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PCBM was synthesized by following the previously reported procedure.[] 1 Preparation of thin PCBM films and STM experiments were carried out in a Ultra-High Vacuum (UHV) chamber with a base pressure of 5× 10-10 Torr, and equipped with standard facilities for metal surface preparation, a lowtemperature effusion cell and an Aarhus-type variable-temperature, fast scanning STM purchased from SPECS. Atomically clean, crystalline Au (111) surfaces were prepared by standard sputter/anneal procedures (sputter at 1 kV for 15 min followed by annealing to 800 K for another 15 min), which resulted in large terraces (about 200 nm wide), separated by monoatomic steps and displaying the 22×√ 3 herringbone reconstruction. PCBM was deposited from a resistively heated glass crucible at 400 K onto the clean Au (111) substrate held at room temperature.

Once the sample has been prepared by the aforementioned protocol, it is transferred to the STM, which has been previously cooled to 150 K. Measurements are carried out at low temperatures in order to freeze molecular diffusion. Tunneling conditions are chosen so as to not disturb individual PCBM molecules (V≈ 1 V bias and I≈ 100-500 pA). Theoretical calculations were performed in the framework of the density functional theory (DFT) with the B3LYP fuctional for exchange and correlation. Due to the large size of PCBM, all calculations were done for molecules in the gas phase without considering the surface. Geometry optimizations for PCBM monomer and dimers were performed with 6-31G* basis set, which has been shown to provide accurate geometries and relative energies of …