Unoccupied Electronic States at the Interface of Oligo(phenylene-vinylene) Films with Oxidized Silicon

Ultrathin films of trioligo(phenylene-vinylene) and of dinitro-substituted trioligo(phenylene-vinylene) end terminated by dibutyl-thiole were thermally deposited in an ultrahigh vacuum (UHV) on an oxidized silicon substrate (SiO2)n-Si. The surface work function and the density of the unoccupied electron states (DOUS) located 5–20 eV above the Fermi level (EF) were studied during the film deposition using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. The electronic structure typical for the organic films studied was formed, while the organic deposit thickness increased up to 5–6 nm. The DOUS peak structure of the organic films studied obtained from the TCS results showed good correspondence to the DOUS peaks obtained from the density functional theory (DFT) calculations. The calculations were made for the model oligo(phenylene-vinylene) films where the neighboring molecules had relatively weak interaction, which is also characteristic of the disordered films studied experimentally. The comparison of the DOUS spectra of the unsubstituted film and of the dinitro-substituted film showed that the substitution had a minor effect on the DOUS for the energies higher than 10 eV above EF, while in the energy range from 5 to 10 eV above EF a significant difference between DOUS of the two films studied was observed. The 0.3 and 0.7 eV increase of the surface work function values was observed during the formation of the unsubstituted trioligo(phenylene-vinylene) and of the dinitro-substituted trioligo(phenylene-vinylene) deposits on the substrate surface, respectively. The interfacial charge transfer was characterized by the formation of the 2 nm thick polarization layer in the organic films and by the negative charge transfer to the organic layer from the (SiO2)n-Si substrate.

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