Surface-enhanced Raman scattering studies on C60 fullerene self-assemblies

Surface-enhanced Raman scattering (SERS) was used to investigate C-60 self-assembling in solvents like pyrrolidine (Py) and Nmethyl-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 C-60 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 C-60 and the Au substrate, appearing as a chemical component in SERS generation, is mainly evidenced by a band at similar to342cm(-1). In the aggregated phase, the intermolecular interactions lead to a reduction in the parent I-h C-60 symmetry as observed by a modified phonon spectrum. As a general feature, the spectral range below 800cm(-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 C-60 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 C-60 cage. These complexes are noticed in the SERS spectrum by the replacement of the original H-g(l) band at similar to269cm(-1) with two others at similar to255 and similar to246cm(-1). In the aggregated phase, when using NMP and P as a solvent, the Raman spectrum reveals new bands that appear around 94 and 110-118cm(-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. (C) 2004 Published by Elsevier Ltd.