| Summary: | This dissertation concerns the development and use of a novel form of field emission electron source in a medium-voltage (100 kV) scanning transmission electron microscope (STEM). It is the properties of the electron source, and particularly its brightness, which determine the extent to which interference effects are observable at the detection plane. These features directly contribute to high resolution image contrast. Interest was stimulated into the possibilities of improving the brightness of electron sources by at least an order of magnitude, by Hans-Werner Fink in the late eighties. He developed a process whereby a tungsten emitter could be produced which terminated in a single atom (a so-called nano-tip). The electron beam from such a source has been shown to exhibit high coherence, good current stability, a small cone angle and confinement of the emission surface to the atomically-sized apex. Experiments, however, were confined to tips in simple field emission chambers and the low-voltage point-projection microscope. In this dissertation, nanometre-sized protrusions have been grown at the apeces of tungsten emitters using a method whereby the surface metallic layer of atoms is melted under the action of strong electrostatic fields and applied heat. Such tips have shown emission characteristics, within an un-modified 100 kV STEM, which correspond with those previously observed from nano-tips. An evaluation of the suitability of the STEM for future use of these tips is included. In addition to requiring low levels of electrical instabilities, a mechanically reinforced tip base assembly was necessary. This was designed, constructed and shown to be an improvement over that of the commercially available source. A method to measure absolute beam coherence values at the specimen plane, and hence characterise tips, was also evaluated.
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