Adsorption of sulfur on a molybdenum field emitter

Abstract

The adsorption system molybdenum/sulfur has been studied with a probe-hole field emission microscope. Elemental sulfur (S 2) impinging onto a clean molybdenum surface at room temperature is captured with high efficiency in a mobile precursor state. During dosing spreading of sulfur is anisotropic, with the highest rate occurring near the 〈111〉 zones and on the {110} vicinals. The precursor is transformed into chemisorbed sulfur on clean molybdenum surfaces of any orientation. On the saturated chemisorption layer additional sulfur can be physisorbed. On several surface orientations reaction to molybdenum sulfide occurs at slightly elevated temperatures. Work function measurements of the clean molybdenum surface result in φ 110 = 5.0 ± 0.15 eV, φ 100 = 4.70 ±0.03 eV, φ 211 = 4.64 ± 0.03 eV, φ 111 = 4.18 ± 0.01 eV and φ 544 = 4.28 ± 0.04 eV based on an average work function φ = 4.2 eV. In general, adsorption of sulfur increases the work function. After heating to 1000 K which effects decomposition of molybdenum sulfide the saturated chemisorption layer increases the work function by Δφ = 0.8−1.2 eV depending on the orientation. On heating to higher temperatures, work function changes indicate substrate rearrangement on several orientations and gradual desorption. Complete desorption occurs between 1680 K ({111} face) and 1800 K ({110} and {100} faces). Diffusion of chemisorbed sulfur exhibits distinct face specifity. The temperatures where migration becomes observable range from 320 K at the 〈111〉 zones up to 500–600 K near the {100} faces. Activation energies of diffusion in the low-concentration chemisorption layer from 56 kJ/mol on the {110} vicinals up to 142 kJ/mol on the {100} faces have been determined.