Sulfur Structures on Bare and Graphene-Covered Ir(111)

We present a study of sulfur adsorption on bare Ir(111). Two well-defined superstructures are found: a (3×3)R30°and a c(4 × 2) S-adlayer. Moreover, we also investigate sulfur intercalation of graphene on Ir(111). For adsorption, sulfur is provided either in the form of the precursor molecule H₂S or as elemental sulfur through sublimation from FeS₂ heated in a Knudsen cell. On the basis of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED), as well as density functional theory calculations (DFT), we present a model for the c(4 × 2) superstructure consistent with surface relaxations. We show that above a graphene coverage threshold, when islands of the two-dimensional (2D) material start to coalesce, the sulfur superstructure intercalated below graphene depends on the form in which sulfur is provided: c(4 × 2) forms in the case of exposure to elemental sulfur, while the (3×3)R30° superstructure forms in the case of H₂S exposure. The two intercalation structures influence the graphene moiré corrugation in different ways. We have used DFT calculations to determine sulfur adsorption energies, surface relaxations, and the influence of sulfur intercalation on the density of electronic states of graphene on Ir(111).