HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY ON C-60 AND C-70 ULTRATHIN FILMS

Vibrational spectra of C-60 and C-70 monolayer films grown on cleaved surfaces of MoS2 have been measured by high-resolution electron energy loss spectroscopy, In the case of a C-60 monolayer film, a dipole-act ive peak, which was not observed in the case of a thick film, was obse rved under the specular reflection condition, This indicates that the symmetry of C-60 molecules is broken by the existence of the surface o f MoS2. The spectrum of C-60 monolayer film under the off-specular ref lection condition, on the other hand, resembles the spectrum of a thic k film, indicating that the lowering of symmetry of C-60 molecules is neither caused by charge transfer nor by chemical bonding between C-60 and MoS2. In the case of C-70, an IR-active mode which has a dipole m oment parallel to the 5-fold axis disappeared in the spectrum taken un der the specular reflection condition, This indicates that the 5-fold axis of C-70 molecules are parallel to the MoS2 surface.