Surface morphology and adlayer structure of Se on Rh(111).

The deposition of Se from SeO solutions was examined in the submonolayer regime by cyclic voltammetry, scanning tunneling microscopy and atomic force microscopy. Up to a coverage of ca. 0.5 (Se atoms to substrate atoms) a smooth adlayer is obtained with a 2 × √3 structure. When the coverage is increased, at around 0.55 V further deposition is paralleled by a roughening starting at the monoatomic steps. The adsorbed Se is stable in the SeO free solution. For coverages below 0.25, separate domains for the Se covered regions and Se free regions were observed for potentials above 0.6 V. Since this is the potential of the spike corresponding to adsorption of OH at the clean Rh(111) surface in HClO, we have to assume that Se and OH adsorb in separate domains. At lower potentials, where OH is desorbed, Se spreads over the complete surface which then appears completely smooth in the STM images. When the coverage is about 0.25 or above, the roughening is also observed in SeO free solution, demonstrating that the rough structures are not due to disordered deposition, but really due to a roughening by place exchange.