Quantitative Convergent Beam Electron Diffraction and its Applications in Studying Crystal Structure and Bonding.
The aim of the research in this dissertation has been to develop new experimental methods for the highly accurate measurement of the amplitudes and phases of the Fourier coefficients of the ground state potential in crystals, and applying them to study the charge distribution of crystals and their bonding. The basic experimental technique used to achieve this goal is the quantitative Transmission Convergent Beam Electron Diffraction (TCBED). In the process of this research, a new technique was developed which allows the phases of low order reflections to be measured in non-centrosymmetric crystals. The theory foundations of all applications in this dissertation was laid, and the program 'TCBED', based on the described theory, was developed to calculate the many-beam electron dynamical diffraction for all applications of this dissertation. Various intensity measurement methods are studied. A powerful electron intensity measurement system was developed, which use an Energy Loss Spectrometer for detection of electrons so that the electron intensity of different energies can be selectively measured. The CBED systematics method for measuring structure factor amplitudes was described and applied to the measurements of low-order structure factors of GaAs. The experimental results were compared to theory and used to study crystal bonding of GaAs. Good agreements were found between the experimental results and the theoretical results by the pseudopotential method by Nielson and Martin. Two techniques for the accurate measurement of the phase of structure factor were developed. with an understanding of the three -beam electron diffraction. One is the critical voltage CBED systematics method, and the other is the CBED nonsystematics method. Both techniques were applied to the phase invariant measurement of CdS. The advantages and disadvantages of both methods were compared and discussed. Some conclusions and suggestions are presented.
- Pub Date:
- Physics: Condensed Matter