C60 on strain-relief patterns of Ag/Pt(111):: Film orientation governed by template superstructure

The adsorption, growth and overlayer formation of C-60 fullerene molecules on strain-relief patterns induced by 2 monolayers of Ag on Pt(111) (Ag(2ML)/Pt(111)) is investigated by means of scanning tunneling microscopy. The Ag(2ML)/Pt(111) template surface consists of periodic crossing dislocations separating face-centered cubic (fcc) and hexagonal close-packed (hcp) stacking domains. At room temperature, C-60 molecules are found to be sufficiently mobile on the template surface to cross the dislocations and to self-assemble into large hexagonally close-packed two-dimensional islands. The nucleation and growth of the C-60 islands takes place not only at the step-edges, but also in the middle of terraces. The islands cover as well fcc as hcp domains of the template, in a near commensurate fashion with respect to the Ag(2ML)/Pt(111) superstructure. This commensurability manifests itself by a transfer of the strain relief superstructure into the molecular film via a "lock to register" mechanism. The Ag(2ML)/Pt(111) superstructure acts on the molecular film by orientating the C-60 close-packed rows along the Ag < 1(1) over bar 0 > direction. This is different from the usual orientation of a C-60 film on a standard and unreconstructed Ag(111) substrate, namely it is rotated by 30 degrees. The C-60 "lock to register" phenomenon can find interpretation in a strong substrate-molecule interaction, which can result in a significant compression of the C-60 lattice. Here, a special role is played by Ag(2ML)/Pt(111) small hcp stacking domains enclosed by strong discommensuration lines, foremost responsible for the "lock to register" mechanism.