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 -: art. no. 154703

Near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy was adopted to probe the unoccupied electronic states of C-60 anchored onto an organized assembly of 11-amino-1-undecane thiol on Au(111). The polarization dependence of the intensity of p* resonance associated with C-60 p 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 C-60 and/or 1-ML C-60/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 P-z of -NH2 group of thiolate SAM and p levels of C60, resulting in a nucleophilic addition with a change in the symmetry of C-60 from I-h to C1 in the SAM. Occurrence of a new feature at 285.3 eV in the NEXAFS spectrum, assigned previously to p* graphitic LUMO, signified the formation of aggregated clusters, (C-60)(n) of C-60 monomer. Low tunneling current scanning tunneling microscopy confirmed them to be spherical and stable aggregates with n similar to 5. 2005 American Institute of Physics.