Electronic structure of the conduction band of the interface region of ultrathin films of substituted perylenedicarboximides and the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band and the interfacial potential barrier during the formation of interfaces of dioctyl-substituted perylenedicarboximide (PTCDI-C$_8$) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films with the oxidized germanium surface have been presented. The experimental results have been obtained using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode at energies in the range from 5 to 20 eV above the Fermi level $E_{\operatorname{F}}$. The positions of the maxima of the fine structure of total current spectra (FSTCS) of the PTCDI-C$_8$ and PTCDI-Ph films differ significantly in the energy range from 9 to 20 eV above the Fermi level $E_{\operatorname{F}}$, which can be associated with the difference between the substituents of the chosen molecules, dioctyl- and diphenyl-, respectively. At the same time, the positions of the lowenergy maxima in the FSTCS spectra at an energy 6–7 eV above the Fermi level $E_{\operatorname{F}}$ for the PTCDI-C$_8$ and PTCDI-Ph films almost coincide with each other. It has been suggested that these maxima are attributed to the electronic states of the perylene core of the molecules under investigation. The process of the formation of interfacial potential barriers of the PTCDI-C8 and PTCDI-Ph films with the oxidized germanium surface has been analyzed. It has been found that the work functions of the surface, $E_{\operatorname{vac}}$–$E_{\operatorname{F}}$, differ little from 4.6 $\pm$ 0.1 eV over the entire range of organic coating thicknesses from 0 to 6 nm.