by Cheung Man Kit. / Errata slip inserted / Bibliography: leaves 83-84 / Thesis (M.Ph.)--Chinese University of Hong Kong, 1985

by Chu Hon Yue. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1990. / Bibliography: leaves 91-93. / ACKNOWLEDGEMENT --- p.i / ABSTRACT --- p.ii / Chapter I. --- INTRODUCTION --- p.1 / Chapter II. --- LOW ENERGY ELECTRON DIFFRACTION --- p.4 / Chapter 2.1. --- LEED EXPERIMENT --- p.4 / Chapter 2.2. --- LEED CRYSTALLOGRAPHY：I(E) CURVE --- p.7 / Chapter 2.3. --- AN OUTLINE OF THE PHYSICS OF LEED --- p.8 / Chapter 2.3.1. --- LEED Process In One-Dimensional Surface --- p.8 / Chapter 2.3.2. --- LEED Process In Three-Dimensional Surface --- p.11 / Chapter 2.3.3. --- Surface With An Overlayer --- p.12 / Chapter 2.3.4. --- The Muffin-Tin Model And The Inner Potential --- p.13 / Chapter 2.3.5. --- The Phase Shift --- p.17 / Chapter 2.3.6. --- Thermal Effects --- p.19 / Chapter III. --- DYNAMICAL LEED CALCULATION： THE RENORMALIZED FORWARD SCATTERING PERTURBATION METHOD --- p.22 / Chapter IV. --- THE ZANAZZI-JONA RELIABILITY FACTOR --- p.30 / Chapter V. --- CALCULATIONS AND RESULTS --- p.33 / Chapter 5.1. --- EXPERIMENTAL I(E) CURVES --- p.33 / Chapter 5.2. --- CALCULATED I(E) CURVES --- p.36 / Chapter 5.2.1. --- Top Site --- p.4o / Chapter 5.2.2. --- Bridge Site --- p.41 / Chapter 5.2.3. --- Hollow Site --- p.42 / Chapter VI. --- DISCUSSIONS AND CONCLUSIONS --- p.44 / APPENDIX I --- p.51 / TABLES --- p.52 / I(E) CURVES AND CONTOUR MAPS --- p.61 / APPENDIX II --- p.79 / MAIN PROGRAMME FOR Rh(100)-c(2x2) -S --- p.80 / INPUT PARAMETERS FOR TOP SITE --- p.85 / INPUT PARAMETERS FOR BRIDGE SITE --- p.87 / INPUT PARAMETERS FOR HOLLOW SITE --- p.89 / REFERENCES --- p.91

Thesis (M.Phil.)--Chinese University of Hong Kong. / Bibliography: leaves 88-90.

This work presents findings from experiments carried out using the neutron Laue method in tandem with laboratory source X-ray diffraction to characterise a series of organic molecular complexes which exhibit interesting, and potentially “tunable”, temperature dependent charge transfer effects, such as proton migration and proton disorder within hydrogen bonded networks. These subtle processes are studied by variable temperature neutron diffraction studies, allowing the positional and anisotropic displacement parameters of the hydrogen atoms to be refined accurately and their evolution with temperature followed. The hydrogen atom behaviour is found to be influenced by the local environment, including weak intermolecular interactions in the vicinity of the hydrogen bond under study. Complexes of urea and methyl substituted ureas with small organic acids are presented, which show robust and reproducible structural motifs. In favourable circumstances, these contain short, strong hydrogen bonds (SSHBs) within which the proton may undergo temperature dependent migration. By synthesising a number of complexes containing SSHBs, potential routes to the design of proton migration complexes are found, which utilise crystal engineering principles and pKa matching. Variable temperature studies conducted on these complexes also show unusual thermal expansion properties and phase transitions in urea-acid complexes which do not display proton migration. Systems containing hydrogen bonded dimers of 3,5-dinitrobenzoic acid are also studied, and shown to contain temperature proton disorder within moderate strength hydrogen bonds linking the dimers. The presence and potential onset temperature of any disorder is found to be influenced by interactions around the acid dimers and potential routes to controlling proton disorder are discussed. Complexes of the proton sponge, 1,8-bis(dimethylamino)napthalene (DMAN), with organic acids are also presented, in which the structures have been determined using neutron diffraction. DMAN readily accepts a proton from the acid co-molecules used in forming the complexes, forming a strong intramolecular SSHB within the protonated DMAN. Strong intermolecular hydrogen bonds are also induced between the acid molecules in many cases. The neutron studies presented here investigate the effect of weak interactions on the behaviour of hydrogen atoms located within these SSHBs, and also indicate over what distance such interactions significantly affect the hydrogen atom behaviour.

