low frequency vibrational modes in a number of different systems using ultrafast spectroscopic techniques. These consist of biological systems, including the enzyme, morphinone reductase (MR) and the related biomolecules, riboflavin (Rb) and flavin mononucleotide (FMN), as well as non-biological systems, including the semiconductor gallium nitride (GaN) and gold nanoparticles (Au NPs). The term low frequency refers to terahertz (THz) frequencies, where vibrational modes exist at the molecular level, with molecular rotations, lattice vibrations and inter- and intra-molecular vibrations occurring in the THz spectral range. These vibrational modes occur on sub-picosecond timescales and therefore ultrafast techniques utilising femtosecond laser pulses provide a means of studying these modes, and are employed throughout this thesis. The two ultrafast techniques of transient absorption (TA) spectroscopy and terahertz time-domain spectroscopy (THz-TDS) were used. Firstly, a high-repetition rate transient absorption (HRRTA) spectrometer was commissioned to perform pump-probe measurements with an ultraviolet pump and broadband visible probe. The performance of the HRRTA spectrometer was benchmarked using Au NPs and used to investigate the existence of a promoting vibration in MR contributing to the catalysis process, predicted to occur at THz frequencies. Weak oscillations were detected in the charge-transfer absorption band of MR bound to the non-reactive cofactor 1,4,5,6-tetra-hydro-nicotinamide adenine dinucleotide (NADH4), with a frequency of approximately 1.5 THz and provide evidence of the first direct observation of a promoting vibration in an enzyme. To complement the TA measurements, THz-TDS was also used to obtain direct measurements of the absorption at THz frequencies. Due to the challenge of studying water-based biological samples, an initial investigation was performed on a wurtzite GaN wafer, which exhibited optical phonon modes in the THz frequency range that were found to determine the dielectric response of the semi-insulating semiconductor wafer. Use of a non-polar m-plane wafer allowed the anisotropic nature to be observed and values of 9.22 ± 0.02 and 10.32 ± 0.03 for the static dielectric constants were obtained for the THz electric field polarised both perpendicular and parallel to the c-axis of the wurtzite GaN wafer, respectively. Finally, biological studies using THz-TDS were performed with measurements on Rb pellets and films revealing vibrational modes in the THz region. The sharp absorption features were not observed in FMN, despite a small difference in molecular content from Rb, and dehydration was required to reveal small amplitude absorption features. Final measurements on MR and MR-NADH4 films were carried out and evidence of absorption features in the THz frequency range were observed, however further work is required to determine the precise origin of these features.

Zhang Ailing. / "September 2004." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the academic requirements for the degree of Doctor of Philosophy. Johannesburg, 2014. / Unable to load abstract.

This thesis describes the synthesis, characterization and photocatalytic applications of carbon nitride (C3N4) and titanium dioxide (TiO2) materials. C3N4 was prepared from the thermal decomposition of a trichloromelamine (TCM) precursor. Several different reactor designs and decomposition temperatures were used to produce chemically and thermally stable orange powders. These methods included a low temperature glass Schlenk reactor, a high mass scale stainless steel reactor, and decomposition at higher temperatures by the immersion of a Schlenk tube into a furnace. These products share many of the same structural and chemical properties when produced by these different methods compared to products from more common alternate precursors in the literature, determined by infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and elemental analysis. C3N4 is capable of utilizing light for photocatalysis due to its moderate band gap (Eg), measured to be between 2.2 and 2.5 eV. This enables C3N4 to be used in the photocatalytic degradation of organic dyes and the production of hydrogen via the water-splitting reaction. C3N4 degraded methylene blue dye to less than 10% of its initial concentration in less than an hour of UV light illumination and 60% under filtered visible light in 150 minutes. It also degraded methyl orange dye to below 20% in 70 minutes under UV light and below 60% in 150 minutes under visible light. Using precious metal co-catalysts (Pt, Pd, and Ag) photo-reduced onto the surface of C3N4, hydrogen was produced from a 10% aqueous solution of triethanolamine at rates as high as 260 μmol h-1 g-1. C3N4 was also modified by mixing the precursor with different salts (NaCl, KBr, KI, KSCN, and NH4SCN) as hard templates. Many of these salts reacted with TCM by exchanging the anion with the chlorine in TCM. The products were mostly prepared using the high temperature Schlenk tube reactor, and resulted in yellow, orange, or tan-brown products with Eg values between 2.2 and 2.7 eV. Each of these products had subtle differences in the IR spectra and elemental composition. The morphology of these C3N4 products appeared to be more porous than unmodified C3N4, and the surface area for some increased by a factor of 4. These products demonstrated increased activity for photocatalytic hydrogen evolution, with the product from TCM-KI reaching a peak rate as high as 1,300 µmol h-1 g-1. C3N4 was coated onto metal oxide supports (SiO2, Al2O3, TiO2, and WO3) with the goal of utilizing enhanced surface area of the support or synergy between two different semiconductors. These products typically required higher temperature synthesis conditions in order to fully form. The compositions of the SiO2 and Al2O3 products were richer in nitrogen and hydrogen compared to unmodified C3N4. The higher temperature reactions with C3N4 and WO3 resulted in the formation of the HxWO3 phase, and an alternate approach of coating WO3 on C3N4 was used. The degradation of methyl orange showed a significant increase in adsorption of dye for the composites with SiO2 and Al2O3, which was not seen with any of the individual components. The composite between C3N4 and TiO2 showed improved activity for hydrogen evolution compared to unmodified C3N4. The surface of TiO2 was modified by the reductive photodeposition of several first row transition metals (Mn, Fe, Co, Ni, and Cu). This process resulted in the slight color change of the white powder to shades of light yellow, blue or grey. Bulk elemental analysis showed that these products contained between 0.04-0.6 at% of the added metal, which was lower than the targeted deposit amount. The Cu modified TiO2 had the largest enhancement of photocatalytic hydrogen evolution activity with a rate of 8,500 µmol h-1 g-1, a factor of 17 higher than unmodified TiO2.

