Structural investigation of the interaction of molecular sulfur with Ag(111)

Scanning tunneling microscopy (STM) and normal-incidence X-ray standing wave (NIXSW) analysis, together with low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS), have been used to investigate the structure of surface phases formed by the dissociation of molecular sulfur (82) on Ag(111) under ultrahigh vacuum conditions. Both a complex low coverage phase and the well-known Ag(111)(√7×√7)R19°-S surface have been investigated. In the case of the low coverage phase, the long-range periodicity seen in STM and the LEED pattern have been reconciled, while NIXSW data indicate a significant local coherence of the S atoms on the substrate. However, the complexity of this phase, with a minimum estimate of 11 S atoms per unit mesh, is too great for complete structural interpretation. A model for the (√7×√7)R190°-S phase, based on epitaxial growth of the high-temperature fee phase of Ag₂S(111), is found to be consistent with all the experimental data, while NIXSW provides information on the nature of the Ag(111)/Ag₂S(111) interface. In particular, this interface is proposed to involve S atoms in sites between the outermost unreconstructed Ag(111) surface and a 3/7 ML reconstructed Ag layer with the same periodicity as in the Ag₂S overlayer above. Some issues concerning the nature of the atomic-scale STM imaging, and the relationship to electrochemical studies of this interface, are discussed.