Interface formation between oligo(phenylele–vinylene) films and highly ordered pyrolytic graphite and Ge(1 1 1) surfaces

Abstract

Thin films of trioligo(phenylene–vinylene) end terminated by dibutylthiole (tOPV) were thermally deposited in UHV on highly ordered pyrolytic graphite (HOPG) and on Ge (1 1 1) substrates. The surface potential and the structure of unoccupied electron states (DOUS) located 1–25 eV above vacuum level were monitored during the film deposition, using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. The electronic work function of the tOPV films changed during the film deposition until it reached a stable value of 4.3±0.1 eV at the film thickness of 8–10 nm. Deposition of the tOPV under 3 nm led to formation of intermediate DOUS structures that were different for the cases of the two substrates used in the study. The intermediate structures got replaced by a DOUS structure of the tOPV films which was the same for the two substrates, along with the increase of the deposit thickness to 8–10 nm. A significant electron transfer from the tOPV film to Ge (1 1 1) surface was observed and the charge transfer layer in the film extended up to 10 nm. No significant charge transfer was observed at the interfaces between the tOPV film and the HOPG substrate. A substantial reconfiguration of the electronic structure of the tOPV films due to the interaction with the Ge(1 1 1) and also the HOPG surfaces was deduced and the charge transfer at the tOPV–Ge(1 1 1) interface was found to be consistent with the extended interface dipole model.