The Theory And Application Of Electron Energy Loss Spectroscopy In Anisotropic Materials

Due to the complex orientation effect, it is almost impossible toquantitatively study the electronic structure of anisotropic materials byelectron energy loss spectroscopy (EELS). In this thesis, we haveestablished a model to describe the inelastic scattering in the anisotropicsystem, using the inelastic scattering theory. Our model provides a physicalexplanation for the orientation effect in the core-level electron energy lossspectroscopy. It firstly describes the quantitative dependence of the EELSfine structure on both the experimental collection condition and the sampleorientation. At the same time, our model predicts the angular distribution ofthe inelastic scattering electrons following the excitation of an anisotropicelectronic transition. More importantly, we derived the magic angle (MA)condition which eliminates the orientation effect in EELS experiments foranisotropic systems. This provides a universal and directly interpretablemethod of investigating intrinsic electronic structure of materials even atnano-scale.The introduction of relativistic effect in our coherent model has led us torecognize the importance of the quantum interference effect between thelongitudinal and transverse interactions between the fast electron and thematter. The quantum interference term, which is ignored in the traditionalinelastic scattering theories, has been found to be essential for the anisotropicsystem.Based on a precise determination of experiment parameters and thecareful processing of the experimental results, we have measured systemicallythe angular distribution of inelastic electron and the magic angle condition forEELS. The dependence of magic angle condition, on both the initial electronenergy and the energy loss in inelastic scattering, has been found to be in agood agreement with the prediction of our coherent model. They give thedirect experimental evidence that the quantum interference term exists.The quantitative understanding of the connection between the anisotropicelectron structure, the sample-orientation information, and the experimentalconditions has allowed us to develop a series of new experimental methods toinvestigate the anisotropic materials. For example, we have shown that theanisotropic EELS could be expressed as the sum of the magic angle EELS(MAEELS) and the dichroic EELS (DEELS), whose physical meaning andmeasurement have been detailed in our thesis. As an example of usefulapplications, we have shown that the EELS signal can allow us to determinethe existence of a preferred orientation of sp2 bonded atoms in a pulsed laserdeposited amorphous carbon thin film and its disappearance with metal atomco-deposition. Such information allows us to relate the change in theirphysical properties to modification in the microstructure of the amorphousthin films. Lastly, through an understanding of the optics of the EELSspectrometer, we have suggested that it is possible to extract the 3-Dorientation information from the 2-D CCD recording of the EELS signals.Through an experiment with BN crystal particles, we have shown that thismethod has the potential of precise orientation determination with very highspatial resolution.