Oligothiophene films under electron irradiation: electron mobility and contact potentials

This work compares mobility in various films of alpha-oligothiophenes with chainlengths ranging from four to six thiophene rings (4T, 5T and 6T), which were studied by high-resolution electron energy-loss spectroscopy (HREELS) in the same conditions of beam intensity and geometrical configuration. The increase of electron mobility going from 4T to 6T was estimated from an analysis of the energy extent in the complete spectra of the electrons backscattered from the sample surface. The same method was still applied to two films of 6T deposited on different substrates, gold and highly oriented pyrolitic graphite (HOPG), where a change in the molecular orientation is expected. The same value was found for the electron mobility in both films. However, different contact potentials were observed and no charge accumulation was detected. On the other hand, work functions were estimated for both films using the work functions values found in the literature for gold and HOPG, which are 4.6 and 4.5 eV, respectively. In this case, it was not possible to conclude if a different molecular orientation produced a change in the work function or not, as the difference between the work function values of the substrates given in the literature is larger than 0.1 eV. (C) 2002 Elsevier Science B.V. All rights reserved.