Self-assembly of C ₆₀ on a ZnTPP/Fe(001)– p (1 × 1)O substrate: observation of a quasi-freestanding C ₆₀ monolayer

Fullerene (C ₆₀ ) has been deposited in ultrahigh vacuum on top of a zinc tetraphenylporphyrin (ZnTPP) monolayer self-assembled on a Fe(001)– p (1 × 1)O substrate. The nanoscale morphology and the electronic properties of the C ₆₀ /ZnTPP/Fe(001)– p (1 × 1)O heterostructure have been investigated by scanning tunneling microscopy/spectroscopy and ultraviolet photoemission spectroscopy. C ₆₀ nucleates compact and well-ordered hexagonal domains on top of the ZnTPP buffer layer, suggesting a high surface diffusivity of C ₆₀ and a weak coupling between the overlayer and the substrate. Accordingly, work function measurements reveal a negligible charge transfer at the C ₆₀ /ZnTPP interface. Finally, the difference between the energy of the lowest unoccupied molecular orbital (LUMO) and that of the highest occupied molecular orbital (HOMO) measured on C ₆₀ is about 3.75 eV, a value remarkably higher than those found in fullerene films stabilized directly on metal surfaces. Our results unveil a model system that could be useful in applications in which a quasi-freestanding monolayer of C ₆₀ interfaced with a metallic electrode is required.