Polarized near-edge x-ray-absorption fine structure spectroscopy of $C60$-functionalized 11-amino-1-undecane thiol self-assembled monolayer: Molecular orientation and Evidence for $C60$ aggregation

Near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy was adopted to probe the unoccupied electronic states of $C60$ anchored onto an organized assembly of 11-amino-1-undecane thiol on Au(111). The polarization dependence of the intensity of $π*$ resonance associated with $C60$ $π$ network revealed the self-assembled monolayer (SAM) system to be oriented with an average molecular tilt angle of 57° with respect to the surface normal. Invoking the absence of solid-state band dispersion effects and in comparison to solid $C60$ and /or 1-ML $C60$/Au(111), the electronic structure of the resulting assembly was found dominated by spectral position shift and linewidth and intensity changes of the lowest unoccupied molecular orbital (LUMO), $LUMO+1$⁠, and $LUMO+2$ orbitals. The latter implied hybridization between N $Pz$ of $−NH2$ group of thiolate SAM and $π$ levels of $C60$⁠, resulting in a nucleophilic addition with a change in the symmetry of $C60$ from $Ih$ to $C1$ in the SAM. Occurrence of a new feature at 285.3 eV in the NEXAFS spectrum, assigned previously to $π*$ graphitic LUMO, signified the formation of aggregated clusters, $(C60)n$ of $C60$ monomer. Low tunneling current scanning tunneling microscopy confirmed them to be spherical and stable aggregates with $n∼5$⁠.