STM study of the adsorption of single C-60 molecules on the Ge(111)-c(2x8) surface

The interaction of isolated C-60 molecules with the Ge(111)-c(2 x 8) surface is studied using scanning tunneling microscopy. Following room temperature deposition, the C-60 molecules seem to be randomly distributed on the surface. However, in most cases the adatom arrangement of the c(2 x 8) structure is perturbed close to the C-60 molecules. A rather typical defect is the shift of a part of a Ge adatom row by one surface lattice constant. We present strong evidence that a large number of these defects is produced upon C-60 adsorption. We show that in most cases the defective adatom arrangement in the vicinity of the C-60 must be accompanied by unsaturated Ge surface atoms, which exhibit a half-filled dangling bond state, underneath the molecules. We propose that the C-60-substrate bond is mediated by a charge transfer from these substrate dangling bond states to the C-60 molecules.