CHARACTERIZATION OF OLIGOTHIOPHENE FILMS BY HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY

Electronic structures of quinquethiophene (5T) and sexithiophene (6T) polycrystalline films deposited by evaporation on Sold substrates were studied by high resolution electron energy loss spectroscopy (HREELS) . Vibrational spectra were compared to infrared spectroscopy and Raman scattering results. They reveal that surfaces are not rough and are f ree from contaminants. Energy gap values of 2.38 +/- 0.02 and 2.27 +/- 0.02 eV for 5T and 6T films, respectively, were evaluated from electr onic spectra. Comparison with optical spectroscopy enabled the assignm ent of losses corresponding to optically allowed and forbidden transit ions. Some of the latter ones located at 1.2, 1.7 and 2.0 eV were obse rved in 5T films and were assigned to S-o-->T-n transitions. Different ial cross section analysis reveals the presence of resonance processes in the excitation mechanism. Ionization potentials were estimated to he 4.7 +/- 0.5 eV for both films. (C) 1998 Elsevier Science S.A. All r ights reserved.