Thermal rearrangement of tungsten field emitters in the presence of adsorbed oxygen

Abstract

The effect of heating at temperatures in the range 1000–1500°K on oxygen-covered tungsten surfaces has been studied by field emission and field ion microscopy. The shape of the tungsten emitter changed from the initial field evaporated form to a more polygonal form, which varied with the oxygen coverage. The form of the field emission patterns was determined by electron emission from the highly curved regions of the surface between the enlarged (211), (100), (310) and (110) facets. The intensity of electron emission and the final shape of the emitter indicated that for sub-monolayer coverages the equilibrium coverage of the (100) region was less than for other surface regions, showing that the binding energy is least for oxygen adsorbed on (100) surfaces. The change in shape of partly oxygen-covered emitters occurred by the same type of surface tungsten atom displacement as rearrangement of clean field-evaporated tungsten emitters. Adsorbed oxygen hindered surface diffusion of tungsten.