Surface-enhanced Raman scattering studies on C60 fullerene self-assemblies Original Research Article

Surface-enhanced Raman scattering (SERS) was used to investigate C60 self-assembling in solvents like pyrrolidine (Py) and N-methyl-2-pyrrolidinone (NMP) as well as in binary mixtures of o-dichlorobenzene (DCB)/acetonitrile (ACN) and DCB/NMP. For a correct evaluation of the modifications of Raman spectra induced by the C60 aggregation, we have also presented the variations due to the measuring method, i.e., the signal dependence of the metallic support type and the surface roughness. The interaction between C60 and the Au substrate, appearing as a chemical component in SERS generation, is mainly evidenced by a band at ∼342 cm−1. In the aggregated phase, the intermolecular interactions lead to a reduction in the parent Ih C60 symmetry as observed by a modified phonon spectrum. As a general feature, the spectral range below 800 cm−1 is the most diagnostic for the aggregate assignment, the main indicative being the disappearance of the Raman bands associated to the radial vibration modes. SERS measurements have revealed two stages in the self-assembling of C60 in NMP. In the beginning, charge-transfer molecular complexes that associate slowly in stable aggregates are formed by the binding of an NMP molecule to the C60 cage. These complexes are noticed in the SERS spectrum by the replacement of the original Hg(1) band at ∼269 cm−1 with two others at ∼255 and ∼246 cm−1. In the aggregated phase, when using NMP and P as a solvent, the Raman spectrum reveals new bands that appear around 94 and 110–118 cm−1, which are associated with the interball interactions. In a DCB/ACN solvent mixture, the self-assembling process is driven by weak van der Waals type forces and resembles a precipitation, yielding C60 clusters of different size.