Conduction band electronic states of ultrathin layers of thiophene/phenylene co-oligomers on an oxidized silicon surface

Abstract The atomic composition and the electronic properties of ultrathin films of the four-conjugated rings thiophene/phenylene co-oligomers end-terminated by the −CH 3 and by the -CF 3 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 were found to correspond to the chemical formulae of the CH 3 -phenylene-thiophene-thiophene-phenylene−CH 3 (CH 3 -PTTP−CH 3 ) and CF 3 -phenylene-thiophene-thiophene-phenylene-CF 3 (CF 3 -PTTP-CF 3 ) molecules studied. The TCS measurements during the increase of the organic layer thickness up to 6 nm on a (SiO 2 )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 E F . DOUS of the films investigated was also obtained theoretically by means of ab initio calculations. According to the calculations, the energy position of the lowest unoccupied molecular orbital (LUMO) peak was found at 4.4 eV in the case of the CH 3 -PTTP−CH 3 film and at 3.9 eV in the case of the CF 3 -PTTP-CF 3 film. The effect of fluorine substitution on the DOUS was determined as approximately 2 eV peak shift towards low electron energies in the case of the CF 3 -PTTP-CF 3 film compared to the case of the CH 3 -PTTP−CH 3 film in the electron energy range from 10 eV to 22 eV above E F and as a more complex peak restructuring in the energy range from 5 eV to 10 eV above E F . The DOUS peaks of the two films studied were assigned to molecular orbitals with π* character in the 5–9.5 eV energy range and molecular orbitals with σ* character in the 9.5–22 eV energy range above E F .