Unoccupied electronic band structure of conjugated molecular films interfacing polycrystalline gold surface

Abstract

Thin films of Cu-phthalocyanine (CuPc) and of tri-oligo(phenylene–vinylene) end terminated by di-butyl-thiole (tOPV) were thermally deposited in situ in UHV on polycrystalline Au substrate surfaces. The surface potential, the surface work function and the structure of density of unoccupied electron states (DOUS) located 5–25 eV above the Fermi level ( E F) were monitored during the film deposition, using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Auger electron spectroscopy (AES) was used to monitor atomic composition of the surfaces under study. The deposition of the CuPc films resulted in formation of the DOUS structure typical for these films while the deposition of tOPV films of less than 2 nm thickness resulted in formation of an intermediate DOUS structure which was replaced by a stable DOUS of tOPV along with a further increase of the deposit thickness. The electronic work function of the CuPc and the tOPV films changed during the film deposition until it reached a stable value of 4.4 ± 0.1 and 4.3 ± 0.1 eV, respectively, at a film thickness of 8–10 nm. The width of the interface dipole layer in the CuPc and the tOPV films interfacing with the Au surface did not exceed 2 nm. Analysis of the TCS data allowed us to assign the major DOUS features of the CuPc and tOPV films under study.