Perovskite photovoltaics is one of the fastest growing opto-electronic technologies with device efficiencies currently exceeding 23%. The opportunity to deposit these abundant materials with large area solution processing techniques could make perovskites viable for low-cost production. However, since perovskite materials are prone to degradation, their lifetime needs to be improved to that of silicon solar cells before these devices can be commercialized. Moreover, unlike most semiconductors, trap densities in polycrystalline perovskite films in high-performing devices have been determined to be relatively large, suggesting a remarkable defect tolerance in perovskite films that needs to be understood in the context of the nature of the trap states and any residual non- radiative losses. These non-radiative losses are observed as photoluminescence heterogeneity within perovskite films, even for high-performing perovskite systems. In this work, we explore the degradation kinetics of perovskite devices under stress conditions and find that further stability improvements should focus on the mitigation of trap generation during ageing. Furthermore, we fabricate perovskite solar cells with a novel back-contact structure, in which electron- and hole-selective electrodes are co-positioned on the back side of the cell and spaced by 100 μm. By utilising grazing-incidence X-ray diffraction, we show that even in the earliest stages of conversion of precursors to perovskite we achieve remarkably high open-circuit voltages, suggesting that the defect tolerance of perovskites appears at an early stage in the conversion process. Moreover, we employ scanning X-ray diffraction with nanofocused beam and obtain detailed information, revealing overlapping grains located at different depths within perovskite films. We find that the critical grain size is the longer-range structural super-grains rather than the grains viewed with conventional microscopy techniques. These findings further highlight the presence of structural defects in perovskite materials and provide important insights towards improving the optoelectronic behaviour of these materials.

Glass-ceramic composites and nanocrystalline glass-ceramics containing barium titanate (BT) or potassium sodium niobate (KNN) ferroelectric phases have been studied, with particular regard to their use as dielectric energy storage materials. Barium borosilicate glass (BBS) was used as a sintering aid for BT and KNN, producing glass-ceramic composites. The temperatures required to achieve densification were reduced from 1400 to 1200 °C for BCZT (Ca, Zr-doped BT) and 1170 to 1050 °C for KBN (Bi, Na, K, Zr-doped KNN) by the use of glass additives. An unexpected observation, found in both BCZT and KBN systems, was the heterogeneous dissolution of dopant elements into the glass, inducing additional anomalies in the relative permittivity-temperature relationships. For BCZT, the orthorhombic-tetragonal phase transformation temperature shifted upwards to ≈ 50 °C, which was attributed to modification of the Ca/Zr ratio by preferential dissolution of Ca into the glass phase. Similarly, for KBN the dopant elements appeared to be leached into the liquid phase during sintering, resulting in relative permittivity-temperature characteristics similar to those of pure KNN. A modified BBS glass having various KNN contents was prepared by the conventional melt-quenching method and then heat-treated to induce crystallisation, producing nanocrystalline glass-ceramics. It is shown that crystallisation of an intermediate barium niobate phase initiates at temperatures in the region of 650 °C; this is subsequently converted into perovskite KNN together with a second phase of Ba3Nb5O15 at temperatures from 700 to 800 °C. The final crystallite size was in the region of 30±7 nm. The highest dielectric energy storage density of 0.134(4) J cm-3 was obtained for a glass-modified BT ceramic at an electric field level of 5 kV mm-1. However, the energy storage efficiency of the BT-based ceramics was relatively poor and they displayed a general tendency for saturation, indicating potentially poor performance at higher field levels. On the other hand, the KNN-based ceramics exhibited slightly lower energy storage density values, up to 0.108(1) J cm-3, but with much improved linearity and energy storage efficiency. Therefore, the latter is considered to be more suitable as energy storage dielectrics. The BBS-KNN glass-ceramics yielded relatively low energy storage density, 0.035(2) J cm-3, but the dielectric linearity and storage efficiency were similar to or better than those of the KNN ceramics, indicating good potential for use as energy storage dielectrics at very high electric field levels as a result of their nanocrystalline microstructures.

