Two-dimensional superstructure formation of fluorinated fullerene on Au(111): A scanning tunneling microscopy study

A two-dimensional fluorinated fullerene (C ₆₀F ₃₆) superstructure has been successfully formed on Au(111) and was investigated using scanning tunneling microscopy (STM) and density functional theory calculations. Although there exist three isomers (C ₃, C ₁, and T) in our molecular source, STM images of the molecules in the well-ordered region all appear identical, with 3-fold symmetry. This observation together with the differences in the calculated lowest unoccupied molecular orbital (LUMO) distribution among the three isomers suggests that a well-ordered monolayer consists of only the C ₃ isomer. Because of the strong electron-accepting ability of C ₆₀F ₃₆, the adsorption orientation can be explained by localized distribution of its LUMO, where partial electron transfer from Au(111) occurs. Intermolecular C-F·π electrostatic interactions are the other important factor in the formation of the superstructure, which determines the lateral orientation of C ₆₀F ₃₆ molecules on Au(111). On the basis of scanning tunneling spectra obtained inside the superstructure, we found that the LUMO is located at 1.0 eV above the Fermi level (E _F), while the highest occupied molecular orbital (HOMO) is at 4.6 eV below the E _F. This large energy gap with the very deep HOMO as well as uniform electronic structure in the molecular layer implies a potential for application of C ₆₀F ₃₆ to an electron transport layer in organic electronic devices.