Methylthiolate on Au(111): adsorption and desorption kinetics

Abstract

Low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy and line of sight mass spectrometry have been used to study the adsorption and desorption of dimethyldisulfide (DMDS) on Au(111). At 300 K adsorption is dissociative, forming a chemisorbed adlayer of methylthiolate with a 1/3 ML, (√3 × √3)R30°, structure. At 100 K adsorption is molecular, with dissociation to form the 1/3 ML (√3 × √3)R30° methylthiolate structure occurring at 138–160 K. A physisorbed DMDS layer, with a coverage of 1/6 ML of DMDS, forms on top of the (√3 × √3)R30° chemisorbed MT surface for T ≤ 180 K, with multilayers forming for T ≤ 150 K. In temperature programmed desorption, multilayers of DMDS desorbed with zero order kinetics and an activation energy of 41 kJ mol⁻¹; the physisorbed layer desorbed with first order kinetics, exhibiting repulsive lateral interactions with an activation energy which varied from 63 kJ mol⁻¹ (θ = 0) to 51 kJ mol⁻¹ (θ = 1); the chemisorbed methylthiolate layer desorbed associatively as DMDS via the physisorbed layer, the activation energy for the reaction, 2 methylthiolate → physisorbed DMDS, exhibiting repulsive lateral interactions with an activation energy which varied from 65 kJ mol⁻¹ (θ = 0) to 61 kJ mol⁻¹ (θ = 1). The physisorbed disulfide layer explains the pre-cursor state adsorption kinetics observed in sticking probability measurement, while its relatively facile formation provides a mechanism by which thiolate self-assembled monolayers can become mobile at room temperature.