Atomic and molecular hydrogen superstructures on Co(111) nanoislands grown on Cu(111)

Atomic and molecular hydrogen superstructures on Co(111) nanoislands grown on Cu(111) Muriel Sicot Eindhoven University of Technology jeudi 22 mai 2008 , 18h00 Conférence présentée à l'atelier (2008) Surface structures and related electronic properties of flat Co nanoislands supported on Cu(111) exposed to molecular hydrogen at low temperature have been investigated by scanning tunneling microscopy/spectroscopy (STM/STS). First, prior to hydrogen adsorption, the Co islands and the Cu substrate exhibit surface states. Moreover, the Co islands are shown to be magnetic at T=5K by spin-polarized STM. Second, the Co/Cu(111) system is exposed to a low dose of hydrogen. At adsorption temperature Tads around 10K, STM/STS analysis show that Cu is a hydrogen free surface. Contrary to Cu, STM images of the Co surface show a hydrogen adlayer forming a p(2 x 2) superstructure. The corresponding STS reveals a shift of the Co d surface state from -0.3 eV to -0.5 eV. At Tads=78K, the p(2 x 2) superstructure is again observed on Co. These results suggest that the p(2 x 2) structure is induced by dissociative chemisorption of hydrogen on cobalt. Preferential adsorption areas on a Co nanoisland were monitored by sequential STM-imaging carried out at T=78 K. The real-time STM observation enabled visualization of the adsorption process. It clearly demonstrates that hydrogen first adsorbs at the edges prior to fill progressively the inner region of the island. Third, the Co/Cu(111) system is exposed to a high dose of H2. At Tads around 10K, STS spectrum measured on both Co and Cu reveals a prominent peak of differential conductance centered around zero bias ascribed to the presence of physisorbed molecules. Our results suggest that H2 first dissociatively chemisorbs on Co forming an atomic H adlayer. Subsequently, molecules impinging on that H-terminated Co surface are physisorbed. Upon high exposures, different phases on Co are formed and a mixture of (2 x 2) and (3 x 3) phases is identified on Cu. A model describing the adsorption sites of the molecules is proposed.