An Oriented Cluster Formation of Endohedral Y@C82 Metallofullerenes on Clean Surfaces

An oriented head-to-tail cluster formation of an endohedral yttrium metallofullerene, Y@C{sub 82}, on a Cu(111)I x 1 clean surface has been observed directly by the use of a scanning tunneling microscopy (STM) under ultrahigh-vacuum conditions. The STM observation reveals that the Y@C{sub 82} molecules preferentially form dimers and one-dimensional clusters at the step edge of the Cu(111) surface. The observed intermolecular distance (11.2 {+-} 0.5 A) is smaller than that of the simple Y@C{sub 82}, Y@C{sub 82} van der Waals distance (11.4 A), indicating the presence of a strong interfullerene interaction. The STM results suggest the presence of strong dipole-dipole and charge-transfer interactions among Y@C{sub 82} fullerenes. Such a molecule has a great similarity to the `superatom` features proposed theoretically in a semiconductor heterostructure. 30 refs., 5 figs.