Investigation of a two-dimensional dysprosium germanide on Ge(111): A medium-energy ion-scattering study including direct observation of a reversal of top-layer buckling upon adsorption of hydrogen

A two-dimensional dysprosium germanide of stoichiometry DyGe2 on the Ge(111 ) surface, with a nominal Dy coverage of one monolayer, has been investigat ed using medium-energy ion scattering. A quantitative structural analysis h as revealed that the Dy atoms reside beneath a well-ordered Ge(111) bilayer , rotated by 180 degrees with respect to the bulk. Upon adsorption of hydro gen, the buckling of the top layer was seen to reverse with a corresponding expansion of the Dy-Ge bond length. Possible generalizations for other rar e earth/semiconductor systems are discussed.