Conduction band electronic states of ultrathin layers of thiophene/phenylene cooligomers on an oxidized silicon surface

The atomic composition and the electronic properties of the ultrathin vacuum deposited films of the four-unit thiophene/phenylene co-oligomers end-terminated by the -СH3 and by the -СF3 radicals vacuum deposited onto the oxidized silicon surface were studied using the x-ray photoelectron spectroscopy (XPS) and total current spectroscopy (TCS) techniques. The relative concentrations of the C, S and F atoms measured from the film deposits were evaluated and it was found to correspond to the chemical formulae of the СH3-phenylene-thiophene-thiophene-phenylene-СH3 (СH3-PTTP-СH3) and СF3-phenylene-thiophene-thiophene-phenylene-СF3 (СF3-PTTP-СF3) molecules studied. The TCS measurements during the increase of the organic layer thickness up to 6 nm on (SiO2)n-Si substrate were used to determine the conduction band peak structure related to the density of the unoccupied electronic states (DOUS) in the range from 5 eV to 22 eV above EF. The theoretical calculations at the B3LYP/6-31G(d)) level were used to determine the DOUS of the two films studied in a broader energy range from 2 eV to 22 eV above EF, which made it possible to determine the position of the lowest unoccupied molecular orbital (LUMO) peak at 4.4 eV in the case of the СH3-PTTP-СH3 film and at 3.9 eV in the case of the СF3-PTTP-СF3 film. The effect of the fluorine substitution on the DOUS was determined as approximately 2 eV peak shift towards low electron energies in the case of the СF3-PTTP-СF3 film compared to the case of the СH3-PTTP-СH3 film in the electron energy range from 10 eV to 22 eV above EF and as a more complex peak restructuring in the energy range from 5 eV to 10 eV above EF. The DOUS peaks of the two films studied were assigned to have * character in energy range from 9.5 eV to 22 eV and to have * character in energy range from 5 eV to 9.5 eV above EF.