Surface electronic properties of Cs-fullerides

Electronic properties of fullerides were monitored during the intermixing of Cs atoms and C-60 molecules in solid phase. By combining the ultraviolet photoionization spectroscopy (UPS) and metastable de-excitation spectroscopy (MDS) techniques, the properties of the outermost layer were extracted out of the subsurface region. Cs/C-60 heterostructures, differing in the number of Cs atoms per one C-60 molecule, were obtained by applying various preparative methods. Different preparation methods may result in formation of the same bulk material (in the sense of UPS), however, the resulting surfaces exhibit quite different electronic properties as probed by MDS. In particular, the presence and the state of excess Cs atoms on top of a saturated fulleride phase (Cs-x*C-60, x* = 6) depend strongly on the formation method used. An electronically unique state of Cs atoms on top of the saturated phase was found and attributed to small metallic Cs clusters. The surface electronic properties result from competition between occupation of the C-60 t(1u)-derived state and the delocalization degree of the Ca 6s electrons. Both properties are strongly affected by thermal treatment of the fulleride phase. (C) 1997 Elsevier Science B.V.