An oriented head-to-tail cluster formation of an endohedral yttrium metallofullerene, Y@c82, on a Cu( 11 1) 1 x 1 clean surface has been observed directly by the use of a scanning tunneling microscopy (STM) under ultrahighvacuum conditions. The STM observation reveals that the Y@c82 molecules preferentially form dimers and one-dimensional clusters at the step edge of the Cu ( 1 1 1) surface. The observed intermolecular distance (1 1.2 f 0.5 A) is smaller than that of the simple Y@c82---Y@& van der Waals distance (1 1.4 A), indicating the presence of a strong interfullerene interaction. The STM results suggest the presence of strong dipoledipole and charge-transfer interactions among Y@c82 fullerenes. Such a molecule has a great similarity to the "superatom" features proposed theoretically in a semiconductor heterostructure.

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