An oriented head-to-tail cluster formation of an endohedral yttrium metallofullerene, YC82, on a Cu(111)I × 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 YC82 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 YC82, YC82 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 YC82 fullerenes. Such a molecule has a great similarity to the 'superatom' features proposed theoretically in a semiconductor heterostructure.
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