Abstract not available

Thesis (M.S.)--State University of New York at Binghamton, Thomas J. Watson School of Engineering and Applied Sciences, Systems Science and Industrial Engineering Department, 2006. / Includes bibliographical references.

<p><p>In this work growth of cubic GaN in the selective area (SA) MOVPE process is</p><p>simulated. The simulations are restricted to small pattern SA MOVPE growth.</p><p>In this case the traditional MOVPE growth and the enhanced growth caused by</p><p>surface diffusion are important growth factors. The lateral vapor phase diffusion</p><p>is ignored while this process only has a small impact on the enhanced growth in</p><p>the small pattern SA growth. The model is build for simulation of anisotropic</p><p>growth. It has been shown that different type of anisotropic growth occurs when</p><p>the mask pattern are orientated in different directions on the substrate. While</p><p>the anisotropic growth is not well understood two different models are studied in</p><p>this work.</p><p>The simulation is restricted to the geometrical growth characteristics such</p><p>as mask and crystal width, mask alignment and surface diffusion on the crystal.</p><p>The reactor geometry, pressure and growth temperature are not investigated that</p><p>closely and are only treated as constants in the model.</p><p>The model used in this simulation gives good results for short time simulations</p><p>for some certain cases. The model shows that the fill factor has a greater</p><p>impact on the grown shapes than the individual mask and crystal width. But</p><p>there are problems with the anisotropic and flux from mask modeling while some</p><p>facets do not appear and the lateral growth along the mask show doubtful results.</p><p>The model show good results in short time growth and predict some important</p><p>characteristics in SA MOVPE.</p></p>

Uniformly distributed ZnO nanorods with diameter 70-100 nm and 1-2μm long have been successfully grown at low temperatures on GaN by using the inexpensive aqueous solution method. The formation of the ZnO nanorods and the growth parameters are controlled by reactant concentration, temperature and pH. No catalyst is required. The XRD studies show that the ZnO nanorods are single crystals and that they grow along the c axis of the crystal plane. The room temperature photoluminescence measurements have shown ultraviolet peaks at 388nm with high intensity, which are comparable to those found in high quality ZnO films. The mechanism of the nanorod growth in the aqueous solution is proposed. The dependence of the ZnO nanorods on the growth parameters was also investigated. While changing the growth temperature from 60°C to 150°C, the morphology of the ZnO nanorods changed from sharp tip (needle shape) to flat tip (rod shape). These kinds of structure are useful in laser and field emission application. / Singapore-MIT Alliance (SMA)

High density, uniform GaN nanodot arrays with controllable size have been synthesized by using template-assisted selective growth. The GaN nanodots with average diameter 40nm, 80nm and 120nm were selectively grown by metalorganic chemical vapor deposition (MOCVD) on a nano-patterned SiO2/GaN template. The nanoporous SiO2 on GaN surface was created by inductively coupled plasma etching (ICP) using anodic aluminum oxide (AAO) template as a mask. This selective regrowth results in highly crystalline GaN nanodots confirmed by high resolution transmission electron microscopy. The narrow size distribution and uniform spatial position of the nanoscale dots offer potential advantages over self-assembled dots grown by the Stranski–Krastanow mode. / Singapore-MIT Alliance (SMA)

In this work growth of cubic GaN in the selective area (SA) MOVPE process is simulated. The simulations are restricted to small pattern SA MOVPE growth. In this case the traditional MOVPE growth and the enhanced growth caused by surface diffusion are important growth factors. The lateral vapor phase diffusion is ignored while this process only has a small impact on the enhanced growth in the small pattern SA growth. The model is build for simulation of anisotropic growth. It has been shown that different type of anisotropic growth occurs when the mask pattern are orientated in different directions on the substrate. While the anisotropic growth is not well understood two different models are studied in this work. The simulation is restricted to the geometrical growth characteristics such as mask and crystal width, mask alignment and surface diffusion on the crystal. The reactor geometry, pressure and growth temperature are not investigated that closely and are only treated as constants in the model. The model used in this simulation gives good results for short time simulations for some certain cases. The model shows that the fill factor has a greater impact on the grown shapes than the individual mask and crystal width. But there are problems with the anisotropic and flux from mask modeling while some facets do not appear and the lateral growth along the mask show doubtful results. The model show good results in short time growth and predict some important characteristics in SA MOVPE.