Fiber Bragg grating sensors have been constantly researched for the last ten years and have finally begun to find use in the commercial market. However, one of the major factors limiting their widespread use is their system cost. Their lightweight, flexibility, electromagnetic immunity, and small size make fiber Bragg grating (FBG) sensors feasible in hostile environments where electrical and mechanical sensors may not function effectively. These sensor systems utilize moderately expensive light sources and detectors at telecommunication wavelengths of 1300 nm and 1550 nm. These are the center wavelengths of the mass-produced FBGs and FBG phase masks. This thesis addresses the development of a lower cost short wavelength fiber Bragg grating strain sensor system using gratings written at 790 nm and 850 nm with the modified phase mask method recently developed at Oregon State University. Short wavelength gratings allow the use of less expensive semiconductor sources and silicon detectors, greatly reducing the overall cost of a strain sensor system from approximately $1600 for a 1300 nm system to $1000 for a 790 nm system. First, the fundamental properties and historical background of fiber Bragg gratings were reviewed. Followed by a literature review of the structures, fabrication methods, and applications of FBGs including sensor applications. The design, manufacture, and assembly of the new short wavelength strain sensor system were described including the production of pigtailed super-luminescent edge emitting light emitting diodes (SELED) from commercial laser diodes, a fiber recoater, and multiple attempts to write a fiber Bragg grating in the 750-850 nm wavelength region. Finally, the short wavelength strain sensor system was compared with a 1300 nm strain sensor detailing the potential cost savings with the short wavelength system. / Graduation date: 2003

Thesis (M.S.)--Ohio State University, 1984. / Includes bibliographical references (leaves 118-119). Available online via OhioLINK's ETD Center

Fluorescence microscopy offers the opportunity to image noninvasively biological samples in real time. However, the phenomenon of diffraction limits the resolution of conventional fluorescence microscopes to submicrometer dimensions in both the horizontal and vertical directions. This limitation can be overcome by photoswitchable fluorescent probes able to undergo reversible saturable optically linear fluorescence transitions (RESOLFT). In this study, firstly, a photoswitchable fluorescent probe based on BODIPY fluorophore and Spiropyran photochrome were designed and its photophysical and photochemical properties were investigated in organic and aqueous environments. Also, its imaging with patterned illumination was showed by trapping them in PMMA matrix. Secondly, photochromic [1,3]oxazines with different substituents as well as polymers incorporating them were synthesized and their photochemical and photophysical properties were investigated. Thirdly, to improve the switching speeds and fatigue resistance of the BODIPY-Spiropyran conjugate, the photochromic part was replaced by [1,3]oxazines and dyads incorporating BODIPY fluorophore and [1,3]oxazine photochromes were synthesized. Lastly, a new strategy was designed to switch the fluorescence of fluorophores by a modular approach. It is based on photoinduced elongation of the absorption wavelength of a fluorescent chromophore with the aid of an appended photochromic auxochrome.

Ultrafast electron diffraction (UED) is a method of directly imaging system dynamics at the atomic scale with picosecond time resolution. In this thesis I present theoretical analyses of the experimental processes, and construct models in order to better understand UED experiments and to guide future refinements. In particular, I derive a model of electron bunch propagation and a model of electron bunch diffraction, where both models take into account all bunch parameters. To analyse the propagation of electron bunches, I present a mean-field analytic Gaussian (AG) model. I derive a system of ordinary differential equations that are solved quickly and easily to give the bunch dynamics. The AG model is compared to N -body numerical simulations of initially Gaussian bunches, and I demonstrate excellent agreement between the two result sets. I also present a comparison of the AG model with numerical simulations of quasi-Gaussian and non-Gaussian distributions, extending the applicability of the AG model to the propagation of ``real-world'' bunches. During propagation, electron bunches can be shaped by electron-optic devices, which are necessary to attain high brightness, sub-100 fs bunches. I investigate two types of electron-optic devices: one is a magnetic lens used for collimating or focusing bunches, the other is a bunch compressor. I derive bunch parameter transformations for each of the electron-optic devices, and present numerical calculations using these transformations along with the AG model showing the effects of the devices on the evolution of the bunch parameters. To analyse electron bunch diffraction in UED experiments, I present a general scattering formalism. Using single-scattering and far-field approximations, I derive an expression for the diffracted signal that depends on the electron bunch properties just before scattering. Using this expression I identify the transverse and longitudinal coherence lengths and discuss the importance of these length scales in diffraction pattern formation. I also discuss the effects of different bunch parameters on the measured diffracted flux, and present sample numerical calculations for scattering by nanosize particles based on this model. This simulation demonstrates the cumulative effects of the bunch parameters, and shows the complex interplay of the bunch and target properties on the diffracted signal.