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. © 2005 American Institute of Physics.