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

A. S. Komolov, E. F. Lazneva, N. B. Gerasimova, Yu A. Panina, V. S. Sobolev, A. V. Koroleva, S. A. Pshenichnyuk, N. L. Asfandiarov, A. Modelli, B. Handke, O. V. Borshchev, S. A. Ponomarenko

Research output

Abstract

The atomic composition and the electronic properties of ultrathin films of the four-conjugated rings thiophene/phenylene co-oligomers end-terminated by the −CH3 and by the -CF3 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 CH3-phenylene-thiophene-thiophene-phenylene−CH3 (CH3-PTTP−CH3) and CF3-phenylene-thiophene-thiophene-phenylene-CF3 (CF3-PTTP-CF3) molecules studied. The TCS measurements during the increase of the organic layer thickness up to 6 nm on a (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. 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 CH3-PTTP−CH3 film and at 3.9 eV in the case of the CF3-PTTP-CF3 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 CF3-PTTP-CF3 film compared to the case of the CH3-PTTP−CH3 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 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 EF.

Original languageEnglish
Pages (from-to)40-45
Number of pages6
JournalJournal of Electron Spectroscopy and Related Phenomena
Volume235
DOIs
Publication statusPublished - 1 Aug 2019

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Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Radiation
  • Atomic and Molecular Physics, and Optics
  • Condensed Matter Physics
  • Spectroscopy
  • Physical and Theoretical Chemistry

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