Thesis (M. Phil.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

An 89 mm-diameter vertical tubular-flow reactor was used to study the kinetics of nano-sized silicon nitride powder synthesis via the animonolysis of SiO vapor at temperatures ranging from 1300°C to 1400°C. The SiO generation rate was controlled by adjusting the mass of SiO particles initially charged in the SiO generator, when the flow rate of carrier gas argon was maintained unchanged. The molar feed ratio of NH₃/SiO at the feeder outlets was maintained in large excess of the stoichiometric ratio ranging from about 100 to 1200 mol NH₃/mol SiO. The SiO-NH₃ reaction yielded two different morphologies of silicon nitride products at different locations in the reactor: nano-sized powder with an averaged particle size of about 17 nm and whiskers with a variety of shapes and diameters of a few micrometers. Nano-sized powder was the dominant product in the system and its mass fraction over the total product varied from 83% to 100%, depending on operating conditions. The contact pattern between SiO vapor and NH₃ inside the reacting zone was one of the most important parameters that affected Si₃N₄ formation kinetics. When a small single tube was employed for feeding NH₃ (flow condition J), a highest efficiency of SiO vapor utilization was achieved at a high level of SiO conversion. The SiO conversion increased from 72% to 91% with an increase in the residence time from 0.17 s to 0.69 s, indicating that the SiO-NH₃ reaction was not instantaneous but was relatively fast. When the molar feed rate of NH₃ was 2-3 orders of magnitude greater than that of SiO vapor, the rate of nano-sized powder synthesis was independent of NH₃ concentration and of first order with respect to the SiO concentration. A pseudo-first order rate expression was proposed, and the apparent activation energy was determined to be 180 kJ/mol. The gas flow in the reactor simulated with a computational fluid dynamic program revealed that whisker formed where the stagnation of gas flow formed. A power law rate expression for whisker formation was proposed based on measured rates of whisker formation and simulation-predicted reactant-gas concentrations. / Graduation date: 2004

Herein, a program of research is detailed related to transport through the Si metal oxide semiconductor (MOS) quantum dots fabricated in a process flow compatible with modern ULSI (ultra large scale integrated circuit). Silicon quantum dots were fabricated by placing split gates within a MOSFET structure. Quantum dots of several sizes and geometries were fabricated by this process for the purpose of investigating the effects of size and shape on quantized transport through the dots. The transport properties of the different quantum dot sizes and shapes were investigated at low temperatures, and compared to normal MOSFETs fabricated by the same technology. Equilibrium measurements with the device biased in the regime from the onset of weak inversion to just past the onset of strong inversion revealed strongly oscillatory behavior in the tunneling conductance. The conductance peaks appear to map an energy level spectrum in the dot as the inversion and depletion gates are separately swept. Symmetric devices, biased both symmetrically and asymmetrically, show two groups of "branches" which evolve with different slopes in the V[subscript Inv]-V[subscript Depl] plane. An asymmetric device studied shows three groups of branches. In addition, a fine structure is observed in the conductance peak behavior of two devices. This apparent energy level structure is compared to the body of literature on the so-called artificial atoms, as well as self-consistent three dimensional quantum mechanical solutions for the energy levels in the same dot structure, which qualitatively agree with the overall slope of the observed data. However, the calculations reveal only the multiple sets of slopes when asymmetrically biased. These multiple slopes are postulated to arise due to the splitting of the degenerate states of the symmetric structure as the bias makes the structure increasingly asymmetric. Finally, a simplified model is presented which shows how slight asymmetry in the dot confining potential can give rise to both a fine structure and multiple slopes in the branches, and several alternative mechanisms are presented to explain the origin of the fine structure observed. / Graduation date: 2000

Graduation date: 1996

Graduation date: 1996

Graduation date: 1995

Introduction: To evaluate the effectiveness of smoking cessation intervention, reliable outcome is essential. Exhaled nitric oxide (ENO) is decreased in smokers, tends to normalize after cessation and might be a good tool to evaluate abstinence. Objective: To evaluate changes in ENO after smoking abstinence of 7 or more days. Methods: 58 smokers in a cessation attempt and 12 non-smokers were recruited: 7 visits for smokers and 2 for non-smokers. Carbon monoxide and cotinine were used to detect smoking status. Results: ENO is decreased in smokers compared to non-smokers (10.8 vs. 20.1 ppb, p<0.001). There was no significant difference in ENO pre and post quitting (p=0.080) although there was a trend to increase as early as 3 days after abstinence (10.78 vs. 15.11, p>0.05). There were no differences in nasal NO measurements (p=0.278). Conclusion: ENO doesn’t seem to be a reliable marker of short-term abstinence.

Introduction: To evaluate the effectiveness of smoking cessation intervention, reliable outcome is essential. Exhaled nitric oxide (ENO) is decreased in smokers, tends to normalize after cessation and might be a good tool to evaluate abstinence. Objective: To evaluate changes in ENO after smoking abstinence of 7 or more days. Methods: 58 smokers in a cessation attempt and 12 non-smokers were recruited: 7 visits for smokers and 2 for non-smokers. Carbon monoxide and cotinine were used to detect smoking status. Results: ENO is decreased in smokers compared to non-smokers (10.8 vs. 20.1 ppb, p<0.001). There was no significant difference in ENO pre and post quitting (p=0.080) although there was a trend to increase as early as 3 days after abstinence (10.78 vs. 15.11, p>0.05). There were no differences in nasal NO measurements (p=0.278). Conclusion: ENO doesn’t seem to be a reliable marker of short-term abstinence.