Hydrogen superstructures on Co nanoislands and Cu(1 1 1)

Surface structures and related electronic properties of flat Co nanoislands supported on Cu(1 1 1) are studied before and after adsorbing different doses of molecular hydrogen at 10 K by low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) at 5 K. It is found that the adsorption process on Co consists in two steps. First, H2 dissociatively chemisorbs on Co forming an atomic H adlayer. Subsequently, molecules impinging on that H-terminated Co surface are physisorbed. When adsorbing a low dose of hydrogen on the Co/Cu(1 1 1) system, STM analysis shows that the chemisorbed hydrogen adlayer on Co forms a p( 2 × 2 ) superstructure. On Cu surface however, no superstructure is observed. When adsorbing a high dose of H2, hydrogen-induced superstructures are observed on both Co and Cu surfaces. These superstructures are ascribed to the presence of physisorbed molecules as revealed by the STS spectra. A ( 3 × 3 ) superstructure is observed on Co and a mixture of ( 2 × 2 ) and ( 3 × 3 ) is identified on Cu. A model describing the adsorption sites of molecules is proposed: H2 molecules occupy on-top sites when arranged as a ( 2 × 2 ) phase and they occupy on-top and bridge sites when arranged as a ( 3 × 3 ) phase. Besides, STM-induced desorption is used to desorb atomic hydrogen from selected Co nanoisland surface. The desorption is visualized in terms of a disappearance of the p( 2 × 2 ) superstructure.