thicker or of higher density regions will scatter more strong and will appear darker in image [4]. This is known as a bright field image. For a crystalline specimen, the objective aperture can used to selected either the deflected beam and hence generate a bright-field image or a specific diffracted beam to form the final image, thus giving strong contrast to show the regions that are diffracting strongly. It is possible therefore to obtain important features about crystal defects such as stacking faults, dislocations and precipitates. As in the absence of a specimen the background appears dark, the final image is called dark field image [4]. Image contrast for thin specimens can be enhanced by using small objective apertures (mass-thickness and diffraction contrast). However, use of small apertures limits the resolution achievement in the final image inevitably since many scattered electrons may be stopped by the objective aperture. For high resolution imaging, a large objective aperture is therefore desirable in order to maximize resolution. The most significant contrast mechanism is then that of phase contrast produced by phase changes introduced by the inherent aberrations of the microscope lens system and the degree of image defocus [11]. 3.2.2 High resolution EM HREM is one of the most important ways to directly study the crystallographic structure and property of material at the atomic-scale. In HREM, many diffracted beams are allowed to pass through a large objective aperture. These beams then interact with the transmitted beam to form an interference pattern. For a suitably orientated crystalline sample the net result is the formation of periodic lattice fringe patterns in the image plane which can be interpreted on the basis of the spacing and the corresponding intersection angles of crystallographic lattice planes [1, 11]. The physics of image formation in high resolution in high resolution TEM is a vast subject [28]. However, for the purposes of this thesis an explanation using the phase object approximation will be considered. In the phase object approximation, it is assumed that the amplitude changes experienced by the transmitted wave (due to inelastic scattering) are small. Further simplifications which are sometimes made are: (i) that the electron waves are only scattered once on their way through the specimen and that (ii) the diffracted wave amplitudes are extremely weak compared with the
thicker or of higher density regions will scatter more strong and will appear darker in image [4]. This is known as a bright field image. For a crystalline specimen, the objective aperture can used to selected either the beam and hence generate a bright-field image or a specific diffracted beam to form the final image, thus giving strong contrast to show the regions that are diffracting strongly. It is possible therefore to obtain important features about crystal defects such as stacking faults, dislocations and precipitates. As in the absence of a specimen the background appears dark, the final image is called dark field image [4]. Image contrast for thin specimens can be enhanced by using small objective apertures (mass-thickness and diffraction contrast). However, use of small apertures limits the resolution achievement in the final image inevitably since many scattered electrons may be stopped by the objective aperture. For high resolution imaging, a large objective aperture is therefore desirable in order to maximize resolution. The most significant contrast mechanism is then that of phase contrast produced by phase changes introduced by the inherent aberrations of the microscope lens system and the degree of image defocus [11]. 3.2.2 High resolution EM HREM is one of the most important ways to directly study the crystallographic structure and property of material at the atomic-scale. In HREM, many diffracted beams are allowed to pass through a large objective aperture. These beams then interact with the transmitted beam to form an interference pattern. For a suitably orientated crystalline sample the net result is the formation of periodic lattice fringe patterns in the image plane which can be interpreted on the basis of the spacing and the corresponding intersection angles of crystallographic lattice planes [1, 11]. The physics of image formation in high resolution in high resolution TEM is a vast subject [28]. However, for the purposes of this thesis an explanation using the phase object approximation will be considered. In the phase object approximation, it is assumed that the amplitude changes experienced by the transmitted wave (due to inelastic scattering) are small. Further simplifications which are sometimes made are: (i) that the electron waves are only scattered once on their way through the specimen and that (ii) the diffracted wave amplitudes are extremely weak compared with the deflected