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

A two-dimensional fluorinated fullerene (C(60)F(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(60)F(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(60)F(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(60)F(36) to an electron transport layer in organic electronic devices.