FIELD-EMISSION FROM ATOMIC SIZE SOURCES

The source area of electron field emission from ultrasharp [111]-oriented tungsten tips terminated by a few (down to one single) atoms is confined to the very end of the tip, thus forming an atomic size electron source. The most remarkable emission property of such tips, the angular dispersion of the emitted electron beam, is measured both qualitatively and quantitatively for well-defined atomic arrangements at the tip apex and also for a wide range of applied electric fields and currents. It is shown that only pyramidally shaped tips emit electrons in sharp beams, with small opening angles down to less than 3-degrees. The results suggest, in contrast to existing theories, that mesoscopic parameters, as the overall shape of the tip apex (on a scale of a few nanometers) and the associated patch field distribution, might play a crucial role in focusing the beam.