Adsorption of sulfur on a tungsten field emitter

Abstract

The adsorption of sulfur on a tungsten field emitter has been studied with a probe hole field emission microscope. If a beam of S 2 molecules impinges on the tip at a slanting angle from the rear a chemisorbed layer is formed at the observable part of the tip via mobile adsorbed molecules. The highest mobility of these adsorbed molecules is observed on the smooth regions near the (211) and (110) faces, whereas barriers hinder the diffusion onto the (110) and (111) faces. The diffusion of chemisorbed sulfur has been investigated by heating tips with spatially non-uniform sulfur deposits. These experiments indicate distinctly face specific and coverage dependent diffusion of adsorbed sulfur. On a fully sulfur covered tungsten tip after heating to 300°C the work functions are increased by ΔΦ (110) = 0.4 eV, ΔΦ (100) = 0.45–0.5 eV, ΔΦ (111) = 0.8 eV and ΔΦ (310) = 0.85–0.9 eV. The Fowler-Nordheim parameters and the FEM images obtained after further stepwise heating indicate face specific changes in the adlayer. Complete desorption of sulfur is achieved between 1300° and 1550°C depending on the substrate orientation. The Fowler-Nordheim data and the diffusion experiments suggest the existence of several distinct adsorption states.